Specialized Molding Techniques: Application, Design, Materials and Processing

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Edited

by

Hans-Peter

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Heim

H. Potente Plastics Design Library

Copyright © 2001, Plastics Design Library. All rights reserved. ISBN 1-884207-91-X Library of Congress Control Number: 2001091835

Published in the United States of America, Norwich, NY by Plastics Design Library a division of William Andrew Inc. Information in this document is subject to change without notice and does not represent a commitment on the part of Plastics Design Library. No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information retrieval and storage system, for any purpose without the written permission of Plastics Design Library. Comments, criticism and suggestions are invited and should be forwarded to Plastics Design Library. Plastics Design Library and its logo are trademarks of William Andrew Inc.

Please Note: Great care is taken in the compilation and production of this volume, but it should be made clear that no warranties, express or implied, are given in connection with the accuracy or completeness of this publication, and no responsibility can be taken for any claims that may arise. In any individual case of application, the respective user must check the correctness by consulting other relevant sources of information. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

Manufactured in the United States of America.

Plastics Design Library, 13 Eaton Avenue, Norwich, NY 13815 Tel: 607/337-5080 Fax: 607/337-5090

Preface

Over the next few decades, the development of our society will be characterized by two particular factors: 1. the increasing importance of the medical sector, coupled with growing requirements on medical engineering products. 2. a continuing key role for mobility within society, with growing emphasis on finding a solution to the associated ecological problems. Plastics will play a crucial role in both cases. With properties ranges that can be widely adjusted and ease of processing, plastics can be used to produce highly-integrated, customized product solutions for medical-engineering applications, as well as products for the automotive sector and telecommunications. The plastics sector is far from having exhausted the innovation potential that exists. What is required are material, process engineering, and mechanical engineering-based approaches to innovation which will make it possible to respond to ever more demanding applications or the substitution of other materials by plastics. The development trends currently emerging in plastics engineering are essentially the combination of different processes (e.g., injection molding and surface refinement), the combination of different materials (e.g., plastic/metal composites), the integration of a wide range of functions within a single component, the improvement of surface and/or optical properties, and also reduced material consumption and the recyclability of the materials employed. Much of the innovation potential lies in the injection molding process. Special injection molding processes such as the thin-wall technique, micro-molding, hybrid processes and multi-component processes, are playing an increasingly large role in the processing of plastics. At the same time, the requirements placed on product quality, production quality and also part precision are rising, while costs are expected to be reduced wherever possible. This combination of objectives can only be achieved by an appropriate increase in productivity from process-engineering innovations and a reduction in the number of process steps. From the microeconomic angle, the challenge facing a plastic-processing company today is one of retaining its market position through readiness and ability to innovate. This applies particularly to the opening up of new application potential and the application technology that this requires, as well as to the crucial aspect of employee skills. Specialist know-how is essential in order to survive on the market – in other words, injection molding

vi

Preface

companies are called upon to acquire know-how on new materials and material combinations as well as on process variants and special injection molding processes. It is at this point that the book commences. It constitutes a collection of experimental and theoretical studies in the field of special injection molding processes. The papers that have been brought together in this book were presented at International SPE (Society of Plastic Engineers) Conferences over the period 1996 to 2000. The presenters of the papers are scientists and representatives from eminent institutes and companies who have made a key contribution towards the continuing development of injection molding technology through their work. The manuscripts printed in the book represent an extract of their current work and research results. This book is not therefore intended as a textbook but rather as specialist reading, progressing far beyond the basic principles of application technology in injection molding. A broad range of specialized subjects is covered, including process engineering, material science, mechanical engineering, and mold engineering. The topics treated are divided into the following subject areas, which follow on from an initial overview of the special injection molding processes that are currently in use: Gas Assisted Injection Molding (GAIM) • influence of process parameters • molded part design and mold layout • the special processes of powder injection and reaction injection molding Thin Wall Molding • influences on molded part quality • avoidance of errors during application • influence on cycle time and costs Molding of Micro Parts and Micro Structures • reproduction of high-precision surface structures • optical applications • simulation techniques and mechanical engineering Improving Material Properties • modified plastics for automotive applications • improved molded part properties through vibrational molding Molding of Composites • compression molding • vacuum assisted liquid molding • resin transfer molding • inmold decoration

Preface

vii

Mold Making and Plasticisation automation technology mold engineering for special injection molding processes twin-screw injection molding screw optimization This book presents the state of the art in all the areas presented. The authors of the individual contributions give their views on the uses and limitations of new injection molding technologies. They thus offer the know-how that is required to exploit the innovation potential of plastics engineering. • • • •

Helmut Potente and Hans-Peter Heim Paderborn, June 2001

Table of Contents

Preface Helmut Potente and Hans-Peter Heim One-shot Manufacturing: What is Possible with New Molding Technologies James F. Stevenson

v 1

Chapter 1: Gas Assisted Injection Molding

15

Gas Assist Injection Molding. The North American Legacy Jack Avery Flow Directions in the Gas Assisted Injection Molding Technology Young Soo Soh and Chan Hong Chung Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties Kurt W Koelling and Ronald C Kaminski Cover Part as an Application Example for Gas-assisted Injection Molded Parts Michael Hansen Molded Part Design for the Gas Injection Technique H. Potente and H.-P. Heim Design Optimization of Gas Channels for an Air Cleaner Assembly Using CAE Simulations D.M. Gao, A. Garcia-Rejon, G. Salloum and D. Baylis The Occurrence of Fiber Exposure in Gas Assist Injection Molded Nylon Composites Shih-Jung Liu and Jer-Haur Chang Saving Costs and Time by Means of Gas-assisted Powder Injection Molding Christian Hopmann, Walter Michaeli Gas-assisted Reaction Injection Molding (GRIM): Application of the Gas Injection Technology to the Manufacturing of Hollow Polyurethane Parts I. Kleba, E. Haberstroh

15 27

35 43 51

57

65 73

79

Chapter 2: Thin Wall Molding

89

Thin Wall Processing of Engineering Resins: Issues and Answers Larry Cosma

89

ii

Table of Contents

Effects of Processing Conditions and Material Models on the Injection Pressure and Flow Length in Thinwall Parts A. J. Poslinski 10 Common Pitfalls in Thin-Wall Plastic Part Design Timothy A. Palmer Flow Instabilities in Thin-wall Injection Molding of Thermoplastic Polyurethane Christian D. Smialek, Christopher L. Simpson Pressure Loss in Thin Wall Moldings John W. Bozzelli, Jim Cardinal, and Bill Fierens Integrating Thin Wall Molder’s Needs into Polymer Manufacturing W. G. Todd, H. K. Williams, D. L. Wise Thinning Injection Molded Computer Walls Lee Hornberger and Ken Lown

Chapter 3: Molding Micro Parts and Micro Structures

99 107 113 121 127 133

143

Transcription of Small Surface Structures in Injection Molding – an Experimental Study 143 Uffe R. Arlø, Erik M. Kjær Injection Molding of Sub- µ m Grating Optical Elements 149 R. Wimberger-Friedl Process Analysis and Injection Molding of Microstructures 157 Alrun Spennemann and Walter Michaeli Simualtion of the Micro Injection Molding Process 163 Oliver Kemmann, Lutz Weber, Cécile Jeggy, Olivier Magotte, and François Dupret

Chapter 4: Manufacturing of Composites Melt Compression Molding (MCM) a One-shot Process for In-mold Lamination and Compression Molding by Melt Strip Deposition Georg H. Kuhlmann In-mold Lamination Back Compression Molding Thomas Huber Analysis and Characterization of Flow Channels during Manufacturing of Composites by Resin Transfer Molding R. V. Mohan, K. K. Tamma, S. Bickerton, S. G. Advani and D. R. Shires Optimization of Channel Design in VARTM Processing Roopesh Mathur, Suresh G. Advani and Bruce K. Fink

171 171 187

193 209

Table of Contents

Injection Compression Molding. A Low Pressure Process for Manufacturing Textile-Covered Mouldings Carsten Brockmann, Walter Michaeli Kurz-Hastings Inmold Decoration Roy Bomberger

Chapter 5: Improving Material Properties High Impact Strength Reinforced Polyester Engineering Resins for Automotive Applications Mengshi Lu, Kevin Manning, Suzanne Nelsen, and Steve Leyrer Control of Internal Stresses in Injection Molded Parts Through the Use of Vibrational Molding, “RHEOMOLDINGSM”, Technology Akihisa Kikuchi, Marc Galop, Harold L. Brown, and Alexander Bubel Experimental Determination of Optimized Vibration-assisted Injection Molding Processing Parameters for Atactic Polystyrene Alan M. Tom, Akihisa Kikuchi, and John P. Coulter Vibrated Gas Assist Molding: Its Benefits in Injection Molding J.P. Ibar

Chapter 6: Mold Making and Plasticisation

iii

215 223

229 229

237

245 253

259

Advances in Stack Molding Technology 259 Vincent Travaglini and Henry Rozema Advanced Valve Gate Technology for Use in Specialty Injection Molding 267 John Blundy, David Reitan, and Jack Steele In-mold Labeling for High Speed, Thin Wall Injection Molding 273 Gary Fong Advances in Fusible Core Technique 281 E. Schmachtenberg and O. Schröder Processing Glass-filled Polyethylene on a Twin-screw Injection Molding Extruder 287 David Bigio, Rajath Mudalamane, Yue Huang and Saeid Zerafati Injection Molding by Direct Compounding 295 Bernd Klotz Improvement of the Molded Part Quality: Optimization of the Plastification Unit 301 S. Boelinger and W. Michaeli

iv

Table of Contents

Non-return Valve with Distributive and Dispersive Mixing Capability Chris Rauwendaal

307

Index

313

One-shot Manufacturing: What is Possible with New Molding Technologies James F. Stevenson GenCorp Technology Center, Akron, OH 44305, USA

INTRODUCTION New molding technologies1 together with a revolution in thinking about how to design and manufacture products2-3 have merged to open exciting new possibilities in polymer part manufacturing. The new technologies offer versatile, cost effective forms of materials and more unified, efficient production methods. Even greater benefits, especially from consolidation, exist for parts previously made of metal. These processes move forward and, in some cases, realize the goal of forming a complex product in a single manufacturing step, or one-shot manufacturing. Conventional manufacturing processes generally employ homogeneous materials and simple primary shaping processes to form components which were then assembled by various joining methods. These subassemblies then go through various secondary operations and ultimately are combined to form the finished product. This sequential process is labor intensive, time consuming, and costly; it requires large inventories and long changeover times and is prone to produce scrap. This paper presents and analyzes common features of ten of the new material-process technologies. These technologies, along with chapters on predicting orientation and warpage by T.A. Osswald and on lean molding by Colin Austin are presented in greater depth in a recent book Innovation in Polymer Processing: Molding.1 All of these process and material innovations are based on antecedent technology and well-known physical principles. The key to realizing these innovations was conceptual.

NEW TECHNOLOGIES The innovative molding technologies are summarized in Tables A1-A10 which also include a listing of advantages/disadvantages, applications, and materials. These tables are self contained; readers are referred to them as independent sources of information and as descriptive material for the processes cited in Tables 1-3.

2

Special Molding Techniques

PRODUCT ELEMENTS The framework given in Tables 1-3 facilitates classification and comparison of the new molding technologies. It serves both as a guide to applications and as a means of locating opportunities and gaps in the new technologies. Technologies at similar locations in the tables can be considered as potential alternatives for each other. In the tables materials are divided into polymers, consisting of rubber and plastics, and nonpolymers, primarily solids or gases. Solids, typically fibers, serve as reinforcements, whereas gases reduce density or increase stiffness for a given cross-sectional area by distributing material to increase the moment of inertia. The term macroscopic refers to dimensions that are on the order of the thin part dimension, e.g., a 2-mm diameter void in a 3-mm rod. Microscopic means dimensions two orders of magnitude or more smaller (e.g., a 300-layer laminate) than the part thickness. Achieving one-shot manufacturing requires an optimal combination of MATERIALS, PROCESS, and GEOMETRY. MATERIALS In terms of MATERIALS, the innovative molding technologies are classified according to COMPOSITION, one or more POLYMERS with or without NONPOLYMERS, and SCALE, MICROSCOPIC or MACROSCOPIC, as shown in Table 1. Table 1. Materials: composition and scale Composition Scale

Polymers combined with Other polymers

Gas

Solids

Macroscopic

Blow Molding Coinjection Molding (MMP)* Multimaterial Molding (MMP) In-Mold Coating (MMP) Dual Molding (LPM)*

Gas-Assisted Molding Liquid-Gas Molding (LPM) Blow Molding

Laminate Molding (LPM)

Microscopic

Lamellar Molding

Microcellular Plastics Controlled Density (LPM)

Sheet Composites Reactive Liquid Molding

* Processes designated LPM (Low Pressure Molding) are described in Table A4; those designated MMP (Multimaterial Multiprocess) are given in Table A10.

One-shot Manufacturing

3

Two of the innovative processes shown in Table 1, lamellar molding and microcellular plastics, serve primarily as the means of generating a unique material on the microscopic scale and secondarily for shaping the material. Other processes listed in the table combine polymers and nonpolymers on a microscopic or macroscopic scale to from products where specific properties of the multiple materials meet local functional needs. This localized optimization ultimately enhances overall product performance. PROCESSING PROCESSING is the bridge between the unshaped raw MATERIALS and the GEOMETRY (MACROSCOPIC STRUCTURE and SIZE & SHAPE) of the product as shown in Table 2. Table 2. Processing: materials and geometry Geometry Materials

Macroscopic structure Laminate

Polymers

Polymers and nonpolymers

In-Mold Coating Coinjection Blow Molding Dual Molding (LPM)

Segmented Multimaterial Molding (MMP), Blow Molding

Size & shape Large Sheet Composites In-Mold Coating (MMP) Injection-Compression (MMP)

Solid

Laminate Molding (LPM) Reactive Liquid Molding

Laminate Molding (LPM) Sheet Composites Reactive Liquid Molding

Gas

Blow Molding Gas-Assisted Molding Liquid Gas Molding (LPM) Fusible Core

Blow Molding Controlled Density Molding (LPM)

Hollow Dual Molding (LPM) Blow Molding

Blow Molding Gas-Assisted Molding Liquid Gas Molding (LPM) Fusible Core

Many processes under MACROSCOPIC STRUCTURE in Table 2 allow the combination of a polymer with other polymer(s) or nonpolymers on a macroscopic scale to give

4

Special Molding Techniques

which have uniform properties over the surface of the part but variable properties through the thickness. SEGMENTED PARTS which exhibit a variation of properties along the surface but are uniform over a given cross-section. Combinations of laminated and segmented parts are possible, for example gas-assisted parts with hollow sections only in certain regions of the part. In terms of part SIZE & SHAPE, innovative technologies are generally needed when the part is LARGE, especially when a cosmetic surface is required, or when the part has a COMPLEX, often hollow shape, particularly when it is load bearing. LAMINATES

GEOMETRY Part GEOMETRY can be considered in terms of FUNCTION where the part is located on a scale ranging from ENCLOSURE (containers or panels, often with cosmetic surfaces) to LOAD BEARING, and COMPLEXITY which allows for geometric complexity ranging from SYMMETRIC (planar or axisymmetric) to fully THREE DIMENSIONAL. Table 3. Geometry: function and complexity

One-shot Manufacturing

5

Table 3 suggests that molded parts and the associated innovative processes to make them generally range from more or less symmetric enclosures in the upper left to three dimensional load bearing parts in the lower right.

REFERENCES 1 2 3

Stevenson, J.F. (Ed.) Innovation in Polymer Processing: Molding, Hanser, Munich, Hanser-Gardner, Cincinnati (1996). Womack, J.P., Jones, D.T., and Roos, D., The Machine that Changed the World, Macmillan, New York (1990). Gooch, J., George, M., and Montgomery, D., America Can Compete, Institute of Business Technology, Dallas (1987).

6

Special Molding Techniques

APPENDIX Table A1 and A2. Gas-assisted injection molding: process [1.1] and simulation [2.1] PROCESS: Nitrogen gas under high pressure is injected through the nozzle or mold wall into plastic partially filling a mold. The gas flows preferentially through local thick sections with hot interiors and pushes the plastic ahead to fill the mold. SIMULATION: Commercial software now available to predict gas flow paths, polymer thickness, clamp force, and contraction during cooling for various geometries and process variables including gas pressure, injection time, and prefilled polymer volume. Simulations and experiment generally show • increasing gas pressure decreases fill time, gas penetration distance, and (by conservation of mass) polymer wall thickness, • melt temperature has a variable effect on gas penetration length, • increasing delay time before the start of gas injection increases wall thickness and gas penetration length, • increasing gas injection time increases gas penetration distance, • decreasing the prefilled polymer volume fraction increases the penetration length until a critical level when gas blows through. • increasing gas pressure level and time decreases shrinkage. Simulations are generally able to predict undesirable air traps and gas penetration into thin sections. Simulation of a freezer bottom part converted to gas-assisted molding showed a 70% reduction in packing pressure, feasibility of using a less expensive material, and reduced warpage due to lower, more uniform pressure and higher part stiffness. Eight design guidelines are given based on both experiment and computer simulation [2.1]. Advantages/Disadvantages

Applications

Materials

PROCESS: Part weight and cooling time can be reduced up to 50%. Sink marks are eliminated. Warpage is reduced. Clamp force and injection pressure are lower. Part stiffness is increased because of the higher moment of inertia. Licensing is necessary. SIMULATION: Simulation helps identify optimal process conditions including runner layout and size, and location and timing of gas introduction. Software is available but new.

Handles, Panels with Ribs, Appliance/machine Housings (TV benzels) Automotive Parts (Clutch Pedals, Mirror Housings)

ABS, PA, PE, PP, PS, PPO, PC, PBTP, PC/PBTP, SAN, TPE, TPU

One-shot Manufacturing

7

Table A3. Fusible core injection molding [3.1] Complex hollow parts are formed by injection molding plastic around a fusible alloy core which is subsequently removed by melting. The fusible (or lost) core typically is cast form a bismuth-tin alloy with a eutectic melting point of 138oC. The molten metal fills a split steel mold from the bottom and then cools for 2 min to produce a heavy core with a mirrorlike surface. The still-hot core is positioned by a robot in a steel mold and plastic is injected. Flow channels are designed to balance forces around the core during filling to prevent core movement. For thermoplastics the injection temperature, e.g. 290oC for polyamide, can be well above the melting point of the core since the relatively high thermal diffusivity of the metal maintains a low interface temperature. After demolding, cores are melted out in a large bath or by induction heating or by injecting heat transfer fluid inside hollow cores. Advantages/Disadvantages

Applications

Materials

Plastic parts made by fusible core technology have a weight and cost advantage over metal parts. Fusible core molding eliminates the need for mechanically complex molds or joining separately molded parts. Interior surfaces of fusible core parts are smooth which increases gas flow. Disadvantages are loss or oxidation of expensive core metal and need for robots to handle heavy cores.

Air intake manifolds, tennis racquets, pump parts

PA Poly(etherarylketone)

Table A4. Low pressure molding [4.1] Low pressure molding, as developed by Siebolt Hettinga, enables a number of other molding technologies.In Low Pressure Molding (LPM) the mold cavity is filled at low speed through large gates with a controlled pressure profile in the shape of a broad inverted U. LPM has no packing stage and no cushion. The melt temperature profile is controlled by adjusting screw speed and flow resistance during plastication. LPM works better with low viscosity semicrystalline materials and is not suitable for thin-wall parts. Slow injection, lower melt and higher mold temperatures reduce residual stress to allow demolding at a higher temperature to maintain or reduce cycle times. For larger parts, low clamp force can be achieved using multiple valve gates with programmed opening [4.2]. Lower clamp force allows use of self clamping molds and multistation injectors. Laminate Molding involves molding plastic at low pressure directly behind textile, film, or metal. In Liquid Gas Injection Molding, a volatile liquid is injected at low pressure into the melt and then vaporizes to form hollow channels in the part. The liquid condenses and is absorbed in the part. Dual Molding, similar to Bayer’s Multishell Molding, forms an integrated hollow part by overmolding at low pressure an assembly formed from separately molded parts. In Controlled Density Molding the mold is partially opened once a skin has formed to give a low density interior.

8

Special Molding Techniques

Table A4. Low pressure molding [4.1]

Advantages/Disadvantages

Applications

Materials

Substantial capital costs savings result from the use of presses with a lower camp force or self clamping molds. Laminate Molding saves on assembly and adhesive costs in fabric/plastic laminates.

Low Pressure Molding: Interior Vehicle Panels, Bumper Fascia. Laminate Molding: Fabric/Plastic Seats, Vehicle Trim Panels. Liquid-Gas Assist Molding: Large Chairs, Chair Bases. Dual and Shell Molding: Manifolds, Pump Bodies, Valves, and Fittings. Low Density Molding: Fittings, Electronic Enclosures, Table Tops.

Thermoplastics, especially polyolefins, thermosets

Table A5. Advanced blow molding [5.1] The advanced blow molding technologies described below have greatly extended the versatility and facilitated product design. Deep-Draw Double-Wall Molding employs a mold with four hinged slides and an advancing core which close in a programmed manner around a partially inflated parison to shape a deep draw part. Press Blow Molding is used to form panels between shallow male and female mold halves which press together certain sections and inflate other sections to form hollow stiffening ribs.Three Dimensional (3D) Blow Molding forms serpentine three-dimensional parts without excessive scrap by manipulating the parison and positioning it in a convoluted mold cavity. Positioning the parison can be accomplished by (1) translating in two directions the mold which is titled at an angle, (2) movement of the parison by robotic arms in a mold with multiple sections which close sequentially, and (3) guiding the parison through the mold by sucking air along the length of the mold. Multimaterial Blow Molding employs multiple materials sequentially along the part length, in layers over the part thickness, or on opposite sides of the parison. New material developments include molding of 0.3-in fiber reinforced materials and foam layers [5.2]. Computer simulation of blow molding has been developed by A.C. Technology, Ithaca, NY in cooperation with G.E. Advantages/Disadvantages

Applications

Materials

Deep Draw Technology increases draw (depth-to-length) ratio from 0,3 to 0.7 and allows forming of parts with undercuts, ribs, and noncircular crosssections. Multimaterial applications allow soft surfaces on structural parts, flexible conduits with rigid connectors, or parts with opposite sides of different properties. 3D Blow Molding consolidates complex parts and enhances function.

Insulated containers with foam, Planters, Conduits, Air Ducts, Bumpers, Equipment Panels, Instrument Panels, Portable Toilets, Golf Cases, Arm Rests, Gasoline Filler Tubes, Gas Tanks.

PE, PS, PP, POA, ABS, PPE elastomers

One-shot Manufacturing

9

Table A6. Thermoplastic sheet composites [6.1] Production of thermoplastic sheet composites involves two steps: (1) FORMATION of fiber reinforced sheets (prepreg) by polymer impregnation and sheet consolidation and (2) SHAPING of the sheets. Large volume competitive sheet FORMATION processes are continuous Melt Impregnation (e.g. Azdel sheet by extruding polypropylene onto a continuous “swirled’ fiber glass mat), and Slurry Deposition in which long fibers and polymer powder with dispersing agents are deposited on a moving screen similar to paper making. Other processes are Powder Impregnation (powder and fiber consolidated by pultrusion, double belt press, or compression molding), Reactive Pultrusion, and Commingling (intertwining different fibers) and Coweaving polymer fibers, and reinforcing fibers. SHAPING techniques for consolidated prepreg include Melt-Phase Stamping (prepreg covering the mold cavity is heated with infrared and shaped in a fast closing press), Fast Compression Molding (thick charge flows during mold closing), and Solid State Stamping (semicrystalline plastics below their melting point are stamped into parts with simple geometries in 15 sec. Other shaping methods include Pultrusion of prepreg tapes, Diaphragm Molding (preform between plastically deformable diaphragms shaped by hard tooling), Rubber Pad Molding, Hydroforming (rubber bladder inflated hydraulically, Vacuum Forming in an autoclave, and Flexible Resin Transfer Molding (sheets of resin and fiber between elastomeric diaphragms are consolidated, then shaped). Advantages/Disadvantages

Applications

Materials

Extrusion melt impregnation allows high fiber contents and longer fibers which give improved mechanical properties. Slurry deposition employs shorter fibers which allow greater flow and more complex parts. Cycle times are short.

Automotive Body Panels, Other Components, Aircraft Components

PP, PE, PA, PBT, PET, PVC, PC, PEEK, PSU, PPS

10

Special Molding Techniques

Table A7. Reactive liquid composite molding [7.1] Reactive Liquid Composite Molding (RLCM) proceeds in two steps: (1) PREFORM FORMATION by organizing loose fibers into a shaped preform, and (2) IMPREGNATION of the fibers with a low viscosity reacting liquid. The reacting material may be thermally activated by heat transfer in the mold or mixing activated by impingement of two reactive streams. Simulations of flow and reaction, a recent innovation in RLCM, allow determination of vent and weld line locations, fill times, and control of ‘racetracking’ in terms of gate locations, mat permeability, and processing conditions. Commercial success requires (1) fast reaction and (2) efficient preform formation. Cycle time for thermally active systems can be decreased by using higher mold temperatures and heating the preform. Innovative processes for PREFORMING include: Thermoformable Mat heated by IR to melt the binder and pressed into shape by one or two moving platens while supported by a hold/slip edge clamp to reduce wrinkling. Automated Directed Fiber Performers employ multiple delivery systems to create a surface veil, a chopped roving layer, and continuous roving with loops, all of which are fused by hot air. The SCRIMP process channels resin flow between layers of fibers or along internal networks. Water Slurry Deposition positions fibers by water flow through a contoured screen and sets them with hot air. Innovations to reduce costs by combining process steps include: Direct Part Forming combines sheet formation and shaping, e.g. heating porous sheet and then consolidating and shaping in a compression mold. The Hot Air Preformer produces performs by either directed fiber or thermoplastic mat forming. The Cut-N-Shoot process combines preforming and molding steps consecutively in the same tool. Bladder inflation inside a mold shapes the preform and forms the mold wall during filling. Advantages/Disadvantages

Applications

Materials

Low pressure and temperature processing by RLCM allow the use of inexpensive light-weight tools, especially for prototyping. RLCM allows customizing reinforcement to give desired local properties and part consolidation via complex 3D geometries.

Marine and Poolside Products, Sanitary ware, Caskets, Automotive Panels, Vehicle Suspension Links

Isocyanate based resins (mixing activated) Unsaturated polyester and styrene (thermal activated)

One-shot Manufacturing

11

Table A8. MIcrocellular plastics: formation and shaping [8.1] Extremely small closed cells from 0.1 to 10 microns in diameter can be formed in most plastics by dissolving gas in the plastic, typically supercritical CO2, and then rapidly reducing the pressure and increasing temperature in a controlled manner to cause homogeneous and likely heterogeneous nucleation and growth of gas bubbles. The bubbles are to be smaller than naturally occurring flaws in the polymer so mechanical properties are not compromised. Particles in PS(HI) can be sites for heterogeneous nucleation [9.2]. Short diffusion paths, elevated temperatures, and gases in the supercritical state are necessary to achieve the high diffusion rate and high gas concentration needed for commercial use. Extrusion with gas injection is an efficient process to saturate polymer with gas. Manufacturing issues are determining sequence of pressure, temperature, and shaping geometries to nucleate and form cells without disruption and to shape product without distortion. Microcellular technology is covered by several patents and is offered for licensing by Axiomatics, Woburn, MA Advantages/Disadvantages

Applications

Materials

The extremely small bubbles give weight reductions of 10-90%, no reduction in specific mechanical properties, appearance of a solid opaque surface, and foaming of thin sections. Fatigue resistance is observed to increase. Environmental advantages are use of atmospheric gases and lower material use

Siding, Pipes, Aircraft Parts, Athletic Equipment, Machine Housings, Automotive Components, Food Containers, Artificial Paper, Thermal Insulation, Fibers for Apparel and Carpets

ABS, PE, PET, PMMAPS, PS(HI), PP, PUPVC, SMC, Fluoropolymers, Poly(methylpentene)

Table A9. Lamellar injection molding [9.1] In Lamellar Injection Molding (LIM), two (or three) materials are extruded separately and combined with a 3 (or 5)-layer feedblock with multipliers to form a melt stream with hundreds of layers. This stream is injection molded to from parts with an irregular lamination pattern. The third material may be an adhesive. The layer structure, as assessed by oxygen permeability, shows (1) undesirable high permeability when too few layers allow easy passage around barrier layers, (2) low permeability (300-fold reduction) at 60-600 layers equal to theoretical minimum for lamellar structure, and (3) increased permeability as extremely thin laminates break up to from discontinuous domains (blends). LIM technology is offered for licensing by the Dow Chemical Company

12

Special Molding Techniques

Table A9. Lamellar injection molding [9.1]

Advantages/Disadvantages

Applications

Materials

Only machine modifications needed are addition of feedblock and multipliers. LIM does not require multiple channels or sequenced valving used in coinjection molding and can easily be applied to complex parts or multicavity molds. Parts can be molded with high barrier properties to gases and hydrocarbons at lower costs than monolayer materials. Scrap can be recycled by incorporation into the major component or by conventional methods since LIM materials are compatible. Optical clarity (reduced haze) is improved compared to blends because the ordered LIM morphology reduces light scattering. LIM structure, with sheetlike continuous component selected for specific properties (controlled thermal expansion, increased load bearing, and temperature resistance), offers distinct property enhancements compared to blends.

Structural Parts (dimensional stability, temperature/ chemical resistance) Housewares/Durables (clarity, temperature and solvent resistance) Containers for Food and Chemicals (gas. hydrocarbon barriers) Automotive Reservoirs (fluid/heat resistance).

PC/PET, PC/ PBT, PO/ad/ EVOH, PET/ PEN, PO/ad/ PA, PS/PA6, PC/TPU, TP/ T-LCP, filled/ unfilled, brittle/ductile, virgin/recycle

Table A10. Multimaterial multiprocess (MMP) technology [10.1] The use of multiple materials and processes is the overarching technology in achieving one-hot manufacturing for large and/or complex parts. Material thermal expansion differences can be dealt with by flexible joints, (adhesives), process sequence to minimize distortion, sliding at interfaces (incompatible materials), and design for minimum distortion. Common multimaterial multiprocess technologies include Injection Compression Molding in which resin is injected into a partially open mold which closes, requiring less clamp force and producing less residual stress in the part. Multimaterial Molding in which a material is shaped, the mold is altered, and a second (or subsequent) material is shaped. Shaping processes are combinations of injection and compression molding and stamping. In-Mold Coating in which a thin thermoset coating is injected onto an injection or compression molded part in a closed mold. ‘Mono-Sandwich’ Coinjection Injection Molding in which a small extruder, operated intermittently, pumps a skin layer into the front of the main injection unit for subsequent coinjection. The Alpha 1 machine at GE Plastics, with two injection units, a long stroke vertical press, and shuttle table, allows combinations of compression molding, (gas-assisted) injection molding, and stamping. Other MMP technologies are described in tables on Low Pressure Molding, Advanced Blow Molding, and Lamellar Molding.

One-shot Manufacturing

13

Table A10. Multimaterial multiprocess (MMP) technology [10.1]

Advantages/Disadvantages

Applications

Materials

The advantages of more than one material and/or process include design flexibility, tailored performance, effective material use, lower labor costs, improved quality through automation, reduced secondary operations, less auxiliary equipment, and more recycle use. Multiple materials allow advantageous combinations such as multiple colors (automotive lens), flexible/rigid (conduits with connectors), and consolidated/strong (plastic/metal composite) and cost /barrier or strength (laminate structure).

Telephone booth molded from 132 lbs of structural foam on a 2500-ton press with three injection units Air vent with molded movable louvers made from incompatible materials. Automotive bumper with injection molded fascia over a stamped beam. Multicolor automotive taillights.

Combinations of thermoplastic, thermoset, and reinforcements subject to constraints of product performance, limitations on distortion, and interface requirements (adherent or incompatible)

TABLE ABBREVIATIONS ad ABS EVOH PA PBT PC PE PEN PET PMMA PO PPS PP PS PSU PPO PPE SAN SMC T-LCP TPU TPE

adhesive Acrylonitrile-butadiene-styrene copolymer Poly(ethylene-co-vinyl alcohol) Polyamide Poly(butylene terephthalate) Polycarbonate Polyethylene Poly(ethylene 2,6-nathalenedicarboxylate) Poly(ethylene terephthalate) Poly(methylmethacrylate) Polyolefin Poly(phenylene sulfide) Polypropylene Polystyrene Polysulfone Polyphenyleneoxide Poly(phenylene ether) Poly(styrene-co-acrylonitrile) Sheet Molding Compound Thermotropic Liquid Crystal Polymer Thermoplastic Polyurethane Thermoplastic Elastomer

TABLE REFERENCES 1.1 2.1 3.1 4.1 4.2

Eckardt, H., “Gas-Assisted Injection Molding,” Ref. [1]. Turng, L.S. “Computer-Aided Engineering for the Gas-Assisted Injection Molding Process,” Ref [1] Hauck, C. , Schneiders, A., “Injection Molding with Fusible Core Technology,” Ref. [1]. Hettinga, S., “Controlled Low Pressure Injection Molding,” Ref[1]. Turng, L.S., Chiang, H., Stevenson, J.F., Plast. Eng., p.33, Oct. 1995.

14

5.1 5.2 6.1 7.1 8.1 8.2 9.1 10.1

Special Molding Techniques

Sugiura, S., “Developments in Advanced Blow Molding,” Ref. [1]. Myers, J., Mod. Plast., p.64, June 1995. Bigg, D.M., “Manufacturing and Formation of Thermoplastic Sheet Composites,” Ref. [1]. Castro, J.M., “Reactive Liquid Composite Molding,” Ref. [1]. Suh, N.P., “Microcellular Plastic,” Ref. [1]. Campbell, G.A. and Rasussen, D.H., U.S. Patents 5,369135, 5,358,675 (1994). Barger, M.A., Schrenk, W.J., “Lamellar Injection Molding Process for Multiphase Polymer Systems,” Ref. [1]. Avery, J.A., “Multimaterial Multiprocess Technology,” Ref. [1].

Chapter 1: Gas Assisted Injection Molding Gas Assist Injection Molding The North American Legacy

Jack Avery GE Plastics, USA Let's look at some of the specifics. What impact will the expiration of the original Frederich patent have? As you would expect, there are a variety of opinions. After having discussed this with a variety of people, it is my opinion that it will have little actual impact on the development of gas-assist injection molding. Why? Most applications use in-runner or in-article options. These variations provide the most flexibility to introduce gas into the part and consequently the most flexibility to optimize both the design of the component and the utilization of the process. Design will dictate the best solution for gas injection. But there will always be some applications where through the nozzle technology is the best choice. Typically these are handles, symmetrical components, and similar type applications. Typical applications of gas-assist injection molding include: • Business machine chassis/ housings • Material handling pallets • Furniture: chairs, tables • Handles • Automobile bumpers • Automobile trim • Television housings • Golf club shafts Not only has progress been made in licensing and utilization of gas-assist injection molding technology, advancements in computer simulation, design and variations in the technology continue.

GAS ASSIST INJECTION MOLDING TECHNOLOGIES Several variations of gas-assisted injection exist. Most are patented and require licensing to practice.

16

Special Molding Techniques

Commercial Technologies License Required Cinpres Yes GAIN Yes Johnson Controls No Airmould (Battenfeld) No Helga (Hettinga Industries) Yes Another trend is that machine manufacturers are entering into licensing agreements with Cinpres or GAIN or both. This enables them to integrate the control system for gas-assist injection into their machines as an option. Companies having such agreements include: • Cincinnati Milacron Cinpres • Engle Machinery - Worldwide GAIN • HPM Industries GAIN • Husky Cinpres Gas assist-injection molding is a global process. One of the drivers is that global OEM's recognize its value and are beginning to apply the technology on a broad base. Examples are: • Samsung • Mitsubishi Consumer Electronics • Ford • Xerox Another significant factor in the globalization of gas-assist injection molding is that the technology suppliers have a global presence. Battenfeld, Cinpres and GAIN all have relationships that enable them to serve the worldwide market, either directly or on a primary licenser basis. In addition, Cinpres is opening an office in Singapore to serve Asia (Table 1). Table 1. N. America

Europe

Japan

Airmould

Battenfeld

Battenfeld

Tsukishima

Cinpres

Cinpres Ltd.

Cinpres Ltd.

Mitsubishi Gas Chemical

GAIN

GAIN

GAIN

Asahi

Helga

Hettinga

Hettinga

Toray

Johnson Controls

JCI

JCI

JCI

Gas Assist Injection Molding

17

LICENSING GAS-ASSIST INJECTION TECHNOLOGIES A significant difference between the use of gas-assist injection technology in Europe and the rest of the world is the need for licensing. Two companies, Cinpres and GAIN hold a variety of patents on this technology. To use any of these patented technologies anywhere in the world, and to ship a product manufactured using gas-assist technology anywhere in the world, a license is required. Three licensing options are available: • Patent only license • Development license • Full technology license A patent only license provides protection to use technology patented by the licenser. An example of a patent only license is Engle who requires that a GAIN Technologies patent only license be taken prior to or in conjunction with purchase of Engle equipment. Machine manufacturers with these agreements include: Cinpres GAIN Engle X X Husky X Klockner Ferromatik Desma X Mannesman Demag X A development license can be taken for the development phase of an application. Under this arrangement, the technology is in your facility and access to support from the technology supplier is available. In some cases, this license is used to evaluate the technology for a specific application. The development license must be converted to a full technology or full manufacturing license prior to going into production. Each technology supplier has variations on the type of license and fees. A summary of the license fees and "hardware" cost associated with using gas assist technology in production is in Table 2. It is necessary to review each individual case with the technology licenser to determine actual costs and conditions.

18

Special Molding Techniques

Table 2. Gas-assist injection molding licensing information(1) Manufacturing license fee

"Gas injection" equipment

Additional costs "royalties"

Airmold (Battenfeld)(6)

None

Single Machine, Single Injection Point, Base Price $110 000, Expandable(7)

None

Cinpres(5)

$60,000

Single $35,000, Multiple $58 - 95,000

Based on: Material Usage or Tooling Fee or Flat Fee for Parts

Epcon

-

Single $55,000 Multiple $77,500

None

GAIN

Per mold $1.5 -15,000/yr per facility $25 - 250,000/yr(2)

Single $25 - 50,000 Multiple $35 - 85,000

None

HELGA (Hettinga)(3)

None

HELGA Package $70 - 75,000

None

Johnson Controls Multinozzle/ Sequential Gas Assist(4)

None

Integrated into machine controls $30 -50,000

None

Nitrojection

$25,000

$45 - 85,000

None

(1) (2) (3) (4) (5) (6) (7)

Licensing fees and details vary depending upon each application and supplier. It is essential to obtain from each supplier relative to a specific application Per mold license is also available on a lifetime basis: $12.5 - 75,000 "HELGA" Package includes required equipment and rights to practice Available only as an option on new or as a retrofit on existing Johnson Controls' machines Patents pending Airmold process per Airmold specification, no license required Includes pressure generator which can be used with additional machines

APPLICATION DEVELOPMENT What is different in the application development process? The most important step is to determine if the application is appropriate for the gas-assist injection process.

Gas Assist Injection Molding

19

How is this accomplished? The first step is to complete a thorough assessment of the performance requirements. Then the material, process and design of the component can be determined. Factors which make gas assist injection the process of choice include: • High stiffness to weight ratio is required • Part design allows for hollow rib geometry • Tight tolerances are required • A hole in the part surface can be tolerated, or the hole can be sealed • Improved surface is desirable vs. structural foam molded parts Once the performance requirements have been determined and gas assist injection has been selected as the appropriate process, part and tool design proceed. It is important to take into consideration details which are different from standard injection molding. They are: PART DESIGN CONSIDERATIONS • • •

Sizing of gas channels Gas channel layout Location of gas injection point(s) TOOL DESIGN CONSIDERATIONS

• •

Gate size for the through-the-nozzle or in runner gas injection Gas nozzle location - for in-runner and in-article gas injection, material must cover the gas nozzle prior to gas introduction • Location of gas nozzles in the tool to prevent interference with cooling lines, slides, ejector systems, etc. Where can you obtain assistance? Four primary sources of assistance exist: 1) Gas-assist injection technology supplier (Cinpres, GAIN, Battenfeld, etc.) 2) Material supplies may provide assistance if the application is a fit for their materials 3) For OEM's the third source may be a molder who has experience with gas assist technology 4) Consultants who specialize in gas-assist technology (such as Caropreso Associates) or firms that can provide modeling assistance, i.e. Plastics and Computer As a general comment, licensees find that even though assistance is available, the final test is to learn through the experience of putting parts into production. A useful recommendation of many molders is to do a prototype tool for the first few applications. This will provide you with some leeway for changes prior to the production tool. Also, in many cases, the prototype tool can be used for process validation, preproduction or initial parts while the production tool is being completed. Another option is to prototype a section of the part, i.e.

20

Special Molding Techniques

one-quarter. This could provide information critical to design and construction of the mold but would not provide parts for evaluation. Each application needs must be considered.

ADDED COST USING GAS-ASSIST TECHNOLOGY One factor often overlooked in the development of a program using gas-assist injection molding technology is the added cost involved. In addition to the licensing fees and royalties, equipment costs plus the nitrogen used in the process must be taken into account. Also, the cost of the mold may be higher than for standard injection molding since, except for through the nozzle technology, gasinjection nozzles must be integrated into the mold. These added costs must be recovered. Some factors which may contribute to recovering costs are: • Parts consolidation resulting in fewer molds, less machine utilization and reduced or elimination of assembly • Use of lower tonnage machines • Improved part quality • Reduced cycle time • Less scrap • Lower weight (lighter and less material) Delphi Interior and Lighting Systems had significant challenges to overcome to meet GM requirements for door systems for new vehicles. Some of the requirements were: • Reduced systems cost • Systems mass reduction • Reduced assembly time • Improved system quality Gas-assisted injection molding was selected as the process technology to meet these demanding requirements due to the benefits it offered: • High strength-to-weight ratio • Low molded-in stress provides excellent dimensional stability • Parts consolidation opportunities • Weight reduction • Design flexibility • Reduced part cost When Delphi initiated the program in the early 1990's, gas-assist injection molding had not demonstrated the capability to deliver all of these benefits in a production environment. What did Delphi do to reduce the risk of employing a new technology in this high visibility program? They brought in a gas-assist technology supplier and a material supplier who was actively developing gas-assist design and process technology for their materials.

Gas Assist Injection Molding

21

A four year collaboration produced the following results: Parts consolidation: 61 parts to 1 part Assembly time reduction from 330 sec to about 60 sec Lighter weight (up to 1.5 Kg/vehicle) Reduced tooling requirements and investment costs Improved material handling Better assembly ergonomics Improved quality (fewer squeaks and rattles) Reduced operational noise Improved corrosion resistance Best-in-class serviceability Up to 10% piece price savings This sounds like a successful program. The most important lesson is that the key participants were involved from the inception and worked through the development program together. • • • • • • • • • • •

TECHNOLOGY DEVELOPMENTS Technology developments continue. Design guidelines have been developed and published. The lead has been taken by material suppliers who have developed this information for use with their customers in developing applications that employ gas-assist injection molding technology. In addition, the Structural Plastics Division of the Society of the Plastics Industry has a compilation of papers relating to gas-assist injection molding technology which have been presented at their annual conferences. Process technology development continues. Mitsubishi Gas Chemical has developed a variation of the Cinpres process - the "full shot" process. Instead of short shooting the mold and packing it out with gas, in the "full shot" process, the mold is filled with polymer and the gas is used only for packing. Nozzle design is one of the critical areas of gas-assist injection molding. For this to be a commercially viable process, injection of the gas through the nozzle must be as trouble free as injecting the material. This is an area of ongoing development by a wide variety of suppliers. Cinpres has introduced a new "directional" nozzle design that ensures the flow of gas into the mold in the same direction as resin flow. A nozzle with a 90 degree tip whereby the gas exits at right angles, ensures the flow of nitrogen into the mold in the same direction as resin flow. This configuration is claimed to prevent blemishes on the surface of the part that appear opposite the nozzle.

22

Special Molding Techniques

Xaloy Inc. has introduced an upgraded nozzle that is claimed to increase durability, and reduce cost. This redesigned nozzle uses a hydraulically or pneumatically actuated needle to shut off melt flow when the hold pressure is released at the end of the injection cycle. This prevents gas in the sprue and runner system from flowing back into the nozzle. Nitrogen source and recovery play a critical role in the cost and quality of components produced using gas-assist injection technology. Studies underway indicate that nitrogen purity is critical, especially when using engineering thermoplastic resins. Impurities (i.e., oxygen) can result in oxidation and burning in the mold. Several on-site nitrogen separation systems are available for prices ranging from $10,000 - $70,000 depending on volume required. Gas recovery is a topic of intense discussion, as it offers cost reduction opportunities. Recovery rates of 70-90% are attainable with some resins. However, volatiles can be picked up by the gas flowing through the molten materials resulting in contamination or fouling of the recovery system and/or clogging of gas injection needles. Much more work needs to be done in this area. Design, analysis and material optimization are also critical elements. The full potential of gas-assist technology will not be realized without continuing developments in these areas. Dow, DuPont, GE Plastics all are working to optimize design for their materials. A.C. Technology, Plastics and Computers Inc. and Moldflow provide specialized software for design optimization and process simulation. Work continues as new releases of software incorporating more sophisticated approaches to the gas-assist technology continue to be commercialized. In addition, gas-assist injection technology development continues. Cinpres and GAIN continue their developmental work, and variations of the basic technology continue to be introduced. Another approach was introduced in late 1995 by EPCON Gas Systems Inc. This approach is based on three key elements: • Shot control • Simplified gas pins • Pressure control EPCON has applied for patents on their approach to shot control. It is based upon "strategically" located thermocouples at points in the mold where the material flow front needs to stop before gas pressure completes the mold filling. The thermocouple simultaneously signals the gas injection unit to begin the gas flow and the molding machine to stop its hydraulic pumps which end the injection forward sequence. Their approach to gas pins is similar to the concept used for mold vents - the holes are small enough so that the material skins cover but allows the gas to pass through. According

Gas Assist Injection Molding

23

to EPCON, "Microscopic holes have been incorporated to allow enough volume of gas to pass through." To date, I am not aware of any EPCON systems in commercial production. It remains to be seen how well this technology performs. Their approach to gas pins is similar to the concept used for mold vents - the holes are small enough so that the material skins cover but allows the gas to pass through. According to EPCON, "Microscopic holes have been incorporated to allow enough volume of gas to pass through."

EQUIPMENT Variations on gas-assist injection molding continue to evolve. The latest is Battenfeld's Monomodule. This unit is for use when gas is to be injected at a single point. Then only one pressure regulator is required. The handheld unit provides the capability to be programmed directly. The data is stored in the Monomodule.

SUMMARY We all know that gas-assist injection molding technology languished for several years in North America due to litigation threats. How was this "fear" overcome? As with any industry, there are leaders and there are followers. The leaders were convinced that the upside for productivity and design flexibility were worth the risk. They stepped forward and developed applications using gas-assist injection molding technology. The initial applications took longer to develop than standard injection molding or with structural foam. But once they have developed the technology, they have an advantage. This "confidence factor" is a competitive advantage that can be exploited and translated into a bottom-line advantage over the followers. My opinion as to why gas-injection molding technology is developing so rapidly in North America, has been the collaboration of the members of the supply chain involved in developing an application. In Europe, the machine suppliers have been the primary drivers of gas-assist technology. Even though they had an earlier start, I feel that North America is moving faster and has applied the technology to more challenging applications. How have we been able to do this? The OEM's and material suppliers recognized the potential this technology offered and joined forces with technology suppliers to apply it to applications that offered a high return. The GM-Delphi Super Plug application is an excellent example of this. For this process technology to become mainstream, progress must be made in several areas.

24

Special Molding Techniques

1) The "knowledge gap" must be overcome. It must be able to be done right the first time. The development cycle for components molded using gas-assist technology must be no longer than for a standard injection or structural foam molded component. 2) Recognition that gas-assist injection is not a "fix-it" for existing problems. As with any process, to optimize value, the product must be optimally designed for a material and process. 3) Additional costs of using gas-assist injection must be understood and minimized. The value of components molded using this technology must be recovered. 4) Equipment consistency and reliability issues (gas injection control and nozzle) must be eliminated. 5) "Fear of litigation" must be eliminated! Commercialization of gas-assist injection technology can take several paths. Three of which might be: 1) Technology becomes widely understood and low cost to implement. Most converters become "experts" and utilize. 2) A group of converters specialize in gas-assist injection molding. Their expertise enables them to develop components quickly and cost effectively, positioning them as the suppliers of choice for gas-assisted injection molded components. 3) Captive converters become the largest users of gas-assist injection. They have the large volume applications which enable them to amortize the added cost. What is needed for this technology to prosper? Higher levels of collaboration. The OEM must involve the processor, material supplier and tool maker at the program inception. This will ensure that all required input is provided early and that costly changes and/or compromises are eliminated or minimized. This will also develop an increased sense of ownership by the members of the supply chain. Another area of need is increased education of the design community. For proper application of gas-assist injection molding, the designer must understand the process, its strengths and weaknesses in order to minimize design flaws which result in process, performance and economic issues. Will the rapid growth continue? Not unless the technology becomes widely understood and is demonstrated to provide cost effective solutions. Designers must become confident not only in designing components, but also that mold makers and converters have the capability to implement the technology.

SOURCES Battenfeld of America, Inc., 31 James P., Murphy Industrial Highway, W. Warwick, R.I. 02893, Phone (401) 823-0700, Fax (401) 823-5641

Gas Assist Injection Molding

25

Cinpres Limited, Waterworks Plaza, Building, 3135 South State Street, Suite 108, Ann Arbor, MI 48108, Phone (313) 663-7700, Fax (313) 663-7615 Epcon Gas Systems, Rochester Hills, MI, Phone (810) 651-9661, Fax (812) 650-8293 Gain Technologies, Inc. 6400 Sterling Drive, North, Sterling Heights, MI 48312, Phone (810) 826-8900, Fax (8I0) 826-8906 Hettinga Technologies, Inc., 2123 N.W., 11th Street, Des Moines, IA 50325, Phone (515) 270-6900, Fax (515) 270-1333 Johnson Controls, Inc., 10501 Highway, M-52, Manchester, MI 48158, Phone (313) 428-8371, Fax (313) 428-0143 Kontor- JPI Technologies Inc., 35 Gibson, Lake Dr., Box 220, Palgrove, ON, LON 1PO, Canada Phone (905) 880-2600, Fax (905) 880-2599 AC Technology/C-Mold, 11492 Bluegrass, Parkway, Suite 100, Louisville, Ky 40299, Phone (502) 266-6727, Fax (502) 266-6654 Moldflow Pty, Limited, 4341 S. Westnedge, Suite 2208, Kalamazoo, MI 49008, Phone (616) 345-4812, Fax (616) 345-4816 Plastics & Computers, 14001 Dallas, Parkway, Suite 1200, Dallas, TX 75240, Phone (214) 934-6705, Fax (214) 934-6755

Flow Directions in the Gas Assisted Injection Molding Technology

Young Soo Soh and Chan Hong Chung Department of Chemical Engineering, Kyungbook, 712-714 Korea

INTRODUCTION Gas assisted injection molding process produces parts with many advantages including Class A finish with no sink marks, reduced cycle time, lower injection pressure, lower clamping tonnage, reduced part warpage, greater design freedom, and more. In the gas assisted process, an inert gas is injected into the center of the flow of plastic. A combination of the high surface tension of the plastic and the lower viscosity of the hotter molten material in the center of thicker sections, such as ribs and bosses, confines the gas to form hollow area in the thicker sections of the parts. Most gas and assisted injection molded parts may be categorized into two types: The parts consist merely of single thick section through which the gas penetrates, and the parts consists of a nominal thin wall with gas channels traversing the parts. The latter are more difficult to design and process because the gas may not just flow through the channel but penetrate into the nominal thin walled. These parts are expected to be designed such that the gas cores out all the channels without penetrating into the thin walls. To design molds such that the gas cores out all the thick sections and not the thin walls, one needs to predict and understand the preferred direction of gas in the process. In this paper, we use a method to relate the preferred gas direction with the process variables. The method requires a knowledge on the relations between resistance for the gas flow and processes variables such as resin flow length, cross section area of cavity, melt temperature, and existence of short shot. A simulation package was used to confirm the method. Commercial packages simulate the flow of gas. At a mold design stage, the commercial package plays a very important role to prevent blow through or fingering phenomena by simulating the gas flow. At the pilot production or first mold trial stage, the package also plays an important role to make perfect parts. If, however, the packages are not available at the molding shop or instant solution to prevent blow-through or fingering is necessary at the mold trial stage, the equations in this paper are very useful to treat the troubles. When a trouble shooting engineer modify the virgin mold without a flow analysis package, initially

28

Special Molding Techniques

he needs qualitative information described in the theory, not quantitative. After qualitative questions are answered, the quantitative solution comes from trial and error. For example, if the theory tells that the channel size be enlarged, the shop mold maintenance technician will enlarge the channel diameter a little and try the molding cycle, which then will be followed by another small channel enlargement, if necessary, until satisfactory parts come out of the mold.

THEORY Although the process is unsteady state, steady state flow equations may be used to explain the gas flow directions. The equation for the steady state flow of a Newtonian fluid between infinite parallel flat plates is given by1 ----------------∆p = 12µVL 2 a

[1]

where L a

length of plate in direction of flow distance between plates ∆p pressure drop across the distance average velocity V neglecting end effects. The equation for the steady state flow of pseudo plastic liquids between infinite parallel flat plates is given by Q ( 3n + 1 ) n m  ∆p ------- =  -------------------------  -----------------   ( 3n + 1 ) πn 2L a

[2]

where m, n power law indices Q flow rate The steady state flow of a Newtonian liquid through conduit with diameter D is given by1 ----------------∆p = 32µVL 2 D

[3]

The steady state flow of pseudo plastic liquids through conduit with radius R is given by Q ( 3n + 1 ) n m  ∆p ------- =  -------------------------  ------------------   ( 3n + 1 ) πn 2L R

[4]

When the direction of gas path is discussed for the gas assisted injection molding, the term "the direction of least resistance" is commonly used. When more than two paths are

Flow Directions

29

competing for the direction of gas, the gas prefers the direction of less resistance. Very often, this resistance is explained by temperature, length, or distance between plates. The degree of resistance is proportional to the pressure drop requirement in equation [1] through [4]. Using these equations, the preferred gas directions can be predicted. Equations [1] and [3] are easier than equations [2] [4] to use, and equations [1] and [3] are as correct as equations [2] and [4] to answer qualitative questions such as "which one is the least resistance path?" Theoretically, there exists pressure drop both along the gas phase and along the polymer melt phase. However viscosity of gas is less than 0.1% of apparent viscosity of polymer resin, and the pressure drop along the gas phase can be considered negligible. Thus the pressure of the gas may be considered the same for all regions of gas not only in the stage of stationary packing stage, which comes from fluid statics theory, and but also in the phase of first dynamic stage. Hence, only the pressure drop of the resin is necessary for the discussion of the degree of the resistance. The resistance increases with the increase of viscosity, path length between gas front and melt front, velocity of the melt, and decreases with the increase of path cross section area. The path cross section area here excludes the frozen layer of mold cavity

RESULTS AND DISCUSSIONS Consider that a polymer melt was injected in the middle of a 8 mm diameter pipe to a total melt length of 60 mm, followed by gas injection at the point 0.5 mm left of the melt injection point, and we now try to pick the preferred direction of gas. With the equation 1, one can calculate that the resistance to the left hand side for the gas to move with velocity, V, is proportional to 32µVL ( ∆p ) L = -------------------L2 D

where ( ∆ p)L is pressure drop requirement to the left hand side direction and LL is distance between gas injection point and left melt front. The resistance to the right hand side for the gas to move with velocity, V, is proportional to 32µVL ( ∆p ) R = -------------------R2 D

One only needs to compare LL with LR as all the remaining variables are the same. LL is less than LR and ( ∆ p)L is less than ( ∆ p)R. Thus the resistance of the flow to the left hand side direction is smaller, and the left hand side direction is the preferred direction for gas flow.

30

Special Molding Techniques

In Figure 1, a Mold Flow simulation results are shown, which is consistent with the method given here. GAS INJECTION Consider another case where a pipe with 8 mm diameter is connected to a pipe with 4 mm diameter. The melt length from the center is the polymer polymer same for both directions. The gas is injected at the center and we now try to pick the preferred Figure 1. Gas injection - Case I. direction of gas. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 8mm

D = 8mm

29.5mm

30.5mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 32µVL ( ∆p ) R = -------------------R2 DR

Thus ( ∆p ) L = 32µV ( 30 ⁄ 16 )

and ( ∆p ) R = 32µV ( 30 ⁄ 64 )

( ∆ p)L is less than ( ∆ p)R. and the resistance of the flow to the right hand side direction is smaller, and the right hand side direction is preferred direction of gas flow. In Figure 2, Mold GAS INJECTION Flow simulation is shown for the case, which is consistent with the method given here. Consider the third case where a pipe with 8 polymer polymer mm diameter is connected to a pipe with 4 mm diameter, where gas is injected at the point Figure 2. Gas injection - Case II. where two pipes are connected. The melt length from the gas injection point is different at each side. We now try to pick the preferred direction of gas. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 4mm

30mm

D = 8mm

30mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to

Flow Directions

31

32µVL ( ∆p ) R = -------------------R2 DR

Thus ( ∆p ) L = 32µV ( 10 ⁄ 16 )

and ( ∆p ) R = 32µV ( 100 ⁄ 64 )

( ∆ p)L is smaller than ( ∆ p)R and the resistance of the flow to the right hand side direction is greater, and the left hand side direction is preferred direction of gas flow. In Figure 3, Mold GAS INJECTION Flow simulation results is shown for the case, which is consistent with the method given here. Consider case 4, where a pipe with diamepolymer ter 7 mm is connected to a cavity of 7 mm thickFigure 3. Gas injection - Case III. ness formed by two parallel plates, where gas is injected at the point where two cavities are connected. The melt length from the gas injection point is the same at each side. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 8mm

D = 4mm

10mm

100mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 12µVL ( ∆p ) R = -------------------R2 a

Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

POLYMER SHUT OFF

and ( ∆p ) R = 12µV ( 20 ⁄ 49 ) 20mm 20mm

GAS INJECTION

polymer

polymer

Figure 4. Gas injection - Case IV.

( ∆ p)L is greater than ( ∆ p)R and the resistance of the flow to the left hand side direction is greater, and the right hand side direction is preferred direction of gas flow. In Figure 4, Mold Flow simulation is shown for the case, which is consistent with the method given here.

32

Special Molding Techniques

Consider case 5, where a 7 mm diameter pipe is connected to a cavity of 7 mm thickness formed by two parallel plates, where gas is injected at the point where two cavities are connected. The melt length from the gas injection point is 40 mm at the plates and the same for the pipe. With the equation 1, one can calculate that the resistance to the left hand side is proportional to 32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 12µVL ( ∆p ) R = -------------------R2 a

Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

and ( ∆p ) R = 12µV ( 40 ⁄ 49 )

( ∆ p)L is greater than ( ∆ p)R and the resistance of the flow to the left hand side direction is greater, and the right hand side direction is preferred direction of gas flow. In Figure 5, Mold Flow simulation is shown for the case, which is conGAS INJECTION sistent with the method given here. Consider case 6, where a 7 mm diameter pipe is connected to a cavity of 7 mm thickness polymer formed by two parallel plates, where gas is injected at the point where two cavities are conFigure 5. Gas injection - Case V. nected. The melt length from the gas injection point is longer at the plates with 60 mm. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 7mm

20mm

40mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 12µVL ( ∆p ) R = -------------------R2 a

Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

Flow Directions

33

and ( ∆p ) R = 32µV ( 60 ⁄ 49 )

( ∆ p)L is smaller than ( ∆ p)R and the resistance of the flow to the right hand side direction is greater, and the left hand side direction is the preferred direction of gas flow. In figure 6, Mold Flow simulation is shown for the case, which is consistent with the method given here.

REFERENCES 1

W. L. McCabe, J. C. Smith, and P. Harriot, Unit Operations of Chemical Engineering, 4 th Ed. McGraw-Hill, 1986.

Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties

Kurt W Koelling Dept. of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210, USA Ronald C Kaminski The Geon Company, One Geon Center, Avon Lake, Ohio 44012, USA

INTRODUCTION In general, gas-assisted injection molding can be described by a simple three-step process.1 A short shot of molten polymer initially fills 75-98% of the mold cavity under the ram speed control of the injection molding machine. After a short delay period, compressed nitrogen gas cores out the molten polymer, filling the remainder of the mold. The third step, or the gas packing stage, occurs as a result of the volumetric shrinkage of the polymer melt. As the plastic solidifies, the gas expands into the volume created by shrinkage, locally packing out the part. In 1935, Fairbrother conducted the first experiments investigating this flow phenomena using a viscous Newtonian solution. He found that m, the fractional coverage or fraction of liquid deposited on the walls of the tube after bubble penetration, is a function of the capillary number, Ca, for capillary numbers up to 0.009.2 The fractional coverage, m, is defined for tube-shaped geometries as: m = Ap/ At = 1 - (rb/R) 2 [1] where Ap is the polymer cross-sectional area, At is the tube cross-sectional area, rb is the radius of the gas bubble, and R is the radius of the tube. The capillary number is defined as the product of the bubble velocity, Ub, and the viscosity of the fluid, η , divided by the fluid surface tension, Γ , or: Ca = η Ub / Γ [2] Taylor investigated this problem further in 1961 and ran experiments that extended to capillary numbers of two.3 Cox found that the fractional coverage reached an asymptotic value of m = 0.60 for capillary numbers greater than ten for viscous Newtonian fluids.4 With interest renewed in this problem because of gas injection molding, Poslinski and Stokes conducted similar isothermal experiments using silicone liquid pastes that behaved as Bingham fluids.5,6 Capillary numbers of up to 800 were obtained and showed good cor-

36

Special Molding Techniques

relation with previous work at low capillary numbers. However, the fractional coverage ultimately approached a value of m = 0.564 at high capillary numbers.5 An isothermal computer model was developed to simulate the gas-liquid dynamics to validate the experimental results. It was discovered that the hydrodynamic layer of molten polymer deposited by the passage of the gas bubble was much larger than the associated frozen layer developed at the mold-melt interface. Polymer solutions, including a viscous, Newtonian fluid, an elastic Boger fluid with Newtonian shear viscosities, and a shear-thinning polymer solution have been used in isothermal experiments to characterize the role of melt rheology in the gas-assist process.6 Fractional coverage data from the shear-thinning fluid compared well with the Newtonian data at very low capillary numbers, but dropped abruptly at a capillary number of two and approached a limiting value much lower than m = 0.60. The Boger fluid also showed good agreement at low capillary numbers, but began to climb at a capillary number of approximately six, ultimately reaching a value of m = 0.75. Simple correlations for the actual gas-assisted injection molding process have also been developed.7-11 It was shown that the wall thickness of the molded part has a dependence on the residual time of the gas bubble.7,8 The residual time is defined for each point on the flow path of the gas bubble as the difference in time between the passing polymer front and the moving bubble tip. This research indicates that increasing residual times result in higher fractional coverage. Others, however, have performed research to demonstrate a capillary number dependence by accounting for the solid wall thickness build-up as a function of residual time.9-11

EXPERIMENTAL BACKGROUND CCD video camera

75 ton injection molding machine

High resolution monitor,S-VHS video recorder, and 486 DX2 PC with data acquisition board

Controlled volume gas injection unit

Figure 1 - Experimental apparatus for gas-assisted injection molding trials.

A spiral tube mold cavity of 0.0127 m (0.5 in) diameter with a flowlength of 0.585 m (23 in) was utilized for the molding experiments. Six Kistler melt pressure transducers, model 6159A and 6157 type, were mounted flush in the full spiral mold cavity. The placement of the first four transducers allowed measurements of the advancing gas bubble in the region filled during the polymer injection step of the process. The fifth and sixth transducers were situated in a region filled during the gas filling step of the cycle.

Influence of Processing Conditions

37

η (Pa-s)

The trials were conducted at the laboratory facilities of the Ohio State University Engineering Research Center for Net Shape Manufacturing (ERC/NSM). The equipment used to mold the test parts is shown in Figure 1. An all-electric 75 ton ACT-B Cincinnati Milacron injection molding machine was utilized in conjuction with a single cylinder Cinpres gas injection unit equipped with a Cinpres II gas nozzle for a constant volume GIM process. The melt pressure transducer measurements of the gas bubble advancement were verified in preliminary trials by replacing the moving side core plate with a two-inch thick, circular, borosilicate glass plate, fixtured into a protective steel frame. This produced a crosssection that was one-half of the full tube. A mirror positioned at a 45 degree angle was placed directly behind the glass window. Mounted above the mirror was a Cohu CCD video camera, model 4915-2001. The video image captured by the camera was recorded by a high resolution, S-VHS videotape recorder at a rate of 60 frames/second. While the video images from the GIM process were recorded, the signals from five melt pressure transducers, amplified by Kistler model 5012 charge amplifiers, were fed to a data acquisition system. A Keithley MetraByte DAS - 1600 series data acquisition board, installed in an IBM-compatible 486 DX2 computer, sampled the amplified signals at a rate of 200 samples/second per channel. Comparisons between the videotaped images and the pressure profiles, were then performed to verify the measurement technique. During the molding experiments, an electronic Mettler AE-100 balance was used to weigh each set of full spiral parts, ensuring as much repeatability as possible during the trials by monitoring the shot weight. A Sheffield model RS30 co-ordinate measuring machine (CMM) evaluated the gas bubble area of each cross-section from the molded spirals. Three points were measured on the part exterior, along with one point near the gas bubble center. Twenty-five points around the circumference of the bubble surface were then measured. These points were connected to the initial probe position to calculate the bubble area by dividing it into twenty-five small triangles. Three transparent injection molding grade 10000 compounds were utilized in this study: a general PS PVC purpose polystyrene (Dow Styron 685 D), a rigid 1000 poly(vinyl chloride) (GEON 87781), and a high PC 100 viscosity polycarbonate (GE Lexan 101). Each 10 compound provided the opportunity to examine the effects of polymer rheology on the resulting 1 0.01 0.1 1 10 100 1000 10000 100000 fractional coverage or wall thickness of the γ (sec ) molded test spirals. Figure 2 displays the shear Figure 2. Cross-exponential viscosity characterizations rate dependent viscosity of each material at its' of experimental materials. base melt temperature, as predicted by the Cross-1

38

Figure 3. Fractional coverage vs. flowlength for three gas piston speeds using polystyrene (Tmelt = 487 deg F, Gas delay = 2.4 sec).

Special Molding Techniques

Figure 4. Fractional coverage vs. flowlength for three gas piston speeds using polycarbonate (Tmelt = 624 deg F, Gas delay = 2.4 sec).

exponential model. Model parameters were taken from the material selection database of AC Technology's injection molding simulation software.

RESULTS AND DISCUSSION The role of the melt rheology and the gas bubble velocity during the deposition of the hydrodynamic polymer layer is shown by the response of the wall thickness to changes in the gas compression rate. Figures 3 and 4 show the fractional coverage as a function of distance down the spiral for three different piston speeds of the gas cylinder. In Figure 3, a large change in wall thickness is shown for the polystyrene depending on the piston speed of the gas cylinder. The slowest gas compression rate created a more uniform wall thickness distribution, before dropping off in the region filled during gas bubble penetration. Conversely, the fastest gas piston speed caused a non-uniform, decreasing wall thickness distribution. In contrast to the polystyrene, the response of the polycarbonate to the changes in piston speeds was negligible until the end of the gas bubble advancement. As Figure 4 shows, there was no change in wall thickness until a flowlength of 0.375 m was reached. After that point, slow piston speeds cause slightly thicker walls and fast piston speeds cause the walls to thin, as with the polystyrene. Subsequent trials were performed with pressure transducers mounted in the spiral mold. Since the times at which the polymer and gas fronts reached each transducer could be determined from the melt pressure profiles, residual times as a function of flowlength were obtained. The time delay between the end of polymer injection and the beginning of gas injection was the same for all three gas piston speeds for polycarbonate and polystyrene. The residual times, however, change as the gas compression rate changes. The lowest gas piston speed produced the highest, most uniform distribution of residual times along the spiral flow path. In contrast, the highest setting lowered the residual times, with a resulting distribution that is less uniform than either of the other two settings.

39

100

100

10

10

Ub (m/sec)

Ub (m/sec)

Influence of Processing Conditions

1 1.8 in/sec 3.1 in/sec 5 in/sec

0.1

0.01 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1 1.8 in/sec 0.1

0.50

Flowlength (m)

Figure 5. Bubble velocities vs. flowlength for three gas piston speeds using polystyrene (Tmelt = 492 deg F, Gas delay = 2.65 sec).

0.01 0.00

3.1 in/sec 6.2 in/sec 0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Flowlength (m)

Figure 6. Bubble velocities vs. flowlength for three gas piston speeds using polycarbonate (Tmelt = 637 deg F, Gas delay = 2.05 sec).

Figures 5 and 6 demonstrate gas bubble velocities which were calculated from the residual times discussed above. Recalling the large changes in fractional coverage and the shear-thinning behavior of the polystyrene, the velocities for the three settings in Figure 5 are not surprising. The interaction between the polymer melt rheology and the advancing gas front can be seen, as the velocities for the high setting diverge from those for the low and base settings. As the gas bubble accelerates due to the decreasing resistance associated with polymer deposition on the mold walls far in front of the bubble tip, the melt shearthins. This further decreases the resistance to gas bubble acceleration, causing faster bubble acceleration, higher shear rates and continually decreasing viscosities and resistances to flow. Since the polycarbonate has a large, upper Newtonian viscosity before a transition to a shear thinning regime and the polystyrene displays shear-thinning behavior over the entire range of interest, it is reasonable to expect that the polycarbonate would show little response to changes in gas piston speed and bubble velocities until they were large enough to cause the melt to shear-thin. As expected, the polycarbonate bubble velocities in Figure 6 showed very different rates of gas front acceleration from those of the polystyrene. Comparing well with the bubble velocities, the fractional coverage did not begin to change as a function of the gas piston speed until the bubble velocities began to diverge and rapidly accelerate. The issue of the relative importance of the gas bubble velocity in comparison to the thermal influences in the mold cavity may be addressed through plots of Fourier number vs. fractional coverage. The Fourier number, Fo, is defined as: Fo = α tr / Rt2 where α is the thermal diffusivity, tr is the residual time, and Rt is the tube radius. Figures 7 and 8 show the dependence of the wall thickness on the Fourier number. These plots are divided into two regions where the polymer deposition on the mold walls is either as a result

Special Molding Techniques

0.8 0.75

F illed u n d er G a s C on t r ol

0.8

F illed u n d er P olym er C on t r ol

0.7

0.65 0.6

0.55

F illed u n d er G a s C on t r ol

F illed u n d er P olym er C on t r ol

0.7 0.6

0.5

0.5

0.45 0.4 0.001

F r a ct ion a l cover a ge

F r a ct ion a l cover a ge

40

0.01

0.1

1

Figure 7. Depende nce of wall thickn ess on Fou r ier nu mber u sing polycarb ona te.

0.4 0.00001

0.0001

0.001

0.01

0.1

Figure 8. Dependence of wall thickness on Fouri er nu mber us ing polystyr ene.

of a polymer or a gas bubbl e dr iving for ce. In th e r egion filled under polymer contr ol where small bubble velocities are common, ther mal inf luen ces cluster th e fr act ional coverage r esults in a genera lly incr easing tr end . In th e r egion filled under gas contr ol where th e gas velocities are highly varied, it is appare nt t hat the wall thi ckness shows litt le or no de pen dence on t he res idual time.

CONCLUSIONS Gas -assisted injection molding experi ment s wer e per for med u sing a spir al tub e mold and thre e comm on injection mol ding gr ade compo unds : po lystyr ene, polyvinyl chlori de, a nd polycar bona te. By meas uri ng th e wall thickne ss along the flow path of the gas bubble, the r esidua l time, gas bubbl e velocity, and m ateri al pr opert ies were found to be int er r elated and r esponsible for c han ges in fr actional wall coati ng t hickn ess of as muc h as 20%. The sha pe of the shear r ate dep ende nt viscosity curve for ea ch ma ter ial was found to p r ovide an i ndi cation of the ability to cha nge the wall th ickness cover age by direc t manipul ati on of the gas bu bbl e velocity thr ough pr oce ss pa ram eters su ch as gas pi ston spe ed and pr e-cha rge gas pr essure . Polymer melts which b egin to shear -th in at low shear rates ar e mor e sensitive to chang es in gas pr essure and gas piston speed, while those polymers th at ha ve significan t up per Newtonian r egions ar e r elatively insensitive to the se cha nges.

ACKNOWLEDGMENT Thi s wor k was sup por ted b y the En gineer ing Researc h Ce nte r for Net Shape Manufacturing at the Ohio State Univer sity and The Geon Company . The aut hor s would like to thank D r. Clive Cops ey and Mr . Scott Weir for the ir help in pr oviding t echni cal ad vice and a ssistanc e and Mr . Alfr ed Geiger f or hi s effor ts in collecting and summarizi ng resul ts fr om the PVC exper iments .

Influence of Processing Conditions

41

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

Zheng, T., Glozer, G. and Altan, T, ERC for Net Shape Manufacturing at OSU Report, 1992. Fairbrother, F. and Stubbs, A.E , J. Chem. Soc., 1, 1935, pp. 527-529. Taylor, G.I., J. Fluid Mech., 10, 1961, pp. 161-165. Cox, B.G., J. Fluid Mech., 14, 1962, pp. 81-96. Poslinki, A.J. and Stokes, V.K , ANTEC Proceedings, 39, 1993, pp. 68-73. Huzyak, P., and Koelling, K., 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Findeisen, H., Diploma thesis at the IKV Aachen, 1991, Betreuer: A. Lanvers. Findeisen, H., 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Brockmann, C., Zheng, T., and Altan, T., ERC for Net Shape Manufacturing at OSU Report, 1994. Zheng, T., Koskey, J., and Altan, T, 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Zheng, T., and Altan, T., ERC for Net Shape Manufacturing at OSU Report, 1994.

Cover Part as an Application Example for Gas-assisted Injection Molded Parts

Michael Hansen Mack Molding Company, 608 Warm Brook Road, Arlington, VT 05250, USA

INTRODUCTION The gas -assisted inj ection m olding pro cess is in use now for sev er al year s offer ing new t echnical and c r eative possibilities for inj ection mol ding. After a bri ef surve y of th e pr inciple sequenc e of the process and ba sic pr ocess phy sics this paper c omm ents on Injection of an applicat ion example for a cover part and pr ovides solutions for the melt the pr oblems found d uri ng t he pr ocess of fixing existing issues on thi s tool. In Figur e 1, the pr inciple sequen ce of the pro cess is shown. In this pr ocess, the pr essur ized nitrogen gas is injected into the Gas injection melt to penetrat e the part via a network of thicker cro ss-sectioned and subsequent gas-chann els. The pr ocess consists of ever ything from a partial to follow-up pressure a volumetric filling of a cavity with polymer melt, as in compact inj ection mol ding. This phase is followed by the inj ection of compressed ga s, usual ly nitr ogen, be cau se of its availability, cost a nd iner tn ess. Ther e is a vari ety of gas-assisted injection molding pr oVenting and part release cesses. In most of the processes, the gas is inj ected into th e hot cor e of the melt through the nozzle an d th e sprue /r unne r s ystem, Figur e 1. Sequence of th e gas- or dire ctly int o the cavity via one or more gas needles. Due to an assisted injection molding pr o- almost consta nt pre ssure in a ll areas with g as p enetr at ion, a good cess. and even pr essur e distri but ion a nd t r ansmission is gua ranteed across the molded parts. After the end of the gassing phase, the pressure is released either by gas r ecycling or blowing the gas into th e at mosphere . As soon as ambient p r essur e is r eached , th e molded p ar t can be ejected . The r e ar e thre e main basic cat egori es of appl icat ions and some combi nat ions of t hese. The categories are as in the following. Th e first gro up in cludes tube - or r od-sha ped pa rt s such as for example clothes hangers and grab handles. The second category consists of large

44

Special Molding Techniques

-Design and layo11t of the part based on e1."perlence . .CoRstJ11ction of a prototype

tool

. -Te6iing or tht pl'Ototype tool

~

... -Detection

of weaknesses

. -Rem011'lll ofweaknesies tltronglt modiflcat1on I adaptation (gfomcil'Y , illjcction point locatiOlls, 2})5 ~ectlon locatIon)

Figure 3. Influential factors for the production of gasassisted injection molded parts.

.Layout or a nlM~produdiOlltool bum on thefmdingsfium the prototype tool n,-outs

cover-shaped structural parts with a network of gas channels often combined with the rib struc. ture of those parts such as e.g. business machme Figure2. Partandtooldesignfor gas-assisted injection housings, automotive panels and outdoor furnimoldedparts. ture. The third group encircles complex parts consisting of both thin and thick sections where the process is often used mostly for part integration by consolidating several assembled parts into a single design. Typical examples are armrests or mirrors as designed for automotive applications. The characteristic of the gas-assisted injection molding process leads to both various advantages and inherent problems associated with the design and processing. The key factor in being successful in producing with one of the gas-assisted injection molding processes is to use the advantages to your benefits and to avoid the difficulties by optimizing part design, material selection for the application as well as to choose the optimum technique.

STRATEGIES

FOR PART AND TOOL

DESIGN

In Figure 2, the particular steps for the optimization of tool and part design are listed. The building of a mass production tool is based on the knowledge and experience gained from experiments. In many cases,the use of programs for mold filling simulation is a good help to optimize the final design. INFLUENTIAL

FACTORS

There are four different basic groups of influential factors involved in the production of gasassisted injection molded parts as shown in Figure 3. These groups are material properties, processing parameters, part design and the gas-assistedinjection molding technique. Substantial is a part design adapted to the gas-assisted injection molding process. It is very important, that designers take the specific process conditions and requirements associated

Cover Part as an Application Example

45

with these techniques into account in advance while developing new part designs. The necessary adaptation work and the costs associated herewith can be reduced to a minimum. Furthermore the knowledge about interdependencies/interactions between part design, runner/gate-type and location, course and size of the gas-channels in connection with the filling pattern after melt injection and processing parameters are a very important factor for achieving an optimized and stable production process. The above-described procedure can be used for all application examples and offers solutions for all the process-related phenomena/problems in each category of parts. The course to optimize every single part differs a little bit from application to application. But the key principle is always to analyze the influential factors and adjust and improve those as much as necessary. With this better understanding of the process more and more of the advantages can be used in order to utilize the high and still growing potential of the gasassisted injection molding process in daily production and for new future developments. In the following the experiences in fixing an existing production tool is discussed. The analysis starts in this case with a try-out to determine the problems associated with tool/ design. After the weaknesses are determined the necessary adaptations are will be implemented and tested in a subsequent try-out. In order to achieve the best possible result for the production all the above-mentioned influential factors were investigated while testing and analyzing the tool.

GAS–ASSISTED INJECTION MOLDED COVER PART Because of the physics of the gas-assisted injection molding process cover parts require a network of gas-channels to provide the guidance for the gas entering into the hot melt through nozzle or gas pin. The more than one-dimensional flow path of cover-shaped structural parts ask for an excellent balancing of the injection point location, the gassing location and the melt filling pattern before gas injection. These are only some of the boundary conditions in connection with a cover application, which need to be taken into consideration. The size of the part reviewed in the following is 985 mm x 560 mm with a 3 mm wall thickness. The material used in this application is a glass-fiber filled PPE/PS resin. The material was chosen for this application because of structural requirements, heat and chemical resistance. The cavity surface of the finished part is a class A textured surface. The problem with the glass-fibers is to create an acceptable surface finish for the subsequent texture painting process without showing visible sink, hesitation or flow marks. The material solidifies quickly once the flow front stagnates. In areas with weld lines problems can occur to produce an acceptable surface without glass-fibers showing up at the surface. If those areas show up another secondary filling/sanding operation would be necessary.

46

Special Molding Techniques

PROBLEM DESCRIPTION

Figure 4. Core side of cover part with gas-channel network.

In case of this cover application the main issues are as described in the following. The melt filling pattern before gas-injection has to match the gas-channel network along the structural ribs. The task is to determine the appropriate degree of pre-fill with polymer before gas-injection, which makes possible to produce parts without the above-mentioned surface imperfections. The configuration of the injection sprue, gas pin and the network of gas- channels are shown in Figure 4. This con-figuration of gas and melt injection creates certain boundary conditions for the pre-fill with melt, which are illustrated in the following chapter. Figure 5 shows the injection sprue and the gas pin location as well as one of the filling stages before gas injection. Very important for the production of the part is to avoid sink marks on the cavity as occurring in areas around screw bosses and thick ribs running across the length of the part in center and on the side opposite to the screw boss locations.

FINDINGS FROM PRACTICAL MOLDING EXPERIMENTS In the try-outs with the existing tool the next step was to find weaknesses/necessary adaptations for Figure 5. Filling pattern showing the location of sprue correcting the problems occurring in the mass and gas pin. production. Prerequisite for running a successful production is to establish a stable molding process. One important prerequisite for the production of gas-assisted injection molded parts are the filling pattern before gas injection. This needs to be adapted to the planned or existing course of the gas-channel network. The part design is dependent upon size and shape, as well as the course and length of the gas-channels. The information for the final layout can be obtained from short shot studies as well as from mold filling simulation programs.

Cover Part as an Application Example

47

Another important factor is to check the process for repeatability in production. To compensate for slight fluctuations in the injection stage with polymer it is very important to ensure the same conditions for the subsequent gas injection via gas pin. Changes in the short shot sequence can create a narrow processing window for the gas-assisted injection molding process and under certain circumstances it results in a borderline running process. Due to the lower flow resistance there is a significant foreflow visible in areas where gas-channels are running along ribs or across surfaces. This flow behavior determines the degree of pre-fill with polymer before gas injection. As a result of the gas pin position and the gas-channel connecting the gassing location the whole surface area around the gas pin location needs to be completely filled before gas injection. Otherwise a gas blow-through would occur. The height (51 mm) and a 3 mm wall thickness of the main high rib running almost across the whole center portion of the part results in a sink mark potential on the later texture painted surface. There is also a thick rib running almost all the way along the part on the side of the 6 hinges as shown in Figure 4. This rib is a bigger problem from a processing prospective because there are more than one gas–channel intersecting with this rib. A sink mark far away from the gassing location can only be avoided when the gas hollows out the area where the rib intersects with the main surface. To achieve this there are at least 3 different gas-bubbles necessary created via the different gas-channels attached to the rib. The most critical factor in producing these parts is to figure out a way to keep the gas bubble penetrating through the hot melt always in the same areas of the part. This is the key to a successful and stable serial production. Slight fluctuations in gas-bubble extension and path can lead to significant changes in the molded part. The ability to keep the gas-bubble penetration in certain limits is the boundary line between running a successful application or having a constant fight with a production problem. In connection with the processing there is a very important issue with gas-assisted injection molded parts, which is often even more critical than the process itself. It is the design of the molded part. In the case of this part there are 4 areas with screw bosses and a rib pattern as shown in Figure 6 a) that create a mounting plain. These create one of the production problems. The rib pattern around the screw boss can result in a significant visible sink mark on the surface. All those areas are located far away from the melt and gas injection and represent areas which are filled and packed late in the process of producing the part. The gas-channels need as shown in Figure 6 b) to be at least close enough to pack the areas around the boss areas. In Figure 7 a) the course of the gas penetrating the melt is shown. The gas flows from the gas pin in direction to the high center rib and the gas flow is diverted into 6 different directions following the gas-channel network of the part. The gas flow is split up is more

48

Special Molding Techniques

Figure 6. Screw boss areas with rib pattern for creating a mounting plane.

often diverted flowing towards the last filled areas of the part as shown in Figure 7 b). The Figure 7. Gas distribution from the gas injection point following the gas-channel network. more flow paths are provided for the gas, the more problem areas can occur on the molded part. The most critical areas from showing sink marks are the thick ribs running along the length of the part. Due to the wall thickness and height the gas pressure needs to prevent those areas from sinking in. The rib on the side opposite the boss areas is very critical for packing. As shown in Figure 8 a) and b) the gas-bubble splits up reaching the thick rib and flows in two opposite directions. Important here is to achieve a stable gas-channel pattern with a gas-bubble entering the root of the rib from both sides (see Figure 8 b) packing out the melt to avoid a sink mark. This can be achieved by extending the gassing time and pressure to allow the gas to compensate the shrinkage in those thick-walled areas.

Cover Part as an Application Example

49

CONCLUDING REMARKS The application example shows both the difficulties of distributing the gas along several different paths as well as the ability to pack areas far away from the gas injection location. To make sure that sink mark can be avoided it is necessary to provide a consistent compact injection molding phase. It also demonstrates that extending the pin length in those screw boss areas were the key to void sink marks along with an adapted gassing time and pressure. As long as the gas-channel can penetrate close or into those areas, visible sink marks can be avoided as shown in Figure 7 b). The nitrogen gas has the ability to pack ribs far away from the gas injection without leaving a visible sink mark on the surface. Prerequisite to achieve this goal is to provide the consistency of metering and constant conditions in the compact filling phase. Then the gas always follows the same flow Figure 8. Gas-channel course close to boss areas and paths and the gas-channel distribution along ribs along the rib parallel to main cross rib. and mass accumulations is nearly constant. Based on those results very little adaptations to the gas-channel network were necessary. Basically only one gas-channel needed to be tapered down at the very end to avoid slight sink marks. In other areas a slight chamfer helped to guide the gas in certain locations. The high rib running across the length of the part didn’t show any visible marks on the cavity surface after optimizing the gas processing parameters. This part is a very good example that in a lot of applications the sometimes so-called minor side issues are creating the biggest production problems and those can be identified by analyzing all the influential factors, which contribute to the production of a gas-assisted injection molded part.

REFERENCES 1

2

Hansen, M. “Anwendungsbeispiele fuer Gas-innendruckformteile,” (Topic: “Application Examples for the Gas-Assisted Injection Molding Process”), seminar “Processing Technology in Injection Molding”, at the SKZ in Wuerzburg, Germany, June 1997 Hansen, M. “Verfahrenstechnische Grundlagen zur Auslegung von Gasinnendruck-formteilen”, (Topic: “Processing Basics for the Design of Gas-Assisted Injection Molded Parts”), Ph.D. thesis, Shaker Publishers, Aachen, 1996

50

3

4 5

Special Molding Techniques

Potente, H. “Anwendung des GID-Verfahrens am Hansen, M. Beispiel eines Haltegriffes”, (Topic: Burgdorf, D. Application of the Gas-Assisted Injection Molding Technology exemplary for an Oven Handle”), Plastverarbeiter 46 (1995) 3, p. 40-51 Hansen, M. Application Examples for Gas-Assisted Injection Molded Parts Structural Plastics ‘99, Boston, p. 95-108 Hansen, M. Application Examples for Gas-Assisted Injection Molded Parts Journal of Injection Molding Technology, Vol. 3, No. 3, p. 141-153.

H. Potente lnstitut

fur Kunststofftechnik,

Universitiit-GH

and H.-P. Heim

Paderborn,

Germany

INTRODUCTION Over the past few years, a large number of variants of the injection moulding process have been introduced offering properties optimally tailored to the applications and targets in question. The gas injection technique (GIT) is a multi-component process, similar to the sandwich technique, which involves different raw materials being successively injected into the mould, leading to a layered moulding structure. By injecting inert gas into the molten plastic, hollow spacesare created in areas of the moulded part with higher wall thicknesses. The use of this process can be motivated either on purely process engineering grounds or for design reasons.In many cases,design requirements are placed on parts which only permit cost-efficient production by the gas injection technique and, at the same time, the process offers a number of technical advantages.Various drawbacks have to be set against these advantages -especially the considerably more difficult design of the part and the more complex process control.7 The different variants of the gas injection technique can be distinguished on the basis of the nature of the gas introduction or the type of melt injection. The gas can be introduced either via the machine nozzle or via a mould injector.8 Both processes have their specific advantagesand drawbacks for the particular application involved.7 To form the hollow spaces,melt is pushed out of the liquid centre of the moulded part and replaced by gas, and hence it is necessary for a similar-sized volume to be available or to be created in the moulded part in order to receive this displaced material. The mould cavity is only filled partially to begin with, by the short-shot process, and the residual filling with gas conducted in such a way that melt is conveyed into the as yet unfilled volume by the gas guidance geometry (thick-walled area of the moulded part).5 In the ancillary cavity process, by contrast, the moulded part undergoes complete volumetric filling and, when the gas is injected in, a connected volume is opened up by the machine control system. The melt is conveyed into this area by the gas guidance geometry as the gas is being introduced.7 Both casescall for highly elaborate balancing of the displaced volume and the volume to be filled.

52

Special Molding Techniques

The gas injection technique has found a large number of areas of application over the past few years in particular. The moulded part geometries for which the gas injection technique is used similarly vary over a broad range. It has become established practice to distinguish between .rod-shaped, thick-walled moulded parts .thin-walled moulded parts with gas guidance ribs and .thin-walled moulded parts with thick points in certain parts. The actual shape of a moulded part in these categories results from the requirements placed on the part design and function, making it necessaryfor the process employed to be adapted to the properties desired in the moulded part being produced, with consideration to design guidelines. The cause-and-effect correlations for the gas injection technique are correspondingly complex.

PROBLEM When it comes to the design optimization of a gas injection moulding, different publications contain information on the design of the thick-walled areas of the moulded part in respect of cross-section and wall thickness ratios as well as in terms of the radii employed. This provides design guidelines, which have already been incorporated in a systematic design procedure in the main.7 These guidelines relate to the gas guidance geometries, for which highly detailed design recommendations are given. They constitute the basis for what is set out below and ought to be borne in mind whenever a GIT moulding is being de-signed. POSITION OF THE MEL T AND GAS INJECTION POINTS In conjunction with the structural layout of the moulded part, it must be stated that the position of the melt and gas injection points is of particular importance, in addition to the design guidelines.2,6 The position of the melt gate must be optimized with allowance for the melt flow and coordinated with the position of the gas injection point.l Since the moulding compound can only be displaced to-wards the end of the flow path, i.e. towards unfilled areas of the moulding, the fosition of the melt gate and the gas injection point determine the course of the gas bubble. Apart from this, the associatedhigh pressure requirement causesmolten plastic to be displaced from the thick-walled gas guidance geometry to thin-walled areas of the moulding, which can lead to instabilities in the process sequence. In order to completely fill the thin-walled areas, a sufficiently high pressure must be provided with the gas. The problem here is the stagnation of the melt after its injection, while the gas pressure is being built up, which is inherent to the process. Figure 1 shows the pressure development at different measurement points in the moulded part. A pressure gradient is clearly evident along the flow path. This is only eliminated

53

Molded Part Design

Penetration of the gas in the flat area of the moulding and blow-through

gas guidance rib

residuallyfilled area

gas injection

Figure I. Pressure development over time in the gas injection process, from Michaeli, W., Lanvers, A.: Gasinjektionsverfahren, p. 248.

Figure 2. Schematic diagram of an unfavourable combination of gas guidance rib position and residually-filled area.

as the gas bubble advances, giving an isobaric state prevails along the length of the gas channel.4 Areas of the moulded part that are a long way from the gas injection point first have a relatively low pressure acting on them at the time the gas is introduced. If this is not sufficient to displace the melt, the flow front will freeze, producing a gramophone record effect on the surface. the moulded part will only be fully shaped with a high gas pressure. It can be concluded that this effect will be all the stronger the higher the pressure requirement in the residually-filled area is. The gas follows the course of least resistance. The higher the pressure requirement for residual filling, therefore, the higher the tendency for fingers to form in areas adjacent to the hollow space guidance rib. In extreme cases, the course followed by the gas can deviate completely from the gas guidance rib if there are other areas of the moulding that are easier to displace. The schematic diagram in Figure 2 shows an example of such a case. The position of the so-called residually-filled area, i.e. the part of the mould cavity that is not filled with melt prior to the injection of the gas, is thus of decisive importance. Prefilling thus constitutes a crucial aspect of moulded part design. In critical cases,just a slight variation in the amount of compound initially injected in can lead to the problems that have been indicated. Two requirements can thus be placed on the layout of GIT mouldings: .the residually-filled area must be located at the end of the desired gas channel .thin-walled areasmust be completely filled with melt before the gas is introduced.

54

Special Molding Techniques

PRESENTATION

OF THE PROBLEM TAKING A SAMPLE MOULDED PART

,\rea...fthec".jtynotIiIlcd priortothe.tartofg~rod..o:tjOn -/ ~ "

The moulded part shown in Figure 3 was developed for different studies of the gas injection ~echnique.Thi~ is a test mold whic.h de~ibera~ely mcorporates different process engmeenng diffi-

culties, such as the thick-walled annular region, and also the gas guidance geometry divided into two flow directions. In order to produce a lowwarpage and low-shrinkage moulded part, the (;as n..'.'dl" thick-walled areas of the moulding are to be holposition lowed-out by the gas injection technique. First of all, it is necessary to establish the Figure 3. Presentation of the residually-filled areas gas guidance geometry and the required position of the residually-tilled area(s), so that the melt and gas injection points can be determined. After different preliminary considerations, it was decided that gas injection should be performed in the annular area and that the gas bubble should propagate around the circular ring and in the two rod-shaped, thick-walled areas. It must be borne in mind here that it is, of course, not possible to achieve a ring fully surrounded by a gas bubble and, hence, apart from the two residually-tilled areas in the front domes, an unfilled area must also remain in the ring after partial filling. The target residually-tilled areas are shown in Figure 3. The requirements for the establishment of one or more appropriate melt injection points are thus: .partial filling with the unfilled areasthat are marked in Figure 3 .proportional distribution of the volumes of the un-filled areas in accordance with the volume of melt to be displaced (with different degrees of pre-filling) and .complete filling of the thin-walled area during partial filling.

ESTABLISHMENT

OF OPTIMUM MEL T INJECTION POINTS WITH A SAMPLE MOULDED PART

To determine the melt injection points, use was made of finite element calculations to simulate filling behavior with different gate positions. Different calculations were performed with melt injection in the central thin-walled area, inter alia. By contrast to injection in the edge area, this position also constitutes an advantage in mould engineering terms. Figure 4 shows the calculation results for three different gate positions.

55

Molded Part Design

Figure 4. Filling simulation for three different gate positions (marked by the arrow).

mc11injoction po;n",

0 impro,""

0

artick ~comctty

2

Figure 5. Presentation of the improved article geometry and melt injection points.

It is clear that these injection points will not give satisfactory results. The flow pattern shows Figure6. Presentation of differentdegrees of filling for that the desired division of the residually-filled theoptimised articlegeometry. areas is not possible on account of the flow direction dictated by the perforations. The left-hand picture shows an excessively high filling level in the thick-walled domes. When the injection point is moved towards the centre of the moulded part, disproportionately high filling of the ring area results. This effect can be attributed to the melt guidance through the bars in the thin-walled area of the moulding. Since a different gate position cannot be expected to improve the flow pattern, structural changes were made to the moulded part. Figure 5 shows the changed article geometry. As the Figure 5 shows, additional bars were incorporated in the region of the perforations in order to guide the melt. This produces a clearly improved flow front course which better meets the requirements on the flow behavior defined at the outset. As Figure 6 shows, virtually symmetric mould filling results in the thin-walled region. The depiction of different degrees of filling shows that the requirements in respect of the residually-filled areas can also be fulfilled. An estimate of the unfilled residual volumes for

56

Special Molding Techniques

different degrees of filling shows that the three unfilled areas are more or less proportional to the displaced volume. This ensures that the prefilling can be aligned to the different gas bubble cross-sections to be expected. CONCLUSIONS The example outlined illustrates the basic approach to the establishment of appropriate melt injection points for the gas injection technique. It becomes clear that even slight changes to the article geometry and the position of the gate can lead to changes in the course of the flow front, which have an extremely negative impact on application of the gas injection technique. The flow pattern is thus one of the decisive parameters for the process reliability that can be attained with the GIT and for the quality of the moulded parts produced. Filling simulations employing finite element calculations represent an appropriate method for obtaining information on the application of the GIT right at the development phase. Through appropriate balancing of the filling behavior and the unfilled areas of the cavity after partial filling, it can be ensured that the conditions for application of the GIT, i.e. the three main requirements on appropriate melt injection points, can be fulfilled. The results presented deal exclusively with the filling behavior of the moulded parts. The extent to which other results of an FE simulation, such as pressure and temperature distribution, can be used and interpreted specifically for the GIT is to be checked in further investigations.

REFERENCES 1 2 3 4

5 6

7 8

Eckardt, Helmut: Gas-Assisted Injection Molding, in: Stevenson, J. F.: Innovation in Polymer Processing: Molding, Carl Hanser Verlag, Munich Vienna New York, 1996. Jaroschek, Christoph: Elegant? Preiswert? Oder sogar beides?, Kunststoffe 87 (1997) 9, pp. 1172-1176. K1otz, B.: Voraussetzungen im Bereich der Formteilgestaltung fUr die Anwendung des Gasinnendruckver-fahrens, Transferzentrum Aachen Kunststofftechnik, SpritzgieBtechnisches Kolloquium 1990, pp. 36-57. Michaeli, W.; Lanvers, A.: SpritzgieBen transparent gernacht -Neue Entwick1ungen bei der ProzeBsimulation, Teil1: CAE- Techniken fUr das Zweikomponen-tenspritzgieBen und das Gasinjektionsverfahren, Plaste und Kautschuk 39 (1992) 7, pp. 241-248. Moritzer, Elmar: Ph1lnomenorientierte ProzeB-und Formteiloptimierung von thermoplastischen Gasinjektions-( GIT)SpritzgieBartikeln, Dissertation an der Universitlit-GH Paderborn, Shaker Verlag, Aachen 1997. Renger, M.: Das Gasinnendruckverfahren -eine SpritzgieBvariante mit besonderen Mt\glichkeiten, SUddeutsches Kunststoff-Zentrum WUrzburg, Fachtagung 18.-19. September 1990, pp. 101-136. Rennefeld, Christoph: Konstruktive Optimierung von Thermoplastformtei1en und SpritzgieBwerkzeugen fUr die Gasinnendrucktechnik, Dissertation an der Universitlit GH Paderborn, Shaker Ver1ag, Aachen, 1996. Shah, Suresh: Gas Injection Molding: Current Practices, ANTEC '91, pp. 1494-1506.

Design Optimization of Gas Channels for an Air Cleaner Assembly Using CAE Simulations

D.M. Gao, A. Garcia-Rejon, G. Salloum and D. Baylis Industrial Materials Institute - National Research Council Canada, 75, blvd. de Mortagne, Boucherville, Quebec J4B 6Y4, Canada

INTRODUCTION The typical gas-assisted injection moulding process (short shot process) can be subdivided into the following steps: a) polymer filling to a predetermined percentage cavity filling b) gas injection; and c) packing stage. During polymer filling the cavity is partly filled (up to 80%). Shortly after the end of the polymer injection, the gas is injected to hollow-out the gas channels until the cavity is completely filled. The relative melt/gas flowrate and the switch-over time between polymer and gas injections will determine the amount to be hollowed. Due to the gas penetration inside the polymer melt during the gas injection phase, the amount of material needed, the level of injection pressure as well as the clamp tonnage required, the resulting shrinkage/warpage and sink marks on the part can be greatly reduced. Due to its versatility in the production of mouldings of much greater complexity which combine thick and thin walls, hollow sections and elongated shapes, gas assisted injection moulding has resulted in the production of single parts replacing multipart assemblies and therefore a substantial reduction in manufacturing costs. Designers now have much greater freedom to incorporate thick and thin sections in the same moulding that will result in ribs and flow leaders; higher stiffness to weight ratios; reduced cycle times; and higher dimensional stability. The air cleaner assembly, analyzed in this project, is a good example of an open-channel part. These parts have a thin wall with the gas channels traversing the part similar to conventional ribs. These parts are more difficult to design and mould because the gas may penetrate into the thin walled sections of the part (fingering). The optimal layout of the gas channels - relative location of gas channels and gas injection points to polymer gates - within a cavity should create a polymer filling pattern in which the lowest pressure will be located near the end of the channels. In order to avoid fingering the channels should be oriented in the direction of the melt flow. Channel size and geometry have to be chosen in such a way to minimize race track effects while maintaining

58

Special Molding Techniques

the structural advantages offered by gas assisted injection moulding. In the case of multiple gas channels it is also very important to avoid an unbalanced gas penetration in the different channels. The use of computer aided flow analysis in the case of gas assisted injection moulding can be of great help to the part designer, mould maker and processor in the determination of parameters such as: i) gas channel design; ii) % of polymer to be injected and its optimal injection point; iii) gas injection locations; iv) preset volumes and pressures for gas injection; and v) filling patterns and operating conditions for optimal polymer wall thickness; best quality surface and minimal shrinkage and warpage. The primary objectives of this project are to determine, through computer aided flow analysis, the gas channel design (size and location) as well as the optimal moldability diagram (polymer injection speed, flow rate, temperature range, gas pressure, etc.) for an air cleaner assembly moulded using a 30% glass fibre reinforced polypropylene. In order to provide a useful information to the part designer, mould maker and moulder, several gas channel designs as well as different moulding conditions are considered. A number of important factors such as polymer flow pattern, gas penetration, tooling constraints, etc. are taken into consideration in the optimization of the moulding conditions.

NUMERICAL MODEL During the gas penetration into the polymer melt, three distinct flow regions during the gas injection stage can be identified: 1) gas penetration region, 2) polymer melt region, and 3) empty or unfilled region. Region 1 is initially filled by the polymer. The gas penetrates into the polymer melt and creates a gas core. During the gas penetration, the gas is transmitting the pressure required to advance the melt front. The polymer skin layer between the gas and the mould walls is stagnant. It is assumed that the skin layer consists of a solidified layer and an adhered layer. The solidified layer is formed by the polymer freezing upon contact with the cold mould. The adherence between the polymer flow and the solidified layer creates the adhered layer. Regions 2 and 3 are identical to those encountered in conventional injection mould filling except that two moving boundaries for the polymer melt region are present. In this study, the polymer melt is considered as a Generalized Newtonian Fluid, i.e. the viscosity is a function of shear rate and temperature. The flow is assumed to be quasi-steady state and the inertia terms are neglected due to the low Reynolds numbers encountered in molten polymer flow. Since most parts produced by gas-assisted injection moulding have a shell like geometry, i.e. the part thickness is much smaller than other part dimensions, the lubrication approximation (Hele-Shaw flow)1 can be used for modelling the global flow behavior in the mould cavity. A dimensional analysis of the energy equation shows that the

Design Optimization

59

heat conduction in the flow direction can be neglected since the thickness of the cavity is much smaller than the other two dimensions. The convection in the gapwise direction is also neglected. The pressure equation is solved using the Galerkin finite element method. A three node triangular element was chosen to approximate the pressure. The energy equation is discretized using the finite difference method. The time dependent derivative of the temperature is approximated by backward finite difference. A control volume approach has been employed to track both, the flow front advancement as well as the gas-polymer interface. A thickness fraction of polymer skin (Fs) is associated to each control volume in order to represent the three distinct regions present during the filling phase. Fs is defined as the ratio of the thickness of the polymer skin to the total thickness of the part. Fs = 1 represents an element completely filled with polymer, while Fs = 0 represents an empty element. 0< Fs <1 represents an element where the gas has penetrated through and therefore the polymer layer has become a skin layer. Details concerning the simulation model development and its numerical implementation are given elsewhere.2-4

SIMULATION RESULTS GEOMETRY Figure 1 shows the geometry of the air cleaner assembly. Two gas channel network designs are presented here in order to illustrate the design optimization procedure. In both cases square gas channels having a 10x10 mm cross section were used. Even though the gas channel size is generally small compared to the overall part geometry, as the flow velocity is mucher high inside the gas channels than in the thinner sections, a local fine mesh is needed to provide an adequate representation of the gas penetration which is the primary concern in gas channel design. MATERIAL PROPERTIES The material used for the moulding trials is a 30% glass reinforced polypropylene (ThermofilP6-30FG-0153). The Carreau-WLF model was used to describe the melt viscosity as function of the shear rate and temperature.

Figure 1. Geometry of air cleaner assembly.

60

Special Molding Techniques

η0 At η = --------------------------· n ( 1 + λA t γ ) 8.86 ( T ref – T s ) 8.86 ( T – T s ) log A t = -------------------------------------------- – -------------------------------------( 101.6 + T ref – T s ) ( 101.6 + T – T s )

The coefficients for the Carreau-WLF model are given in Table 1. Table 1. Carreau model coefficients (P6-30FG-0153) Zero shear rate viscosity, η0, Pa.s

1.1213 E+04

Reciprocal shear rate, λ, 1/s

6.4200 E-01

Exponent, n

6.8750 E-01

Reference temperature, Tref, oC

2.0035 E+02

Standard temperature, Ts , oC

4.9050 E+01

No flow temperature, oC

173

Melt density, g/cm3

0.84

Thermal conductivity, kcal/mhC

0.211

Specific heat, J/gC

1.49

RESULTS FOR DESIGN 1 The first gas channel design comprises three primary independent gas channels in which two of them have transversal branches touching the part edges (See Figure 2). This design allows maximum gas penetration. The gas penetration in all gas channels generates a uniform pressure over the entire part, therefore reducing the cooling time and the parts’s warpage. A centre polymer gate is considered to provide a balanced filling prior to the gas penetration. Three gas injection points were used to core the three primary gas channels. During the actual moulding this design will require a three gas cylinder machine. The location of the polymer gate and gas injection points are shown in Figure 2. Figure 3 shows the polymer flow front advancement before the gas injection. The grey scale represents the filled domain as a function of time. For this gas channel network, the polymer filled region is stretched in the same direction as the orientation of the primary gas channels. This is due to the fact that the polymer follows the path of least resistance, e.g. inside the gas channels.

Design Optimization

61

Polymer Gate

Gas injection points

Gas channels

Figure 2. Polymer and gas injection locations for Design 1.

Gas finguering

Gas channel 3

Gas channel 2 Last filled area

Gas channel 1

Figure 4. Final gas penetration for Design 1.

Figure 3. Polymer prefill filling pattern for Design 1.

After the polymer injection, gas was simultaneously injected from the three gas injection points. The final gas penetration is represented in Figure 4. The last filled region was located near the exhaust pipe (top left of the Figure). Since the gas channel 3 (at the right of Figure 4) is located across the part, the gas penetrated the entire gas channel and then leaked out. Due to the strongly stretched polymer prefill pattern (see Figure 3), the gas penetration inside the center gas channel was blocked by the stagnant polymer meeting the part edges. Therefore the centre gas channel has no practical use in this design. RESULTS FOR DESIGN 2

Figure 5 shows the second design studied in this project. The centre gas channel has been removed and three transverse gas channels have been added to connect the two previously independent gas channels and to increase the part rigidity. A centre polymer gate is used and the gas will be injected through two gas injection points located at the low end of the two primary gas channels (see Figure 5). The final gas penetration is shown in Figure 6. Accord-

62

Figure 5. Polymer and gas injection locations for Design 2.

Figure 7. Pressure distribution for Design 2 (unit: psi).

Special Molding Techniques

Figure 6. Final gas penetration for Design 2.

Figure 8. Temperature distribution for Design 2 (unit: oC)

ing to the resulting filling pattern (not presented here), the gas advanced at almost the same speed in both gas channels, therefore ensuring an even penetration. With this modified design the two primary gas channels are entirely hollowed. The transverse branches are also hollowed by the gas except for the centre portions which are being further investigated. As expected, the pressure distribution is quite uniform due to the uniform gas penetration (see Figure 7). Figure 8 shows the temperature field. It can be seen that the region around the centre polymer gate has cooled down faster. The polymer skin contained inside the gas channels, being hotter, will cool down during the gas packing phase.

Design Optimization

63

CONCLUSIONS In this work the gas channel design optimization of an air cleaner assembly was conducted using a numerical model for gas assisted injection moulding developed at NRC's Industrial Material Institute. Two gas channel designs are presented as well as the simulation results on gas penetration, pressure distribution and temperature field. The CAE simulation techniques have shown their effectiveness when used for the optimization of this industrial part.

ACKNOWLEDGEMENTS The authors would like to thank Siemens Electric Ltd for allowing to present the results of this project.

REFERENCES 1 2 3 4

C.A. Hieber and S.F. Shen, J. Non-Newtonian Fluid Mechanics, 7, (1980). D.M. Gao, K.T. Nguyen, A. Garcia-Rejon and G. Salloum, Proceedings SPE ANTEC’96, 638-643, Indianapolis (1996). D.M. Gao, K.T. Nguyen and G. Salloum, NUMIFORM'95, Eds. Shan-Fu Shen & Paul Dawson, 1125-1130, Ithaca, NY (1995). K.T. Nguyen and D.M. Gao, ASME Annual Meeting, MD-Vol. 49/HTD-Vol. 283, 89-103, Chicago (1994).

The Occurrence of Fiber Exposure in Gas Assist Injection Molded Nylon Composites

Shih-Jung Liu and Jer-Haur Chang Polymer Rheology and Processing Lab, Mechanical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan, R.O.C.

INTRODUCTION Gas assist injection molding1,2 has received extensive attention in recent years, due to its flexibility in the design and manufacture of plastic parts. In this process, the mold cavity is partially filled with the polymer melt followed by the injection of inert gas into the core of the polymer melt. Gas assist injection molding can produce parts incorporating both thick and thin sections with less shrinkage and warpage and better surface finish. It requires lower clamping force than the conventional injection molding process.3-5 Today, short glass-fiber reinforced thermoplastic composites have the fastest rate of development and represent 30% of the composite market. Gas assist injection molding of glass-fiber reinforced composites has the capability of producing parts having thick and thin sections with a good structured rigidity. However there are still some unsolved problems that confound the overall success of this technology. Parts roughness occurring on the surface of molded composites caused by inappropriate mold design and processing condition is one of them. To the best knowledge of the authors, no research paper has ever discussed the abovementioned problem. The purpose of this report was to study the surface roughness phenomenon occurring in the gas assist injection molded composite parts. The materials used were short glass-fiber filled Nylon-6 composites. Experiments were carried out on an 80-ton injection molding machine equipped with a high pressure nitrogen-gas injection unit. Two “float-shape” axisymmetric cavities were used. Various processing variables were studied in terms of their influence on parts surface quality. The final goal of this research is to explain the mechanisms of rough surface formation, so that steps can be taken to improve the surface quality of molded composites.

66

Special Molding Techniques

EXPERIMENTAL The resins used in this study were commercially available grade 15% and 35% E-glassfiber filled Nylon-6 composites (Ginar Chem., Taiwan).6 The fiber in the composites has a diameter of 10 µ m and an approximate aspect ratio of 10, as measured by the resin supplier.6 Table 1 lists the characteristics of the composite materials. Gas assist injection molding experiments were conducted on an 80-ton injection molding machine (Taichung Mach. VS-80, Taiwan). A high-pressure nitrogen gas injection unit (Gas Injection model PPC1000, U.K.) was attached to the machine.6 Table 1. The processing variables as well as the values used in the experiments. a

b

c

d

e

f

Melt temperature, oC

Mold temperature, oC

Melt filling speed,%

Shortshot size, mm

Gas pressure, bar

Gas injection delay time, s

1

280

65

55

42

50

5.0

2

285

80

65

43

60

6.0

3

290

95

75

44

70

7.0

4

295

110

85

45

80

8.0

5

300

125

95

46

90

9.0

Processing parameters

Figure 1. Layout and dimensions of mold cavity.

Two “float-shape” cavities were used in this study. Figure 1 shows the dimensions of these cavities. One of Figure 2. Schematically the distribution of parts roughness and the positioning of the them has a diameter d measuring probe. (=9 mm) at the ends and a diameter 2d at the center portion, i.e. d-2d-d, while the other has the same geometry except a size variation of d-4dd. The mold has two movable plugs in the runners. By turning the plug, the way the flow enters each cavity can be con-

The Occurrence of Fiber Exposure

67

trolled. Only one cavity was used for the experiments each time. The temperature of the mold was regulated by an oil-circulating mold temperature control unit. After molding, the surface roughness of molded composites was measured. A roughness meter (Hommelwerke model T2000, Germany) with a measurable range of ±200 µ m was used. Eight lines equally spaced across the surface length of a cylinder (see Figure 2) were selected for evaluation. The length of each line was 8 mm, which was the longest measurable distance of the roughness meter at a time. Both the maximum individual peak-to-peak height Rmax and the arithmetic average roughness Ra were used for parts evaluation. Rmax is defined as the maximum peak-to-valley dimension obtained from the five sampling length le within the evaluation length lm (ISO 4287/1). The arithmetic average value Ra of filtered roughness profile is defined by the following equation (ISO 4287/1): lm

1 R a = ----- ∫ y dx lm

[1]

0

which was determined from deviation about the center line within the evaluation length lm. The measured eight roughness Rmax and Ra were integrated to obtain the final surface roughness values. Various processing variables were studied in terms of their influence on parts surface quality: melt temperature, mold temperature, melt filling speed, short-shot size, gas pressure, and gas injection delay time. Table 1 lists these processing variables as well as the values used in the experiments.

RESULTS Gas assist injection molding experiments were conducted with an 80-ton injection molding machine equipped with a high-pressure nitrogen gas injection unit.6 All molded composites exhibited severe surface roughness, beginning from the divergent portion to beyond the center areas of the parts. The d-2d-d parts showed a spiral roughness distribution on the surface. The d-4d-d parts exhibited a wider and more severe distribution of roughness on the surface. Figure 2 shows schematically the rough surface distribution of the parts. Since the molded d-4d-d part exhibited a greater severity of the surface roughness phenomenon, it was used for the subsequent investigations.

68

Special Molding Techniques

Various processing variables were studied in terms of their influence on surface roughness of the molded parts. Table 1 lists these processing variables as well as the values used in the experiments. To mold the parts, one arbitrary processing condition was chosen as a reference Table 1). After molding, a roughness meter was used to measure the surface quality of the parts. A total of eight lines (8mm long for each line) equally spaced across the surface length of a cylinder (Figure 2) were selected for evaluation. By changing one of the parameters in each test, we were able to understand the effect of every factor on the surface roughness of gas assist injection molded composites. CONTENT OF GLASS FIBER The effect of the content of glass fiber on the parts surface roughness was studied. Virgin Nylon, 15% and 35% glass fiber reinforced Nylon composites were used to mold the parts. It was found that the 35% filled composites exhibited the most severe rough surfaces, while the virgin Nylon exhibited none of the surface roughness phenomenon. By setting one arbitrary set of processing conditions in Table 1 (the shaded one) as an example, the measured roughness values (Ra) were 1.6, 3.8 and 8.5 respectively for the molded virgin Nylon, 15% and 35% glass fiber filled Nylon parts. Here the 35% filled Nylon material exhibited the most serious surface roughness; it was therefore selected for the subsequent experimental investigations MELT TEMPERATURE AND MOLD TEMPERATURE

Figure 3. Effect of melt temperature on the surface roughness of gas assist injection molded composites.

Figure 4. Effect of mold temperature on the surface roughness of gas assist injection molded composites.

The effect of melt temperature on parts surface roughness was investigated. Thirty five percent glass fiber reinforced Nylon was used for the experiments. Five temperatures of the polymer melt were set in the gas assist injection molding experiments: 280, 285, 290, 295 and 300oC. The measured results in Figure 3 shows that part surface roughness decreased

The Occurrence of Fiber Exposure

69

with melt temperature. The effect of mold temperature was also studied. Five different mold temperatures were selected, from 65 to 125oC (Table 1). The experimental result in Figure 4 shows that surface roughness of gas assist injection molded Nylon composites decreased with mold temperature. SHORT-SHOT SIZE The gas assist injection molded composites in the experiments were subjected to different short-shot size melt fillings (screw displacement ranges from 42 to 46 mm). The measured result does not show an obvious effect of short-shot size on the surface roughness of gas assist injection molded composites. MELT FILLING SPEED The effect of melt filling speed on the surface quality of molded composites was also studied. Different filling speeds, from 55 to 95% of the maximum available speed of the injection molding machine, were selected. The experimental result shows that the surface roughness of the parts generally decreased with increased melt filling speed. GAS INJECTION PRESSURE AND GAS INJECTION DELAY TIME Parts were molded with different gas injection pressures. Five different gas Table 2. Effect of processing parameters pressures (50 to 90 bars) were selected to on the surface roughness of gas assist mold the composites. The result suggests injection molded composite parts that one can decrease the surface roughness by increasing the gas injection presSurface sure. The effect of gas injection delay Processing parameter roughness time on the surface roughness was also studied. The gas delay times selected for Content of glass fiber () () the experiments were between 5 to 9 secMelt temperature () () onds. Parts were not moldable for the gas Mold temperature () () delay time shorter than 5 seconds in this study. The measured surface roughness of Melt filling speed () () the molded parts in The result suggests Gas pressure () () that one can improve the surface quality of the parts by shortening the gas injection Gas injection delay time () () delay time. 7DEOH lists the effect of various processing parameters on the surface roughness of gas assist injection molded composite parts, based on the above experimental results.

70

Special Molding Techniques

DISCUSSION A series of short-shot experiments of the polymer filling have been completed. As a polymer melt is injected from a small size gate or runner into a large size cavity, the phenomenon of “jetting” or irregular flow can be observed.6 The melt emerging from the gate forms a jet that rapidly advances until it is stopped by the mold wall. Regular flow forward filling commences subsequently. It has been experimentally observed6 that jetting can occur whenever the dimension of the fluid stream is smaller than the smallest dimension in the plane perpendicular to the flow. It is thus related both to the gate size and to the degree of extrudate swelling of the melt, rather than to the level of that axial momentum. Filled polymers, which swell less than unfilled melts, exhibit jetting at lower filling rates.6 In this study, both d-2d-d and d-4d-d parts exhibited jetting flows during filling. Parts molded by the d-4d-d cavity exhibited more severe jetting and irregular flow than those molded by the d-2d-d cavity. This might be due to the fact that the d-4d-d cavity has geometry of larger size-variation from the end to the center. In this study, jetting and irregular flows6 occur in the molded composites. The polymer melt emerging from one end of the mold forms a jet that rapidly advances until it is stopped by the mold wall. As soon as the jetted melt contacts the mold wall it tends to cool. The polymer shrinks much more than the glass fiber6 and leaves the fiber exposed to the parts surface. Parts roughness may thus form at the jetting and irregular flow filled surfaces. Regular flow forward filling commences subsequently after the jetting flow filling.6 As the melt filling stops, the high-pressure gas is injected and pushes the composite against the mold wall.1 This minimizes the shrinkage of the materials and the consequent parts roughness except at the jet filled areas, which have been cooled down and left the fibers exposed to the parts surface. In the experiments, the d-4d-d parts were found to have more serious jet filling than the d-2d-d parts. This might explain why the d-4d-d parts exhibited more severe surface roughness. For the factors selected in the experiments, a higher melt temperature was found to improve the surface quality of the molded composites (Figure 3). Increasing the melt temperature keeps the material hot for a longer time for the gas pressure packing. This will minimize the shrinkage of the polymer3 as well as the surface roughness. As soon as the melt begins to enter the cavity it starts to cool. In order to minimize the surface roughness, the temperature of the melt must remain high enough for a period sufficient for the gas to pack the composite.6 This is aided by having a.high mold temperature so that the melt will not cool too rapidly (Figure 4). In gas assist injection molding, the gas pressure acts as a holding pressure just as in that of conventional injection molding.2 One can decrease the surface roughness by increasing the gas pressure. It is mainly due to the fact that increasing the gas pressure decreases the

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shrinkage of the Nylon matrix1 as well as the possible exposure of the glass fibers. This will make the final part surface smoother. Increasing the gas injection delay time increases the cooling time of the polymer melt. It becomes more difficult for the composites to be packed by the subsequent Nitrogen gas. Filling the mold cavity as rapidly as possible should minimize the surface roughness and obtain parts with the best surface quality.

CONCLUSIONS This report has examined the effect of different processing factors on surface roughness of gas assist injection molded composites. The following conclusions can be drawn based on the current study. • The occurring surface roughness mainly resulted from the exposure of glass fiber on the surface of gas assist injection molded Nylon composites. • When designing a part, one should avoid the occurrence of jetting or irregular flows (e.g., by adopting a smaller size-variation geometry design) during filling to eliminate the surface roughness phenomenon. • One can also improve the surface quality of a gas assist injection molded composites by: increasing the melt and mold temperature, increasing the melt filling speed, increasing the gas pressures, or shortening gas injection delay time. In this study, the mechanism of surface roughness formation has been explained and steps can thus be taken to ensure that the roughness can be minimized. This provides significant advantages in improving product quality of gas assist injection molded composites.

ACKNOWLEDGEMENT The authors would like to express their gratitude to the National Science Council of Taiwan, R.O.C. for their funding support under the grant NSC89-2216-E-182-005.

REFERENCES 1 2 3 4 5 6

S. Shah, SPE-ANTEC Tech. Paper, 37, 1494(1991). L. S. Turng, SPE-ANTEC Tech. Paper, 39, 74(1993). S.C. Chen, K.F. Hsu and K.S. Hsu, Num. Heat Trans., 28, 121(1995). S.H. Parng and S.Y. Yang, Inter. Polym. Proc., 13, 318(1998). S.C. Chen, N.T. Cheng and S.Y Hu, J. Appl. Polym. Sci., 67, 1553(1998). S.J. Liu and J.H. Chang, Polym. Comp., (in press) (1999).

Saving Costs and Time by Means of Gas-assisted Powder Injection Molding

Christian Hopmann, Walter Michaeli Institute of Plastics Processing (IKV), Aachen, Germany

INTRODUCTION Powder injection molding is a near net-shape manufacturing technique for complex metal or ceramic components. The first step in the manufacturing process (Figure 1) is the careful choice Injection Moulding Compounding of the ingredients of the compound that in general consists of a polymeric binder system and the metal or ceramic powder. Binder system is a mixture of all additives and auxiliaries necessary to Sintering Debinding allow the feedstock being processed by an injection molding machine. It may contain plasticizers, lubricants, mold release agents, wetting Figure 1. Powder injection molding process. agents and other. The binder system has to fulfil three tasks. First, it has to provide a sufficient flowability to enable the processing by injection molding. Afterwards the binder has to provide mechanical stability to the green part to allow for handling. Finally the binder has to be removable from the part without any residue. The feedstock typically has a volume loading of 60%. Following the compounding the second step is the injection molding of the feedstock. The machine equipment and processing is very similar to conventional injection molding of thermoplastics. Due to the high powder content the protection against wear of the barrel, nozzle and mold has to be improved. Regarding the process there are some peculiarities primarily concerning the course of cavity pressure over time. This is because of the rheological behavior of the feedstock as discussed below. Injection molding was conducted on a machine with a clamping force of 600 kN and a three section screw with a diameter of 25 mm and a L/D-ratio of 20. The injection molding stage has a considerable influence on the dimensional stability of the moldings. The aim is

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to achieve a homogeneous density distribution, since any density differences will immediately result in inhomogeneous shrinkage and warpage. Finally the binder is removed from the molding and the part is sintered to its final density. There are several possibilities to extract the binder from the molding i.e. thermal treatment and catalytic debinding. However binder removal and sintering may take several hours up to some days.

CHARACTERI ZATIO N OF THE FEEDSTOCK Viscocity [Pas]

104

2

10

υm,PP =235 C υm,CIM =135 C

1

10

Table 1. Thermal properties of CIMfeedstock in comparison to pure polypropylene

PP Ceramicfeedstock

3

10

1

10

3

10 10 Shear rate [1/s] 2

Figure 2. Flowability of CIM-feedstock.

4

10

density ρ, g/cm3 heat capacity cp, kJ/kg thermal conductivity λ, W/mK

PP

CIM

0.9

2

2

1

0.2

1.4

In order to process ceramic feedstock it is 0.1 0.7 very important to consider its specific rheo- thermal diffusivity a, mm2/s logical and thermal properties.1,2 As to the high filler content of about 60% there is a significant reduction in the flowability of the feedstock which should be met by adding additives of low viscosity to the binder. Figure 2 shows a comparison of the viscosity of a ceramic feedstock and a polypropylene. The binder bases on a wax/PP-system. It can be shown that the feedstock's viscosity is slightly higher than that of the polymer but it is within the processing range of common thermoplastics. Besides the flowability of the feedstock, thermal properties are influenced by the high powder loading (Table 1). Regarding the thermal material properties density, thermal conductivity and heat capacity or rather the thermal diffusivity derived from these three properties, it is obvious that it is seven times higher for ceramic feedstock than for polypropylene. Because thermal diffusivity characterizes the velocity of temperature adjustment within the injection molding tool it is proved with regards to processing that the feedstock solidifies very fast. The effective holding pressure time is substantially reduced which demands for a higher filling and holding pressure in order to produce moldings of high quality.

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75

Furthermore, the outer layers in the flow channel are considerably thicker than those occurring in the processing of pure thermoplastics and a rapid sealing of the gate can be expected. Due to a complete filling of the cavity the injection speed has to be on a level that is sufficiently high. On the other hand an excessive injection speed results in jetting of material and separation effects of powder from binder having negative effect on part's quality. Altogether the relevant parameters of the powder injection molding process have to remain in a fairly narrow window.3

WHY GAS-ASSISTED INJECTION MOLDING? Using gas-assisted injection molding, which leads to inner hollow sections, the consumption of raw material can be cut while the stiffness of the part remains on a high level. It is possible as well to increase part's volume while keeping the consumption of material constant. For these reasons gas-assisted injection molding results in a more cost effective production of plastics. Altogether the use of gas-assisted injection molding in powder injection molding provides the same advantages as in the processing of thermoplastics.4 Moreover the transfer of gas-assisted injection molding to powder injection molding has some additional advantages: 1. Due to the increased surface and the lower material consumption besides the saving of cycle time a considerable saving for debinding and sintering can be expected. This leads to an improvement of cost effectiveness because these periods are clearly higher than the cycle time of the injection molding process. 2. During injection molding of compact parts molecular orientations and internal stresses are induced by flow effects for the entire holding pressure phase in order to compensate volume shrinkage. Preferably they can be found near the gate. These orientations and internal stresses may relax during debinding and result in irreversible defect of the molding. Using gas-assisted injection molding, orientations caused by holding pressure phase are eliminated due to the constant pressure level over flow length. In this phase mass flow is reduced to very low level so that resulting moldings have a better dimensional stability and are low-stress and low-warpage in comparison to compact parts. This has not only a favorable effect on the quality of the moldings but also on the error rate. 3. Due to the gas pressure on constant level moldings with higher differences in wall thickness are producible. Internal stresses, which may cause the damage of the part during the following process steps, are avoided to a great extent. In addition to these advantages the use of gas-assisted injection molding in powder injection molding offers a new range of application as well as an increased degree of integration. Nevertheless, up to now the use of gas-assisted injection molding in powder injection molding has not been a matter of research. There are only a few references that certify

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the feasibility of this technique.5,6 For this reason IKV is about to gain systematic knowledge about the use of gas-assisted injection molding in powder injection molding. Figure 3. Defects caused by jetting of material.

OBJECTIVE S flow front of the gas

Within the scope of research at IKV the combination of gasassisted injection molding and powder injection molding will be realized. This will include the analysis of all process steps from compounding to the ceramic part. This enables the solidified detection of all relevant process parameters and their conseouter layer quences on the quality of the part. It is essential to focus the results on the properties of the ceramic product and not only to consider the quality of the green part. The relevant quality features are the error rate and the resulting wall thickness as well as their distribution over the flow path of melt. Figure 4. Acceleration of the gas bubWhen using gas-assisted injection molding in powder ble. injection molding the very poor elastic properties of the melt are a main problem (Figure 3). This behavior favours jetting of material which results in a poor reproducibility of cavity filling particularly when producing moldings with high crosssections. This point has to be met by solutions relating to the mold and relating to the process engineering. Beyond the elastic properties elongational viscosity and shear viscosity play an important role with respect to the processability of the feedstock. Considering the high shear viscosity the acceleration of the gas bubble caused by a decreasing pressure difference between the flow front of the feedstock and the gas bubble (Figure 4) leads to a crack initiation and propagation. On the other hand the elongational viscosity hinders the frontal flow at the flow front and brings about the risk of defects on the flow front. flow front of the melt

INV ESTIGATI ONS First, some fundamental investigations about the feasibility of "gas-assisted powder injection molding (GAPIM)" have been conducted. A consisting injection molding tool, equipped for gas-assisted injection molding but not adapted to ceramic feedstock, has been used in order to gain some basic information about the behavior or ceramic feedstock during this process. This information has been very important for a successful design of an optimized mold.

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Furthermore a new injection molding tool has been designed and manufactured with spegas nozzle cial respect to gas-assisted powder injection gas bubble molding. The cavity shows a molding that is very similar to a screw and has different diameters (Figure 5). It enables getting information about.possible size of molding and critical areas melt like changing diameters. In order to analyze difFigure 5. Design of the GAPIM-Tool. ferent variations of gas-assisted injection molding in powder injection molding the use of an overspill cavity is possible. Following the fixing of a suitable operating parameter set systematical analyses about the connection between process parameters and molding's properties had to be found. The relevant process parameters are varied close to the operating parameter set and the effect on part’s properties is investigated. The diameter of the gas bubble respectively the resulting wall thickness of the molding, the length of the gas bubble and the distribution of the gas bubble over the flow length are considered as the most relevant quality characteristics. Investigations about possible defects like cracks or voids complement the research work. gate

overspill cavity (optional)

CONCLUSIONS The feasibility of combination of gas-assisted injection molding and powder injection molding has been found out within a first step. Using an existing injection mold parts could be produced (Figure 6). However, due to the fact that this mold has not been adapted to the properties of ceramic feedstock demolding was unsuccessful Figure 6. Sample (detail). again and again and a lot of moldings have been destroyed. Nevertheless it has been proved that it is possible to combine gas-assisted injection molding and powder injection molding using the selected feedstock. Regarding the first results it is predictable that resulting wall thickness will be very thin in comparison to gas-assisted injection molding of unfilled thermoplastics. Due to the high elongational viscosity the feedstock used preferably tends to a solid flow profile over flow channel height. This leads to a displacement of the layer near the wall by the gas-bubble.

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REFERENCES 1 2 3 4 5 6 7

Kurzbeck, S., Kaschta, J., Münstedt, H.: Rheological behavior of a filled wax system, Rheologica Acta 35 (1996) 5, S. 446-457 Münker, M.: Untersuchung zur Spritzgießverarbeitung keramikpulvergefüllter Formmassen und rechnerische Simulation des Formteilbildungsprozesses, unveröffentlichte Diplomarbeit am IKV, RWTH Aachen, 1995 Hopmann, C., Knothe, J.: Verarbeitung von ultrahoch-gefüllten organischen Silizium/Siliziumnitrid-Dispersionen nach dem Pulverspritzgießverfahren, Abschlußbericht zum BMBF-Verbundprojekt 03 N 3000 C, IKV Aachen, 1997 Lanvers, P.: Analyse und Simulation des Kunststoff-Formteilbildungsprozesses bei der Gasinjektionstechnik (GIT), Dissertation an der RWTH Aachen, 1992 N.N.: Powder injection offers new choice for molders, Modern Plastics (1997) 12, S. 14 N.N.: Spritzguß mit Hohlräumen, DKG 75 (1998) 6, S. 7 Pohl, T.: Einsatz der Gasinjektionstechnik beim Pulverspritzgiessen, unpublished thesis, IKV, RWTH Aachen, released soon, supervisor: C. Hopmann.

Gas-assisted Reaction Injection Molding (GRIM): Application of the Gas Injection Technology to the Manufacturing of Hollow Polyurethane Parts

I. Kleba, E. Haberstroh Institut für Kunststoffverarbeitung (IKV), Pontstraße 49, D-52062 Aachen, Germany

INTRODUCTION Gas-assisted Injection Molding (GIM) is one of the most promising special injection molding technologies for processing thermoplastics and continues to gain market shares. Between 1997 and 1998 a growth in use of 10% by North American molders has been stated.1 Once limited to thick-walled moldings, nowadays this processing technology is applied to manufacture a broad range of plastic products including complex and thin-walled parts. As its main advantages raw material and weight reduction, shorter cycle times, minimization of warp phenomena as well as compensation of sink marks can be cited. Moreover recent investigations have shown that the gas injection technology is capable to realize parts with functional hollow spaces, e.g., media pipes. This could be demonstrated for injection molding of thermoplastics as well as hot-curing liquid silicone rubber (LSR).2,3 These advantages already indicate the great potential of application of a gas-assisted reaction injection molding (GRIM) technology for reactive polyurethane (PU) systems. Polyurethanes are known for their broad range of material properties which can be varied from soft and elastic to hard and stiff. Hence the combination of these material advantages with the processing potential of the gas injection technology could lead to an improvement of existing PU processes and products. As an example, the GRIM technology could be used to improve the surface quality due to the possibility to apply an internal gas holding pressure. In addition it could lead to weight, raw material and hence cost reduction similar to the benefits of GIM. Moreover the GRIM technology could also enable interesting new PU applications. For example, it could be used to manufacture gas pockets in soles of shoes in a one-step process to realize special elastic properties. However, due to the significant differences in the material behavior during the molding process between thermoplastics and reactive polyurethane systems the transference of the experiences gathered in gas-assisted injection molding of thermoplastics has to be regarded

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critically. Against this background numerous investigations concerning the material behavior during the gas-assisted molding process have been performed.

DESCRIPTION OF THE PROCESS The processing steps of the GRIM concept are shown in Figure 1. In accordance with the GIM process the first step is a short shot with the reactive PU mixture, characterized by the degree of filling FG. After a certain period of time, the socalled (gas) delay time, td, which enables a pregas injector mixing head reaction of the PU mixture the gas is injected MH MH MH MH A B into the core area of the PU mixture. During this process the gas bubble propagates inside the center of the mixture while due to the fountain Figure 1. Schematic description of the Gas-assisted flow at the flow front the PU skin material is disReaction Injection Molding (GRIM) process. placed from the inside to the boundary area of the cavity. When the cavity is filled volumetric a gas holding pressure is applied until an appropriate degree of curing is reached. After this holding pressure time, th, the gas pressure is released and the part can be demolded. gas injection short shot with reactive PU mixture volumetric filling

gas holding pressure curing

pressure release demolding

RHEOKINETIC MATERIAL BEHAVIOR One of the most important influencing parameters of a stable gas bubble propagation is the viscosity. For thermoplastics the change of viscosity during the molding process is mainly influenced by cooling effects of a hot melt in a cooled mold. This solidification behavior leads to a lower viscosity and flow resistance in the middle area of the cavity so that a centered gas bubble propagation is preferred as desired. In the opposite the molding of reactive PU systems is influenced by an exothermic chemical coupled with physical processes. Thus the rheological behavior of PU systems is depending on temperature but also on the degree of conversion. This so-called rheokinetic material behavior leads to rather complex viscosity profiles inside the mold depending on time, mold temperature and the reaction kinetics of the PU system. Moreover PU systems are generally characterized by a significant lower viscosity level compared to thermoplastic melts. Since it is obvious that the realization of a stable gas bubble propagation is easier in higher viscous liquids the GRIM process can be expected to be more difficult to control. This underlines the significance of the pre-reaction phase. One consequence of the lower viscosity is that a short shot and gas injection against the gravity is preferred as shown in Figure 1.

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81

These pre-considerations indicate the importance of analyzing the rheokinetic material behavior especially during the pre-reaction phase to realize a stable gas bubble propagation. Since the rheological properties inside the mold are inaccessible for measurements a mathematical model has to be developed. This implies a mathematical description of the temperature field which is influenced by heat conduction and heat generation effects due to the exothermic reaction. It can be modeled by the equation4 · ρ ⋅ c p ∂ϑ ------- = ∇λ∇ϑ + Φ''' ∂t

[1]

–EA · n Φ''' = ∆ϑ adb ρc p k 0 exp --------- ( 1 – r ) RT

[2]

where λ , cp, and ρ are the thermal conductivity, heat capacity, and density, respectively. It · can be shown that assuming a constant density and heat capacity the heat source Φ''' for the PU specific isocyanate/polyol reaction close to stoichiometry can be approximated by5

with

r k0 EA R n T

degree of conversion, absolute rate constant of the reaction, activation energy, gas constant, order of reaction, absolute temperature, ∆ϑ adb maximal temperature difference ϑ max - ϑ 0 under adiabatic conditions. Taking into account equation 2 and assuming the thermal conductivity to be constant, equation 1 can be written as – EA n ∂ϑ ------- = ∆ϑ adb k 0 exp --------- ( 1 – r ) RT ∂t

λ 2 ∇ ϑ + -------ρc p

[3]

The parameters k0, EA, n and ∆ϑ adb have been obtained by evaluating the exothermic temperature rise of the PU system under adiabatic conditions.6 The thermal material characteristics λ , cp, and ρ have been determined by means of a pvT- and DSC-analysis of the cured material. Equation [3] as well as the determined material characteristics have been implemented into the Finite Element program ABAQUS. A comparison of the calculated temperature development inside a circular mold (D = 20 mm) with appropriate temperature measurements has shown a good agreement. Moreover several FTIR measurements under isothermal conditions at different temperatures have been performed to determine the degree of conversion versus time. Also in this case a good agreement with the calculated results could be observed. To model the change of viscosity η as a function of temperature ϑ and degree of conversion r the following empirical equation7 has been used

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η ( ϑ, r ) = η 0 ( ϑ ) exp ( Br )

[4] where η 0 is the temperature dependent initial viscosity of the mixture. The functionality η 0 ( ϑ ) has been determined by measuring the viscosity of the initial reactants at various temperatures and by applying a mixing rule. In addition numerous measurements concerning the time depending isothermal viscosity development of the PU mixture at different temperatures have been performed. Analyzing these data it could be shown that the coefficient B of equation [4] is a rather complex function of degree of conversion and temperature. The FEA code for the temperature field has been extended by equation [4] as well as by the resulting dependencies of the parameters η 0 and B. To verify this algorithm the results of calculation for the viscosity developD = 8 mm D = 20 mm ment under isothermal conditions of the reacting PU system at different temperatures have been compared with the appropriate measurements. In all cases a good approximation of the measurements could be registered. To sum up it can be said that the developed model and the determined material characteristics enable a good approximation of the viscosity -4 -2 0 2 4 -10 -5 0 5 10 field during the pre-reaction phase. Figure 2 radius r (mm) radius r (mm) shows some calculations of viscosity profiles Figure 2. Calculated viscosity profiles at distinct reaction over the radius of a tubular molding at distinct times and different molding diameters ( ϑ m = 60°C). reaction times for two different molding cross sections (D = 8 and 20 mm) and a mold temperature of 60°C. In the following these viscosity data will be exemplarily used to interpret the observations of the molding experiments.

MOLDING INVESTIGATIONS PROCEDURE, SET-UP AND QUALITY CRITERIA The delay time as one of the most important influencing parameters of the GRIM process has already been mentioned. Furthermore the mold temperature, ϑ W, has a significant influence on the reaction kinetics, the temperature development and hence on the viscosity rise. Both of these process parameters have been varied in a broad range and their influence on the part quality has been analyzed. Furthermore the influence of the gas injection pressure, pGas, or the velocity of gas bubble propagation, respectively, has been investigated. The molding experiments have been performed using a modular mold which enables to vary the geometry of the part from D = 5 to 30 mm (circular cross section) and from B = 40

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83

to 60 mm (rectangular cross section) using different mold inserts. The length of the parts was kept constant with L = 500 mm. The PU system used was a slow reacting model formulation. It shows a marked thermoplastic phase during its curing reaction, is characterized by a low degree of cross-linking and belongs to the group of segmented flexible polyurethanes. Dosing and mixing of the initial reactants were performed manually and the injection of the mixture into the cavity using a pressure pot or a plunger injection unit. As an appropriate degree of filling a value of FG=65% has been identified in pre-investigations and has been assessed for any further investigation. As the main quality criterion the thickness of the PU skin of the final hollow article, designated as residual wall thickness r, at different cross sections over the entire length (or height) of the gas bubble has been determined. Furthermore the specific residual wall thickness, rspec, which is defined as the ratio of residual wall thickness and radius of the part R=D/2, as well as the means r’ and r ’spec have been used to characterize the part quality.

RESULTS AND DISCUSSION The investigations concerning the variation of the gas injection pressure (1 - 10*105 Pa) have shown that in the entire pressure range a stable gas bubble propagation is possible. However, at a pressure level of 1 - 3*105 Pa a comparably large deviation from the mean residual wall thickness could be observed while for pGas > 3*105 Pa the parts show a homogenous residual wall thickness over the entire length of the gas bubble with no significant influence of the presFigure 3. Influence of delay time on the (specific) resid- sure on the mean residual wall thickness. ual wall thickness for different molding diameters ( ϑ m = Against this background for all further investiga60°C). tion a gas pressure of 5*105 Pa has been applied. Figure 3 (upper diagrams) shows the (specific) residual wall thickness as a function of molding diameter D for different delay times as well as the appropriate maximum and minimum values ( ϑ W = 60°C). It can be seen that at a long delay time of td = 360 s and for larger part diameters (D ≥ 15 mm) the resulting deviations within the residual wall thickness are small. In contrast to this for D ≤ 15 mm a gas injection even at higher pressure levels was impossible. This can be attributed to a faster increase of viscosity over the entire cross section (see Figure 2) due to an increasing influence of the mold temperature on the rate of curing with decreasing molding diameters.

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Moreover it could be observed that for D = 8 mm a gas delay time of 300 s already leads to a very slow gas bubble propagation and to a significant decrease of the residual wall thickness from the beginning to the upper end of the gas bubble. In consideration of the calculated viscosity data shown in Figure 2 it can be said that in this case the maximum viscosity level (800 Pa s near the mold wall and 1400 Pa s in the middle of the part) is reached. At a medium delay time of 270 s a good gas bubble structure with a low deviation from the mean residual wall thickness can be observed for all investigated cross sections. A similar observation has been made for short delay times of 180 s and molding diameters ≤ 15 mm (Figure 3, lower diagrams). Furthermore Figure 3 demonstrates that for short delay times and molding diameters of D ≤ 10 mm a marked good part quality could be realized. For molding diameters larger than 15 mm and short delay times an significant increase of deviation of the residual wall thickness with increasing molding diameters as well as secondary gas bubbles in the bottom area of the part were observed. Moreover it can be said that in this case the residual wall thickness decreases from the bottom of the main gas bubble to the top end. These phenomena can be attributed to an increasing influence of gravity. It can be expected that during and after the gas injection process the relatively low viscose PU mixture flows back to the bottom of the part and in front of the gas injection gate. As a consequence for voluminous moldings a longer delay time and hence higher viscosity level is necessary to reach a homogenous gas bubble structure as can be observed for td = 270 s 360 s (Figure 3). As an example, for D = 20 the minimal delay time was found to be 270 s. This corresponds with a viscosity between 15 Pa s in the middle of the part and 170 Pa s at the mold walls (Figure 2). The phenomenon of back flow of the skin material has also been observed for the molding experiments with the larger rectangular cross section (B = 40 mm) as illustrated in Figure 4. On the one hand at a short delay time of 225 s this back flow leads to the generation of the mentioned secondary gas bubbles during the gas injection process. On the other hand in the first stage of the holding pressure period the back flow due to the still low viscosity level leads to an accumulation of skin material at the bottom of the part. However, an increase of delay time of 45 s is already sufficient to prevent the back flow effect. Moreover, Figure 4 illustrates that with an increasing delay time the Figure 4. Influence of delay time on the residual wall thickness (B = 40 mm, ϑ m = 60°C).

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mean residual wall thickness as well as the deviation decrease. This could also been observed for the other molding diameters. To sum up it can be said that for long delay times the processing window is limited by an upper viscosity level and the resulting flow resistance. For the investigated PU system the mold temperature is dominating the reaction kinetics and the change of viscosity (see Figure 2). Thus for smaller cross sections the upper limit of the processing window concerning the delay time is reached at shorter delay times. In contrast to this the lower limit is effected by an increasing influence of gravity with a decreasing viscosity level. Figure 5 shows the mean residual wall thickness as a function of molding diameter for different mold temperatures (20°C and 60°C) and a medium delay time of 270 s. As mentioned above for ϑ m = 60°C the gas bubble shows a good quality with a low deviation of the residual wall thickness for each of the investigated cross sections. In contrast to this at a marked low mold temperature of 20 °C just the moldings with a diameter of D ≤ 12 mm are showing a comparably good gas bubble quality. For larger molding Figure 5. Residual wall thickness vs. molding diameter diameters a more significant deviation of residat different mold temperatures (td = 270 s). ual wall thickness can be observed, especially for the largest cross section of D = 30 mm. In this case an uneven gas bubble structure in the bottom area of the part can be seen. This phenomenon can also be attributed to the above described back flow effect due to a slower reaction and lower viscosity level under these mold temperature conditions. For the larger rectangular cross sections a further interesting effect could be observed. At low mold temperatures the moldings show a local decrease of residual wall thickness in the Figure 6. Influence of mold temperature on the residual height of the mixture level of the short shot (Figwall thickness for different molding geometry (td = 270 ure 6, top). This decrease of residual wall thicks). ness decreases with increasing mold temperatures and is absent for mold temperatures higher than 60°C. An explanation for this effect is that due to the thermal conditions (low mold and air temperature on top of the mix-

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ture level, exothermic heat generation in the middle area of the molding) the material close to the mixture level shows a slower viscosity rise due to a slower reaction compared to the material in the middle area of the residual part. At the beginning of the gas injection process this low viscose mixture is displaced to the mold walls. During the gas bubble propagation this material is continuously cleared away due to the shear flow resulting in the observed gas bubble extension when it passes this area. In the opposite at higher mold temperatures a more homogenous mixture can be expected since the mold temperature was found to be in the range of the temperature level in the middle area of the residual part. Hence in this case the extension phenomenon is absent. A further result of the process investigations concerning the influence of mold temperature is that below a molding diameter of 15 mm the mean residual wall thickness seems to be independent from the mold temperature while for D ≥ 15 mm an increase of the residual wall thickness with the mold temperature could be observed (Figure 6, bottom). This behavior can be attributed to the faster reaction and viscosity growth close to the mold wall while for smaller cross sections the mold temperature is influencing the development of viscosity over the entire cross section resulting in more homogenous properties of the PU mixture.

CONCLUSIONS It could be proved that for various cross sections the realization of a gas injection process for reactive PU systems is possible. Depending on the processing parameters, in particular the gas delay time and the mold temperature, a very good part quality can be obtained. Moreover it has been shown that the developed mathematical description of the temperature and viscosity field is capable to explain the observed processing behavior. However, at the moment these issues are restricted to simple tubular molding geometries and to slow reacting PU systems with a marked thermoplastic phase during the solidification reaction. In further investigations it has to be proved in what respect the GRIM process can be realized for more complex, praxis relevant parts as well as with faster reacting and with non-segmented, highly crosslinking PU systems. An additional interesting task for future investigations is the use of foaming PU systems.

ACKNOWLEDGMENT The investigations set out in this report received financial support from the Bayer AG, Leverkusen (Germany), to whom we extend our special thanks.

REFERENCES 1 2

William, N., Gas-assist continues to make advances, Modern Plastics Int., (1999) 5, p. 34-36. Michaeli, W. and Brunswick, A., Herstellung medienführender Leitungen mit GIT”, Kunststoffe 88 (1998) 1, p. 34-39.

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3 4 5 6 7

87

Michaeli, W., Brunswick, A. and Henze E., LSR-Bauteile mit funktionellen Hohlräumen, Kunststoffe 88 (1998) 9, p. 1404. N.N., VDI-Wärmeatlas, VDI-Verlag, Düsseldorf, 1984. Fleischer, D., Sandwichstrukturen aus randschicht-verstärktem Polyurethan-Hartschaum, PHD thesis at the RWTH Aachen, 1997 Macosko, C. W., RIM – Fundamentals of Reaction Injection Molding, Hanser Publisher, Munich, Vienna, New York, 1989 Malkin, A.Y., Michaeli, W. et al., Rheologie reaktiver Systeme: Ermittlung von Berechnungsgrundlagen für die Herstellung verstärkter Kunststoffe, final report of the research project (Volkswagen-Stiftung) No. I/71477, Aachen, 1998.

Chapter 2: Thin Wall Molding Thin Wall Processing of Engineering Resins: Issues and Answers Larry Cosma Principal Processing Engineer, GE Plastics

FILL TIME AND FLOW LENGTH: CRUCIAL CONCERNS The most important factor in successful thin wall molding, around which all other processing issues revolve, is cavity fill time. As a wall section decreases, it becomes more difficult to get the material to flow the distances required for success in thin wall molding. The flow length of a resin is the maximum distance a material can flow before the melt front stops moving. One way to understand this is as a ratio that compares the length a material must travel into the mold with the thickness of the part's wall section. If the total length of flow for a part is 250 mm (10 in) and the wall section is 2.5 mm (0.100 in), the Length to Thickness Ratio (L:T) is 100:1. At conventional wall thicknesses such as 3.0 mm (0.118 in), most molders can easily achieve this ratio, even when using PC and PC/ABS resins. As wall sections decrease to 2.0 mm and under, the "skins" formed also will be reduced, but proportionately not nearly as much as the overall wall. The ratio of frozen-skin layer to molten-core layer increases. When this happens, there is relatively less material in the molten core or cross-section of the part to finish flowing and pack out the part. Therefore, the once easy-to-hit 100:1 ratio becomes more difficult to reach during processing. And when wall sections drop to 1.0 mm (0.040 in), it becomes difficult to achieve an L:T ratio of even 70:1. For greater success at thinner walls in a given application, it's best to use conservative L:T values for material flow. The most common techniques for reducing the L:T ratio include use of multiple gates or placing a gate near the center of the part. Designing with a low L:T ratio also will help reduce molded-in stress levels, increase the ability to pack out sinks at the end flow, and help ensure a more even shrinkage rate throughout the part, resulting in less warpage.

INCREASED INJECTION PRESSURES Molding a part with thinner walls requires shorter fill time because walls freeze and close off the channel of molten material faster. Since the amount of material in the wall section is less, heat dissipates more quickly, causing the material to "freeze off." Therefore, for the

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injected material to properly fill the mold, less time is available than for parts with thicker Pressure walls. Injection pressures of 16,000 to 20,000 psi Time are typical for applications such as notebook computer housings, that is, parts with 1.2 mm to 2.0 mm walls (0.050 in to 0.080 in). For walls below 1.2 mm (0.050 in), pressures up to 35,000 psi may be required. Typically, to reduce fill time, injection pressure must be increased. For WALL THICKNESS example, consider a part with a 3 mm (0.118 in) wall section and a maximum flow length of 225 Figure 1. Injection pressure and fill time vs. wall thickmm (8.9 in). The part's L:T ratio is 75:1. ness. Depending on the material used and the melt temperatures employed, this part could be filled in a leisurely three seconds and packed afterward. In contrast, a similar part with a 1.0 mm (0.040 in) wall section would be limited to a maximum flow length of 75 mm (2.95 in) to maintain the L:T ratio of 75:1. Even with the much shorter flow length, this becomes difficult to do: the fill time in this situation would have to take place within 0.5 seconds. As wall sections drop below 0:5 mm (0.020 in), the fill time may drop below 0.1 seconds (Figure 1). In short, all other recommendations, techniques, and guidelines stem from the need for complete and accurate cavity fill in very little time. 1mm

2mm

3mm

EQUIPMENT PROBLEMS AT THIN WALL CONDITIONS When fill times drop below 1.0 second, several issues arise regarding molding equipment. These include: • Moving the screw forward at sufficient speed • Having sufficient pressure to move the screw forward at the required speed • Controlling the hydraulics to stop the screw's forward momentum when needed • Controlling the material pressure in the cavity to prevent overpacking • Platen flexure Another machine-related problem too often overlooked is material residence at higher temperatures. As a general recommendation for its own materials, GE Plastics suggests using 40 percent to 70 percent of the barrel shot capacity on every cycle. GE developed this guideline by calculating the total time the material spends in the barrel while it is being processed.

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Since polymer bonds break down with exposure to too much heat for too long a time, it is important that the material not remain at elevated temperatures for too long. Because thin wall applications require less material to form parts with the same relative projected area, most machines now have barrel shot sizes that are too large for the thin walled parts being molded in them. Downsizing barrels for thin wall applications may be required under some circumstances. As an example, one leading thin wall molder has 180 t (200 T) molding machines fitted with a shot capacity as small as 142 g (5 oz). These equipment issues can be resolved, but possibly not on existing equipment. Customized machinery for thin wall molding is now being offered by several leading machinery suppliers. In general, standard microprocessor-controlled machines with closed-loop functions are suitable for thin wall applications such as notebook computer housings, which are in the 1.2 mm to 2.0 mm (0.050 in and 0.080 in) range. These microprocessor-controlled machines usually give fast and accurate response to operator commands. Presses to mold parts with walls below 1.2 mm (0.050 in) are more specialized than those typically found in custom-molding facilities.

TOOLING PROBLEMS AT THIN WALL CONDITIONS The fast injection speeds and higher pressures of thin wall molding also create tooling issues. These problems include venting, tool erosion, core-cavity alignment, plate flexure, and part release. A closer look at each of these problems, and their remedies, will be instructive. VENTING Venting is an issue in thin wall molding. Because of the high injection speeds. there is less time for gases to escape from the cavity. A pressure buildup of hot gases in the cavity causes difficulty in mold filling. This buildup of heated gas can be so great as to cause actual burning or carbonization of the plastic melt. In extreme cases, the buildup of heat, pressure, and volatiles can even etch the steel of the mold. The problem is solved by adding vents from the mold cavity, especially where flow fronts converge and trap gases. Vents need not be any deeper or wider than in conventional tooling. There simply needs to be more of them. It also may be useful to vent core pins, ribs, bosses, and ejector pins. Another solution is vacuum venting. Instead of venting into the atmosphere, vents enter a tightly sealed system with a vacuum pump attached to it. Each time the mold closes, atmospheric air is vacuumed out of the cavity before injection starts.

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TOOL EROSION Material flowing into a mold can gradually wear away the tool steel. Most erosion generally takes place near the gates that fill the part. Erosion takes place in the molding of conventional parts, and is exacerbated in thinner walls because of the higher fill rates. Filled grades can accelerate erosion. Erosion can be minimized through the use of hardened tool steels such as A-2, D-2, and M-2. CORE-CAVITY ALIGNMENT In thin wall processing, the high pressures tend to push the core and cavity in different directions. This can result in parts with changes in wall thickness at different locations. Crucial tolerances, as well as the ability to fill the cavity, can be compromised. Core and cavity can be aligned through the use of mold interlocks. Typically, two or four interlocks are used in the parting line surfaces of the mold. PLATE FLEXURE Mold plates must be thick enough and bolted together tightly enough so they don't move. But at high pressures, movement can occur unless this is accounted for in tool design. Using extra heavy steel plates, support pillars, dowel pins, and bolts adds the necessary stability. PART RELEASE Thinwall Ejector Layout

Figure 2. Part ejection.

Conventional Ejector Layout

The tighter the cavity is packed, the more the plastic tends to adhere to features in the mold. If the mold is overpacked, the material can grip surface features. Also, some materials adhere to the tool steel more than others. Using more and larger ejector pins and sleeves is necessary (Figure 2). Commercially available mold coatings added to the surface of the tool steel will also help in releasing parts.

AESTHETICS

Portable telecommunications devices such as notebook computers and cellular phones have become popular consumer items. They must be attractive, particularly at the point of purchase. Gate vestiges are often undesirable. and direct gating often will leave gate blush and other flow marks. Valve gating is an excellent way to improve surface appearance. Careful

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93

site selection of the valve gate may allow for direct gating to the appearance side of a part in an area that subsequently will be covered by a decal or other secondary operation. New valve gates, such as those supplied by Kona Corp., have shown exceptional part aesthetics when placed opposite an appearance surface in 1 mm (0.040 in) walls. Thinner wall sections often will reduce sink marks seen near ribs and bosses, but "ghosting" from wall transitions may be more evident. Painting can hide flaws, but it will increase the cost of the part and has environmental concerns. Often, adding texture is a better and far less expensive way to disguise surface flaws or make them less noticeable. The heavier the texture, the more it will hide cosmetic blemishes. Another decorative option drawing much attention has been in-mold decoration. In-mold decoration has been used for years in the automotive industry, especially in back-lit instrument panel fascias. Now, the process is being explored to decorate portable computers. Through in-mold decoration, notebook and subnotebook computer housings can be customized with a virtually unlimited numbers of colors, prints, designs, and logos. In this process, thin, clear, pre-printed films are inserted into the mold before injection. The film adheres to the plastic, forming a single part with a superior even dramatic, appearance. Films comes in a variety of materials, such as polycarbonate, vinyl, and polyester. They range in thickness from 0.25 mm to 0.75 mm (0.010 in to 0.030 in). Depending on the geometry of the surface to be covered, the film may be a flat sheet which forms to the minor contours of the cavity or is preformed to the contours of the part. Film can be held in place during the molding process in several ways, including vacuuming, registration pins, static electricity, and core geometry. Costs are figured on a per square-foot basis. As an example, a polycarbonate film 0.25 mm (0.010 in) printed and cut to fit the cover of a notebook computer may cost about 50 cents. Total costs would include film loading and adjustments in cycle time. Another issue in thin wall aesthetics can be knit lines. While multiple gating may help improve L:T ratios, they create more knit lines. Again, texture will help hide knit lines. Also, multiple "live feed" processes such as the SCORIMTM process can reduce or completely eliminate knit lines. In this licensed process, material is delivered by two runner systems to opposite sides of the part. Two pistons, agitating in alternating movement, keep the molten core "live" longer and minimize if not eliminate unattractive knit lines. Achieving a good surface appearance when using glass- or mineral-filled materials can be a further challenge to high aesthetics. Fast injection rates, hot mold temperatures, and heavy textures can help hide the fillers in these materials. Some resins, such as LEXAN SP polycarbonate resins with 10 or 20 percent glass filler, flow very well with little or no glass showing on the surface.

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MATERIALS Choosing the right materials, of course, is crucial for any application. The correct material, one that will perform according to all of the design and processing criteria, can mean the difference between success or failure in a part. The proper material can enhance product design and improve manufacturing by creating a broader processing window. On the other hand, the incorrect resin can render an otherwise good design unworkable and difficult to process. Material selection is a process of elimination. Key application requirements are defined and ranked, and these requirements are translated into physical properties. The properties are then compared to the resin families offered by materials suppliers. In many cases, the actual molding of parts will help finalize material selection. Every application is different and each presents unique design and processing challenges. But from surveys of product designers, a list of general materials requirements, in order of importance, can be presented: FLOW LENGTH The most critical property of a thin wall part is the flow length. The material must be able to fill the mold. Often, resin suppliers compare relative flow lengths of resins by using spiral flow test numbers. However, designing parts and molds using spiral flow data can be misleading and dangerous to the success of an application. Spiral flow data represents the maximum flow of a material through a channel. Seldom will an application exhibit pure channel flow. So for design purposes, it's best to be conservative. IMPACT STRENGTH A part bets its durability and toughness from the material. It is important to understand the impact requirements of the assembled part, especially portable devices. The resin's physical performance will generally fall within specifications offered by the supplier, assuming it was properly processed within recommended guidelines. Unfortunately, this can be a big assumption when it comes to thin wall molding. AESTHETICS The part must look good. Since the majority of designers prefer the cost advantages of unpainted parts, additional processing challenges are created.

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STIFFNESS Stiffness of the application becomes a critical, issue as the part gets smaller and the wall thickness is reduced. Stiffness is most often achieved with part design and assembly techniques rather than the flexural modulus of the resin. HEAT RESISTANCE The kinds of portable applications typical of thin wall molding (e.g., notebook computers) require better thermal performance than stationary applications (desktops) because of the different end-use requirements and smaller package size. FLAME RETARDANCE Because FR materials create added headaches for molders, they should be used only when necessary. From a design standpoint, there are few differences between FR and non-FR products. Typically, cellular phones do not require FR materials, while notebook computers require UL 94* V-1. MECHANICAL INTEGRITY As it relates to the final assembly of the part, mechanical integrity is often overlooked. But assembly methods and material selected can greatly impact the weight of the part, as well as its total cost. Materials used for thin wall applications require, processing freedom and superior performance to withstand rigorous molding environments and sometimes abusive enduse conditions.

COMMON MOLDING ERRORS The most common error made in processing thin wall applications relates to temperature. To make mold filling easier, processors will often raise material temperatures above recommendations. Turning up barrel temperature reduces the viscosity of the material, enhancing flow, and since thin wall parts are more difficult to fill, the temptation is always to increase heat. However, higher temperatures can result in loss of physical properties, and the first property to suffer is impact. Degraded material will generally produce brittle parts; ductility values will not be as great as those reported by the materials supplier. The best guideline here is to stay within the material supplier's recommendations for drying time and temperatures, barrel temperatures, and residence time in the barrel and hot runner system. Another common mistake is thinking a part can be filled with more pressure, when what it really needs is a shorter fill time. Fill time is a dynamic process. Added pressure

96

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alone won't guarantee optimum processing. In fact, it can cause problems such as molded-in stress, anisotropic shrinkage, and warpage.

FASTER CYCLE TIMES Getting the heat out of a part in a short amount of time has a notable advantage: cycle times can be reduced. The 3.0 mm part (0.118 in) mentioned earlier may have a cycle time of 40 seconds or greater. But when the walls are reduced to 1.0 mm (0.040 in), the cycle can be expected to be below 20 seconds (Figure 3). Some molders are achieving cycle times under seven seconds. These cycles are so fast that robots must be used to remove parts, since gravity cannot clear them from the mold halves fast enough. Cycles this fast are easy to achieve with high performance molding machinery equipped with accumulators on the clamp and injection Figure 3. Typical cycle time range. units, plus advanced hydraulics and valves. Careful attention must be paid to the cold runner system or sprue for fastest cycle times. The faster times of thin wall molding also influence the heat history of the material within the barrel. In addition, the guideline of a 40 percent to 70 percent shot size vs. capacity may not always apply. Since the cycle time may be one third of that used for conventional molding, a lower percentage of barrel capacity may be acceptable in some applications. Molders should verify this by testing properties of the molded parts they are producing, regardless of the machinery sizes and cycle times.

CONCLUSIONS As consumer demand grows for smaller, lighter computers, cellular phones, and other telecommunications and data storage products, the need for advancements in thin wall technology will grow proportionately. Material suppliers are responding by delivering new resins that will meet the imposing physical demands of the OEM designer. They are producing resins that flow well, offer impact strength, and provide good aesthetics, stiffness, and heat properties. For their part, designers are beginning to better understand the differences in thin wall design, particularly with respect to the issues of impact, stiffness, and manufacturability.

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However, the greatest gain in successful processing of thin wall applications rests with the tool designer and the molder. Increasing flow length can best be influenced by using very fast injection speeds coupled with the special tooling techniques and machinery discussed here. Thin wall technology can't be learned overnight. It requires an investment of time as well as money, but it's an investment that potentially offers an enormous return.

Effects of Processing Conditions and Material Models on the Injection Pressure and Flow Length in Thinwall Parts

A. J. Poslinski GE Corporate Research and Development, Schenectady, New York, 12309, USA

INTRODUCTION Tradeoffs between machine capabilities, production rates, and structural performance usually result in plastic parts that are about 2 to 3.5 mm (80 to 140 mil) thick. However, with the recent trends toward portable communication and computer miniaturization, the conventional design envelope no longer applies to products such as personal pagers, cellular phones, and notebooks. Plastic shells on the order of 1 to 2 mm (40 to 80 mil) have been successfully integrated with electronic components that provide the necessary stiffness.1 Because the tool modifications corresponding to this step change in wall thickness result in faster cooling and greater deformation of the molten plastic during processing, one goal of this work is to compare the skin and core structure of conventional and thinwall parts. The highly oriented skin and the extent of the randomly oriented core strongly influence the potential for anisotropic shrinkage and nonuniform warpage. The main difficulty facing thinwall designers and processors is achieving longer flow lengths without introducing knitlines caused by multiple gates. Inevitably, elevated melt temperatures, faster injection speeds, and higher injection pressures must be used to reach the current 100:1 and 150:1 flow length to wall thickness ratios for cellular phones and notebooks.1 The wrong combination of thinwall process settings may lead to material degradation and an increased number of shows that the rheological and impact performance of these polycarbonate materials does not significantly change within the temperature range recommended by resin manufacturers. However, thinwall parts molded at higher temperatures are more likely to fail in a brittle manner when exposed to sudden impact conditions. Clearly, thinwall molding is best accomplished by maintaining the melt temperature within the recommended limits. The present work expands this concept further by performing a parametric analysis to identify the primary variables affecting the injection pressure and the maximum flow length of thinwall resins and to suggest the optimum combination of process settings for thinwall molding.

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MATERIAL PROPERTIES An accurate prediction of mold filling requires a viscosity model that is valid over the entire range of shear rates and temperatures encountered during processing. An equation that provides a good fit of the viscosity data is the generalized Cross model:3 · α  η o γ η = η o 1 +  ---------  τ∗ 

(n – 1) ⁄ n

[1]

where η o denotes the zero-shear viscosity, and α , n, and τ * are material constants. The temperature and pressure dependence of the viscosity is indirectly captured through η o with a simple exponential or the Williams-Landel-Ferry (WLF) expression η o = E1 exp ( E 2 ⁄ T ) exp ( E 3 P ) [2] –W ( T – Tg – W 3 P ) η o = W 1 exp ---------------------------------------------51.6 + ( T – T g )

[3]

where Ei and Wi are material constants, P is the pressure, T is the temperature, and Tg is the ambient glass transition temperature. Because data on the pressure dependence of the viscosity is not readily available, an average value for the coefficient E3 can be estimated over the temperature range of interest.4 Likewise, the temperature data since it represents the pressure dependence of the glass transition temperature. The thermodynamic properties that influence the heat transfer taking place during mold filling include the specific heat, the thermal conductivity and the density. Constant properties are used if the molten plastic is assumed incompressible. When this approximation is relaxed, the specific heat and the thermal conductivity remain unchanged, but the temperature and pressure dependence of the density is described with the double-domain Tait equation of state:5 V = 1 ⁄ ρ = V o [ 1 – 0.0894 ln ( 1 + P ⁄ B ) ] V o = B 1 + B2 ( T – T g )

[4]

B = B 3 exp [ – B 4 ( T – T g ) ]

The specific volume V is equivalent to the reciprocal of the density ρ , Vo is the specific volume under ambient pressure, and Bi are material constants. The Tait equation is applied separately in the molten and solid states, yielding two sets of coefficients above and below the glass transition temperature of Tg - W3P. The numerical calculations presented in this report are based on the viscosity data of an impact-modified polycarbonate resin (LEXAN® UL6339R grade). Table 1 lists the material constants for the Cross exponential and WLF temperature models. In both cases, the power law index n is set to 0.2, and the shear stress constant τ *, which is related to the onset of shear-thinning behavior, is fixed at 5.17 x 105 Pa. A constant density of 1000 kg/m3 is used

Effect of Processing Conditions

101

to enforce incompressibility; otherwise, the spatial variation of the density is predicted with the Tait equation of state and the corresponding material constants listed in Table 2. A constant specific heat of 2056 I/kg K, a constant thermal conductivity of 0.25 W /m K, and an ambient glass transition temperature of 144°C complete the required set of material properties. Table 1. Material constants Type

1

Exponential

l.l9xlO-7

WLF

5.05xlO12

Table 2. Material constants

in SI units for the Cross model 2 l.26xlO4

30.9

3 l.7xlO-8

l.9xlO-7

in SI units for the Tait equation

1

2

3

4

solid

8.54xlO-4

l.59xlO-7

2.99xlO8

1.71xl0-3

melt

8.54xlO-4

5.62xlO-7

l.83xlO8

3.99xlO-3

Type

Including the pressure dependence of the zero-shear viscosity in the material model requires further consideration. Normally, the pressure coefficient E3 or W3 is set to zero when the exponential function in Equation [2] or the WLF function in Equation [3] is fit to the experimental data. The other two constants El and E2 or W 1 and W 2 are then associated with the ambient pressure, which is set to zero as a reference point. Independently changing E3 or W 3 results in the viscosity increasing above the measured values for pressures larger than zero. Because viscosity measurements are usually pressures, the reference point for the viscosity isobars should be greater than zero. This simply means that the viscosity level, which is controlled by El or W 1 needs to be adjusted when nonzero values of E3 or W 3 are imposed. One way to account for the pressures generated during the viscosity tests is to f1rst calculate the average mean pressure in the capillary die and then modify the viscosity coefficients accordingly. In the case of the polycarbonate resin considered in this study, the average mean pressure based on capillary viscosity data6 at a temperature of 305°C (580°F) is approximately 17 MPa (2.5 ksi) for shear rates ranging from 1000 to 2000 s-I. Using this value as the reference point, the exponential and WLF material constants, El and Wl in Table 1, are reduced to 8.88 x 10-8 Pa s and 3.135 x 1012 Pa s, respectively. The corrected values shift the viscosity isobars downward, so

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WKDW WKH\ FURVV RYHU DW WKH UHIHUHQFH SUHVVXUH DQG WKH YLVFRVLW\ OHYHO PHDVXUHG ZLWK WKH FDSLOODU\ UKHRPHWHU 1/2 in

2.5 in

NUMERICAL CONSIDERATIONS

2 in

Injection molding simulations are performed with a process model that simulates the flow of molten plastic into 6 in 5/16 in any arbitrary molding geometry.7 The part geometry is shown in Figure 1. Specifically, the main components include a machine nozzle, a sprue, and a quarter disk. The quarter disk represents the radial flow in actual mold filling; its size and thickness matches the typical 0.04 in aspect ratio of thinwall parts and the mold geometry used in a related experimental study.8 The additional pressure drop in the sprue is modeled with a cold runner, Figure 1. Schematic drawing of the part geom- and the additional shear heating at higher flow rates in etry. the nozzle is modeled with a hot runner. The hot material forming the shot in front of the nozzle and the dynamics of the injection unit are replaced with constant melt temperature and variable screw velocity conditions at the nozzle entrance. 1/4 in

SKIN AND CORE STRUCTURE Figure 2 shows a surface and contour plot of the cross-sectional temperature distribution predicted for an 82 cm3/s (5 in3/s) flow rate and a wall thickness of 1 mm (40 mils) at the end of filling. The radial and axial coordinates correspond to the flow and thickness directions, respectively. In particular, the left and right edges represent the gate entrance and the end of flow; whereas, the upper and lower edges represent the mold wall and the midplane of the quarter disk. The surface topology shows that the temperatures at the gate entrance and near the midplane are almost identical due to convection of heat in the flow direction, and a steep temperature gradient develops near the mold wall as a caused by high shear rates raises the temperature by as much as 30°C (80°F) near the upper left corner of the domain. The contours reveal that a band of higher temperatures extends from this region along the diagonal toward the lower right corner. The temperature spike increases at higher flow rates, disappears completely when the molten plastic is injected at approximately 16 cm3/s (1 in3/ s), and is not affected by wall thickness. The temperature distributions can be used to identify the solidified material in contact with the mold wall. The solid thickness is practically nonexistent near the gate entrance; it is also reduced near the melt front as a result the fountain flow approximations. Because the

Effect of Processing Conditions

103

Temperature (F)

temperature distribution shown in Figure 2 is not strongly affected by the wall thickness, the 500 400 numerical calculations confirm that the solid 300 layer is similar for both conventional and thin200 wall parts, constituting about 2% of the total 100 thickness. At the lower flow rates of 16 to 82 20 16 li ) cm3/s (1 to 5 in3/s), the solid layer is on the order 0 (m 8 ce n 2 of 10%. sta Radial D 4 Di 0 istance ial 6 (In) Another component that makes up the skin Ax region is the shear zone next to the solid layer. Figure 2. Cross-sectional temperature distribution for an 82 cm3/s (5 in3/s) flow rate and a wall thickness of 1 mm The size of the shear zone is similar for both (40 mils) at the end of filling. conventional and thinwall parts; however, considerably higher shear rates are observed in the latter case. The shear zone makes up approximately 30% of the total thickness, so that the total skin region is on the order of 40%. 360

270

210

350

310

INJECTION PRESSURE PREDICTIONS If mold filling is not constrained by the limitations of the injection unit, the pressure at the sprue entrance increases in an approximately linear manner as the melt front advances further into the part. The final value required to completely fill the entire mold cavity is defined as the injection pressure. The results indicate that the lower viscosities at higher melt temperatures and faster injection speeds reduce the pressure requirements. Whereas the minimum pressures for conventional molding are obtained around 164 cm3/s (10 in3/s), the injection pressures for thinwall molding continue to decrease at flow rates even as high as 492 cm3/s (30 in3/s). Evidently, the additional heat dissipation at the higher shear rates in thinwall parts compensates for the higher pressures required to achieve faster injection speeds. The effect of various material models on the injection pressure at 82 cm3/s (5 in3/s) and 305°C (580°F) demonstrates that slightly higher predictions are obtained when the spatial variation of the density is taken into account because some of the pressure is used to compress the material. However, including the effect of compression work raises the melt temperature slightly, so that the injection pressures are reduced back to the levels predicted with the incompressible material model. Significantly higher pressures are predicted when the pressure dependence of the viscosity is included; although, the correction for the pressures generated during capillary measurements results in somewhat lower values. Furthermore, the difference between the exponential and WLF temperature models becomes more appar-

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ent; the WLF function predicts a steeper viscosity rise with pressure, and thus higher injection pressures.

FLOW LENGTH PREDICTIONS If mold filling is constrained by the limitations of the injection unit, the flowrate decreases after the injection pressure reaches the maximum allowable value. When the molten plastic stops flowing and does not reach the end of the mold cavity, the flow length is defined as the farthest distance from the gate attained by a short shot. The numerical calculations indicate that longer flow lengths are obtained at higher melt temperatures. The flow length is not strongly affected by the flow rate, except at the higher pressure limits, when the longer flow path provides more time for the melt to heat up at the higher flow rates. An examination of the cross-sectional temperatures reveals that the short shot is not caused by freeze off; rather, the molding machine does not provide enough force to push the material further. The flow length trends obtained with various material models are similar to the injection pressure trends. The pressure dependence of the viscosity has the greatest effect, resulting in shorter flow lengths.

SUMMARY A parametric analysis has been performed to investigate the process mechanics of thinwall molding.: Numerical calculations confirm that the skin and coy structure in thinwall parts is similar to the structure i conventional parts; however, considerably higher shear rates are observed in the shear zone of the skin region The injection pressure is reduced with higher temperatures and faster injection speeds, and the resin flow length is increased with higher melt temperatures and larger machine capabilities. The spatial variations of the density and the additional compression heat resulting from higher thinwall pressures do not significantly affect these two variables. However, experimental evidence is needed to validate the higher injection pressures and shorter flow lengths caused by the viscosity pressure dependence. The present analysis suggests that thinwall molding is best accomplished within the temperature range recommended by resin manufacturers and on molding machines that provide faster injection speeds and higher pressure limits.

ACKNOWLEDGEMENTS This work was supported by GE Plastics. The authors also wish to thank Jack Berkery for performing the numerical simulations. Also, helpful modeling suggestions by Toni Gennari are duly appreciated.

Effect of Processing Conditions

105

REFERENCES 1 2 3 4 5 6 7 8

Thinwall Technical Guide, 2nd ed., Wireless Network Telecommunications Group, GE Plastics, Pittsfield, Massachusetts (1995). A. J. Poslinski, L. O'Connell, P. R. Oehler, SPE Annual Technical Papers 42, submitted (1996). C. A. Hieber and H. H. Chiang, Rheologica Acta 28, 321-332 (1989). C. A. Hieber, in Injection and Compression Molding Fundamentals, A. 1. Isayev, ed., 1-136, Marcel Dekker, Inc., New York (1987). P. Zoller, in Polymer Handbook, J. Brandrup and E. H. Immergut, eds., John Wiley and Sons, New York (1989). Engineering Design Database and Design Guide, Commercial Technology Division, GE Plastics, Pittsfield, Massachusetts (1989). C -MOLD CAE Software, AC Technology, Ithaca, New York (1995). A. J. Poslinski and G. Tremblay, SPE Annual Technical Papers 42, submitted (1996).

10 Common Pitfalls in Thin-Wall Plastic Part Design

Timothy A. Palmer Bayer Corporation, 100 Bayer Road, Pittsburgh, PA 15205, USA

DEFINITION OF THIN-WALL For the purposes of this paper, a thin-wall part is defined as one injection molded in an engineering thermoplastic resin (e.g. PC, PC/ABS, PA6), having projected area greater than 8 square inches and nominal wall thickness less than 0.060" (1.5 mm). Today, many thin-wall applications push beyond this defined limit and use nominal wall thicknesses less than 0.040" (1.0 mm).

PITFALL #1: DESIGNING WITH TOO MUCH VARIATION FROM THE NOMINAL WALL THICKNESS After the molten resin is injected into the mold cavity, different areas of the plastic part experience different levels of volumetric shrinkage proportional to wall thickness. In conventional moldings packing pressure is applied to force more molten material into the thicker areas, minimizing the effects of differential shrinkage. Unlike conventional parts, molten resin in thin-wall parts solidifies only a few seconds after the end of fill, giving packing pressure little time to act. The thinnest walls solidify before significant volumetric shrinkage can occur. Thicker areas take longer to freeze, experiencing very high volumetric shrinkage. In the worst case, material around the gate can solidify before any area of the part can be adequately packed-out. The notion that molten plastic follows the path of least resistance is especially true in thin-wall housings. Often, advancing flow will simply not fill the thinnest areas of a part, creating either non-fill or gas entrapment. Because of these difficulties, thin-wall parts should be designed with uniform wall thickness as much as possible. This allows molded parts with low differential volumetric shrinkage, improved dimensional quality and reduced chance of cosmetic problems caused by non-fill or gas entrapment. However, the decision to use nominal wall design must be made early in the design cycle due to the restrictions it may impose. Often, additional wall thickness must be added to the inside of a housing opposite areas such as label recesses to

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maintain the nominal wall thickness. Note that as with conventional parts, sharp edges in the flow path and at internal corners should be avoided.

PITFALL #2: USING IMPROPER RIB TO WALL THICKNESS RATIO The thick section formed by the intersection of a rib and the nominal wall tends to experience greater volumetric shrinkage than the rest of the part, causing sink opposite the rib. In conventional housings, rib base thickness is based on a percentage of the attached nominal wall, varying from 50 to 66% depending on the degree of cosmetic perfection desired. This design practice acts to reduce the thick section and make it easier to pack-out, largely eliminating visible sink. When standard rib design rules are applied to thin-wall parts, the resulting rib designs are usually too thin to fill properly, especially after draft is added. If the ribs can be filled, freeze-off usually occurs well before the rest of the part, with shrinkage much different than in the attaching nominal wall. To allow the ribs to fill properly, a 1:1 rib to wall thickness ratio can be used in walls less than about 0.050" thick. Any resulting sink marks tend to be much less noticeable than with conventional parts, especially if the opposing surface is textured. In a thin-wall part, there is much less material at the rib/wall intersection to shrink and cause sink than in conventional molded parts.

PITFALL #3: CONSIDERING ONLY EASY-FLOW RESINS FOR THIN-WALL APPLICATIONS Thermoplastic resins are often available in a range of molecular weights. Grades with lower molecular weight typically have lower melt viscosity and flow farther under the same pressure than their higher molecular weight counterparts. Unfortunately, easier flow usually comes at the expense of physical properties such as yield strength and impact strength. In addition, a material's resistance to UV light and chemical attack are reduced with decreasing molecular weight. Because thin-wall applications can be difficult to fill, the expected flow properties of low molecular weight resins seem desirable. Figure 1 shows the difference in predicted filling pressure between high and low molecular weight grades of polycarbonate for a sample housing. Mold-filling analysis results for the 0.040" (1.0 mm) nominal wall show that regardless of molecular weight, high-performance injection molding equipment is probably required. In this case, using a lower molecular weight resin may sacrifice material properties without significantly reducing production costs.

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109

Filling Pressure vs. Wall Thickness Injection Rate - 15 cu. in./sec. Injection Pressure (psi)

35000

9.0 "

12.0"

30000

25000

20000

15000

0.750 "

10000

5000

0

Housing Nominal Wall Thickness (in.) Resin Melt Flow Index (g/10min) 35

20

12

Centrally Gated Housing, Single Hot Drop Figure 1.

PITFALL #4: RELYING ON FIBER-REINFORCED RESINS TO PROVIDE RIGIDITY The structural rigidity of a thin-wall housing is greatly reduced versus its thick-wall counterpart due to the reduction in section modulus. From the standard engineering beam bending formula (w/both ends simply supported), the maximum deflection is inversely proportional to the thickness cubed, so under identical loads, a beam 0.040" thick has deflection 8 times a wall 0.080" thick. A potential solution for thin-wall housings is to use a fiber-filled resin, which typically increases the material modulus by about 50% (10% glass fiber-filled). However, maximum deflection is only inversely proportional to the material modulus, so the unfilled beam only deflects 1.5 times more than the fiber-filled one. Because the wall thickness effect dominates over the effect of fiber reinforcement, the rigidity of thin-wall housings cannot be expected to compare to thick-wall, conventional housings. Rigidity of thin-wall applications will still depend on assembly with the product's other internal components, regardless of the resin used. Impact properties are also important for thin-wall housings given their widespread use in hand-held products prone to being dropped. Fortunately, thinner walls may perform slightly better in a drop impact because more flexible walls have better energy absorption. However, the addition of fillers can sharply reduce these properties. For example, the notched izod impact strength of 0.125" thick polycarbonate is reduced from 17 ft lb/in to 2 ft lb/in when 10% glass is added. These examples suggest that the liabilities of fiber-filled materials may outweigh their benefits in most thin-wall parts.

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Special Molding Techniques

PITFALL #5: IMPROPERLY LOCATING GATES Thin-wall applications push thermoplastic resins and standard injection molding equipment to their respective limits, but properly locating gates is often overlooked as a way to widen the available processing window. Unfortunately, gate locations are often chosen after part designs are finalized, leaving only a few locations where gate vestige is allowed. A better approach is to pick gate locations early in the design cycle to optimize filling, and then position label areas or other styling to conceal any remnant of vestige. In conventional as well as thin-wall parts, filling pressure is minimized when all of the last areas to fill do so simultaneously. This phenomenon is known as balanced filling and promotes uniform solidification and packing of the part. When wall thickness is uniform in a thin-wall part, gate locations should be chosen so that the longest flow paths from all gates are equal in length. However, if a thin-wall part has non-uniform wall thickness, truly balanced filling is difficult to achieve. In fact, some degree of filling imbalance may actually improve the moldability of a non-uniform wall part. Mold-filling analysis is required to optimize such cases. When analyzing a thin-wall part, the mold-filling analyst should always consider the part and the delivery system (e.g., three-plate runner, hot manifold), because pressure consumed in these components can have a much greater effect on flow balance in thin-wall parts than in conventional designs.

PITFALL #6: USING SLOW INJECTION RATES While high injection pressures are required to fill thin-walled parts, delivering the molten resin at a sufficient injection rate is also an important parameter. To prevent early freeze-off, the molding machine must inject material at a rate high enough to produce shear heating at the flow-front. Once the flow-front temperature begins to drop, the pressure required to advance it can quickly exceed press capabilities, resulting in non-fill. Today's closed-loop, electronic controls allow nearly any injection rate to be set at the press, but close examination of the actual ram velocity vs. position trace may show that the desired injection rate can only be achieved over a small portion of the injection cycle, if at all. In this case, a "high-performance" injection molding press designed specifically for high injection rates will be required. Such machines have the ability to deliver high pressure at very high injection rates through the use of accumulators or other methods.

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111

PITFALL #7: USING MORE GATES THAN NECESSARY In many thin-wall applications, numerous gates ar e used when fewer would be suitable because R Input the material is not expected to flow more than a Flow Rate, Q R/2 few inches beyond the gate. However, as menQ tioned in #6, significant flow in thin walls is posQ/4 sible when flow front velocity is high enough. t v v The rapid freeze-off expected in thin walls typically occurs because the flow front velocity is R R V=Q/2πRt too low to genera te shear heat ing. V=Q/4πRt While the ability to maintain high Figure 2. flow-front velocity is largely dependent on the capabilities of the injection molding press, the number of gates used also plays an important role. Assuming radial flow from a pin-point style gate, the flow front velocity is inversely proportional to the distance flowed. If a squar e housing is fed through a centrally located gate (Figure 2), flow front velocity at the end of fill is Q/2 π Rt, where Q is injection rat e, R is the radial distance flowed and t is part thickness. When multiple gates are used to fill the part, flow distance is reduced, but the input flow rate must be divided among the gates. In this example, the four gate system has half the flow front velocity of the single gate system at the end of fill. The par t with a single, center gate has higher flow front velocity at the end of fill, no major knitlines and avoids gas entrapment at the center of the part.

PITFALL #8: UNDERSIZING GATES Because higher injection rat es are used in thin-wall molding, larger gates are required to prevent cosmetic damage caused by excessive gate shear. Externa lly heated hot drops or valve-gated drops allow large gate diameters with clean degating. The following formula can be used to estimate the required pin-point or hot-tip gate orifice diameter. D =

3

32Q ---------nπγ

Here, the diameter D is a function of Q, the volumetric flow rate from the nozzle, n, the number of gates and γ the shear rate limit. For engineering thermoplastics the shear rate limit is usually 20,000-40,000 1/s, depending on the shear-sensitivity of the resin. Use a limit of 20,000 1/s for shear -sensitive resins. Note that this formula assumes equal flow

112

Special Molding Techniques

~1.0 mm Nominal Wall Thickness ~2.0 mm Gate Diameter

Reinforcing Dome

Figure 3. Suggested pin-point gate detail for thin-wall parts requiring large gates.

passes through each gate. It can also be used to size tunnel gates, which should have at least a 20° included angle and be at a 45° angle to the parting line. If a three-plate runner is used, large gates may cause damage the thin nominal wall during degating. This can be avoided if a reinforcing dome is used opposite the gate as shown in Figure 3. Keep in mind that pressure imbalance between multiple drops in cold, three-plate runners may be more than with hot runner systems.

PITFALL #9: UNDERESTIMATING CLAMP TONNAGE REQUIREMENTS In thin-wall molding, it is not uncommon for the process window to be limited by the mold blowing open due to high cavity pressures. With conventional parts, clamp tonnage estimates of 3 tons per square inch are often adequate. Thin-wall applications must typically allow for more than 5 tons of clamp per square inch of the mold cavity projected area. If the part to be filled is large, the mold and backup plates should be about twice as thick as conventional parts to prevent flexing during high-pressure injection.

PITFALL #10: INADEQUATE VENTING IN THE TOOL The fast injection rates used in thin-wall molding require larger parting line vents, primarily to prevent flow hesitation as air is pushed from the cavity at the end of fill. However, the higher injection pressures and better flowing resins used increase the risk of parting line flash. A mold designed with a generous number of thinner vents may be the best compromise. Proper venting in the areas where air is chased at the end of fill is especially critical. Air trapped ahead of a quickly converging flow front can significantly increase filling pressure requirements.

Flow Instabilities in Thin-wall Injection Molding of Thermoplastic Polyurethane

Christian D. Smialek, Christopher L. Simpson Plastics Engineering Dept., University of Massachusetts, Lowell, USA

INTRODUCTION Thin-wall molding is conventionally defined as molding parts that have a thickness of 1 mm or less and a surface area of at least 50 cm2.5 Thin-wall molding has been around for decades, but due to the narrow processing window it did not catch on. The earliest applications for thin-wall molded parts were in the container and packaging industries. Recent reinterest in thin-wall injection molding is due to economic and environmental concerns when it was realized that products could be made lighter, more compact, and less expensive, as well as made quicker due to the reduced time for cooling the part during processing.7 In the past processing equipment could not generate the high pressures that are needed in thin-wall injection molding. As processing equipment became more robust and as there were revolutionary advancements in resin and in process control equipment, thin-wall molding was realized as a viable alternative to conventional molding. Advancements in process control systems have enabled the processor to have more precise control over the entire molding process. This enabled the production of parts with tighter and higher tolerances. While process control systems allowed one to have better control over our processing window, the introduction of high flow resins in the 80’s and the single site catalyst of the 90’s allowed for expansion of the previously found narrow processing window.1 Resins with narrow molecular weight distribution exhibit better properties and more stability at high rates of flow. This makes them ideal for thin-walled applications were high injection pressures and flow-rates are the norm. Market demands of the computer and telecommunications industries has fueled the latest interest in thin-wall molding.7 Each new product line demands smaller and lighter products and as a result the thin-walled parts have been attractive by providing housings at a reduced weight, size, and cost. Thin-wall elastomeric injection molding, specifically TPU, has not been evaluated, extensively, to date. Reasons for this center on the (still existing) processing difficulties associated with the resiliency and high viscosity of the material. However, it should be real-

114

Special Molding Techniques

Table 1. Experimental processing variables

Table 2. Experimental constants Parameter

Melt temperature

Fill pressure (melt pressure)

238°C

Experimental value

Fill time

2.0 s

59 MPa

Pack/hold time

6.0 s

232°C

61 MPa

Cooling time

8.0 s

224°C

61, 85, 109 MPa

Mold temperature

43°C

215°C

85, 109, 133 MPa

Pack/hold pressure

55 MPa (melt pressure)

207°C

133 MPa

Back pressure

0.7 MPa (hydraulic pres.)

ized that gaskets, seals, and other products can now be made thinner and less expensively with thin-wall injection molding.

EXPERIMENTAL The virgin resin chosen for this experiment was Desmopan – 453 (Miles; lot no: 392223828-1). Before processing the resin was dried for 12 hours at 88°C in a desiccant drier (Novatec NPH), since polyurethane is a hygroscopic material. The cavity was 140 mm in length, 51 mm in width and had a uniform thickness of 1 mm. The sprue was conical and had an average diameter of 7.5 mm and a length of 70 mm. The runner was full round with a diameter of 10 mm and length of 15 mm. The resin was molded on a 150-ton injection molding machine (Reed, 5-ounce, TGII series). Fill pressure and melt temperature were changed for each trial that was tested. Fill time, pack/hold time, pack/hold pressure, mold temperature, cooling time, and back pressure remained constant throughout the experiment. The melt temperatures and fill pressures for the experiment are listed in Table 1, while the constant parameters are listed in Table 2. The molding trials began at the lowest temperature and highest pressure. While molding at a set temperature, the pressures were varied until all the pressures for that temperature had been tested. When the pressure was changed after one trial had been completed (at constant temperature), the machine was kept in cycle and the first three parts were discarded to ensure proper conditions. After all of the trials had been completed at any one temperature, the machine was taken out of cycle and material was purged so that the actual melt temperature could be measured with a portable thermocouple device (Atkins digital thermocouple;

Flow Instabilities

115

Type J, model 396). After this, the temperature profile was changed, the molding cycle was resumed and samples were produced until the temperatures had reached the new set-points and stabilized. Ten samples were kept for each trial. This same part, sprue and runner included, was also modeled in full, (no simplifications) on MOLDFLOW, an injection molding analysis software package. A three-dimensional multi-laminate filling analysis was performed at all of the processing temperatures used in the actual experiment. The fill time used for this computer-aided analysis was 0.35 seconds, the filling time output by the RPC (process analysis) feature on the injection molding machine. That is, the actual time that it took to fill the mold, not the time that was used for fill time. Desmopan 386 was used for the software analysis since the 453 resin was not available on the database and this resin was the most similar TPU with respect to thermal, rheological, and physical properties. In addition to the processing and software analysis equations predicting shear rate, γ , shear stress, τ , and pressure drop, ∆ P, were solved. All three analyses were compared. Tables pertinent to this report are displayed within the report while figures appear at the end of the text.

RESULTS PROCESSING 240

230

220

210

200 0

50 {MPa}100

150

Figure 1. Processing window (melt temperature vs. fill pressure) for conventional and thin-wall injection molding of Desmopan – 453.

During processing it was determined that there was a minimum temperature and pressure required for fully filling the cavity. The experiment began at low temperatures, and until the temperature reached 224°C it was not possible to fill the cavity, even when the pressure was set at 133 MPa. The temperature increased as each trial was completed, and it was found that at 238°C, when the material showed significant degradation, the minimum pressure needed to fill the cavity was 59 MPa. It was also determined that the acceptable processing range of the melt temperature (was) from 224-232°C while the pressure to fill ranged from 109-61 MPa in that range. Figure 1 displays the processing window for thinwall molding, with respect to melt temperature and fill pressure superimposed on the processing window suggested by the resin supplier for conventional molding. Additionally, it was noted that when the injection pressure was 109 MPa or greater, the surface texture of

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Special Molding Techniques

the part was not smooth like the surface of the mold, but rather marked with surface depressions, such as those found on the surface of a golf ball. SOFTWARE ANALYSIS The computer model indicated that the model would fill at all temperatures other than 207°C, which was the lowest temperature used in the experiment. The software was capable of providing values of the required pressure for filling, Pmax, maximum shear rate, γ max, maximum shear stress, τ max, percent skin, % skin, and percent throughput, % thrpt, for each analysis. Table 3 displays these data for each trial. Table 3. Computer aided filling results Tmelt, oC

Pmax, MPa

τ max, MPa

γ max, s-1

% Skin

% Thrpt

238

43

0.24

40,500

22.0

100

232

51

0.27

42,150

22.5

100

224

65

0.31

43,600

24.3

100

315

87

0.55

52,400

26.5

100

207

100

-

-

38.3

runner only

The maximum allowable values for γ and for the material as well as a generic filling pressure maximum were also provided by the software package. Table 4 exhibits these values. τ

Table 4. Computer based maxima Parameter

Maximum value

THEORETICAL EQUATIONS

Since traditional equations for determining shear rate, γ , pressure drop, ∆ P, and shear stress, τ , are geometry specific, the mold was 0.300 MPa τ max split into distinct regions, each of unique geomeFilling pressure 100 MPa try. The first region consisted of the sprue and the runner. These two sections were modeled together, as a cylinder of 7.5 mm diameter and 85 mm length. The other section was the actual cavity, which was modeled as a rectangular slit. For the shear rate calculation, the gate (also a slit) dimensions of 10 mm width and 1 mm depth were used. The following equations were used to estimate the shear rate though each section. γ max

40,000 s-1

Flow Instabilities

117

Cylinder

γ a = 32Q ⁄ πd

Slit

6Q γ a = --------2 wh

3

where γ a is the apparent shear rate (s-1), Q is the volumetric flow rate (mm3/s), d is the cylinder diameter (mm), w is the slit width (mm), and h is the slit height (mm).4 Since elastomers, as are most thermoplastics, are characterized as exhibiting pseudoplastic flow, the following correction was made to the apparent shear rate: ( 3n + 1 ) γ c = γ a -------------------4n

where γ c is the corrected shear rate (s-1), γ a is the apparent shear rate (s-1), and n is the power law index. From a plot of viscosity, η , vs. shear rate, γ , the slope of the graph is equal to n-1. Said plot was provided by the supplier.8 This TPU has a power law index of 0.35. Once the corrected shear rates were found and the viscosity through each section was determined from the same plot of η vs. γ , the pressure drop through each section could be calculated. The following equations were used to perform this operation:

Cylinder

L ∆P = 8Qη -------4πr

Slit

φ ∆P = 12QηL ---------3 wh

where ∆ P is the pressure drop (MPa), η is the viscosity (Pa-s), L is the length of the section (mm), r is the radius of the cylinder (mm),4 and is the geometry factor (for a slit φ = 1.5).6 In the same fashion, when ∆ P was found it was possible to determine the shear stress, τ , developed in the cavity during filling. The shear stress developed at minimum pressure to fill and maximum pressure available to fill, as well as the filling pressure at which τ is equal to the critical τ were determined using the following equation; dP h τ = ------- --dx 2

where τ is the shear stress (MPa), dP/dx is the pressure gradient in the cavity (MPa/mm), and h is the height of the cavity (mm).3 Table 5 demonstrates the values that were calculated during this analysis. TPU FLOW TYPE A very interesting observation noted during this experiment was the flow-profile of the TPU. A dual-plug profile was observed in most short-shot trials, rather than the traditional single, centered plug profile. In parts that were not filled more than 20 mm, there was not

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Special Molding Techniques

Table 5. Theoretical results Parameter

Result

γ c (sprue)

12,410 s-1

γ c (gate)

17,810 s-1

η (sprue)

100 Pa-s

η (gate,cavity)

50 Pa-s

enough time or space for the profile to develop and the flow that was observed was due to entrance effects at the gate. That is, the material was fanning out. In all other shots that were not full, this dual profile could be observed.

DISCUSSION FILLING PRESSURE, TEMPERATURE, SHEAR RATE

The computer-aided filling analysis suggested that the cavity could be filled with 43 MPa at the 2.3 MPa ∆ P (sprue) highest temperature, 238°C, while the generalized equations predicted that 54 MPa would be 51.7 MPa ∆ P (cavity) required. In actual processing 59 MPa was the 54.0 MPa ∆ P (total) minimum pressure needed at 238°C. However, at this temperature the resin was severely 0.19 MPa τ (at min. pressure) degraded, so in actuality, 61 MPa was the mini0 .49 MPa τ (at max. pressure) mum pressure needed in order to produce a good part. P (at τ = τ critical) 84.0 MPa The software also indicated that the cavity could be filled when the melt temperature was 215°C. During processing it was found that the cavity could not be filled until the temperature of the melt was 224°C. Although the software understated the minimum pressure and temperature required for filling, the analysis of the shear rate and shear stress were, most likely, more accurate, especially when compared to the predictive equations. This is due to the fact that the software was able to take into account the skin layer that forms and entrance effects at the gate. SHEAR STRESS During flow a shear stress distribution develops in the cavity, with the shear stress at the freezing skin layer near the wall at a maximum value, retreating to a minimum value in the hot flowing center.5 The critical shear stress represents the value above which primary bonds in the polymer backbone can be broken during flow. If the shear force of flow is too high it will overcome the frictional force between the mold wall and the skin layer. This will act to tear some of the skin layer away from already frozen polymer (slip-stick phenomena).5 This leads to serious cosmetic defects in the molding. This effect is magnified in thinwall molding.

Flow Instabilities

Flow Front

Frozen Layer

FOUNTAIN FLOW Figure 2. Typical flow in a rectangular slit cavity. Flow History

Gate

Flow Direction

119

Upon subsequent analysis of the molded parts it was found that some of the samples molded at pressures greater than 109 MPa demonstrated this slip-stick phenomena. These parts were characterized by depressions in the top and bottom surfaces, along the whole length of the part. This pressure of 109 MPa was greater than the 84 MPa that was predicted to induce the onset of slippage. To investigate this effect, the core side of the mold was surfaced with a fine aluminum-oxide powder (vapor-honed) which acted to increase the roughness of the surface, and coefficient of sliding friction between the polymer and the cavity surface. Samples were then molded and analyzed. It was found that the rougher surface led to a more stable flow, and a more cosmetically appealing surface on the molding. Thus, from a practical standpoint, it is better to create a mold with a rougher surface to prevent slippage during filling.

Figure 3. Typical melt front profile experienced during thin-wall molding of polyurethane elastomer.

TPU FLOW TYPE

Typically, fountain flow profiles are observed in thermoplastic injection molding. This flow-profile is characterAir Trap ized by parabolic velocity profiles at the melt front with respect to the width, with the maximum melt velocity at the center and zero velocity at either wall, as demonstrated in Figure 2.2 However, as illustrated in Figure 3, the TPU studied herein showed what appeared to be a dual-plug profile with the maximum velocity of the melt Weld Line Gate at each wall retreating to a reduced velocity in the center. This observation appears to be totally unique and was Figure 4. Weld line and air trap location in completely filled samples. unreported in previous literature. The implications of such unique rheology may explain the appearance of gas traps in the part, rather than gas burns at the corners as would normally be expected for a mold without venting. In addition this flow suggests the presence of a weld line running down the center of the part even though there was only one gate for the part and there were no obstructions during flow. Both the air trap and weld line are shown in Figure 4.

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This behavior could represent a new challenge in mold design and processing for TPU’s. The formation of this flow-type and possible theories for its formation will continue to be researched and discussed.

CONCLUSIONS Although there are uncertainties in with thin-wall molding of TPU’s, such as the flow profile, it was found that it is possible to mold TPU’s for thin-wall applications. It was also found that while it is possible to predict behavior during filling, the results of the predictions should be used only as a starting point for processing.

FUTURE WORK Future experiments will be designed to study thin-wall injection molding of other TPU’s as well as thermoplastic polyolefin elastomers (TPO’s). These experiments will center on investigating the unique flow profile found in this experiment. Additionally, physical properties, such as tensile properties, will be determined and related to the processing conditions at which the samples were produced.

ACKNOWLEDGEMENTS The authors would like to extend sincere thanks to our advisor Dr. Nick R. Schott of UMASS-Lowell, Dr. S. J. Grossman (also from Lowell), Stephen Guberski, and to Lee Plastics for providing the mold, machine, and material for this experiment. We would also like to thank the following people at Lee Plastics for their assistance: Leo Montagna Jr., Dan Wagner, and Steve Leele.

REFERENCES 1 2 3 4 5 6 7 8

Belcher, Don and Whetten, Alan (1994), Processing Effects for High Speed Thin Wall Injection Molding of Polyethylene Improved Processing (IP) Resins, ANTEC’94, p.593 Kramer, Nanda M. G., How Plastics Flow Into and Within a Cavity, KONA – The News, October 1993. McKelvey, James M., Polymer Processing, Wiley, New York, 1962. Oehler, Peter R. (1996), Estimation of Machine Requirements and Process Optimization of Thinwall Injection Molding, ANTEC’96, p.572 Schott, Nick R., Thin Wall Injection Molding: Strategies for Processing and Applications for Consumer Electronics, Lowell, 1995 Stevens, M. J., and Covas, J. A., Extruder Principles and Operation, Chapman & Hall, New York, 1995. Thinwall® Technical Guidebook for Electronics Applications, GE Plastics, October 1995. Urethane Elastomer Engineering Handbook, Miles Polymer Division, October 1992.

Pressure Loss in Thin Wall Moldings

John W. Bozzelli Injection Molding Solutions Jim Cardinal, Bill Fierens General Polymers Division, Ashland Chemical Company

INTRODUCTION As the injection molding industry continues to mature, the development trends begin to emerge. One of the many trends today is this drive to thinner-wall parts. Designers are driving more functions into parts to save assembly costs. Thinner walls are used to save weight and plastic which also reduces part costs. These trends do save costs push a molders' processing window narrower and narrower. What can the molder do to cope with this trend with respect to machines and processing strategy? Within processing there are four key plastic variables that define a part: • Plastic temperature • Plastic pressure • Plastic flow rate • Plastics cooling rate and time This list is not in order of any priority and of these this paper will address only the key variable, plastic pressure as it relates to part complexity and thin walling. This is not to say that plastic pressure is the most important. We are simply singling out plastic pressure with respect to a part becoming more complex and thinner walled. The point being that both of these trends require more melt pressure to fill and pack the mold as well as higher clamp tonnage to keep the mold closed. This paper compares the pressure requirements to fill a rectangular cavity and pack it to normal molding pressures as the nominal wall decreases from 2.54 - 1.27 mm (0.100 0.050 inch). The study merely reports experimental pressure readings for three different resins. Two polystyrenes, one crystal general purpose and one high impact, along with a polypropylene. While the general trend toward higher required pressure is well accepted there is a scarcity of actual data published. This data is needed to correctly specify machines and understand the magnitude of clamp tonnage required to do thin-wall molding. With the proper machine and the correct processing strategies for thin-wall molding the molder may increase his processing capabilities and maximize profits.

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Special Molding Techniques

EXPERIMENTAL Tests were done under controlled conditions. Molding was done on a 100 metric ton Mannesman Demag digital hydraulic molding machine with a 15.6 intensification ratio. The mold was a wedge action variable depth mold built by DME. Part dimensions were 38.1 mm (1.50 inches) wide by 152.4 mm (6.00 inches) long with the nominal wall thickness stated in the data tables. Melt temperature setpoints were at 232oC (450oF) which yielded and actual melt temperature of 235oC (455oF). Mold temperature was set at 38oC (100oF) Filling the part was done under velocity control. There was always abundant pressure on the pump side of the flow control valve to assure velocity control. Hydraulic pressure in the injection cylinder varied shot to shot compensating for viscosity variations during fill. Fill was terminated at a stroke position that provided a part 99% full. Hydraulic pressure was monitored at a 100 hertz through a strain gauge hydraulic pressure transducer. Data was plotted and stored via computer. Hydraulic pressure at transfer was converted to plastic pressure in the nozzle via the machine's intensification ratio and reported as the pressure to fill the part 99% full. Cavity pressures were measured at two locations along the parts center line near the gate and near the last area to fill. The measurements were made via strain gauge transducers behind 6.04 mm (0.125 inch) ejector pins. The distance between the ejector pins was 140 mm (5.50 inches). This is the distance used to calculate the pressure loss reported in the data Tables 1-3.

RESULTS AND DISCUSSION Tables 1 through 3 present the data in both SI and English units. Data is presented for fill times, time to fill the parts 99%, from 1.44 seconds to 0.42 seconds, at nominal wall thickness' of 2.54 mm (0.100 inch) and 1.27 mm (0.050 inch). This nominal wall change represents two length to thickness ratios: 60: 1 for the 2.54 mm thickness and 120:1 for the 1.27 mm thickness. The highest of which barely qualifies as thin wall molding as sometimes defined. Thin wall molding length to thickness ratios can go to 300:1 ratios. Table 1. Pressure loss vs. nominal wall thickness and flow rate data for high impact polystyrene (STYRON 484); (SI units) Nominal wall, mm

Fill time, s

Inject pressure to fill part, MPa

Hold melt pressure, MPa

2.54

1.44

60.9

48.6

Post gate pressure, MPa

31.8

Last fill pressure, MPa

21.4

Pressure loss across part, MPa

Pressure loss per cm of flow, MPa

10.4

5.0

Pressure Loss

123

Table 1. Pressure loss vs. nominal wall thickness and flow rate data for high impact polystyrene (STYRON 484); (SI units) Nominal wall, mm

Fill time, s

Inject pressure to fill part, MPa

Hold melt pressure, MPa

2.54

0.82

65.6

35.0

31.0

2.54

0.42

76.3

31.0

1.27

1.44

95.0

1.27

0.83

1.27

0.43

Post gate pressure, MPa

Last fill pressure, MPa

Pressure loss across part, MPa

Pressure loss per cm of flow, MPa

22.0

9.0

4.4

31.8

22.9

8.9

4.3

60.3

49.4

17.3

32.1

15.5

89.8

49.4

41.0

15.1

25.9

12.5

93.7

37.2

40.7

19.3

21.5

10.4

Table 1A. Pressure loss vs. nominal wall thickness and flow rate data for high impact polystyrene (STYRON 484); (English units) Nominal wall, in.

Fill time, s

Inject pressure to fill part, psi

Hold melt pressure, psi

Post gate pressure, psi

0.100

1.44

8,827

7,045

4,618

0.100

0.82

9,513

5,075

0.100

0.42

11,070

0.050

1.44

0.050 0.050

Last fill pressure, psi

Pressure loss across part, psi

Pressure loss per inch of flow, psi

3,111

1,507

287

4,501

3,195

1,306

249

4,500

4,610

3,318

1,292

246

13,784

8,744

7,161

2,511

4,650

886

0.83

13,027

7,162

5,941

2,190

3,751

714

0.43

13,588

5,392

5,905

2,793

3,112

593

Table 2. Pressure loss vs. nominal wall thickness and flow rate data for polypropylene (SB-823; 20 MRF); (SI units) Nominal wall, mm

Fill time, s

Inject pressure to fill part, MPa

Hold melt pressure, MPa

2.54

0.82

42.1

30.3

29.0

2.54

0.44

52.8

33.5

31.9

Post gate pressure, MPa

Last fill pressure, MPa

Pressure loss across part, MPa

Pressure loss per cm of flow, MPa

20.6

8.4

4.1

22.7

9.2

4.5

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Special Molding Techniques

Table 2. Pressure loss vs. nominal wall thickness and flow rate data for polypropylene (SB-823; 20 MRF); (SI units) Nominal wall, mm

Fill time, s

Inject pressure to fill part, MPa

Hold melt pressure, MPa

1.27

0.90

60.2

55.5

55.3

1.27

0.40

69.8

46.5

46.8

Post gate pressure, MPa

Last fill pressure, MPa

Pressure loss across part, MPa

Pressure loss per cm of flow, MPa

30.9

24.4

11.8

24.4

22.3

10.8

Table 2A. Pressure loss vs. nominal wall thickness and flow rate data for polypropylene (SB-823; 20 MRF); (English units) Nominal wall, in.

Fill time, s

Inject pressure to fill part, psi

Hold melt pressure, psi

Post gate pressure, psi

0.100

0.82

6,100

4,400

4,200

0.100

0.44

7,658

4,853

0.050

0.90

8,726

0.050

0.40

10,120

Last fill pressure, psi

Pressure loss across part, psi

Pressure loss per inch of flow, psi

2.985

1,215

231

4,628

3,290

1,338

255

8,053

8,025

4,485

3,540

674

6,740

6,781

3,543

3,238

617

Table 3. Pressure loss vs. nominal wall thickness and flow rate data for crystal general purpose polystyrene (STYRON 685D, 2MFR); (SI units) Nominal wall, mm

Fill time, s

Inject pressure to fill part, MPa

Hold melt pressure, MPa

2.54

0.42

73.9

45.2

33.6

1.27

0.43

99.6

51.7

42.1

Post gate pressure, MPa

Last fill pressure, MPa

Pressure loss across part, MPa

Pressure loss per cm of flow, MPa

22.4

11.2

5.4

17.0

25.1

12.1

Pressure Loss

125

Table 3A. Pressure loss vs. nominal wall thickness and flow rate data for crystal general polystyrene (STYRON 685D, 2 MFR); (English units) Nominal wall, in.

Fill time, s

Inject pressure to fill part, psi

Hold melt pressure, psi

Post gate pressure, psi

0.100

0.42

10,713

6,552

4,872

0.050

0.43

14,442

7,502

6,113

Last fill pressure, psi

Pressure loss across part, psi

Pressure loss per inch of flow, psi

3,248

1,624

309

2,472

3,641

694

Interpretation of the data provides some insights to requirements for thin-wall molding. First, the data indicates the thinner the nominal wall the greater the injection melt pressures required to fill and pack the cavity. This is expected, however the magnitude of the pressure increase is noteworthy. In each case the pressure loss more than doubled, in some cases it tripled, that of the thicker wall data. Higher thin-wall ratios would drive this pressure loss even higher. This brings to question the typical rules of thumb for calculating required clamp tonnage. The data demonstrates the need for significantly higher clamp tonnage. Normal requirements typically stated for these relatively easy-flow commodity resins will be elevated to tonnage quoted for engineering grade resins. Processors need to have available exact pressure loss data for a given distance relative a specified wall thickness so they can properly specify machine clamp requirements. The large pressure loss is the cause for the higher clamp pressure requirements. To minimize pressure loss through the part, various injection rates were tried. While data is not complete for each resin a trend is clear: fast injection rates produce less pressure loss through the part. In going from the slow 1.44 second fill time to the faster 0.43 second fill time, pressure loss decreased dramatically. This provides better part uniformity which may provide better performance, less dimensional variances and less warp. However, faster injection rates do require greater melt pressures to drive the plastic into the cavity. This is a critical point in specifying thin wall molding machines. Melt pressures of 140 MPa (20,000 psi) are common today but may not be sufficient to tomorrow's downsized thin wall part. This data showing that faster fills allow for lower pressure losses provide a strategy for filling thin-wall molds: faster fill rates produce parts with less pressure gradient from gate to last area to fill. The molder can improve his process capability by using fast fill speeds.

SUMMARY Data has been presented that shows the effect of reducing nominal wall thickness on the pressure requirements of molding three commodity resins. Going from a 60:1 to 120:1 flowlength to thickness ratios more than doubled the pressure loss in the cavity. Data also shows

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Special Molding Techniques

that faster injection rates provide significantly lower pressure loss within the cavity while requiring higher injection melt pressures. This data supports the position that for thin-wall molding higher clamp pressures and higher melt pressure will be required of molding machines to control thin-wall processing. It is important that more of this type of data be generated for both commodity and engineering grade resins so that molders and processors can correctly specify molding equipment and processing strategies.

ACKNOWLEDGMENTS The authors would like to thank Ashland Chemical and Dow Plastics, particularly Gary Rademacher for their support of this work.

Integrating Thin Wall Molder’s Needs into Polymer Manufacturing

W. G. Todd, H. K. Williams, D. L. Wise Equistar Chemicals, LP

INTRODUCTION One of the most frustrating problems for resin manufacturers is how to relate injectionmolding parameters back to manufacturing synthesis conditions and laboratory quality control (QC) measurements. This article describes a unique use of existing QC-measured resin properties to predict relative molding cycle times for high-flow polyethylene (PE) resins. The introduction of the “Isometric Spiral Flow Chart” (Figure 4) provides the basis for this new approach. A nomogram for optimizing injection molding melt temperatures when transitioning from lot-to-lot is also presented.

DISCUSSION Molders of rigid food packaging containers and promotional drink cups generally have well-defined processing needs and related methods to measure process consistency and molded part performance. Likewise, the polymer manufacturer has well-defined manufacturing and analytical methods for characterizing resin properties and physical properties. How well a resin supplier is able to translate polymer manufacturing measurements back to the molder’s process and the end-use applications often determines the degree of success for both the resin supplier and the molder. Table 1 attempts to define these inter-relationships between the molder’s processing requirements and the polymer producer’s process measurements. The information in Table 1 shows the injection molder can easily measure some of his needs and the manufacturer can relate those needs through TS (Technical Service) laboratory measurement. In other areas, the customer does not have a well-defined measurement of his needs. The problem is further compounded Figure 1. Typical cycle times versus spiral flow number. because even if the customer’s measurement can

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Special Molding Techniques

be correlated to a measured TS lab measurement, how does the TS measurement relate to a plant QC measurement? For example, molding cycle time correlates well to TS laboratory spiral flow measurements as illustrated in Figure 1. The Equistar spiral flow number (SFN) is the number of centimeters of flow produced when molten resin at 227oC is injected into a long, spiral-channel insert (half-round 0.635 x 0.157 x 127 cm) at a constant pressure of 6.9 MPa. Estimated shear rate is approximately 10,000 reciprocal seconds. The equivalent ASTM Method for SFN is D3123. Table 1. Molding requirements versus resin physical properties Molder End-use requirements

Polymer producer How measured

Tech. service lab. measurement

Plant QCmeasurement

Production rates

Cycle time

Spiral flow

MI2* and MFR**

Stacking strength

Top load

Flex modulus

Density, MI2 and MFR

Toughness

Drop impact

Izod impact - ambient

Density, MI2 and MFR

Cold temp. impact

Drop impact

Izod impact – freezer

Density, MI2 and MFR

Dimensional control

Shrinkage, lid fit, nesting

ASTM shrinkage

Density, MI2 and MFR

Warpage

Visual, printability

Part deformation

Density, MI2 and MFR

* Melt Index (MI2) ** Melt Flow Ratio (MI20/MI2)

Now that we have established a relationship between cycle time and SFN, how do polyethylene resin properties influence SFN? Typically a resin manufacturer changes polymerization catalyst systems, modifies reactor configuration or adjusts reactor-operating parameters, such as temperature, ethylene and hydrogen concentrations, to vary molecular weight (MW) and molecular weight distribution (MWD). Melt index, MI2, is measured in the QC lab and is used as an indication of resin molecular weight. It is defined as the number of grams of polymer extruded in ten minutes as measured by ASTM Method D1238. The higher the melt index, the lower the molecular weight and melt viscosity which means the resin processes more easily. Melt flow ratio (MFR or MI20/MI2) is a calculated QC lab number, which is used as an indication of MWD. It is calculated by dividing a melt index measured at a high shear rate (MI20) by a melt index measured at a low shear rate (MI2). A low MFR indicates a narrow MWD; conversely a larger number indicates a broad MWD

Integrating Thin Wall Molder’s Needs

129

Figure 2. Commercially available high flow resins.

Figure 3. Spiral flow number versus MI20 correlation.

polymer. In general, a broader MWD resin flows easier than a narrow MWD resin at a given melt index. This article defines high-flow polyethylene resins as those resins with melt indices above 20 and MFR’s between 20 and 40. Figure 2 plots commercially available highflow resins as functions of melt index and MFR. Individual resins are identified by their MI2. Regression of Equistar laboratory spiral flow data for the resins shown in Figure 2 resulted in the correlation shown in Figure 3, which is defined by the following equation: [1] SFN = 10.44 + 1.016 x Sqrt (MI20) Remembering that [2] MI20 = MI2 x MFR

Melt Index, g/10 min

140 120 SFN, cm

100

70

80

65 60

60

55 50 45 40 35 30

40 20 0 18

20

22

24

26

28

30

32

34

36

38

Melt Flow Ratio MI20/MI2

Figure 4. Isometric spiral flow chart commercially available high flow resins.

Isometric spiral flow values were superimposed on Figure 2, as shown in Figure 4. With this easy-to-read chart, one can rapidly determine how one resin performs versus another one with regard to cycle time. The effect of melt index and MFR on spiral flow is

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Special Molding Techniques

160

Melt index, g/10 min

140

120

Aspect Ratio

100

440

80

400

60

360 320 280 240 200

40 20 0 18

20

22

24

26

28

30

32

34

36

38

Melt Flow Ratio, MI20/MI2 Figure 5. Isometric aspect ratio commercially available high flow resins.

Spiral Flow number, cm

65 60 55

Lines of Constant Melt Viscocity

50

45 40 35 30 25 20 210

230

250

270

290

310

330

Melt Temperature, C Figure 6. Melt viscosity as function of spiral flow and melt temperature.

very graphic. Molders can easily determine which resins satisfy their required cycle times. An equally valuable tool would be a chart that predicts how a given resin fills a particular mold. To accomplish this, the isometric SFN values in Figure 4 were converted into mold aspect ratios as shown in Figure 5. The aspect ratio of a mold is calculated by dividing the length of melt flow by the average wall thickness of the part. Using Figure 5, the customer and resin manufacturer can easily determine the range of MI2s and MFRs that will fill a given mold. Recently, Equistar established spiral flow specifications for all high-flow HDPE resins and began reporting the spiral flow number for each lot on the shipping Certificate of Analysis (COA). The customer can compare the spiral flow of an incoming lot of resin with the

Integrating Thin Wall Molder’s Needs

131

32.5

100

180 190

37.5

210

40.0 42.5 45.0

200

80

B D

C

240 250

47.5 50.0 52.5 55.0 57.5 60.0 62.5 65.0 67.5 70.0 72.5

220 230

60

A

40

20

0

E

260 270 280 290 300 310 320 330 340

Melt Temperature, C

Spiral Flow Number, cm

35.0

Figure 7. Spiral flow temperature adjustment nomogram.

spiral flow of the lot on-hand and readily estimate how the new lot will process relative to the lot currently in production. For example, if the lot currently being run has a SFN of 50 cm and the new lot has a SFN of 55 cm, the new lot should process at an approximately 10% faster rate. To further aid the customer in adjusting his or her processing conditions, Figure 6, Isometric Melt Viscosity Chart as a function of SFN and melt temperature, was developed from laboratory, capillary Rheometer, melt viscosity data. For a given SFN and melt temperature, a point can be located on a constant melt viscosity line. By tracing along this viscosity line to the new incoming lot’s SFN, the required melt temperature to compensate for the difference in SFN's between the two lots can be read. Adjusting the melt temperature to compensate for the difference in SFNs minimizes changes in cycle time. To simplify this compensation process, the special nomogram shown in Figure 7 was developed. The left-hand vertical line represents the SFN of a given lot of resin and the right vertical line is the extruder melt temperature used to process the resin. The center vertical line is a Polymer Melt Viscosity Index, which is a relative scale from 0 percent to 100 percent of the melt viscosities used to develop the nomogram. To use the nomogram, the customer draws a straight line between the SFN of the lot currently being run and the extruder melt temperature. This locates a fixed point on the center Melt Viscosity Index. The line is then rotated about this fixed Melt Viscosity Index point to the new incoming resin lot’s SFN. The recommended new extruder melt temperature is read from the right side Polymer Melt Temperature Line. For example, if a molder was transitioning from a resin with a SFN of 50 cm at a melt temperature of 230oC to a resin with a SFN of 45 cm, the melt temperature required to

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Special Molding Techniques

maintain the same cyclic time would determined by: 1) drawing a straight line (solid line) between Point A, SFN of 50 cm, and Point B, 230oC melt temperature; 2) locating Point C, 50%, on the Viscosity Index line; 3) extend a straight line (dashed line) from Point D, new SFN of 45 cm, through Point C to Point E, which gives a new melt temperature of 251oC.

SUMMARY The use of SFN’s to describe the flow properties of a given resin in thin wall injection molds uniquely combines the effect of both the resin’s MI2 and MWD. In addition, superimposing isometric SFN’s and Aspect Ratios onto resin product maps graphically depicts how one resin will perform in a given mold versus another resin. Furthermore, the thin wall molder can minimize transition losses between resin lots and between resin grades by using the Isometric Melt Viscosity Chart or the associated Nomogram to adjust injection melt temperatures.

ACKNOWLEDGEMENTS The authors would like to acknowledge the following Equistar Chemicals associates who developed the laboratory data used to generate the correlations presented in this article: Jean Merrick-Mack, Scott Nolan, Charlie Smith, Kirby Perry, Jim Hale and Mark Gregurek.

Thinning Injection Molded Computer Walls

Lee Hornberger and Ken Lown Santa Clara University, Santa Clara, CA, USA

BACKGROUND Several new plastic materials and processes have been developed in the last few years which facilitate the production of high quality "thin" walled plastic parts. These new technologies have enabled the production of cosmetic injection molded parts with wall thicknesses less than 1 mm.1,2 This form of molding has been readily adopted by the cellular phone and portable computer industry because thin walled parts provide valuable product weight and cost savings. But, designers of larger parts such desktop or workstation computer housings have had little interest in thinning the walls of their products and have continued to design their parts with wall thickness in the range of 3 mm. However, the current market trend of reducing computer cost and the environmental impact of large quantities of plastic material makes it worth exploring the potential material savings of thinner walled products.

REQUIREMENTS FOR COMPUTER HOUSINGS Typical computer housings such as those made by Sun Microsystems are made from two to four parts each in the order of 400 mm long by 400 mm wide. A typical computer housing panel designed by Sun is illustrated in Figure 1. Housing panels such as this one must be structurally sound, cosmetically attractive, resistant to weathering and meet flammability requirements for office equipment. They must also be easy to mold consistently for a price well below $10 with volumes in the order of 100,000 parts. Consequently, most computer housings are made from engineering resins such as flame Figure 1. Sun’s housing panel. retardant ABS, PC/ABS blends or Polycarbonate which meet these specifications at reasonable cost.

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Special Molding Techniques

DESIGNING THINNER WALLED HOUSINGS Of the many requirements for computer housings three typically drive the design: structural properties, cosmetics and cost. Thinning the nominal wall effects all three of these areas. The first and minimum structural requirement for a housing is stiffness. The stiffness of the computer is a measure of its ability to withstand the loads it is subjected to without a noticeable change in shape. The loads on a housing are varied. They may be due to the weight of a monitor or spring loads from EMI shields or even customer abuse so the housing must have some minimum stiffness in all directions. The stiffness of the housing is a combination of its geometric stiffness, measured by its moment of inertia, I, and its material stiffness, measured by the modulus of elasticity, E, of its plastic. The overall stiffness of a box can be quantified by its "EI" product. The higher this product the stiffer the housing. Thinning the housing wall dramatically decreases its moment of inertia which is proportional to the cube of wall thickness. The designer can compensate, somewhat, for a loss in geometric stiffness by changing to a stiffer material. Another important structural property of a housing is its load bearing capacity. This is of particular importance for detailed design features such as snaps and bosses. The load a feature can bear is proportional to its material yield strength and cross-sectional area. Consequently, thinning feature walls reduces the load they can tolerate and must be compensated for by changing to stronger materials. Impact strength is another important structural property in computer housings as they are often subjected to intentional or unintentional physical blows from their human masters. Designers enhance the ruggedness of the boxes by selecting materials with high impact strengths. Most office computers and telephones are made from materials with notched impact strengths of 160 J/m (3 ft-lbf) or greater and this seems a reasonable range for housing materials. Decreasing the wall thickness of a housing does not directly change its impact strength. However, impact strength may be lost when stiffer, stronger materials are substituted in these applications. The cosmetics or appearance of a housing may also be affected by thinning its nominal wall if it increases the occurrence of sinkmarks or weldlines. In conventional 3 mm design, the wall thickness of internal ribs and bosses are specified as 60% of the nominal wall in order to minimize visual sink. Using this same approach in the design of thinner walled parts yields much weaker ribs and bosses which may not support their loads. Designers may then be forced to increase wall thickness and trade sinkmarks for strength. However, it is reported that 1 mm thick panels made from glass filled materials can have rib widths 100% of the nominal wall without sink.3 Another cosmetic issue impacted by the use of a thinner walls in housings is weldlines. Weldlines occur when melt fronts meet within the part. Normally, designers minimize weld-

Thinning Injection Molded Computer Walls

135

lines by limiting the number and location of gates needed to fill a part. Housing tool designers typically use one or two direct gates to mold these parts in three millimeters. However, more gates are needed to fill thinner walled parts as the thinner walls constrict the flow of material and accelerate the cooling of the material. The final, and most important design element which is affected by thinning the nominal wall is the manufacturing cost of the part. This cost is made of three components: material cost, processing cost, and tooling cost. Thinning the wall reduces the reduces material cost by decreasing the volume of material required to fill the part. It decreases process cost by reducing cooling and injection time. Tooling cost, however, are reported to increase when molds are designed for thinner walled parts1,3-5 as more steel is required to resist the higher pressures needed to push these materials into the mold. In addition, if glass filled materials are used for these designs, harder, more expensive steels must be used due to resist the abrasive nature of these materials.3

TRADE-OFF STUDY METHODOLOGY Manufacturers of computer housings such as Sun Microsystems, have been reluctant to use thinner walls in their products because the benefits relative to the apparent risk have been unclear. As part of Sun’s study of thin wall design, the authors designed a Sun housing panel with 1, 2 and 3 mm walls and evaluated these panels relative to their resultant structure, cosmetics and cost. The panel analyzed was that sketched in Figure 1. The structural properties of the three panels were compared by analyzing their relative stiffness, flexural strength and impact strength. In order to do this, the geometric stiffness of each of the three panels was calculated from its wall thickness and the width of its largest cross-section. For comparison purposes, this value was calculated with and without the six ribs that straddled the part. The modulus, E, used to evaluate the EI product varied for each as different materials were recommended by General Electric and industry standards for each wall thickness.2,3 For the 3 mm wall a flame retardant KJB ABS was chosen as it is commonly used in these applications. For the 2 mm design, a PC/ABS high flow 2950 Cycoloy3 was recommended as it provided acceptable flammability resistance at this thickness, and high flow at the high pressures and temperatures used in this type of molding. It also has increased stiffness, impact and yield strength over ABS. For the 1 mm panel a high modulus Lexan SP76043 was recommended as by GE, Shieldmate and Apple for its structural value and cosmetics in this wall thickness. The relative load bearing capacity of the three panels was compared by evaluating their performance in a standard cantilever snap feature under load. The snap feature was modeled by a 25.4 mm long by 12.7 mm wide cantilever beam clamped at one end with a concen-

136

Special Molding Techniques

trated load applied at the free end. The maximum load, P, the beam can withstand without yielding was calculated from the standard cantilever beam formula: P= Sy wt2 / (6l) where: tensile yield strength of the plastic Sy t wall thickness of the panel l length of cantilever (25.4 mm) w width of cantilever (12.7 mm) The impact strength of the panels was assumed to be equal to their notched Izod impact strengths which are shown in Table 1. These values were listed on the data sheets for these materials. Table 1. Properties of plastic materials Property

Lexan SP 7604

Cycoloy 2950

Cycolac KJB ABS

6.08

2.58

2.27

Yield strength, Sy, MPa

74

64

38

Impact strength, notched, J/m

53

458

213

Density, 10-6 kg/mm3

1.36

1.18

1.22

Market price, $/kg

11.86

8.87

6.47

Modulus, E, GPa

To evaluate the relative cosmetics of the panels, the authors estimated the number of gates needed to fill each part based on recommended L/T ratios for thin wall molding by General Electric.3 To minimize sinksmarks, the parts were designed so that the rib to wall thickness ratio in the 1 mm parts were 1:1 and in the 2 and 3 mm parts were 0.6:1 (60%) as recommended.3 Manufacturing cost were calculated from material, processing and tooling cost through the following relationships: Part Cost = Mp+ Pp + Tp where: Material cost per part Mp Processing cost per part Pp

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137

Tp Tooling cost per part Material cost, Mp, were calculated from the following equation and the material properties in Table 1: Mp=VpxρmxCm where: the total part volume Vp the weight density of the material (kg/mm3) ρm cost of the material per kg in custom colors. Cm The processing cost per part was calculated from the molding cycle time and assumed molding machine rates. Secondary cost were not included in this evaluation. Cycle time was estimated using the methods of Poli and Dixon6 which calculates cycle time from considerations of wall thickness, part complexity and required surface finish. Injection machine cost were assumed to be $75/hour for a standard 3 mm wall part, $100/hour for a 2 mm wall and $120/hour for a 1 mm wall. Molding thinner walls requires higher injection pressures and more expensive higher tonnage injection molding machines. In addition, 1 mm parts need specialized machines with smaller barrels and programmed injection to aid flow and minimize degradation of these materials.3 Rates were chosen from the authors experience in California area. The tooling cost per part was derived from total tool cost distributed over 100,000 parts. Tooling cost was estimated with Poli and Dixon’s methodology.7 In this technique the complexity of the part due to features such as ribs, holes and bosses as well as its size and texture requirements are quantified relative to a baseline part (a simple plastic washer). The resultant complexity factor becomes a multiplier of the known cost of the washer tooling. The baseline washer tool was specified by Dixon and Poli’s as requiring 200 hours8 of machining and approximately $1000 of material. To estimate the washer tooling cost for this study, a machining cost of $75/hour was used which resulted in a total tool cost of $16,000. A 30% surcharge was added to the estimated cost of the 1 mm tool as this was the reported burden for increasing the strength of the mold for high pressure molding, increasing its hardness for the high materials and for the increased number of ejector pins needed to remove the fragile part.3-5

RESULTS The calculated structural properties, cosmetics and cost for the three designs are displayed in Tables 2, 3 and 4. From Table 2 it is obvious that the loss of geometric rigidity, I, with decreasing wall thickness is difficult to compensate for even with a glass filled polycarbonate which has a modulus more than 2.5 times that of the ABS. Even with the higher

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Special Molding Techniques

modulus material, the net stiffness, EI, for the 1 mm walled part was 10 times less than that of the original 3 mm walled part in both the unribbed and ribbed configurations. Table 2. Structural properties of housing panels Wall thickness, mm

1

2

3

Stiffness, I, mm4 (6 ribs)

110

460

1362

Stiffness, I, mm4 (no ribs)

39

312

1053

Stiffness, EI, GPa-mm4 (ribbed)

669

1187

3097

Snap load capacity, Newtons

7.3

21.3

28.38

1

2

3

Flow length, mm

90-150

180-300

30

Minimum gates

3

2

1

Minimum weldlines

2

1

0

1

2

3

Material cost, $

3.55

4.60

4.83

Processing cost, $

1.60

1.75

1.61

Tooling cost, $

1.26

0.97

0.97

Total part cost, $

6.41

7.32

7.41

Table 3. Cosmetics of housing panels Wall thickness, mm

Table 4. Manufacturing cost of housing panels Wall thickness, mm

The loss in stiffness, however, in going to a 2 mm from a 3 mm wall was fairly minor. Impact strength actually improved when Cycoloy replaced ABS in the 2 mm version of the part. The reverse of this result occurred when the glass-filled polycarbonate was used in the 1 mm part. Here, a large loss in impact strength was the price paid for the gain in stiffness. The load bearing capacity of the model snap feature (reported in Table 2) was diminished by 25% in the 2 mm part and 75% in the 1 mm part relative to the 3 mm version. This

Thinning Injection Molded Computer Walls

139

loss in strength and impact coupled with an increase in stiffness of the snap severely limits its allowable deflection and utility in a 1 mm part. The decrease in load capacity in the 2 mm nominal wall design is not as severe and is somewhat offset by the lower modulus and higher impact strength of the PC/ABS. The cosmetics of the part also suffer as its wall is thinned to 1 mm. As evidenced in Table 3, the flow length predictions for the 1 mm thick walls are nearly half of that for the 2 and 3 mm walls and this increases the number of gates required to fill this part. The three gates and two weldlines estimated to fill the 1 mm walled part listed in Table 3 are probably a low estimate as this calculation did not fully account for the poor flow of the glass filled material It is apparent from the data in Table 4 that the most beneficial asset of molding a 1 mm part is the decrease in part cost. The estimated total manufacturing cost for the 1 mm thick version of the part is a dollar less than that of its 3 mm counterpart. This 14% savings comes from the decrease in material needed to fill the part. Additional cost savings resulted from a decrease in cycle time for the thinner parts Cycle time for the 1 mm part was estimated, in this study, to be 48 seconds for the 1 mm, 63 seconds for the 2 mm and 77 seconds for the 3 mm. Cycle time savings, however, were offset by the increased molding machine cost and tooling for the thinner walled parts. No unique modifications had to be made to the 3 mm tool to produce 2 mm parts assuming high quality steel and conservative tooling design was used for 3 mm walls. Consequently, the cost for 2 mm tool was identical to that of 3 mm. Although there were savings on material volume and cycle time, the net cost of a 2 mm made from the PC/ABS blend was only 1.5% less than the original 3 mm part made from FR ABS. If FR ABS is acceptable for the 2 mm part than the manufacturing cost can be reduced by 20% to a total of $5.41.

REALITY CHECK A number of companies like Sun are in the process of evaluating the potential of thin wall molding and their experiences appear to support and supplement the quantitative data of this analysis. Shieldmate has had extensive experience molding small thin walled parts and found that their cycle times were significantly lowered. They were able to mold small 2 mm parts in 30 seconds. This is nearly half of the value estimated in this study but their parts were less than 150 mm in overall length and width. Shieldmate has also molded 1 mm parts 200 mm x 50 mm with a single gate but have found that they needed high quality tools and more expensive customized molding machines to produce quality parts.1

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Ron Lenox of Triquest11 also reported cycle times in the order of 35-45 seconds for 1.5 mm wall thickness parts which were roughly 200x280 mm He was able to use PC/ABS in this application but noted a significant increase in tooling cost. Apple successfully molded 1.5 and 2 mm walls in their laptop products without painting but they found that they needed glass fillers in their material to maintain the structural rigidity of the housing. It also increased their tooling cost. This result paralleled the findings of this report.9 Eastman Kodak has experimented with thinning the walls on large plastic parts (greater than 300x 600 mm).10 They saw a decrease in cycle time between 30 and 40% when they decreased their wall thickness from 3 to 2.5 mm. However, they noted significant manufacturing yield problems due to warp and burning when they made parts below 3 mm thick. Kodak predicts that these losses would be diminished with molding machines with programmed injection and plastic raw material with tightly controlled specifications.

ADDITIONAL TRADE-OFFS There are other issues that are equally important in the design decision relative to thin wall molding that are more difficult to quantify than the three design areas discussed in this paper. Warpage of parts is one of these. As reported in the Kodak test, decreasing the wall thickness may increase warpage of large parts by altering its cooling. Glass fillers in thin wall materials may also change the way in which the part warps. Manufacturing yield during molding was another factor difficult to quantify in this study. Whenever, a new technology is implemented in manufacturing there are always startup problems and these lower production yields. As reported by Kodak, this is also the case with thin walled parts. The degradation of the mechanical properties of the material in thin wall molding is a third issue of concern. In order to lower the viscosity of the plastic during molding and increase its flow through thin walls the molders generally increases the melt temperature of the material and raise injection velocity. Pushing these process variables to their limits can lead to degradation of the material and the loss of its stiffness and impact strength.3

SUMMARY & CONCLUSIONS Clearly, there is a large potential for cost savings by thinning the walls of a plastic part. According to the costing model used in this study a savings between 14 and 20% is possible on large computer housing parts produced in the range of 100,000 parts. Currently designers and molders are shying away from this approach because these savings do not appear to offset the loss of stiffness, impact strength, and load bearing capacity shown in this paper.

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However, the movement toward thinning the walls in larger housings appears to be inevitable as the benefits in cost are so attractive. The steps in this direction may be very gradual. Designers will slim the walls by tenths of millimeters rather than by whole millimeters. This gradual approach which enables continuous improvements and cost savings worked well in thinning smaller parts such as cellular phones and cameras10 and may work well in the computer industry. Important for the implementation of thin walls in computer housings is the development of grades of materials that can overcome the limitations discussed in this paper. Materials are needed with high stiffness, impact, strength, and flow properties which can be consistently produced. As was evidenced in the trade-off analysis of this study, it will be the development of these materials rather than tooling or molding machines which will drive this technology.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

Matecki, S. Shieldmate, Personal conversation. Fassett, J., "Thin Wall Molding: Differences in Processing over Standard Injection Molding," Proceedings of ANTEC ‘96, Indianapolis, May 1996, p 430. "Thinwall: Technical Guide for Electronics Applications," GE Plastics Technical Bulletin. Marques, R, SPM, Personal conversation. Rowe, D., Apple Computer, Personal conversation. Dixon, John R. and Poli, Corrado, Engineering Design and Design for Manufacturing, Field Stone Publishers , Conway, Mass., 1995. p15-1 and 11- 1NBC-575, October 1995. P.7-1 Ibid, p.11-1. Ibid, p.15-2. Sabi Khan, Apple Computer, Personal conversation. Bill Moncha, Eastman Kodak, Personal conversation. Ron Lenox, Triquest, Personal conversation.

Chapter 3: Molding Micro Parts and Micro Structures Transcription of Small Surface Structures in Injection Molding – an Experimental Study

Uffe R. Arlø, Erik M. Kjær Danish Polymer Centre, Technical University of Denmark

THEORY PHYSICAL MODEL (HYPOTHESIS) During filling of the cavity the plastic melt is transported from the center towards the cold mold wall by fountain flow where the melt solidifies. At the time the melt reaches the mold wall only a low pressure is present (the pressure in the melt front is atmospheric). Later in the process the frozen layer at the surface grows and pressure is built up. Ideal conditions for a good surface transcription would be • Plastic material with a high temperature (a thin frozen layer) • Large forces to press the plastic material around the surface structures, i.e. high pressure. However, these two criteria are not met at the same time. As described above the plastic frozen layer grows with time and so does the pressure. Therefore surface transcription can be regarded as a compromise between temperature and pressure. It is hypothesized that surface transcription is formed by three mechanisms: • An immediate formation where the molten plastic flows in the surface valleys and over the peaks as it flows over the surface. This mechanism takes place during the filling stage. This mechanism will be denoted flow formation. • A formation that takes place after the molten plastic has passed over the surface during the subsequent pressure build-up. This formation could be regarded as a thermoforming-like deformation of the frozen layer around the surface structures. The mechanism is predominant in the post-filling stage. This mechanism will be denoted press formation. • A modification of the surface structure due to thermal effects such as shrinkage and stress relaxation as the heat from the plastic parts core is transported through the surface. This mechanism will be denoted thermal modification. The first two mechanisms are the basis for this study.

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PROCESS PARAMETERS In practice the hypothesized physical phenomena are controlled by process parameters on the injection molding machine and equipment. A parameter space for the process could consist of • Melt temperature (controlled by barrel heater bands) • Mould temperature (controlled by coolant flow and/or temperature) • Injection rate • Switch over point • Holding pressure • Holding pressure time • Cooling time • Other parameter controlling plastification and mold opening. In agreement with the aim of studying the mechanisms described above, temperature and (holding) pressure are selected. The temperature of interest is of course the temperature of the plastic, where the melt temperature as set on the machine and the mold temperature corresponds to the initial value and boundary condition of the heat transfer problem. In other words both melt temperature and mold temperature are means of controlling the development of temperature in the plastic material during the process. For this study the melt temperature is selected. Further the injection rate is included influencing both the temperature of the plastic (due to friction and time for cooling) and the pressure gradient.

EXPERIMENTAL PROCEDURE PROCESS EQUIPMENT AND MATERIAL The experiment was carried out with a 2 mm thick ruler type part with an edge gate and a cold runner in a two plate mold (see Figure 1). Opposite the gate end, the cavity had a band with a rough spark eroded surface. Both CMOLD simulation and the physical parts show that the melt front almost forms a straight line as is passes the rough surface band hence assuring uniform orientation over the band. The part was made in polystyrene BASF 143E and produced in an Engel 135 injection molding machine.

Figure 1. Part and runner (fill pattern CMOLD).

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DESIGN OF EXPERIMENT I order to investigate the two mechanisms the design of the experiment was divided into two sets, one to study the flow formation and one to study the press formation. In the first set injection rate was varied over three levels and the melt temperature over two levels while holding pressure was kept at a minimum (as close to zero as the machine allowed). In the second set the holding pressure was varied over three levels and the melt temperature over two levels while injection rate was set at maximum. The experiments were carried out with full factorial variation. Process parameters are varied over a range corresponding to processing intervals recommended by the material supplier. SURFACE CHARACTERIZATION The surface roughness was measured on a Talysurf Surtronic 2D mechanical stylus instrument. As roughness parameter the Ra value was selected due to its wide spread application. The application of stylus characterization for comparing mold surfaces with plastic part surfaces suffers an inherent error in that valleys on the mold surface transcribes into peaks on the plastic part surface. Due to its physical dimension the stylus is unable to register deep valleys correctly while is capable of registering high peaks. This problem can be overcome by making a silicone replica of the mold surface and using this for comparison with the plastic part. However, since the aim of this study is to compare the plastic parts produced under different process conditions with each other such a replication has not been carried out. The rough mold surface has an Ra value of 12.6 µ m. As a supplement to the stylus measurement scanning electron microscope images has been processed for the mold wall and selected parts.

RESULTS

Figure 2. Effects during filling.

The experiment reveals that surface transcription consists of both the hypothesized flow formation and press formation (see Figures 2 and 3). From the first set of experiments where holding pressure is set to a minimum it can be seen that the transcription is improved progressively with higher injection rate. This can be contributed to the fact that high injection rates reduces time for cooling of the melt and/ or the fact that high injection rates can

146

Figure 3. Effects during post-filling.

Figure 4. Rough spark eroded mold surface, SEM image.

Figure 5. Plastic part surface “max. settings”, SEM image.

Special Molding Techniques

induce higher pressure gradients. In the second set of experiments where press formation is investigated it is evident that higher pressure up to a certain point improves transcription. It is surprising that the transcription quality falls at the highest level of holding pressure. This phenomenon is observed for both temperatures and cannot be explained by measurement uncertainty of process variance. In both sets of experiments the increased temperature improves transcription quality. Higher temperatures are observed to shift the roughness upwards. The shift is more pronounced for the experiments with minimum holding pressure than for the experiments with holding pressure suggesting that the flow formation mechanism is more sensitive to (the initial) melt temperature than the press formation is. Recently a study1 at The Technical University of Denmark concerning transcription of specific micro structures on the surfaces showed results supporting some of these findings. By comparing SEM images of the plastic parts produced with “max. setting” (high temperature, high injection rate, high holding pressure) and “min. settings” (low temperature, low injection rate, low holding pressure) it is obvious that the “max. setting” part has a greater roughness than the “min. setting” sample (see Figures 4, 5 and 6).

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CONCLUSIONS

Figure 6. Plastic part surface “min. settings”, SEM image.

Surface transcription in injection molding represents complex mechanisms and depends on pressure, temperature and the viscoelastic properties of the plastic. Two mechanisms have been identified: Flow formation and press formation. Generally increased temperature results in better surface transcription, while increased pressure leads to better surface transcription up to a certain (high) level of pressure. Surprisingly the transcription pressure dependence shows a local maximum and the transcription quality drops for the highest level of pressure.

ACKNOWLEDGEMENTS The authors wish to thank Lotte Due Teilade, Ph.D. for a critical review of this paper.

REFERENCES 1

Sørensen, Johansen: Micro injection molding, 1999, Technical University of Denmark.

Injection Molding of Sub-µm Grating Optical Elements

R. Wimberger-Friedl Philips Research Laboratories, Eindhoven, The Netherlands

INTRODUCTION Injection molding of optical components has become a high volume business mainly due to the enormous success of optical recording. The requirements with respect to the optical performance are very stringent for all parts in the optical light path, viz., the recording substrate and a number of components in the optical pick up. This requires a very accurate shape replication and low optical anisotropy as induced by the molding related stresses. Microstructure replication is important for both the media as well as components in the player. The information on the substrate is present in the form of small pit structures or as grooves in the case of recordable media. The pits are of sub-micron dimension but rather shallow. Still the replication is challenging for high recording density substrates like the DVD because of the large area and the thin substrate which makes filling difficult. In the optical pick up of the player also diffractive optical elements are used for instance for splitting of the main laser beam into three beams for track following but also for the deflection of the returning beam onto the detector area. Such diffractive gratings can be produced in high volume by injection molding already. In certain players an optical element with l/4 retardation is necessary for optimized beam splitting. Such elements are usually made from polished quartz laminates which are intrinsically expensive to produce. It is known that grating structures with a pitch considerably smaller than the wavelength of light do behave as retardation elements due to a so-called form birefringence effect which makes the effective refractive index in the structure depend on the direction of polarization.1 By injection molding such structures one could replace the expensive quartz element by a cost effective plastic component. In the following the design, fabrication of structured mold surfaces and the results of the replication by straight injection molding will be presented.

Special Molding Techniques

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Figure 1. Calculated effect of the pitch variation on intensity and retardation of transmitted 0th -order beams.

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depth (microns) retardation

Int Te

IntTm

785nm; n= 1.573 dc= 0.6; p= 480nm

Figure 2. Calculated effect of structure depth on retardation and intensity of transmitted 0th-order beams.

GRATING DESIGN

The grating geometry was designed with the aid of a numerical package called Cyclop developed at our laboratory by P. Urbach.2 This is an exact treatment of the problem by solving the Maxwell equations in a finite element representation of the physical grating geometry. The simplest geometry is a rectangular grating with pitch p, height h and duty cycle dc (polymer to air ratio). In Figure 1 the effect of the pitch is demonstrated for the retardation and the intensity of the emerging 0th order beams with polarization parallel and perpendicular to the grating direction Te and Tm, respectively. As can be seen for the indicated wavelength of 785 nm and refractive index of Polycarbonate, above a pitch of 0.5 µ m the intensities drop quickly due to upcoming higher order diffraction. Below the retardation increases strongly with pitch. Therefore a pitch just below 0.5 seems the best choice. In Figure 2 the effect of the grating depth is illustrated. As expected, the retardation increases linearly with depth. The required depth for a retardation of λ /4 is 1.56 µ m. One can also see that the transmission oscillates with depth. This means that such grating structures have anti-reflective properties due to interference. Higher values of transmission could be achieved with non-rectangular shapes at the expense of efficiency in retardation. The third design parameter is the duty cycle. Simulation shows an asymmetry in the behavior with a maximum retardation at dc = 0.6. The resulting design will have an aspect ratio of > 5! This is clearly a challenge for the insert production as well as the replication. One possibility to facilitate manufacture is to split the function in two λ /8 grating structures for the top and bottom surface, respectively. In that case the depth is reduced to about one half of the value shown above.

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EXPERIMENTAL OPTICAL INSERT MANUFACTURE Several techniques were used to obtain the structures. The basic step in obtaining such structures is E-beam lithography. The E-beam was used either for making a lithographic mask for the wafer stepper or an in-situ mask for consecutive etching steps or to write the desired structure directly in a thick photo-resist layer. The resist itself is not considered strong enough to be used in a mold. Therefore the structures first were replicated in a different material. One obvious way of doing this is by electroplating Ni. In the case the photoresist is only used as etching mask the structures were obtained by etching a 80 nm thick Cr layer. The Cr structure then served as an etching mask for the reactive-ion etching (RIE) in SiO2.3 The fused quartz structures can then be used directly as mold insert or replicated first in electroplated Ni as well. The resist was Success ST 3, a DUV resist of BASF. 0.8 mm thick resist structures were also obtained from DUV exposure with a wafer stepper ASML PAS 5500/90 from a mask written with E-beam. MOLDING The structures as obtained on typically 2 mm thick substrates were attached to steel rods. The shape of the cavity which determines the product shape is sketched in Figure 3. The diameter of the area containing the grating structure was 5 mm in the largest case, the thickness typically 1 mm. The molding experiments were carried out on a 35 tons Engel machine with 20 mm screw at Philips PMF in Eindhoven. The polymer was Makrolon CD 2000 (polycarbonate of Bayer AG, FRG). Its thermorheological properties are described elsewhere.4 The Figure 3. Schematic mold construction. molding parameters, viz. the injection rate and temperature, mold temperature and packing pressure, were varied in a limited range only. In contrast to other research groups5 we did not use thermal cycling of the mold but employed a constant mold temperature in a conventional molding process.

RESULTS INSERTS Three different inserts were used for molding, i.e., RI-etched quartz with a depth of 1.6 and 0.8 µ m and electroplated Ni with a depth of 0.8 µ m. The etching of quartz in a depth of 1.6 µ m turned out to be very difficult. The trenches either were not open completely or the

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walls tumbled. The processing window is not broad enough for safe industrial production. The structures of 0.8 µ m were less critical although one always needs to etch an array of structures with steps in etching time due bad predictability of the etch depth. The structures written by E-beam into thick resist always showed a negative slope and were therefore not used for replication. Instead DUV lithography was used. Ni replica’s were obtained from the resist structures for use in the mold. Figure 4 shows SEM micrographs of fractured cross-sections of the insert structures obtained by different techniques, as indicated. MOLDING

depth [nm]

For the replication of microstructures by a thermoplastic material the contact temperature is of major importance. The polymer Figure 4. Insert structures as melt has to flow into the channels of sub-micron height. The heat obtained by RI etching (top) and transfer to the walls occurs within a microsecond. The filling DUV lithography (bottom). would have to occur considerably faster in order 1000 to prevent freeze-off. Only when the contact temperature is above the no-flow temperature 800 enough time is left for complete filling. The contact temperature with a given polymer melt is 600 only determined by the mold temperature and the thermal properties of the mold material. The 400 effect of mold temperature is demonstrated clearly in Figure 5, where the depth of the repli200 cated grating is plotted as a function of the mold 0 temperature for the case of a Ni insert with 0.8 146 144 148 150 154 152 T mold [degC] µ m. Only at a mold temperature of 152oC, which is above the dilatometric Tg, the strucFigure 5. Measured depth of the replicated structures in PC vs. mold temperature as obtained from Ni insert. tures are completely filled. Despite the high mold temperature it is still possible to conduct a regular molding cycle but the products are not according to specs with respect to flatness and birefringence of the PC bulk. The molded structures are very good replicas of the mold insert as can be seen in Figure 6. Even the standing waves which are typical for DUV lithography are still visible on the walls of the PC grating. The duty cycle, however, is lower, i.e. the walls are thinner than according to the mold structure. This is attributed to shrinkage.

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Figure 7. Fracture surface and surface SEM micrographs of PC structure as replicated from SiO2 insert.

With a mold insert of fused quartz the contact temperaFigure 6. SEM micrographs of ture will be much fractured edge of PC structures. higher at the same mold temperature. Therefore the filling will be much easier. In fact no filling problems were encountered with mold temperature of 140oC. SEM micrographs of cross-sections of structures molded in this way are shown in Figure 7. As can be seen the structures are completely filled but there is a shape deviation. The structures are thicker at the top than at the bottom. At the moment this is not quite clear whether this is a filling phenomenon or a consequence of some elongation during the release of the structures. The secondary structure at the fracture surface is due to the fracture which indeed is very difficult to achieve with Polycarbonate. Sofar only structures of 0.8 µ m have been shown. But surprisingly enough we were also able to mold structures with double the depth. A major probFigure 8. SEM micrographs of defect (top) and FIB lem turned out to be the observation of the rep- machined detail at 45 degrees of 2 µ m deep grating. lication fidelity as such structures cannot be investigated easily. One technique which worked well is milling by a focussed ion beam. There the structure is first coated by a W layer and then ablated at a certain area. In this way

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it becomes possible to view the structures from aside. A typical result is shown in Figure 8. The contrast in the SEM is not very good but one can clearly see that the structures are approximately 2 µ m deep! To our best knowledge this is a record for replication with injection molding. As can be seen also in Figure 8 is that the walls start tumbling which will affect the local duty cycle and consequently the optical function. The inserts are very vulnerable and in practice we always found local defects like missing walls. Also the etching is very critical as mentioned above. The mold temperature is also important for the total shrinkage of the polymer after vitrification. The shrinkage of the bulk leads to a relative displacement of the replicated structures with respect to the mold structures and will destroy the fragile walls. Therefore the mold temperature has to be chosen close to the vitrification temperature anyway.

CONCLUSIONS • • •

•

• •

Grating structures with l/4 retardation have been designed for use in CD recorders with 785 nm wavelength. Such rectangular structures have a pitch below 0.5 µ m and depths in excess of 1.5 µ m. Inserts with these dimensions were indeed manufactured by RI-etching in SiO2. The etching process window, however, is very narrow and defects are always observed in the structures. By splitting the function over the two surfaces of the retardation element the depth of the structure is reduced to approx. 0.8 µ m. Such structures can be produced in both Ni and SiO2. The replication from SiO2 inserts by straight injection molding yields a stable and feasible process for mass fabrication. Structures with a record depth of 2 µ m and a pitch of 0.5 µ m were replicated by injection molding with polycarbonate.

ACKNOWLEDGEMENT The author likes to acknowledge the contribution of many colleagues at Philips which was essential for success of the project. W. Ophey, J. van Haren and R. Merkx for their assistance with the optical simulations, H. van Helleputte and E. van der Heuvel for the lithography and etching of the structures on the mold inserts, J.Godfried of PMF for help with the molding experiments, W. Coumans for the electroplating and J. de Bruin for the fabrication of the inserts.

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REFERENCES 1 2

3 4 5

Born, M., and Wolf, E., Principles of Optics, Pergamon, New York, 1980. Urbach, P.H., and Merkx, R.T.M., Finite Element Simulation of Electromagnetic Plane Wave at Gratings for Arbitrary Angles of Incidence, in Mathematical and Numerical Aspects of Wave Propagation Phenomena, Eds. Cohen, G., Halpern, L., and Joly, P., SIAM, 1991. Chapman, B., Glow Discharge Processes, J. Wiley, New York, 1980. Wimberger-Friedl, R., and de Bruin, J., Rheol. Acta, 30 (1991), 329 and ibid. 419. e.g. Ehrig, F. Klein, H., Rogalla, A., Ziegmann, C., Micro technology: New Dimensions in Plastics Processing, in 19 Kunststofftechnisches Kolloquium des IKV, 1998, 1.

Process Analysis and Injection Molding of Microstructures

Alrun Spennemann and Walter Michaeli Institute for Plastics Processing (IKV), Aachen, Germany

INTRODUCTION The injection molding of microstructures represents a key technology for the economic production of medium and large series of microstructured moldings and the assembly of micro-systems. During the last years fundamental research on the injection molding of microstructures has been done at the Institute for Plastics Processing (IKV) at Aachen University of Technology (RWTH Aachen). For these investigations a suitable injection molding machine and an appropriate mold technology were provided. A modular stem mold was designed for various different mold inserts. This mold contains a vario-thermal tempering system.Thus the mold is first heated close to melt temperature before injection with an electric heating and then cooled down to ejection temperature by a fluid tempering unit.1 In a first step demonstrator cavities were used to analyze the suitability of selected low viscous materials to fill a microstructured mould easily. These demonstrators, e.g. honey comb structures, were moulded in LiGA-cavities. The LiGA-technique is a special technology often used in micro technology that allows the precise production of microstructures with very high aspect ratio and high quality surfaces. For aspect ratios < 5, traditional processes, e.g. micro-cutting, micro spark-erosion and laser erosion are supposed to be a less expensive alternative.1

INJECTION MOLDING OF MICROSTRUCTURED PARTS This paper will compare the three processing technologies mentioned concentrating on the technological limits (smallest size of structures, cavity materials, freedom of design,...) and the quality of molded parts. Systematic trials were carried out to find a process window of injection molding parameters. After 500 shots the quality of the cavities was controlled. Ten different cavities were manufactured by microcutting, micro spark-erosion and laser-erosion. The geometry of these cavities varies in width and depth of the ditches and the width of the bridges. Moreover, different ejection slopes are realised in some cavities. The mould inserts are 8 mm in diameter with a structured area of 6 mm in diameter.

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variation of geometry in the cavity: width of width of depth of ditches bridges ditches (µm) (µm) (µm) left: 10 to 40

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Figure 1. REM shot of a molded part (standard molding parameters, POM, spark-eroded cavity).

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Figure 2. Variation of cavity temperature (POM, sparkeroded cavity).

500 µm cavity: laser-eroded material: PC

Figure 3. REM shots pf molded parts.

The design of the cavities follows IKV investigations and hints from publications1-3 as well as advice from the manufacturer (Fraunhofer Institut fur Produktionstechnologie (IPT), Aachen; Fraunhofer Institut fuer Lasertechnik (ILT), Aachen; Laboratorium fuer Werkzeugmaschinen and Betriebslehre (WZL), Aachen; Ronda AG, Schweiz). For systematic injection molding trials, standard parameters were defined for two thermoplastic materials: polyacetal (POM) and polycarbonate (PC). The parameters for the processing of POM (Hostaform C52021, Ticona GmbH, Frankfurt) are: mass temperature of 220°C, temperature of the oil tempering unit 90°C (i.e. a cavity temperature of about 190°C because of the additional electric heating) and injection pressure 250 bar. PC (Makrolon 2205, Bayer AG, Leverkusen) was molded at 310°C mass temperature, 500 bar injection pressure and the oil tempering unit was set at 120°C. Figure 1 gives an example of a molded part (POM) from a spark-eroded cavity. The cycle times vary from 40 to 60 s. The injection molding parameters were varied systematically to analyze their influence on the process. Parts molded at a higher temperature of the oil tempering unit (105°C) are overfilled (Figure 2, left), but when the oil temperature was set as low as 60°C, the surface is not molded well (Figure 2, right). After 500 shots, the small bridges (20 µ m) in the cavity

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showed damages, because the mold material was thermally influenced by the spark-erosion process. Therefore, bridges in spark-eroded cavities should at least have a width of 30 µ m. Against that, the micro-cut cavity is rather tough. A disadvantage of micro-cutting is the limitation to non-ferrous metal for the processing with diamond tools. The molded parts (POM) are a detailed copy of the cavity SPAN 3 (Figure 3, left) and were ejected without any problems. The laser erosion process provides variability as well in design of the structures as in the choice of the cavity material. Unfortunately, the erosion process was not optimized, so that the surface of the structures is rather rough. The molded part (Figure 3, right) reproduces these surface faults exactly. As the cavities do not show any damage after the injection molding trials, this process technology should be improved and is then very suitable for the production of micro cavities.

DEVELOPMENT OF A NEW MACHINE TECHNOLOGY In micro injection molding another task besides the injection molding of small parts (> 1 g) with microstructured details as described above has to be considered: the direct production of micro parts, i.e. parts with a part weight down to a milligram (mg). Until now there are no suitable injection molding machines available for the production of single micro parts, so injection molders produce big, but precise sprues to achieve the necessary shot weight.4-6 Figure 4 gives an example: The two parts shown are raytracing elements in the headlights of a Maerklin Mini Club railway engine. Made from polymethylmethacrylate (PMMA) both of them together have a part weight of 0.0335 g, but the shot weight including the gate is 0.5549 g, so that the weight of a single part is only about 3% of the total shot weight. The regrind of the sprue cannot be used for the same article, as the quality of the recycling material is not good Figure 4. Molded parts (0.0335 g), gate enough for raytracing. So 94% of the material are wasted. (0.5214 g) and PMMA granule (0.024 Considering costs of up to $60/kg of special material e.g. for g). medical applications, this waste can be an important cost factor. Figure 4 also illustrates the specific problems that come along with such small shot weights: The size of the granules used in standard injection molding limits the size of the plastification screws to 14 mm diameter minimum, i.e. that when the screw moves just 1 mm, about 0.185 g plastic material are injected. And even just one granule of PMMA weighs 0.024 g, which is more than one of the parts shown in Figure 4.

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Until now several machine builders offer modified standard machines with very small screws for the injection molding of small parts from 5 g to 0.5 g shot weight. One machine builder presents a suitable injection molding machine with integrated quality control and handling for micro parts (part weight down to mg, but - including the gate - higher shot weights about 0.01 g) on the Plastics and Rubber Fair K '98, Duesseldorf.6 Since several years the IKV is involved in the development of special plastification units for small shot weights. A combined screw plastification and plunger injection system was developed by IKV and Ferromatik Milacron Maschinenbau GmbH, Malterdingen, Germany and is sold with Ferromatik machines since 1994. This plastification unit has a preferred shot weight of 0.1 to 1 g.1 To open new dimensions in the size of minimum shot weight (< 0.01 g), IKV is now developing a micro injection molding machine that meets the molder's demands. These demands were defined by reference molders and by the experiences made during the process analyses with the different cavities as described above. The injection pressure varies between 150 and 600 bar. The cavity has to be evacuated to 0.5 bar to avoid burn marks and soiling. Mass temperatures go up to 400°C for some engineering plastics. The mold tempering (fluid tempering) varies between 60 and 180°C and an additional local heating of the cavity up to mass temperature has to be realised. So the components of the new machine concept have to consider the following demands: the plastic material must not melt in the material feeding, but in a small metering zone to avoid material degradation. The dosing has to be controlled properly without soaking in air. A homogeneous tempering of the plastification unit with a good thermal separation of nozzle and mold is important. For the injection of mg-shot weights the dosing has to be very exact and the material has to be injected fast and without leakage. All components should be dismantled and cleaned easily. In the following the important elements and functions of the new machine and the injection molding process are explained.7 Figure 5 shows nozzle [1] and mold (two plates [2] and [3]) in detail. The conical nozzle is tempered separately and well insulated [4] against the other machine components. The spree plate [3] is very narrow to keep the spree volume [5] as small as possible. The plate [2] on the movable platen side transmits the clamping force onto spree plate and nozzle. The process starts with the injection of the molten mass [6] into the cavity [7] using an injection plunger (see Figure 6). The plunger is driven by an electric motor. Nozzle and mold plates Figure 5. Nozzle and mold. are heated up to the temperature of the molten mass. After injec-

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tion, the holding pressure avoids shrinkage while the cavity is cooled and the plastic freezes. At the end of the holding phase, the nozzle is cooled down rapidly by the injection of liquid gas (CO2), so that the mass in the nozzle freezes at once and shrinks (see Figure 7a). At this moment new molFigure 6. Injection molding machine - start of process ten mass can be metered. Then the gate is ejected cycle. and teared off the molten mass in the cavity. Figure 7b explains the ejection of the molded part that is cooled down by a fluid tempering. To eject the part, the mold plates [2 and 3] have to be opened. During ejection the mold plate [3] is on the nozzle to heat up again and to hold back leaking melt. Then the mold closes and is heated up to the temperature of the molten mass. So the next production cycle can start. As the concept of the plastification and the injection unit is new and completely different from those available on the market, IKV has applied for a patent.7

Figure 7. Dosing, ejection of the gate (a) and of the molded part (b).

OUTLOOK In the next steps of the project, the machine components will be designed and built according to the described concept. A suitable mold technology has to be adapted following the IKV experiences in micro injection molding. An important aim is the reduction of the size of the mold to realize an effective and homogeneous tempering. The behavior of the new machine/mold system has to be tested. Investigations about the produced part quality will follow.

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ACKNOWLEDGEMENTS The investigations set out in this report received financial support by the Deutsche Forschungsgemeinschaft (DFG), to whom we extend our thanks.

REFERENCES 1 2 3 4 5 6 7

A. Rogalla: Analyse des Spritzgiessens mikrostrukturierter Bauteile aus Thermoplasten, Ph. D. Thesis, RWTH Aachen, 1998. W. Michaeli, A. Rogalla, A. Spennemann, C. Ziegmann: Mikrostrukturierte Formteile aus Kunststoffgestalten, F & M Feinwerktechnik, Mikrotechnik, Mikroelektronik, 106 (1998) 9, p. 642-645. M. Weck, S. Fischer: Ultraprazisionstechnik fur die Werkzeugbearbeitung, Froc. IKV-Seminar Innovative Produktionstechnologien fur das Spritzgiel3en von Klein- and Mikrostrukturbauteilen aus Kunststoff, Aachen, 1997. W. Gotz: Mikroteile in der halben Zykluszeit herstellen, Industrieanzeiger 18 (1998), p. 40-41. M. Kleinebrahm: Der Weg zum Mikrospritzgiessen, Proc. Micro Engineering, Stuttgart, 1998. C. Kukla, H. Loibl, H. Detter, W. Hannenheim: Mikrospritzgiessen - Ziele einer Projektpartnerschaft, Kunststoffe 88 (1998) 9, p. 1331-1336. W. Michaeli, A. Spennemann, B. Lindner, E. Koning, J. Zabold: Verfahren zum Spritzgiessen von Mikroformteilen aus thermoplastischen Kunststoffen mit einer geringen Angussmasse, applied for patent, 1998.

Simulation of the Micro Injection Molding Process

Oliver Kemmann and Lutz Weber Institut für Mikrotechnik Mainz GmbH, Germany Cécile Jeggy, Olivier Magotte, and François Dupret Université Catholique de Louvain, Belgium

INTRODUCTION

Figure 1. Micro motor, with gear box.

Figure 2. Optical switch.

Market analysis for microsystems1 show, that 40 billion US$ will be spent on micro devices mainly in the automotive and communication industries until the year 2002. Key products are acceleration and pressure sensors and increasingly components for the computer industry (read/write-heads for hard disks, flat display monitors etc.). Even if polymer parts have not taken over the market of these silicon based products, yet, they are already performing excellently in the fields of medical technology, biotechnology or passive plastic components for optical networks. Examples include micro motors and gears (Figure 1), optical switches (Figure 2), glucose and blood pressure sensors as well as components for minimal invasive surgery. A multibillion dollar market for microstructured parts with typical part-structure dimensions from several micrometers up to 100 µm can be expected. And injection molding is still the most common process for cost effective mass production even in the field of microstructured parts.

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Tools like software packages for the injection molding process are very common in industry, since the early eighties,2-4 because development time and cost must be decreased permanently. Filling, post-filling even shrinkage and warpage of plastic parts can therefore be calculated. All producers of plastic parts are using simulation tools to decrease the development costs, select proper machines and choose the right material from material data bases all in the sense of a cost effective mass production of plastic parts. Fast running 2½D codes provide excellent results with flat and thin, so-called standard injection molding parts. However, to conquer the very new market of micro injection molding it is extremely important to learn from the huge amount of experience in the field of standard injection molding and to provide the same tools. Therefore initial simulations with commercial software were run at the Institute for Microtechnology Mainz (IMM) to check their suitability for the microinjection molding process. The characteristic shapes and extremely small dimensions of micro parts as well as unique flow front shapes with different materials quickly reveal that a 2½D solution is not sufficient anymore to describe all the effects. Therefore, to describe the filling of micro structures a 3D transient code is under development at the Université Catholique de Louvain (UCL).5

DIFFERENCES BETWEEN STANDARD AND MICRO INJECTION MOLDING In general micro injection molding is the production of plastic parts with structure dimensions in the micron or sub-micron range. Micro structured mold inserts are produced with the help of ultra precision processes. These inserts are attached to standard molds as known from conventional injection molding. Especially the LIGA technology6 allows the production of metal mold inserts with structures in the micron or even sub micron range, e.g. by attaching many micro test structures to one base plate. To achieve proper filling of micro structured parts significant modifications to the standard process must be made. Due to the extremely low surface roughness of the mold cavity walls, demolding without a standard draft of 3° becomes feasible. However, during demolding any lateral offset has to be avoided. Otherwise it can be observed, that structures are ripped or sheared off the ground plate. To support the filling of small cavities, especially with a high aspect ratio (height against smallest lateral dimension) the so-called variothermal heating is used. In contrast to standard cooling, which keeps the mold temperature at a certain temperature below transition temperature, the surface of the mold insert is heated up with the help of an inductive heating7 almost to the melt temperature in order to gain a lower melt viscosity during filling. Compressed air causes problems, too. So the air in the mold must be evacuated by a vacuum pump. This is necessary to provide complete part fill-

Simulation of the Micro Injection Molding

Figure 3. Test structure molding, POM.

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Figure 4. Simulation result, C-MOLD filling.

ing as well as to prevent the “Diesel effect” where polymer is burned by the compressed, hot air at the bottom of the insert. The state of the art in micromolding allows the development of microstructured plastic parts with hardly any restriction in design. Suitable machines and molds are available. On the other hand, there is a lack of basic understanding of the flow behavior of plastics in microstructures. Hence, suited test structures as well as simulation tools must be developed.

INITIAL 2½D SIMULATIONS AND MOLDING TRIALS Almost three years ago, IMM started with initial simulations of the microstructure filling. Using the C-MOLD software,8 the filling of test structures (Figure 3)9 was simulated (Figure 4). These test structures were filled via a ground plate, while the two walls building a cross were 50 µm and 10 µm thick and 100 µm high. The material was POM. As observed from the performed short shots, the thinner wall was simultaneously filled from the ground plate and the thicker wall, thus was causing a weld line through the smaller wall structure. Another effect is, that the upper right corner is filled last. The calculation results show these effects sufficiently but, taking a closer look at the edges of the structure walls, one can see melt front effects, which are in the direction of flow, and cannot be predicted by usual 2½D software tools. To understand this, it is necessary to take a look at the simplifications made when describing the filling behavior with conventional codes. The usually flat and thin character of conventional injection molding parts allows to make these simplifications, which make the calculations fast and easy. The dimensional character of the micro structures is however not thin and flat anymore, and therefore does not fulfill the requirements needed to allow the simplifications used in standard simulation packages. Also visco-elastic- or surface-tension-

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effects are not taken into account since they are not important for standard parts.10 Even the meshing of micro structures with 2D mesh generators is difficult and mostly impossible. Modeling e.g. blind hole parts as they are typical for LIGA components reveal the problem of double generating volumes, since the real micro structure is much smaller than the stabilizing ground plate. So the volume around the connecting node is generated twice. A similar effect is known from standard parts as the “step effect” where a sudden change of wall thickness cannot be modeled since a mid-plane 2D mesh cannot describe asymmetrical wall thickness distributions. Beside these modeling and numerical simplifications which lead to incorrect prediction of the filling, the material (especially the rheological data used) is taken from data bases for macroscopic applications and scaled down to the sub-millimeter range. To provide more suitable data, especially for viscosity, and to be able to verify the simulation results, it is thus necessary to investigate the filling of microstructures directly, by using different materials. Therefore, a proper simulation of the micro injection molding process requires a 3D code and mesh generator as well as accurate data for micro range applications. In the following sections, a first approach to a simulation software suitable even for LIGA parts, which are right now the smallest molded parts, is developed while the injection molding tests are performed to provide the data needed to verify the results from the simulations.

3D SIMULATION APPROACH In the 3D model, two simulation scales, the ground plate and the micro-part scales, must be considered in order to predict numerically the injection molding of LIGA-produced microparts. Two meshes must thus be generated in order to simulate the filling process for such micro-part. First, the filling of the ground plate is predicted using a mesh, whose finite element scale is quite larger than the micro-part size. Hence, the number of nodal unknowns is not prohibitively high (it should be noted that conventional injection molding software could generally be used to perform this first simulation). Secondly, the filling of the microstructure is performed using a 3D mesh. Both the micro-structure and a reduced part of the ground plate are covered by this second mesh, in order to allow imposing boundary conditions obtained from the first simulation. However, the second simulation certainly cannot be performed using conventional injection molding software, since the second mesh clearly exhibits three-dimensional features. The objective of performing filling simulations is to predict the motion of the flow front(s) (which are true surfaces of arbitrary shape), together with the evolution of the unknown fields. For that purpose, the 3D time-dependant software uses two basic modules (viz. the so-called flow and geometrical solvers), which are processed using a decoupled algorithm.

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The flow solver is devoted to calculate the velocity, pressure and temperature fields using a generalized Newtonian model with a temperature and shear rate dependent viscosity. As this approach turns out to be not completely satisfactory for simulating the filling of truly 3D micro-parts, visco-elastic effects will be considered at a later stage. While classical finite elements are used for space discretization, Eulerian integration schemes are used for time integration. The geometrical solver is devoted to move the front(s), track front-front and front-wall meetings if any, and to create a finite element mesh covering the flow domain occupied by the fluid at every time step of the simulation. Moreover, an “extrapolation mesh” covering the region located between the fronts at successive times tn and tn+1 is generated in order to allow extrapolation of the fields calculated by the flow solver onto the new flow domain (this is required because Eulerian time integration is performed). A similar method was used11 in order to perform 2 ½ D molding simulations. The re-meshing algorithm is based on the Delaunay triangulation principles and implemented using the node-insertion scheme developed by George.12 Exact geometry algorithms are used in order to avoid the dramatic effect of round-off errors occurring in computational geometry procedures.

NEW MICRO INJECTION MOLDING TRIALS In order to provide sufficient material and flow data for the simulation of the micro injection molding process it is extremely necessary to investigate the flow behavior of various thermoplastic engineering polymers. Therefore, typically used flow spirals are replaced by newly designed structures based on the experience gained in former investigations at IMM. New inserts are derived from these guidelines to investigate the limits of part filling in micro injection molding. The trials are carried out with different engineering polymers with known excellent flow characteristics. Test structures with lateral dimensions between 2.5 µm and 20 µm (Figure 5) are mounted onto a ground plate and filled via a regular runner system. A film gate connects the ground plate with the runner system. With systematic injection molding tests the flow behavior of the polymer through the molded structures has been investigated. An injection molding machine (ARBURG Allrounder 370C 800 – 100) and the following materials have been used: polyoxymethylene (POM) as a standard material for micro injection molding and an unfilled polyphenylenesulfide (PPS)

Figure 5. LIGA test mold inserts.

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because of its extremely low viscosity at melting temperature. The final parts have been investigated with the help of a SEM by observing the degree as well as the quality of the filling. The POM parts (Figure 6, left) show very good filling behavior. Even the thin Figure 6. POM molding results. channels of the part are filled very well. Taking a closer look at the smallest ring (Figure 6, right) shows, that the demolding process is bending the thin structure. Compared to the POM parts, the PPS structures are less well replicated. Only the two outer circles are sufficiently filled (Figure 7, left). Taking a closer look at the melt front of the largest ring Figure 7. PPS molding results. (Figure 7, right) reveals a unique shape with a lot of small weld lines in it. Showing such a significant difference between the different parts, forces the need for a simulation software helping to foresee such behavior for future applications.

OUTLOOK With the increasing use of micro injection molding in micro fabrication, the need for a software tool providing important information during the design stage of the part development will increase, too. Therefore the ground has to be prepared to develop a specific simulation software for micro injection molding. Moreover, the results gained from injection molding tests with many different materials should be collected in a database, and software programmers will take all the differences between conventional and micro injection molding into account by using this data base. Within the rush into 3D software tools for the standard injection molding process, the special role of micro injection molding must not be forgotten.

ACKNOWLEDGMENTS The authors wish to thank the European Commission and the consortium of the BRITEEuram Project BRPR-CT97-0430.

REFERENCES 1

R. Wechsung, J. C. Eloy, Market Analysis for Microsystems – an interim report from NEXUS Task Force, Proc. EUROSENSORS XI, Warschau (1997).

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H.H. Chiang, C.A. Hieber and K.K. Wang, Polym. Eng. Sci., 31 (1991) 116;125. C.A. Hieber and S.F. Chen, J. Non-Newtonian Fluid Mech., 7 (1980) 1. K.K. Wang and V.W. Wang, in : A.I. Isayev (ed.), Injection and Compression Molding Fundamentals, Marcel Dekker, New York, 1987. C. Jeggy, O. Magotte and F. Dupret, Numerical simulation of the micro-injection moulding process , in: Proc. 2nd ESAFORM Conference on Material Forming, Guimarães, Portugal (April 1999), 117;120. W. Ehrfeld, D. Münchmeyer, Nucl. Inst. and Meth. in Phys. Research, A303, p. 523-531, (1991) C. Schaumburg, W. Ehrfeld, W. Schinköthe, Th. Walther. L. Weber, Microsystems Technology 98, Proceedings, Berlin, p. 679, (1998) C-Mold, User Manual, Advanced CAE Technology, Inc. Ithaca, (1997) M. Hörr, Diploma Thesis, FH Darmstadt and the Institute for Microtechnology Mainz GmbH, (1997) J. Zachert, Analysis and Simulation of Three-Dimensional Polymer-Flow in Injection Moulding, Aachen (1998) F. Dupret and al., Modelling and simulation of injection molding, in Advances in the Flow and Rheology of Non-Newtonian Fluids, D.A. Siginer, D. De Kee and R.P. Chhabra (ed.), Rheology Series, Elsevier, 1998. P.-L. George and H. Borouchaki, Triangulation de Delaunay et maillage, Hermes edition, (1997).

Chapter 4: Manufacuring of Composites Melt Compression Molding (MCM) a One-shot Process for In-mold Lamination and Compression Molding by Melt Strip Deposition Georg H. Kuhlmann Dieffenbacher UTW

PREFACE Since about 10 years processes for simultaneous moulding of carriers and decorative lamination (IML - in-mold lamination) are steadily replacing conventional methods. This development was primarily initiated by the automotive industry with the objective to be prepared for future trends such as: • growing demands for better and more comfortably appointed interiors of passenger cars and - to a lesser extent - of vans, busses, and trucks achievable e.g. by an increased application of textile coverstock and leather substitutes both preferably with a soft touch • the necessity of cost reduction i.e. by fewer manufacturing steps and less manual labour including finishing • more safety e.g. by application of materials with higher impact and without splinters or sharp rupture lines after accidents as well as the use of foam paddings • ecological concerns to be overcome by lamination without adhesives i.e. solvent matters and yet with better adhesion of the laminate, furthermore by composites suitable for recycling or uncritical incineration of waste or used parts. • preservation of fossil energy by reduced vehicle weights also easing the strain on traffic surfaces as well as by substitution of processes heavy on energy like GMT (Azdel) preperation and forming • a fair chance for agriculturally orientated economies replacing industrial fiber by regenerative fibers. The technologies described by the term "low pressure injection moulding" can substantially contribute to achieve these objectives. Meanwhile other industries i.e. not connected with the automotive industry e.g. furniture and packaging material manufactures are successfully applying the processes - a trend gaining forceful momentum by excellent results obtained by compression moulding of melt strips of long glass fiber reinforced thermoplastics (LFT) into technical, non-laminated parts.

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LOW PRESSURE INJECTION MOULDING TECHNICS

Figure 1. Low pressure injection molding process.

• •

Low pressure injection moulding technics have a lot in common to justify the definition. They are not a fundamentally new technology but the clever combination of known technical methods further developed and improved for the purpose. At present three technics are steadily gaining importance - a development with growing momentum since wider use reveals the outstanding capabilities of the processes. These innovative processes are: • backinjection including the injection/compression

melt flow compression moulding and backcompression by melt strip deposition for two applications i.e. in-mold lamination (IML) and compression forming of fiber reinforced thermoplastics (LFT). Low pressure injection moulding technics have a lot in common to justify the definition: • Predominantly hydraulic clamping units - vertical or horizontal - are applied, modified from clamping unit for conventional injection moulding. • Plastication occurs by means of a single screw extruder. • The melt is injected into a mould - closed or open - by a conventional injection unit adapted for high plastication and injection rates. • All low pressure injection moulding processes are capable of in-mould lamination (IML) of decorative coverstock. • Part forming is performed at low internal mould pressure originally not exceeding approx. 100 bar (i.e. 1450 psi) also established as the borderline for economical inmould lamination (IML) i.e. about the maximum sustained by coverstock materials. With the advent of LFT (long fiber reinforced thermoplastics) compression moulding internal mould pressures up to 200 bar (i.e. 2900 psi) are applied - an acceptable demarcation line between low and high pressure injection moulding. Generally speaking the internal mould pressure for the low pressure technologies amounts to 15 to 60% of high pressure applications. • Most development efforts are dedicated to the reduction and limitation of internal mould pressure during the forming cycle. These are areas influenced by the machines and the pertinent software e.g. - melt injection profiles - pressure build-up and compression speed profiles - clamping force decompression profiles - reduction of flow length

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by sequential gate valve actuation (cascade valve control) or variable melt strip deposition. Of course mould design is a decisive factor for the moulding success e.g. by dimensioning and location of the sprue gates, dimensioning of shear edges, flow aids, cooling and ejector technics, etc. This paper is primarily concentrating on technics based on the melt strip deposition. As there are many different terms for the various low pressure injection moulding technics, it appears to be useful to briefly identify the other methods.

BACKINJECTION From the many denominations "backinjection" seems to be the most descriptive and probably the most popular. The process is performed on conventional, mainly horizontal injection moulding machines or - in increasing numbers on special machines 3 4 with relative to the clamping force - large mould mounting areas and purpose built injection units with high injection rate and low injection pressure. The coverstock is inserted and located in an open mould - a shear edge mould permitting Figure 2. Principle of backinjection cycle. 1 - insert covdraw-in of the coverstock during the closing erstock, 2 - clamping and injection, 3 - cooling, 4 demolding. cycle to avoid wrinkles and damage by stretching of the fabric and yet flash-tight during the injection. In order to prevent weakening joint lines (also a potential source of wrinkles), melt penetration, destruction of any foam backs and/or special textile effects like piles, plush finish or leather grain embossing on foils injection occurs through carefully arranged gates with pneumatic needle shut-off nozzles which are actuated for injection in a specific sequence described as cascade control. Moulds for backinjection are quite sophisticated. Apart from a complicated hot runner system incorporating the shut-off nozzles with their pertinent drives also all other mould elements like ejector, core pulls and slides have to be accommodated in the injection side mould half. Ejectors etc. are not acceptable on the decorative side. A variant is the injection/compression cycle during which - sometimes after a preforming stroke for the coverstock - the carrier material is injected in a partially open mould. 1

2

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By closing the gap the part is formed and laminated. The mould corresponds to a backinjection mould. The method has similarities with melt flow compression moulding. By backinjection remarkable results can be achieved provided the limitations of the technology which are especially valid for larger parts are recognized e.g. • restricted influence on coverstock preservation - fabric or foil - without barrier back finish • back finish is required for a save process • significant effect on foam layers • no genuine soft touch • sensitive process because of risk of wrinkles or damage to the coverstock • rather complicated mould system which may be heavy on maintenance.

MELT FLOW COMPRESSION MOULDING Melt flow moulding for short (ignoring all the other names) is performed on vertical clamping units. The coverstock - perhaps preformed for deep parts - is inserted into an open mould. Then the mould is partially closed. The carrier stock is injected from below through a hot runner system and several generously dimensioned gates with pneumatically actuated needle shut-off nozzles. The melt available as cakes around the gates is compression formed into the part by closing the remaining mould gap. Shear edge moulds with hot runner systems similar to those for backinjection are applied. Mould cost especially for large parts is probably the only drawback for a wider acceptance of the process. Another may be a rather diffuse patent Figure 3. Principle of flow compression cycle. 1 - insert coverstock, 2 - partial closing and injection, 3 - compres- situation in some countries. sion forming and cooling, 4 - demolding. A melt flow compression moulding plant is similar to backcompression equipment. However normally fitted with one injection unit only which is permanently attached to the lateral inlet of the hot runner block i.e. motion axes as with backcompression equipment are not required. Backcompression machines are available equipped for melt flow compression moulding.

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BACKCOMPRESSION - MCM

Figure 4. Principle of MCM-IML cycle. 1 - melt strip deposition, 2 - insert coverstock, 3 - compression forming and cooling, 4 - demolding.

The term "backcompression" is quite well accepted for a process based on compression moulding of a melt strip deposited in an open mould. Backcompression describes the process during which a coverstock cutting is placed on a melt strip for simultaneous compression moulding and lamination (IML) of interior automotive parts. In recent months the application also based on melt strip deposition of mostly fiber reinforced thermoplastic stock (LFT) with subsequent compression moulding e.g. of nonlaminated structural car parts attracts growing attention. MCM, short for melt compression moulding, appears to be a more comprehensive name with the distinction IML for the previous backcompression process and LIFT for all long fiber reinforced applications.

MCM-IML A typical MCM-IML cycle is performed as follows • The cycle starts with an open mould in a vertical press. • A horizontal injector equipped with a deposition head moves into the mould depositing a melt strip in the lower mould half during the retraction movement (x-axis). • A flat or preformed decorative cutting is placed on the melt strip. • The press closes moulding the part by compression. • At the end of the cooling cycle the press opens for part demolding. Thermoplastics are used as carrier materials - predominantly PP unfilled or talc filled and to a lesser extent ABS, ABS/PC alloys, and PA. There is a vast variety of coverstock materials e.g. • woven and non-woven fabrics with various finish like pile and plush, and many colors including sensitive dark blue. Barrier layers are the exception even for fabrics as light as <200 g/m2 (i.e. <0.7 oz/ft2)

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foils (TPO, PVC) with an embossed surface are increasingly used with excellent preservation of the grain. The coverstock is normally placed on top of the melt strip in order to minimize the exposure time to the heat of the melt. With today's fast cycles coverstock insertion prior to the melt strip deposition is possible however excluded for materials with foamback or foils without back protection (barrier layer). Lower stock temperature, intensified mould cooling, and thinner walls have lead to significantly shorter cycle intervals. Lower temperature and decreased internal mould pressures more recently supported by ram decompression and a length controlled retraction cycle are protecting the coverstock most efficiently. With cycle intervals around 60 s MCM-IML is revealing its versatile capabilities and economical feasibility. Originally mainly intended for large area parts like complete door trims there is a rapidly increasing interest in applications for smaller parts e.g. B and C pillars, map holders, IP flaps, etc. Since the introduction of MCM-IML for industrial production less than 6 years ago the process has seen significant progress in application engineering as shown on the chart below. area of change previously today back protection ofsame as for depending on nature coverstock backinjection of material none or little moulding of assembly simple shapes only all shapes required fixtures including elements of 120 mm (4.7 in)of depth and with undercuts stock temperature (PP) at 230-240°C 180-190°C deposition die orifice (450-460°F) (360-375°F) internal mould pressure min. 60 bar 50 bar (870 psi) (725 psi) melt strip deposition speed < 100 mm/s > 300 mm/s (< 4 in/s) (> 12 in/s) mould temperature upper/ 40°/40-70°C 10-20°/20°C lower half (100°/100-160°F) (50-70°/70°F) wall thickness achievable 2.5 - 3.5 mm 1.8 - 2.0 mm (0.10-0.14 in) (0.07-0.08 in) typical cycle time with 120 (105) s 67 (55) s manual (robotic) insertion and demolding

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Multi-cavity moulds are common already e.g. 2 left + 2 right center console parts for BMW series 5 with pile carpet lamination i.e. 4 parts per cycle of 57 s or • 2 left + 2 right door trim inserts for BMW series 3 i.e. 4 parts per cycle of 60 s both with robotic coverstock placing and parts demolding. There are many valid reasons for the ready acceptance of the MCM-IML process by the industry like • numerous degrees of freedom of parts design • moulding of high/deep assembly fixtures including undercuts • elimination of 180° back wraps previously necessary for optical reasons • 90° back wraps also with recess • sensitive treatment of delicate coverstock e.g. fabrics with pile and plush finish, embossed foils, also material with foam back • genuine uniform soft touch also on large area parts • none or little protective backfinish of decorative coverstock • lamination in one heat (and cycle) i.e. IML resulting in • absence of adhesives i.e. of aggressive solvent matters • secure intimate durable bonding of lamination • low content of manual labour • low internal stresses because of the compression cycle • most rejects, waste, and used parts are recyclable or suitable for incineration i.e. no waste for classified disposal • lower cost of investment as with conventional process • uniform industrially reproducible high quality In fact there are few compelling reasons advising against a wide use of the MCM-IML process. Such reasons are • parts with areas too small for controlled deposition of the melt strip • significant undercuts at the fringes of the laminated area and • unavailability of space - even after tilting of the core - to securely deposit the melt strip. It is appropriate to say that a comparison of cost i.e. for investment and manufacturing of parts produced by conventional high pressure moulding with subsequent press lamination to parts produced by the integrated MCM-IML method will result in cost savings up to 40%. On average a cost reduction of 20 to 25% is a realistic assumption. •

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MCM-LFT The request from within the car component industry to develop the MCM-LFT process is motivated by the intention to replace glass mat reinforced thermoplastics (GMT or Azdel) and to an extent SMC wherever possible for reasons like • cost reduction (material and processing) • simplification of process • reduction of thermal stress on the PP polymer • lower cost of investment • floor space requirements • more flexibility in regard to material analysis and characteristics etc. At present there are two methods which have matured to continuous production application. The processing of long glass fiber reinforced granules (LFG) which come as little rods and are readily available in Europe in lengths from > 10 mm (0.4 in) to 25 mm (1 in) from sources like (in alphabetical order) • Appryl (Elf-Atochem) of France, brand name "Pryltex" • Borealis of Finland, brand name "Nepol" • DSM of the Netherlands, brand name "Stamylan" and • Ticona (Hoechst) of Germany, brand name "Compel" The pertinent equipment will also process recycled GMT (Azdel) material or blends with LFG. Parts already produced are bumper carriers, undercovers, and battery holders. Instrument panel carriers are under development. In-line compounding and subsequent forming of LFT starting from roving. The process runs through the following stages: • Roving filaments are unwound from bobbins under rupture control and guided through tubes to a preheating station. • The rovings are pulled through a pultrusion head fed by a single screw extruder with PP melt for roving impregnation. • The impregnated roving is taken in by screws of a corotating twinscrew extruder at a point where the main matrix is already molten. • Fiber length is determined by screw rotation. • The LFT melt is extruded into preforms to be placed in a compression mould or discharged into an injection unit. Recycled material (GMT-Azdel, LFT) may be added by means of an additional single screw extruder at the end of the compounding phase. Fiber lengths are varying however come with a length concentration around 10 mm (0.4 in) to 80 mm (3.1 in).

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The process is successfully applied for the production of VW Passat frontends since a longer period of time. A typical MCM-LFT cycle is identical to the MCM-IML cycle exclusive of the coverstock placing phase. The stock temperature at the orifice of the deposition die will be around 240°C to 260°C (- 460° to 500°F). However the cooling time aided by the better heat conductivity of glass fiber and the absence of coverstock will be about the same as with the MCM-IML technique. Based on a wall thickness of 2 to 3 mm (- 0.08 to 0.12 in) a total cycle time of 60 sec by manual demolding and 45 sec by robotic demolding can be achieved. Early demolding with subsequent postcooling will shorten the cycle and help to control internal stresses. The internal mould pressure required may vary from 80 bar (- 116 psi) to approx. 150 bar (- 2180 psi) depending on the structure of the part, fiber lengths, and fiber content and hence the flowing properties of the stock. For the MCM-LFT process most development and application efforts are directed to the preservation of the original fiber lengths. Glass fiber appears to be mainly affected by pressure resulting in immediate contact of the fibers. Detrimental pressure may result from pressure development in the final flights and at the tip of plastication screws i.e. backpressure and/or by narrowing of the melt passage for extrusion or injection. Adequate conditions during extrusion are provided for LFG (pellet) processing by preparing the melt on a single screw extruder with the following characteristics: • low compression, low friction screw of 30 D length, screw diameter min 90 mm (3.5 in) preferably 130 mm (5.1 in) with the low rotation speed of max 30 rpm • deep cut, thermolator controlled intake zone with tangential intake pocket • external heating by heater collars on the barrel supported by a screw core temperature control • axially movable screw for back pressure control and melt ejection. The above extruder is also capable of processing recycled material from GMT-Azdel or LFG. Due to the most careful plastication conditions output capacities are in the range of 160 kg/h (350 lb/h) for screw diameter 90 mm (3.5 in) to 240 kg/h (530 lb/h) for a 130 mm (5.1 in) screw. Extruders with larger screws are available. Twin screw extruders with corotating screw are designed for high mixing capacities at low stock pressure i.e. they are high efficiency compounding extruders. They are not suitable to process LFG or recycled materials.

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MCM EQUIPMENT In principle the main units of a MCM-IML plant are identical to those for MCM-LFT, adapted only for specific application engineering requirements.

Figure 5. Schematic drawing of MCM-plant. 1. clamping unit, 2. injection unit, 3. melt strip deposition die, 4. hydraulic systems, 5. electrical and electronic control units.

CLAMPING UNIT MCM clamping units are designed as hydraulic vertical 4 column presses. As cycles start with an open mould for subsequent compression moulding special emphasis is laid on a mechanically sturdy design including platens with low deflection, generously dimensioned columns with extent and precise guidings of the upper moving platen. The guidings are about 2.5 times the length compared to conventional clamping units plus additional stiffening elements. During MCM-IML and most MCM-LFT applications none or moderate tilting momenta occur which will be absorbed by the mechanical design features. In rare cases i.e. involving parts with a complicated structure, melt with poor flowing properties, etc. requiring higher internal mould pressures along with critical tilting momenta a frame press with closed loop platen parallelism control may be necessary. MCM clamping units are available with two hydraulic systems (reference: 8000 kN i.e. 880 US-t):

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Hydraulic system based on variable displacement pumps with the following data relevant to the process closing speed mm/s 500 in/s 19.7 opening speed mm/s 330 in/s 13.0 compression speed mm/s 25 in/s 1.0 pressure build-up s <2 s <2 This specification is adequate for most MCM-IML applications. Whereas closing and opening speeds have marginal influence on the performance only, pressure build-up and hence compression speed may have decisive bearing on the moulding quality because of the instantaneous flow in a cold mould and under high friction. Therefore an accumulator assisted hydraulic system is advisable for certain MCM-IML applications and is essential for most MCM-LFT applications. The pertinent critical data for a Accumulator hydraulic system are closing speed mm/s 800 in/sec 31.5 opening speed mm/s 800 in/sec 31.5 compression speed mm/s =50 in/sec =2 pressure build-up s 0.9 sec 0.9 Clamping units with clamping forces from 4000 kN (440 US-t) to 15 000 kN (1650 US-t) are available. At present units of 8000 kN (880 US-t) and 10 000 kN (1100 US-t) appear to be preferred. Platens are still mostly dimensioned to customer's needs. However platens with a mould mounting area (x by y) of 1400x1900 mm (i.e. 55x75 in) are much in demand as being suitable to accommodate two cavity moulds for standard left/right door trims. The clamping units do not require pits. INJECTION UNIT MCM injection units follow the design principles of conventional injection unit modified for increased plastication capacity and high injection rates at moderate injection pressures. For MCM-IML plants the following frame work of data are an average specification screw diameter mm 70 - 110 in 2.8 - 4.3 screw length /' 20 - 24 /' 20 - 24 rpm 140 - 170 screw rotation speed min-1 140 - 170 3 3 in /s 3.3 - 6.7 plastication capacity cm /s 54 - 110 injection pressure bar = 700 psi =10 150 in3/s 42.7 - 97.6 injection rate cm3/s 700 - 1600 1100- 4100 in3 67.1 - 250.2 injection volume cm3

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For MCM-LFT the corresponding data are screw diameter mm 90 - 160 in 3.5 - 6.3 screw length /' 28 - 30 /' 28 - 30 rpm < 30 screw rotation speed min-1 < 30 in3/sec 2.4 - 4.3 plastication capacity cm3/s 40 - 70 injection pressure bar = 400 psi =5800 3 3 in /sec 43.9 - 128.2 injection rate cm /s 720 - 2100 1700 - 8000 m3 103.7 - 488.2 injection volume cm3 Screw core temperature control and pellet preheating devices will increase the plastication capacity. For the left and right part feature during one cycle MCM-IML injection unit are usually equipped with 2 injection units. For the melt deposition movements the injection units are mounted on motion axes i.e. x-axis (horizontal motion parallel with screw axis) driven by servo valve controlled hydraulic cylinder speed max. mm/s 1500 in/s 59.1 y-axis (horizontal motion perpendicular to screw axis) driven by ball screw spindle with servo AC drive speed max. mm/s 500 in/s 19.7 z-axis (vertical motion perpendicular to screw axis) driven by ball screw spindle with servo AC drive speed max. mm/s 200 in/s 7.9 In most cases the z-axis is executed for mold height adjustment only driven by synchronized lifting spindles with AC motor speed max. mm/s 10 in/s 0.4 Strokes of axes are defined by the platen size or mould mounting area respectively if 2 injection units are provided x- and y-axes move independently. User demands for larger injection volume and LFT capabilities are calling for larger i.e. heavier injection unit. This comes along with the request for faster movements of the axes. This has lead to new developments separating the plastication function from injection. The plastication unit is a stationary single screw extruder purpose built for the specific task i.e. thermoplastic or LFT processing. Size and weight is no restricting factor. The screw is still axially movable for fast melt transfer into the injectors. Melt strip deposition is performed by one or two separate plunger injectors which are mounted on individual 3-axes coordinate table. As the weight of the injector is only about 15% of an average size integrated plastication/injection unit fast dynamic movements by

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Figure 6. MCM-plant with stationary extruder and 2 injection units (side view-rear view). 1 - stationary extruder, 2 - plunger injector, 3 - axes coordinate table, 4 - melt strip deposition die, 5 - hydraulic station, 6 - control cabinet.

Figure 7. MCM-plant with stationary extruder and 2 injection units (top view). 1 - stationary extruder, 2 plunger injector, 3 - axes coordinate table, 4 - melt strip deposition die, 5 - hydraulic station, 6 - control cabinet.

servo controlled rack and pinion drives become reality e.g. x-axis 1800 mm/s i.e. 71.0 in/s y-axis 1000 mm/s i.e. 39.4 in/s z-axis 500 mm/s i.e. 19.7 in/s speeds which will help to shorten cycle intervalls. The drive elements applied permit high accuracy of positioning of the deposition die in the mould (± 0.2 mm i.e. ± 0.008 in). Plunger injection units yield high shot precision and consistency. Melt transfer occurs under first-in-first-out conditions. During melt transfer from extruder to injection chamber the plunger is retracted in a controlled movement

for accurate dosing of the shot size. As there is no friction from a full screw as with screw injection a rather low injection pressure is applicable i.e. < 250 bar i.e. < 3625 psi - conditions especially favorable for LFT processing.

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MELT STRIP DEPOSITION DIE Melt strip deposition dies are basically flat extrusion dies. They consist of two mould halves (sandwich design) with a melt feeding channel followed by a coat hanger type computer optimized distribution channel. The standard version has a fixed lip gap and working width. The actual strip dimensioning is achieved by interchangeable orifice attachments mounted on the die body by a quick mounting device. In order to obtain a short distance between the orifice and the deposition surface of the mould the feeding channel is arranged in the injector side mould half i.e. deflected by 90° to permit a perpendicular position of the die in relation to the mould. Lips and die body are heated. The melt strip cutter consists of a cutting blade horizontally sliding over the orifice by a pneumatic drive. To prevent heat build-up the blade stays in the retracted position between the cutting cycle. A hydraulic rotary shut-off valve in the adaptor between injector and die retains the melt under pressure before injection thus preventing melt leakage from the die. The importance of reduction of flow lengths was discussed earlier. Deposition of melt strips close to the fringes of the cavity is a proven method. Deposition dies with a continuous width control will contribute to this target. A patented device attached to a deposition die as described above is equipped with a number of narrow cutting blades instead of one only. Each blade is actuated by an individual pneumatic drive. The cutting and retracting pattern is freely programmable interfacing with injection rate to account for the changing cross-section of the melt strip. The device also allows to spare islands. For melt accumulation in certain part areas a most reactive program to increase the injection rate i.e. to accelerate screw or piston by a speed profile is available. This feature can be further supported by a speed profile of the x-axis. This unit has been successfully used to produce door trims without insert i.e. some kind of a frame. Fig. 8 shows a MCM plant now being used in continuous production. The plant includes a coverstock loading and demolding robot. The plant represents the present state of the art as described above.

OUTLOOK The future will see developments in the following areas: 1. refinement of the MCM-IML backcompression process to the efficiency totally comparable to the basic conventional high pressure injection moulding process in regard to quality, productivity, including automation 2. replacement of the GMT-Azdel process by MCM-LFT for most application

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3. introduction of natural fiber to partly substitute glass fiber and for improvement of parts not yet reinforced and 4. replacement of some SMC applications by using the melt strip deposition methods with new BMC type materials. MCM will be an important process of the future.

Figure 8. MCM equipment layout.

In-mold Lamination Back Compression Molding

Thomas Huber Swiss Impulse, Inc., the partnership of Georg Kaufmann AG, Switzerland and Delta Tooling, Auburn Hills, MI, USA

INTRODUCTION During the process the coverstock is mechanically fused to the substrate through the influence of heat. Peeling the coverstock off the substrate is no longer possible unless a weak layer (e.g. foam) can be split. The application has the following advantages: • no gluing or welding required • no special plant ventilation necessary • no solvent recovery necessary • no surface preparation needed • no tooling cost for blank substrate • no tooling cost for fixtures, vacuum forms, etc. • no storage of blank substrates • no fogging from interior car parts • exceeds all standard peel and pull tests • lower overall price per piece (up to 30% savings possible) • if coverstock and substrate are from the same family of plastic (e.g. PP) the part can be recycled The application has the following disadvantages: • small parts cannot be properly filled by the melt strip • difficult tooling for significant undercuts at the edge of the parts • vertical press and special plastifier unit needed The application has one big challenge: • coverstock

BACKGROUND Conventional compression molding has been practiced for years. Best known application is sheet compression and GMT (Glass Mat Technology). In the GMT process heated squares

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of material or sheets are formed into shape with a mold installed in a fast closing, powerful vertical press. Back compression molding is also related to back injection molding, also known as low pressure or reversed molding. However, pressure (up to 80/100 bars at the main nozzle) and high temperatures (220oC) are still needed to fill the mold. The coverstock is placed into the mold before closing. Especially at the nozzles the coverstock is fully exposed during the filling of the mold and has to withstand local elevated pressure and heat. Good mold makers can alleviate some of these challenges, but for larger parts there is no alternative to a specialized hot runner system and controlled valve gating.1 Production and demand for back compression molded parts are rapidly growing in Europe. We estimate that approximately 40,000 to 75,000 parts are formed in this fashion every day. In the US, Ford Motor Company is producing almost all the door panels in a similar process which, however, requires a specially designed three layer vinyl coverstock. At least a dozen US companies are experimenting with IML or are already in production.

PROCESS PARAMETERS In back compression molding the clamping pressure can be significantly reduced, which allows for presses with large platen and reasonably sized hydraulic units. Clamping force for back compression molding is 5,000 to 7,000 kN for each m2 of the projected cavity surface.2 There are no good measurements available, but it is estimated that the actual local pressure interacting with the compressed coverstock is only a fraction of the numbers above and certainly far less than the local peak pressure seen at the drops during the filling and compacting of the mold with a hot runner. The preferred clamping speed should be on the slow side to achieve an even “rollout” of the melt strip, also avoiding abrupt forces on the coverstock. CAVITY The molten plastic (melt strip) is placed DECOR into the open mold (at zero pressure), onto the NOZZLE COOLING core side through a wide nozzle, or it can be HYDRAULIC CYLINDER perked up through distribution channels (hot runner) into the open or semi-closed mold. The EJECTOR PLATE polypropylene melt strip has a temperature of 180-190oC. That is a 15% to 20% reduction in Figure 1.

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energy, when compared to injection molding. As a result, we have less internal stress, faster cooling cycles, and considerable energy savings (Figure 1). The coverstock is either placed manually or by a robot on top of the molten plastic or hung in a frame attached to the moving cavity side. There is ample time (approximately 20 seconds) to place the coverstock and close the press after the extrusion head is retracted.

PROCESS EQUIPMENT Several machine manufacturers are offering equipment for the process, including a double nozzle for filling simultaneously two cavity molds. Most often used is a 8,000 kN vertical press. However, larger units are already on order from various parts’ producers. The injection unit (plastifier) can be moved along 2 or 3 axis. Special wide nozzle dies with a cut-off device are needed. Such dies have heated and adjustable lips with an opening range of 3 to 10 mm.

APPLICATIONS Testing of the parts by the European automobile and furniture manufacturers produced excellent results: • Lower internal stress in the substrate, resulting in less warping. BMW and Mercedes are now installing most of these parts in the luggage compartments of their station wagons. Less warping also means that these parts do not need an additional trim to cover up problems, which might develop later on. • Quality and price lead to the decision for Mercedes to use the process for all the different doors in the new A-Class van. • Robust appearance, easier installation, and price convinced a large US truck manufacturer to retool for IML parts. • Many flat parts (door inserts, console inserts, side wall panels) are excellent applications for IML with back compression molding. • Economical coverstock (felt) for trunk parts e.g. access doors and floor sections can be used in connection with a high content of recycled material.

ADVANTAGES The following advantages can be listed for the back compression molding process: • less thermal exposure for the coverstock • no peak pressure, when compared with the pressure generated at the nozzles of injection molding

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• • • • • • • • • •

uniform wall thickness not a must any longer especially suited for carpet and foam backed cover-stock (vinyl, fabric) excellent results with larger parts (rear door claddings measuring 2 by 5 feet across) tooling cost comparable to a conventional injection mold for the blank substrate no hot runner system, no valve gating, no heating elements adjustable filling (fill location and amount) for best results less tool maintenance in the substrates a high content of recycled materials can be used with proper tooling a high degree of detailing in back of substrate possible and last but not least: 20% to 30% reduction in piece price One of the European automobile executives is quoted: “Presently we see no other technology that can achieve the same quality for a lower price!”

MOLD DESIGN It is impossible to list or discuss all the design features of in-mold laminating tools. The tooling is less complicated and less crowded, since the entire hot runner system does not exist. Complex detailing is often requested by the designers, especially in the trunk section of the car. Walls, clips, hooks, “dog houses”, etc. are designed into the back of the substrate. Such IML parts can double as holders of first aid kits, carriers for lifting jacks, or other tools. The line of draw is very important, since only the movement and force of the press is filling the last corner of the mold. These forces have to be properly vectored, otherwise, bonding between coverstock and the substrate is put in jeopardy. With IML the pressure and wear and tear on the diving edges3 are greater than in back injection molding. Forming the part and closing the mold occur during the same time. Special guide plates are necessary to equalize the forces. The ejector plate is hydraulically operated and has to be fully retracted prior to deposit the melt strip onto the mold. Cooling is very important to minimize internal stress and discoloration of the coverstock. During the cooling cycle the coverstock acts as an insulation, and an asymmetric heat profile will occur in the wallstock. The distribution and layout of the cooling channels is very important (Figure 2). Conventional design experiences and computer software are not applicable. There is no mold flow software available for this technology. Instinct and experience with IML molding is important. Every part is unique in its demand. Collected design criteria for back injection molding can only serve as a guideline. Mold design for back compression molding has become an art again. Common sense and experience with the application is important.

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CAVITY

191

SIDE

Figure

Core Side Figure 2.

PART DESIGN

In-mold lamination is only going to be successful if the designers and stylists of the car companies and tier one suppliers can be convinced to take full advantage of the process. In Europe many designers have been visiting GK for special design courses and discussions for the best solutions and compromises. It is important that these discussions are taking place as early as possible during the design phase. The crucial points remain the same: Raw Edges and 180° Folds (Figure 3) Figure 4. IML is producing a strong bond between coverstock and substrate all the way to the edge. There is no danger that the coverstock will peel off at the edge! IML parts (without hand-wrap) are possible for even the most discriminating taste, for appearanceand rattle free performance. Wall Thickness, Ratios to Ribs (Figure 4) In general the same rules apply as for normal injection molding. However, a ratio of 1 to 1 without any sink marks is not uncommon. Walls up to a depths of 120 mm are possible without filling problems. It becomespossible to design parts with different wall thickness. The drawings shows a back rest for an office chair.4 Ribs are needed for structural stiffness. Different Coverstock Materials (Figure 5) The practical range of different coverstock applications is restricted by the diving edges of the mold. It is therefore important to know what coverstock material will be used.

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OFFICE CHAIR BACKREST

However, it is conceivable that different mold halves are used for different production. Such halves could also be designed for parts which will be flocked or covered with leather. Toady's CAD/CAM systems allow extreme close tolerances, which make interchangeable halves possible.

COVERSTOCK Figure 5.

The selection of the proper coverstock is impor-

tant. The specifications from the car manufacturers usually do not reflect in-mold lamination material. As a mold maker we are not an expert in textile and vinyl. We recommend, however, that every molder is lobbying with the coverstock suppliers for close cooperation regarding IML: .all materials have to have a stretch factor, which should be omni directional .if the material is not dense enough or sensitive to heat, a protective layer is needed (e.g. fabric or fleece) which is laminated to the back of the coverstock .colors have to withstand fading while exposed to the heat during the molding process ."stiff' coverstock combined with curved parts most likely have to be preformed .the back, or back layer, of the coverstock has to be compatible with plastic used for forming the substrate in order to achieve proper bonding .thin film (e.g. on exterior car parts) has to be extremely flexible, not showing any stretch marks when elongated CONCLUSIONS IML back compression molding already has a proven track record in Europe and has established itself as an economical solution to produce a variety of interior car parts. As more vertical pressesand plastifier units are placed into production the technology will also find its way into other applications, like the furniture industry.

REFERENCES Plast Europe 4/94, Vol 84, G. Bagusche, Unitemp SA, Switzerland. Example: A one cavity door panel projection is 0.5 m2. Necessary clamping force (or press size) is max. 3,500 kN, or in other words, you need a press size of approx. 400 US tons. Diving edge is the specially designed parting line to accommodate the coverstock without leaving any gaps for flashing. Plast Europe 7/97, Volume 87; M. Zwetz, Waldshut, Germany.

Analysis and Characterization of Flow Channels during Manufacturing of Composites by Resin Transfer Molding

R. V. Mohan, K. K. Tamma, S. Bickerton, S. G. Advani and D. R. Shires Processes and Properties Branch, Materials Division, U. S. Army Research Laboratory, Aberdeen Proving Grounds, MD 21005, USA

INTRODUCTION The success of Liquid Composite Molding (LCM) processes such as Resin Transfer Molding (RTM) depends on the complete impregnation of the fibrous reinforcing preform. Variation in volume fraction around bends and corners creates gaps around the fiber preform within the mold cavity. The deformation of the fiber mat in the preforming stage,1-3 caused by bending, shearing and stretching, creates variations in permeabilities and reductions in thickness in the sheared preform regions. These physical variations of the preform during preform lay-up in the mold create regions of higher porosity, hence increasing the local permeability. Such regions offer the injected resin paths of least resistance, significantly altering the shape of the resin flow front, and the injection and mold pressures. This phenomenon is called race tracking4,5 and often leads to dry spot formation and other defects. While race tracking has traditionally been viewed as an undesired effect in RTM, related processes such as Vacuum Assisted RTM (VARTM) have employed race tracking effects to improve and facilitate mold filling. Race tracking may be purposely created within a mold through the use of “flow channels” or “runners” to improve the flow distribution, and direct the resin to areas that may be difficult to fill successfully. Pre-determined flow channels may also reduce the time required to fill the mold under constant pressure injection, and reduce the injection and mold pressures in constant flow rate injection. The beneficial effects of flow channels have not been explored fully in the context of RTM, even though the potential exists. Process simulation studies demonstrating the effect of flow channels on the flow front progression and pressure histories in RTM structural components relevant to army applications are presented by Mohan et al.6 A clear understanding of the effects of mold filling due to race tracking caused by channels is needed to fully explore the benefits obtainable through the use of flow channels.

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Manufacturing process simulations under the paradigm of Virtual Manufacturing have been successfully employed for RTM processes. Various manufacturing process modeling simulation tools analyzing the progression of the resin inside a mold cavity are currently available and have been applied successfully to various net shape parts manufactured by RTM.7-10 Race tracking effects due to channels can be accounted for in mold filling simulations by applying constant values of higher permeability within regions of a mold representing the flow channels.11-13 The process simulation predictions depend on the models employed to predict the permeabilities applied to the flow channels. Experimental studies involving the flow visualization in transient molds as well as investigation of flow characteristics using Laser Doppler Anemometry (LDA) techniques to experimentally measure the velocity profiles of the fully developed race tracking flow have been conducted earlier11,14,15 to study the race tracking phenomenon in liquid composites molding. These studies correlate and justify the use of equivalent permeability models with the experimental observations.11 Experimental investigations studying flow behavior due to the presence of gaps along the edges of a planar rectangular mold cavity have been conducted and correlated with simulations. These studies involved the application of equivalent permeabilities to the air gaps present within the mold, justifying their use in process simulations.12 The present work experimentally investigates mold filling in the presence of flow channels within a simple mold configuration. Investigations involve different sized square channels on the face of a simple rectangular plate mold cavity. Experimental flow front and pressure measurements (injection and mold pressures) are employed to analyze and characterize the flow through channels and determine the optimal equivalent permeability model for the flow channels. Experimental observations based on transient flow fronts and transient pressures are compared with simulations based on various equivalent permeability models which are employed in simulations. The equivalent permeability models provide a good effective process design tool to understand the flow and pressure behavior when flow channels are involved and provide a simulation tool to ensure successful impregnation of fiber preforms. The experimental data and numerical simulations presented, demonstrate and validate the effect of flow channels in reducing the injection and mold pressures and redistributing the flow.

EXPERIMENTAL STUDY: EFFECT OF FLOW CHANNELS Experiments to study and understand the effects of flow channels on flow front progression and pressure histories were completed using a simple two piece mold, the male and female pieces constructed from 0.0254 meters (1 inch) clear acrylic plate. The mold cavity is a planar square 0.3556 by 0.3556 meters (14 by 14 inches), having a thickness of approximately 0.003175 meters (1/8 of an inch). There is a single injection port at the center of the plate,

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ALL DIMENSIONS IN INCHES

ALL DIMENSIONS IN INCHES

(a) Flat plate part - no channels

(a) Male mold piece with grooves

(b) Experimental setup (b) Flat plate part - with channels

Figure 2. Schematic of male mold piece and experimental setup.

while a hole of 0.0127 meters (1/2 inch) diameter is cut in the center of each preform. The cut in the preform eliminates the need for the fluid to flow through the thickness of the preform at the injection site. Pressure data was collected using pressure transducers screwed into the female portion. The pressure transducers were located at the injection port, 2 inches and 3 inches away from the injection ports along the central symmetric lines. To study the effect of channels, grooves were cut on the face of the male mold plate. The injection port is at the center of the male piece. The flow channels have square cross-sections and two different sized channels were employed. The cross-sectional dimensions of these channels are 0.003175 x 0.003175 meters (1/8 x 1/8 inches) and 0.001984 x 0.001984 meters (5/64 x 5/6 inches). The geometry of the part with and without channels is shown in Figure 1. The male piece with the channels cut is shown in Figure 2 (a). With the injection at the center of the plate, these channels create a preferential path for the resin flow.

Figure l. Flat plate mold geometry.

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Experiments were completed with the constant flowrate injection system represented schematically in Figure 2 (b). The transparent mold permitted the fluid flow front to be visible throughout the experiment and recorded to video tape. A video camera was placed directly above the mold capturing a plan view of the flow front progression. The experimental system consists of a control mechanism which continually monitors the pressures drops and adjusts the pressure applied to the pressure pot to give a constant flowrate injection.16 The experimental system permits continuous measurement of fluid flowrate into the mold and pressure just prior to the injection port. The pressure transducers attached to the female mold piece measured pressures directly within the mold. Due to the corrosive nature, and the cleaning challenges provided by the actual resin systems, a substitute fluid was used. Since we are concerned only with the mold filling stage of RTM, curing need not be considered. The fluid chosen for the flow visualization studies was a mixture of corn syrup, water and clothing dye. The corn syrup mixture is Newtonian and the viscosity is easily characterized. Several experiments were conducted to study the effect of channels on flow front progression and mold pressure histories. Two different styles of preform were studied, continuous strand random fiberglass mat, and a continuous strand stitched fiberglass mat, with tows running in the 0, +45, and -45 degree directions. Two volume fractions were studied for each material, 19% and 26% for the random mat, and 44% and 51% for the stitched mat respectively. Each fiber preform configuration involved three sets of experiments, one with no channels in the male mold and the two experiments with the two different sized channels. The viscosity and flowrate were maintained to be nearly equal within each set of experiments. The resulting cavity thicknesses for each experiment were measured, as this is an important parameter provided to the numerical simulations presented later. Due to varying amounts of flexure of the acrylic mold pieces, the nominal cavity thickness of 0.003175 meters (1/8 of an inch) could not be assumed. Digitized video frames from several experiments are presented in Figures 3 and 4. Figure 3 presents frames captured at 4, 10, and 21 seconds into the 19% volume fraction random mat experiments. Frames are shown from each of the three separate experiments. Figure 4 presents frames captured at 15, 37, and 49 seconds into the 44% volume fraction stitched mat experiments. The effect of the channels on the flow distribution is clearly seen from these video images. These figures also clearly demonstrate the increased deformation of the flow front as the channel sizes are increased. Figure 5 compares the three injection pressures and pressures at the 2 inch transducer location obtained from the random mat experiments presented in Figure 3. Figure 6 compares the three injection pressures and pressures at the 3 inch transducer location obtained from the stitched mat experiments presented in Figure 4. The pressure histories shown here

Analysis and Characterization of Flow Channels

4 Seconds

10 Seconds

21 Seconds

Figure 3. Video frames of flow fronts - Random mat; Top row (No channels); Middle row (5/64 inch channels); Bottom row (1/8 inch channels).

197

15 Seconds

17 Seconds

49 Seconds

Figure 4. Video frames of flow fronts - Stitched mat; Top row (No channels); Middle row (5/64 inch channels); Bottom row (1/8 inch channels).

clearly illustrate the reduction in injection and mold pressure due to the presence of the channel. This reduction could be used effectively in making large composite structures with low pressure RTM.

FLOW MODELING The flow modeling simulations for RTM are based on modeling the resin flow through a fiber preform as a pressure driven flow through a porous medium characterized by Darcy's law.17 Darcy's law relates the velocity field to the pressure gradient, through the fluid viscosity and the fiber preform permeability. Permeability is a measure of the resistance a fluid experiences when flowing through a porous media and is an important material characteristic involved in flow simulations. The mold filling simulations involve solving both the pressures and tracking of the flow fronts which can be multiple as well as converging and diverging inside a mold cavity, modeled by a Eulerian numerical mesh geometry. Mold filling simulations based on finite element-control volume approaches4,18,19,20 are based on solving the transient mold filling problem as a quasi-steady state problem along with an explicit time advancement scheme for advancing the flow front and tracking the filled regions.

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90000 70000

30000

60000 50000 40000 30000

25000 20000 15000

20000

10000

10000

5000

0

0

5

10

15

20

Time, s

No Channels 1/8 inch channels 5/64 inch channels

35000

Pressure, Pa

Pressure, Pa

40000

No Channels 1/8 inch channels 5/64 inch channels

80000

25

30

0

35

0

(a) Injection pressures

25000 20000 15000 10000

40

50

60

70

60

70

8000 6000 4000 2000

5000 0

30

Time, s

No channels 1/8 inch channels 5/64 inch channels

10000

Pressure, Pa

Pressure, Pa

12000

No channels 1/8 inch channels 5/64 inch channels

30000

20

(a) Injection pressures

40000 35000

10

0

5

10

15

20

Time, s

25

30

35

(b) Pressures at 2 inch transducer location Figure 5. Effect of channels on pressures : ra ndom mat experimental compar isons.

0

0

10

20

30

40

Time, s

50

(b) Pressures at 3 inch transducer location Figure 6. Effect of channels on pressures stitched mat experimental compar isons.

The flow modeling simulations utilized in this paper for tracking the flow and pressures inside a closed mold cavity ar e based on a pure finite element based methodology for RTM simulations developed by Mohan et al.7 These computational developments for the RTM flows ar e based on a transient mass balance equation for the resin mass while employing Darcy's law for the velocity fields in conjunction with an implicit filling technique to track the flow fronts and filled regions. The transient mass balance equation for the resin mass inside the mold cavity involves the unknown variables pressure and fill factor, Ψ , which denotes the fill nature of the mold cavity region. Finite element discretizations ar e introduced for the pressure and the fill factor field to yield a system of discretized equations.

Analysis and Characterization of Flow Channels

199

The resulting discretized system of equations for the pressure and fill factor are solved in an iterative manner till mass convergence is obtained. The theoretical and computational details and the numerical and physical advantages of this methodology are discussed elsewhere.7,8,21 One significant advantage of the flow modeling strategy employed here is that the computed location of the flow front at any instant of time does not depend on the time step size employed to reach that stage. With effective models to characterize the permeabilities of flow channels, the flow modeling simulations can be effectively used to study the fiber impregnation and mold pressures when the flow channels are involved to determine and investigate optimal process parameters and mold designs.6

FLOW CHANNELS AND RTM SIMULATIONS The flow channels are open regions, not containing any porous material. The existing RTM process modeling simulation tools can be effectively employed by characterizing the flow in the channels in terms of average flow capacity in the channel cross-sectional area. The channels are thus treated as a porous medium whose material characteristics are quantified by a variable called equivalent permeability. Equivalent permeability approaches have been verified both theoretically and experimentally11,14,15 and have been employed effectively in RTM flow simulations to model the constitutive flow behavior due to race tracking in simple one-dimensional mold geometries.12,11 In this paper, flow in channels is modeled based on equivalent permeability models. Permeabilities of flow channels in our numerical models have been quantified with equivalent channel permeabilities. The equivalent permeability in terms of Darcy's law is defined as the average velocity divided by the driving force. The steady state flow in channels can be described by a fully developed pressure driven flow as described by the Poisson equation, with zero-velocity at the mold surface, and slip velocity at the interface.11,22 For the type of square channels involved in this study, two different permeability models are available and the equivalent permeabilities of these models are presented in Table 1. Other models, including the parallel plate model, tend to be over predictive. Model 2, presented in Table 1, includes effects on flow resistance in the channel due to the presence of the permeable wall of the preform.11 However, the resulting expression for equivalent permeability is complex, and requires the specification of the permeability of the preform, and alpha, an empirical slip constant. Though preform permeability is typically well known, alpha is very difficult to measure, and has no real physical significance.14,15 A significant simplification can be made if it is assumed that we have zero velocity at the permeable wall of the preform. The resulting formulation based on a fully developed duct flow through a rectangular cross-sectional channel is presented as model 1 in Table 1.23 When employing this expression, both the preform permeability and alpha are not required, and

200

Table 1. Equivalent

Special Molding Techniques

permeability

for rectangular

channel sections

the formulation is simpler to use. Studies indicated that there was some error in the expression presented in reference11for model I. All of the simulated results presented in this paper are based on equivalent permeabilities calculated from model I. Simulations were carried using both, however it was found that there was never more than an 8% diffference in permeability for various alpha values. This resulted in a small adjustment to the predicted pressure histories and flow front progressions. Also, as we had no measurementsof alpha, little justification could be found in applying the permeable wall model. ANAL VSIS AND DISCUSSION In this section, the flow fronts and pressure histories from the numerical process modeling simulations are compared and discussed with the experimental results, to understand the behavior of the equivalent permeability models in simulations. The process modeling simulations are based on 2.5-D thin mold geometries and flow models. Simulations presented here were carried out to compare the flow fronts and pressure histories with the experimental observations and measurements. Since the experiments were considered to be completed before the fluid reaches the end of the preform, we are interested only in the predicted flow front progression and pressure histories before the flow fronts reach the edge of our simulation model. The fiber preforms employed in the experiments were square sections of dimensions 0.3302 x 0.3302 meters (13 x 13 inches), which corresponds to a half width of 0.1651 meters (6.5 inches). Hence for comparisons with experiments, we are interested in simulated flow fronts up until the point that they have travelled a maximum of 0.1651 meters (6.5 inches) from the injec-

Analysis and Characterization of Flow Channels

201

tion point. Taking advantage of symmetry, simulations were completed using a mesh having a quarter circular plate of radius 0.2032 meters (8.0 inches), composed of 4 noded quadrilateral elements with appropriate thicknesses representing the cavities measured from the experiments. Two-dimensional quadrilateral elements are employed to model channels. Table 2. Physical measured parameters for simulations Random mat experiment Permeability Experiment K11,

m2

K22,

m2

Flow rate, m3/s

Viscosity, Pas

Thickness, m

No channels

4.56E-09

4.56E-09

9.552E-06

0.228

0.0038862

1/8 channels

4.56E-09

4.56E-09

9.643E-06

0.226

0.0038862

5/64 channels

4.56E-09

4.56E-09

9.562E-06

0.228

0.0038862

Stitched mat experiment No channels

2.16E-09

5.657E-10

1.953E-06

0.092

0.0032512

1/8 channels

2.16E-09

5.657E-10

1.947E-06

0.0925

0.0032512

5/64 channels

2.16E-09

5.657E-10

1.947E-06

0.0925

0.0032512

Figure 7 shows the flow front contours when no channels are involved for the random mat and stitched experiments detailed earlier. All the simulated flow front contours presented are staggered to show details on a finer scale during the time of interest for which the experimental results are available. Simulations are Random Mat Stitched Mat based on the physical parameters listed in Table Figure 7. Simulated flow fronts - No channels; (Refined 2, which are measured during the experiments. contours represent flow fronts at successive 2.0 second intervals for random mat and 4.3 second intervals for The fiber volume fraction for the random mat is stitched mats). 0.197 and that of the stitched mat is 0.444. The channel permeabilities for the 0.003175 meters (1/8 inch) channels is computed to be 3.5427E-07 m2, and for the 0.001984 meters (5/64 inch) channels is 1.3838E-07 m2 based on model 1 discussed earlier.

202

Special Molding Techniques

Isometric View

Plan View

Figure 8. Simulated flow fronts - 1/8 inch channels ; Random mat; (Refined contours represent flow fronts at successive 2.0 second intervals).

Exp. - Inj. Sim. - Inj.

Figure 10. Random mat: Comparison of pressure histories; 1/8 inch channels.

Isometric View

Plan View

Figure 9. Simulated flow fronts - 1/8 inch channels ; Stitched mat; (Refined contours represent flow fronts at successive 4.3 second intervals).

Exp. - Inj. Sim. - Inj.

Figure 11. Random mat: Comparison of pressure histories; 5/64 inch channels.

From Figure 7, it is clear that the simulated flow fronts with no channels match the

202

Special Molding Techniques

Isometric View

Isometric View

Plan V;ew

Figure 8. Simulated flow fronts -118 inch channels ; Random mat; (Refined contours represent flow fronts at successive 2.0 second intervals).

PlanView

Figure 9. Simulated flow fronts -118 inch channels ; Stitched mat; (Refined contours represent flow fronts at successive 4.3 second intervals). 60000

40000 EXp. Sim.

35000

.Inj. .Inj.

. -

Exp. Sim.

50000

"' 30000 0. .25000 QJ ~ 20000 14 ~ 15000 0. 10000

-Inj -Inj

~ Q, 40000 4) ~

:;

30000

III 01 GI

Ioi 20000 Q, 10000

5000 0

0 0

5

10

15 20 'l'i1\1e, S

25

30

35

0

5

10

15 Time,

20 s

10

15 Time,

20

25

30

35

25

30

35

8

(a) Injection pressures

(a) Injection pressures

0

5

25

30

35

(b) Pressuresat 2, 3 inch transducer locations Figure 10. Random mat: Comparison of pressure histories; 1/8 inch channels.

0

s

10

15 Time,

20 s

(b) Pressuresat 2, 3 inch transducer locations Figure 11. Random mat: Comparison of pressure histories; 5/64 inch channels.

From Figure 7, it is clear that the simulated flow fronts with no channels match the

Analysis and Characterization of Flow Channels

203

experimental results well. The simulated and experimental pressure histories (not presented here) were in agreement. These results provide the authors with confidence in the experimental program and mold filling simulation process. The simulated flow fronts of the random mat experiment having 0.003175 m (1/8 inch) square section channels are shown in Figure 8, and those for the stitched mat experiments are shown in Figure 9. From these figures and the experimental observations presented earlier, it is clear that the simulated flow fronts provide a very good qualitative match. The simulated flow fronts based on the smaller 0.001984 m square section channels showed similar behavior and are not presented here. Pressure histories, which is a more rigorous test, based on simulations are compared with experimental pressure histories for both channel sizes in Figures 10 and 11 for the case of random mat, and in Figures 12 and 13 for the stitched mat experiments. Comparisons of the experimental and numerically predicted flow fronts indicate that the fluid flows faster through the channels in the Figure 12. Stitched mat: Comparison of pressure histoexperiments, compared to simulations. This may ries; 1/8 inch channels. be attributed to how the injection conditions are modeled in simulations. Simulations are based on distributing the injected flow through the nodes comprising the inner radius and channels; Hence it is seen that there is some flow into the fiber preforms even before the channels are filled. In reality, it is more likely that the majority of fluid will flow into the channels, with a smaller amount flowing directly into the preform. Injection conditions must be modeled very elaborately to capture this behavior. The 2.5-D thin cavity flow simulations cannot capture accurately the 3-D flow effects seen in the experiments. The nature of the simulation geometry used drives the flow into the preform as soon as the channels are filled, while during the experiments, the fluid must flow through the thickness of the preform after leaving the channels and spreading into the fiber preform. This effect is more noticeable in preforms with higher volume fractions, as resistance to flow through the thickness increases.

204

Special Molding Techniques

Figure 13. Stitched mat: Comparison of pressure histories; 5/64 inch channels.

By further improving the modeling of flow at the injection ports, the flow fronts based on the simulations may be further improved, thus providing better comparisons with experimental observations. A full 3-D flow simulation would take into account the flow through the thickness, but will be computationally expensive and would require transverse permeability data which is difficult to measure.24,25 However, the thin 2.5-D flow simulations, used in conjunction with equivalent permeability models, provide very good qualitative comparisons that improve as the experiments proceed, and the flow fronts move away from the injection point and flow channels. The equivalent permeability models based on duct flow are based on steady state models while mold filling flow is highly transient. This is true more in the initial stages, which are not effectively simulated. A comparison of the experimental and simulated pressure histories provide some interesting observations. For the case of the random fiber mat with lower volume fraction, the experimental pressures at the 2 and 3 inch pressure locations start early compared to the simulations. This start can be attributed to the early differences in flow front progression seen between the experimental and simulated flow fronts, and the lower volume fraction of the random mat providing little resistance to flow through the thickness of the preform. These properties allow the fluid to reach the pressure transducers quickly. In the stitched mat experiments presented here, the experimental pressures at the 2 and 3 inch locations lag behind the simulated pressures. This is interesting to note, as fluid fills the channels very quickly during these experiments. This can be attributed to the finite time it takes for the fluid to penetrate through the thickness of the preform, initiating a delayed time response of the transducers located on the face of the female mold below the fiber preform in the experiments. This time lag could be effectively addressed if full 3-D flow simulations were used.

Analysis and Characterization of Flow Channels

205

The pressure histories also show significant differences in the slope and characteristics between experiments and simulations which can be attributed to the combination of effects discussed earlier. Experimental uncertainties in the measurement of cavity thickness, viscosity, flowrate and other general errors during experimentation, can also attribute to differences in pressure curve characteristics seen in the experiments and the simulations. Furthermore, despite more sophisticated analysis required to accurately capture the transient and 3-D flow effects with channels, the 2.5-D process simulation models, used in conjunction with equivalent permeability models, provide effective process design and analysis tools for understanding the effects of channels on mold filling. These simulations can be used as an effective tool to predict problems than may occur during mold filling. These allow the designer to make modifications before the mold designs have been finalized in a virtual environment, ultimately leading to process maturation.

CONCLUDING REMARKS The positive effects of controlled race tracking during composite manufacturing by RTM have been demonstrated through experiments involving a simple flat plate mold geometry. While race tracking is normally considered to be undesirable, flow channels can be effectively employed in RTM mold designs to improve the flow distribution and reduce mold pressures. As channels serve to reduce pressure within a mold cavity, injection flowrates may be increased while keeping pressures within permissible levels thereby improving the process cycle times. Experimental flow visualization results presented have clearly demonstrated the dramatic effect of channels, even in the simple mold cavity geometry employed here. The experimental flow front progressions and pressure histories have been compared with process simulation results based on a pure finite element based methodology. The flow in the channels is modeled to be Darcian, quantified by equivalent permeabilities, modeled on steady state fully developed flow through a rectangular cross-sectional duct geometry. Comparisons indicate that process simulation models based on 2.5-D thin geometries and flow models, in conjunction with equivalent permeability models, provide effective process design and analysis tools for understanding the influence of channels during mold filling. However, a more elaborate analysis of the injection conditions for simulations, equivalent permeability characterization models to account for the transient flow nature, and threedimensional models to take into account physically observed 3-D flow effects are needed for a more accurate analysis and understanding of the channel effects on the local flow and pressure histories near a fiber preform. Further investigations involving constant pressure experiments and analyses to include 3-D flow effects through full three-dimensional effects are planned.

206

Special Molding Techniques

ACKNOWLEDGMENTS This work is supported by ARO grant number DAAH04-96-1-0172 through University of Minnesota, and by ARL grant number DAAL01-95-K-0086 through University of Delaware. Special thanks to Mr. Nam Ngo of University of Minnesota for the earlier work on flow modeling studies. Thanks are also due to Dr. Andrew Mark, Mr. Walter Roy and Dr. Shawn Walsh of U. S. Army Research Laboratory for their support and encouragement.

REFERENCES

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

L. Fong and S. G. Advani. The role of drapability of fiber preforms in resin transfer molding. In 9th Americal Society for Composites, page 1246, 1994. S. Bickerton, S. G. Advani, L. Fong, and, K. Fickie. Effect of draping of fiber preform on process parameters during manufacturing with resin transfer molding.. In 11 th Annual ESD Advanced Composites Conference and Exposition, Dearborn, MI, November 1995. P. Simacek and S. G. Advani. Permeability model for a woven fabric. Polymer Composites (in press). M. V. Bruschke and S. G. Advani. A finite element/control volume approach to mold filling in anisotropic porous media. Polymer Composites, 11(6):398-405, December 1990. K. Han, C. H. Wu, and L. J. Lee. Characterization and simulation of resin transfer molding - race tracking and dry spot formation. In 9th Annual ASM-ESD Advanced Composites Conference and Exposition, pages 286-300, Dearborn, MI, 1993. R. V. Mohan, D. R. Shires, K. K. Tamma, and N. D. Ngo. Flow channels/fiber impregnation studies for the process modeling/analysis of complex engineering structures manufactured by resin transfer molding. In ASME International Mechanical Engineering Congress and Exposition, Atlanta, GA, November 1996. R. V. Mohan, N. D. Ngo, K. K. Tamma, and K. D. Fickie. On a pure finite element based methodology for resin transfer mold filling simulations. In R. W. Lewis and P. Durbeta, editors, Numerical Methods for Thermal Problems, volume IX, pages 1287-1310, Atlanta, GA, July 1995. Pineridge Press. R. V. Mohan, N. D. Ngo, K. K. Tamma, and K. D. Fickie. A pure finite element based methodology for resin transfer mold filling simulations. Technical Report ARL-TR-975, U. S. Army Research Laboratory, March 1996. M. V. Bruschke and S. G. Advani. A numerical approach to model non-isothermal, viscous flow with free surfaces through fibrous media. International Journal of Numerical Methods in Fluids, 19:579-603, 1994. B. Liu, S. Bickerton, and S. G. Advani. Modeling and simulation of resin transfer molding : Gate control, venting and dry spot prediction. Composites - Part A, 27A:135-141, 1996. J. Ni, Y. Zhao, L. J. Lee, and S. Nakamura. Analysis of race tracking phenomenon in liquid composite molding. In 11th Annual ESD Advanced Composites Conference and Exposition, Dearborn, MI, November 1995. S. Bickerton and S. G. Advani. Characterization of corner and edge permeabilities during mold filling in resin transfer molding. In ASME AMD-MD Summer Annual Meeting, Los Angeles, CA, June 1995. S. Bickerton and S. G. Advani. Experimental investigation and flow visualization of the resin transfer molding process in a non-planar geometry. Composite Science and Technology (in press). S. Gupte and S. G. Advani. Non-darcy flow near the permeable boundary of a porous medium: An experimental investigation using Ida. Experiments in Fluids (in press). S. Gupte and S. G. Advani. Flow near the permeable boundary of aligned fiber preforms. Polymer Composites (in press). J. Mogavero. Compression characterization and resin infiltration of multi layered preforms in resin transfer molding. Master's thesis, University of Delaware, October 1996.

Analysis and Characterization of Flow Channels

17 18 19 20 21 22 23 24 25

207

H. Darcy. Les Fontaines Publiques de la Ville de Dijon. Delmont, Paris, 1856. C. A. Fracchia, J. Castro, and C. L. Tucker. A finite element/control volume simulation of resin transfer mold filling. In American Society for Composites - 4 th Technical Conference, Lancaster, PA, 1995. W. B. Young, K. Han, L. Fong, and L. J. Lee. Flow simulation in molds with preplaced fiber mats. Polymer Composites, 12(6):391403, December 1991. F. Trouchu, R. Gauvin, and D. M. Gao. Numerical analysis of the resin transfer molding process by the finite element method. Advances in Polymer Technology, 12(4):329-342, 1993. R. V. Mohan et al. Process modeling and implicit tracking of moving fronts for threedimensional thick composites manufacturing. In AIAA-96-0725, 34th Aerospace Sciences Meeting, Reno, NV, January 1996. G. S. Beavers and D. D. Joseph. Boundary conditions at a natually permeable wall. Journal of Fluid Mechanics, 30(1):197-207, 1967. S. Bickertorn et al. Important mold filling issues in liquid composite molding processes Modeling and experiments. In To appear in the Proceedings of the Annual Meeting of the Society of Plastics Engineers : ANTEC 97, Toronto, Ontario, May 1997. R. S. Parnas, J. G. Howard, T. L. Luce, and S: G. Advani. Permeability characterization. part 1: A proposed standard reference fabric for permeability. Polymer Composites, 16(6):429-445, December 1995. T. L. Luce, S. G. Advani, J. G. Howard, and R. S. Parnas. Permeability characterization. part 2: Flow behavior in multiple-layer preforms. Polymer Composites, 16(6):446-458, December 1995.

Optimization of Channel Design in VARTM Processing

Roopesh Mathur and Suresh G. Advani Center for Composite Materials, and Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA Bruce K. Fink Army Research Laboratory, Aberdeen Proving Ground, MD 21005, USA

INTRODUCTION The Vacuum Assisted Liquid Molding VARTM (Figure 1) process1,2 has been used for the manufacture of large composite parts. In this process, a preform is placed in an open mold and a plastic vacuum bag placed on top of the mold. A Vacuum pump Resin impregnates Cured part Resin injection fibers and cures vacuum is created in the mold using a vacuum pump. A resin source is connected to the mold. As vacuum is drawn through the mold, resin infuses into the preform. VARTM is finding increasing application in Figure 1. Vacuum assisted resin transfer molding. the manufacture of large parts with complex geometry such as panels of all-composite buses, railroad cars and armored vehicle components.3 These parts are manufactured after the process has gone through a costly and time consuming development cycle. This development cycle is empirical and experimental and requires considerable amount of effort and expertise. However, it is unclear whether the manufacturing process is efficient and cost-effective. The actual processing conditions may differ from that of the development cycle on a part-to-part basis. Hence there is a need for the scientific study of the manufacturing process and the development of objective process efficiency criteria that will facilitate cost-effective manufacturing. The increasing complexity of components and new processes such as Coinjection Resin Transfer Molding4 will lead to an increasing need for optimization of RTM based processes. The filling and cure of the part can be simulated by several packages Fiber preform

Resin injection

Mold

Fiber preform under vacuum

210

Special Molding Techniques

developed for this purpose, such as Liquid Injection Molding Software or LIMS,5 which has been developed at the University of Delaware. The process of resin infusion of large parts with complex geometry and varying preform properties have been examined. Often, such parts have channels cut in them in order to ensure that resin reaches all sections of the part. The configurations of the channels determine the mold filling time and minimize dry spot formation. However since the volume occupied by the channels contains pure resin, the weight of the finished part is higher. The costs of tooling and setting-up a complex system of channels should also be taken into account. A channel configuration having wide channels reaching all parts of the mold would be able to fill the part in less time. But the costs in weight and tooling have to be accounted for, to reflect manufacturing needs. A Process Performance Index is proposed here, which provides an objective measure of manufacturing efficiency and enables the comparison of different channel configurations. The performance index has been tested out on a test part, which has complex shape, large size and varying material properties in the preform and the results are discussed.

A PROCESS PERFORMANCE INDEX A Process Performance Index can be defined as a quantification of manufacturing efficiency and cost effectiveness. It should contain important parameters and criteria that effect the manufacturing process. In this study, three different criteria have been considered: the time to fill the part, the area of the surface area of the part occupied by the channels and the length of the channels. The time to fill is the most important of the three factors and it reflects the fact that the configuration of the channels with respect to the fibers in the preform determines the time it takes to fill. The area of the channels and their length is proportional to the dead weight of resin in the channels, which should be kept to a minimum. The length of the channels reflects the cost of machining the channels into the part as well as the pressure required to deliver the resin to the channel. These have different magnitudes and need to be scaled to the same magnitude for the purpose of comparison. Hence a simple Process Performance Index can be formulated, which includes all the three criteria as well as their scaling factors: t fill A channels l channels - + ------------------------- + ----------------------J = ---------∗ ∗ t fill A channels l channels∗

[1]

The scaling factor for the time to fill is the time taken for the filling if the entire part had one value of permeability and the resin was injected along the width. Thus the flow is one-dimensional along the length and the time to fill can be calculated by an analytical expression:

Optimization of Channel Design

211

2

φµl t fill∗ = ------------2k∆P

[2]

The characteristic length is the perimeter of the part and the characteristic area of the channels is taken as the product of the highest dimension of the preform and the average thickness of the preform. In practice, some of the manufacturing criteria merit more importance than the others. The time to fill a given part may be an overriding concern while the cost of tooling and weight are unimportant. For applications where the dead weight of resin is important, such as aerospace applications, the second criteria should be given more weight. The tooling costs may be significant for manufacture of a complex part which requires a complicated channel configuration with a number of resin inlets. This is reflected in the following Process Performance Index containing weights for the different criteria. t fill A channels lchannels J = λ 1 ----------- + λ 2 -------------------------- + λ 3 -----------------------∗ ∗ tfill A channels l channels∗

[3]

1.22m 0.17m

Wheel Base (thick section)

APPLICATION

6.1m

Figure 2. Section of Northrop Grumman ATTB.

The part evaluated here is a two-dimensional derivation from a section of an all-composite bus that is under development by Northrop Grumman. It has the dimensions as shown (Figure 2) and has thick sections made of a different material which corresponds to the wheel wells on the actual part. Thus we have a large complex part with different permeability properties in different sections. The part is symmetrical about the centerline thus enabling the detailed analysis of the flow of resin in one half of the part. For this part, the characteristic time to fill is 6 hours, the characteristic length is 14.6 m and the char-

acteristic area is 0.16 m2. Six possible channel configurations have been evaluated here using the Process Performance Index defined in the previous sections (Figure 3). The permeability of the material in the thick section was simulated by considering it as 25 plies of random-mat material compressed down to a thickness of 7.6 cm. The permeability of the material in the rest of the part was determined experimentally. It is a stitched fabric having two-dimensional anisotropy with the ratio Kxx/Kyy=0.5. The effective permeability of the channels was determined

212

Special Molding Techniques

Figure 3. Channel configurations for optimization study.

using the relation for flow in a rectangular channel. The values of the permeabilities, volume fractions and thickness of each section are given in Table 1. Table 1. Properties of the preform material Permeability K11, m2

Permeability K22, m2

Thickness, cm

Volume fraction

Woven material (1)

10-9

2x10-9

2.54

0.5

Random material (2)

10-10

10-10

7.62

0.3

Material

Table 2. Times to fill, length of channel and area occupied for different channel configurations Configuration number

Fill time, h

t/t*

Length of channels, m

l/l*

Area of channels, m2

A/A*

1

5.00

0.84

6.1

0.42

0.16

1.0

2

3.93

0.66

12.2

0.83

0.16

1.0

3

0.41

0.07

6.1

0.42

0.16

1.0

4

5.41

0.91

9.1

0.63

0.12

0.75

5

5.78

0.968

9.62

0.66

0.16

1.0

The six channel configurations were meshed using PATRAN and the simulations performed using LIMS. The filling of the part with one of the channel configurations are given

Optimization of Channel Design

Figure 4. Channel configuration 3: TECPLOT result of flow simulation using LIMS (time to fill: 0.41 hrs).

213

as a TECPLOT contour plot (Figure 4). The filling times were determined and the value of the Process Performance Index evaluated with different sets of parameters. The times to fill are given in Table 2. The time to fill, the area occupied on the part face by the channels and the length of the channels are scaled down using the scaling factors described earlier. The PPI is calculated with the following set of weights: All equally important: λ 1 = λ 2 = λ 3 = 1 Length unimportant: λ 1 = λ 3 = 1, λ 2 = 0.1 Areas unimportant: λ 1 = λ 2 = 1, λ 3 = 0.1 The values of the PPIs for the 3 different sets of parameters are given in Table 3.

Table 3. Process performance index for 3 different sets of parameters Configuration number

λ1 = λ 2 = λ 3 = 1

λ 1 = λ3 = 1, λ 2 = 0.1

λ 1 = λ 2 = 1, λ 3 = 0.1

1

2.25

1.88

1.36

2

2.16

1.74

1.59

3

1.49

1.11

0.59

4

2.28

1.72

1.61

5

2.63

2.03

1.73

RESULTS AND DISCUSSION From the results, it can be seen that the channel configuration number 3 gives the best value for the performance index in all three cases. This is due to the higher number of injection locations and the location of the channels such that the resin has a smaller distance to travel than in the other cases. Configuration 2 has a lower time to fill than configuration 1, but has a higher value of PPI. Thus if the dead weight of the resin and the tooling costs are taken into account, then the second configuration is probably more inefficient. If an penalty is assigned for the higher number of injection ports, then the configuration 3 would have a comparable PPI. Channel configurations 4 and 5 have comparable or higher values of the PPI than the other configurations. Thus the branching configurations employed don’t seem to have any effect

214

Special Molding Techniques

on the efficiency of the manufacturing process. This is due to the length that the resin has to traverse to reach all parts of the mold with one injection port only and hence the mold filling times are higher.

CONCLUSIONS A Process Performance Index to measure the efficiency of the VARTM manufacturing process with channels was postulated. Different configurations of channels were evaluated for the filling a large part with complex geometry and varying material properties using the PPI. The PPI was found to provide a good measure of the cost-effectiveness of manufacturing and enables a quantitative evaluation of the variables vital for efficient manufacturing.

ACKNOWLEDGMENTS This work was prepared through the participation in the Composite Materials Research Collaborative Program sponsored by the Army Research Laboratory under Cooperative Agreement DAAL01-96-2-0048. The authors would like to acknowledge the help of Mr. Rod Don of the Army Research Laboratory and Mr. Simon Bickerton of Mechanical Engineering, University of Delaware with this work.

REFERENCES 1 2 3 4 5

Gallez, X.E. and S.G. Advani, “Numerical Simulations for Impregnation of Fiber Preforms in Composites Manufacturing, Fourth Intntl. Conference on Flow Processes in Composite Materials, University of Wales, 1996 Seemann II, “ Plastic Transfer Molding Techniques for the Production of Fiber Reinforced Plastic Structures”, United States Patent 4,902,215, Feb 20, 1990. Pike, T., McArthur, M. and Schade, D. , “Vacuum Assisted Resin Transfer Molding of a Layered Structural Laminate for Application on Ground Combat Vehicles”, Proc. of 2th Intntl. SAMPE Tech. Conference, 1996 Gillio, E.F. et. al., “ Manufacturing of Composites with the Con-Injection Process”, 38th Structures, Structural Dynamics and Materials Conference, AIAA, 1997 M. V. Bruschke and S. G. Advani, "A Numerical Approach to Model Non-isothermal, Viscous flow with Free Surfaces through Fibrous Media, International Journal of Numerical Methods in Fluids, 19, pp.575-603 (1994).

Injection Compression Molding. A Low Pressure Process for Manufacturing Textile-Covered Moldings

Carsten Brockmann, Walter Michaeli Institut für Kunststoffverarbeitung, Pontstraße 49, Aachen D-52062, Germany

INTRODUCTION Especially in the automotive, but also in the furniture industry, there is a growing demand for decorated mouldings.1,2 Decorated mouldings can be produced on a standard injection moulding machine, using conventional injection moulding (IM) or injection compression moulding (ICM). Both processes have some advantages compared to the decoration of mouldings by glueing, because both are one step processes with a good reproducibility and the avoidance of adhesives. When producing decorated mouldings directly in an injection mould, the decoration material is stressed by high temperatures and cavity pressures. This very often leads to strong decoration material damage.1,3-5 One major type of damage is the collapsing of the foam layer of the decoration materials, which are typically used in the car interior. This foam layer shall provide a "soft touch"-effect to give the car interior a comfortable appearance. If the foam layer collapses this effect is lost. ICM is a special process of IM, which is able to reduce the cavity pressure.3,6,7 This process starts with the closing of the mould until the compression gap is reached. At this point of time the mould is already sealed, because it is equipped with shear edges. In the next step the melt volume, which is needed to fill the moulding, is injected into the cavity and the shut-off nozzle is closed afterwards. In the last step the mould is closed completely by the compression movement. Due to this movement the melt is spread throughout the cavity until it is filled. Finally the melt is compressed, because the injected melt volume is usually higher than the cavity volume. This is necessary to compensate the volume shrinkage. In conventional IM this is done by the packing phase, which is only exceptionally used in ICM. The major goal of the experiments introduced in this paper is to compare conventional IM and ICM in regard of cavity pressure reduction and the quality of decorated mouldings.

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Figure 1. Molding and mold concept.

Futhermore the major process parameters of the injection compression moulding process, which influence the quality of the decorated mouldings, shall be found out and optimized.

EXPERIMENTAL SETUP The experiments are carried out on a 150 t Mannesmann Demag Ergotech IM machine, which is equipped with an ICM control. A special test mould (Figure 1), equipped with shear edges and facilities to preform and fix the decoration material is used. Four pressure transducers are mounted into the mould for cavity pressure measurements. These measurements are stored and visualized with a PC based data acquisition system. All experiments are run with the polypropylene Vestolen P2000, which is an easily flowing PP-type with a MVI of 55 (Tm = 230°C, load 2,16 kg). Four different textile decoration materials are used, which are provided by Viktor Achter GmbH & Co KG, Viersen. All of them consist of three layers including a PU foam layer.

RESULTS AND DISCUSSION The process conditions of ICM and conventional IM differ strongly in some points. Whereas the injection speed is the most important influence on the cavity pressure in IM, in ICM the clamping force and the injected melt volume have strong effects. This is due to the final compression phase. The higher the clamping force, the higher is the cavity pressure, as can be seen in Figure 2. The first experiments were run without decoration material. To be able to compare both processes, parts with the same weights have been produced.8 The influence of the melt volume and clamping force becomes clear when the process is explained in a p,v,T-diagram, as shown in Figure 3.8-10 From 1 to 2 the isothermal injec-

IM: injection speed . . . 20 mm/s __ 60 mm/s ____ 100mm/s

time (s)

pressure (bar)

ICM: clamp force chosen (measured) ____ 90kN (246kN) __ 51kN (175kN) . . . 25kN (120kN)

time (s)

Figure 2. Influencing parameters for IM and ICM.

217

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Injection Compression Molding

temperature Figure 3. ICM in a p,v,T diagram.

tion phase can be found, which is connected with the first pressure increase. Due to the compression gap the cavity is not filled completely. Therefore the pressure drops in the delay time between injection and compression phase (point 3). With the beginning of the compression phase the melt starts to flow again, which leads to another pressure increase until point 4 is reached. Here the cavity is filled completely and the melt is now compressed by the clamping force until point 5 is reached. Finally the pressure decreases again (5 to 6) because of the cooling of the melt and the coupled volume shrinkage. Point 7 represents the opening of the mould. If an IM machine with optional pressure-controlled clamp force is used, the course in the p,v,T-diagram is drawn from 4' to 5'. In this case the pressure stays constant once a given clamp force is reached. For in-mould surface-decoration the volume shrinkage is usually not as important as it is for conventional IM. The reason for this is the decoration material, which covers optical problems such as sinkmarks. Therefore only small or no packing pressures are applied in IM process. In analogy to this it is possible to reduce the cavity pressure for ICM by a smaller injected melt volume. The course in the p,v,T-diagram would then look like as drawn by the

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mould position

pressure (bar)

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injection moulding

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Figure 4. Comparison of pressure courses for IM and ICM with decoration material.

compression speed: . . . 5% (0.9 mm/s) __ 15% (2.7 mm/s) ____ 25% (4.5 mm/s)

pressure (bar)

injection compression moulding

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compression speed: . . . 5% (0.9 mm/s) __ 15% (2.7 mm/s) ____ 25% (4.5 mm/s)

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Figure 5. Influence of compression speed on the cavity pressure.

dashed line from 5* to 6*. Figure 4 shows a comparison of the pressure courses, in the case that decoration material is used. Another process parameter for the ICM process, which has strong influence on the cavity pressure is the compression speed. With higher speed the pressure to fill the cavity increases as well as the maximum pressure, caused by the melt compression (Figure 5). The first effect is due to lower shear rates and the second due to a longer cooling time and which leads to a reduced melt volume, when the compression phase starts. The height of the compression gap influences the pressure course only during the injection phase. The differences are, compared to the pressure maximum, relative small. In the first experiments the IM parameter injection speed was the same for both processes, conventional IM and ICM. With this setup it was easier to isolate the influences of IMC process, because the injection moulding parameters stay constant. But this experimental setup did not take the stress time into account, which was found to be very important. The stress time is defined as the time, when the decoration material is loaded with pressure and high temperatures.

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3.5 residual foam thickness (mm)

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Figure 7. Comparison of residual foam thickness for IM and ICM (equal stress times). Figure 6. Comparison of residual foam thickness for IM and ICM (different stress times).

residual foam thickness (%)

Figure 6 shows the residual foam thickness displayed over the flowpath. The residual foam 100 90 thickness is measured one week after moulding. 80 This Figure shows important information for the 70 60 producer. He is interested in the absolute thickScrew 50 Position ness of the foam layer but also in the distribution 40 30 over the part. Strong differences in thickness 20 lead to visual marks on the parts surface. In Fig10 Compression Gap = 2 mm 0 ure 6 results of experiments with constant injec0 20 40 60 80 100 120 flow path length (mm) tion speed are shown. This means, that all ICM experiments have longer stress times than the Figure 8. Residual foam thickness for simultaneous comIM experiments, because after the injection pression. phase, which takes the same time for both processes, there is the compression phase. This results in residual foam thicknesses for ICM, which are as small as for IM or even smaller, although the cavity pressures are lower a can be seen in Figure 4. In Figure 7 results of later experiments are shown.11 Here the ICM experiments have the same stress times as the ones done by IM. For these experiments at first a reference part is produced by IM. The injection time for this part is then used as the reference processing time for all following ICM experiments. This means, that the injection time plus the compression time can only take as long as this reference time. Therefore higher injection speeds are used for the ICM experiments and the compression phase takes place in the remaining time. The results of ICM are in this case much more uniform. In case of a compression gap of 3 mm the residual foam thickness is between 60 and 80% (compression gap = 3 mm) of the original thickness of the decoration material. For decreasing compression gaps the parts quality is getting worse. There are bigger differences in the thickness distribution and the 40 mm 45 mm

50 mm

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minimum thickness is decreasing. The curve with 0 mm compression gap represents conventional IM and has strong differences over the flow path. Further improvement of the parts quality can be reached by using the option "simultaneous compression". In this case the compression phase starts already during the injection phase. The starting point of compression can be defined by the screw position. The stress time can further be decreased with this option, which leads to a more uniform and higher residual foam thickness. In addition the melt flow becomes more uniform without the typical stopping between the injection and compression phase. Figure 8 shows the influence of a starting point variation on the residual foam thickness. The compression gap (2 mm) and the injection speed (150 mm/s) stay constant. For all different starting positions the foam thickness is more uniform and the absolute values are higher than in the experiment shown in Figure 7.

CONCLUSIONS In-mould surface-decoration is an economic way of producing textile decorated mouldings. But still there are quality problems, like decoration material damage, especially if the size of the moulding exceeds certain bounds. Responsible for this damage are high cavity pressure and the melt temperature. Especially the cavity pressure can be reduced by the use of ICM. Furthermore the experiments show, that the stress time is an important parameter for final parts quality. With ICM it is, compared to conventional IM, possible to reduce cavity pressures and at the same time the stress time. Further reduction of the stress time is possible by using simultaneous compression. With this option a further improvement of the quality of the decorated moulding is possible. An optimization of the ICM process is possible especially by changing the parameters compression gap and compression speed.

ACKNOWLEDGEMENTS We appreciate the help of Mannesmann Demag Kunststofftechnik, Viktor Achter GmbH & Co KG and Vestolen GmbH. The investigations set out in this report received financial support from the Ministry of Economics (BMWi) and from the AiF e.V., to whom we extend our thanks.

REFERENCES 1 2

Annen, D., Analysis of the part developement process for the production of decorated mouldings by in-mould surface-decoration, unpublished diploma thesis at the IKV, Aachen, 1993, supervisor: S. Galuschka. Galuschka, S., In-mould surface-decoration - manufacturing of textile-covered injection mouldings, dissertation at the RWTH, Aachen, 1994.

Injection Compression Molding

3 4 5 6 7 8

9 10 11

221

Michaeli, W., Galuschka, S., In-mould surface-decoration: Analysis of the boundary conditions. Teil 1: Plastverarbeiter 44 (1993) 3, S. 102-106, Teil 2: Plastverarbeiter 44 (1993) 4, S. 62-68. Münker, M., Developement and set-up of two tests for the characterisation of decoration materials, internal report at the IKV, Aachen, 1996 supervisor: C. Brockmann, A. Oelgarth Steinbichler, G., Gießauf, J., In-mould surface-decoration with textiles and films, Kunststoffberater 41 (1996) 3, p. 16-21. Jaeger, A., e.a., Machine technique and processing for in-mould surface-decoration, Kunststoffe 81 (1991) 10, p. 869-874. Hansen, J., Analysis of injection compression moulding in respect to the opportunity to decrease pressures during mould filling, unpublished thesis at the IKV, Aachen, 1994, supervisor: S. Galuschka. Wodke, T., Optimizatioin and comparison of the injection moulding and the injection compresion moulding process in respect to the part in in-mould surface-decoration, unpublished diploma thesis at the IKV, Aachen, 1996, supervisor: C. Brockmann. Yang, S.Y., Lien, L. Experimental Study on the Injection Compression Molding of Parts with Precision Contours, International Polymer Processing XI (1996) 2, p. 188-190. Knappe, W., Lampl, A., About the cycle course in injection compression moulding of thermoplasts, Kunststoffe 74 (1984) 2, S. 79-83. Kuckertz, M., Comparison of injection moulding and injection compression moulding in respect to the suitability for in-mould surface-decoration, unpublished thesis at the IKV, Aachen, 1996, supervisor: C. Brockmann.

Kurz-Hastings Inmold Decoration

Roy Bomberger Kurz-Hastings, Inc. The Kurz World Group is the "State of the Art" in Decoration Technology. Technical and Cost improvements as well as automation are the factors most important to any modern injection molding plant. Kurz-Hastings IMD process makes is possible to produce finished decorated parts in one operation. This process has tremendous advantages to any other decorating methods for suitable applications. In many cases the appearance of the piece part has to be improved. This can be done by decorating their surface and thus achieving a simulated appearance of woodgrains, marbles, metallizations or various combinations. Some products may require special artwork or special coating on the top surface. There are many ways to rationalize the additional operations to decorate parts such as pick and place from a molding machine to hot stamp machine or heat transfer machine. This requires secondary operations and additional space. The idea of decorating inmold is not new in the USA. Parts have been produced for two decades by placing pre-cut foil inserts in the mold. This method was done with random or continuous patterns. It has its limitations due to position accuracy. The IMD combines the mold and hot stamping technology into a single process developed in foil manufacturing and design of the parts. The IMD process offers a number of important advantages when compared with the traditional method such as printing, painting, metallizing (chrome plating) and conventional hot stamping. 1. The process is environment friendly. 2. The decorated area forms an integral bond to the substrate offering better surface properties. 3. The IMD process will not add any additional time to the molding cycle. Because most people use robotics to remove parts, the film will be advanced at the same time as the parts are removed. 4. Multi-color decoration is achievable in a single cycle. 5. This easy change of foil during molding permits trouble free production of different surface decorations. Also, it permits you to change foil to give a different look to the product you are running.

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THE IMD PROCESS The IMD process uses special modified hot stamp foils. These foils are guided through the mold in its open position. During injection, the pressure and heat of the molten material transfers the printed layer from the polyester carrier and bonds it to the plastic substrate. A foil feeding and positioning device is used along with special modified foil with registration marks for positioning of film. The foil feed device is attached to the moveable platen of the injection molding machine. The molding press must have two stage injection. Auxiliary equipment such as mold temperature controller and hopper dryers, ensure optimal molding conditions.

THE SEQUENCE OF THE IMD PROCESS The roll of IMD foil is held by the upper part of the device. The foil is fed from top to bottom of the mold by this device and used film is rewound by the bottom roller. The printed layers of this film are transferred by the hot plastics being injected against it. No die cutting or trimming of the foil is required in this process. As the press opens, a robot removes the finished part from the mold and the foil advances to the next image.

THE FOIL FEED DEVICE The device and its control cabinet are supplied in a unit. The mounted unit will be affixed to the molding machine by a base plate (20 mm thick). The Kurz IMD device can be used with any injection molding machine of modern design. Three types of IMD devices can be used depending on the positioning tolerances and feeding requirements. IMD-JB TYPE Due to a very precise feeding system this offers positioning tolerances of ± 0.05 mm in each of two X/Y directions. The required positioning registration mark printed on the foil is different from the MA device (see below). This IB type provides the best possible positioning accuracy of the decoration. IMD-MA TYPE This device is also equipped with sensors for separate positioning in the X/Y direction. The maintained positioning accuracy of this device is plus minus 0.1 mm. Both devices IB & MA feed the foil with an adjustable speed until the sensor governing the vertical positioning reads the start of the printed positioning mark. The final posi-

Kurz-Hastings Inmold Decoration

225

tioning, with a slower speed, follows until the end of the mark is read. The machine closes after the foil is in position (controlled by the device) and the mold cycle starts. Each of the devices offer three control possibilities: a. Controlled by infra-red sensor. This must be used when individual images are processed. b. Control by timing. In this case, the foil-feed amount is a result of the combination of speed and time entered into the control cabinet of the device. c. Manual control. Used basically for setting up. IMD-T The unit is a time device only which can be used for continuous pattern. NOTE: This unit can not be converted to a sensor unit. All of the IMD devices are available in three sizes corresponding with the width of foil: 250 mm, 350 mm and 400 mm. Devices tailored for special applications are available on request.

IMD FOIL The IMD foils are specially modified hot stamping foils using a polyester carrier film 23 to 75 micron thick. The choice of the suitable carrier film thickness depends on the decoration depth and geometric shape, the size of the part, the position and the types of gating and type of plastic material. We have developed several special foils for the process. They are: a. Metallized high gloss foil (Gold, Silver and Metallic colors) b. Single color (Unicolor) foil c. Woodgrain, Marble, Granite of various designs d. Brushed Silvers, Golds & Matte e. Multicolor pictures - 7 to 9 colors printed in register f. Continuous patterns g. Selective or partial metallization The modern printing machines at Kurz permit multiple tones and shades and partial metallization, also full metallization. We can also have in the image matte and gloss surface effects. The adhesion layer of the IMD foil is specifically formulated for use with different molding material thus ensuring good bond between the plastic and the decoration layer of foil in every case. Foil suitable for the following plastics are available: 1. PS 2. SAN 3. ABS

7. PC/ABS 8. PBTP 9. PPE

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4. PP 10. ABSAN 5. PPO 11. PMMA (acrylic) 6. PA 6 PA6.6 12. PC * (under consideration of special parameters)

TOOLING FOR INMOLD When designing new tools for IMD process, the following points should be observed: 1. The foil must have an unobstructed passage between the guide pillars and pass close to the parting surface of the mold. 2. The gating must ensure that the foil presses against the cavity surface during injection. This is usually done with edge or submarine (tunnel) gates. Also the hot tip runner system works well. 3. Regular types as well as 3 plate molds are in use for IMD. Slides and cams, etc. are possible provided they are designed in a way preventing the damage or pinching of the foil during their operation. 4. Since the inmold feeding device is attached to the moveable platen of the molding machine, the decorated surface of the part must be formed by a cavity or cavities located in the moving half of the mold. This construction requires the ejectors to operate from the fixed half (stationary side) of mold. Mechanical or hydraulic activation of ejector system is used. 5. All cavities in multiple cavity mold must be covered by the width of foil. Also foil can be run partially if located in front of the injection point. 6. The part can be filled by one or more gates. Particular care must be taken not to create excessive weld lines where two or more streams of plastic meet. The foil will not cover nor disguise such weld lines and appearance faults will result. 7. Adequate venting (0.0007 to 0.004 depth) must be provided to permit the air trapped between the carrier film and the surface of the cavity to escape. The position and width of these vents depends on the gate and gating position and also on the shape of the molded part. Moldmakers should realize that they do not need as much cavity pressure as normal because of the foil in the mold. 8. All cores forming openings in the decorated surface must be located in the fixed and moveable half of the foil (core split). 9. Cavities can use textured finish or high polish surface depending on the required appearance. 10. The decoration of 3 dimension surfaces must not result in excessive elongation of the IMD foil. Local overstretching of the foil can be prevented by providing for sufficiently large radius, chamfer and soft transition when designing the surface to be decorated. The parting line of the decorated part must also be taken into account.

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227

11. Temperature controllers should be available to maintain mold temperature. This will range from 85 degrees to 180°F Sometimes a separate controller may be needed on both halves of the mold. 12. Both photo cell sensors can be accommodated within the mold frame in suitable cut outs in the mold frame or located outside the mold.

POINTS TO CONSIDER WHEN DESIGNING IMD PARTS & MOLDS 1. The Depth of Decoration: The corner radii of square or rectangular parts should at least match the decoration depth (if decoration depth is 2 mm, the minimum radius should also be 2 mm). A minimum 10 degrees draft in the molding direction will help separate a carrier film at mold opening. All inmold radii should be polished. This allows the foil to move while the cavity fills. The decoration depth of minimum 0.020 should be maintained even in cases of totally flat decorated surfaces. This tensioning of the foil over the edge of the part puts tension on the carrier and this prevents the foil from wrinkling. The gating should be located as centrally as possible for parts of irregular shape. Round parts are basically gated in the center to help in maintaining the diameter tolerance. In cases requiring multiple gating, the gates should be located as close to each other as practical to minimize the effect of flow and weld lines in the part. 2. Parts with textured surfaces can be achieved by using foil with texture and/or texturing the mold surface. Textured IMD foil can be matte, brushed or have area of mixed appearance (this will require a separate operation during manufacturing) such as high gloss and matte together. The depth of pattern in the mold surface must not exceed 0.006 with edges and corners having radii of at least 0.030 inch. 3. Plastic material that will be suitable for IMD. Resin with a high flow and easy melt are preferred. (It is best to fill the mold relatively fast. This helps maintain a uniform temperature on the foil surface allowing the adhesive to bond to the plastic part). 4. Clamping pressure of molding machine should be as low as possible. This permits a little movement of the foil in case of decorating a large area. 5. Elongation of foil: Test results have shown that the stretchability of nonmetallized foils must not exceed 20%. This has been achieved under optimal molding conditions. However, each part design should be evaluated separately taking into account its material, shape, size and the specific decoration being applied. 6. Gates should be as large as possible to facilitate even and fast filling of the cavities. 7. Wall thickness of the parts should be at least 0.040. Thinner wall section may not insure proper adherence of the foil to the plastic.

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8. Positioning of the part (parts) in mold; long or narrow parts should be positioned horizontally in the mold. This orientation will assist in shortening mold cycle.

THE ECONOMICS OF THE IMD PROCESS The IMD process combines several traditional production and decorating steps in one operation. Therefore, when comparing the cost of IMD with the costs of traditionally decorated parts, examples should be used where multiple operations and multiple colors would otherwise be required. This IMD process is favorable in cost reduction because of much lower reject rates and the elimination of storage and handling between operation. The estimated savings in real terms vary between 10% and 50% depending on the individual part. The IMD process eliminates environmental problems as no paints or solvents are involved so there are no concerns of VOC's.

Chapter 5: Improving Material Properties High Impact Strength Reinforced Polyester Engineering Resins for Automotive Applications Mengshi Lu, Kevin Manning Hoechst Research and Technology Suzanne Nelsen, Steve Leyrer Ticona, Summit, NJ

Figure 1. The correlation between impact strength and flex modulus for some commercially available polyester resins.

Figure 2. The correlation between break elongation and flex modulus for some commercially available polyester resins.

INTRODUCTION For engineering plastics to be used in large parts applications such as automobiles, the material must meet the performance requirements such as stiffness and toughness. In addition, the material should have good processability to be injection moldable. For short-fiber reinforced engineering plastic materials, as a rule of thumb, stiffer materials tend to be more brittle. A review of mechanical properties of a variety grades of polyester engineering resins manufactured by Ticona supports this generalization. Figure 1 illustrates the relationship between impact strength and flexural modulus for several polyester materials. Unfilled Vandar thermoplastic alloys such as Vandar 4602 and Vandar 6000 are super-tough at room temperature but have low flexural modulus compared to neat PBT. A similar trend exists between break elongation and flex modulus as shown in Figure 2.

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Vandar 4361, a 30% glass filled and impact modified PBT resin, has a good combination of stiffness and toughness. One objective of this work was to generate a PET based resin which has high impact strength and maximum break elongation while maintains the highest possible flexural modulus. Semi-crystalline polymers such as polyesters are pseudo-ductile materials, i.e., they exhibit high crack initiation energy but low crack propagation energy under impact. When there is a sharp notch or crack present, the parts made from this kind of material may fail in a catastrophic manner without absorbing much energy during impact. Toughening semicrystalline polymers including polyester is a common practice in the plastics industry. The general approach is to create a second dispersed rubbery phase which helps initiate matrix plastic deformation leading to dissipating a large amount of energy before fracture. To be effective the rubber particle size has to be within a certain range. The optimal rubber particle size range varies from polymer to polymer. In general, for aromatic polyesters like PET and PBT or polyamides like nylon 6 and nylon 66, the optimal rubber particle size range is between 0.1 to 1 micron.1-3 To achieve such fine rubber dispersion, some kind of reactive compatibilization is usually needed since most rubbers are neither miscible nor compatible with polyesters or polyamides.4,5 Selection or design of the proper impact modifier and proper compatibilizer is the key to developing a material which has the desired properties. In this report, we discuss the development of Impet Hi 430, a toughened, reinforced PET based engineering resin. This material has both high toughness and good processability to allow molding of large parts. The effect of rubber level and glass level on the mechanical properties will be reviewed. Structure/property relationship will.also be discussed.

EXPERIMENTAL The polyethylene terephthalate used, Impet® 100, has an IV of 0.68 and melt viscosity of 1560 poise at 280oC and 1000 1/sec. A 14 µ m fiberglass is used as reinforcement. Three elastomers were evaluated as impact modifiers. The first two, designated as A and B, have functional groups that can react with the PET carboxyl end groups. The third elastomer, C, does not contain any reactive groups. All the materials were compounded using either a 30 mm or a 40 mm Werner Pfleiderer ZSK twin screw co-rotating extruder. All components except glass were preblended using a tumbler mixer before compounding. The glass was fed downstream. The extrudates were cooled in a water trough before being pelletized. Normally, prior to molding the pellets were dried at 135oC for 4 hours or at 93oC overnight using a dehumidifying oven. Various standard injection molding machines like a 30 ton BOY machine were used to mold test bars in accordance with either ASTM or ISO standards. All samples were conditioned at 23oC and 50% relative humidity for 48 hours before testing.

High Impact Strength

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Figure 3. The melt viscosity of 15% glass reinforced resins as a function of rubber content of elastomers A, B and C.

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Figure 4. The notched Izod impact strength of 15% glass reinforced resins as a function of rubber content of elastomers A, B and C.

The apparent melt viscosity and melt stability were measured by Kayeness Rheometer at 280oC and 1000 1/sec. All samples were dried in a vacuum oven at 150oC for one and a half hours prior to testing. The rubber morphology of selected compounds were examined on fracture surface using a Jeol JSM-T200 Scanning Electron Microscopy. To enhance phase contrast the rubber phase was selectively etched by soaking the samples in xylene for half a hour.

RESULTS AND DISCUSSION One existing material, Impet 320, was first reviewed. The impact strength and modulus of this resin are shown in Figure 1. The impact strength was deemed inadequate for structural applications where greater toughness is desired. This material contains 15% glass and low level nonfunctionalized impact modifier. It seemed necessary to use a higher level of functionalized impact modifier to achieve better toughness. Elastomers A and B both contain reactive groups which can chemically interact with PET. This interaction may help generate proper rubber morphology necessary for toughening. For comparison, the nonfunctionalized elastomer C was also used. All the modifications were based on 15% glass reinforced formulation. Figure 3 illustrates the response of melt viscosity of the resins as a function of the level of these three elastomers. The rheological information gives a good indication of the interaction between the elastomers and PET matrix. Elastomers A and B both seem to have strong interaction with PET while elastomer C seems to have little interaction with PET. Figure 4 shows the notched Izod impact strength of these resins versus the elastomer level for each elastomer. For elastomers which have a strong interaction with PET, i.e., A and B, increasing the loading level

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Figure 6. The melt viscosity of 15% glass reinforced resins as a function of the concentration of elastomer A in the total rubber content.

leads to higher impact strength. On the other hand, increasing the loading level of elastomer C from 5 to 20%, seems to have no effect on impact strength. Elastomer A imparts the best toughening improvement. The morphology of the resins that contains 20% of elastomers A, B, and C are shown in Figures 5a to 5c. When elastomers A and B were used, the average rubber particle size was below 1 µ m. While particles of elastomer C are irregular in shape with an average particle size well above 1 µ m. The difference in morphology may explain why elastomers A and B are better Figure 5. The SEM graphs of 15% glass filled resin conimpact modifiers than elastomer C. Using elastaining 20% rubber. tomer A renders a better impact strength, it also increases the melt viscosity of the resin significantly. A resin with good impact strength and a relatively lower melt viscosity is more desirable from a processing point of view. The chemical structure of elastomer A has a similar backbone to that of elastomer C. It is suspected that the two elastomers are at least compatible (if not miscible) due to this similarity in structure. One idea was to explore the possibility of using a combination of elastomers A and C as impact modifiers. The hypothesis was that elastomer A could serve as both an impact modifier and as a compatibilizer between the PET matrix and elastomer C. Diluting elastomer A with C could lead to lower melt viscosity. While keeping the total rub-

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Figure 7. The tensile strength and flex modulus of 15% glass reinforced resins as a function of the concentration of elastomer A in the total rubber content.

233

Figure 8. The notched and unnotched Izod impact strength of 15% glass reinforced resins as a function of the concentration of elastomer A in the total rubber content.

ber content constant at 20%, various combination of elastomers A and C were used as impact modifiers for the 15% glass reinforced resin. Figure 6 shows the melt viscosity versus the concentration of A in the total rubber. In general, as more elastomer A was used, the resin had higher melt viscosity. Figure 7 shows the tensile strength and the flexural modulus of these resins as a function of the concentration of A. When the rubber is composed of 100% of the nonreactive elastomer C, the compound has low tensile strength and low flexural modulus. The addition of even small amount of the functional elastomer A leads to immediate increase in both properties. Further increase in elastomer A concentration, however, imparts no additional improvement. It appears that there is a minimum amount of A (perhaps ~ 20% in the total rubber content) that is needed to provide compatibilization for the nonfunctionalized elastomer C with the PET matrix. Adding more than this minimum amount of elastomer A provides no further improvement in tensile properties. The impact properties of these compounds is plotted as a function of elastomer A concentration in the total rubber content, see Figure 8. Similar to the tensile properties, adding a small amount of elastomer A to elastomer C leads to a significant increase in both notched and unnotched Izod impact strength. Adding more than 20% of elastomer A in the total rubber content seems to impart little further improvement in impact strength. There is a clear advantage of using a combination of elastomers A and C, preferably 20% of A in the total rubber content. First, it has notched Izod impact strength of 190 J/m, only slightly lower than using 100% of elastomer A. Second, it has almost exact tensile properties as using 100% of elastomer A. Third, the melt viscosity of this compound is much lower than using 100% of elastomer A. The rubber morphology of this compound is shown in Figure 9. It is interesting to point out that the

234

Figure 9. The SEM graph of 15% glass filled resin containing 4% A and 16% C.

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Figure 10. The stress-strain curves of Impet Hi 430 at various temperatures.

average rubber particle size is less than 1 µ m. The particles appears to be uniform in size, confirming the hypothesis that elastomers A and C are compatible. One would expect a bi-modal particle size distribution if they were not compatible. This resin is reinforced and yet it exhibits a yield point even at sub-ambient temperatures as shown in Figure 10. The existence of a yield point indicates ductile fracture behavior which is Figure 11. The impact strength and flex modulus of 4/16 not common for reinforced resins. A/C toughened resins as a function of glass level. Figure 11 shows the effect of glass content on the flexural modulus and impact strength of resins which contain a combination of 4/16 A/C elastomers. While the impact strength seem to stay more or less the same, the flexural modulus increases almost linearly as the glass level is increased.

CONCLUSIONS We have successfully developed a new PET based engineering resin product line, i.e., Impet Hi. The first product, Impet Hi 430, has a good combination of stiffness and toughness. This resin also has excellent processibility. This resin was successfully molded into very large parts. This product can be UV stabilized for exterior applications, and the natural product can be colored using available concentrates. This material finds use in large molded parts that experience a large temperature range in use. Other potential applications include recreation vehicles, automotive assemblies, furniture and appliances.

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ACKNOWLEDGMENT The authors would like to thank Mr. A. Towne, Dr. Rong Chen, Physical Testing Lab of HR&T and Polymer Processing Team of Ticona for technical assistance.

REFERENCES 1 2 3 4 5

Epstein, B. N. U.S. Patent, 4,172,859, 1979. Wu, S. Polymer, 1985, 26, 1855. Flexman, E. A., in “Toughened Plastics I: Science and Engineering”, Ed. Riew C. K. and Kinloch, A. J., Advances in Chemistry Series 233, American Chemical Society, Washington, DC 1993. Lu, M., Keskkula, H. and Paul, D. R., Polymer, 1993, 34, 1874. Lu, M., Keskkula, H. and Paul, D. R., J. Appl. Polym. Sci., 1995, 58, 1175.

Control of Internal Stresses in Injection Molded Parts Through the Use of Vibrational Molding, “RHEOMOLDINGSM”, Technology

Akihisa Kikuchi, Marc Galop, Harold L. Brown, Alexander Bubel TherMold Partners L.P., Stamford, Connecticut, USA

INTRODUCTION In the injection molding processes, there are many parameters of importance. These variables can be divided into two major types: 1) controls of a molding machine; and 2) polymer rheological behavior. Vibrational molding technology was applied to an injection molding process in an effort to control rheological behavior during processing and to improve the properties of materials. Use of the vibration technique results in more ordered structures from part to part and reduces the effect of certain independent variables during processing.1

A PROBLEM IN INJECTION MOLDING PRACTICE In molded products, the internal stresses, which are sometimes referred to as residual stresses or molded-in stresses, are the result of the morphologies of a molded part. In simple terms, a part has high internal stresses if molecules are more oriented; a part has low internal stresses if molecules are more relaxed. The morphologies which determine the properties of molded parts are the result of the rheological behavior of polymer during processing. The rheological behavior is controlled by the processing temperature, applied pressure to the polymer melt, and shear stress resulting from polymer flow. In general, these factors are controlled poorly during injection molding practice. During processing the molecules tend to be more oriented than relaxed, particularly when Gate sheared, as during injection molding. Figure 1 schematically depicts the cavity melt flow looking at a part’s thickness during the injection molding processes.2 Orientation consists of a Surface Highly Oriented

Flow Front

Velocity Profile

Core Orientation From Bulk Shear

Extensionally Oriented Skin From Stretching

Sub-Surface Orientation From High Shear Near Wall

Figure 1. Cavity melt flow looking at a part’s thickness.

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controlled system of stretching plastic molecules to improve their strength, stiffness, optical, electrical, and other properties. For example, molecular orientation results in increased stiffness, strength, and toughness as well as liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation.3 However, orientation resulting from the melt flow during processing can be deliberate or undesirable depending on the direction of applied load or stresses when used as a finished product. When the orientation is accidental and unfavorable, which is very common in injection molding practice, it is typically referred to as so-called residual stresses or molded-in stresses. The term residual stresses identifies the system of stresses that are in effect locked into a part, even without external forces acting on it. Orientation or residual stresses play an important role in toughness enhancement because toughness is primary based on the mechanics of craze formation and shear band (craze and flaws) formation. The shear bands determine the fracture mode and toughness of a polymer when subjected to impact loads. The amount of energy dissipated depends on whether the material surrounding the flaws deforms plastically. For toughness enhancement the residual stresses play an important role in the suppression of craze formation, by avoiding the stress state that promotes brittle fracture. For instance, at room temperature an oriented PS is a brittle, glassy, amorphous polymer, whereas a uniaxial oriented PS is highly anisotropic. High tensile strength, elongation, and resistance to environmental stress crazing and cracking are achieved in the direction of orientation. However, an oriented PS is weaker and more susceptible to stress crazing in its transverse direction than is an unoriented PS. Biaxially oriented PS is strong and tough in all directions.4 Many factors influence orientation or residual stresses, such as the design of the part, the design of the mold, and the processing conditions. There are three typical causes of residual stresses in the injection molding process: 1) Nonuniform shear action - as can be seen in Figure 1, molecules are highly oriented near the mold walls as compared to the bulk and this shear tendency due to the melt flow behavior results in nonuniform orientation through a part thickness and part width with respect to the flow direction; 2) Nonuniform heating and cooling - the introduction of residual stresses can be the result of nonhomogeneous plastic deformation occurring during thermal and mechanical actions, arising from changes in either volume or shape. Thermal treatments like quenching and annealing introduce changes in physical and mechanical properties. For example, with sheet plastic the stresses created by quenching are the result of uneven cooling, when the surfaces cool faster than the core. This produces nonuniform volume changes and properties throughout the thickness. The compressive stresses on the surfaces of the quenched plastic produce tensile stresses in the core, which maintain the equilibrium of the forces;4 3) Nonuniform part dimension - varying wall thickness from thick and thin sections in a part induces residual

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stresses. This is due to the different rates of shrinkage (causing warpage), and possibly void formation in the thick portion. Since the parts in a mold solidify from their outer surfaces toward the center, sinks will tend to form on the surface of a thick portion.5

VIBRATIONAL MOLDING CONCEPT With the problems in mind, vibrational molding technology has been applied to the injection molding process. The patents for the specific types of vibrational molding technology are owned by TherMold Partners L.P., Stamford, Connecticut.6,7,8 The concept of vibrational molding is based on the oscillation of a molten polymer to control rheological behavior of polymers within the mold cavity during processing. Oscillating polymer melt within the cavity controls the effects obtained from shear stresses, such as molecular orientation. This concept was verified by using a compression molding technique. When the vibrational molding technology is applied to the injection molding, the vibration force can be induced during two stages, the injection stage and holding stage. When the vibration is induced during the injection stage, the melt flow behavior is altered as compared to the flow in the injection molding process. When the vibration is induced during the packing stage, it generates a different type of pressure gradient/distribution as compared to the gradient/distribution in the injection molding process. This pressure gradient generated by vibrational molding results in "In-Mold Flow" and controls the internal friction.9 "InMold Flow" modifies the flow advancement. There is induced a distinct flow pattern as compared to the flow pattern resulting from the conventional molding. Also, the amounts of shear effects on the melt can be increased or decreased because vibration frequencies and amplitudes control the internal friction which results from shear stresses and hydrostatic pressure.8 This vibration allows for the simulation of various cooling rates that can be higher than the values obtainable by conventional conductive cooling. In addition, high pressure processing is achieved by controlling hydrostatic pressure. Therefore, the effects of high cooling rates or the effects of lower temperature processing as well as high pressure processing can be obtained by inducing vibrations while maintaining normal molding conditions. The cooling rates through the thermokinetic transition temperatures, such as melting temperatures, Tm, or glass transition temperatures, Tg, can be high, and these transition temperatures can be raised. Additionally, more uniform temperature gradients are obtainable since the vibration energy is transmitted to the entire part. This internal friction controls molecular orientation/relaxation which in turn result in controlling internal stresses as well as molded part properties.

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Figure 2. Schematics of VGS I.

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Figure 3. Schematics of VGS II & II-A.

VIBRATIONAL MOLDING APPLICATION METHODS There are five types of vibrational molding application methodologies, which are referred to as Rheojector (Vibration Generating System - VGS) I, II, II-A, III, IV and IV-A, however the VGS IV and IV-A systems, which are much more advanced and more flexible than other vibration Figure 4. Schematics of VGS III. generating systems, are not disclosed since they are pending patent approval. Figure 2 depicts the schematic of the VGS I system with the rotary valve in its initial position. The VGS I system was installed between the mold die and the barrel, and this system was used for the proof of concept. This system is very similar to the two stage injection molding machine. The injection screw injects the melt into the chamber when the rotary valve is in the initial position, then the rotary valve rotates 90 degrees and the plunger injects the melt into the mold cavity. During the injection and/or holding stage, the plunger vibrates to manipulate the melt. Figure 3 depicts the schematic of the VGS II and II-A. The VGS II system embeds piston(s) within the mold cavity or the mold, and these pistons manipulate the melt. To install the VGS II system, the mold die needs to be modified. The VGS II-A system uses the same concept as the VGS II system, however the mold modification is very minimal. Instead of placing the pistons within the mold, the VGS II-A system places the pistons at the parting line of the mold. Figure 4 depicts the schematic of the VGS III system. The concept of the VGS III system is very similar to the VGS I system, however the rotary valve used for the VGS I was removed in order to have two stage processing. The sensors, the controls of VGS I and VGS II, and the controls of the injection machine were interfaced

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through the data acquisition board to a personal computer. The entire process which was the combination of vibrational molding technique and the injection molding was monitored and controlled by the external computer.

EXPERIMENTAL PROCEDURE The objective of this study was to enhance the part properties by applying the vibrational molding technique to the injection molding process. The materials used for this particular study were Polystyrene (PS), Polycarbonate (PC) and Polyethylene Terephthalate (PET). The VGS IV system was used for this study to implement the vibrational molding (VMT) technology to injection molding processes. The molding conditions were the same for both VMT and injection molded specimens. TSC internal stress measurements were performed for both VMT and reference samples under identical conditions. At first, the reference samples were produced under the conditions which meets the resin manufacturers’ performance specification. The VMT samples were produced just by simply applying the vibration while using the processing conditions obtained during the reference sample production. The internal stresses were measured by Thermally Stimulated Current (TSC) Spectrometer provided by TherMold Partners L.P., Thermal Analysis Instrumentation Division. Figure 5 depicts the picture of TSC experiment station. TSC measures molecular mobility by observing the displacement current generated by dipole motion. Typically molecules are oriented in certain directions during processing and trapped within a part after solidification. When a part is heated up to a certain temperature (i.e. Tg or Tm), molecules relax and the displacement curFigure 5. Picture of thermally stimulated current (TSC) spectrometer. rents measured in a proportion to the amount of stress originally in the specimen. The results is a spectrum of current versus temperature. The total amount of internal stresses in a part can be derived from the area under the curve of current-temperature curve obtained in the TSC data. In layman’s term, a part has low internal stresses if the area under the TSC curve is small, and a part has high internal stresses if the area under the TSC curve is large.11

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Figure 6. Stress-strain curve for PS.

Special Molding Techniques

Figure 7. Stress-strain curve for PC.

Figure 9. Birefringence pattern of PC lens.

Figure 8. TSC measurement data for PC injection molding VMT.

RESULTS AND DISCUSSION

The results shown in Figure 6 was obtained from PS specimens. Figure 6 depicts the comparison of the stress-strain curve between the vibrational molding (VMT) specimen and the injection molded specimen. As seen in Figure 6, the VMT specimen clearly shows the improvement in toughness (area under the stress-strain curve) as well as tensile strength. For instance, the elongation at break was boosted up from 6.17 to 8.19% by VMT, and the improvement was 32.7%. The tensile strength was boosted up from 45.36 MPa (6578 psi) to 55.47 MPa (8043 psi), and the improvement was 22.3%. The results shown in Figure 7, 8 and 9 were obtained from PC specimens. Figure 7 depicts the comparison of the stress-strain curve between the vibrational molding (VMT) specimen and the injection molded specimen. Figure 8 shows the TSC’s internal stress measurement data for the same specimens. Figure 10 shows the birefringence pattern for injection molded PC lens and VMT PC lens, respectively. As seen in Figure 7, the VMT specimen clearly shows the improvement in toughness (area under the stress-strain curve) as well as tensile strength. For instance, the elongation at break was boosted up from 75.19% to 104.00% by VMT, and the improvement was 38.3%. The tensile strength was boosted up

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Figure 10. Stress-strain curve for PET.

from 55.75 MPa (8084 psi) to 69.39 MPa (10061 Figure 11. TSC measurement data for PET. psi), and the improvement was 24.5%. Figure 8 clearly shows the difference in the area under the current-temperature curve between the VMT sample and the injection molded sample. By comparing the birefringence pattern of injection molded lens and VMT lens in Figure 9, significant differences in the formation of birefringence are seen. The injection molded lens has more discontinuity in birefringence, which reveals nonuniform morphology. However, the VMT lens part has more uniformity, which reveals relatively more uniform morphology. The results shown in Figure 10 and 11 were obtained from PET specimens. Figure 10 depicts the comparison of the stress-strain curve between the vibrational molding (VMT) specimen and the injection molded specimen. Figure 11 shows the comparison of the TSC’s internal stress measurement data for the VMT (4 curves as indicated as Vibrational Molding) and injection molded specimens (4 curves as indicated as Regular Molding). As seen in Figure 10, the VMT specimen shows slight improvement in toughness (area under the stress-strain curve). The elongation at break was improved from 216.3 to 226.9% by VMT, and the improvement was 4.9%. However, it must be noted that the primary objective of producing the PET samples was to reduce the processing (melt) temperature while maintaining the part clarity. The test results shown in Figure 11 clearly shows the difference in the amount of internal stresses (area under the current-temperature curve) between the VMT sample and the injection molded sample. The VMT curves (indicated as Vibrational Molding) were more like flat lines as compared with the injection molding (Regular Molding) curves. The reduction of the internal stresses due to VMT was as much as 80%. In addition, the VMT curves show more repeatability than the injection molding. This results indicates that VMT had less deviation in the amount of internal stresses in the part than the injection molding. By considering the test data and the earlier theoretical explanation, reducing the internal stresses in a part (or making a more relaxed part) improves the toughness. However,

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according to the earlier explanation the more oriented part, which has more internal stresses, should have higher strength than a more relaxed part. This was not seen in our experimental data. This may be because vibrational molding produces parts with more ordered structures while maintaining the relaxed structures instead of having the molecules under tension and/ or compression within a part.

CONCLUSION Based on the test results, using vibrational molding can provide the following advantages: 1) vibrational molding can reduce the internal stresses without having secondary operation such as annealing. 2) vibrational molding can reduce the internal stresses to improve part’s toughness without sacrificing other tensile properties such as tensile strength. 3) vibrational molding can produce a part with improved performance without sacrificing a cycle time.

REFERENCE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Akihisa Kikuchi and Robert F. Callahan, "Quality Improvements Resulting From RHEOMOLDING Technology During Injection Molding Processes", SPE ANTEC’96 proceedings, pp. 764-768. Donald V. Rosato et al, 1991, Designing with Plastics and Composites; A Handbook. New York: Van Nostrand Reinhold, pp. 617. Donald V. Rosato et al, 1991, Designing with Plastics and Composites; A Handbook. New York: Van Nostrand Reinhold, pp. 118. Donald V. Rosato et al, 1991, Designing with Plastics and Composites; A Handbook. New York: Van Nostrand Reinhold, pp. 791-794. Donald V. Rosato et al, 1991, Designing with Plastics and Composites; A Handbook. New York: Van Nostrand Reinhold, pp. 791-794. Doug Smock, Nov. 1993, ''Good vibes boost part properties in quantum leap; 'Rheomolding' will greatly expand application for plastics. Developer is seeking license agreement now.", Plastics World, Vol. 51, pp. 14-15. Sara Ferris, Dec. 1993, "New technology vibrates melt, affects plastic microstructure.", Plastics Machinery & Equipment, Vol. 22, No. 12, pp.37. John De Gaspari, Mar. 1994, "Melt Flow Oscillation Improves Part Properties", Plastics Technology, Vol. 40, No. 3, pp.21-23. Akihisa Kikuchi and Robert F. Callahan, "Quality Improvements Resulting From RHEOMOLDING Technology During Injection Molding Processes", SPE ANTEC’96 proceedings, pp. 764-768. J. P. Ibar, 1981, "Rheomolding: A new Process to mold polymeric materials", Polym. Plast. Technol. Eng. Marc Galop, “Characterization of Internal Stresses Using The Thermally Stimulated Current Technique”, prepared for publication.

Experimental Determination of Optimized Vibration-assisted Injection Molding Processing Parameters for Atactic Polystyrene

Alan M. Tom, Akihisa Kikuchi, John P. Coulter Lehigh University, USA

INTRODUCTION As manufacturing becomes increasingly competitive, improved processing technology is continuously needed. One such new technology, vibration assisted injection molding, appears promising. Several mechanisms through which to implement the technology have been studied during the past decade, but the complete understanding and effective deployment of the concept remains to be realized. Vibrational Assisted Injection Molding (VAIM) was initially implemented and introduced experimentally to the injection molding process in the early 1980’s. To date, the concept has shown promising results in its ability to improve aesthetic, mechanical, thermal, and optical properties of molded parts. Although conventional injection molding techniques had made polymer manufacturing popular, associated final products are often not defect free. Typical final product defects include sinks and voids within the material, warpage from residual stresses created upon solidification, and weldlines that affect appearance and structural integrity. One of VAIM’s potentially biggest contributions to the manufacturing industry related to injection molded plastic parts will be in its ability to reduce material property variations, and hence enhance quality control through the reduction of rejects. In it’s simplest form, VAIM applies mechanically induced vibrational forces to polymer flow melts during the injection molding process. Once applied to the plastic, the vibrational forces can be controlled to enhance and change the micro as well as the macro structures of the polymer material either at specific locations within the part or throughout the molded part. Improved properties of plastic molded parts include mechanical properties (tensile and impact strength), thermal properties (glass transition temperature and melt temperature), optical properties (transparency and birefringence), and aesthetic properties (weldlines, warpage, and color uniformity). Current modern day systems that have been developed include SCORIM (Shear-Controlled Orientation Injection Molding) by Scortec Inc.,1-4 Push-Pull process,3,5-7 Injection Spin Process,8,9 Moving Boundary technique,10,11

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and Rheomolding by TherMold Partners.2,12-18 These modern VAIM systems developed within the past decade have been experimentally implemented, but a complete understanding on a scientific level of both the mechanical and material property improvements have yet to be realized. For instance, it is known theoretically that process conditions such as temperature, pressure and cooling rate affect the degree of crystallinity, the size, type and distribution between crystallites and the amorphous phases. These crystal structures in turn affect the physical and mechanical properties of the final plastic parts such as polymer glass transition temperature and melt temperature. However, experimental results using VAIM fall short of scientifically explaining what effects specific processing variables such as melt oscillation type, frequency, initiation time, and duration have on material properties, specifically glass transition and melt temperatures. Therefore, a controlled scientific experiment was necessary to evaluate and determine optimal molding conditions and the effects of all processing conditions on final product properties.

EXPERIMENTAL PROCEDURE EXTERNAL HARDWARE As shown in Figure 1, a Gateway 2000 GP6 series personal computer was used as an external medium to run LabVIEW software. LabVIEW software enabled the user to vary BOY 15 S vibrational molding parameters such as freInjection Molding Machine Reglomat RT20 quency, amplitude, vibration start and stop time Thermolater Gateway 2000 GP6 by interacting with a 15 ton BOY injection Personal Computer molding machine via a National Instruments A/ Figure 1. Experimental hardware and software setup. D converter and DAQ relay board. Two power amplifier relays were used to ramp up control output digital signals emitted by the DAQ board in an effort to assure sufficient electrical power was being generated to operate the hydraulic compression and decompression valves of the injection molding machine. Mold temperatures were controlled through the use of a Reglomat RT20 thermolater that supplied heated water flow in a closed loop network of hoses to an ASTM standard tensile test mold. Tensile test specimens produced in the shape of dogbones were in compliance with ASTM D638-91, standard test methods for tensile testing of plastics. Control Box with DAQ and Amplifier

VAIM SOFTWARE DEVELOPMENT In an effort to supply oscillatory pressure vibrations to polymer melt, computer software was written with the aid of LabVIEW to physically simulate the manual operation of push-

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ing and releasing both injection and decompression buttons on the injection molding machine in an alternating fashion. Choosing to control the injection molding machine in this manner, versus writing software that utilized a square wave Decompression function generator, allowed for the advantage of doing experimental vibrational tests with constant stroke amplitude while still being able to vary other machine processing variables such as Figure 2. Injection and decompression process. vibration frequency. The software activation of screw injection and decompression motions is shown in Figure 2. Compression

CONTROL SPECIMEN TESTING Polystyrene single gated control test specimens were molded using conventional injection molding procedures and were used as a baseline reference for comparative measures against those molded with mechanical vibration. In an effort to obtain optimal machine molding parameters for baseline tensile test specimens, Taguchi’s method was implemented on six machine parameters at three levels of operation, thereby producing 729 separate experiments. Five test specimens were produced for each control experiment for an overall total of 3,645 polystyrene dogbones. TESTING OF VAIM SPECIMENS In a similar fashion, vibrational molded samples were produced and experimentally tested for optimal molding conditions within a particular processing window. Four molding parameters were chosen to investigate the preliminary optimal VAIM molding conditions. These were Delay Time to Begin Vibration, Oscillation Frequency, Vibration Duration, and Duty Cycle. Each parameter was tested at three levels of operation producing a total of 81 experiments. Each experiment produced 5 VAIM samples for an overall total of 405 test specimens. The Delay Time to Begin Vibration variable is the amount of time that the machine remained in idle after polymer had been injected into the mold cavity before mechanical vibrations were activated. This variable was tested at 0, 0.5, and 1.0 seconds. A Delay Time of 0 seconds represents an immediate activation of the vibration control mechanism following complete polymer injection. Previous studies have indicated that large increases in polymer viscosity occur at low oscillating frequencies. Therefore, it was determined that 1, 2, and 3 Hz frequencies would be tested. The VAIM Vibration Duration parameter was tested at 5, 10, and 15 seconds and

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represents the length of time that vibration oscillations were applied to polymer melts. The Duty Cycle, Tinj/Tdec, represents the ratio of machine injection time to decompression time for one complete cycle of vibration. Time (sec) Time (sec) Figures 3(a) and 3(b) depict examples of duty (a) (b) cycles for a 2:1 (i.e. 2 to 1) ratio of injection Figure 3. (a) 2:1 Duty cycle for a 1 Hz frequency. (b) 2:1 time to decompression time for 1 Hz and 2 Hz Duty cycle for a 2 Hz frequency. frequencies, respectively. For a 1Hz frequency, the period of one complete cycle will be 1 second, and therefore during a 2:1 duty cycle, the machine injection and decompression positions will be sustained for 0.66 s and 0.33 s respectively. The fact that the injection time was always longer relative to the decompression time was necessitated by the observation that any duty cycle ratio less than one produced parts that were not completely filled and exhibited sink marks. This was a direct result of less than minimal packing pressure needed to counteract the effects of the observable defects. As shown in Figure 3(b), for a 2 Hz frequency, the period of one vibration cycle was reduced to 0.5 s and the machine injection and decompression positions should then be held for 0.33 s and 0.17 s respectively. 2:1 Duty Cycle for 2Hz Frequency

Injection Signal

Injection Signal

2:1 Duty Cycle for 1Hz Frequency

+1

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+1 0

-1

-1

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0.66

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TENSILE TEST SPECIMEN AND INSTRUMENTATION A single gated ASTM tensile test specimen in the shape of a ‘dogbone’ was used to determine the optimized injection molding parameters for control and VAIM experiments. Test specimens were produced and tested under the guidelines of ASTM D638-91 on an MTI Phoenix-486 tensile test machine with a load cell capacity of 10,000 lbs and a cross head speed of 0.1 in/min. Dimensions of each specimen, for the purpose of determining cross sectional area, were measured and recorded using a digital caliper.

PRELIMINARY OPTIMAL MOLDING PARAMETERS Preliminary results for the optimal molding conditions on VAIM single gated, polystyrene, tensile test specimens proved to be conclusive in setting a general trend that could be used as guidelines for the further study necessary to completely optimize VAIM molding conditions. Utilizing Taguchi’s method, a main effects analysis was done on the ultimate tensile strength (UTS) results of 405 VAIM specimens and plotted against 4 individual molding parameters shown in Figure 4(a), (b), (c), and (d). Note that each data point plotted is representative of an average UTS determined from 135 samples. With molding machine parameters set at the same settings as the optimized control sample settings, the results of optimal Delay Time to Begin Vibration is observed to be 0.5

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Main Effects Analysis UTS vs. Frequency

7200

Ultimate Tensile Strength (psi)

Ultimate Tensile Strength (psi)

Main Effects Analysis UTS vs. Delay Time to Begin Vibration

7100 7000 6900 6800 6700

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Main Effects Analysis

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Ultimate Tensile Test (psi)

Main Effects Analysis UTS vs. Vibration Duration

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7100

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6900

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Figure 4. Preliminary experimental results showing VAIM parameters and their effects on final product ultimate tensile strength (UTS).

s. As shown in Figure 4(a), the longer it took to initiate vibratory motion, the greater the risk for polymer solidification within the mold cavity which in turn reduced the effects of VAIM. The VAIM Vibrational Frequency parameter in Figure 4(b), showed increasing results with increasing frequency and therefore a 3 Hz frequency was chosen as the optimal value. This is partly due to the effects of shear thinning which reduces polymer viscosity and increases the flow characteristics of the polymer material being molded. The optimal molding condition for Vibration Duration was determined to be 5 s, as depicted by the results of Figure 4(c). There is a high probability in existence that any machine vibratory motion exerted after this time period (i.e. 10 s or 15 s) would not be felt locally by the solidified polymer within the mold. In fact, VAIM test specimens created with 10 s and 15 s duration’s exhibited sink marks that were a direct result of insufficient packing pressure during solidification. This suggests that at some critical time during the polymer injection process, oscillatory vibrational motion should cease in order to allow for a normal, necessary packing procedure. Any polymer packing within the mold done by oscillatory pressure vibrations after this critical time was futile and actually was observed to degrade polymer strength. A VAIM Duty Cycle of 2:1 was chosen as the optimal injection and decompres-

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Main Effects:Part 2 UTS vs. Vibration Duration

8200 8100 8000 7900 7800 7700 7600 7500 7400

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Ultimate Tensile Strength (psi)

Main Effects:Part 2 UTS vs. Frequency

3

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7

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Figure 5. Refined experimental results showing optimal VAIM frequency of 8 Hz and maximum vibration duration of 6 seconds correlating to tensile strengths of 8157 and 7867psi, respectively.

sion time ratio that produced parts with the highest ultimate tensile strength, as shown in Figure 4(d).

REFINED VAIM OPTIMAL MOLDING PARAMETERS During a secondary phase of the research, refined VAIM optimized molding conditions Table 1. Refined optimal VAIM were determined for a processing window that molding conditions for polystyrene included vibrational molding machine frequencies of up to 8 Hz. From an observed general Delay time to begin vibration 0.5 s trend previously obtained during the preliminary study, it was determined that the four levels of Vibration frequency 8 Hz parameter testing could now be narrowed down Vibration duration 6.0 s to 2 by eliminating variations in Duty Cycle and Delay Time to Begin Vibration to 2 to 1 and 0.5 2:1 Duty cycle (Tinj/Tdec) seconds, respectively. The results of the remaining two parameters, frequency and vibration duration, versus UTS are plotted in Figure 5. In summary, the experimental results obtained in the determination of refined optimal VAIM processing conditions on single gated polystyrene test specimens are shown in Table 1. It is interesting to note that VAIM polystyrene test specimens produced an average tensile strength at 8 Hz of 8,293 psi which is an increase in tensile strength of 1,463 psi when compared to control samples previously tested that exhibited an average maximum tensile strength of 6,830 psi. This corresponded to a 17.6% increase in ultimate tensile strength. Also, reference control specimens exhibited a 273 psi variation in tensile strength while VAIM specimens molded at optimal conditions exhibited only 106 psi strength variations. This is a reduction in tensile strength variation of over 60% and definitely shows that VAIM

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deserves some merit as to improving final part quality. Also worthy of discussing is the fact that out of 81 VAIM experiments completed, only 3 produced ultimate tensile strengths that were actually lower than the average ultimate tensile strength result obtained from control specimens. The common factor shared by all three cases was that they were all molded with a Delay Time to Begin Vibration of 1 second. Again, this shows a minimal affect on polymer rheology that was translated to a less than minimal affect on the polymer final morphology.

CONCLUSION AND FUTURE WORK The application of mechanical pressure vibrations to conventional injection molding processes has shown definite advantages and improvements to final product mechanical properties as well as a 60% decrease in product quality variations. Besides showing a maximum increased result in ultimate tensile strength of 21% for a commercial grade polystyrene material, VAIM has partially confirmed the theoretical effects of low vibrational frequencies and their impact on material morphology. Although an amorphous polymer, such as Polystyrene used in this experiment, exhibited improvements in tensile strength, the results should not be considered equally true for crystalline and reinforced polymers since they have not been experimentally tested. Also, their molecular structure and flow property characteristics that characterize these types of materials vary substantially from amorphous polymers. Initial attempts at determining the validity of VAIM is in its infancy and the groundwork has been set for future studies. Future work with this novel injection molding process will require determining VAIM effects on other polymer materials and expanding the range of testing conditions. For example, it was confirmed from experimental results that theoretical low frequencies contributed greatly to final product morphology, but there is a concern as to the limitations of what exactly “low frequencies” mean. There is an experimental trend in VAIM data showing increased strength for increasing frequencies, but surely there is a critical frequency at which the advantages of expending more energy to exert higher vibratory oscillations will not out weigh the cost or effort of producing a stronger part. This general line of thinking can also be applied to other variables such as Vibration Duration, and Tinj/Tdec, which must have the outer limits of their processing window set. Also, future studies should include the effects of amplitude dependency if any. Future studies will include the aid of pressure transducers equipped within the mold, and continued extensive research on polymer behavior to determine a scientific knowledge of the local characteristic variations and their relationship to the applied external global parameters.

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REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Grossman, E.M. Scorim - Principles, Capabilities and Applications in ANTEC '95. 1995: Society of Plastics Engineers. p. 461-476. Ibar, J.P., Control of Polymer Properties by Melt Vibration Technology: A Review. Polymer Engineering and Science, 1998. 38(1): p. 1-20. Kalay, G. and M.J. Bevis, The Effect of Shear Controlled Orientation in Injection Moulding on the Mechanical Properties of An Aliphatic Polyketone. Journal of Polymer Science Part B - Polymer Physics, 1997. 35(3): p. 415-430. Malloy, R., G. Gardner, and E. Grossman. Improving Weld Line Strengths Using A Multi-Live Feed Injection Molding Process. in ANTEC '93. 1993: Society of Plastics Engineers. p. 521-529. Theberge, J., IM alternatives produces performance advantages, in Plastics Engineering. 1991. p. 27-31. Ludwig, H.-C., G. Fischer, and H. Becker, Quantitative Comparison of Morphology and Fibre Orientation in Push-Pull Processed and Conventional Injection Moulded Parts. Composites Science and Technology, 1995. 53(2): p. 235-239. Michaeli, W. and S. Galuschka. Procedure for Increasing the Weldline Strength of Injection Molded Parts. in ANTEC '93. 1993: Society of Plastics Engineers. p. 534-542. Cao, B., et al. Injection-Spin Process. in ANTEC '94. 1994: Society of Plastics Engineers. p. 2603-2606. Cao, B., et al., Injection-Spin Process, in Plastics Engineering. 1994, Society of Plastics Engineers. p. 47-49. Gardner, G. and R. Malloy. A Moving Boundary Technique To Strengthen WeldLine In Injection Molding. in ANTEC '94. 1994: Society of Plastics Engineers. p. 626-630. Gardner, G.P. and R.A. Malloy. Use of The Moving Boundary Molding Technique to Straighten Weld Lines. in ANTEC '96. 1996: Society of Plastics Engineers. p. 685-691. Ibar, J.P. Control of Performance of Polymers and their Blends through Melt Vibration Technology. Critical Review. in ANTEC '96. 1996: Society of Plastics Engineers. p. 769-773. Ibar, J.P., Control of performance of polymers and their blends through melt vibration technology. Critical review. Progress in Rubber & Plastics Technology, 1997. 13(1): p. 17-25. Kikuchi, A. and R.F. Callahan. Enhancement of Molded Product Properties Through The Use of Rheomolding Technology on Injection Molding Processes. in RETEC (PD3). 1996: Society of Plastics Engineers. p. 165-181. Kikuchi, A. and R.F. Callahan. Quality Improvements Resulting From Rheomolding Technology During Injection Molding Processes. in ANTEC '96. 1996: Society of PLastics Engineers. p. 764-768. Kikuchi, A., et al. Molded Product Properties Enhancement through the Use of Rheomolding Technology on Injection Molding Processes. in ANTEC '97. 1997: Society of PLastics Engineers. p. 436-440. Kikuchi, A., et al. Control of Internal Stresses In Injection Molded Parts Through The Use of Vibrational Molding, "Rheomolding", Technology. in ANTEC '98. 1998: Society of Plastics Engineers. p. 2233-2237. Kikuchi, A., J.P. Coulter, and P. Santiago. Vibration-Assisted Injection Molding Technology For Improved Manufacturing. in Competing in a Global Manufacturing Environment. 1999. Behtlehem, Pennsylvania: Lehigh University Center for Manufacturing Systems Engineering. p. 79-87.

Vibrated Gas Assist Molding: Its Benefits in Injection Molding

J.P. Ibar EKNET Research, P.O. Box 385, New Canaan, CT 06840, USA

INTRODUCTION It is well known to those skilled in molding polymeric materials that such defects as weld lines, sink marks, and warpage of the final part are caused by melt fronts collision, unbalanced flow, uneven cooling, non-uniform internal stress and non-homogenous nucleation and growth of crystals as the part solidifies. Varying the processing parameters (e.g., temperature, pressure, flow rates, filling and packing time in the case of injection molding etc.) can result in the modification of the molded part outlook and final product's physical properties, but the modifications are often slight and not quantified, and they also rely, to a large extent, upon the expertise of the molding operator who uses his experience and art to determine the molding processing parameters, the so-called "processing window". Unfortunately, more often than less, the conventional wisdom for a good processing window involves increasing the clamp tonnage, sometimes as high as 25-50,000 psi of pressure, which substantially contributes to the price of the injection molding equipment. The methods of gas assist molding have demonstrated their great usefulness in injection molding to hollow parts out and induce an excellent surface finish. Most of the benefits from applying vibrational energy to the molding process are well-documented.1 The idea of using pressurized gases to transmit vibrational energy to plastic as it enters and fills the mold - and thereby improve the physical and mechanical properties of the molded item - is the subject of a recent patent.2 The technology, called Vibrogaim (Vibration Gas Injection Molding) is the next generation vibration molding technique providing melt manipulation capabilities during molding. Vibrogaim's innovation comes from the use of pressurized gas as the tool for delivering the vibrations from generating devices into the plastic melt. The gas is the means of controlling and maintaining vibration in the system. The present paper explores the various benefits of vibrating gas during injection molding of plastics under gas vibration. Gas can be inserted in the mold prior to melt injection, and vibrated from the subsonic to the arsenic range in order to modify the filling process. Gas can also be inserted

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prior to injection and, acting like a pressurized vibrated gas spring, help induce orientation benefits during filling completion. Vibrated air pressure, localized in specifically designed air-runners distributed around the runners and inside the mold, helps fill and pack the mold, core out hollow parts and balance flow in multicavity molds. Compared to earlier processes that apply the vibrational energy to the entire mold in order to affect the melt1,2 Vibrogaim, by employing pressurized gases to induce the resonance, opens up new ways to influence molding results. One unique advantage of this technology is the ability to alter the morphology of the plastic and upgrade one or more mechanical properties, such as tensile strength, modulus and impact. Other benefits include improved surface finish and the potential to control nucleation and crystal growth rates, reduce internal stresses and warpage, and cut down on sink marks and voids. Based on developmental work with Vibro-molding technology, up to a 25% gain in crystallinity in polypropylene can be achieved with a resulting improvement in low temperature impact and clarity. Also other advantages in molding process itself, such as smoother melt flow and new latitude in controlling mold flow mechanisms. The basic idea in Vibrogaim is its use of pressurized gas to manipulate the melt as it enters and fills the mold in order to upgrade or otherwise alter the properties of the part. The pressurized gas fills the mold assembly, vibrations generated by electrical or mechanical transducers. The gas may be introduced directly into the mold from the back of the cavity and/ or through the injection nozzle. It also can be introduced, via channels or shaped chambers, at specific locations in the mold where the vibration effects are wanted, near the runners and gates. A chemically inert gas, such as nitrogen, is used wherever there will be contact with the melt (cavity, runner system, nozzle etc.). Air is used in the channels and chambers and other passages that are external to the flow path. The Vibrogaim process can apply many types of vibratory energy to the molding process, depending on the effect wanted on the part or the process. The energy modes range from subsonic, low frequency vibration (1 to 30 Hertz), easily obtained with help of pneumatic piston actuators, going all the way up to the ultra-sonic range15,000 to 20,000 Hertz. In many cases, several waves of vibration are used simultaneously or in a sequence at various locations. The advantage of using air/gas is that it is quite straightforward to combine different modes of various frequencies, various amplitudes. The different modes combine their effect on the plastic melt, increasing the elasticity of the melt for the shear oscillation of low frequency, and modifying the mechanism of flow through the runners and in the cavity, through the higher frequency modes. Such flexibility in melt manipulation is not available from other techniques which do not use gas as transfer material.

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VIBRATED GAS MODIFIES THE FILLING PROCESS In a second application of the use of vibrated gas (air in this instance), vibrated air inside a network of air runners located in the mold, near the gate and around plastic runners, is activated to produce a reduction of the friction coefficient at the wall surfaces, allowing either a better flow, a different type of flow or greater throughput. Alternatively, as already mentioned, gas inserted in the cavity prior to filling can be put into oscillation and/or resonance to influence the mechanism of filling, away from the common fountain flow at the tip. In the case of knit lines, a short shot just ahead of the knit line is suspended by the gas pressure and high frequency vibrations propagated towards the flow front to achieve superior welding results. The gas is then released to allow the completion of the filling process.

VIBRATED GAS ACTS AS SPRING TO MODIFY MELT One of the most novel applications of the technology involves pressurizing the mold cavity before the shot. The compressed gas (which may vary from 50 to 3,000 psi) acts like a spring, keeping the melt "floating" for a few seconds before it is allowed to completely fill the cavity. During that time, the melt is subjected to oscillation by a series of low-frequency (10 to 30 Hz) pressure pulses which induce a large increase of its elasticity. The vibratory energy pumped into the melt changes the mechanism of deformation during subsequent filling of the cavity, in particular it favors flow-imposed alignment of the polymer molecules, so that a higher percentage of the molecular bonds are forced into an orientation deformation mechanism. Orientation of the molecular bonds is the basis for existing strengthening by orientation processes and has been shown to directly affect the mechanical and physical properties of the molded plastic. Tensile strength, modulus and other mechanical properties can be improved, depending on the specific process parameters. The process of providing.orientation benefits during injection molding requires the right combination of process variables, or the properties' improvements are non-existent. The gas pressure and the vibration characteristics must be in the right range to oscillate and reorient the melt. Also, the plastic’s rheological temperature cannot be too warm (no more than 45oC above its (frequency dependent) glass transition temperature, so it won't have time to relax and loose the orientation before it can fill and pack the mold. Gas pressure can be used in other ways inside the mold besides molecular orientation: helping to fill and pack the mold, core out hollow parts, and help balance flow in multi-cavity molds. These applications require precise control of temperature and pressure as well as instantaneous timing.

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In addition to the low frequency vibrations that produce the molecular reorientation described above, the Vibrogaim process also uses vibration frequencies up in the ultrasonic range (15 kHz to 20 kHz or higher). These frequencies have other types of effects on the properties of the molded part or process. For example, ultrasonics can be used to induce a local fusion of the weld lines or adjust the balance between the rate of crystallization and crystal growth in semi-crystalline plastics. Smaller crystals can boost impact resistance, light transmission and chemical resistance, while larger crystals can improve mechanical strength. In addition, high-frequency vibrations accelerate the stress relaxation of semicrystalline and amorphous plastics. The faster the return to the equilibrium state, the less the chances of molded-in stress. Another benefit is reduced friction between the melt and the runner walls and other areas in the flow path. Minimizing friction translates into faster molding and better part surfaces as well as lower shear stress and, in the case of optical products, birefringence.

CONTROLS Most of the benefits from applying vibrational energy to the molding process are well-documented.1 Vibrogaim's innovation comes from the use of pressurized gas as the tool for delivering the vibrations from generating devices into the plastic melt. The gas is the means of controlling and maintaining resonance in the system. Resonance is the condition in which the components of a system - in this case, the plastic, mold cavity, mold passageways, etc. - vibrate at the natural frequency of the system. The trick is to find the natural frequency, or to tune the system in order to extract the energy from the vibrations and produce a physical change in the targeted item. This is done step by step with use of an acoustic chamber connected to the mold cavity during filling. The resonance frequency is determined as a function of time as the cavity is filled with hot melt. The Vibrogaim technology uses a sophisticated computer control system designed to monitor and maintain resonant conditions in the mold assembly or other machine components. The closed loop, high speed system integrates inputs from the pressure, temperature and vibration sensors and adjusts these parameters instantly to track with the events of the molding process. Stored programs in the computer, specific for each job, dictate the sequence of control actions, including when the gas is injected into the mold or nozzle or the air into the mold channels, at what pressure, when venting occurs, when vibration is initiated and at what frequency and power level. The principal hardware items for the Vibrogaim technology consists of the vibration sources, gas mixing units, gas and air pressurizing/injection units, valves and other control devices. The process hardware and the computer system are sup-

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plied as a portable package, capable of being used on any molding press. Specific applications in the automotive industry will be presented at the meeting.

CONCLUSIONS The present paper explores the processing of injection molded plastics under gas vibration. Vibrated gas can be used for several purposes. 1. Gas can be inserted and vibrated in the mold prior to melt injection to modify the filling process mechanism, fuse knit lines, heal sink lines and other defects due to flow imperfections. 2. Compressed Vibrated Gas can act like a pressurized vibrated gas spring, which helps induce orientation benefits in the short shot during filling completion. 3. Vibrated air pressure, localized in specifically designed air-runners distributed around the runners and inside the mold, helps fill and pack the mold, core out hollow parts and balance flow in multi-cavity molds. 4. Vibrated Gas can also be used to tag parts for recognition during recycling or later inspection. Vibrogaim technology uses pressurized and vibrated (or pulsed) gas to manipulate the melt as it enters and fills the mold in order to modify the flow pattern (especially when knit lines form) and/or alter the properties of the part. The pressurized gas fills the mold assembly, vibrations are.generated by pneumatic, electrical or mechanical transducers. The gas may be introduced directly into the mold from the back of the cavity and/ or through the injection nozzle. It also can be introduced, via channels or shaped chambers, at specific locations in the mold where the vibration effects are wanted, near the runners and gates. The paper reviews hardware and controls requirements to apply this novel technique to injection molding.

REFERENCES 1 2

3 4 5 6

Plastics Blow Molding Handbook, Norman Lee Editor, Van Nostrand Reinhold, New York (1990). P. J. Zuber, Relationship of Materials and Design to Gas-Assist Injection Molding Application Development in Molding 95, ECM Fith International Conference and Exhibit, March 27-29, 1995, New Orleans. Also see: Gas-Assist Injection Molding, "Design and Processing guide for GE Resins", a GE Plastics publication. H. Eckardt, Annual Conference, SPI Structural Division, 18, 57. S. Shah and D. Hlavaty, ANTEC 91 Reprints, 1479 (1991). S. Shah, "Gas Injection Molding:Current Practices", ANTEC 91 Reprints, 1494 (1991). K. Beattie, "Developments in Cinpres Gas Injection Nozzles", Molding 95, Fith International Conference and Exhibits, March 27-29 1995. P.L. Medina, L.S. Turng, V.W. Wang, "Understanding and Evaluating Gas-Assisted Injection Molding Applications via Computer Simulation", paper presented in the Structural Plastics 19th Annual Conference, Society of the Plastics Industry, Atlanta, Georgia, April 1991. J.P. Ibar, Control of Polymer Properties by Melt Vibration Technology. A Review. Polym. Eng. Sci., 38(1), 1 (1998). The References provides a list of all Patents. J.P. Ibar, Method for Exerting Stress Tensor to Molding Material, US Patent 5,543,092. J.P. Ibar, U.S. Patent 5,605,707 and PCT Application, Method and Apparatus for Controlling Gas Assisted Injection Molding to Produce Hollow and Non-Hollow Plastic Parts and Modify Their Physical Characteristics (1995). B. Miller, Closed-Loop Controller Aids Gas-Assist Control, Plastics World, July 1995, p.13.

Chpater 6: Mold Making and Plasticization Advances in Stack Molding Technology

Vincent Travaglini and Henry Rozema Tradesco Mold Limited, Rexdale, Ontario, Canada

INTRODUCTION In any industry, increasing competition coupled with the need for higher levels of productivity motivates suppliers to provide products with higher quality, in a shorter time and at a lower price. In an industry where processes are already very efficient, the most effective way to accomplish this is through technical innovation. With respect to injection mold making, one of the major technological advancements was the development of the conventional two face stack mold. The ability to mold on two faces provides twice the output from the same molding machine. Prior to this parts were typically molded on one face (single face) in either single or multi cavity configurations. Molding on two faces has been widely accepted over the last two decades. Over the last few years, there have been further technological advances that improve both the productivity and the flexibility of molding operations. These advances include: 1. Four face (level) stack molds. 2. Quick Product Change, QPC, systems. 3. Two cavity stack molds for large parts. Four level stack molds essentially quadruple the output from single face molds. QPC molds allow molders to switch from one product to another in less than one hour in both single face and two face stack mold configurations. Two cavity stack molds allow large parts to be molded in a back to back configuration, thus doubling the machine capacity. The key to each mold configuration is based on the development of the valveless melt transfer system. For each of the stack mold configurations listed, the following paper will detail the engineering principles and technical application of this new hot runner technology.

CONVENTIONAL TECHNOLOGY The simplest mold design, from a runner point of view, is the single cavity, single face mold. The machine nozzle injects plastic directly into the cavity. The single face mold can also be

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SPRUE BAR

MELT PAIR

SINGLE FACE MOLD

CONVENTIONAL STACK MOLD VMTS

VMTS

QUICK PRODUCT CHANGE STACK MOLD

VMTS SPRUE BAR

FOUR FACE STACK MOLD

VMTS

TWO CAVITY STACK MOLD

Figure 1. Runner layout.

extended to a multi cavity layout. In this case, the machine nozzle injects the melt into a runner system that feeds each individual cavity. With the introduction of the conventional two face stack mold, parts could now be molded on two faces. This mold design has been widely accepted in the thin wall injection molding industry, typically with hot runner designs. The melt is injected into an extended sprue (sprue Figure 2. 8 cavity (2x4) conventional stack mold. bar) which is attached to a manifold system. The manifold feeds each cavity on both molding levels (see Figure 1 & Figure 2) From a rheological standpoint, since the runner has increased in length, both residence time and pressure losses in the runner system have increased. Attention must be paid to ensure the hot runner is balanced and all cavities are fed at the same pressure and temperature. The largest advantage to this design is the doubling of machine capacity. By placing the cavities back to back, the stack mold takes advantage of injection force cancellation such that the same machine as a single face mold can be used. A 10-15% increase in tonnage is required to offset the force that the machine nozzle applies to the sprue bar.

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The sprue bar in the conventional stack mold design is typically located on the centerline of the mold. This results in two major drawbacks. First, the cavities cannot be placed in the center of the mold, they must be placed around it. Secondly, parts can be damaged upon ejection since they may come in contact with the hot sprue bar on the first molding level.

TECHNOLOGICAL ADVANCEMENT

Figure 3. Hot runner system for a two cavity stack mold.

The engineering challenge arose with the idea to extend the stack mold design from two to four levels. A Valveless Melt Transfer System [VMTS] was designed in order to pass the melt across the mold parting line. This VMTS is engineered to provide a tight seal each time the mold is closed and the melt is injected at pressures in the range of 20,000 psi. It is a self compensating design that allows for variations in nozzle thermal expansion. When the mold is opened and the VMTS is disengaged, drool is avoided due to the self decompression of the central hot runner system. There are no heavy wear items as associated with valve shut off systems (see Figure 3).

APPLICATIONS FOUR FACE STACK MOLDS The four face stack mold is essentially two stack molds placed back to back. Due to increased mold shut height and plasticizing requirements, it is well suited for the high volume production of shallow parts such as thin wall lids, for the packaging industry (see Figure 4). The first application of the VMTS was in the four face stack mold design. The melt is injected into a heated sprue bar along the mold centerline. Figure 4. 96 cavity (4x24) four level stack mold. It is then transferred to the non operator side of the mold where it splits in two directions. In each direction the melt passes through the VMTS as it crosses the parting line and enters into its respective hot runner block. From there it is routed through a fully balanced hot runner manifold where it feeds individual nozzles for injection into the cavities (see Figure 1). The entire hot runner system is thermally and rheologically balanced such that the first cavity on

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the first level and the last cavity on the fourth level are filled at the same time, temperature and pressure. With the extended runner length, shear rates, pressure drops and shear heat temperature rise are considered when sizing all runner lines. The primary advantage to the four level stack mold is increased plant efficiency. Without modifying the injection molding machine, output is essentially quadrupled over a single face mold. Savings are realized by decreased manufacturing costs which can be passed on to the customer. Cycle times are extended by fractions of a second since more time is required to open and close the mold over all four faces. As in the conventional stack mold, the four level stack mold also takes advantage of the force cancellation by positioning the cavities in a back to back arrangement. By placing an additional two levels in the mold, clamp tonnage is not increased over the conventional stack mold. Other technical features include synchronized opening provided by either a harmonic linkage system or series of splines. The linkage system not only ensures equal openings on all four levels but also activates stripper plates to eject the parts. If the spline centering device is used, standard part ejection can be performed by cam actuated linkages or through the use of internal hydraulic or pneumatic cylinders. Due to additional mold weight, the three center sections of the mold are supported on the machine tie bars using adjustable mold supports. Since its inception in 1991, the four face stack mold has been successfully designed and manufactured in various cavitations. After the first 8 cavity (4x2) mold there have been numerous additional four level stack molds produced; the latest being a 96 cavity (4x24) mold running at a six second cycle. With each new design, thermally and rheologically balanced hot runner systems are engineered to meet the increasing demands of molding environments. QUICK PRODUCT CHANGE, QPC, SYSTEMS QPC refers to tooling designed for the quick changeover of core and cavity module sets without having to change over the entire mold. The main technological objective in the development of this mold design was to reduce the time required to change from one product to another using a large multi cavity stack mold. The intention is to produce groups of relatively low volume products in an efficient stack molding environment. Technical features include: • QPC mold operates in the same molding machine as a conventional stack mold • QPC mold operates at the same cycle time as a conventional mold • the hot runner system is thermally and rheologically balanced

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•

263

shot to shot change over time is less than one hour for both mechanically and air ejected parts in the stack mold configuration • the mold design allows upgrading from a single face mold to a stack mold • machine shut height remains the same from one product to the next The QPC system consists of two basic components: a unique hot runner carrier frame and interchangeable core and cavity sets which are housed in plate modules. All necessary water, air and electrical connections are designed to remain permanently within the carrier frame. Thus when a product change is made, there is no time lost in re-installing these services (see Figure 5). Figure 5. 24 cavity (2x12) Quick Product Change, QPC, system. The sprue bar in the conventional stack mold design presents an obstacle in applying a quick mold change technique. It is virtually impossible to access and remove plates or modules from the stationary side without removing the entire mold. With the application of the VMTS, this problem is eliminated. The melt is injected at the mold centerline and is immediately transferred around the core and cavity modules via a hot runner manifold. The plastic then crosses the mold parting line with the VMTS and passes the melt to a fully balanced, central distribution manifold. This manifold feeds the individual nozzles for injection into the cavities (see Figure 1). The QPC design s major advantage is the flexibility that the quick mold changeovers offers molders. This includes the ability to minimize inventory levels and costs and maintain $Just in Time# delivery schedules. Molders with high product changeovers will realize lower manufacturing costs per part due to the time savings in mold changeovers. The modular system allows molders to start projects initially on a small scale. Expansion from a single face to a stack mold can be done using the original core and cavity module sets. As a result molders can compete with high efficiency tooling without the high initial capital investment. Latest advances include modular QPC stack molds for the production of cutlery items. These molds apply hot and cold runner technology along with cam followed ejector systems on both molding levels. In addition, a design of a four level stack mold incorporating the QPC system has been completed.

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TWO CAVITY STACK MOLDS FOR LARGE PARTS Typically large parts are molded in single cavity tools due to tonnage and platen size limitations. Given conventional technology, in order to increase the cavitation of the mold in either the single face or stack mold configuration, a larger tonnage machine is required. The technical challenge was to increase the cavitation of the mold without having to change the press. Similar to the QPC hot runner design, the VMTS was applied to the two cavity stack mold. By placing Figure 6. 2 cavity (2x1) stack mold. the cavities in a back to back configuration, the result is a two cavity stack mold that runs in the same molding machine as does the single cavity (see Figure 6). As with the QPC design, the melt is injected at the mold centerline into a hot runner manifold housed within the stationary core backing plate and transferred around the core and cavity set on the stationary side. It then crosses the VMTS into a simple hot runner manifold that feeds both parts (see Figure 1). This concept has also been expanded to feeding back to back valve gates. This technological advancement provides molders with a considerable advantage over their single cavity competitors. Output is doubled without having to invest into a larger machine to increase cavitation. As with the four level mold, plant efficiency increases and manufacturing costs decrease dramatically. This technology has been initially applied to the low pressure molding of heavier wall items with low L/t ratios. Recently, it has been applied to molding thin wall containers in a back to back configuration with a wall thickness of 0.7 mm (0.028") and a resulting L/t ratio of 300.

CONCLUSIONS The development of the valveless melt transfer system has opened numerous doors in stack mold design. The ability to transfer high pressure melt around cavities and across parting lines has provided for the #next generation# stack molds such as: 1. Four face (level) stack molds. 2. Quick Product Change, QPC, systems. 3. Two cavity stack molds for large parts.

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The four level and two cavity stack molds provide molders with significantly lower manufacturing costs and increased productivity from existing molding machines. Quick Product Change molds offer flexibility to molding operations, making shorter runs more economical.

Advanced Valve Gate Technology for Use in Specialty Injection Molding

John Blundy, David Reitan, Jack Steele Incoe Corporation

INTRODUCTION The use of valve gate hot runner systems has been generally accepted in our industry as a more precise method of controlling the gate vestige, allowing the user positive open/close capability. The use of larger orifices would allow for faster fill and part stress reduction. However, due to the complexity, these systems do present areas of concern. Consideration for operating the mechanism either hydraulically or pneumatically must be properly designed to provide reliability. Contamination generated by material passing around the shut-off pin can cause flow lines on the finished part with some resins. Absolute control of the open/close position, which is usually based on time, can present difficulties. Advances in system design, computer technology, materials and manufacturing processes have allowed great improvements in valve gate systems. These advanced systems will be reviewed in the following four sections of this report.

CO-INJECTION Co-injection, the molding of two similar or dissimilar resins in the same molded part creating a separate skin of one material and a separate core of a second material, requires the use of valve gates. Advanced valve gate systems are key to successful single cavity, multi-cavity or sequential molding for co-injection applications. The Figure 1. special valve gate cylinder provides for three unique positions for resin flow and controls the volume of skin and core material (Figure 1). Position one starts the injection of the skin (surface) layer into the cavity. This is followed by the second position which starts the flow of the core material while continuing the flow of skin material. After the desired amount of core is achieved, the pin moves to the secondary position allowing the skin material to fully encapsulate the core. Finally, pack pressure POSITION 1

POSITION 2

POSITION 3

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is applied prior to shutting off both skin and core materials. This unique three position actuator, along with the shut-off pin and mixing pin, provide the necessary mechanical flow channels and precise shut-off sequence for successful coinjection processing. This advanced system is the result of a joint technology between INCOE Corporation and Bemis Corporation. INCOE’s patented valve gate technology along with Bemis’ processing “know how”, provide the stimulus for improved co-injection technology. This part (Figure 2) illustrates a part performance advantage in that the overall rigidity of the arm rest was improved while allowing the Figure 2. desired cosmetic requirements to be maintained. Various features and benefits can be realized by the co-injection process including improved engineering properties, lower product cost and cycle time reduction.

GAS-ASSIST VALVE GATE

Figure 3.

When applying gas-assist technology for applications where the gas is introduced in the mold, in either the part or runner system, valve gate technology is, in most cases, required. This process requires a valve gate to allow for proper pack-out and to keep the plastic from flowing back into the manifold. This example (Figure 3) is typical of any gas-assist system and can be duplicated for multiple cavities or for sequential fill applications. The systems are designed such that all valve gates are closed prior to the gas injection cycle.

CLEAR-FLOTM VALVE GATE The development of the Clear-FloTM valve gate system (Figure 4) has advanced the technology by removing the valve gate shut-off pin from the material until the exact point of close. This advanced design concept opens the door to the use of shear/heat sensitive materials in valve gate applications. The flow channel is increased and flow separation “which is inherent in conventional valve gates” is eliminated. This larger flow channel also minimizes flow induced material stress. The design allows the material to maintain a consistent veloc-

Advanced Valve Gate Technology

269

Figure 5.

Figure 6.

ity from the manifold entrance to the gate area. These advantages corrected a processing problem shown in this lawn tractor hood application (Figure 5). The quality issue was a shadow or streak created by shear sensitive colored resin. The shut-off pin separates the material flow creating a difference in velocity which would cause the color concentrate to be somewhat darker, thus, creating an imperfection on the part. The larger flow channel provided a better condition for the engineering resin. The rate of reject was reduced from 18% to near zero. Color changes are dramatically improved as the absence of the shut-off pin eliminates the flow separation condition as well as creating an area that is not directly removed by the new resin. This advanced design also incorporates several other advantages. First, a straight shut-off pin is used (Figure 6). This concept provides positive seal between the shut-off pin and pin channel, which virtually eliminates plastic leaks, as well as providing for an improved gate appearance. Additionally, the actuator provides for improved reliability by reducing the components from Figure 7. 19 to 9 and uses either pneumatic or hydraulic power (Figure 7). Figure 4.

270

Special Molding Techniques

SEQUENTIAL GATE CONTROL SYSTEM

Figure 8.

Figure 9.

Further, improving the technology available in the area of valve gate pin position control has been achieved using a newly developed Sequential Gate Control System. It is the result of developing and using proprietary software with the latest in Computer technology. Based on Windows Software, this system can be used on any valve gate system and can be interfaced with all injection molding machines. This development is especially beneficial for sequential molding, co-injection or the Clear-FloTM systems (Figure 8) since pin position directly influences molded part quality. The controller provides precise control based on linear movement of the injection screw. The system is designed to allow for two open and close sequences per valve gate cycle. The sequential valve gate system can be activated by time and/or linear position using either the inch or metric scale. The system includes many unique features such as system ready protection, modem hook-up for trouble shoot-

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271

ing or upgrade, mold file storage for quick set-up security code and 24 hour printable run history. The system can control up to 40 separate valve gates for positive open and close actuations (Figure 9).

SUMMARY Valve gate systems continue to be a very important part of many of today’s molding applications. Valve gates are being used for co-injection both single cavity and multi-cavity molds. Valve gates are, in most cases, mandatory for gas-assisted molding. Valve gate systems are also used for sequential molding, family molds and over molding. The refinements in valve gate technology will continue to advance the capabilities of the plastic molding industry.

In-mold Labeling for High Speed, Thin Wall Injection Molding

Gary Fong Tradesco Mold Ltd.

INTRODUCTION This paper introduces a simple and cost effective IML system developed for high speed, multi-cavity injection molds. After this brief introduction, some of the basic as well as specific design issues will be discussed. In no way can this article cover the many facets of IML system design. To conclude, a brief overview of future design considerations will be presented. The concept of in-mold labeling has existed for over 14 years.1 IML have been applied to all forms of plastic packaging: thermoforming, blow molding, and injection molding. IML technology can be found in European, Asian, and other foreign molding shops. IML systems are readily available for high volume packaging applications. However, IML systems for high speed, multi-cavity injection molding applications are rather expensive. The technology ranges from simple mechanical swing arms, to complex robotic arms with computer control. The type of IML system to incorporate into a process largely depends on the production level, as well as the cost of post-printing. In general, an IML system will increase the cost of a mold by 10%-40%.2 The main drive for this project was to develop an IML system that was designed in conjunction with the mold. Currently, it is the trend for the injection machine supplier, or a robotics supplier to implement the IML system. This leads to a larger IML development team, involving multiple companies, and a certain amount of confusion. It is believed that the concurrent design of the mold and IML system will result in a synergistic molding system. This IML system was developed for a four-cavity lid mold. However, future adaptations of this system could see its use in multi-level, multi-cavity molds or even single cavity, single face tools.

TECHNICAL ISSUES There are many issues involved with an IML system, some of which are

274

• • • • • • • •

Special Molding Techniques

label properties spatial limitations mechanically or electrically driven robustness of the IML system material selection of IML components path of transfer arm into the mold space label positions at start and end of cycle continuous label feed mechanism

LABELS The labels used for this project were made from oriented polypropylene (OPP), pre-cut to size. The surface was treated with an antistatic coating to facilitate label separation by the IML system. The labels were sized at 91.80 mm [3.614”] by 111.5 mm [4.389”], with four corner fillets of 25.40 mm [1.000”]. The thickness were 0.080 mm [0.003”]. No static charging devices were used to assist label handling in this system. Rather, a vacuum was used wherever the label was required to ‘adhere’ to a surface, namely the pickup plate and the cavity surface.

BASIC DESIGN CONSIDERATIONS Spatial limitations are a major concern for an IML system. The IML system requires sufficient room to move its transfer arm from the label holder into the mold space. The path of entry into the mold space can be accomplished either from the top or the side of the mold. In general, a mechanism at the bottom of the mold will interfere with falling parts during the part ejection stage. The working space for the IML system also dictates its design. In general, the larger the work space, the simpler the design requirements. The available space is dictated by the tie-bar space, mold size, as well as molding environment. A large tie-bar space, with a small mold and a very spacious molding environment will provide the most workspace for the IML system. In reality, this situation does not occur very often. This IML design was chosen to enter through the side of the mold space. Room was made available during the design of the mold to accommodate an IML system. The molder was also willing to further accommodate the IML system, by increasing the size of the safety gates on the machine. The complete removal of the gate was avoided, as only a few inches of extra space was needed. The key criteria when choosing an IML system are speed, reliability, simplicity, and robustness.

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275

The core system for this IML system is based on an electro-mechanical design. The various pneumatic pistons and vacuum generators are controlled with a PLC. The motion of the transfer arms are mechanically controlled by a spline, driven by the moving plates of the injection machine. The speed of this IML system is controlled by the rate at which the mold opens and closes. By tying the system directly to the mold, it does not increase the cycle time. The speed of the IML system is very important for high-speed injection molds. A delay, due to an IML system’s slower speed, can strongly affect the molder’s bottom-line. The mechanical control over the motions of the transfer arm provides the molder with peace of mind, as the sliding arm should never be caught between the closing plates. As well, the repeatability of the transfer arm’s motion is maintained due to its mechanical connection to the mold. The ‘Mechanical Part Removal System’ was chosen as the core technology behind the prototype IML design. The ‘Mechanical Part Removal System’ was originally designed to remove plastic lids from a multi-level, multi-cavity mold. This system was mounted directly onto the side of the mold. The simplistic design of this mechanism allows for easy maintenance whenever required. Perhaps one of the greater concerns for an IML system is robustness. The system should, ideally, last forever. The ‘Mechanical Parts Removal System’ has successfully operated in the field through 3 years of continual service. This in itself is a good measure of the basic system’s robustness.

SPECIFIC DESIGN CONSIDERATIONS Simply speaking, most IML systems have both a transfer arm and a label holder. Both components in this IML prototype are bolted to the side of the mold with holder arms. The mold is first dropped into the machine, then the IML assembly is attached to the side of the mold. Assembly of the various IML components must be manageable by a lone technician. Thus the system should be lightweight and compact. LABEL TRANSFER ASSEMBLY The following is a quick overview of the label transfer assembly components; the assembly includes the transfer arm, pick-up plates, linkage, bearing housing, and spline. Their function, and the material choices made for the components are discussed. Refer to Figure 1 for part identification. The system’s transfer arm is made from aluminum to reduce the inertial loads being applied onto the links. Though some loading is applied to segments of the arm, it wasn’t felt to be significant enough to warrant concern. A steel transfer arm would have created prob-

276

Special Molding Techniques

bearing housing tie bar spline

magazine

linkage

Mold linear guide rails

pick-up plate

Direction of Motion

transfer arm

Figure 1. Picture of the in-mold labeling transfer arm assembly.

lems with regards to the sliding wear and linkage strengths. The linear motion of the transfer arm is controlled through a set of linear guide rails. The linear guides limit the sliding motion to a lateral direction. The pick-up plate is attached to the transfer arm with four pins and a piston. The piston is pneumatically controlled to drive the plate forward and back while the pins maintain alignment. The labels are affixed to the plate through the use of vacuum holes. The pick-up plate is made from aluminum, for its light weight and its low strength as compared to the mold steel. If the label plate, for whatever reason strikes the molding surface, then the plate would deform rather than the tool. This protects the tool. The linkage system transmits the force from the bearing housing to the transfer arm. It is made from steel to maintain its strength and durability. The wear at the joints is reduced through the use of special washers and self-lubricating bushings. As these components potentially cycle between 5-8 seconds, wear issues were of some concern. The weight of the linkage arms was also an issue, and as such, the linkage arms were designed for maximum strength, while maintaining a reasonable weight.

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277

The bearing housing, in turn, drives the linkage system. The bearing housing is fixed onto the moving side of the mold. The bearing housing is made from both steel and aluminum parts. Dynamic components in the bearing housing follow the profile of the spline to provide the motion needed to drive the transfer arm. The bearings used in the housing were chosen to maximize, rather than reduce, contact area. A small bearing contact would gouge into the surface of the spline in long-term applications. The spline is the source of power for this mechanical system. Figure 2. Picture of the in-mold labeling system on the four cav- The spline is designed to provide the desired angle of twist to the ity mold. bearing housing. Figure 3. shows the spline used in the prototype. CNC tool paths created from the 3D geometry were used on a steel rod to obtain the desired profile. After the piece was completed, it was.checked with the CMM machine to ensure that the proper rotational angle was imbued upon the spline. One end of the spline is fixed onto the stationary side of the mold. The other end is held by the bearing housing, which is fixed to the moving side of the mold. When the mold opens and closes, the linear force from the injection machine is transformed into a rotational force by the spline’s profile. This rotational motion is then transmitted by the bearing housing, through the linkage system and Figure 3. Spline used to drive the IML systransformed back into a linear force by the linear guide tem. rails on the transfer arm. Thus any error in the spline profile would affect the transfer arm’s linear movement. This potential problem is addressed later in this article. LABEL HOLDER The label holder for this IML system was designed to accommodate lids of various sizes. Though further development is required, this label holder design provides some insight into the various requirements for a label holder. The label holder will also be referred to as the magazine. Figure 4 shows an assembly of the magazine. Several factors were considered for the design of the magazine: ‘quick change’ capability, and the label position with respect to the pick-up plates. Since the labels themselves are relatively small, the use of a magazine system is not too unreasonable.

278

Special Molding Techniques

The magazine was designed to hold approximately 300 labels. The labels can be changed during production by simply replacing one magazine with another. The magazine slide along grooves in the holder arms, which are fixed to the sides of the mold plates. This ‘quick change’ characteristic was required for this IML system, as the molder had multiple label designs for the same part. The label holder was made from aluminum to minimize the handling weight. The labels are held within four thin plates. These four plates can be adjusted to change the label’s position in the magazine, which also changes the label’s position on the cavity surface. As mentioned earlier, an error in the spline profile will cause the transfer arm to deviate Figure 4. Fully adjustable label holder (magazine). from its desired position. This deviation, though, should be fairly constant as the mold is generally opened and closed at a consistent distance. This deviation can be corrected by adjusting the four plates in the magazine. For example, if the transfer arm is deviated by X mm, then the four plates can be adjusted by –X mm to correct the problem. The labels would still be aligned to the center of the cavity surface. This methodology can be applied to fix other errors in the mechanical system as well. There is an advantage and disadvantage for this method of alignment. On one hand, full flexibility is provided to correct alignment issues. On the other hand, it is very time consuming to adjust each of the four plates for each magazine. Future developments will determine a median solution that allows positioning control, while minimizing adjustment time.

FUTURE DESIGN CONSIDERATIONS Some of the goals in the next phase of this project would be to improve the label holder units, reduce the weight of the system, and further optimize the system’s design. Since space was available along the sides of the mold, a concept of a label magazine was simple to adopt. However, if the labels were much larger, the magazines would become a problem. The use of a lighter material would be needed for the magazine.

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279

A different approach for large label applications could involve the use of intermediate label handling systems. The intermediate handling systems would transfer the labels from outside the injection machine space to a fixed position for the transfer arm. By moving the labels outside the injection machine, problems with label size, storage, and orientation would be reduced significantly. Unfortunately, the design of a reliable and robust intermediate handling system could prove difficult. Development of an ‘infinite’ label supply system will also be considered. The main objective being to facilitate the refilling of labels, without stopping the injection machine. The prototype magazine design is considered a continuous label feed mechanism, though only for a relatively short period of time. Attempts will also be made to reduce the size of the various components in this system. This decrease in size will reduce the loads on the system, as well as reduce the cost from material and machining requirements. Strength and robustness of the system will not be compromised. Scalability is a large issue for this IML system, as adaptation to a large or small mold is desired. Further investigation into making the transfer arm components scalable will be included. There are many ways to design an IML system. There are even more ways to improve on one. However, with the introduction of this specific system, it is envisioned that in-mold labeling will soon be cost effective for all types of molds, operating at various production levels.

ACKNOWLEDGMENTS I would like to thank Vincent Travaglini, Don Hersey, Henry Rozema, and Joe Klanfar, at Tradesco Mold Ltd., for their support throughout this project. I would also like to thank Michael Sava, an advisor from the National Research Council Canada, for his support.

REFERENCES 1 2

Mike Fairley, Overview of the in-mould label market- size, growth, and types; presented at In Mould Labelling Innovations and Opportunities, March 25-26, 1996. Achim Franken, Injection-moulding equipment and robotics; presented at In Mould Labelling Innovations and Opportunities, March 25-26, 1996.

Advances in Fusible Core Technique

E. Schmachtenberg, O. Schröder University of Essen, Institute for Plastics in Mechanical Engineering (IKM), Altendorfer Str. 3, 45127 Essen, Germany

INTRODUCTION The basic production sequence of fusible core technology is shown in Figure 1. A metal core is produced from a low-melting metal alloy, usuencapsulating ally a eutectic tin/bismuth alloy. It is then core removal raw materials inserted into an injection mold where it is encapsulated by the polymer. Afterwards, the core is cleaning of parts casting of removed in a heating bath where the plastic part cores comes into contact with a heated liquid medium, part which is removed in the subsequent cleaning Figure 1. Basic production sequence of fusible core tech- process.1 nology. In the product development, the feasibility studies must take into account each process step and the interactions between the materials used (polymer, core material, mold materials and heating medium). For example, the core material must not melt or soften during injection molding. The polymer must be compatible to the heating medium, and the molten metal must not corrode the material of the core-casting mold.

SUITABILITY OF RAW MATERIALS Melting out of the core usually takes place in a bath of modified polyglycolether as heattransfer medium. Melt removal can be assisted by induction coils to make this process step faster. Nevertheless, the same heat-transfer medium is used. Thus, using conventional production plants and methods the product designer has to make sure that the polymer is compatible to the modified polyglycolether at a temperature of 160°C. Table 1 shows a survey of the materials investigated so far at the University of Essen/ Germany, concerning the compatibility to the heat-transfer medium. The materials classified as suitable are currently undergoing further feasibility studies concerning the process

282

Special Molding Techniques

step of injection molding. These studies have take into account the aspects described Table 1. Compatibility of raw materials in the following. with heating medium. Glass fibre content

PA6 and PA66

30 to 50%

Suitable

PPA

33%

Suitable

PBT

none/30%

Suitable

PET

30%

Suitable

PPE

20 to 30%

Suitable

PPE + S/B

20%

Suitable

PVDF

none

Suitable

PFA

none

Suitable

Assessment

PREVENTING CORE MELTING AND CORE SOFTENING

stress (N/mm2 )

Raw material

strain (%)

Figure 2. Stress-strain-curves of tin/bismuth.

During injection molding there are two major problems which may occur: shifting PP 30% Restricted suitable of the core and melting of the core. The problem of shifting of the core in POM cop. none unsuitable fusible core technology can be dealt with in PES none unsuitable a similar way to that used in conventional injection molding. It should be taken into PSU none/30% unsuitable account that the core material has a comparatively low stiffness and strength which are highly temperature dependent. Figure 2 shows the corresponding stress/strain curves. It is evident that core shifting in fusible core technology is a question of temperature. The higher the maximum temperature of the core the more high mechanical loading of the core has to be avoided. A appropriate choice of the gate position and additional core supports may be necessary. Core melting occurs as a result of a local and/or global temperature increase of the core. The local temperature increase is particularly prevalent in the gate region of the cavity. This Problem can be estimated by the contact temperature: POM

none

b p ϑp + bC ϑC ϑ contact = ------------------------------bp + bC

Restricted suitable

[1]

Advances in Fusible Core Technique

283

with λ i ρ i c p, i

[2] The index "p" stands for "polymer" where as the index "c" stands for "core". The contact temperature ϑ contact depends on the temperatures of the polymer melt ϑ p and the temperature of the core ϑ c which is approximately the temperature of the cavity. Moreover, the contact temperature is influenced by the properties of the materials, thermal conductivity, λ , thermal diffusivity, a, density, ρ, and the specific heat capacity cp. However, equitation [1] takes not into account the global increase of temperature of the core during injection molding. This effect can be theoretically detected by calculating a thermal balance across the core. A characteristic parameter, the melting index (MI), can be defined for the risk of melting: bi =

∆h p ρ MI = -----p --------------------------------------ρ c c p, c ( ϑ m, c – ϑ 0 )

[3]

d core diameter D outer diameter

It is a function of the characteristic parameters of the materials used, and the process parameters. The specific enthalpy difference ∆ hp of the polymer from melt to solid state is dependent on both, material properties and process parameters. The temperature of the injection molding cavity can be regarded as the starting temperature, ϑ 0, the melting point of the core material as ϑ m,c. As the melting index increases, the core thickness must increase in relation to the wall thickness of the plastic part to prevent impermisnon-critical range sible heating of the core. Figure 3 shows the critmixed range ical and non-critical geometrical proportions calculated from the thermal balance for a part critical range (cylindrical core) produced by fusible core technique. Since this is a simplified but practicable Melting Indax (MI) approximation, the diagram shows a relatively Figure 3. Number of non-plastic elements for different wide scattering range, for which the theory does distribution rules. not give a clear prediction.1 Practical tests at the University of Essen, in which the temperature profiles in cores for different geometrical proportions and process parameters were recorded show, however, that the "mixed range" can be regarded as largely non-critical as regards core melting.

COSTS Fusible core technology is generally in competition with manufacturing processes in which parts injection molded without undercuts are subsequently joined by welding, snap connec-

284

Figure 4. Intake manifolds (Photos: MANN+HUMMEL and SIEMENS AT).

Figure 5. Hot water applications.

Special Molding Techniques

tion, screwing or overmolding. If parts can still meet specifications, production costs for parts produced by multicomponent technology are often expected to be lower. Parts produced by fusible core technology are characterized by high technical requirements, which cannot be achieved in the same way by multicomponent technology. Prevalent reasons for manufacturing plastic parts by fusible core technology are high demands on the accuracy of the inner geometries and the lifetime of the part (high temperatures and media). Moreover, fusible core technology provides the possibility to integrate functions [2]. Typical parts which such requirements are shown in Figure 4 and Figure 5. The intake manifolds shown in Figure 4 are parts that can be produced in large quantities on highly automated plants. These production facilities require investments of the order of US $3 to 5 million. Figure 5 shows pump housings that are manufactured in small to medium production runs. They require smaller and more flexible production plants, which usually have a lower degree of automation. These plants are not designed for a specific part, but can be used for different parts. Investment strongly depends on the degree of automation and is only about US $0.5 to 1 million, making them eco-

nomic.

DEVELOPMENT POTENTIAL In the plant conception, it must be taken into account that the cycle time for core production is often significantly longer than the cycle time for injection molding. This is a factor that significantly impairs the economy of the process chain. Usually twice as many production units must therefore be purchased for core casting than for injection molding. Current R+D work is concentrated on reducing the cycle time for core casting. In particular, researches are investigating the use of alternative mold materials and material coatings for core casting molds. Basically, the use of materials with high thermal conductivity reduces cycle times. However, many such materials are corroded by the molten tin/bismuth alloy, or themselves change the properties of the molten metal. Furthermore, materials with high thermal conductivity lead to low contact temperatures in the core casting process, as Figure 7 shows.

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285

contact temperature (oc)

The contact temperature on first contact of the molten metal with the mold has, in turn, a significant effect on the surface quality. rotating shaft of tested material Therefore, at the University of Essen an analytical heated cylindre instrument has been developed for investigation the compatibility of mold materials and material coatings that can be tin/bismuth used (Figure 6). The first investigations show that materials such as copper, brass and aluminium are attacked after even comparatively short contact with the molten metal. With the use of aluminium, there is the added problem that the molten Figure 6. Testing of mold materials. tin/bismuth alloy undergoes a significant viscosity increase and eventually becomes unusable. 160 140 Anodized aluminium on the other hand has a melting point of tin/bismuth 120 much higher resistance. This indicates that it 100 80 may be a suitable alternative to tool steel. The PTFE 60 aluminum, anodized anodized layer also leads to higher contact tem40 steel peratures. Practical tests with molds made from 20 aluminum 0 anodized aluminium show that better surface 10 50 80 20 40 90 100 30 60 70 quality of the cores can be reached compared to steel molds. Ongoing research work at the IKM Figure 7. Contact temperatures of mold materials. is investigating this mold material in detail. Different mold coatings are also being investigated. If a surface coating is found with a sufficiently high lifetime and significantly lower thermal diffusivity a further cycle time reduce with a simultaneous improvement in surface quality can be achieved. Figure 7 shows that, for example, a PTFE coating always leads to contact temperatures above the melting point of tin/bismuth. As tests show, this leads to a higher freedom of shape of the core, since flow turbulences during filling do not LQHYLWDEO\ lead to surface defects. Moreover, a thin coating does not affect cycle time. However, pure PTFE coatings only have a poor lifetime.3 A wealth of experience is available in the processing of PA66 by fusible core technology. For almost all other materials, there is a question mark over their suitability for this special process. Extensive investigations are therefore underway into the suitability of alternative polymer materials.

CONCLUSION Although fusible core technology is often regarded sceptically as regards production costs, technically demanding parts can be manufactured economically. The exploitation of the

286

Special Molding Techniques

potential available with mold technology and polymeric materials will in future make fusible core technology suitable for further products and product groups.

REFERENCES 1 2 3

Polifke, M.: Ph.D. Thesis, University of Essen, Germany, 1999 Schmachtenberg, E., Polifke, M.: From an Intake Manifold to Pump Casing, Kunststoffe (plast europe) 86 (1996) 3, p 16-17 Schmachtenberg, E., Schröder, O.: Fusible Core Technology, Kunststoffe (plast europe) 89 (1999) 9, p 36-38.

Processing Glass-filled Polyethylene on a Twin-screw Injection Molding Extruder

David Bigio, Rajath Mudalamane, Yue Huang University of Maryland Saeid Zerafati Elf Atochem

BACKGROUND Glass fibers are a common filler added to polymers in order to improve mechanical properties of the raw polymer, especially the impact strength and stiffness. The traditional route to producing fiber reinforced parts involves blending the fibers into raw polymer in a TwinScrew Extruder followed by pelletization. The pellets are then molded using an Injection Molding Machine to form the final parts. All these steps cause fiber attrition. The focus of these experiments is to determine final glass fiber lengths under different operating conditions and compare these lengths to results reported by other researchers who have used a conventional route. One such paper is by Averous et al.10 They blended 4.5 mm glass fiber strands into polypropylene using a twin screw extruder and then injection molded tensile testing bars. They then conducted fiber length and distribution studies on the parts. They report final glass fiber lengths of 0.47 mm (number average) and 0.7 mm (weight average). In their recent paper, Fu et al.1 present a review of literature on the effect of processing on fiber length retention. The important conclusions they make are: 1) most of the fiber damage occurs in the injection molding machine, rather than in the compounder, 2) low screw speeds and relatively high barrel temperatures minimized fiber breakage, 3) lower back pressures and generous gate and runner dimensions are recommended to preserve the fiber aspect ratio during molding, 4) back pressure has a more dramatic effect upon the fiber length than does the injection speed. This is understandable because the pellets containing glass fibers have to go through the melting section in the molding machine, which is a region of high shear and high frictional forces.

THE TIME The Twin-screw Injection Molding Extruder (TIME) is a novel injection molding machine that is capable of both blending and extrusion in one step. Because it is a one step process,

288

Special Molding Techniques

the fibers never go through the entire extrusion process as well as the pelletization which limits the fiber size and nor the melting section in the (a) TIME, but are blended into the molten polymer before injection. This machine is explained in (b) Compounding detail in the following sections. The screw part cylinders of this machine is based on a Non-Intermeshing, counter-rotating Twin-Screw Extruder (NITSE). Figure 1. Schematic diagram of the TIME. Figure 1 shows a schematic diagram of this machine. This machine differs from a NITSE in that one of the screws is capable of axial movement and has a non-return valve on the end. This enables the screw to inject and mold parts. Material flow direction

Feed 2

Melt seals

Feed 1

BACKGROUND OF PROCESSING ON THE NITSE Stellar3

compounded 1/8" glass fibers into nylon 6 on a NITSE. They chose Nichols and that device because it can be operated at low shear rates, even at high screw speeds. They also expected higher glass fiber aspect ratios due to the higher free volume of the tangential screws. They found that tensile strength, flexural modulus, impact strength were weakly affected by the operating conditions but varied linear with increasing fiber content which ranged from 10% to 40 wt.%. Distributive mixing refers to the process of blending materials when there is no resistance to the process (e.g., paints) whereas dispersive mixing is needed when there is resistance in the nature of interfacial tension (immiscible polymers) or solids agglomerates (e.g., carbon black). The mixing of glass fibers needs to overcome any possible agglomeration, without breaking the glass, and then distribute throughout the polymer evenly. The NITSE has been shown to have superior distributive mixing characteristics to the SSE4 and Co-rotating TSE5 especially when the flights of the screws are set at a stagger of 50%.6,7 The screw to.screw fluid transfer, with the resultant reorientation of the flow, which is important for efficient mixing, has been shown to be due to a local pressure gradient across the nip region.8 Dispersive mixing is accomplished through cylinders or reverse flight elements.8,9 The advantage of this design is that a high shear stress can be applied evenly to the flow. This condition is optimal for redistributing the glass fibers throughout the material without high stresses that could break glass fibers. DESCRIPTION OF THE TIME A single screw, 50-ton (clamp force), all electric IMM built by Cincinnati Milacron was used as the base machine. The barrel and screw were removed from the machine and a new transmission was fabricated and installed in their place. A long drive shaft connects the

Processing Glass-filled Polyethylene

289

injection screw or main screw to the injection drive unit of the molding machine. This allows the injection screw to move axially to inject material. An auxiliary shaft can also be driven by the transmission, thus converting it to a twin screw machine. The auxiliary screw drive was slaved to the main screw drive so that it turns the auxiliary screw at the same speed as the main screw whenever the extruder/plasticizer axis is activated. Thus, the TIME operates essentially as a single screw machine during injection and Figure 2. Drawing of the last two barrel sections showing pack, when the screws don’t turn. During the the transition from twin to single screw (not to scale). retraction step, both screws turn to melt and blend material. The auxiliary shaft cannot move axially but can rotate to perform blending duties in a NITSE mode. The most important factors that influenced the design of this transmission were 1) the close spacing between the two screws, 2) one screw needs to move axially with respect to the other. A barrel is constructed by assembling barrel sections, supplied by the NFM-Welding Engineers and the transmission, supplied by Spirex Corp. These sections are NITSE standard 0.8”(20.32 mm), which are mounted on to the face of the transmission and secured with tie rods. The screw sections are made by NFM-Welding Engineers and their outer diameter is 0.8”(20.32 mm). The last barrel section is single screw and the auxiliary screw stops before this section. The main screw continues into the last barrel section. The pool of molten material for injection is formed in this section. A non-return valve on the end of the main screw prevents material from flowing backwards into the twin-screw region (see Figure (2)). It also builds up pressure that pushes the injection screw back to obtain the required shot size.

EXPERIMENTS MATERIALS The raw fibers have a mean length of 5mm. The polyethylene used in the tests was an injection molding grade Alathon® made by DuPont. PART PRODUCTION Polymer was fed using a loss-in-weight feeder. This resin goes through a melting section with tapering screws and compounding cylinders (restrictive elements) for pressure generation. Glass fibers are then added to the molten polymer through a downstream feed port.

290

Special Molding Techniques

Since glass-fibers require relatively gently mixing, no special mixing, elements are used. The gently mixing screw-to-screw transfer flow accomplishes the required dispersion of glass fibers. The barrel temperatures are set at 320ºF for all zones. Dumbbells shaped tensile testing specimens are molded under different blending conditions. A range of glass fiber contents from 10% to 40% was tested and twenty parts were made at each fiber content. A range of screw speeds (during screw retraction) from 100 to 250 rpm was also tested at each fiber composition. The changing screw speed at the same feed rates results in a different amount of screw fill. SCREW DESIGN

Figure 3. Photograph showing fully assembled screws mounted on to the transmission.

Figure (3) shows the screw design used for this process. Since there are no special kneading elements that are used in a NITSE, a simple plot of screw root diameter v/s length provides ample information about the screw design. Compounding cylinders are used as restrictive elements and are labeled. The melting section consists of a gradually tapering screw that pressurizes material to push it over the cylinders. The melting section is followed by the second feed region where deep screws (11.48 mm R.D.) are used to accommodate the addition of low-bulk density

Processing Glass-filled Polyethylene

291

X-Axis: Screw Length (inches) Y-Axis: Root diameter (inches)

Compounding cylinders

Main Screw

Pressure Ports

Feed Port

Auxiliary Screw

4

3

Feed Port

2 Barrel

Barrel IV

18

16

14

12

Barrel II

10

8

Barrel I

6

4

2

1

0

0

Figure 4. The screw design used for these tests.

Figure 5. Image of a fiber bundle prior to processing. This bundle is 5 mm long.

Figure 6. Image of the fibers after blending into polyethylene and separation by pyrolysis.

glass-fiber (~0.45 g/cc). As it was mentioned before, the mixing flow caused by screw-toscrew transfer is used to blend the fibers into the polymer. The final section has shallow conveying elements (17.37 mm R.D.) which generate enough pressure to push material through the non-return valve. It also serves to seal the material in the melt pool and prevent it from flowing up through the second feed port.

292

Special Molding Techniques

FIBER LENGTH DETERMINATION The polypropylene was burned off in an oven in two hours at 900ºF. The glass fibers left behind were spread on microscope slides and studied under an optical microscope. Images of the fibers were captured using a CCD camera mounted on the microscope and processed in an image analysis program to determine fiber lengths. A population of about 300 fibers was included in the average for each measurement. Examination of the samples shows that the chosen screw design is successful in incorporating and distributing the glass fibers into the matrix. This was accomplished in a very mild screw design which imparts little stress. The mixing was due to the transfer from one to another in the apex region. Results from preliminary studies of glass fiber lengths are presented at this time. Preliminary tests have shown average final fiber size of 0.814 mm. This is a 30% improvement over the lengths observed by other researchers. The lengths were shorter than expected. This is attributed to the fact that a standard non-return valve was used in the machine. This has a restricted flow path and can cause fiber attrition. For future tests, an improvised screw tip, similar to a smear-tip will be used.

CONCLUSIONS This paper has demonstrated that it is capable of processing and injecting glass fibers in the single step process on the TIME. Glass fiber length improvement of 30% compared to reported lengths has been realized in initial tests. Future tests will incorporate a new design of the non-return valve to remove the limiting flow path.

ACKNOWLEDGEMENTS The authors express their gratitude to the Spirex Co. for their continued support and speedy handling of the fabrication process and repairs. Thanks are also due to the Adell Plastics Inc., for generously supplying the materials for testing. The invaluable assistance of Marcel Meissel, Detlef Knoeller and Mauricio Justiniano is also appreciated.

BIBLIOGRAPHY 1

2 3 4 5 6

Fu S. Y., Lauke, B., 0äder E., Hu. X., Yue, C.Y., “Fracture resistance of short-glass-fiber-reinforced and short-carbonfiber-reinforced polypropylene under Charpy impact load an its dependence on processing”, J. Mat. Processing Tech., 89-90 (1999), p501-507. Mudalamane, R., Bigio, D.I., Tomayko, D.C., Zerafati, S., ‘Development of a Twin-screw Injection Molding Extruder’, ANTEC ’99. Nichols, R. and Stellar M., Plastics Compounding, Vol. 9, No. 4,(1986) Howland, C. and L. Erwin, SPE ANTEC, 113-115, 1982. Bigio, D. I., K. Cassidy, M. DeLappa, and Baim, W., ‘Starve-Fed Flow in Co-Rotating Twin Screw Extruders’, International Polymer Processing Journal, Vol. VII, 2, p. 111-116 June, 1992. Conners, M., BS Thesis, MIT, 1985

Processing Glass-filled Polyethylene

7 8 9 10

293

Bigio, D.I. and Baim W., ‘A Study of the Mixing Abilities of the Counter-rotating, Non-intermeshing Twin Screw Extruder Using a Newtonian Fluid’, Adv. in Poly. Tech., Vol. 11, No. 1, p. 135-141, Feb, 1992 Bigio, D., M. Ramanthan, and P. Herman, Adv. in Poly. Tech., Vol. 12, No 4, 353-360 (1993) Hagberg, C., M. Shah, and D. Bigio, SPE-ANTEC, (1995) Avérous, L., Quantin, J-C. and Crespy, A., ‘Determination of the microtexture of reinforced thermoplastics by image analysis’, Composites Sci. and Tech., 58(1998), p. 377-387.

Injection Molding by Direct Compounding

Bernd Klotz Krauss-Maffei Kunststofftechnik, Germany

INTRODUCTION Direct compounding has long since been established in sheet-, profile- and pipe-extrusion, where the high cost-advantages of single-stage article production are appreciated.5 Compared to extrusion applications, direct compounding of injection molded articles is comparatively unknown territory. The IMC-process (Injection Molding Compounding) enables filled or reinforced plastics to be direct-compounded immediately before injection molding. This offers two advantages to the molder: his material costs are reduced and he gains in flexibility.

SAVINGS POTENTIAL The economic success of injection molding to the greater part relies on this process producing fully finished articles during each cycle, employing gas-injection technology, multicomponent injection molding, the lost core technique1 or the back-injection method, amongst others. All these processes have in common, that they become effective, once the melt has been compounded. Cost-savings result especially with the production of fully finished and/ or assembled articles. Because expenditure for downstream finishing stages is either dispensed with or reduced considerably. Contrary to downstream finishing, rationalization of material costs has hardly been attempted, if pigmentation at the machine is ignored. But that example in particular shows, that molders would be able to save the greater part of the added costs for colored granulate, and would in addition become decidedly more flexible, where store keeping, color metering as well as shade of color are concerned. These arguments apply to a much greater degree to the compounding of filler and reinforcement materials. The savings potential becomes obvious, if one considers, that the material costs represent roughly 80% of the production costs for consumer articles. Even with technical components, they still amount to about 50%.

296

Special Molding Techniques

Vast amounts of chalk- or talcum-filled polypropylene for instance are molded into the most varied products. Whereas an injection molding screw disperses pigments added as masterbatch very well during granulate plasticizing, a twin-screw compounding extruder is required for producing blends, due to the larger amounts of fillers added in powder-form.3,4 These extruders, that have proved themselves superbly with the continuous production of blends, are poorly suited however for discontinuous operation, characteristic for the injection molding process.

KEY-FUNCTION OF THE DOUBLE-SHOT POT INJECTION SYSTEM With the IMC-process, a double-shot pot is the link between the continuous compounding process and the discontinuous injection molding side. This allows the high melt quality of compounding to be directly employed for injection molding. As subsequently explained in detail, this novel combination of compounding extruder, double shot pot and conventional clamping unit, allows • filled plastics materials to be compounded directly on the modified injection molding machine, • the compounding extruder to be run continuously and yet to • achieve for the injection molding machine as a whole the machine performance customary for injection molding, even if operation is interrupted. It is an additional advantage of this machine design, that due to the separation of the twin-screw’s melt compounding from the injection molding sequence, the two units can be optimized independently of each other.

CONTROL CONCEPT Parameters are set through the machine control system. All the usual compounding-specific setting parameters for the twin-screw extruder are to be found in the control system’s parameter list, so that the compounding expert is unrestricted as well, when optimizing. Settings of the dicontinuous molding process - injection, holding pressure and cooling, up to the demolding stage – as well as for the mold movement on the injection molding machine, are unrestricted, compared to the standard machine, with regard to the kind as well as the variety of the setting possibilities. Parameters of the “double-shot pot” injection unit are set on the two-component machine’s control system. The parameter governing the metering stroke of the double-shot pot’s specific filling weight, in conjunction with the actual cycle-time, is converted directly into an output capacity m° (kg/h), that has to be achieved. Conventional injection molding machine-setting philosophies thus remain intact for the machine setter.

Injection Molding by Direct Compounding

297

THE “IMC”-RANGE OF MACHINES Such machine-modules as gravimetric metering units, twin-screw extruders, double-shot pots can in almost every case be adapted to virtually every clamping unit by simple design modifications.

“IMC”INJECTION- AND PLASTICIZING UNIT The “IMC” injection- and plasticizing unit is a modular combination of twin-screw extruder and double-shot pot, that have been matched to each other with regard to achievable shot weights. There is a choice of 4 units, that are listed in the Table 1 below. Table 1 SP3500/40 IMC

SP5700/50 IMC

SP14700/60 IMC

SP19000/60 IMC

max. swept volume, ccm

1953

3611

7632

11133

maximum injection pressure, bar

1742

1580

1931

1707

AREAS OF APPLICATION The advantages of the IMC-machine range come into their own during long-term production runs, typical for garden-furniture- or carbody-components, where materials or molds are seldomly changed. Thus, for instance, a manufacturer of leisure furniture processes several thousand tons of chalk-filled PP annually into garden chairs and tables. That was the reason, why the first prototype application was designed for the production of armrests for garden chairs, at a shot weight of about 1000 grams.

FUNCTIONS SEQUENCE Based on the example of the prototype application, the subsequent text describes the flowpath of the basic material components, starting in the gravimetric metering unit and finishing as the completed article. The conventional single-screw plasticizing unit has been replaced with a close-meshing, co-rotating twin-screw compounding extruder (40Ax36D). It produces the compound for the armrests from polypropylene, pigmented masterbatch and chalk, delivering the mix

298

Special Molding Techniques

as a homogenous melt. The extruder is equipped with degassing, in order to dispense with having to pre-dry the PP-granulates, as well as the chalk. A gravimetric mixing- and metering unit transfers the pigmented masterbatch-enriched PP-granulate to the extruder’s feed-throat opening. So that the feed-throat is not subjected to hard wear, and in order to achieve good distribution without any agglomerate forming, the chalk is added only, when the granulate has already melted. A feed position some way down the side of the extruder barrel serves that purpose. Chalk is fed into the melt flow by a second gravimetric metering unit, and a twin-screw conveying unit (model ZSFE 34). Due to the dispersing effect of the co-rotating twin-screws, the chalk is broken down very finely and uniformly distributed, so that a homogeneously melted compound results. The compounded melt is metered continuously into one of the double-shot pot’s cylinders by the extruder, through a transfer-line and a two-way valve. The double-shot pot unit has been designed as plunger system, for efficient scavenging of the cylinders. Heaterbands keep the melt in the transfer pipes, the valve and the injection cylinders at processing temperature. During metering to the first-in-first-out principle, the plunger moves back inside the injection cylinder. Once the metering time has elapsed, the two-way valve changes over to the second cylinder without interruption, while the extruder keeps running, thereby changing the cylinder functions. Immediately afterwards, the plunger of the freshly charged cylinder starts a production cycle by injecting the melt. During that phase the plunger/cylinder unit operates in the same way as the injection unit of a plunger-type injection molding machine; this is followed by the process stages of holding pressure, cooling, article demolding and nozzle contact for the next injection cycle. So that the compounding extruder continues reliably in continuous operation, the metering time is set, so that it is only fractionally longer than the injection molding cycle time. Hydraulic drives have been installed for generating the metering stroke or the injection pressure, aided by the plungers. The injection molding compounder operates in a very material-protective manner, due to • the single-step working operation, which dispenses with the additional thermal stress encountered with separate compounding, • the twin-screw extruder having a very tight residence-time profile, in contrast to the triple-zone injection molding screw, • no locally high shear-forces occurring either in the extruder or in the buffer store, so that there is no risk of material overheating and • the melt-conducting channels and buffer stores having been designed according to rheological aspects.

Injection Molding by Direct Compounding

299

PRODUCING WITH IMC When starting-up the plant from cold, the procedure is similar to that of running-up an injection molding machine. First of all, the extruder, the double-shot pot and the connecting pipes are brought up to temperature. Then gravimetric metering opens the supply of the granulate components, and plasticizing starts. The initially produced melt is discharged through a special valve, installed downstream of the extruder. As soon as stable operating conditions have been established in the extruder, the start-up valve changes over to its home-position, the melt charges one of the injection cylinders, and the earlier mentioned production sequence starts. The pre-heated injection molding compounder can be started-up almost as quickly, as an injection molding machine of identical size. Under normal running conditions, i. e. during trouble-free production of molded articles, the two cylinders operate in synchronization with the cycle time, either as melt buffer or as injection unit, as described. If operation had to be interrupted for a brief period, one of the cylinders will additionally have been charged with the material volume of the second cylinder’s storage capacity, once metering is completed. It is possible to lower the extruder’s output rate to a material-specific minimum by closed-loop control during such an interruption of operation. To that end, the control system throttles down the material supply and the screw RPM, so that the filling rate of the extruder - and thus of the melt quality – remains constant. Only when an interruption takes longer than 30 minutes will the plant have to be shut down. When processing PP, it is not necessary to lower the operating temperature. Once the fault has been rectified, the plant is re-started within a few minutes, as described above. By and large, the injection molding compounder therefore shows a behavior in all the important operating conditions - like start-up, normal operation, malfunction -that comes close to that of the injection molding machine’s performance during production runs.

PROFITABILITY, FLEXIBILITY As mentioned at the beginning, profitability of direct compounding arises from the achievable savings in material costs. That potential must therefore be determined specifically for each application. In the case of chalk-filled PP, a compound containing chalk at 30 percent by weight, available in the trade, presently costs about 1.80 DM/kg in white, and colored blue about 3.40 DM/kg. Based on prices of 1.35 DM/kg for unfilled PP and 0.40 DM/kg for chalk, the costs for material produced on the injection molding compounder amount to 0.7 kg PP × 0.3 kg chalk × 1 kg of compound

1.35 DM/kg 0.23 DM/kg

= =

0.95 DM 0.07 DM 1.02 DM

300

Special Molding Techniques

That represents a cost difference across the board.from 0.78 DM/kg for white material to 2.38 DM/kg for colored compounds, (without even taking the costs for the color-masterbatch into consideration). These amounts are available for amortization of the machine’s additional equipment as well as for possibly covering any other additional outlay, such as quality assurance. For typical products, an amortization period of from less than 1 and 3 years results, which is clearly reduced with rising annual material costs. In addition to the cost advantages, the molder gains flexibility with the purchasing and stockpiling of materials, as well as the determining of amounts of filler additives and reinforcing media. Indirect cost advantages result, when the cycle time is reduced, due to a higher content of filler (the cooling time gets shorter, because the lower proportion of PP means, that there is less to be dissipated from the polypropylene melt), so that productivity increases.

OUTLOOK During the production of medium-sized to larger moldings from modified plastics materials, the injection molding compounder opens up savings opportunities, which have hitherto been impossible to exploit. This for instance applies to filled materials, or those containing flame-retardant additives, as employed by the electrical appliances industry. Even large moldings consisting of different compounds (e. g. glass-fibre reinforced plastics) such as bumpers, fascias, internal door coverings, rear-trunk claddings, can be produced by this method. Ultimately the injection molding compounder will be able to open up yet another area of application: the gentle, material-protective compounding of difficult to process, high quality plastics of narrow processing lee-ways. During the melting process of such materials, single screw plasticizing units can reach their limits - amongst other things, due to their broad residence time profile. The injection molding compounder on the other hand is capable of homogeneously melting these plastics materials, custom-made as reactor-blend, for instance. Transferred for injection to the double-shot pot at high plasticizing rates and without causing thermo-mechanical damage.

LITERATURE 1 2 3 4 5

Rothe, J.: Sonderverfahren des Spritzgießens. Kunststoffe 87 (1997) 11, 1564-1582. Bürkle, E.; Rehm, G.; Eyerer, P.: Hinterspritzen und Hinterpressen – Lagebericht zur Niederdrucktechnik. Kunststoffe 86 (1996) 3, 298-307. Allen, P. S.; Bevis, M. J.; Hornby, P. R.: New direct compounding/injection unit for molding composites. Modern Plastics International, April 1987, 38-39. DE 40 21 922 A 1. Putsch, P.: Kombiniertes Compoundier-Spritzgußverfahren und Vorrichtung zur Durchführung dieses Verfahrens. Offenlegungsschrift vom 16. 1. 92. Horst Kurrer: Gefüllte Polyolefine direkt extrudieren, Kunststoffe 83 (1993) 17-21.

Improvement of the Molded Part Quality: Optimization of the Plastification Unit

S. Boelinger, W. Michaeli Institut für Kunststoffverarbeitung (IKV), Pontstr. 49, D-52062 Aachen, Germany

INTRODUCTION Because of its varied area of responsibility the plastification unit is very important for the injection molding process. The process steps carried out by the plastification unit are the feeding of the material, the conveying of the material, the melting and the homogenization of the melt. As consequence the following demands are made on the plastification unit:1-6 • the supply of the necessary mass flow with a high melting capacity, • a good energetic operation behavior, • a good feeding behavior and a constant convey of the material over the whole screw displacement, • the achievement of a good thermal, mechanical and material melt homogeneity, • a good residence time of the material to reduce the thermal degradation, • a good reproducibility of the melting process and • a wide area of use. Traditionally three-zone plastification screws, also called standard screws, were used in the injection molding process. The injection molding materials and the machine technology are more and more adapted to the different process groups. Because of this fact standard screws are limited frequently in their efficiency so that not all demands on the plastification unit can be fulfilled. One important industrial process group are fast running processes like the production of packaging. The use of multi-cavity and multi-floor mould bases requires a high plasticising output from the plastification unit. Furthermore new developments in the mould technology lead to a quicker movement of the mould base during the injection cycle. The optimization of the control technology of the injection molding machines allow the production with a higher injection speed so that cycle time can be reduced, too. But the reduction of the cycle time can be limited by the melting capacity of the plastification unit, if the dosing is the dominant operation step of the injection molding process and if a high melt vol-

302

Special Molding Techniques

ume is needed. In this case the necessary amount and quality of the melt can often be not achieved using the established screw geometries. In contrast to the fast running processes other product groups, like optical parts, require an extremely good thermal homogeneity of the melt. Good quality parts can only be produced using extremely slow injection speeds and a controlled temperature in the barrel. Injection molding materials used for the application of optical parts, like PC, PMMA, COC or transparent PA, react very sensitive against heat. Due to the long cycle times which are normal during the production of optical parts the use of standard screws is limited often. It is a problem for the design of plastification screws, that the target values “melt quality” and “the minimization of the cycle time” behave in different ways. As design criteria the application of the screw has to be taken into account. It must be the aim to reach an optimized area of both demands, cycle time and melt quality. Furthermore it should be possible to use the screw design for a varied choice of injection molding materials.

BASIC INVESTIGATIONS OF DIFFERENT SCREW CONCEPTS In cooperation with an injection molding machine manufacturer several screw geometries were tested with different injection molding materials.1 During the investigations the influence of several dosing parameters on the melting capacity, the energy consumption of the screw and the melt homogeneity were analyzed. Furthermore it was the aim to quantify the modification of the screw geometry which is normally based on internal company experience. The used screw geometries were a three zone screw (standard screw), a multi-threaded screw and three barrier screws. Differences of the three barrier screw concepts (I-III) were the thread gradient of the main and the barrier flight and the length of the screw zones. As basic screw geometries a diameter (D) of 60 mm and a length of 20 D were chosen. To analyze the influence of shear and mixing elements the basic screws were lengthened with additional elements to a new screw length of 25 D. For the investigations a defined volume of 10 liters was injection molded with the materials PP and PE. Besides the parameters back pressure, rotational speed and melt temperature the residence time of the material in the barrel was varied. Therefore the dosing time and the cycle time were changed in combination. In this first investigations the influence of the parameters was analyzed separately. But the interactions of the parameters were taken into account qualitatively at the interpretation of the results. Some examples of the results are explained below. The main emphasis is put on the analysis of the plasticising output (melting capacity) of the plastification unit in correlation with the varied parameters. Figure 1 shows the correlation between the plasticising output

Improvement of the Molded Part Quality

Figure 1. Plasticizing output as function of the dosing position (D = 60 mm; L = 25 D; material PP).

303

Figure 2. Plasticizing output as function of the residence time (D = 60 mm; L = 25 D; material PP).

and the dosing position (scaled as multiple of the screw diameter D) for the screws with the length 25D and the material PP. For the use of the barrier screw concepts it was found that a degressive profile of the barrel temperatures is good for processing. Because of this fact only the results for a degressive barrel temperature profile from the feeding zone to the Figure 3. Plasticising output as function of the residence nozzle (250°C-230°C) are shown. For the use of time (D = 60 mm; L = 25 D; material PE) barrier screw the barrier screw concepts the plasticising output III. was reduced with increasing dosing volumes. Furthermore differences in the plasticising output can be noticed for the different barrier screw geometries. The use of the barrier concept I leads to a small drop of the plasticising output whereas the barrier screw concept II reduces the plasticising output about 15%. In comparison to the barrier screw concepts the investigated standard screw and the multithreaded screw only show slight changes in the plasticising output. In a second step the influence of the residence time and as consequence the influence of the heat conduction was investigated. Figure 3 shows the plasticising output as function of the residences time for PP and the 25D screws. Here could be noticed that for short residence times the plasticising outputs are close together. But with growing residence times there are differences in the achieved plasticising outputs. For the standard and the multithreaded screw the plasticising output is not much influenced by the residence time. In contrast to this fact the plasticising output increases with longer residence times using the barrier screw concepts. Again there is a difference between the different types of barrier screws. The increase in the plasticising output comes to 20% for the barrier screw concept I.

304

Special Molding Techniques

100

-..,I-'.--P'oI'«2~~~.l4~:..:CI-.

I

W

I

"""""" ~..,. , ~ ..,-,

80

70

i

. -

:

0

20 30

,

10

O

-w

0

20

0

00

80

R.aide-

100

120

1imO

10160

160

200

220

j I I ~~(m'SI

1&1

Figure 4. Plastizising oputput as function of the residence time (D = 60 mm; L = 20 D; material PE).

-

Figure 5. Medium energy consumption as function of the circumferential speed (in principle).

The barrier screw concept II even comes to an increase in the plasticising output of 40%. As conclusion with the use of barrier screws an improvement of the plasticising output about 56% (concept II) referring to the plasticising output of the standard screw can be achieved. The characteristic curves of the plasticising output in dependency on the residence time for PE and the 25 D screws are shown in Figure 3. In principle the same correlations can be noticed as they are shown above for the material PP.In comparison to the standard screw the plasticising output could be increased about 78% using the barrier concept II. All in all the achieved plasticising outputs are higher for the injection molding material PE than for PP. Reasons for this fact are the different material properties, like the material density and the melting enthalpy. The results for the plastification screws without shear and mixing elements (20D screws) are shown in Figure 4. Here again the plasticising output is given as function of the residence time for the use of the material PP. Again the courses are similar to the courses of the 25D screws as they are shown on Figure 3. Another important aspect for the evaluation of screw concepts is the medium energy consumption during the dosing. Figure 5 shows the medium power consumption as function of the circumferential speed. The graph only shows the courses for the different screw concepts in principle. With increasing circumferential speed a higher energy consumption is required. Though all courses which are shown on Figure 5 are close to each other there is a tendency of a lower energy consumption for the use of barrier screws.

FURTHER INVESTIGATIONS

BASED ON A MODULAR DESIGN OF THE PLASTIFICATION SCREW

The basic investigations have revealed that changes in the screw geometry have a great influence on the plasticising output. But due to the high costs of the screw manufacturing

Improvement of the Molded Part Quality

305

the number of screws used for the investigations is limited. One solution for this problem is the design of a modular screw. Components of this modular screw are the load-bearing shaft, also called mandrel, and the screw elements which are on the mandrel (Figscrew element ure 6). Important is the protection of the screw elements against twisting. Therefore feathers, multi-wedge toothing or a profile splinted shaft can be used. Addishaft nut load-bearing shaft (mandrel) feather tionally the screw elements are fixed Figure 6. Load-bearing shaft (mandrel) with fixed screw ele- with the screw tip. Due to the heat in the ments.8 barrel the screw elements and the mandrel might show differences in their elongation. This effect is compensated using springs.7 This screw concept has been used in the area of extrusion, especially for compounding, for a long time. It is necessary for compounding to adapt the screw geometry to the extrusion material. In this area threading elements with different geometries, shear and mixing elements are combined useful. Furthermore it is possible to vary the material of the screw elements, if abrasive materials are.used for extrusion. For example nitride steel can be replaced by full-hardened tool steel.7 For the injection molding process the adaptation of the screw geometry to the process conditions is important, too. Especially for laboratory tests advantages can be gained using modular screws. This type of screws allows to change the screw elements purposefully to vary characteristics of the screw geometry, e.g. thread gradients or depths. Further advantages are the possibilities to change the length of the screw zones and to use hardened or coated screw elements, if this is useful for the application.9-11 Injection molding materials taken to produce high quality optical parts for example show the tendency to stick on the screw materials normally used for standard applications. This stick effect could be reduced using surface coated screw materials. For the adaptation of the modular screw from the extrusion to the injection molding process it is important to work out a new screw design. In contrast to the extrusion process the plasticising is not a continuous process step and uneven load is put on the screw. One task of the injection screw is the dosing. During this process step the screw moves backwards with additional rotation. Afterwards the screw is responsible for the injection process and it functions as a piston. The melt is injected into the mould base by an axial progressive movement of the screw. For the final design of the modular screw the trigger of the changing torque and the buckling behavior of the screw must be taken into account. Furthermore

306

Special Molding Techniques

the screw elements are not allowed to be sheared off the mandrel during the injection molding process.

CONCLUSIONS AND NEW GOALS Because of the possibility to change the single screw elements purposely, the modular screw concept is a good basis for the investigation of the plastification behavior of injection molding machines in laboratory tests. For the experiments with the modular screw the statistical method for experiment design will be used so that interactions of the parameters can be taken into account. As objective the plasticising output and the melt quality will be investigated in dependency on the process parameters and the screw geometry. In addition to the material homogeneity of the melt the axial and radial temperature distribution of the melt will be evaluated.

ACKNOWLEDGEMENT The presented basic investigations of different screw concepts were carried out in cooperation with the company Mannesmann Demag Ergotech GmbH, Schwaig, Germany.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

Mehler, C., Untersuchung verschiedener Schnek-kengeometrien von Spritzgießmaschinen; not published diploma thesis at the IKV, Aachen, Germany (1998), Supervisor: S. Bölinger, S. Pahlke (Demag Ergotech GmbH, Schwaig). Mehler, C., Schimmel, D., Pahlke, S., Dreizonenschnecke oder Barriereschnecke für Polyolefine?, ERGOpress 2:1 10-13 (1999) 1. Bürkle, E., Qualitätssteigerung beim Spritzgießen als Aufgabe des Plastifiziersytems, Kunststoffe 78:4 289-295 (1989) 4. Bürkle, E., Bauer, M., Würtele, M., Spritzgießschnecken - Kompromisse definieren ihr Einsatzspektrum, Kunststoffe 87:10 1272-1286 (1997). Potente, H., Entwicklung bei der Auslegung von Plastifizierschnecken, Plastverarbeiter 43:11 118-127 (1992). Wortberg, J., Mahlke, M., Effen, N., Barriereschnecken steigern Homogenität der Schmelze, Kunststoffe 84:9 1131-1138 (1994). Uhland, E., Kunststoff-Extrusionstechnik I Maschinenbauliche Grundlagen – Doppel-schneckenextruder, gleichläufig, Carl Hanser Verlag, München, Wien, 527-535 (1989). Meier, U., Kunststoff-Extrusionstechnik I Maschinenbauliche Grundlagen - Andere Extruder, Carl Hanser Verlag, München, Wien, 536-539 (1989). Peters, H., Mit flammgespritzten Schnecken gegen abrasiven Verschleiß, Kunststoff-Berater 12 891-892 (1973). Lülsdorf P., Verschleißschutz bei Spritzgieß-maschinen, Kunststoffe 86:6 776-782 (1996). N. N., Panzerschichten erhöhen die Lebensdauer von Schnecke und Zylinder, Kunststoff-Berater 12 887-889 (1973).

Non-return Valve with Distributive and Dispersive Mixing Capability

Chris Rauwendaal Rauwendaal Extrusion Engineering, Inc., Los Altos Hills, California 94022, USA

INTRODUCTION The screw of the plasticating unit of an injection molding machine (IMM) typically consists of a single stage, single flighted conveying screw with a non-return valve at the end. Mixing sections are usually not incorporated into the screw design. One reason for this is the fact that most plasticating units a relatively short; the typical length-to-diameter ratio is 20:1 in IMMs. This does not leave much space to incorporate a mixing element. Another reason may be the mistaken believe that mixing is not very important in the injection molding process. A convenient method to improve the mixing capability of the plasticating unit of an IMM is to design the non-return valve (NRV) such that it has mixing capability. Such a dual-purpose NRV allows an increase in mixing capability without affecting the melting and conveying capability of the plasticating unit. This paper will describe a NRV mixer based on the CRD mixing technology developed for single screw extruders.

BACKGROUND In polymer mixing we usually distinguish between distributive and dispersive mixing. Distributive mixing aims to improve the spatial distribution of the components without cohesive resistance playing a role; it is also called simple or extensive mixing. In dispersive mixing cohesive resistances have to be overcome to achieve finer levels of dispersion; dispersive mixing is also called intensive mixing. The cohesive component can consist of agglomerates where a certain minimum stress level is necessary to rupture the agglomerate. It can also be droplets where minimum stresses are required to overcome the interfacial stresses and deform the droplet to cause break-up. Dispersive mixing is usually more difficult to achieve than distributive mixing. Single screw extruders are generally considered to be poor dispersive mixers while twin screw compounding extruders have much better dispersive mixing capability. However, when we analyze the mixing process in co-rotating twin screw extruders,1 it is clear that the main

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barrel

Curved flight tank

Tapered flight slot

Figure 2. Two methods of creating elongational flow in the CRD mixer.

mixing action does not occur in the intermeshing region but in the region between the pushing flight flank and the barrel. This is particularly true when the flight helix angle is large as it is in kneading disks. This being the case, there is no reason that the same mechanism cannot be used in single screw extruders. Figure 1. Dispersion in shear and elongational flow. The reason that twin screw extruders make good dispersive mixers is that the space between the pushing flight flank and the barrel is wedge shaped and thus creates elongational flow as the material is force through the flight clearance. Shear flow is not very efficient in achieving dispersive mixing because particles in the fluid are not only sheared they are also rotated, see Figure 1 top. In elongational flow particles undergo a stretching type of deformation without any rotation; rheologists call this “irrotational flow,” see Figure 1 bottom. In the past it was considered to be difficult to generate elongational flow in mixing devices. It turns out, however, that this is not the case. The new dispersive (CRD) mixers developed by Chris Rauwendaal2-4 create elongational flow two ways, see Figure 2. By using a slanted pushing flight flank so that the material is stretched as it is forced through the flight clearance and by using tapered slots in the flights. The tapered slot accelerates the fluid as it flows through the slots and thus creates elongational deformation. The dashed arrows in Figure 2 indicate the screw velocity, the solid arrows indicate the melt velocity relative to the screw. The tapered slots in the flights serve to increase distributive mixing as well as dispersive mixing. If the material is not randomized in its passage through the mixer, only the outer shells of the fluid will be dispersed leaving the inner shells undispersed.5 Therefore, it is critical to incorporate both distributive and dispersive mixing ability within the mixer. Figure 3 shows the CRD5 mixer with curved flight flanks and tapered slots in the flights.

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The slots divide the flights into wiping.and mixing flight segments. Each wiping segment is followed by three mixing segments. The initial design of the CRD mixer2 was developed using the concept of the passage distribution function,6 while the final geometry was developed using a detailed three-dimensional flow analysis. The 3D flow analysis was performed using BEM flow,7 a boundary element flow analysis package originally developed at the University of Wisconsin, Madison by Professor Osswald and co-workers. The BEM analysis Figure 3. Three dimensional rendering of a CRD5 mixer. allows a complete description of flow, so that the stresses, the number of passes over the mixing flights, the number of passes through the tapered slots, residence time, etc. can be quantified for a large number of particles. The CRD mixer may well be the first complex mixing device developed solely based on engineering calculations and computer modeling.

APPLICATION TO INJECTION MOLDING Mixing is not only important in extrusion, it is equally important in injection molding. CRD mixing elements can be added to an injection screw. Most injection screws have a nonreturn valve (NRV) at the end of the screw to prevent the molten plastic flowing back into the screw during injection. It is possible to incorporate mixing capability into the NRV to combine two functions within one device. Since the NRV is short, there is little room available to incorporate mixing capability into the NRV. The slide ring NRV is the most suitable with respect to adding mixing capability. The action of the ring check valve is illustrated in Figure 4. When the screw rotates, plastic is conveyed forward, melted, and mixed. The plastic melt accumulates at the discharge end of the screw and pushes the screw back against a controlled pressure. As the screw Figure 4. The ring valve in open (a) and closed (b) position. moves backward, the check ring is dragged to the most forward position against a stop at the end of the screw, see Figure 4a. The stop is usually a star shaped shoulder with four to six points. When the check ring rests against the stop plastic melt can flow through the valve.

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When the screw moves forward, the check ring is dragged to the most Threaded stud rearward position against the check ring seat forming a seal, see Figure 4b. In this position the valve is closed and the plastic melt is thus prevented from leakFigure 5. CRD non-return valve for injection molding screws (open ing back into the screw channel during position). injection. Because of the relative movement between the check ring and the stop, stops will wear over time and eventually have to be replaced. Mixing capability can be designed into the slide ring valve by locating mixing pins on the inside of the slide ring as shown in Figure 5. The pins are elongated in the axial direction to achieve an Figure 6. Solid model of the CRD non-return valve (left) with detail acceleration of the fluid as it is passing of just the slide ring (right). between two pins. The resulting elongational flow results in effective dispersive mixing. The same elongated pins are also located on the outside of the stop, resulting in a second exposure to elongational flow with further dispersive mixing action. The large number of pins located on the slide ring and stop induce a large number of splitting and reorientation events, resulting in efficient distributive mixing action. The CRD NRV provides a convenient and cost efficient method to improve the mixing capability of injection molding screws. Good mixing action can be incorporated simply by exchanging the conventional NRV with a CRD NRV. Figure 6 left shows a solid model of the CRD valve with the slide in the forward (open) position. Figure 6 right shows the slide ring with the internal mixing pins. During screw rotation the slide ring will rotate with the screw but at a lower rotational speed. This results in a mixing action somewhat similar to the Twente Mixing Ring.10 Nose

Slide ring Shoulder

Slide ring detail

EXPERIENCE FROM THE FIELD As of November 1999 over forty extruders are running with CRD mixers. The current applications are color concentrates, foamed plastic, post-consumer-reclaim with calcium carbonate, medical applications, heat shrinkable tubing, carbon black dispersion in polyolefins, and blown film extrusion of low melt index metallocenes. All current applications are on single screw extruders in sizes ranging from 19 to 200 mm (0.75 to 8.0 inch). Three screws

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are being manufactured for injection molding and one set of screws for twin screw extrusion. In the manufacture of color concentrates it was possible to produce even the most difficult material, phthalate blue, with good quality using a single screw extruder. The foamed plastic application polystyrene was foamed with a physical blowing agent carbon dioxide, a demanding application requiring very good dispersive and distributive mixing. The foamed product was found to have uniform cell size and excellent surface quality, while the extrusion process was very stable with respect to melt pressure and temperature. In the blown film application major problems were poor film quality and the inability to feed more than 3% fluff with the virgin plastic without generating gels in the film. The CRD mixing screw was able to improve film quality and handle up to 12% fluff without generating gels. The good results with respect to gels indicate that elongational mixing devices can effectively disperse gels as opposed to shear mixing devices. Luciani and Utracki found similar results in experiment using their extensional flow mixer.9 The experience from the field was obtained with a fifth generation mixing device, the CRD5, see Figure 3. This mixer has four parallel flights with tapered slots in the flights. Each wiping flight segment is followed by three mixing flight segments. Tapered slots separate the flight segments. The wiping segments are offset such that the mixer wipes the entire barrel surface. Complete wiping of the barrel surface is important to achieve efficient heat transfer between the plastic melt and the extruder barrel.

CONCLUSIONS Mixing devices that split and reorient the fluid while generating strong elongational flow can achieve both efficient distributive and dispersive mixing. CRD mixers have proven their effectiveness in extrusion;2-4 however, their use is not limited to single screw extruders. The same mixing devices can be used in injection molding. Perhaps the most convenient method to enhance the mixing capability in injection molding is to design mixing elements into the non-return valve. This paper describes a slide ring valve/mixer based on the patented mixing technology.8 The slide ring has multiple internal grooves that are tapered while the nose-piece has external, tapered grooves. The tapered grooves are formed by elongational pins. The pins split and reorient the fluid thus causing efficient distributive mixing. The tapered grooves accelerate the fluid and cause elongational flow with efficient dispersive mixing. The high mixing efficiency of the CRD mixer is due to the generation of strong elongational flow and the fact that all fluid elements make multiple passes through the high stress regions. Elongational flow not only achieves more effective dispersion; it also creates less

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viscous dissipation than shear flow. As a result, the power consumption and temperature rise in the CRD are less than in mixing devices that rely on shear flow. The CRD valve/mixer is easy to manufacture and can be mounted on existing injection screws. Since the non-return valve on many injection screws is removable, it is very easy to replace a conventional valve with a CRD valve/mixer. As a result, the change-over can be done in a short period of time. Actual results of the NRV mixer obtained from injection molding trials will be presented at the ANTEC meeting as well as animations showing the movement of the various components of the mixer during the injection molding cycle.

REFERENCES 1 2 3 4 5 6 7 8 9 10

Polymer Mixing, A Self-Study Guide, C. Rauwendaal, Carl Hanser Verlag, Munich (1998). “A New Dispersive Mixer for Single Screw Extruders,” 56 th SPE ANTEC, Atlanta, GA, 277- 283, Chris Rauwendaal, Tim Osswald, Paul Gramann, and Bruce Davis (1998). “Experimental Study of a New Dispersive Mixer,” Chris Rauwendaal, Tim Osswald, Paul Gramann, Bruce Davis, Maria del Pilar Noriega, and O. A. Estrada, 57 th SPE ANTEC, New York, 167-176 (1999). “Design of Dispersive Mixing Sections,” Chris Rauwendaal, Tim Osswald, Paul Gramann, and Bruce Davis, International Polymer Processing, Volume 13, pp. 28-34 (1999). “Kinematics and Deformation Characteristics as a Mixing Measure in the Screw Extrusion Process,” T.H. Kwon, J.W. Joo, and S.J. Kim, Polym. Eng. Sci., V. 34, N. 3, 174-189 (1994). “The Distribution of Number of Passes Over the Flights in Single Screw Melt Extruders,” Z. Tadmor and I. Manas-Zloczower, Advances in Polymer Technology, V. 3, No. 3, 213-221 (1983). BEMflow, Boundary Element Fluid and Heat Transfer Simulation Program, ©1996 The Madison Group: PPRC. C. Rauwendaal, U.S. Patent 5,932,159. “The Extensional Flow Mixer”, A. Luciani and L. A. Utracki, Intern. Polymer Processing, 9, 4, 299-309 (1996). A.J. Ingen-Housz and S.A. Norden, Intern. Polym. Process., 10, 120 (1995).

Index

A ABS 135 acrylic 194 aesthetic properties 245 agglomerates 288 amorphous 256 anisotropic shrinkage 99 antistatic 274 appearance 134 applications 15 armored vehicle 209 attrition 287 automotive 93, 163, 171, 175, 189, 215 B backcompression 175 barrel 91, 137 beam splitting 149 binder 74 removal 74 Bingham fluids 35 birefringence 243, 245 blood pressure sensors 163 Boger fluid 36 bosses 134 brittle fracture 238 buses 209 C carbon black 288

catalyst system 128 cavity pressure 215 cellular phone 92, 99, 133 ceramic feedstock 74 Cinpres 17 clamp tonnage 121 clamping force 65, 73 clamping speed 188 clarity 254 communication 163 complexity 4 composition 2 compounding 295 compression molding 175, 187 computer 99, 133 containers 113, 127 conversion 81 cooling 164 rate 246 core 99 cosmetic 133 cost 20, 63, 135 craze 238 Cross model 100 crystallinity 246, 254 cycle time 20, 63, 79, 284, 302 D Darcy's law 199 decoration 93, 215

314

defects 193 degradation 99 design guidelines 58 optimisation 58 Desma 17 Diesel effect 165 dimensional analysis 64 stability 63 distributive mixing 288, 308 drink cups 127 drying time 95 DSC 81 E ejector pin 122 elasticity 134 elongated shapes 63 elongational flow 308 EMI shield 134 energy dissipated 238 Engle 17 ergonomics 21 erosion 92 etching 151 Eulerian time integration 167 F fiber length 287 fill time 89, 90 film 93 flow front 76 length 89, 94

Index

marks 45, 92 pattern 61 food packaging 127 forward momentum 90 fountain flow 143 FTIR 81 function 4 furniture 189, 215 fusible core technology 281 G gain 17 gas assist 15, 27, 36, 43, 63, 65, 79 bubble 58 penetration 38 propagation 81 channel 59 injection 45, 57 pins 22 gaskets 114 gate blush 92 geometry 3, 57 glass fiber 45, 287 Glass Mat Technology 187 glass transition temperature 245 globalization 16 guidance rib 59 H heat conduction 65 dissipation 90 Hele-Shaw flow 64 hesitation 45

Index

Hettinga 18 high flow resins 113 hollow sections 63 honey comb structures 157 Husky 17 I immiscible polymers 288 impact strength 94, 287 inductive heating 164 injection molding compounding 295 injection speed 91 inmold decoration 223 labeling 273 lamination 171 innovation 1 instrument panel 93 interfacial tension 288 interlock 92 internal stress 243 K Klockner Ferromatik 17 knit line 93

315

capacity 260 Mannesman Demag 17 mathematical model 81 medical 159 melt 231, 237 filling 45 metering units 297 micro spark-erosion 157 micro-cutting 157 microstructures 157 micro-systems 157 miniaturization 99 mixing capability 307 mold cavity 35 injector 57 molecular weight distribution 113 morphology 237 N Newtonian fluids 28, 35, 64 nitrogen 254 notebook 92, 99 nozzle 57

L lamination 175 Laser Doppler Anemometry 194 laser erosion 157 licensing 15 lithography 151

O one-shot manufacturing 1 optical anisotropy 149 optical components 149 orientation 255 overlooked 90 overpacking 90

M machine 137

P packaging 113, 301

316

PBT 230 permeability 193 PET 230 photo-resist 151 piston speed 38 plant efficiency 262 plasticating unit 301, 307 platen 90 Poisson equation 199 polarization 149 poly(vinyl chloride) 37 polyamide 285, 302 polycarbonate 37, 93, 99, 158, 302 polyesters 230 polyethylene 127 polymer flow 237, 245 polymethylmethacrylate 159, 302 polyoxymethylene 158, 167 polyphenylensulfide 167 polypropylene 121, 188, 254, 274, 287, 296 polystyrene 37, 121, 238, 247 polyurethane 79 porosity 193 powder injection molding 73 pressure 90, 121, 143 control 22 loss 260 processing parameters 253 window 94, 113, 253 pseudo-ductile 230 psuedoplastic flow 117 PTFE 285

Index

Q quality 20 Quick Product Change 263 R race tracking 193 railroad cars 209 ram speed 35 reaction injection molding 79 recordable media 149 repeatability 47 residence time 95, 260 residual stress 237, 245 resin transfer molding 193 resonance 256 reversed molding 188 Reynolds number 64 rheokinetic behavior 81 rheological behavior 237 properties 81 rheology 37, 73 ribs 134 roughness 65 S scale 2 screw speed 90 screws 301 seals 114 SEM 153, 232 semicrystalline 256 shape 58 shear band 238

Index

thinning 36 shot control 22 shrinkage 74 rate 89 silicone 79 simulation 46 single site catalyst 113 sink marks 79, 93 sintering 74 skin 89, 99 slip-stick phenomenon 118 sprue 260 stack mold 259 stiffness 95, 134, 230, 287 stretching deformation 308 T Tait equation 100 temperature 144 tensile strength 255 texture painting 45 thermal conductivity 283 thin-wall 121 tin/bismuth alloy 284 tool erosion 91 tooling 91 toughness 230, 238 TPU 113 transparency 245 twin screw extruder 287, 308 V Vacuum Assisted Liquid Molding 209 Valveless Melt Transfer System 261 variothermal heating 164

317

venting 91 vertical press 189 vibration 254 Vibration Gas Injection Molding 253 vibrational energy 253 force 245 molding 237 viscosity 27, 37, 81, 113, 164, 231 elongational 76 shear 76 volume 57 W wall thickness 38, 135 warpage 1, 63, 74, 89, 99 weight 20 reduction 79 weldline 134