Blown Film Extrusion: An Introduction

Kirk Cantor

Blown Film Extrusion An Introduction

Hanser Publishers, Munich • Hanser Gardner Publications, Cincinnati

The Author: Prof. Kirk Cantor, Pennsylvania College of Technology, One College Avenue, Williamsport, PA 17701, USA Distributed in the USA and in Canada by Hanser Gardner Publications, Inc. 6915 Valley Avenue, Cincinnati, Ohio 45244-3029, USA Fax: (513) 527-8801 Phone: (513) 527-8977 or 1-800-950-8977 www.hansergardner.com Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20, 81631 München, Germany Fax: +49 (89) 98 48 09 www.hanser.de The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Library of Congress Cataloging-in-Publication Data Cantor, Kirk. Blown film extrusion : an introduction / Kirk Cantor. p. cm. Includes bibliographical references and index. ISBN-10: 1-56990-396-4 (hc) ISBN-13: 978-1-56990-396-4 (hc) 1. Plastic films. I. Title. TP1183.F5C36 2006 668.4‘95--dc22 2006000351 Bibliografische Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über abrufbar. ISBN-10: 3-446-22741-5 ISBN-13: 978-3-446-22741-5 All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or by any information storage and retrieval system, without permission in writing from the publisher. © Carl Hanser Verlag, Munich 2006 Production Management: Oswald Immel Typeset by Manuela Treindl, Laaber, Germany Coverconcept: Marc Müller-Bremer, Rebranding, München, Germany Coverdesign: MCP • Susanne Kraus GbR, Holzkirchen, Germany Printed by Kösel, Krugzell, Germany

Acknowledgements

I am grateful for the many people that have supported my efforts to write this book and develop the software included. Funding for these projects was provided through a grant to the Plastics Resources for Educators Program (PREP) by supporters at the National Science Foundation. My colleagues at Pennsylvania College of Technology provided administrative support. I am very thankful for my good friend and co-worker, Tim Weston, for his vision and leadership through PREP. Other PREP colleagues for whom I am thankful are Alex Bierly, who masterfully created all of the graphics for the simulator, and our other very talented artists, Mike Fleck, Matt Byers, and Craig Reitz. Many thanks go to another close friend and mentor, Chris Rauwendaal, for not only helping with the text of this book, but for teaching me so much about extrusion over the years. I am thankful for the text review and insight given to me by Robert Krycki. My friends at Hanser have been very helpful with the creation of the manuscript and artwork. Thanks especially to Christine Strohm for her years of encouragement and assistance. Finally, I am most thankful for the support of my devoted family and the opportunity given to me by God. My lovely wife, Patsy, and my four beautiful daughters, Kristen, Caylee, Kelsey, and Shannon, have patiently endured my hours away and have even cheered me on to completion. January 2006

Kirk Cantor

Contents Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

Materials for Blown Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.1 Polyethylene (PE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.2 Low-Density Polyethylene (LDPE) . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.3 High-Density Polyethylene (HDPE) . . . . . . . . . . . . . . . . . . . . . . . . 8 1.1.4 Linear Low-Density Polyethylene (LLDPE) . . . . . . . . . . . . . . . . . . 9 1.1.5 Metallocene Polyethylene (mPE) . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.1.6 Polypropylene (PP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.1.7 Polystyrene (PS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.1.8 Ethylene Vinyl Acetate (EVA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.1.9 Ethylene Vinyl Alcohol (EVOH). . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.1.10 Polyvinyl Chloride (PVC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.1.11 Polyamide (PA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.1.12 Polyurethane (PU). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.2 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.2.1 Antiblocking Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.2.2 Antioxidants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.2.3 Antistatic Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.2.4 Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.2.5 Lubricants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.2.6 Reinforcements and Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.2.7 Stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1.2.8 Tackifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2

Extrusion Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.1 Extruder Hardware Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.1.2 Drive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.1.2.1 Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.1.2.2 Speed Reducer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.1.2.3 Thrust Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.1.3 Feed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

VIII

Contents 2.1.4

2.2

3

Screw/Barrel System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.1.4.1 Screw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.1.4.2 Barrel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.1.5 Head/Die System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.1.5.1 Head Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.1.5.2 Adapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.1.5.3 Breaker Plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.1.5.4 Melt Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.1.5.5 Die . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.1.6 Instrumentation and Control System . . . . . . . . . . . . . . . . . . . . . . 36 2.1.6.1 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.1.6.2 Head Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 2.1.6.3 Motor Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Extrusion Functional Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.2.1 Solids Conveying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.2.1.2 Gravity-Induced Region . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.2.1.3 Drag-Induced Region . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 2.2.2 Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.2.3 Melt Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 2.2.4 Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 2.2.4.1 Distributive Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 2.2.4.2 Dispersive Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 2.2.4.3 Mixing Devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 2.2.5 Degassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 2.2.6 Die Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Hardware for Blown Film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 3.1 Upstream Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.2 Grooved Feed Throat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.3 Screws for Blown Film Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 3.4 Blown Film Dies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 3.5 Bubble Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 3.6 Bubble Cooling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.7 Bubble Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 3.8 Collapsing Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 3.9 Haul-off. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3.10 Winders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 3.11 Film Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 3.12 Line Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Contents

IX

4

Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 4.1 Process Variables vs. Bubble Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 4.2 Characteristic Bubble Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.3 Process/Structure/Property Relationships . . . . . . . . . . . . . . . . . . . . . . . . . 94

5

Coextrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.1 Dies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 5.2 Interfacial Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6

Film Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103 6.1 Tensile Strength (ASTM D882). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.2 Elongation (ASTM D882) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6.3 Tear Strength (ASTM D1004, ASTM D1922 and D1938) . . . . . . . . . . . 107 6.4 Impact Resistance (ASTM D1709, D3420 and D4272) . . . . . . . . . . . . . 109 6.5 Blocking Load (ASTM D3354) and Coefficient of Friction (ASTM D1894). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 6.6 Gel (Fisheye) Count (ASTM D3351 and D3596) . . . . . . . . . . . . . . . . . . 111 6.7 Low Temperature Brittleness (ASTM D1790) . . . . . . . . . . . . . . . . . . . . . 111 6.8 Gloss (ASTM D2457). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6.9 Transparency (ASTM D1746) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6.10 Haze (ASTM D1003) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 6.11 Density (ASTM D1505). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 6.12 Melt Index (ASTM D1238) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.13 Viscosity by Capillary Rheometry (ASTM D3835). . . . . . . . . . . . . . . . . 116

7

Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119 7.1 Extruder Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 7.1.1 Surging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 7.1.2 High Melt Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 7.1.3 Excessive Cooling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 7.1.4 Low Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 7.2 Film Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 7.2.1 Melt Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 7.2.2 Thickness Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 7.2.3 Die Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 7.2.4 Gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 7.2.5 Low Mechanical Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 7.2.6 Poor Optical Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

X

Contents

Appendix A: The Blown Film Extrusion Simulator. . . . . . . . . . . . . . . . . . . . . . . . . . .135 A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 A.2 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 A.3 Running the Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 A.4 Worksheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Appendix B: Useful Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .155 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .159

Introduction

Blown film extrusion is one of the most significant polymer processing methods. Several billion pounds of polymer, mostly polyethylene, are processed annually by this technique. While some applications for blown film are quite complex, such as scientific balloons (Fig. 1), the majority of products manufactured on blown film equipment are used in commodity applications with low profit margins: grocery sacks, garbage bags, and flexible packaging (Fig. 2). Consequently sophisticated hardware, materials, and processing methods have been developed to yield film at very high output rates with both low dimensional variation and consistent solid-state properties.

Figure 1

A high altitude, scientific balloon being prepared for launch (National Aeronautics and Space Administration)

2

Introduction

Figure 2

Blown film extrusion is used to produce very high volumes of commodity products such as grocery and produce bags

Polymer chemistry and molecular structure are vital in establishing film properties, but bubble geometry resulting from processing conditions is also significant. Molecular orientation and crystalline structure – controlled by bubble dimensions – affect properties such as tensile strength, impact toughness, and clarity. As a manufacturing process, blown film is somewhat unique, even compared with other extrusion processes. Molten polymer generally exits the die vertically in the form of a freely extruded bubble reaching heights of 50 feet (15 meters) or more (Fig. 3). Guides surrounding the bubble may limit its mobility, but it is still quite exposed to dimensional variation compared to the fixed extrudate in most other extrusion processes, which use vacuum sizers, calibrators, rollers, or other techniques. Depending on processing conditions, the blown film bubble has a shape freedom that allows almost any number of profiles within a designed range. Operators must have a relatively high skill level to accurately obtain the required bubble geometry (i.e., the shape resulting in specified product dimensions and properties). The strong interdependence of process variables is another aspect of the process that requires a high level of operator skill and has led to extensive advancements in measurement and control techniques. There are many process variables – screw speed, nip speed, internal bubble air volume, and cooling rate (frost-line height) – that influence bubble geometry and, as a result, film properties. An adjustment to any one

Introduction

Figure 3

3

A blown film extrusion line (Windmoeller & Hoelscher)

of these variables leads to a change in several geometric characteristics of the bubble. For example, an operator may intend only to decrease film thickness by increasing the nip speed; however, if no other control is modified this adjustment will also create an increase in both frost-line height and layflat width. Therefore, the proficient operator is aware of the influence of each process variable on all geometric characteristics of the bubble and can control more than one characteristic at a time.

4

Introduction

From hardware and materials through processing and properties, this book is intended to provide the reader with a comprehensive understanding of blown film extrusion through a useful balance of theory and practice. Included in this book are the answers to why effects behave the way that they do in the blown film process, so you can improve your ability to troubleshoot and improve systems. At the same time, current practices and equipment are emphasized to keep readers up-to-date with the most productive and efficient technology. The companion CD-ROM, The Blown Film Extrusion Simulator, enhances the learning process. This software was developed specifically to teach blown film extrusion equipment operation and processing principles. The realistic graphic interface and intuitive operating techniques were designed to emulate actual processing methods, so learners can quickly move from the simulator to real production equipment. Throughout this book there are exercises (identified with the symbol •) using the simulator to complement the methods and principles explained. It is intended that, when convenient, readers will take a break from reading the book and spend a few minutes with the simulator to enhance their understanding of the content. Before continuing to the next chapter, you may want to skip to Appendix A to learn how to install and operate the simulator.

1

Materials for Blown Film

This chapter covers the various materials extruded by the blown film process. It is divided into two broad categories: polymers and additives. Many types of polymers are extruded into blown film, but various grades of polyethylene comprise the vast majority. Additives are used in most operations to provide performance, appearance, or cost benefits.

1.1

Polymers

For any product it is the desired property set that determines the best material for the application. The most important properties, such as physical, mechanical, optical, thermal, and electrical, provide fitness-for-use in the final product,. In addition to final product (solid-state) properties, processing properties are also very important in material selection. Ease of processing in blown film can be described by characteristics such as good thermal stability, high melt strength (outside of the die), reasonable head pressures, and no melt fracture (film surface imperfections). Finally, minimal cost is a key property. There are numerous applications for blown film, but a very high percentage of film is used in commodity applications, such as packaging and bags. These products require a combination of performance, processing, and cost that make polyethylene (PE) the ideal polymer for most applications. It is lightweight, water-resistant, has a good balance of strength and flexibility, and can provide some clarity. PE is easy to extrude and heat-seal and, perhaps most importantly, is low cost. In addition to these general properties of PE, it is a polymer that is very well understood scientifically, allowing for polymerization techniques that can be designed and controlled to yield specific property values over a very wide range. That is, PE grades can be produced that are much stronger than average, or much clearer, or much more flexible, and so on. Within the broad family of polyethylene, there are several types that find application in blown film. There are, however, other polymers used as well. While a few of these

6

1 Materials for Blown Film

others are used in monolayer specialty films, they are mostly used as part of coextruded multilayer films, where multiple extruders feed a single die that combines the various feed streams into a single, multilayer film. The following sections detail the many polymer types used in blown film extrusion.

1.1.1

Polyethylene (PE)

Polyethylene (PE) is the simplest polymer from a chemical standpoint. It is polymerized from ethylene monomer and consists of a carbon chain backbone with two hydrogen atoms bonded to each carbon atom (Fig. 1.1). Individual molecules, or chains, may be as long as hundreds to tens of thousands of carbon atoms. PE chains may be very linear or may be branched, depending on how the polymer was synthesized. There are many synthesis techniques for polyethylene [1]. H

H

H

H

~C

C

C

C~

H

H

H

H

Figure 1.1 A short section of a polyethylene molecule

Blown film processing methods for PE vary somewhat depending on the grade. Differences will be discussed in the following sections. However, one important similarity is that all PE grades have a high value of specific heat. Specific heat is a measure of the energy required to raise a unit mass of material one degree in temperature. If a polymer has a high value of specific heat, this means that heat removal from the melt is relatively slow. PE has a specific heat of approximately 2 kJ/kg·K compared to approximately 1 kJ/kg·K for most other polymers. This is the reason why cooling towers for blown film polyethylene are very tall. It takes time to remove enough heat from the two layers passing through the nip rollers to prevent them from sticking together (sometimes called blocking).

1.1.2

Low-Density Polyethylene (LDPE)

Polyethylene is often categorized by its density, a measure of the mass per unit volume (e.g., g/cm3 or lb/in3). When any type of polymer cools from the melt state, some of the chains may organize into highly ordered, more dense crystalline regions (Fig. 1.2). This

1.1 Polymers

7

Figure 1.2 Polymer molecules are shown here in ordered, crystalline regions and disordered, amorphous regions

will occur with sections of molecules possessing long, repeating patterns. In sections containing irregular patterns, such as branch points or chain ends, crystallization does not occur and these regions are called amorphous (disordered). Some polymers, such as commercial polystyrene, are completely amorphous due to hindrance of the entire molecular structure to crystallization. Low-density polyethylene (LDPE) is synthesized in such a way that a highly branched polymer is formed. It consists of short chain branches (less than six carbon atoms long) and long chain branches (almost as long as the length of the backbone). Branch points along the chain serve as disruptions to the order of the system and prevent local crystallization. The lower level of crystallinity results in lower density. LDPE is generally in the density range of 0.91 to 0.93 g/cm3. LDPE tends to be relatively easy to process. Compared to other PE grades, it melts at a relatively low temperature (220 to 240 °F, 105 to 115 °C) and does not require as much extruder motor power. LDPE blown film grades are moderately high in viscosity, but the wide range of branching yields a fairly wide processing window and high melt strength in the bubble. This leads to a stable bubble that can be run with a low frost-line height (pocket bubble, or bowl or pear shape, Fig. 1.3). LDPE heat-seals very easily.

8

1 Materials for Blown Film

Pocket bubble

Long stalk bubble

Figure 1.3 Two types of blown film bubbles

The properties of LDPE blown film can be characterized as tough and flexible. The toughness is derived from a good combination of strength and elongation, particularly when processed with high machine and transverse direction orientation. Flexibility results from low crystal content. LDPE bags provide a soft feel compared to the crinkly feel of HDPE bags. However, LDPE is not as stiff or strong as HDPE.

1.1.3

High-Density Polyethylene (HDPE)

High-density polyethylene (HDPE) is synthesized by a method very different from that used for LDPE. As a result, very linear chains are produced. In fact, HDPE is generally polymerized with a small amount of comonomer that leads to a few short chain branches placed intentionally along the main chain to make the polymer easier to process (Fig. 1.4). A high degree of linearity results in a high percentage of crystallinity (i.e., high density). HDPE is generally in the density range of 0.93 to 0.96 g/cm3. Processing HDPE is somewhat different than processing LDPE. Because of its higher degree of crystallinity and more consistent molecular structure, HDPE melts at a higher temperature (265 to 275 °F, 130 to 135 °C) and has a narrower processing window. It also requires higher screw torque, hence more motor power. To promote good solids feeding via high barrel/pellet friction, grooved feed throats are often used.

Figure 1.4 Schematic representation of an HDPE molecule showing high linearity with a small amount of short chain branching

1.1 Polymers

9

One of the most obvious differences between processing HDPE and LDPE is that HDPE is usually run with a high frost-line height (long stalk or wine glass shape, see Fig. 1.3). The frost-line height is generally on the order of eight to ten times the die diameter. HDPE tends to have lower melt strength than LDPE, so bubble stability may be more of a problem. By delaying transverse stretching of the bubble until the melt is cooler (i.e., having a high frost line), the bubble remains more stable. HDPE has high strength and stiffness among polyethylene grades. As a result, there is continual progress in the area of reducing the thickness of HDPE film products (downgauging). Additionally, HDPE has reasonably good barrier properties (resistance to gas permeation) owing to its high degree crystallinity.

1.1.4

Linear Low-Density Polyethylene (LLDPE)

Linear low-density polyethylene (LLDPE) is a variation of HDPE. It is synthesized similarly, but LLDPE has a much higher content of comonomer, such as hexene or octene. Incorporation of comonomer in the chain yields short chain branches of a specified length (Fig. 1.5). By controlling the amount of branch points through comonomer content, degree of crystallinity – hence density – can be controlled. Variants of LLDPE are known as very low-density polyethylene (VLDPE) and ultra low-density polyethylene (ULDPE). LLDPE density is generally in the range of 0.88 to 0.93 g/cm3. With regard to processing, LLDPE is something of a mixed bag (melt temperature = 240 to 260 °F, 115 to 125 °C). Inside the extruder it performs similarly to HDPE, requiring higher torque and often employing a grooved feed throat. However, outside of the die it is generally processed with a pocket bubble like LDPE, even though the melt strength tends to be lower than that of LDPE. This is handled by using a dual lip air ring that aerodynamically stabilizes the bubble while providing a high volume of cooling air. LLDPE solid-state properties also reflect a combination of those of HDPE and LDPE. Its strength is higher than that of LDPE, approaching that of HDPE. However, it has the softer feel and lower stiffness of LDPE.

Figure 1.5 Schematic representation of an LLDPE molecule showing high linearity with a large amount of short chain branching

10

1 Materials for Blown Film

1.1.5

Metallocene Polyethylene (mPE)

Metallocene polyethylene (mPE) is a broad class of polymers made from ethylene that are synthesized using metallocene catalysts during the polymerization process. Also known as constrained geometry catalyst technology and single-site catalyst technology, this relatively new technique provides very precise control over molecular structure, thus allowing the chemist to accurately design a wide class of polyethylene types based on chain length, chain length distribution, and branching structure. While mPE materials are much like other PE grades, they can be produced with a very wide range of properties. One of the key property benefits is the ability to synthesize very soft, flexible grades. These provide some interesting opportunities for final film products, but a feed material that deforms easily and melts quickly requires some process modifications.

1.1.6

Polypropylene (PP)

Traditionally, polypropylene (PP) has been synthesized from propylene monomer by a method similar to that used for HDPE. Since this technique yields a very regular pattern along the chain, PP is able to crystallize. Because propylene monomer is slightly larger than ethylene monomer (the monomer is called a “repeat unit” once it is incorporated as links in the chain, Fig. 1.6), PP crystal structure is somewhat different than PE crystal structure and has a higher melting point. Also, PP is generally stronger and stiffer than PE. So, it can be used in applications requiring a higher use temperature and more strength. Examples include medical bags that can be autoclaved, hot liquid drum liners, and release films for construction materials. Recently many new polypropylene grades have been synthesized using metallocene technology, similar to that used for ethylene. This has widened the range of property offerings in areas such as stiffness, impact strength, melting point, and clarity. Additional property sets are available from PE/PP copolymers, materials synthesized using both ethylene and propylene monomers. These find many applications in food packaging because of its good clarity resulting from low crystallinity. H ~C H

CH3 C~ H

Figure 1.6 A repeat unit of polypropylene contains one methyl group in place of a hydrogen atom

1.1 Polymers

11

Other than a potentially higher temperature profile (melt temperature = 330 °F, 165 °C), processing PP is similar in ease to processing PE. Both materials are thermally stable compared with other polymers, do not require drying, and decrease in viscosity readily at higher screw speeds (shear thinning). One notable exception, however, is that PP generally has a lower melt strength than PE, particularly compared with LDPE. In blown film processing, this can lead to difficulties with bubble stability. PP can be used in both monolayer specialty films and within a multilayer coextruded structure.

1.1.7

Polystyrene (PS)

Polystyrene (PS) is generally synthesized by a method yielding an irregular pattern along the polymer chains (Fig. 1.7), hence preventing crystallization. Since the resulting polymer is completely amorphous (glass transition temperature = 210 °F, 100 °C), it possesses excellent clarity. This, along with its high strength and low cost, are the most important commercial properties of PS. However, because the chains are very stiff, it possesses a detrimental level of brittleness. PS films can crack easily when folded and slight imperfections, such as gels, readily cause tears that propagate rapidly. For this reason, PS is generally blended with some amount of rubber-containing modifier, such as styrene-butadiene copolymer. Although at higher levels the rubber additive may reduce clarity, it makes PS films mechanically tougher and easier to process. Still, blown film processing of PS is significantly more difficult than PE. Even though PS cools more quickly than PE, it is usually processed at a much lower rate because the film is sensitive to damage. Care must be taken to ensure good melt filtering so that contaminants do not cause bubble breaks. Also, winding and slitting equipment must be designed and operated such that film scratching and cracking is prevented.

Figure 1.7 Schematic representation of a polystyrene molecule showing bulky side groups randomly positioned on either side of the chain

12

1 Materials for Blown Film

Applications for single- and multilayer PS films include food and candy packaging, clear gift wrap, and envelope windows. Because of the ability of PS films to allow gas (such as air) to permeate through them, particularly with higher rubber concentrations, they have been used in breathable produce packaging that provides continued ripening on store shelves [2].

1.1.8

Ethylene Vinyl Acetate (EVA)

Ethylene vinyl acetate (EVA) is a copolymer of polyethylene. It is similar in chemistry to PE, but it has some percentage of vinyl acetate (VA) included along the chains (Fig. 1.8). The amount of VA that is included, generally between about 5 and 20%, depends on the desired properties of the polymer. VA adds polarity, or adhesion, to the polymer and, so, improves the compatibility of the polymer with fillers and gives the polymer adhesive properties. Most blown film applications using EVA do so as layers in coextruded products, such as food and electronics packaging. OCOCH3

OCOCH3

OCOCH3 Figure 1.8 Schematic representation of an ethylene vinyl acetate molecule showing acetate side groups randomly located along the chain

1.1.9

Ethylene Vinyl Alcohol (EVOH)

Ethylene vinyl alcohol (EVOH) is similar to EVA in that it is a copolymer of polyethylene with some percentage of the vinyl comonomer along the chain. In fact, EVOH is synthesized by converting VA units along a chain into vinyl alcohol (VOH) units. Grades of EVOH can be purchased with either fully or partially converted VA units to provide a wide property set. Like EVA, EVOH is primarily used in layers of coextruded film structures. EVOH has some important commercial properties. Perhaps the most important property is its resistance to oxygen permeation. For this reason, it is used as a barrier layer in multilayer food-grade films. Another important property of EVOH is that it is watersoluble. Therefore, it is used in applications such as the delivery of laundry detergent via dissolving packaging.

1.1 Polymers

1.1.10

13

Polyvinyl Chloride (PVC)

Worldwide, polyvinyl chloride (PVC) is one of the most extruded polymers by volume. While environmental and health concerns has created lost market share in certain applications, PVC has made large gains in other areas, such as the replacement of wood and aluminum profiles in building construction products. Blown film extrusion is also used in the manufacture of products from PVC. Though PVC has limited thermal stability, it has good melt strength, which lends itself nicely to blown film extrusion. PVC is an amorphous polymer (glass transition temperature = 220 °F, 105 °C). As a result, it has good clarity. Another important characteristic is that it can be extruded either as a rigid material or as a flexible material by adding a plasticizer to the polymer. Rigid films can be metallized and punched into sequins for dressmaking applications. Flexible films are used to overwrap clothing and other textile products. Two other important characteristics of PVC taken advantage of in film applications are barrier properties and heat-shrinkability. Because of these attributes, PVC film finds usage in food packaging, such as candies, as well as nonfood packaging, such as shrink wrapping of auto parts.

1.1.11

Polyamide (PA)

Blown film applications for polyamide (PA), also known as nylon, are primarily barrier layers in multilayer structures. However, PA has quite different processing characteristics than other layers, such as polyethylene, typically found in these coextruded films. For example, PA has a much higher processing temperature (generally > 500 °F, melt temperature = 360 to 480 °F, 180 to 250 °C) so the extrusion system, particularly the die, must be designed to provide proper temperature control to individual flow layers. Also, PA is a hygroscopic material, meaning it absorbs moisture from the air, so it must be dried sufficiently prior to processing. There are also some specialty monolayer films blown from PA. One example is a high temperature film used as bagging material for composites processing.

1.1.12

Polyurethane (PU)

Polyurethane (PU) is a highly versatile material with a wide range of properties, depending on the chemistry of the specific grade. Blown film grades are thermoplastic

14

1 Materials for Blown Film

(TPU) and either aromatic or aliphatic. Aliphatics are more expensive, but generally have better resistance to ultraviolet radiation and are clearer. PUs are also either polyetherbased or polyester-based, where the former has better low temperature flexibility and the latter tends to be tougher and more chemical resistant. Blown film grades of PU are usually easy to process because they are synthesized to have good melt strength. They are generally processed at temperatures between 350 and 400 °F (180 and 205 °C). However, they are highly moisture absorbent and, if not dried adequately, exhibit gels, streaks, and low melt strength. Because of their high elasticity and toughness, PU blown films are generally used in specialty applications. One example is an adhesive laminating layer between fabrics. Another example is inflatable bladders used in rafts and kayaks.

1.2

Additives

Practically all polymers extruded today are combined with at least one type of additive. This important class of materials represents the primary method by which plastics properties, such as strength, clarity, and cost, are fine-tuned for specific applications. Additionally, additives can serve an important role in improving the processability of polymers. Additives are incorporated into polymers several ways: the resin supplier may include them in the raw material, an intermediate compounder may produce additive-enhanced feedstock, or the final extrusion processor may add them directly. When incorporated in final processing, additives may be introduced individually or as part of a masterbatch, a blended compound containing one or more additives in a polymer carrier. A masterbatch material is incorporated most efficiently when its carrier is compatible both chemically (same polymer) and rheologically (same flow characteristics) with the primary resin. Blown film processors should be knowledgeable about the ways in which additives may affect the processing of plastics. For example, some additives are designed to aid processing by reducing viscosity while others may have a detrimental effect on extrusion machinery by causing considerable wear. The following sections describe various additives used with blown film extrusion polymers.

1.2 Additives

1.2.1

15

Antiblocking Agents

Antiblocking agents are additives that inhibit the stickiness (blocking) of polymer surfaces. Blocking is most apparent in films, particularly blown films. After a blown film bubble has passed through the nip roller and the two halves have been pressed together, they may tend to block. Antiblocks help the film to be easily pulled apart. Diatomaceous earth is a commonly used antiblock additive.

1.2.2

Antioxidants

Antioxidants inhibit the oxidation of polymers that leads to molecular degradation. Polymer chains can break down into smaller segments – a process called chain scission – in the presence of oxygen (air), particularly at elevated temperatures encountered during extrusion. Degraded polymer may be discolored and generally has reduced mechanical properties. In some cases crosslinking (or gel formation) can occur during oxidation. Even when no other additives are present, many polymers are supplied with antioxidants included. This ensures the stability of the polymer during processing. The primary mechanism these additives employ to combat oxidative degradation is to capture free radicals (electrons), generated by oxidation, before they can attack the polymer chain. Phenolics and amines are two of the most common categories of antioxidants.

1.2.3

Antistatic Agents

Antistatic agents (antistats) are designed to minimize the buildup of static charges on polymer surfaces. Many polymers are susceptible to static electricity during processing, such as when film passes through a series of rollers on its way to being wound. This may be particularly problematic on days when the humidity is very low. Antistats help dissipate static charges by causing the polymer surface to be more conductive. For example, one method is to attract moisture to the surface. External antistats are applied directly to the polymer surface. The more common internal antistats, such as amines, are compounded into the system.

16

1 Materials for Blown Film

1.2.4

Colorants

Colorants are perhaps the most popular additives used in the plastics industry. In fact, it is the choice of available colors that gives many products their consumer appeal. Although colorants are primarily used for their aesthetic value, secondary benefits may be achieved, such as the ultraviolet absorptive quality of carbon black. Colorants are introduced into polymers in a variety of ways. The four most popular forms are precolored compounds, color concentrates, dry color, and liquid color. Precolored compounds are delivered ready for final processing. Prior to delivery to the extruder, they have been mixed to the final ratio of polymer to colorant. This is the easiest form in which to color polymer because it requires no additional materials or hardware other than what is typical for single-material extrusion. Additionally, the uniformity of color is usually excellent as a result of the compounding process. However, this form is generally expensive because another processor performs the compounding work. Color concentrates are usually pellets that contain a high percentage of colorant in a compatible polymer carrier. These pellets are then “let down” into the base polymer in a proportion that will provide the correct final concentration of colorant to the system. Color concentrate pellets are often let down at approximately 4% of the final system and require only minimal effort and hardware beyond using precolored compounds. If the main concern is adequate mixing in the extruder, this is one of the most popular forms for coloring plastics. Dry color is powdered color concentrate delivered without a polymer carrier. Although it can be used in more than one polymer type, it is somewhat difficult to handle. The very fine particle size can become airborne, like dust, and create a mess. Also, the low bulk density of the material can cause it to clump together in the feed throat and not enter the extruder uniformly. Liquid color contains colorant in a liquid base. It eliminates some of the handling problems associated with dry color, but requires a pump for accurate delivery. The percentage of colorant in the liquid carrier may be quite high so that only a minimal let down ratio (around 1%) is necessary. While these four forms represent the delivery methods for colorants, there are two basic types of colorants incorporated into these forms: dyes and pigments. Dyes are organic compounds that provide the most brilliant colors in polymers. They tend to dissolve in the polymer and so are most easily distributed throughout the system. Dyes tend to have limited thermal stability and must be matched to the polymer processing temperature. They have their primary application in the coloring of transparent products.

1.2 Additives

17

Pigments are small, colored particles that must usually be broken down (dispersed) and distributed for good color uniformity. They may be based on organic or inorganic compounds, but are generally more thermally stable than dyes. They tend to have better opacity (hiding power) than dyes, but not as good brilliance. Historically many pigments were based on heavy metals such as lead and cadmium, but environmental concerns have prompted a change of these types of colorants to being heavy-metal-free.

1.2.5

Lubricants

Lubricants are used with polymers for two main needs: external lubrication and internal lubrication. External lubrication reduces friction between the polymer and the extrusion hardware, such as on the internal flow surfaces of the die. For example, lubricants can help eliminate melt fracture of blown film by reducing stress on the polymer as it passes through the die. Additionally, die drool (or die lip buildup) has been reduced by the use of lubricants. Internal lubrication reduces the friction between flowing polymer molecules, effectively reducing melt viscosity. Use of internal lubricants can reduce the power consumption required for polymers that are difficult to process. Some common lubricants are metal stearates and paraffin waxes.

1.2.6

Reinforcements and Fillers

There is considerable overlap within this category of additives. A particular material, such as glass, may be employed in one application and be considered a reinforcement, but in another application it is called a filler. However, most commonly, a reinforcement is defined as an additive that improves the mechanical properties (e.g., strength) of the polymer system. A filler is defined as an additive that takes up space in the system, generally to reduce cost. A few of the most important additives within this category are described here. Calcium carbonate is a naturally occurring mineral obtained chiefly from limestone. It is available in a wide range of particle sizes. This material is widely used as a filler, but it also serves to increase impact resistance in many polymers and to improve surface appearance. A benefit of using calcium carbonate that is being exploited more frequently today with blown film extrusion is its low heat capacity compared with polyethylene. This leads to improved cooling rates, hence higher throughputs, for films containing calcium carbonate. However, high loadings of calcium carbonate can lead to increased wear of screws, barrels, and material handling systems.

18

1 Materials for Blown Film

Carbon is used in two primary ways as an additive: carbon black and carbon fiber. Both forms absorb moisture and are generally dried prior to extrusion or used with a vented extruder. Carbon black is a powdery substance and is used in many outdoor applications because of its ability to absorb ultraviolet radiation. This extends the service life of the polymer. Additionally, it is a powerful colorant, is relatively inexpensive, and can increase electrical conductivity. One of the major drawbacks to using carbon black is that it is difficult to handle and its dust can be quite messy. For this reason, it is often purchased in a more expensive pellet (masterbatch) form. Carbon fibers are reinforcing agents. Though somewhat costly, they are extremely strong and significantly increase the mechanical properties of polymers. When used in extrusion, they are generally dispersed in the polymer matrix as short, chopped fibers. In addition to their reinforcing properties, the electrical conductivity of carbon fibers leads to their use in applications requiring the dissipation of static charge. Clay is becoming an increasingly popular additive today. It is delivered to the extruder as a powdery material, but at the microscopic (nanometer) level it is comprised of stacked platelets. Clay is employed in so-called nanocomposites, whose name is derived from the fact that the matrix polymer penetrates between the platelets (intercalates). Under proper processing conditions, the platelets may be separated from one another (exfoliated) for further property enhancement. Mechanical and barrier properties (resistance to gas permeation) may be greatly improved by using clay. Glass is incorporated into polymers in two primary forms: glass spheres and glass fibers. Glass spheres may be used as a cost-saving filler, but can provide some additional properties as well. Hollow spheres reduce the density of the polymer system, providing a weight savings, while often improving flexural strength. Solid spheres improve most mechanical properties. They also provide reflectivity, and are therefore used in applications such as highway markings. Glass fibers are very strong, providing excellent reinforcing properties at a reasonable cost. They come in several grades depending on strength, electrical, and chemical requirements. They are generally used in extrusion as short, chopped fibers. Under high loadings of glass, significant screw and barrel wear can occur. Talc is generally considered a mineral filler, but may provide improved mechanical properties as well. Its platelet structure tends to increase stiffness and service temperature. Talc is softer than many other mineral fillers, so it is not as abrasive to extrusion hardware. Milled wood flour is becoming a popular filler for thermoplastics today. It is inexpensive and imparts to the polymer some of the properties of natural wood, for example,

1.2 Additives

19

appearance, texture, and scent. Because of the polymer matrix, wood-filled products can offer a much longer service life than their natural counterparts. Also, they are much more resistant to deterioration due to moisture. To maintain adequate mechanical properties, compatibilizers are generally used with wood-filled products. Compatibilizers work by increasing the adhesion between the filler and the polymer matrix.

1.2.7

Stabilizers

The two main types of stabilizers are heat stabilizers, to protect polymers during processing, and ultraviolet (UV) stabilizers, to protect polymers exposed to excessive solar radiation. Heat stabilizers find predominant application with polyvinyl chloride (PVC). PVC is very sensitive to heat and shear, releasing hydrogen chloride (HCl) when it degrades. Because of the corrosive effects of HCl and the polymer’s tendency to degrade quickly, PVC must be stabilized during extrusion. This is even more critical with rigid PVC because it is processed at higher temperatures than flexible PVC. As one HCl molecule is released from PVC during decomposition, it makes it easier for a nearby HCl molecule to also be released, leading to rapid degradation of the polymer. Stabilizers tend to react with the first HCl molecule, preventing it from assisting the release of any further molecules. Historically, many PVC stabilizers have been based on lead and cadmium. Environmental concerns prompted the development of newer stabilizers, for example those based on barium-zinc and calcium-zinc. UV stabilizers are used with polymers susceptible to degradation from ultraviolet energy. UV energy can cause chain scission in unprotected polymers. Results of this degradation include loss of mechanical properties, color changes, and cracking. Stabilizers are generally very high absorbers of UV radiation, preventing the energy from damaging the polymer. They may also act to trap free radicals formed during degradation processes. Carbon black is a popular UV absorber, but it is limited to one color choice. Hindered amines are efficient free radical trapping molecules.

1.2.8

Tackifiers

Tackifiers are used to promote adhesiveness in film surfaces. They are generally added to products that will be used in stretch/cling applications such as pallet wrap and silage films. One of the most common tackifiers is polyisobutylene (PIB).

20

1 Materials for Blown Film

PIB is a rubbery polymer generally added to the base resin in quantities less than 10%. One of its more popular characteristics with film processors is its tendency to migrate (“bloom”) to the film surface over an extended period of time. Therefore, the film tends to be less tacky during winding and film handling stages in the manufacturing plant, where tackiness is undesirable. However, after storage and shipment, the film exhibits the product’s intended degree of cling in the field.

2

Extrusion Overview

The extruder is the heart of all extrusion processes, continuously heating and mixing the feed material as it supplies a homogeneous melt to the die for shaping. Although the emphasis of this book is on the hardware and methods specific to blown film extrusion, this chapter provides a general overview of the extruder and its functions. Final product quality and production efficiency are highly dependent on the operation of the extruder. This chapter covers two main sections: extruder hardware systems and extrusion functional zones. The first section identifies the primary components of the extruder and provides detail into their capabilities and size ranges. The second section describes how the hardware interacts with the material as it passes through the system. In other words, it provides a look at what is happening inside each zone of the extruder. The descriptions here are purposefully brief, serving as a foundation to support later discussions specific to blown film extrusion. If the reader is interested in pursuing this topic further, there are many books devoted entirely to the extruder and extrusion processes. For example, Polymer Extrusion by Rauwendaal [3] provides an excellent in-depth analysis. Other references are listed in the back of this book [4–6].

2.1

Extruder Hardware Systems

The purpose of the extruder is to feed a die with a homogeneous material at constant temperature and pressure. This definition highlights three primary responsibilities that the extruder must accomplish while delivering material to a shaping die. First, it must homogenize, or satisfactorily mix, the material. Second, the material entering the die must have minimal temperature variation with respect to both time and position within the melt stream. Third, there must be minimal melt pressure variation with time. It is important that the design and operation of an extrusion system consider all three of these objectives to produce a quality product. Extruders are generally rated by screw diameter. Typical production extruders range in size from 2 to 6 in (50 to 150 mm). A fundamental quantity determined by screw

22

2 Extrusion Overview

diameter is the maximum throughput measured in pounds/hour or kilograms/hour. The relationship between throughput and screw diameter is cubic, dependent upon the volume inside the extruder available for polymer. With a certain polymer, a 2inch extruder may have a maximum throughput of 100 lb/hr, while a 4-inch extruder would have a maximum throughput of 800 lb/hr. Therefore, system requirements such as material handling capacity, motor size, cooling capability, and floor space increase rapidly with an increase in screw size. Components of the extruder hardware can be categorized into five systems (Fig. 2.1): • Drive system • Feed system • Screw/barrel system • Head/die system • Instrumentation and control system

Instrumentation & control system

Head/die system

Feed system

Drive system

Screw/ barrel system Figure 2.1 The five extruder hardware systems

•

Simulator Exercise: When the cursor is paused over any extruder component, an identification tag shows the name of that component.

2.1 Extruder Hardware Systems

2.1.2

23

Drive System

The drive system supplies the mechanical energy to the polymeric material by rotating the screw. This system consists of a motor, a speed reducer, and a thrust bearing. 2.1.2.1

Motor

The motor is the source of power to turn the screw. Extruder motors tend to be relatively large due to the high power consumption by the polymer around the screw. Three sources of power consumption are 1) melting of solids via frictional heat generation, 2) conveying of high viscosity molten polymer along the barrel, and 3) pumping of high viscosity molten polymer through the die restriction. A rule-of-thumb for motor size is motor power (HP) ≅ throughput (lb/hr) ÷ 5 Extruder motors are usually electric, but some systems utilize hydraulic motors. For example, injection molding machines use hydraulics to develop clamp tonnage. Electric motors may be of the direct current (DC) or alternating current (AC) types. Traditionally, DC motors, which regulate speed through voltage control, have been more popular because they could provide the necessary power at a lower cost. However, recent advances in frequency control – the technique used to regulate speed in AC motors – have caused this type of motor to become more widely used. 2.1.2.2

Speed Reducer

Electric motors operate most efficiently at high rotational speeds. A typical maximum speed for an extruder motor is 2,000 RPM. However, a screw speed this high would be detrimental to polymeric materials (e.g., it may result in excessive shear heating and polymer degradation). Therefore, the high motor speed is geared down to a lower screw speed through a speed reducer, also known as a gearbox. Gearboxes usually have reduction ratios in the range of 10 : 1 to 20 : 1, resulting in typical maximum screw speeds of 100 to 200 RPM. Besides speed reduction, the gearbox provides an additional advantage of increased torque. This would be analogous to providing rotational leverage. Because of the high power consumption of polymeric materials, high torque is needed to maintain screw speed. Most drive systems are designed to keep screw speed constant even if the torque requirement changes, which could be created, for instance, by a change in material viscosity.

24

2 Extrusion Overview

Figure 2.2 Two types of drive systems [7]

The input shaft of the motor may be either directly or indirectly connected to the speed reducer (Fig. 2.2). When it is hard-coupled directly through gears, the system is called direct drive. Indirect drive systems utilize belts and sheaves to connect the motor to the speed reducer. Each system has its advantages and disadvantages. Direct drive systems have better speed control and are more efficient, but may be more expensive and time consuming to repair in the case of a system breakdown. Indirect drive systems allow more flexibility in motor location and are easier to repair if the problem simply requires a new belt. 2.1.2.3

Thrust Bearing

The output of the gearbox is connected directly to the shank of the extruder screw. A thrust bearing is located at this junction (Fig. 2.3). The thrust bearing absorbs the backwards push of the screw generated by the pressure of the polymer at the output end of the screw.

2.1 Extruder Hardware Systems

25

Gear box Gear Screw shank

Screw

Thrust bearing

Radial bearing

Thrust bearing assembly Figure 2.3 A thrust bearing assembly (Interactive Training Extrusion, Rauwendaal Extrusion Engineering, Inc.)

Without a thrust bearing, it would be very difficult for the screw to rotate because high frictional forces would be generated by the backwards push of the screw into the gearbox. It would be similar to a wheel turning with tens of thousands of pounds of force applied to the hub. The thrust bearing allows the screw to rotate freely and reduces the frictional forces on the shank that are generated by the head pressure on the tip of the screw. To calculate thrust load, multiply the head pressure by the screw tip area. Typical operating head pressures for extruders are in the range of 1,000 to 5,000 psi (7 to 34 MPa). As an example, for a 4-inch extruder operating at 5,000 psi, the thrust load would be π(4/2)2(5,000) = 62,832 lb. Thrust bearings are rated by the B-10 life, which is the number of hours that 90% of identical bearings will last. A typical rating is at least 100,000 hours or approximately 10 years of continuous operation. This rating is generally for a maximum head pressure of 5,000 psi. Operating at excessive head pressures reduces the life of the thrust bearing. Changing a thrust bearing is a major undertaking. Because the barrel must be removed, this task may take many hours. If unprepared when the old one fails, it may take many days! It is usually a good idea to realign the barrel when it is replaced. Always keep the thrust bearing well lubricated and try to determine the life of the thrust bearing when purchasing a used extruder.

26

2 Extrusion Overview

2.1.3

Feed System

The feed system holds the solid material and delivers it to the extruder. The main components are the hopper and the feed throat. A material handling system (e.g., silo and vacuum loader) may transport the raw material to the hopper. The hopper (Fig. 2.4) holds the solids prior to their entering the barrel. Sometimes a dryer is integrated with the hopper. The hopper shape should be designed to prevent any hang-up of material as it flows down into the feed throat. Ideally, all solids move downward uniformly in what is called plug (or mass) flow (i.e., all material at a given height moving at the same speed with no mixing). The design should consider the form of the feed material. For example, pellets may feed acceptably in a hopper with a relatively large cone angle while powder may require a much smaller cone angle. Fluff regrind may need to be fed with an auger in the hopper, known as a crammer feeder. In general, the hopper should be circular and minimize friction (from sources such as rough surfaces or corners) with the feed material. At its base, the hopper empties into the feed throat through an opening. This opening in the feed throat is often circular or square. However, improved feeding has been measured when the opening is elongated along the barrel direction with dimensions of 1.5 screw diameters by 0.7 screw diameters [7].

Figure 2.4 A cutaway view of a hopper filled with plastic material

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Figure 2.5 An extruder screw being flood fed by a full hopper

The hopper is usually maintained at a near-full level since most single screw extruders are flood fed (Fig. 2.5). Flood feeding occurs when the throat area is kept full, allowing the screw channels to completely fill with each screw rotation. Occasionally, single screw extruders are starve fed, an arrangement where solids are trickled into the throat at a metered rate that doesn’t completely fill the screw channels. This may be necessary for difficult to feed materials, such as powders. The feed throat is actually a prebarrel component located under the hopper. It is usually cored with cooling channels. To keep the solids moving along the feed throat so that more solids can follow, the throat is cooled to prevent premature sticking of the solids in this region. When solids do stick together at the base of the hopper, they block flow and form what is known as a bridge. Bridging can often be relieved by using a poker (plastic or wooden stick) to push the blockage down the throat. Similarly, hot solids may form a plug that sticks in the screw channel. This also may be cleared by putting pressure on the solids in the hopper, but in the worst cases it is cleared only by pulling the screw for cleaning. When blockages have been removed, the source of the problem (such as excessive heat or poor flowing feed material) should be determined and fixed to prevent reoccurrence. In some cases the feed throat has shallow grooves machined into the internal surface. The purpose of these grooves is to promote a higher feed rate down the barrel. Because of the high-energy input into the solids here, throat cooling is even more important when using a grooved feed throat.

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2.1.4

Screw/Barrel System

The screw/barrel system (Fig. 2.6) has been called the “heart” of the operation. Not only does it melt the solids and pump the polymer through the die, it also prepares the melt to be homogeneous and of constant temperature and pressure. Any deviations in composition or fluctuations in temperature or pressure lead to variations in the final product.

Figure 2.6 An extruder screw inside a barrel

2.1.4.1

Screw

The screw (Fig. 2.7) is a long shaft with a thread wrapped helically around it. The thread is called a flight. Between adjacent sections of the flight is the channel. There are many different screw designs, but most screws have three primary sections: feed, transition (or compression), and metering section. Channel depth – an important variable to screw compression – is generally largest and constant in the feed section, smallest and constant in the metering section. Channel depth decreases along the transition section.

Flighted length Metering section

Transition section

Feed section

Outside diameter

Flight width Pitch Helix angle

Figure 2.7 An extruder screw

Channel depth

Root diameter

Shank

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The amount of screw compression is quantified by the compression ratio: compression ratio = feed channel depth ÷ metering channel depth Typical values of compression ratio range from 2 : 1 to 4 : 1, depending on the type of polymer and the bulk density of the feed material. Another important characteristic of screw geometry is the L/D ratio: L/D ratio = screw flighted length ÷ screw diameter Typical values of L/D ratio range from 18 : 1 to 32 : 1, with 24 : 1 being the most common. L/D ratio is based on the number of functions that a screw performs. For example, 24 : 1 is normal for the three standard functions: conveying pellets, melting, and pumping (generating pressure to feed the die). Fewer functions would require a shorter screw; more functions, such as mixing and degassing, would require a longer screw. The screw fits closely inside the barrel. Clearance, the distance between the flight tip and the barrel wall, is generally 0.1% of screw diameter (Fig. 2.8). So, a 4-inch screw will have a clearance of 0.004 inches. Clearances smaller than this will result in excessive flight tip wear, while larger clearances reduce the melting and pumping capacities of the screw. Flight

Barrel

Clearance

Screw

Figure 2.8 The clearance between the screw flight and the barrel wall

Screws generally are constructed of 4140 steel. For certain polymers, particularly those that require high temperature or are highly corrosive, special metal alloys are used. Additionally, flight tip hardening or total screw encapsulation with a hardener can extend screw life. Screws can be cored for heat transfer through the screw surface. This may be advantageous because 50% of the surface area in contact with the polymer comes from the screw. Because fitting the shank end with connections for heat transfer fluid or electrical heaters is complex, many screws are “neutral”, which means they use no active heating or cooling.

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2.1.4.2

Barrel

The barrel is a hollow cylinder extending from the end of the feed throat to the tip of the screw. The exit end of the barrel is referred to as the head. The entire inside surface of the barrel is coated by a very hard liner material, such as tungsten-carbide alloy. This lining extends the life of the barrel by reducing wear. Barrel replacement is very costly and time consuming. To minimize wear on the barrel and screw, it is important that the barrel be aligned properly with the gearbox. Today, it is most common to use a laser borescope to accomplish the alignment. Temperature control zones are located along the barrel length. The number of zones depends on the length of the barrel. Each zone usually controls approximately four to five diameters of barrel length (e.g., for a 3-inch screw, about 12 to 15 inches). Cast aluminum heaters are used for many applications, but high temperature systems may require ceramic heaters. It is general practice to employ active cooling in barrel zones. Cooling blowers or heat transfer fluid may be used. However, if an excessive amount of cooling is used in an extrusion operation, it is an indication that energy is being wasted and a change in operation or design may be necessary. Just before the exit end of the barrel and usually located at the six o’clock position (toward the floor) is a hole in the barrel fitted with a device called a rupture disk (Fig. 2.9). This device is an important safety component. If excessive pressure builds up at the head, a weld in the rupture disk will fail, allowing molten polymer to escape through the device onto the floor to relieve the pressure. Since normal operating pressures can approach or exceed 5,000 psi and most barrels are designed for approximately 10,000 psi, rupture disks are usually rated for 7,000 to 9,000 psi. A properly designed screw/barrel system is long-lasting and safely provides the die with excellent melt quality.

Pressure transducer

Barrel Melt thermocouple Rupture disc

Figure 2.9 A typical configuration at the output end of the barrel includes a rupture disc on the bottom side

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Figure 2.10 An extruder with a vent port in the barrel and a two-stage screw

Some extruded materials require vented barrels (Fig. 2.10). The vent port in the barrel allows unwanted gases to escape from the melt before it exits the die, where the gas would cause foaming, voids, and surface defects in the extrudate. A two-stage screw is used in venting operations. The first stage prepares the melt like a conventional screw, while the second stage decompresses the melt for venting and then pumps it through the die. The second stage contains an extraction zone that is used in combination with a vented barrel to accomplish the removal of gases. Under the wrong operating conditions, such as excessive head pressure, polymer can flow out the vent port. This condition is often called vent flow and can be avoided by proper design and operating conditions.

2.1.5

Head/Die System

The head/die system (Fig. 2.11) receives the melt stream as it exits the barrel. Components in this system include the head assembly, adapter, breaker plate, melt filter, and die.

Die

Screen pack

Breaker plate Clamp ring

Figure 2.11 Head and die assembly (Interactive Training Extrusion, Rauwendaal Extrusion Engineering, Inc.)

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2.1.5.1

Head Assembly

The exit end of the barrel is equipped with a flange to which the head assembly is connected. The head assembly may be one of various configurations, including a swing gate or a clamp ring. Swing gates make access to the screw relatively easy by allowing the operator to open the head assembly without many loose parts. Clamp rings, on the other hand, tend to be more cumbersome because they have heavy components that detach from the head. In either case, it is good practice to have all tools, carts, and fixtures on hand before beginning disassembly of the head.

2.1.5.2

Adapter

Held in place by the head assembly is an adapter, which guides the melt stream from the barrel exit into the die entrance. There may be a fixed nozzle in the head, which can be fitted with a custom adapter for each die employed. For the best temperature control, the nozzle and adapter should each have their own heater band. Usually a single temperature control circuit is used for the nozzle/adapter zone, but for a long transfer pipe to the die, multiple circuits may be necessary.

2.1.5.3

Breaker Plate

The breaker plate is a metal disk located perpendicular to the melt flow at the barrel exit. Containing several holes through which the melt passes, it is generally a little larger in diameter than the screw and mounts in a recession in the barrel exit flange. The breaker plate has three main purposes: 1) to seal the end of the barrel, 2) to hold the screen pack, and 3) to straighten out the flow. When the head is assembled, the breaker plate acts as a melt seal in the parting line between the barrel and the head assembly. For this reason, it is important to ensure proper alignment of the breaker plate when it is installed and correct torque on the head bolts during re-assembly and warm-up. Also, the recess in which the breaker plate is held must be cleaned very carefully to avoid any scratches. Any misalignment or damage to the breaker plate or flange recess will lead to polymer leakage. The most common form of melt filtering is the use of screen packs. The screen pack is held in the breaker plate by melt pressure as the polymer exits the barrel. The breaker plate often contains a pocket to hold screens that are nominally the same diameter as the screw.

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As the polymer flows off of the screw, it tends to retain the helical motion that it obtained traveling down the screw channel. This twisting flow could lead to aesthetic as well as structural defects in the extrudate. However, a breaker plate effectively hinders this motion, causing the flow downstream to be predominantly axial. One more use of a breaker plate is as a static mixer. Mixing is necessary to produce the homogeneity required in the melt. Several designs allow the breaker plate to serve its conventional functions while also creating flow dynamics that improve the level of mixing in the melt. These devices can be stacked in a series to create various flow fields for different mixing applications. 2.1.5.4

Melt Filter

Filtering is required to remove contaminants or gels from the melt before they enter the die. Contaminants may enter the system any number of ways, from debris getting into the solid feed to wear of machine components. Gels, also called fisheyes, which look like small bubbles or hard spots in the extrudate, also may originate from various sources. Some sources of gels are high molecular weight species in the polymer, hot spots created along the screw, or moisture captured by the polymer. There are three main types of melt filter: screen packs, discontinuous screen changers, and continuous screen changers. The most common method of melt filtering is the use of screen packs. A screen pack usually consists of three to five screens stacked together onto the breaker plate or other holding device, such as a cartridge. The screens may be of varying mesh size to provide a progressive capture of particles. Wire mesh sizes are generally in the range of 20 to 350, where the mesh number refers to the number of holes per inch. Therefore, as the mesh number increases, the hole size decreases or the particle filtration becomes finer. Most commonly, screens are stacked in the flow direction from coarsest to finest. A coarse screen is often included last in the flow direction for support because a very fine screen with a small wire diameter could potentially be pushed through the holes in the holding device. So, an example screen pack in order of flow direction might be 20–40–100–20. The actual configuration for a given application would depend upon many factors, including the amount of contamination/gels, polymer viscosity, or head pressure. When it is time to change a screen pack, the condition may be indicated by an increase in head pressure, an increase in pressure drop across the breaker plate, a decrease in output, or a defect in the extrudate. Changing a screen pack involves extruder downtime. The extruder must be shut down and the head disassembled. The breaker plate is removed and the old screens are extracted and discarded. At this point, the breaker plate

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should be replaced with a clean one – or cleaned thoroughly – and the new screen pack installed. The head is then reassembled and the head bolts are checked for proper torque as the system components approach operating temperature. To minimize downtime, it is good practice to have all tools and equipment located at the head area before the changeover begins. Screen changers – discontinuous and continuous – are designed to minimize extruder downtime compared to changing screen packs by disassembling the head. A discontinuous screen changer may be manual or automatic. It generally consists of a sliding plate that houses two breaker plate/screen pack assemblies (Fig. 2.12). One assembly is located in the flow stream to provide melt filtering. The other is located outside of the flow stream and is accessible for removal, cleaning, and replacing for later use. When it is time to change screens, the screen changer is activated manually or hydraulically and slides the clean assembly into the flow stream and the used assembly into the cleaning position. Generally, a brief pressure spike will occur in the melt, so it is good practice to perform the changeover during a time when the spike can be tolerated.

Figure 2.12 Hydraulic sliding plate screen changer, shown with the guards removed (Dynisco Extrusion)

2.1 Extruder Hardware Systems

Cool

Heat

Cool

Hydraulic puller

Heat

Heat

Melt flow

Cool

Heat

Sealing bar

Cool

Filter screen

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Figure 2.13 Moving web-type continuous screen changer (Key Filters, Inc.)

The continuous screen changer (Fig. 2.13) provides a method of melt filtering where the filter medium is continually being refreshed. These devices are particularly useful when the melt stream is highly contaminated or when a changeover pressure spike cannot be tolerated. There are two main types of continuous screen changers. One employs a large rotating disk containing filter material, and the other pulls a web of filter material through the melt stream. In either case, the speed at which the filter moves can be controlled by the extruder head pressure, which depends on the level of contamination trapped in the filter. 2.1.5.5

Die

The die has been called the “brains” of the operation because the product’s final shape is most determined by the melt forming that occurs in the die. Many die types are available to produce various extruded products to specific geometric and performance requirements. One thing that all dies have in common is the need for properly designed internal flow geometry, including exit orifice geometry. If the die does not possess the correct flow shape, it may not be possible to produce the polymeric product to specification. Because the shape and surface properties of the flow channels are critical, it is very important that they are not damaged. Scratches or dents in flow channel surfaces can lead to polymer hang-up and degradation or, even worse, cause permanent marring of the extruded product surface. For this reason, special care should be taken when disassembling and cleaning dies, and only soft metals (such as copper and brass) should be used on or near flow surfaces.

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For optimal processing, dies should have their own temperature control circuits. One heater zone is usually adequate for small dies. Larger dies may have a few to several heating zones. Additionally, insulation is often used around the die body to reduce heat loss to the surrounding air and sensitivity of the die temperature to changes in ambient conditions. While die heating control is recommended, active die cooling is generally not necessary.

2.1.6

Instrumentation and Control System

The purpose of the instrumentation and control system (Fig. 2.14) is to measure and control important processing parameters. Without the data provided by this system, it would be very difficult to maintain a safe and efficient process and to troubleshoot extrusion problems. The data provides a “window” to the process, since we cannot see inside the extruder (and probably would not find it very useful if we could). Monitoring an extrusion line has been likened to monitoring a patient in a hospital; we certainly want to measure the patient’s vital signs, such as body temperature and blood pressure. Similarly, we need to monitor extrusion parameters, so the system does not become unstable, leading to a dangerous situation or to the production of costly scrap. There are many variables that can be measured and controlled on an extrusion line, depending on the product being extruded. Upstream of the extruder, some of the variables include hopper fill level, raw material temperature, and raw material moisture content. Downstream of the extruder, variables include line speed (e.g., belt puller, chill

Figure 2.14 A typical extruder control panel

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rolls, or nip rollers), cooling medium temperature, cut length, and various product dimensions (e.g., diameter and thickness). On the extruder itself, variables include screw speed, heater band current, and throat cooling, but the three most important parameters to measure are temperature, head pressure, and motor current. 2.1.6.1

Temperature Control

Today’s extruder temperature control systems are capable of maintaining temperature within a range of ±1 °F. Precision of this level is acceptable for practically all extrusion applications; however, when variation exceeds just a few degrees, variations in product output can result. Therefore, it is important that accurate temperature control circuits are employed. It is common for an extrusion line to be separated into several temperature control zones. The number of zones depends on the length of the barrel, the type of adapter or transfer line to the die, and the size and complexity of the die. An extruder may have as few as three or well over ten zones. Each zone, or circuit, contains up to four of the following components: temperature controller, temperature sensor, heating unit, and cooling unit. Usually only barrel temperature zones utilize a cooling unit. Most temperature controllers are microprocessor based. The primary functions of the controller are to read the input signal (actual temperature) from the temperature sensor, compare this temperature to the set point provided by the operator, make a decision regarding what action is required, and take the appropriate action. Actions that the controller may take include turning on (or off) the heating unit, turning on (or off) the cooling unit, or doing nothing. The controller repeats these functions continually. Simulator Exercise: The temperature controllers have an on/off switch and allow set-point adjustment between 0 and 500 °F. Heating and cooling time is compressed for user-friendliness. To make its action decision, most temperature controllers employ a logic scheme known as PID control. PID stands for the combination of proportional, integral, and derivative control. Proportional control gives the controller the ability to send partial power to the heating or cooling unit. Instead of being only fully on or off, the heating/cooling unit can be on at some level proportional to the difference between the actual temperature and the set-point temperature. This allows the system to reach the set point without much overshoot and to maintain temperature during operation without significant variation about the set point. Some controllers truly send a proportional amount of

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power to the unit while others send timed bursts of full power to achieve the proportional effect. Integral control monitors actual temperature as a function of time. If this varies because of an upset in the system, such as the addition of new hardware or a change in ambient temperature, the controller can reset for the new thermal conditions. Derivative control monitors the rate at which actual temperature changes. Monitoring the rate allows the controller to approach set point rapidly or control the approach to avoid any overshoot. Another component in the circuit is a temperature sensor. The temperature sensor measures the actual temperature in the zone, converting it to a signal that is sent to the controller. The sensor may be in contact with metal or polymer. There are various types of sensors, including thermistors and infrared detectors, but the most common type is the thermocouple. Thermocouples (Fig. 2.15) generally have a two-prong connector at one end: a long flexible metal cable housing the leads and a metal probe at the other end protecting the actual couple junction. The junction is formed from two dissimilar metal wires. When the temperature at the junction differs from a reference temperature maintained electronically in the controller, a small voltage is naturally generated across the leads. This voltage varies linearly with temperature and so is easily calibrated to actual temperature. Thermocouples are categorized by type (e.g., Type J) to indicate the two metals that comprise the junction, determining the useful temperature range of the sensor.

Heating unit Thermocouple

Barrel Temperature controller

Cooling fan

Figure 2.15 A thermocouple is shown as part of a temperature control circuit

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Thermocouples are primarily used to measure the metal temperature in a particular control zone. Usually a well is drilled into the metal component to locate the couple junction as close as possible to the interior surface (where polymer is flowing). When installing a thermocouple, it is good practice to make sure the probe tip is in contact with the base of the well before securing the connection. Melt thermocouples are also used. These devices are made to contact the actual flowing polymer. This information can be the most useful temperature data from an extruder because it is the polymer temperature that is most important, not the metal temperature. One common type is a flush-mounted probe that has a sensor diaphragm mounted along the inside barrel surface, usually at the head end of the barrel. This type only reads the temperature of the melt in contact with the barrel, usually the same temperature as the barrel itself. Because the temperature of the melt away from the barrel may differ by over 30 °F from the surface temperature, some extruders have a more useful immersion thermocouple that penetrates further into the melt stream. This type can be designed to traverse to various depths. The next component in the circuit is a heating unit. When the heating unit is on, it conducts heat through the metal hardware to the polymer. The most common type of heating unit is a heater band that connects onto the outside surface of the hardware. Some dies utilize heater cartridges inserted inside a hole in the hardware. Heating units are rated by power, where the amount required is determined by the mass of metal to which the unit is attached. Heating units are somewhat susceptible to failure. If a unit is bent, twisted, or dropped, the element inside the unit can be damaged and an open circuit will result in failure. Plastic buildup on a heater band can also lead to a failure. Finally, if the unit is on and there is an air gap between the heater and the metal surface, a hot spot can occur on the heater and it will fail. A heat transfer paste can sometimes be used to ensure good conduction to the metal. If a heating element is suspected to have failed, it can be easily checked offline with a continuity tester. A short or open circuit means that the heating unit needs to be replaced. The final component in the system is a cooling unit. Usually only barrel temperature zones have cooling units. The purpose of a cooling unit is to remove heat from the zone when the controller has determined that the temperature is too high. On most systems, blowers provide adequate heat removal. For higher heat removal rates, the extruder can be plumbed for a heat transfer fluid. Heat transfer oil must be used at temperatures that exceed safe levels for the use of water. It is good practice to monitor the amount of cooling needed by the extruder. When excessive cooling is required, this is an indication that more heat is being input to the polymer than necessary and the excessive heat will

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need to be removed. This wasted energy may indicate the need for a change in operating conditions or a design modification, e.g., a redesigned screw. 2.1.6.2

Head Pressure

While the measurement of extruder head pressure is very important for product quality purposes, it is also perhaps the most important measurement from a safety standpoint. Excessive pressure can cause rupture of the barrel, damage to head and die components, and injury to personnel from projected hardware and hot polymer. Therefore, accurate pressure readings, high-pressure motor shut-off capability, and properly rated rupture disks are essential for safe operation. Extruder pressure is generally not directly controlled, as is temperature, for example. While some extruders do have a valve at the output end, most extrusion systems control pressure indirectly through three primary variables: screw rotational speed, polymer melt viscosity, and die flow geometry. An increase in screw speed results in a higher flow rate of polymer through the die and a higher head pressure. For polymers, the rate of increase of pressure is somewhat less than the rate of increase of screw speed, since shear thinning (reduction of viscosity) occurs. Therefore, if screw speed is doubled, head pressure fortunately does not double! When melt viscosity is increased, head pressure also increases. Viscosity depends on the polymer temperature, so temperature can be used to control head pressure through the melt viscosity. When die flow geometry is more restrictive, head pressure increases. Although significant changes to die geometry cannot be made during extrusion, the design of a die must consider the pressure that will result when a given polymer is extruded at a given rate.

•

Simulator Exercise: The head pressure display will show the changing value of pressure with adjustments to the screw speed and/or melt temperature. In the simulator the melt temperature is linked directly to Barrel Zone 3 temperature. However, at high screw speeds shear heating will raise the melt temperature even further. The most common location to measure pressure is at the head of the extruder, just upstream of the breaker plate. The signal from a single pressure transducer located here is usually sent to a display that has alarm capability for lighting a warning indicator or shutting down the motor. Some extruders will also have pressure measurement just downstream of the breaker plate. Pressure readings on each side of the screen pack provide a good indication of when to change screens. Occasionally, extruders will use pressure sensors along the barrel or in the die. This data is most useful for research and development, quality control, and troubleshooting.

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Strain gage pressure transducers are used for most extrusion applications. These devices have a probe tip that is mounted so that its diaphragm is flush with the internal surface of the barrel or die. The polymer melt flowing under pressure pushes against the diaphragm and displaces a fluid column or a rod located behind the diaphragm. The fluid or push rod transmits the pressure (actually the displacement) from the diaphragm in contact with the melt to a second diaphragm located away from the melt that has a strain gage bonded to it. The strain gage is a resistor in a circuit known as a Wheatstone Bridge. Under zero pressure, the bridge produces zero voltage. However, when the strain gage is deflected, its resistance varies and a voltage is produced across the bridge. This voltage is easily calibrated to pressure. It is good practice to remove the pressure transducer when cleaning the extruder barrel and to avoid peeling polymer off of the probe diaphragm. The diaphragm is somewhat fragile and must not be damaged. If it is damaged, the sensor will not give accurate data and, in the case of a fluid type, could leak fluid into the melt stream. It is important to realize that many sensors use mercury, a potentially hazardous substance, as the fluid in the capillary. 2.1.6.3

Motor Current

An important, and often underutilized, extruder measurement is the motor current. This is a measure of the electrical load required to turn the motor that drives the screw, displayed in either amps or percent of maximum load. At a specified rotational speed, current depends primarily on the mechanical resistance exerted by the polymer around the screw. Since the required current depends on the resistance provided by the polymer, it is an indication of the energy, or torque, being transmitted through the screw to the polymer. Further, it provides information on certain inherent properties of the polymer and on the stability of the processing conditions. The energy indicated by motor current is consumed in three primary areas: melting the solids, conveying the high viscosity melt, and generating pressure to overcome the head/die restriction. The third of these areas consumes much less energy than the first two. Changes in motor current are indicative of changes in the raw material, the processing conditions, or both. For example, if the ammeter is stable, but reading particularly high, this could be evidence of an increase in melt viscosity. If the ammeter is changing frequently, this could be associated with solid bed breakup causing an unstable melting process. Simulator Exercise: The ammeter will show changes in motor current when adjustments are made to the mechanical energy input (screw speed) or the polymer viscosity (melt temperature).

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2.2

Extrusion Functional Zones

This section describes the six zones along the extruder in which various system functions are performed. Understanding the functional zones combines knowledge of polymeric materials with knowledge of extrusion hardware. As polymer moves through these functional zones in its various forms (e.g., pellets, solid bed, or melt), it interacts with hardware components (e.g., hopper, screw, or barrel) in a series of unit operations (e.g., melting, mixing, pumping) that contribute to the final extruded product. If any one of these functions is performed improperly, quality of the final product or efficiency of the extrusion process may suffer. Put another way, to optimize extruder throughput, each functional zone must perform optimally. The six functional zones are (Fig. 2.16): • Solids conveying • Melting • Melt pumping • Mixing • Degassing (or devolatilization) • Die forming

Die forming

Melt pumping Mixing

Solids conveying

Melting

Figure 2.16 The six functional zones in an extruder

Devolatilization

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43

These are not to be confused with the geometric extruder sections, including the feed section, transition section, and metering section. Although there is some correlation between functionality and geometric section, the functions are not constrained by the geometry. For example, melting is not constrained to the transition section only; it may begin in the feed section and extend into the metering section. Further, not all extruders perform all six functions. Some extruders are fed melted polymer and, so, do not need to perform solids conveying or melting, and many extruders perform no degassing. However, most extruders perform the majority of these functions and many perform all six. The following sections detail each of the six functional zones.

2.2.1

Solids Conveying

Solids conveying describes the mechanisms for transporting solid feed material. In most extrusion operations, this takes the form of pellets dropping down through a hopper onto the back of a screw and moving along the screw until melting begins. Other solid forms, such as powder, flake, and fluff, are also used. Notice that solids conveying can be divided into two regions: the gravity-induced region where solids drop through the hopper and the drag-induced region where they move along the screw. For information on solids transport occurring prior to the hopper, refer to books on material handling systems, such as Feeding Technology for Plastics Processing by Wilson [8]. 2.2.1.2

Gravity-Induced Region

In most cases, single screw extruders are flood fed. The hopper is full of pellets that are drawn down toward the base of the hopper via gravity as the screw takes pellets away from the base. With respect to the efficiency of feeding in this region, the primary concern is the ability of the solids to flow freely. Several variables affect this flow. Bulk density is the measure of the density of the feed material, including the air between particles. The bulk density value can provide one measure of the solid’s ability to flow freely. Generally, materials with higher bulk density flow more freely than those with low bulk density. Pellets resembling ball bearings that flow easily may have a bulk density of about 0.7 g/cm3. Very fluffy regrind may have a bulk density as low as 0.3 g/cm3 and is more difficult to feed, perhaps requiring special hardware such as a crammer feeder. A simple test of bulk density is the hand squeeze test. After squeezing a handful of feed material and then opening the fingers, material that feeds well will probably spill out easily. However, material that is difficult to feed will often stay clumped together.

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Another condition that promotes good hopper flow is low friction. When friction between the pellets (internal friction) is low and friction between the pellets and the hopper (external friction) is low, solids are more likely to flow freely. Factors that may increase internal and external friction include rough surfaces, poor hopper design, moisture, heat, and static electricity. Therefore, the degree to which solids flow freely depends on both the material and the hopper design. Under proper conditions, solids move in plug (mass) flow, where the first in the top are the first out the bottom, and no mixing takes place. Problems, however, can occur with poorly flowing materials. Bridging (arching) is when a natural bridge forms at the base of the hopper, blocking further flow. The bridge can often be broken up with a plastic or wooden rod, but it is important to remove the cause of the problem at the same time. A common cause of bridging is too much heat in the feed throat. Another problem is funnel flow (“rat-holing”). This occurs when solids do not move near the hopper walls, but exit down through the center of the hopper. Not only can this lead to flow stoppage, but it can also result in segregation (de-mixing) of different materials in the hopper, such as pellets and regrind. Funnel flow may be caused by a large cone angle at the base of the hopper. Various techniques are used to promote good hopper flow with poorly flowing materials. A vibrating pad may be used at the base of the hopper to inhibit bridging. An auger inside the hopper, called a crammer feeder, is used with very fluffy regrind. Finally, tapered barrels have been used to accept low bulk density solids. 2.2.1.3

Drag-Induced Region

After solids have dropped through the base of the hopper, they enter the back of the screw in the feed throat. The solids must not begin melting at this point or no further solids would be able to enter. Therefore, the rotation of the screw must drag the solids out from under the hopper so that more solids can drop down. This mechanism is called drag-induced solids conveying. It seems logical that because screw rotation causes drag-induced flow, a condition for good solids conveyance in this region would be high interaction (friction) between the solids and the screw. But, this is a case for counter intuition! In fact, when there is high friction between the solids and screw, the solids stick to the screw and simply rotate around and around, never moving toward the die. The optimum condition for drag-induced solids conveying is low friction between the pellets and the screw, and high friction between the pellets and the barrel. This condition

2.2 Extrusion Functional Zones

45

is often called “slip on screw, drag on barrel”. Under this condition, the barrel acts to inhibit the pellets from rotating around with the screw, while the progressing screw flight pushes them down the barrel as they slide along the screw root. This mechanism has been compared to a nut on a threaded rod. If the rod spins with no external force applied to the nut, the nut will rotate, but not move forward. Once an external (retarding) force is applied to the nut on the rotating rod, the nut begins to move forward. Therefore, drag-induced solids conveying can be optimized by creating a condition of high friction between the pellets and the barrel. There are several techniques for accomplishing this. One of the simplest ways is to determine the barrel feed zone temperature that promotes the highest throughput. When the feed zone temperature is increased, the pellets in contact with the barrel get stickier, improving friction. However, if the temperature is too high, the pellets can begin to melt and very little friction is obtained. Another condition resulting in higher overall friction is high bulk density. Finally, using a grooved feed throat is an extremely effective way to increase pellet/barrel friction. In addition to increasing pellet/barrel friction, optimization includes decreasing pellet/ screw friction. Common techniques for accomplishing this include highly polishing the screw, specialty plating such as nickel and chrome, and decreasing screw temperature if screw cooling is available. Of course, a clean screw is also necessary for optimum output. The highest solids conveying rate occurs when the difference between the pellet/barrel friction and the pellet/screw friction is maximized.

2.2.2

Melting

Melting is defined here as the change of polymer from a solid to a liquid, with this definition holding for both amorphous and semi-crystalline polymers. This function generally begins about three to five turns down the length of the screw. There are two energy sources that input heat to melt the polymer: conductive heat from the external barrel heaters and mechanical heat from the friction generated during screw rotation. Although the barrel heaters are more observable, it is actually the frictional heat generation that supplies the majority of heat to melt the polymer. On a large extruder over 80% of the heat may come from screw rotation. This is a benefit regarding total energy usage because the screw has to rotate to transport the material anyway. The heater bands have two more significant roles besides providing heat to melt the polymer. First, they supply the initial energy to heat up the hardware during start-up. An extrusion screw cannot be turned until the extruder is up to operating temperature.

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Second, the heater bands assist in temperature control during operation. Because power to the heater bands is controlled by a microprocessor, the bands provide a much more accurate level of control than frictional energy. There are two reasons why frictional energy provides more melting heat than barrel heaters. As described below, melting occurs near the barrel, creating a melted layer between the barrel surface and the solids. Therefore, energy from heater bands has to transfer through both the steel barrel wall and a layer of melted, insulating plastic. This is an inefficient way to get heat into the solids. Also, the friction from the rotating screw is concentrated at the interface of the solids and the liquid, imparting maximum energy right where melting takes place. To understand how melting occurs, remember that the screw rotates in the direction to move polymer forward, which would move the screw itself backward, if it could. Now, consider for this discussion that the screw stays fixed and the barrel rotates around it, in the direction to cause polymer to move forward (Fig. 2.17). This aids in understanding because it is the dragging action of the barrel that causes melting. (The polymer doesn’t know the difference between screw or barrel rotation.) As the barrel drags over the solids, which have packed together into a solid bed, friction is generated creating a thin melt film between the solid bed and the barrel wall (Fig. 2.18). The barrel drags the melt film toward the back of the channel since the flight has a helix angle. The melt is then deposited into a melt pool located at the back (pushing side) of the channel. As this process proceeds along the screw, the solid bed is squeezed toward the front of the channel by the growing melt pool and toward the barrel by the growing screw root diameter. These squeezing actions maintain good friction at the interface of the melt film and the solid bed, eventually melting all the solids. Understanding this mechanism is important for screw design and process troubleshooting. For example, screw compression ratio and transition length should be designed to closely match the melting mechanism. Further, when pressure surging occurs at the extruder head, it is often related to a breakup of the solid bed during the melting process.

Output Figure 2.17 With a stationary screw, the barrel would rotate in the direction shown to drag material in the output direction

2.2 Extrusion Functional Zones

47

Figure 2.18 Melting mechanism showing melt film at the barrel wall and melt pool against the pushing side of the flight

Improving the melting rate can lead to improved throughput. This is accomplished by increasing the rate of heat input to the solid bed, either by friction or conduction. Frictional heat input is increased by raising the screw speed or reducing the flight clearance. Raising the screw speed must, of course, be accommodated by all other extruder functions. Reducing the flight clearance keeps the melt film thin and the shear to the solid bed high. Notice the implication that if the clearance is large (a worn screw flight), then the melting rate – throughput – is reduced. Increasing conductive heat with the heater bands may also improve the melting rate. More heat to the solids will help them melt faster. Under some conditions, however, it is possible that the overall melting rate actually drops with more conductive heat. This is due to the reduction in viscosity of the melt film, which leads to less frictional heat generated.

2.2.3

Melt Pumping

Near the end of the screw transition section, melting should be complete and the melt pumping function takes place. The purpose of this function is to convey melt toward the end of the extruder and generate pressure to overcome the flow resistance of the head and die. Melt moves through the metering section of a screw by combined drag and pressure flows (Fig. 2.19). The friction between the barrel and the melt hinders the movement of polymer as the screw rotates, effectively dragging it forward in the channel toward the die. At the same time, pressure in the melt pushes the polymer backward in the channel. This pressure results from the restriction to downstream flow through the breaker plate, screen(s), adapter, and die. Therefore, opposing mechanisms act on the melt: drag forward and pressure backward. Of course, on average the drag forward predominates so that polymer exits the die, but the local motion at the bottom of the

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2 Extrusion Overview

channel (root of the screw) may actually be backward, depending on the amount of pressure. (This describes the typical situation; however, in cases such as a grooved feed throat, pressure flow as well as drag flow may be forward.)

z-direction

Barrel wall Pressure flow

Drag flow

Screw root z-direction

Output Figure 2.19 Combined flow of drag downstream and pressure upstream

This combination of flow mechanisms leads to a spiraling journey that any fluid element travels on its way through the metering section. For a case of relatively high pressure, a fluid element might take the following path (Fig. 2.20): travel downstream along the top of the channel, encounter the pushing side of the flight and move to the bottom of the channel, move somewhat upstream while crossing the bottom of the channel, encounter the trailing side of the flight and move to the top of the channel again. Higher pressures lead to further upstream travel while at the bottom of the channel.

Figure 2.20 Combination of drag and pressure flow leads to a spiraling journey down the channel for fluid elements

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49

Upstream travel has some significant implications. In general, more upstream travel (higher pressures) means that the polymer experiences a greater residence time in the extruder. On the positive side, this may lead to improved melting because of the greater heat input, or it could result in better overall mixing. On the negative side, melt temperature and polymer degradation are increased, and throughput is reduced. Total output from the extruder is equal to the difference between the volume of melt dragged forward and the volume of melt pushed backward. Additionally, there is a volume of melt that can leak over the flight tip, reducing throughput. This is called leakage flow and generally amounts to less than 1% of total flow. However, for worn screws (clearance > 0.003 D) leakage flow can be significant and should not be neglected.

2.2.4

Mixing

Mixing is considered any spatial rearrangement or change in size of elements within the fluid. Many extruders are equipped with special mixing hardware, such as integral mixing elements machined onto the screw or static mixers located after the barrel. These are considered specific mixing functional zones to perform mixing of two or more material components. Some mixing is even performed by extruders that use a standard metering screw for one polymer only, such as the rearrangement of fluid elements with different temperatures. The study of mixing can be a rich and complex subject, with topics ranging from dry blending different pellet types to investigating microscopic level domain sizes. Several books have been written on the subject, for example Polymer Mixing: A Self Study Guide by Rauwendaal [9]. However, a useful grasp of this topic can be gained quickly through a general understanding of the two fundamental mixing types: distributive mixing and dispersive mixing. 2.2.4.1

Distributive Mixing

Distributive mixing is the process of spatially randomizing the elements of one material within another (Fig. 2.21). For example, consider the process of making a scrambled egg. The contents of a broken eggshell are poured into a bowl. At first, the yolk is concentrated in one section of the bowl, while the egg white is spread across the bottom. With a fork, the mixture is beaten until the yolk is thoroughly distributed within the egg white. The distributive mixing of one polymer melt within another or the addition of short glass fibers into a polymer melt is similar to scrambling an egg.

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Figure 2.21 Distributive mixing spatially rearranges the minor component

The scrambled egg analogy is also useful to identify the important parameters in a distributive mixing process. The egg white represents the primary material, or matrix, into which the yolk (additive, minor component) is distributed. The beating represents energy input into the system and the tines of the fork establish a desirable flow field. (Consider that a spoon would not be as effective as a fork.) This desired flow field is one with many flow direction changes and is called “reoriented flow” or “splitting and recombining”. Hardware elements that provide good distributive mixing generally produce many flow direction changes. Another important variable to consider for good distributive mixing is the viscosity of the matrix. Low matrix viscosity is most efficient for distributing a minor component. If the minor component is a fluid, then close matching of the viscosities of the matrix and the minor component is optimal. 2.2.4.2

Dispersive Mixing

Dispersive mixing is the process of reducing the size of a component within a polymer melt (Fig. 2.22). Consider the addition of color pigment or calcium carbonate to a polymer matrix. These additives are in particulate form and need to be reduced in size (broken up) before they can be adequately distributed throughout the matrix. Dispersive mixing breaks them up.

Figure 2.22 Dispersive mixing reduces the size of the minor component particles

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There are several important parameters to consider for effective dispersive mixing. The individual components, or agglomerates, to be broken up are composed of smaller cohesive particles. For good dispersive mixing, the force applied by the matrix to break the agglomerate must exceed the cohesive force holding the particles together. This is best accomplished with higher matrix viscosity and mixing hardware that promotes breakup flow. Breakup flow transfers a great deal of energy from the matrix to the agglomerate. Both shear flow and elongational flow are used to transfer energy for breakup. Shear flow is easily produced within an extruder and many dispersive mixing elements have been designed to expose the mixture to regions of high shear. However, it has been known for a long time that elongational flow is about ten times more effective at dispersion than shear flow. Recently, dispersive mixers have been designed to take advantage of this effect. By including converging flow channels that stretch the mixture as it flows through, new dispersive mixers have shown great potential for improved dispersion. Dispersive mixing can consume significantly more power than distributive mixing, depending on the types of mixing elements used. This power consumption should be considered in overall energy management and may contribute to a rise in melt temperature, hence material degradation. Sometimes pulling the screw and checking a mixing element for degraded polymer can help determine if this is a potential problem.

2.2.4.3

Mixing Devices

Mixing devices are generally divided into two main categories: static mixers and mixing elements (also known as dynamic mixers). The use of mixing elements on the screw is far more popular than the use of static mixers. Static mixers are distributive mixing devices (Fig. 2.23). They are located after the screw, between the end of the barrel and the entrance to the die. They do not rotate, so they consume pressure from the melt. However, static mixers do provide good final homogenization of the mixture and reduce temperature variations. Mixing elements are integrated into the screw design (Fig. 2.24). They are often located two to three turns from the end of the screw to leave some pumping capability at the end of the screw. However, a mixing element with inherently good pumping capability is best located at the very end of the screw. There are a great variety of mixer types, but these are usually categorized as either distributive or dispersive. Dispersive mixers always provide some level of distribution as well.

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ow

Fl

Mixing elements

Figure 2.23 Static mixer shown with twisted ribbon mixing element (Dynisco Extrusion)

Figure 2.24 Mixing element machined onto the screw

Because of the large number of mixer types, it is good to understand the important characteristics of these devices. First, it must be determined if the mixer produces the type and degree of mixing required. Factors to evaluate here include distributive vs. dispersive and length of mixing section. Second, cost must be considered. A mixing device adds to the cost of a screw, but in properly applied cases this cost is returned very rapidly in material savings. A third consideration is power consumption. It is important to understand if the mixer imposes a large pressure drop or temperature rise in the melt. Finally, the mixer should be evaluated for any locations where material can hang up to degrade and for its ease of cleaning.

2.2 Extrusion Functional Zones

2.2.5

53

Degassing

Degassing (also called devolatilization) is the process of removing unwanted gases from the polymer melt. This is accomplished by removing the gas through a vent in the extruder located upstream of the die. Although the majority of single screw extruders do not employ venting, many extruders do. Gases need to be removed prior to the die when they produce defects in the extrudate. The most common defect caused by gas is foaming. Of course, foamed extrusions are often produced by design, but when foaming is unwanted, the gas must be removed before the die. Defects also take the form of bubbles and surface roughness. Unwanted gas may be present in the polymer due to a variety of reasons. Moisture that has turned into steam is a common cause. Also, some material components release gas as they degrade at relatively high temperatures. Finally, the gas may be the by-product of a chemical reaction designed into the system to provide a particular product property. A special screw and barrel combination is required to perform degassing so that only gas is removed from the system, not polymer. A typical system employs a two-stage screw and a vented barrel that may have a length-to-diameter ratio of 30 or greater (Fig. 2.25). The two-stage screw adds a deep-channeled extraction section after the normal screw sections (feed/transition/metering). The purpose of the extraction section is to decompress the melt at the vent location. Then the gas can be vented to atmosphere or pulled off with a vacuum without the presence of pressure to push polymer through the vent. The unfilled channel in this section also allows exposure of more surface area to aid in gas removal. After the extraction section, a short compression and pumping section are included.

Figure 2.25 Two-stage screw and vented barrel used for degassing

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2 Extrusion Overview

The screw must be designed to prevent polymer from escaping through the vent, often called vent flow. This is normally accomplished by having a greater pumping capability downstream of the vent than upstream. This condition ensures that the polymer is removed from the extraction section before it can build up pressure. However, proper operation often depends on factors such as polymer viscosity, head pressure, and screw speed. It is not unusual for vent flow to occur under off-design conditions, such as start-up.

2.2.6

Die Forming

The purpose of the die is to form the extrudate into a shape that is close to its final shape. There are many types of dies and a vast number of variations within each type. In this book, the different types of blown film dies will be covered in a later section. This section discusses three common characteristics that may occur when polymer flows through any type of die: die swell, melt fracture, and die drool. Die swell is a natural phenomenon where polymer expands as it exits a die. In other words, if polyethylene rod is extruded through a 1-inch diameter orifice, the extrudate will measure greater than 1 inch immediately after the die exit (Fig. 2.26). The effect is also called extrudate swell since it is actually the extrudate that changes size, not the die. The cause of die swell is not intuitively obvious. It is easy to conceive that because the polymer is under pressure inside the die, it should swell when the pressure is removed. However, this is not the case or the same would hold true for water, which, in fact, does not swell. The cause is actually due to the relaxation of oriented molecules (Fig. 2.27).

Figure 2.26 Die swell causes the extrudate to expand upon exiting the die

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Figure 2.27 Die swell occurs when shear-oriented molecules relax outside of the die

In other words, as long chain molecules travel through the flow field inside a die, they become aligned in the direction of flow (orient). When the orienting effect of the die walls is removed upon die exit, the molecules relax back toward random coils, increasing the cross sectional area of the melt stream. Therefore, since different types of polymers relax at different rates, the degree of die swell varies depending on polymer type. The degree of die swell can also be affected by downstream line speed and by factors affecting the relaxation time and rate, such as die land length and melt temperature. Die swell is an expected, natural occurrence and is accounted for in die design. It presents little problem in symmetrical shapes such as rods and tubes because the line speed can be adjusted to provide specified product thickness. However, for complex profiles such as window lineals, the varying geometry results in varying degrees of swell across the profile. Experience in die design is vital to minimize development time. Melt fracture is generally defined as surface roughness on the extrudate, but may include distortion of the entire body of the extrudate (Fig. 2.28). It is most often observed in high-pressure extrusions, like blown film. The appearances of various forms of melt fracture have led to names such as shark skin, orange peel, bamboo, and ripple. Unlike die swell, which is expected and accounted for in the die design, melt fracture should be avoided entirely. This defect is the result of incorrect die design, improper processing conditions, and/or poorly matched material properties. The root cause of melt fracture has been the subject of numerous studies, resulting in a few theories. It is clear, however, that excessive shear stress in the polymer as it passes through the die is one source of the defect. A cause of high shear stress is abrupt changes in flow streamlines. If the internal flow geometry of a die contains sharp corners, this could lead to melt fracture. More often, excessive shear stress is due to high polymer viscosity or high polymer flow rate. Therefore, action that reduces viscosity or flow rate

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Figure 2.28 Melt fracture in the form of surface roughness

may cure a melt fracture problem. Possible ways to reduce viscosity or flow rate include higher die temperature, wider die gap, and lower screw speed. Also, lubricants have been used with some success by reducing the friction between the polymer and die walls. Die drool is the name given to the accumulation of plastics material on the die face during an extrusion run (Fig. 2.29). Depending on the material, this buildup can occur very quickly, needing to be removed hourly, or it may be imperceptible over the course of a run. It is a potential problem for a few reasons: the drool may break loose from the die face and stick to the extrudate, resulting in out-of-specification product.; the buildup may score or scratch the surface of the product as it rubs along the extrudate; and finally, accumulated material on the hot die face degrades, leading to downtime while the die is cleaned.

Figure 2.29 Die drool accumulating at the stripe positions

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57

The development of techniques for removing die drool during a run has recently received some attention. The most common method is to manually scrape the drool loose with a brass tool. However, methods requiring less intervention include directing compressed air or a gas flame at the site of the drool. The cause of die drool is still under investigation. Early studies point to the surface migration of low molecular weight species in the plastics (such as additives). These species stick to the metal surface and accumulate. Therefore, solutions have focused on changing material composition or reducing the friction between the plastic material and the die wall. The latter approach utilizes internal lubricants, Teflon-coated die walls, or flared die lips.

3

Hardware for Blown Film

This chapter covers hardware specific to blown film extrusion. It is organized by the order in which material flows through the extrusion line, beginning upstream of the extruder and proceeding through windup. The major sections in this chapter are: • Upstream components • Grooved feed throat • Screws for blown film extrusion • Blown film dies • Bubble geometry • Bubble cooling • Bubble stabilization • Collapsing frames • Haul-off • Winders • Film treatment • Line control Simulator Exercise: When the cursor is paused over any hardware component, an identification tag shows its name. Maximum extrusion throughput is often the chief priority in the design of blown film system components. Another important objective in design is minimal film dimensional variation. High production volumes coupled with low profit margins means that saving a few pennies per hundred pounds of polymer may return big gains in the long run. Therefore, to achieve the most cost effective operation, all personnel responsible for blown film processing should have an understanding of each of the components in the process, the interplay between the components, and the final film properties.

•

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3.1

Upstream Components

Upstream components are those associated with feeding solid material into the extruder. Solid material enters the extruder in many forms. Pellets, powder, flake, and fluff are all used, but pellets and fluff are the two most common forms for blown film. Pellet feed comes from three primary sources: resin suppliers, compounders, or film reclaim/pelletizing (reprocessing) systems. Pellets obtained from a resin supplier are generally first-generation (also known as virgin) polymer that has only been through the single heat history resulting from initial pelletization. As a result, the variation in polymer properties as well as the variation in pellet size and shape is minimal. Consistent pellet geometry is beneficial for consistent feeding and film properties. Another consideration of pellets from a resin supplier is the inclusion of an additive for processing stabilization, generally an antioxidant. Although this additive is often included in first-generation pellets, the percentage is usually low and provides limited effectiveness when a significant amount of reprocessed material is blended into the mix. Of course, pellets from a resin supplier are generally more expensive than reprocessed pellets. Compounded pellets are those that have been through an intermediate mixing extrusion to improve the properties of the first-generation polymer with additives such as colorants, stabilizers, and fillers. The compounding step gives these pellets an additional heat history, which should be considered as a potential effect on polymer degradation. Film reclaim systems take scrap from the production process and feed it back into the main extruder. A film reclaim/pelletizing system may provide an economic benefit in operations where a large amount of scrap is generated, such as with handle cutouts from a bagmaking machine or when edge trimming is necessary. In a typical operation, this scrap is transported to another part of the plant where it is cram-fed into a chopper and then an extruder that performs remelt, mixing, and pelletization. The disadvantages with this technique are that this polymer has now seen at least two heat histories, leading to some property degradation, and the pellet geometry variation is higher than with firstgeneration pellets. To minimize the effect of these disadvantages, reprocessed pellets may include some additional antioxidant and are generally blended into the production stream as a minor component with first-generation pellets. Feeding reprocessed pellets with first-generation pellets can possibly lead to a detrimental segregation (demixing) or a reduction in the feed rate through the hopper due to the differences in flow properties of the two pellet types. Conditions that can be tracked in

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61

an effort to minimize these effects are the distribution of particle size and shape, the differences in frictional properties, and the composition ratio of the mixture. When film is prepared for feeding into an extruder simply by grinding instead of remelting/pelletizing, the resulting feed is a very low bulk density product known as fluff. As would be expected, this form of polymer does not feed readily into an extruder hopper simply by gravity. Therefore, a device commonly known as a crammer feeder (Fig. 3.1) is used, generally feeding some ratio of fluff with pellets. In some designs the feeder is on a closed-loop circuit with extruder head pressure measurement to control the feeder rate to maintain constant head pressure. The low bulk density of fluff limits the amount that can be incorporated into the mixture to about 10 to 15%.

Figure 3.1 A crammer feeder for pellets and fluff (Foremost Machine Builders, Inc.)

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3.2

Grooved Feed Throat

Located beneath the hopper is the feed throat section of the extruder barrel (Fig. 3.2). This component is on the order of four barrel diameters in length and is attached between the gear box and the barrel. Though the screw passes through this section, it is distinguished from the barrel proper because it is generally cooled instead of heated. Cooling the feed throat prevents solid feed from sticking to the hopper throat walls and restricting flow or stopping flow all together (commonly called bridging). In many extrusion operations, the internal surface of the feed throat is smooth; however, for blown film, it is quite common to employ a grooved feed throat (Fig. 3.3). This is one that has shallow longitudinal, or perhaps helical, grooves machined into it. The grooves taper to zero depth where the feed throat meets the barrel. The purpose of the grooved feed throat is, quite simply, to increase throughput. The grooves act to increase the friction between the pellets and the barrel, allowing the screw flight to force more pellets forward with each revolution of the screw. The forward movement of the solid bed may approach pistonlike efficiency. This will lead to an increase in the rate of melt flow through the die (i.e., more product per hour).

Figure 3.2 Extruder highlighting the feed throat

3.2 Grooved Feed Throat

63

Discharge end

Feed throat

Grooved sleeve

Grooved sleeve Cooling channel Figure 3.3 Grooved feed throat used for high output systems (Interactive Training Extrusion, Rauwendaal Extrusion Engineering, Inc.)

The solids conveyance rate strongly depends on two frictional interactions: 1. that between the feed material and the barrel and 2. that between the feed material and the screw. By maximizing feed/barrel friction and minimizing feed/screw friction, solids conveyance is maximized. Typically, a threefold increase in extruder output is obtained using a grooved feed throat instead of a smooth one at the same screw speed. In addition to increased output, grooved feed throats have a couple of other benefits. A decrease in output variation (i.e., less surging) often results from creating high barrel friction. This is because of a decreased sensitivity to fluctuations in two common variables: 1. pellet frictional properties and 2. extruder head pressure. Also, “slippery” materials such as very high molecular weight polyethylene are often easier to extrude with this system as the grooves catch the pellets allowing the screw flight to force them forward. There are, of course, some drawbacks that must be considered with grooved feed throats. Significantly higher barrel and head pressures are generated with these systems. Barrel pressures in excess of 10,000 psi have been measured, with this typically occurring back near the feed section of the barrel, not at the discharge end. This condition generally requires a redesigned screw with a much lower compression ratio than conventional

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systems. Finally, a large amount of frictional energy (heat) is transferred into the polymer in the feed section, resulting in high heat generation in the feed throat. This often necessitates intensive cooling of the feed throat. A side effect of the high heat in the feed section is polymer degradation. One consequence of this polymer degradation is an increase in gels. These gels form due to degradation of the polymer, which creates high molecular weight spots in the film. For some extrusion operations, the drawbacks of grooved feed throats discount any benefits. For the blown film extrusion of polyethylene, however, the benefits generally far outweigh the drawbacks. First, for the very significant case of bag production, there is a tremendous need for high outputs. Second, coupling high outputs and a relatively viscous material (for high melt strength) with small die gaps will result in high extrusion pressures anyway. Finally, compared to most other polymers, polyethylene is very thermally stable.

3.3

Screws for Blown Film Extrusion

Extrusion screws were discussed generally in an earlier chapter covering extruder hardware. For all types of extrusion processes, there are some performance objectives that every screw should meet. It is always important that the screw design minimizes polymer hang ups on the screw hardware. When polymer remains on the screw for an extended time, it will degrade causing a loss of properties and a discoloration of the product. Hang up is reduced in the design by allowing generous geometric transitions (radii), such as where the flight meets the root and in fluted mixing sections. Another performance objective is that the screw should provide the required output at a minimum melt temperature. High melt temperature can lead to both degradation and wasted energy due to a need for excessive cooling. In many cases, increasing screw speed can increase output. However, this comes with the detrimental effect of increasing shear heating, hence melt temperature. Therefore, a good design provides the desired output at a shear rate that does not overheat the polymer. This means that the screw must have adequate capacity to feed and melt pellets, mix, and generate pressure, all at a speed that does not result in excessive shear. With respect to screws for blown film extrusion, there are specific design objectives in addition to the general ones mentioned above. Because blown film extruders often utilize a grooved feed throat, the conveyance rate of pellets is generally very high. Therefore,

3.4 Blown Film Dies

65

the grooved feed throat is usually coupled with a screw that has a low compression ratio, often less than one. The very high compaction pressure on the pellets provides energy to melt so that the usual compression (from the reducing channel depth on a conventional screw) is not necessary. Also, the deeper channel in the pumping section allows the melt to exit at a lower temperature because of reduced shear heating. Because of the need for high melt strength in the extrudate, blown film polymers usually have high viscosity. As a result, blown film extruder screws are typically designed for high viscosity melts. This leads to optimal screw designs that have deep metering channels and are able to deliver high torque. Unfortunately, deep flighted screws of conventional design (simple conveying screws) are susceptible to unmelt, a condition where particles that are not completely melted reach the end of the screw. Unmelt can be largely eliminated by the use of a barrier-type screw. For this reason, many blown film screws today include a barrier flight. This second flight in the melting section of the screw helps to ensure that no unmelted material exits from the barrel. Because blown film products are very thin, unmelts can be quite problematic, both structurally (as stress concentrators) and aesthetically. Finally, mixing sections are being used extensively for blown film extruder screws. The need for high melt quality in blown film requires that the material leaving the extruder and entering the die be as homogeneous as possible with regard to both compositional and temperature variations.

3.4

Blown Film Dies

The purpose of the blown film die is to receive polymer melt from the extruder and deliver it to the die exit as a thin annular (tubular) film, generally exiting the die gap vertically upward. The delivery of high melt quality is a performance goal not only of the screw, but also of any well-designed extrusion die. This means that the melt flows through the die smoothly without “hanging up” anywhere inside. Further, good melt quality means that no flow lines are created that may result in a visible or structural defect in the film. Finally, good die performance includes flow channels that produce a uniform exit velocity of the melt around the circumference of the die gap. Several types of blown film dies are available, varying in cost, complexity, and purpose. Side fed, bottom fed, and spiral mandrel dies are used for producing monolayer films.

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Coextrusion dies are used for multilayer products. Both monolayer and multilayer films may be extruded using rotating (oscillating) dies. These are all described in the following paragraphs. The side fed die (Fig. 3.4) is the simplest and least expensive die type. In this design the extruder feeds the die through a single side port and the melt flows around each side of a center mandrel located inside the die. On the side of the mandrel opposite the entry port, the divided melt streams flow together, forming a weld (or knit) line. As mentioned above, weld lines may cause weak spots or visual defects in the film. Another drawback of side fed dies is that they make it difficult to maintain tight gauge (thickness) uniformity. The different flow lengths from the entry port to various locations around the die gap circumference can result in different exit velocities of the melt. The bottom fed die (Fig. 3.4) is an improvement over the side fed die, from the standpoint of gauge uniformity. In this design, the entry point from the extruder is at the base of the mandrel. Therefore, melt travels the same distance from the entry to all points around the gap circumference, leading to a uniform exit velocity around the gap. One drawback, however, stems from the way that the die is built. To fix the mandrel in place, it must be attached to the inside surface of the external die body. These attachment points, commonly referred to as spider legs, interrupt the flow, creating weld lines. The spider legs are typically positioned as early in the flow stream as possible to allow the melt maximum time to reknit. Also, a “smear” surface may be machined into the mandrel downstream of the spider legs to cause flow direction changes that help reknit the melt. By far, the most common type of die for blown film is the spiral mandrel die (Fig. 3.5). This is due to its ability to produce high melt quality. In this design, of which there are several variations, the melt flows around the mandrel in a number of helical channels. As the melt spirals upward around the mandrel, it can leak vertically upward out of the channel and into the adjacent one. Tight clearances at the bottom of the mandrel create a predominantly helical flow at first, while increases in clearances toward the top of the mandrel allow a transition to upward axial flow later. This helical to axial transition provides a mixing effect that largely eliminates the negative influence of weld lines formed by converging flow fronts from adjacent ports inside of the die. Also, this technique allows the flow to be distributed uniformly around the mandrel leading to velocity, hence gauge, uniformity at the die exit. In many operations, the product is made up of multiple layers of polymer combined into a single film. Each layer serves a specific purpose, such as reduced cost, low oxygen permeability, strength, printability, heat-seal ability, and so on. These products are processed using a coextrusion die that is fed by two to five (sometimes more) extruders.

3.4 Blown Film Dies

Spreader Bushing

Adj. collar Body Feeder plug

Sleeve Nut collar Adj. nut

Die ring (centering)

Smear device

Spider ring Air inlet

Figure 3.4 Side fed die (top [6]) and bottom fed die (bottom [54]) show various designs based on cost and processing requirements

67

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3 Hardware for Blown Film

Figure 3.5 Spiral mandrel die produces excellent melt quality

There are two main types of coextrusion blown film dies: concentric (also called nested mandrel) and stack (also called pancake). Comparisons of these die types were published in two mid-1990s reports [10, 11] and are summarized below. Concentric dies (Fig. 3.6) are comprised of a series of hollow spiral mandrels nested inside one another. A narrow gap remains between each mandrel and the one directly inside it. Inside these gaps is where polymer flows. The different gaps separate the various polymer layers. Just prior to the die exit, the different polymers flow together into a single flow channel. Their laminar flow behavior mainly keeps the polymers in separate layers through the final product. These types of dies have been used for many years and perform very well. A couple of possible concerns to consider when designing

Figure 3.6 A concentric mandrel coextrusion die (Battenfeld Gloucester)

3.4 Blown Film Dies

69

systems for these dies include potentially long residence times because relatively long transfer pipes are needed to feed the various layers and, if necessary, rearranging the polymer layers is not easy. Stack dies (Fig. 3.7) separate the flow gaps between vertically stacked plates instead of between nested mandrels. Theoretically, any number of plates can be stacked to add one layer for each parting line between plates. This technique yields some potential advantages. Because the various extruders can be located in a fan around the

Figure 3.7 A stack coextrusion die (Brampton Engineering, Inc.)

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3 Hardware for Blown Film

circumference of the die body, each one may have a relatively short transfer pipe feeding its die layer, thus minimizing residence time. Also, this setup leads to an easier changeover to a different coextrusion layer design. Finally, stack dies provide a good opportunity for thermally isolating each layer when running materials with widely varying melt temperatures. Processing temperature differences of up to 300 °F (150 °C) in adjacent layers have been reported [12]. One potential drawback for this configuration is the high plate deflection pressures that can develop with certain designs [13]. The die opening (orifice) through which polymer exits is an annular shape. It is bound on the inside by the mandrel (or pin, or tip) with radius Ri. The die orifice is bound on the outside by the die ring (or bushing) with radius Ro. The die gap (or thickness of the die opening, td) is defined as the difference between the die ring radius and the mandrel radius (td = Ro – Ri). The correct die gap for a given operation depends on many factors, including the material type, die diameter, blow-up ratio, film thickness, etc. For polyethylene, it is common to use smaller die gaps for LDPE and larger die gaps for LLDPE and HDPE. For example, for the production of 1 mil (0.001 in, 25 micron) LDPE film, a 0.040 in (1 mm) die gap might be used. However, a 1 mil LLDPE film might use a 0.100 in (2.5 mm) die gap. When extruded film is not to be converted inline, but will be wound onto a roll for later conversion, a rotating (or oscillating) die is sometimes used. As a die rotates about its center point, any variation in film thickness is distributed uniformly around the circumference, hence across the wound roll. This technique yields a flat roll instead of one with high and low spots.

3.5

Bubble Geometry

Although bubble geometry is not specifically hardware, it is included in this section because the hardware directly affects the bubble’s geometry. The following paragraphs detail the geometric considerations of bubble shape. The specific shape of the bubble (Fig. 3.8) depends on the combined influence of several process parameters. In general, the bubble usually has a small diameter and large thickness at the die exit and transitions to a large diameter and small thickness as it moves upward toward solidification. Above some point, the geometry is “frozen-in” and remains virtually constant.

3.5 Bubble Geometry

71

Nip speed Bubble diameter

Frost line height

Melt speed

Die diameter Plastic Blown film bubble

Figure 3.8 Bubble geometry characteristics

There are several parameters used to describe the geometry of the bubble: • Die diameter • Die gap • Frost-line height • Stalk • Bubble diameter (BD) • Film thickness • Layflat width (LF) The die diameter represents the initial bubble diameter as it leaves the die, and the die gap determines the initial bubble wall thickness. As the bubble travels upward from the

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die face in the molten state, it is cooled and eventually reaches a temperature where it becomes a solid. The distance from the die face to where this solidification takes place is called the frost-line height. The name is derived from operations where it appears that the film is optically frosting as it becomes cloudy due to polymer crystallization. Conventionally, the frost line is defined as the lowest point where the bubble is at its maximum diameter because there is effectively no further stretching above this point. The bubble region below the frost line is known as the stalk or neck, particularly when it is relatively long. Above the frost line, where geometry is effectively frozen-in, the terms bubble diameter and film thickness are simply used for those characteristics. Once the film is collapsed flat and passes through the nip rollers, the two layer web is characterized by a layflat width. Twice the layflat width is equivalent to the circumference of the bubble (or BD = 2 LF/π). In many cases, it is easiest to measure the layflat width, so this equation becomes a handy tool for determining the bubble diameter. Several process variables work together to determine the bubble geometry: • Melt speed • Nip speed • Internal bubble volume • Cooling rate The melt speed is the upward velocity of the polymer as it exits the die gap. It is controlled by the screw speed, but it is not the same as the screw speed (for one thing, the melt speed is linear and the screw speed is rotational). The nip speed (also called film speed, line speed, and take-off speed) is the velocity of the polymer as it travels through the nip rollers. The film travels essentially at the nip speed at all points above the frost line. In all cases, the film increases in velocity from the die face, where it travels at the melt speed, to the frost line, where it travels at the nip speed. This acceleration leads to thinning of the melt curtain to obtain a thin film. The internal bubble volume is the amount of air contained inside the bubble between the die face and the nip rollers. A similar variable that can be used alternatively is the internal bubble pressure. The cooling rate is determined by the speed at which the cooling air impinges on the bubble and the temperature of that air. There are several other process variables that influence bubble geometry, such as process temperatures, die design, feed material composition, and polymer flow properties, but these generally remain constant for a given run. A distinction is often made between two general types of bubble shapes (Fig. 3.9) that are selected by the processor for a given resin type. The pocket bubble has little or no stalk, beginning its expansion almost immediately above the die face. This

3.6 Bubble Cooling

73

Figure 3.9 Pocket bubble on left and high (long) stalk bubble on right (The Blown Film Extrusion Simulator)

shape is mostly used for low-density polyethylene, linear low-density polyethylene, and polypropylene. The pocket bubble tends to be quite stable due to the cooling air providing early solidification. The other shape is the high (or long) stalk bubble. This type is used primarily for highdensity polyethylene due to that material’s relatively low melt strength. In this process, TD stretching is delayed until the polymer reaches a lower temperature, allowing for a more stable melt and providing higher stress during TD stretching.

3.6

Bubble Cooling

Bubble cooling is generally accomplished by blowing a large volume of air on the film as it exits the die (Fig. 3.10). This may take place on only the outside of the bubble or on both the inside and the outside. Additionally, the bubble is kept inflated to remove more heat from the film as it travels up through ambient air in the cooling tower. Bubble cooling deserves much attention. In many blown film operations, it is the limiting factor to maximizing throughput. Therefore, it is important to have a thorough understanding of the variables that influence the removal of heat from the bubble and ways to improve the efficiency of this process.

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3 Hardware for Blown Film

Figure 3.10 Bubble with external cooling and internal cooling

Polyethylene materials tend to retain heat more than most other polymers. A measure of this tendency is the specific heat value. Polyethylene grades have a specific heat in the range of 1.8–2.3 kJ/kg·°C. Most other polymers have a specific heat in the range of 0.9–1.5 kJ/kg·°C. For this reason, high cooling towers are necessary to remove enough heat in the film that the two sides will not adhere together while passing through the nip rollers. Three primary process variables are responsible for the efficiency of cooling: air speed, air temperature, and air humidity. Air speed is generally measured as a volumetric flow, such as cubic feet per minute. At higher air speed, more heat is removed from the film per unit time. Air speed is determined by the capability of the blower unit and by the static pressure at the blower outlet (Fig. 3.11). As static pressure is increased, the blower becomes less efficient, delivering fewer cubic feet per minute at a given motor speed. Factors that affect static pressure are the number, length, and diameter of cooling hoses and flow restrictions within the hoses or air ring. Another variable affecting cooling efficiency is the temperature of the air impinging on the bubble. Cooler air will remove heat more quickly, but using chilled air increases processing costs so a balance must be reached. As a side note, if a chiller is used to cool the air, it is good practice to insulate cooling hoses, manifolds, and air rings because this will minimize moisture condensation. Ambient air temperature around the extrusion line also has a large effect on bubble cooling, even when chilled air is used in the process. This is why frost-line height may change significantly from day shift to night shift in plants that are not airconditioned.

3.6 Bubble Cooling

75

Air delivery [cfm]

Blower delivery vs. static pressure 2200 1800 1400 1000 0

2

4

6

8

10

Static pressure [in] Figure 3.11 As the static pressure on the blower is increased, the blower efficiency decreases

A third cooling variable is air humidity. If a chiller is used, then the air impinging on the bubble is typically quite dry. However, if ambient air is used, the humidity will vary seasonally and may affect cooling efficiency. Most plant personnel report that cooling efficiency goes down when the air humidity is higher. However, humidity is generally higher at the same time that air temperature is higher, so separating the effects simply by observation is difficult. The physics indicate that more humid air would actually be more efficient at cooling the bubble, so further research is needed in this area. The principal hardware component responsible for cooling is the air ring. This device is located just on top of the die with a layer of insulating material (or air) between it and the hot die face (Fig. 3.12). The air ring surrounds the bubble and delivers cooling air directly onto the bubble. It receives air from the blower through, typically, a number of hoses that attach around the circumference of the device. Inside the air ring, a series of baffled flow channels distribute the air in such a way as to produce a uniform airflow (volume and velocity) at all points around the circumference of the bubble. The importance of the insulator between the air ring and the die is often underestimated. A missing, damaged, or poorly chosen insulator will allow the die to transfer a large amount of heat through the air ring and into the cooling air. This results in decreased production efficiency. Additionally, an air ring sitting directly on the die face will draw heat out of the die, making uniform temperature control of the die difficult. There are two main types of air ring designs (Fig. 3.13): single lip (on left) and dual lip (on right). Single lip air rings are relatively inexpensive and are used with materials that are run with a reasonably stable pocket bubble, such as low-density polyethylene. High-density polyethylene, which typically runs with a high stalk, also is usually run with single lip air rings. Dual lip systems are used with materials that are more prone

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3 Hardware for Blown Film

Figure 3.12 A blown film air ring (Future Design, Inc.)

Single lip design

Dual lip design

Figure 3.13 Two air ring designs [54]

to bubble instabilities, such as linear low-density polyethylene. This design provides airflow to both cool the bubble and aerodynamically stabilize it. Further, most dual lip designs provide the operator with good control of the balance of flow streams used for cooling and stability, a feature that can improve film dimensional consistency. Many blown film extrusion lines today, particularly larger ones, employ an internal bubble cooling (IBC) system to increase cooling efficiency, hence production rates (Fig. 3.14). IBC systems, coupled with conventional air rings, provide cooling to both sides of the film instead of only the outside surface. An IBC device is a continuous heat exchanger, bringing cool air into the bubble while removing heated air from inside the bubble. An additional benefit of IBC is the ability to perform closed-loop control of bubble diameter through adjustment of the airflow rates. Also, faster start-ups can result from this automation opportunity.

3.6 Bubble Cooling

77

Figure 3.14 An internal bubble cooling (IBC) system

An alternative technique for bubble cooling utilizes water applied directly to the outside surface of the bubble (Fig. 3.15). This process provides very high heat removal rates yielding significant output increases and reduced polymer crystallinity (clearer film). Although this technique was developed decades ago, it has seen very little commercial application, primarily due to its complexity. Recently, the process has been redeveloped for application to coextruded structures.

Figure 3.15 Water-cooled blown film bubble utilizing downward extrusion (Brampton Engineering, Inc.)

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3.7

Bubble Stabilization

A fifty-foot high, six-foot diameter bubble with a wall thickness of one thousandth of an inch is very susceptible to lateral movement from environmental effects such as drafts. When movement of the bubble occurs, often called “dancing”, the result is nonuniform wall thicknesses, originating at the die lips. For this reason, bubbles are usually stabilized externally using devices such as cages and irises (Fig. 3.16). In some cases, internal stabilization can be performed as well.

Figure 3.16 A bubble stabilizing cage with Teflon-coated rollers

While a benefit is gained using stabilizers, care must also be taken to ensure that the stabilizers are not the cause of defects. Film scratching and marring can occur when these devices do not perform properly. Generally, cage rollers are Teflon-coated and should be checked regularly to make certain they rotate freely. Noncontact (air-bearing) cages are available as well.

3.8

Collapsing Frames

As the bubble moves upward and approaches the nip rollers, it is “preflattened” by the collapsing frame (Fig. 3.17). This device provides a smooth transition from a round tube shape to a flattened tube shape. Collapsing frames utilize wooden slats, metal rollers, Teflon-coated rollers, or an air cushion to perform the shape transition.

3.9 Haul-off

79

Figure 3.17 The collapsing frame is shown transitioning the bubble from a circular shape to a two-layer flat film

In addition to flattening the tube, the collapsing frame also helps eliminate wrinkles in the final product. These devices are generally adjustable for both height and entry angle. Proper positioning of these two adjustments is often used to correct wrinkling problems.

3.9

Haul-off

A pair of nip rollers (the haul-off device) is located at the top of the cooling tower (Fig. 3.18). Their purpose is to pull the film up from the die. Also, the nip serves as an air seal for the top end of the bubble, so, at least one of the rolls is usually rubbercovered. While one of the rollers is located in a fixed position, the other uses pneumatics to be moved laterally into the closed or open position. This allows for the line to be strung between the rollers for start-up. The fixed-position roll is motor-driven to establish the line speed. Since the line speed (nip speed) is a primary control determining film thickness, bubble diameter, and frost-line height, fluctuations in the motor speed should be minimized, generally less than ±1% full scale.

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3 Hardware for Blown Film

Figure 3.18 A pair of nip rollers with a motor-driven chrome roll and a rubber-covered contact roll

The size of the nip rolls and all downstream idle rollers, known as the roll face width, determines the maximum layflat width that the system is capable of producing. The layflat width is related to the bubble diameter by the following equation: BD = 2 LF/π (where BD = bubble diameter and LF = layflat). The web of film coming off of the tower may be converted into product inline, or it may be wound onto rolls for later conversion. In systems where it is wound into roll packages, the entire haul-off assembly is often located on an oscillating turntable. The benefit to oscillating the haul-off is to uniformly distribute any minor gauge (thickness) variations. Otherwise, a wound roll containing high and low spots would be created.

3.10

Winders

Winders (Fig. 3.19) are used to collect the wound roll packages. They generally operate in a manner that produces rolls of constant web tension, as opposed to operating at a constant winding (rotational) speed. There are two primary winder types: surface winders and axial winders.

3.10 Winders

81

Figure 3.19 A dual-station turret winder (Windmoeller & Hoelscher)

Because of their simple, economical design, surface winders tend to be the more popular of the two. They operate by employing a motor-driven contact roll at the film roll surface. The contact roll essentially lays the film onto the rotating film roll. As the film roll diameter increases, the axis of the film roll moves in a path that separates it from the contact roll, maintaining pressure at the surface. This system is used mainly for medium- and high-tension rolls. Axial (or center) winders are often used when low-tension roll packages are required. In these systems, the central axis of the film roll is motor-driven rather than the contact roll. This type of winder tends to be more expensive than the surface winder. A couple of other functions performed by winders include automatic roll changeover and static dissipation. Turret winders are those that have a mechanism for moving a fresh roll (core) into position on-line while moving a full roll off-line. This can be initiated manually by an operator command or automatically based on roll weight or film length. Winders are often equipped with static discharge systems to handle the large amount of static electricity that builds up during blown film winding.

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3.11

Film Treatment

Polyolefins, such as polyethylene and polypropylene, are nonpolar polymers. This means that any polar materials will not easily wet the surface of (adhere to) these polymers. One area where this has important implications is in the printing conversion process. Inks cannot absorb into most films because they are typically nonporous. The only way the ink will stay on the surface of the film is if it wets the polymer rather than puddle up. To promote the ink adhering to the surface, it becomes necessary to treat the film to make it more polar. The most popular treatment process for blown film is the corona discharge method. Corona discharge units (Fig. 3.20) are high voltage devices that ionize the air in a small region through which the film travels. The ionization creates ozone, a very strong oxidizing chemical. The result is that the film is oxidized, creating a polar surface rich with oxygen atoms [14]. The level of oxidation needed for a given film may depend to some extent on the additive package in the raw material. Also, this type of surface treatment is not permanent. In most cases, the film must be printed within a few days of treatment because additives in the plastic may migrate (bloom) to the surface and inhibit the treatment. Safety is a top concern where these high voltage corona discharge systems are employed. Adequate venting is necessary in the vicinity of the corona to extract the ozone created by the discharge unit. Also, a shut-off interlock should be used in the event of a film line break. Treater station

Ground roll

Ozone destruct unit Ground roll

Web Figure 3.20 Corona discharge unit for treating (oxidizing) film surfaces (Pillar Technologies)

3.12 Line Control

3.12

83

Line Control

The goal of line control is to maintain minimum variation in all measurable film quantities with respect to both position and time. In other words, minimal variation is desired in a measurement such as film thickness from one position on the bubble to another and, at a given position, from one time to another. The high interdependence of process variables on film quality makes this an ambitious objective. Sophisticated control systems have been developed in response to the high interdependence of several process variables coupled with the demands of very high output rates [15]. While many lines still utilize primarily manual controls, a growing number of blown film systems depend on computer-based measurement and control of all key process variables. These computer-based systems can make an important contribution to increasing efficiency, reducing costs, and increasing profits on high output lines. This section explains how film dimensional and property consistency results from maintaining process uniformity in four key areas: • Melt from the extruder (or melt quality) • Film thickness (or gauge) • Layflat width (or, alternatively, bubble diameter) • Frost-line height The following paragraphs focus on achieving uniformity in each of these four areas. Providing high melt quality to the die is the primary objective of the extruder. High melt quality can be defined as homogeneous material of constant temperature and pressure. Homogeneity of the melt depends on several factors, such as the uniformity of the raw material, adequate melting capability of the screw, and adequate mixing capability of the screw and/or static mixer. Raw materials are often sampled in-house or at an outside testing facility to ensure minimal lot-to-lot variation. Variations in the composition ratio of solids (the ratio of first-generation, reprocessed, fluff material, and additives) entering the extruder can lead to nonuniformities in the melt, so should be minimized as an objective of the feed system. Finally, screw design is the critical element in making certain the melting of solids along the barrel proceeds stably and the melt leaves the extruder adequately mixed. To maintain a constant melt temperature, we must consider potential variations with respect to both position and time. Position variations refer to temperature differences

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Film thickness gauge

Blown film bubble

Traversing carriage

Figure 3.21 A thickness gauge mounted on a carriage to rotate it around the bubble

between, for example, the wall surface of a melt stream and the internal core. If melt with a large temperature distribution enters the die, it can result in different temperature streams flowing to different positions around the die exit. This leads to gauge variations. Methods to reduce these variations include screw mixing elements, static mixers after the extruder, and flow channel design within the die. Time-dependent temperature variations (large swings from high to low) must be minimized by the extruder temperature control system. Additionally, screw speed stability and feed material consistency is crucial for good temperature control. Constant melt pressure is the final requirement for high melt quality. The pressure of the melt entering the die, or head pressure, is determined by three variables: head/die flow channel geometry, polymer flow rate, and polymer viscosity. In general, the hardware geometry remains fixed so pressure fluctuations are caused by any changes in flow rate or viscosity as described in the next two paragraphs. However, one change related to hardware geometry, or more specifically flow restriction, is the buildup in front of filtering screens. Monitoring this buildup of contamination is important because of its effects on increasing head pressure which impacts safety and process efficiency. Polymer flow rate is kept constant when the screw processes occur stably. That is, the stability of solids conveying, melting, mixing, and melt pumping is necessary to maintain a uniform flow rate through the die. It is not uncommon, particularly with significant

3.12 Line Control

85

amounts of reprocessed feed material, for solid conveying characteristics to change throughout a run, leading to detrimental pressure fluctuations. Polymer viscosity variations may be caused by changes in either the raw material or the feed composition. Additionally, we can see viscosity changes when there are variations in hardware temperature, such as may occur with an unstable temperature control circuit. The next process variable to be controlled is film thickness (gauge). Either on-line or off-line measurements allow us to monitor film gauge. On-line devices usually employ a radiation source, such as a gamma backscatter system. These devices measure thickness by emitting radiation that reflects back to the sensor from both the near and far surfaces of the film. They can be mounted to measure a fixed location on the bubble or on a carriage to traverse around the bubble (Fig. 3.21). Also, a unit traversing back and forth across the flattened web can be used, but this measures two-layer thickness. Off-line devices typically operate on a capacitance principle, so they don’t require the safety practices employed with radiation sources. Several methods are used to adjust thickness. The best practice is to begin with a good die setup. Before each new run, the operator should check to make sure the die ring is centered on the pin. Although adjustments will almost always be necessary after start-up, this provides a good starting place for the extrusion run. Final adjustments are made by observing the shape of the bubble and monitoring film thicknesses. Once any asymmetry in the bubble is observed, the die bolts should be slightly tightened or loosened to move the ring around the pin so that a uniform thickness distribution results. Adjustment of the die gap is an easily misunderstood concept. In some operations it is ignored, resulting in an ongoing attempt to compensate for uneven melt flow by other methods such as air ring adjustments. In other operations, personnel without the experience to appreciate the sensitivity of the process to these corrections perform the adjustment improperly. To make matters worse, blown film dies may be constructed such that tightening a die bolt increases the local die gap or such that tightening a die bolt decreases the local gap, creating confusion for line operators. In summary, it is good practice to make certain that key personnel responsible for process control are well trained on how to adjust specific dies and what conditions warrant a die adjustment. Sometimes, it is best to use other methods to create a symmetrical bubble, such as ensuring uniform die temperature around the die gap. High output lines often employ automatic gauge control. One type of system utilizes a series of small heaters located around the circumference of the die lips. A downstream thickness measurement is reported back through a controller that decides whether to

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increase or decrease heat to a certain die lip zone, thus promoting or restricting flow to that zone. Other types of systems provide small air jets to several locations around the base of the bubble or selectively heat sections of the cooling air stream around the bubble. In these cases, a signal for increased or decreased airflow or temperature leads to a change in stretching of the melt. Layflat width or, alternatively, bubble diameter is another process control variable. Layflat width can be measured manually with a tape measure but is most often measured automatically using a laser micrometer across the web (Fig. 3.22). Bubble diameter is measured by using ultrasonic sensors mounted on bubble guides or irises or by taking video of the film during a run and measuring the diameter from the video. Adjusting the internal bubble volume (or pressure) changes the bubble diameter (or layflat width). IBC systems can perform this automatically, or it can be done manually by opening a valve to allow more air into the bubble to increase the diameter or by making a cut in the bubble to release some of the air, thereby decreasing the diameter. The final control variable is frost-line height. The position of the frost line is very sensitive to any changes in the process. Therefore, this is an excellent parameter to track as a measure of stability during a run. Any changes in ambient or process temperatures, line or screw speed, or material feed conditions will affect the frost-line height. This, of course, will lead to variations in film gauge and layflat as well.

Laser gauge

Figure 3.22 A laser gauge is used to measure layflat width

3.12 Line Control

87

When frost-line height is measured, it is done manually with a tape measure, electronically through an optical scan of the neck height, or by using infrared temperature measurement (Fig. 3.23). To control frost-line height, we can vary the blower motor speed or the temperature of the cooling air passing through the air ring. Again, this can be performed in a closed-loop or a manual mode.

Pull rolls

IR point sensor

Collapsing frame

Air bubble

Blown film Frost line IR point sensor

Linescanner Die Air ring

Windup roll Cooling unit

Heaters

Extruder

Figure 3.23 Online infrared temperature measurement system (Raytek Corporation)

4

Processing

Blown film extrusion offers excellent manufacturing flexibility because of the ease with which product geometry (such as film thickness and/or layflat width) can be changed without a need to change hardware. Perhaps the most important processing characteristic, however, is the ability to impart biaxial orientation (the alignment of the long, chainlike molecules) into the film in a cost effective manner. Proper molecular orientation in the film is one of the most important objectives of this process. For example, adequate impact strength or puncture resistance in a film can be obtained one of two ways, by creating suitable orientation or by increasing the film thickness significantly. As one would expect, every manufacturer would choose the former over the much more expensive latter, every time. Biaxial orientation means that polymer molecules are aligned in the plane of the film, i.e., in both the machine direction (MD, along the long axis of the bubble) and the transverse direction (TD, around the hoop direction of the bubble). The result is a tough film that resists tearing in either direction (a kind of “rip-stop” effect), as opposed to a film that tears easily in one direction (a so called “splitty” film). This molecular structure is produced when melt exiting the die is stretched in both MD and TD at the same time. Therefore, the geometry of the bubble and the process conditions yielding the geometry are crucial to proper orientation. As mentioned earlier, these process conditions are highly interdependent. This chapter provides an overview of the relationships between blown film processing, molecular structure, and solid-state film properties. Herein, blown film processing is characterized by bubble geometry, molecular structure by orientation, and film properties primarily by tensile strength and tear strength. This approach provides an effective technique for understanding and applying the fundamental principles involved in these process/structure/property relationships. However, since this chapter only begins to capture their complexities, Film Processing by Kanai and Campbell [16] is an excellent source for further knowledge.

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4.1

Process Variables vs. Bubble Geometry

Table 4.1 defines the response that increasing each of the four main process variables (nip speed, screw speed, cooling speed, and bubble volume) has on each of the three main bubble geometric variables (film thickness, bubble diameter, and frost-line height). An asterisk identifies the primary response to each increase. Table 4.1 The Effect of Each Main Process Variable on Bubble Geometry

Variable to increase

Film thickness

Bubble diameter

Frost-line height

Nip speed

↓*

↑

↑

Screw speed

↑*

↑

↑

Cooling speed

↑

↓

↓*

Bubble volume

↓

↑*

↓

* Primary response

This table highlights how making one adjustment in the process affects all three of the bubble characteristics. A skilled operator must understand these interrelationships to accurately adjust the process so that all the geometry requirements are within specification. Even though many blown film lines today are automated to adjust for changes in process conditions, the above relationships still exist and it is the responsibility of the system (manual or automatic) to take the proper corrective action.

•

Simulator Exercise: Try increasing each of the four variables in the table individually. During each increase, observe the affect on each quantity in the measurements panel. It may be beneficial to record and plot, using spreadsheet software such as Excel, the values with each change to see which measurements are affected most by each adjustment. In the following paragraphs, a description is given of each of the relationships identified in Table 4.1. The assumptions here are 1. only one process variable increases at a time while the others remain constant, and 2. each response occurs naturally without correction from a closed-loop control (automatic measurement and adjustment) system.

4.1 Process Variables vs. Bubble Geometry

91

When the nip speed increases, the primary effect is for the melt to be stretched more in MD, making the film thinner. As a result of the film traveling past the cooling air more quickly, the height on the bubble where the temperature has dropped to the point of polymer solidification (the frost line) increases. (It is easy to mistakenly think that the frost-line height should decrease because thinner film must cool faster. However, this is not the case because the effect of an increase in film speed is more significant and the frost-line height always increases.) As the frost-line height increases, the small diameter stalk below the frost line lengthens and the air volume in the bubble is displaced more to the top, because the bubble contains a fixed volume of air. This increase in bubble volume above the frost line pushes the bubble outward to a higher diameter, also contributing to film thinning. An increase in screw speed results in an increase to all three bubble geometry variables. The increase in output from the extruder has the primary effect of increasing film thickness. Also, a greater amount of material results in a greater amount of heat that must be removed from the film. This takes a longer time under constant cooling conditions, thus increasing the frost-line height. Again, as the frost line moves upward, the bubble diameter increases. The slight thinning effect due to an increase in bubble diameter is far outweighed by the thickness increase created by greater output. Increasing the cooling air speed causes faster heat removal from the bubble. Because the film reaches solidification temperature sooner, the primary effect is a lowering of the frost line. As a result, the bubble diameter decreases from the constant internal air volume being distributed over a greater distance from frost line to nip rollers. Because a lower bubble diameter means the film is not stretched as much in TD, the film thickness increases. When more air is inserted into the bubble, bubble volume increases, primarily the diameter increases by stretching more in TD. The increased TD stretching results in thinner film. Thinner film cools more quickly, consequently lowering the frost-line height.

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4.2

Characteristic Bubble Ratios

To properly describe and control the bubble-forming process, certain quantities have been developed to characterize the process conditions that influence bubble geometry. These quantities are the take-up ratio (TUR), the blow-up ratio (BUR), and the forming ratio (FR). The TUR is the ratio of film velocity (Vf ) to melt velocity (Vm), i.e., TUR = Vf /Vm. This quantity provides an indication of the amount of stretching, hence molecular orientation, in MD. The film velocity is the upward speed of the film above the frost line and is established by the control system. It is equivalent to the nip speed. The melt velocity is the upward speed of the molten polymer as it exits from the die lips. It is related to, but is not equal to, the screw speed. The melt velocity can be determined experimentally by marking the film and tracking the mark, but an easier method is to employ the principle of conservation of mass. The conservation of mass states that the mass flow rate (pounds/hour) at all points along the bubble is equal. Mathematically, mrate = (ρ A V)nip rollers = (ρ A V)die gap where ρ = density, A = annular area, V = velocity. This equation can be rearranged to the form TUR = Vf / Vm = (ρ A)die gap / (ρ A)nip rollers The area of an annulus (Fig. 4.1) can be calculated from the following equation: Aannulus = π (Ro2 – Ri2) where Ro is the outside radius and Ri is the inside radius of the annulus.

Ro Ri

Figure 4.1 Area of an annulus: Aannulus = π (Ro2 – Ri2)

4.2 Characteristic Bubble Ratios

93

Since the nip speed is always greater than the melt speed, the TUR is always greater than one. The BUR is the ratio of bubble diameter (Db) to die diameter (Dd), i.e., BUR = Db /Dd. This quantity provides an indication of the amount of stretching, hence orientation, in TD. The bubble diameter is established by the control system and can be either measured directly or calculated by measuring the layflat (LF) width directly (Db = 2 LF/π). The die diameter is fixed. Because TUR requires some calculating to obtain, some workers prefer to use the draw down ratio (DDR) to indicate the total degree of film stretching. DDR is determined from three easily obtained measurements, the die gap, final film thickness, and BUR: DDR = td / (tf · BUR) where td is the die gap and tf is the film thickness. However, even though DDR is easy to determine, it does not specifically indicate the degree of MD or TD stretching. These are indicated by the TUR and BUR, respectively. Finally, the forming ratio is the ratio of the TUR to the BUR. This quantity provides an indication of the balance of stretching, and so orientation, between MD and TD. If a film has identical mechanical properties in MD and TD, it is said to have isotropic properties. When a blown film is processed so that the FR approaches one, it is an indication that the properties of the film approach isotropy. There is not an exact relationship between forming ratio, molecular orientation, and property balance; however, the general trends are in the same direction. Employing TUR, BUR, and FR provides extrusion personnel with convenient measures of processing conditions. The following sample problem shows the utility of these processing values: Problem

Determine the forming ratio and throughput (lb/hr) for the following LDPE blown film line: Die diameter Die gap Nip speed Layflat Film thickness LDPE solid density LDPE melt density

6 in 0.040 in 125 ft/min = 90,000 in/hr 36 in 1.5 mil = 0.0015 in 0.92 g/cm3 = 0.033 lb/in3 0.76 g/cm3

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4 Processing

Solution

TUR = Vf / Vm ρdie gap Adie gap

Adie gap ρnip rollers Anip rollers

Anip rollers

= (ρ A)die gap / (ρ A)nip rollers = ρmelt = 0.76 g/cm3 = π (Ro2 – Ri2) Ro die gap = 6 / 2 = 3 in Ri die gap = 3 – 0.040 = 2.960 in = π (32 – 2.9602) = 0.749 in2 = 0.92 g/cm3 = π (Ro2 – Ri2) = 2 LF / π = 2 (36) / π = 23 in Db Ro nip rollers = 23 / 2 = 11.5 in Ri nip rollers = 11.5 – 0.0015 = 11.4985 in = π (11.52 – 11.49852) = 0.108 in2

TUR = (0.76) (0.749) / [(0.92) (0.108)] = 5.73 BUR = Db / Dd = 23 / 6 = 3.83 FR

= TUR / BUR = 5.73 / 3.83 = 1.5

mrate = (ρ A V)nip rollers = (0.033) (0.108) (90000) = 321 lb/hr The preceding section has described a useful technique for characterizing blown film bubbles based on easily calculated processing values. However, as indicated, the technique is generalized and only provides qualitative information about the degree of melt stretching imparted on the film. Many comprehensive studies have been performed to develop more sophisticated models that characterize bubble stresses and kinematics (shape and velocities). For more detailed reading on this subject, the reader is referred to additional resources [17–28].

4.3

Process/Structure/Property Relationships

Molecular structure, as imparted by processing, has a significant influence on the physical properties of extruded products. This is known as a process/structure/property relationship. In the case of blown film, the extruder conditions and the bubble geometry influence the molecular structure, which then affects film performance.

4.3 Process/Structure/Property Relationships

95

While blown film processing can be quantified with TUR, BUR, and FR as described in the previous section, structure is most readily characterized through molecular orientation. When molecules are oriented, they are deformed from their natural configuration (random coil) into a stretched and frozen configuration (Fig. 4.2). The alignment of long chain molecules in the solid film increases tensile strength in the direction of orientation and improves toughness and impact properties. (Other structural features such as degree, type, and orientation of crystallinity are also important in developing properties.)

Stress

Random coil

Oriented

Figure 4.2 On the left, a polymer molecule shown in its preferred, unstressed state called a random coil and on the right, after experiencing stress, in an oriented state

Molecular orientation is measured by various techniques. While most of these methods (such as infrared dichroism and x-ray diffraction) are conducted only in research facilities, a method that is readily conducted by the average film processor is the shrinkage test [29]. In this experiment, square specimens of film are placed on hot oil for a short time, allowed to shrink, and removed to cool. The length of the specimens in MD and TD are then measured to determine directional shrinkage, an indication of original orientation in each direction. While this does not provide a direct measure of molecular orientation, the technique has been shown to correlate well with direct measurement methods [30]. Several important studies have been conducted to obtain direct measurement of molecular structure in blown films [31–38]. In most cases, these projects seek to correlate process conditions to molecular structure, including orientation and crystallinity. Further, an attempt is often made in these types of studies to relate the structure to film physical properties. Many physical properties of blown film are measured to determine fitness for use. The primary mechanical properties of interest are tensile properties (strength, modulus, and elongation) in MD and TD, puncture resistance, impact strength, and MD and TD tear strength. These are all highly dependent on molecular orientation. Other film properties that are measured include brittleness, gel count, and optical clarity. Generalizations can be made about process/structure/property relationships that are helpful to the blown film processor. That is, we can describe in general the properties that

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4 Processing

Exaggerated molecular orientation

MD

TD High TUR

High BUR

FR ≈ 1

Figure 4.3 The type of molecular orientation in the film depends on the type of processing stress – high TUR yields high MD orientation, high BUR yields high TD orientation, forming ratio approaching 1 yields balanced orientation

will result when the process imparts certain molecular structure. For example, an increase in TUR generally results in an increase in MD orientation (Fig. 4.3). Subsequently, MD tensile strength is higher, but MD tear strength is lower because a tear can more easily propagate through the oriented molecules. When the BUR is increased, TD orientation increases. This leads to higher TD tensile strength, but lower TD tear strength. Finally, when FR approaches one, the MD and TD mechanical properties of the film tend toward a balance, i.e., toward isotropy.

•

Simulator Exercise: Try to produce film with the following specifications: Film thickness = 1 (±0.1) mil, bubble diameter = 9 (±0.1) in, frost-line height = 12 (±0.1) in, and throughput = 30 lb/hr. Note that in the simulator, the extruder has a specific throughput of 1 lb/hr/RPM.

5

Coextrusion

A brief coverage of the topic of blown film coextrusion is provided in this chapter. Coextrusion is the process of feeding a single die with two or more different polymer melt streams. Within the die, the various flow streams are combined to form a singleply film comprised of the individual layers (Fig. 5.1). Because of the high viscosity of polymer melts, the individual layers tend not to mix but to retain their positions within the combined flow stream. In some cases, over seven layers of polymer are extruded into a film. For each type of polymer layer in the final structure, a different extruder is connected to the die (Fig. 5.2).

Figure 5.1 Three individual layers are combined here to form a single-ply coextruded film

Figure 5.2 Coextrusion die with a seven-extruder fan feeding the die (Battenfeld Gloucester)

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5 Coextrusion

The main reason for coextruding film is to obtain a particular set of properties in the final film structure that depends on the contributions of multiple polymers. The various layers often contribute properties such as high strength, low permeability (barrier) to oxygen or carbon dioxide, printability, dual colors, heat-seal ability, low cost, and adhesion. A primary application for coextruded film is food packaging, including meats, cheeses, and cereal. Other items packaged in coextruded film are agricultural supplies, medical products, and electronic components [2]. Coextrusion increases the cost and complexity of a blown film line considerably. Significantly higher hardware costs, increased operator training, and more sophisticated machinery and control systems are some of the demands of a coextrusion line. However, the gains resulting from increased product capabilities make most investments in coextrusion profitable. Improved film performance leads to increased market share and may open new markets. Also, material costs may be decreased through the use of thinner layers due to higher product performance characteristics.

5.1

Dies

As discussed in chapter 3, there are two main types of coextrusion dies: concentric and stack. Though both of these types are used in industry and most die manufacturers have a preferred design, there are several factors to consider with regard to the use of coextrusion dies. Most of the important factors regarding coextrusion die design have to do with the flow of the polymers through the die. In general, it is best for polymer molecules to have as low a residence time in the die as possible. This will minimize degradation. Also, a welldesigned die will yield a narrow residence time distribution, meaning that most of the polymer molecules will spend the same amount of time in the die. Minimal pressure drop through die flow channels is advantageous also. This will help minimize the total head pressure needed to be generated at the end of the barrel. Another consideration is the internal flow length after which two or more polymer molecules have joined together. Many polymer types cannot flow together for very long without experiencing penetration across their interface (as discussed in Section 5.2) or undergoing detrimental heat transfer to the lower temperature material. Also, the ability to thermally isolate polymer grades with different melt temperatures is a design feature to consider. Finally, if an extrusion line will be changed-over frequently, the ease with which different coextrusion structures can be set up must be considered.

5.1 Dies

99

Both concentric and stack dies possess a mix of positive and negative characteristics with regard to the above list of important factors. It is best to identify the key material (such as viscosities and melt temperature,) and processing (such as floor space and the need for layer rearrangement) characteristics of a specific coextrusion system and discuss these with various die manufacturers. However, some general benefits of the two die types are that concentric dies tend to have minimal flow length of conjoined layers, and stack dies are easier to reconfigure and tend to have better layer temperature isolation. Dual Spiral Systems, Ontario, Canada, recently developed a novel approach to economically gaining the benefits of increased film layers [39]. The new die (Fig. 5.3) contains a flow-splitting device in each melt stream that doubles the total number of layers in the final film structure. That is, every flow stream is divided into two adjacent streams of identical polymer that have passed through opposite spiraling subchannels. The resulting

Nominal diameter

E D C Side-fed inlet

B A

New concept for a flat plate die Figure 5.3 A coextrusion stack die utilizing a flow-splitting device in each melt stream (Dual Spiral Systems)

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5 Coextrusion

film structure is said to possess improved mechanical properties, reduced pinholing, and increased film thickness uniformity. Further, the system is said to provide reduced gauge distribution in layer pairs because of a matching of high and low spots, yielding improved barrier performance.

5.2

Interfacial Instabilities

An interfacial instability causes distortion of the streamline where two layers of a coextruded film meet (Fig. 5.4). This can affect both the performance and visual properties of the film. Defects such as thickness inconsistencies, reduction in clarity, and even delamination of layers can result. Many studies have been conducted to understand this phenomenon [40–45] without complete agreement regarding its cause, but the most generally accepted explanation for these instabilities is excessive shear stress at the interface. As different polymer streams flow through the die channels and out of the die, they may have different flow behavior. When the stress at the layer interface exceeds a critical value, the differing flow properties of the individual components create the distortion. This is generally material related, but it can be related to the geometry of the converging flow channels as well. There are several factors that may affect the severity of any instability. The percentage that each layer comprises of the total film thickness can influence flow stability. Several studies have shown that the onset of instability correlates with the layer ratio, even when the two polymers at the interface are identical. Also, the flow length within the die after different layers join together has an effect. Generally, minimizing the conjoined flow length is beneficial. Finally, the rheological (or flow) behavior of the individual layers is probably the most significant factor. Several studies indicate that the extensional viscosities of the materials correlate more with interfacial instability than the shear viscosities.

Figure 5.4 Coextruded film showing interfacial instability

5.2 Interfacial Instabilities

101

A technique that has shown potential in eliminating interfacial instability is the use of fluoropolymer-based processing aids. These same additives are used to eliminate melt fracture in blown film extrusion. In one study [46], the incorporation of a processing aid into the skin (outer) layer of a three-layer coextrusion suppressed formation of the instability.

6

Film Properties

This chapter covers many of the most important properties measured by producers of blown film. These include mechanical, thermal, optical, physical, electrical, and rheological. The first five in this list apply primarily to the extruded film and the last one applies to the molten polymer inside the extruder and die. By obtaining the measurement values for these properties, manufacturers gain assurance that their resin or film will perform adequately, whether during manufacture (extrusion or conversion) or in final product form as used by the customer. It is vital that manufacturers document and maintain baseline data on the performance of their incoming materials and the film that they produce. This information provides the most efficient means for solving many problems that may arise in a manufacturing plant. Many extrusion performance issues are related to even slight modifications in raw material composition or processing properties. These modifications are easily identified when prior baseline data is available. Additionally, film performance deficiencies as measured by a reduction in some property, such as impact strength, may lead the technical staff to identify an undesirable drift in processing conditions. Another important reason for maintaining baseline data is that many customers require it. Tests for resin and film property values can be performed in-house, by an outside testing facility, or by a supplier. There are advantages and disadvantages to each. The most important consideration is the accuracy of the data collected. After that, consideration must be given to costs associated with the amount of data needed (equipment, consumables, training, labor, etc.) and the frequency of measurement required. To suitably compare measurement values obtained at different times or locations, there must be assurance that the tests were performed under identical conditions. To accomplish this, a committee of experts in each particular subject area develops standardized test methods. The methods establish the exact conditions for all test parameters by all parties conducting tests to measure the property of interest. A major publisher of test methods covering polymer resins and plastic films is the American Society for Testing and Materials (ASTM), West Conshohocken, PA, USA. In this chapter, ASTM method reference numbers are included in parentheses at the beginning of each property description section.

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6 Film Properties

Although publicly published test methods are invaluable for comparing measurement results, not all tests must conform to published standards. In many cases, processors, their suppliers, or their customers will design and perform customized tests in an attempt to best simulate actual product conditions. An example would be a grocery sack manufacturer that performs a routine quality check by loading a finished bag with a specified weight and then dropping the load onto the floor from a specified height to check for bag failure. Many in-house tests like this are designed and conducted regularly to best model actual use, shipping, or handling conditions. It is crucial, however, that test procedures are documented and followed identically every time so that results comparisons are valid. The following properties relevant to blown film processing comprise the main sections in this chapter: • Tensile strength • Elongation • Tear strength • Impact resistance • Blocking load and coefficient of friction • Gel (fisheye) count • Low temperature brittleness • Gloss • Transparency • Haze • Density • Melt index • Viscosity by capillary rheometry

6.1 Tensile Strength (ASTM D882)

6.1

105

Tensile Strength (ASTM D882)

Tensile strength, also known as tensile stress, provides data regarding the load carrying capability of the film. It is measured on a universal testing machine (Fig. 6.1) that pulls the ends of a rectangular strip at a specified rate in opposite directions until it breaks. Specialized roller-type grips (Fig. 6.2) are generally used to hold thin films instead of the jaw-type grips used for rigid plastics. Specimen strips cut in both the machine and transverse directions (MD and TD) of the film are usually tested. During the test, an in-line load cell measures the force carried by the film at all times during stretching. Tensile strength is reported as the force at a given point divided by the original crosssectional area (width multiplied by thickness) of the test specimen. The value of tensile strength may be reported at any point during the test, but the points most often chosen are the yield point (where the force in the film begins to drop after an initial rise), the highest point achieved (ultimate, which may or may not be at the yield point), and the break point.

Figure 6.1 A universal testing machine used to measure tensile properties (Tinius Olsen)

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6 Film Properties

Figure 6.2 Self tightening roller-type grips for tensile testing thin film (Ametek Inc.)

The actual tensile strength of a given film specimen is dependent on many factors. Most important is the raw material from which it was produced. Different polymer resins have different inherent strengths. Even different grades of the same polymer vary in strength. Additives and fillers included in the film with the polymer can have a significant affect on strength. Some additives, such as glass fiber, are included specifically to reinforce (add strength to) the film, while other additives may be primarily to reduce cost but still influence final strength. Another significant factor affecting tensile strength is the set of processing conditions used to extrude the film. Almost every extrusion variable has an influence on the solidstate properties of blown film. Temperature settings and screw speed work together to add heat to the molten plastic. Excessive heat can cause significant degradation to the polymer, thus reducing all mechanical properties. Even when degradation is negligible, the amount of stretching applied to the film in MD and TD strongly affects polymer orientation, hence tensile properties. In general, as the degree of stretching in a given direction (take-up ratio in MD or blow-up ratio in TD) increases, the molecular orientation and tensile strength in that direction increase. Even the frost-line height

6.3 Tear Strength (ASTM D1004, ASTM D1922 and D1938)

107

can affect tensile strength. The amount of time the polymer takes to cool influences the final crystalline structure, an important contributor to tensile properties. Finally, the test conditions themselves can influence the measured tensile strength of a specimen. For this reason, it is crucial to closely follow standardized test methods. Room temperature and strain rate used during a test will affect the data. Also, any defects in the test specimen or nicks produced during specimen cutting will lead to erroneous results.

6.2

Elongation (ASTM D882)

Another tensile property covered by the same ASTM method as that for tensile strength is elongation. This property describes the ability of the film to stretch prior to breaking or yielding. A material that stretches significantly prior to the break point is called a ductile material while one that breaks after only a small degree of stretching is called brittle. Most applications for blown film, particularly of polyethylene, require ductility. Elongation is generally reported as a percentage and is calculated by dividing the extension achieved in the specimen at the break (or yield) point by the original specimen length and multiplying by 100. Like tensile strength, elongation is highly dependent on raw material type and composition, extrusion processing conditions, and tensile testing conditions. For the case of processing, elongation tends to be inversely related to process stretching, hence molecular orientation. Though increased process stress (orientation) in a given direction yields higher tensile strength, it usually results in lower elongation.

6.3

Tear Strength (ASTM D1004, ASTM D1922 and D1938)

Tear strength is an important property measured for quality control by blown film processors. There are various methods of measuring tear strength in film, as seen by the multiple ASTM methods listed above. These focus on the resistance to either the initiation of a tear (rupture) or the propagation of an existing tear.

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The most popular method used for quality control uses a pendulum device (Fig. 6.3) to measure the resistance to tear propagation (ASTM D1922). For this test, specified test samples are die cut and include a slit at one end. The tabs formed on either side of the slit are clamped into the device so that when the pendulum is released a so-called trouser leg tear force is applied along the specimen in the direction of the slit. The test device reports the maximum resistive force carried in the film. Commonly, ten specimens in both the machine and transverse direction of the film are tested. As with all film mechanical properties, molecular orientation imparted during the bubble-forming process has a significant influence on tear strength. For this reason, quality control tear test data is often gathered soon after film production and the results relayed back to extrusion personnel, particularly when changes to the process are necessary to increase tear test results. In general, tearing loads propagate most easily in a direction of high molecular orientation, effectively tearing down between the aligned molecular chains. Therefore, low tear test values in the machine or transverse direction can often be improved by increasing process stress (orientation) in the perpendicular direction.

Figure 6.3 A pendulum tear tester (Thwing-Albert)

6.4 Impact Resistance (ASTM D1709, D3420 and D4272)

6.4

109

Impact Resistance (ASTM D1709, D3420 and D4272)

In many film applications the product is subjected to puncture or impact loads. These types of loads are applied perpendicular to the plane of the film. Therefore, the stresses act biaxially (in both the machine and transverse directions simultaneously) and are not represented well by a uniaxial test. Impact resistance tests are designed to biaxially load the film to measure its energy absorbing capability. Several impact tests are designed for films. They differ in the types of test machines used (falling dart vs. pendulum) and the target energy measurement (failure initiation vs. total absorption capability). In one popular method (D1709), a five-inch diameter circular film specimen is clamped into the base of an apparatus. Then a blunt, metal dart is dropped onto the specimen from a specified height. If the specimen fails, some weight is removed from the dart and the test is repeated with a new specimen. If the specimen does not fail, some weight is added to the dart and the test is repeated. This process continues until a statistically valid number of specimens are tested resulting in the nominal weight to failure. At that point, the impact failure energy can be determined. It has been shown for this procedure that friction between the dart surface and the film specimen can have a very strong affect on test results (Fig. 6.4). High friction leads to negligible deformation under the dart and a resulting uniaxial load down the sides of the impact pocket of film. Erroneously high energy values result. A proper test will allow for biaxial deformation (slipping and stretching) of the film under the dart surface. This is best achieved by lubricating the dart tip or film. Using a light coating of powdered polyethylene on the film works well.

Figure 6.4 Photograph showing how friction affects dart impact test on film. Left side shows film lubricated with powder deforms biaxially, while right side shows a more uniaxial deformation when there is high friction

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Impact resistance usually improves with increased processing stress (orientation), especially when the forming ratio approaches a value of one. When the film is produced with high, isotropic (balanced) uniaxial properties, impact resistance is usually maximized. Highly anisotropic film tends to be “splitty”, resulting in low impact values.

6.5

Blocking Load (ASTM D3354) and Coefficient of Friction (ASTM D1894)

Blocking is the term used for the tendency of two pieces of film to stick together, such as after the two sides of a blown film bubble are pressed together through the nip rollers. Blocking in films can lead to difficulties in handling and conversion or can simply be an inconvenience to customers. Many processors use an antiblock additive during blown film production to minimize this tendency. The standard blocking load test (ASTM D3354) utilizes two aluminum blocks with two layers of film pressed between them (Fig. 6.5). One film layer is attached to the upper block and the other layer to the lower block. The upper block is pulled away from the lower block with a small initial force that increases constantly over time. When a specified separation distance is achieved between the two layers of film, the separating force is recorded. While the blocking load test measures the force to separate the films perpendicular to their plane, a different method is used to measure the frictional forces required to slide one layer of film over another (ASTM D1894). In this test, a small sled drags one layer of film over another. A load cell measures the force required to initiate a sliding motion (related to the static coefficient of friction) and maintain a sliding motion (related to the kinetic coefficient of friction).

Film layers Figure 6.5 Apparatus for standard blocking load test

Aluminum blocks

6.7 Low Temperature Brittleness (ASTM D1790)

6.6

111

Gel (Fisheye) Count (ASTM D3351 and D3596)

Gels (also known as fisheyes) are small, hard globules sometimes seen in film that are aesthetically unpleasing and can act as stress concentration points. They can be incorporated into first generation resin or created during the extrusion process by overheating or flow stagnation. Gels are made up of highly degraded particles or very high molecular weight (crosslinked) regions, but sometimes look similar to moisture bubbles or unmelted resin. The standard test method (ASTM D3351) for quantifying gel content in film utilizes an overhead projector to magnify film specimens. Samples of specified size are cut then illuminated and magnified with the projector so that the operator can easily count the number of gels in the test area. Because polyvinyl chloride degrades so rapidly, a special method (ASTM D3596) is used for this material that includes a sample preparation technique utilizing a two-roll mill.

6.7

Low Temperature Brittleness (ASTM D1790)

On occasion, products manufactured from blown film are used at very low temperatures. For example, the NASA Scientific Balloon Program sends polyethylene research balloons through the troposphere, which may reach temperatures below –80 °C. Therefore, it may be important to know the temperature below which film will exhibit brittle behavior. This value is measured using a low temperature brittleness test. There are various methods for performing this test. In the ASTM standard, a specimen comprised of a loop of film is subjected to a hammer impact load. The test is repeated over a series of temperatures and the impacted specimens are examined visually to determine the temperature below which failure occurs in a brittle mode. Another type of test, which evaluates brittleness temperature in a similar manner, utilizes a specimen of film that is clamped onto a table over an oval opening and inflated with air until failure. Because of the shape of the opening, this is known as a racetrack test.

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6.8

Gloss (ASTM D2457)

Optical properties provide a measure of important characteristics of film performance, including aesthetic appeal. Gloss provides a measure of how shiny a film surface appears. Surfaces that reflect more light (i.e., are more mirror-like) have a higher gloss value. Those that absorb more light have a lower gloss value. Gloss measurements are also affected by the smoothness and flatness of the film surface. The standard test method utilizes a light source directed toward the sample surface at a specified incidence angle (e.g., 60°). The angle used depends on whether it is a highor low-gloss surface. The quantity of reflected light is measured by a photosensitive receptor and correlates to the gloss value of the film.

6.9

Transparency (ASTM D1746)

Transparency provides a measure of the clarity of a film sample. A transparent film can be read through even when the writing is some distance beyond the film. This is in contrast to a translucent film that allows light to pass through, but is somewhat cloudy in appearance and so cannot be read through. Transparency is a property that highly depends on the degree of crystallinity in the polymer resin. It is common practice to increase the film’s cooling (quench) rate during solidification in an attempt to minimize crystallinity, hence improve clarity. Transparency tests provide a measure of the amount of incident light that passes through a film sample without being scattered. Films with higher values are called transparent; films with lower values are called translucent. In the standard test method, a very narrow beam emitted from a light source is shone on a specimen. A receptor on the opposite side of the specimen measures the amount of light that both passes through and stays within the narrow beam angle. For translucent materials some light passes through the film but is scattered outside of the beam and is not measured by the receptor. Transparency correlates with a high percentage of emitted light reaching the receptor.

6.11 Density (ASTM D1505)

6.10

113

Haze (ASTM D1003)

Haze is a measure of the amount of light that is scattered as it passes through a film. In the standard test method, a hazemeter is used to measure the amount of light scattered outside of a specified beam angle. The meter utilizes the inside surface of a sphere to collect and quantify the amount of scattered light. The greater degree of light scattering, the higher the value of percent haze.

6.11

Density (ASTM D1505)

Density is an inherent material property defined as the mass of a specimen divided by its volume. For all materials, it is a strong function of temperature because temperature changes affect molecular spacing (volume). For solid-state polymers, particularly polyethylene film, the specific value of density is highly dependent on the degree of crystallinity in the specimen. A higher amount of closely packed crystalline regions lead to higher density. In fact, what differentiates high-density polyethylene from low-density polyethylene is primarily the amount of crystallinity. Density testing of blown films is performed for a variety of reasons. Quality control testing can provide assurance that films were processed under proper conditions, since slight variations in heating, cooling, melt stress, etc., can lead to changes in crystallization, hence changes in density. Also, deviations in material composition from mixing or feeding issues can be readily detected through changes in product density. Finally, density testing provides a rapid technique for differentiating between different polymer types, so it is often used for material identification studies. The standard test method utilizes a density gradient column (Fig. 6.6), which is a column of liquid residing in a large test tube. The liquid is actually comprised of two liquids of different density poured into the tube. The bottom of the column has the highest density, and the top has the lowest. The density decreases linearly from the bottom to the top of the column. Beads of various known densities (to four decimal places in g/cc) are added to the column and float at their specific density levels. Thereby a linear calibration curve can be created of density vs. column height. When a specimen of unknown density is added to the column, it will come to float at its density level, which can be determined from the calibration curve. Setting up the column requires experience and patience. However, once the column is established, it provides a quick and easy method of obtaining accurate density measurements.

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Heavy liquid

Light liquid Temperature controller

Temperature-controlled water jacket

Beads of known density

Figure 6.6 Density gradient column comprised of a heavy liquid at the bottom gradually reducing in density to a light liquid at the top

6.12

Melt Index (ASTM D1238)

Rheological data provides information about how a melted material will perform (flow) under processing conditions. Perhaps the most important rheological test used by plastics processors is the melt index test. Though rheologists scoff at this notion, it is nevertheless vitally important because virtually every manufacturer that employs an extruder, injection molder, or blow molder uses the test method or the data it yields. Incidentally, a true rheologist has a right to scoff! The melt index test provides only the smallest amount of useful data from a vast array of information that the field of rheology can provide about a material. Further, the small amount of data yielded by a

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melt indexer can be misleading. Other techniques, such as capillary rheometry described below, provide more useful information. While the melt indexer has its limitations, it provides a single value representing the subject material in a matter of minutes with very little operator training and at very low cost. These are attractive benefits to the manufacturer that require only a little insight into how their material is going to perform on the extrusion line. The melt index value has units of grams per 10 minutes. Therefore, it is a measure of how many grams of melt extruded from the test machine in 10 minutes. The higher the melt index of a material, the lower is its viscosity (i.e., it has a lower resistance to flow) at the test conditions. The melt indexer used for the standard test method (Fig. 6.7) is essentially a small, vertical ram extruder. An insulated barrel is heated to a temperature specified for the test material. Then, an approximately 5 g charge of material is added to the barrel and allowed to preheat while a piston rests on top of the charge. After the melting time (usually six minutes), a specified weight is placed on the piston and melted polymer is pushed by gravity through a standard size orifice at the bottom of the unit. The extrudate is cut by the operator and a timer is started. After a predetermined amount of time, depending on the flow rate of the material, a second cut is made and the timed extrudate is weighed. This allows for any test time where the extruded weight is multiplied accordingly to report the result in grams per 10 minutes. Practically all resin suppliers provide a melt index value with every lot of material sold and most manufacturers use the melt index (either as reported by the supplier or as measured in-house) as both a check on incoming resin and a guide for processing conditions. Slight variations in the melt index (a few tenths) of incoming resin are usually

Weight Barrel Temperature control jacket

Orifice

Plastic extrudate

Figure 6.7 A melt indexer, also known as an extrusion plastometer

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easily accommodated by minor adjustments to process variables such as barrel and die temperatures. A higher melt index (lower viscosity) material will often process better at slightly reduced operating temperatures. The opposite is true for lower melt index materials. Large variations in melt index, however, should warn extrusion personnel of potential difficulty in processing, perhaps even an inability to process the material on plant equipment. A word of caution: as mentioned above, melt index values are potentially misleading. First, know the temperature and weight used to obtain any test data because they have a direct influence on the results. When comparing the melt index of different materials or lots, one must be sure the test conditions were identical. Also, it is important to remember that the melt index test represents only one, very low, shear rate condition. Inside of an extruder, polymer melt is exposed to many different shear rate conditions – some substantially higher than in a melt indexer. Because of these different shearing conditions and the sensitivity of polymer viscosity to shear rate, melt index data does not necessarily correlate well with actual extrusion conditions. Further, two materials may behave identically in a melt index test, but behave very differently on an extrusion line. For these reasons, it is sometimes best to obtain rheological data over a range of shear rates using an instrument such as the capillary rheometer described in the next section.

6.13

Viscosity by Capillary Rheometry (ASTM D3835)

The rheology of a polymer melt is somewhat complex. Because its flow behavior depends on many conditions, including temperature and shear rate, defining the rheological behavior of a polymer with a single number obtained at a single test condition is often inadequate for designing process hardware (such as screws and dies) or troubleshooting process problems. A more complete test is required for those occasions. Capillary rheometry is the most popular test for comprehensively defining the rheology of a polymer melt to be used in extrusion. A capillary rheometer (Fig. 6.8) is similar to a melt indexer. The main difference is that instead of by gravity, the piston is driven by a variable speed motor. Also, a load cell in-line with the piston measures ram force in real time. This configuration allows tests to be run at a controlled shear rate, and even to vary the rate over a large range during a single test run. The piston force and the orifice geometry provide the data necessary

6.13 Viscosity by Capillary Rheometry (ASTM D3835)

117

Figure 6.8 A bench-top capillary rheometer

to calculate the shear stress in the material. Including the ram speed allows calculation of the shear rate. Because viscosity is defined by the shear stress divided by the shear rate, it can be determined at all test conditions. Most polymers are shear thinning, so a capillary rheometer provides a quick and easy technique for measuring both the viscosity and degree of shear thinning of a polymer melt. In Fig. 6.9, typical data from a capillary rheometer is shown for varying shear rate runs over a series of temperatures. One of the reasons for the popularity of melt index data is that rheometer data is significantly more expensive. Because of the higher cost of rheometry equipment and the advanced training needed by operators, most plastics manufacturers do not have a rheometer on site. Nonetheless, the data obtained from a rheometer is quite valuable to processors, so a technique has been developed to use an extruder to obtain more complete rheological data than is available from the melt index test.

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Capillary rheometer data 1000 Viscosity [Pa . s]

Temperature

100 10 1 10

100

1000

10000

Shear rate [1/sec]

Figure 6.9 Capillary rheometer data showing viscosity vs. shear rate for three temperatures

A paper entitled “Obtaining Flow Properties Directly from an Extruder” [47] details a technique for measuring rheometer-type data using extruders on the plant floor. It describes how extrudate samples, collected from an extruder in a matter of minutes, are used to measure throughput. These measurements are then used along with processing values such as screw speed and head pressure to calculate the polymer’s viscosity and power law index.

7

Troubleshooting

This chapter provides troubleshooting suggestions for some of the most common problems encountered in blown film extrusion. It is designed as a quick reference to help the reader identify causes and potential solutions. In many cases, additional details regarding the problem may be found in an earlier section of this book that covers the particular issue. Troubleshooting the Extrusion Process by Noriega and Rauwendaal [48] provides a more comprehensive coverage of the discipline of extrusion troubleshooting, including data collection methodology and case studies. The chapter is divided into two main categories related to blown film production: extruder and film problems. The main sections covered in this chapter are: • Extruder Problems – Surging – High melt temperature – Excessive cooling – Low output • Film problems – Melt fracture – Thickness variation (including bubble instabilities) – Die lines – Gels – Low mechanical properties – Poor optical properties

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7.1

Extruder Problems

The problems discussed in this section are related directly to the extruder and, so, are applicable to most any type of single screw extrusion operation. The significant influence these problems have on production efficiency and the maintenance of specified product dimensions and properties during blown film extrusion earns their place in this section.

7.1.1

Surging

Surging is a condition where the total flow rate (output) through the die opening fluctuates with time. It is most easily observed as large, rapid variations in the head pressure measured at the end of the barrel. Sometimes, surging can be identified by a sudden loss of motor amperage. The fluctuations are generally ±15% or more of head pressure and follow a random pattern. The timeframe between high and low pressure varies and is on the order of seconds to tens of seconds. Surging can be differentiated from screw beat, which is a variation usually less than ±10% of head pressure and matches the time period of screw rotation. Screw beat pulses tend to be dampened through the breaker plate and die. Surging, however, is most problematic due to the thickness variations it causes in the film’s machine direction. There are two primary causes of surging: feeding instabilities and melting instabilities. When the feed material enters the throat and moves along the screw erratically, the effect is translated downstream through the melt to the extruder head. This can easily happen when material sticks to the screw in the feed section. In an efficient system, solids will tend to stick to the barrel walls and be pushed forward by the screw flight, slipping without difficulty along the screw root. The important characteristic here is the ratio of barrel friction to screw friction, which should be high for good solids conveying. However, a hot, unclean, or damaged screw provides opportunity for feed material to stick, increasing the screw friction. Also, variations in feed shape, such as with high regrind content, result in variations in melting rate, which can cause early melting (and sticking) of solids on the screw. When there is material sticking to the screw, for whatever reason, it will have a tendency to pause and then be forced down the channel as pressure builds behind it, leading to pulsations in the system. For blown film extrusion, particularly with polyethylene, the most common technique for preventing variations in feeding is to use a grooved feed throat. This component

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significantly increases the friction between the barrel and feed material, improving both feed stability and overall system throughput. Other solutions for feed instabilities include ensuring a smooth screw surface and using more consistently shaped feedstock. The screw surface should be kept clean of all degraded material and will perform better if it is surface treated to reduce friction (e.g. chrome plating, Teflon impregnation, etc.) and polished. Consistency of feedstock shape will result in more uniform melting, reducing the chance of material sticking to the screw. Melting instabilities are the other primary cause of surging. As the bed of solid material moves down the screw, it should stay intact and melt uniformly. Under certain conditions, however, the bed will break apart and cause surging downstream. This effect is known as solid bed breakup. In addition to the output surging it causes, solid bed breakup results in a significant drop in average specific throughput (i.e., throughput per RPM). Solid bed breakup is generally associated with an inadequate melting rate of the solids, which leads to plugging of the screw channel. For example, the compression ratio of the screw may be so high that the volume available for material at positions further downstream is so reduced that the screw channel may not be able to accommodate both the remaining solids and the melt pool. In this case, the solid bed continues to be broken and dislodged. Solutions to solid bed breakup focus on increasing the heat energy into the solids. This may require a change in screw speed (it could be boosted to increase shear heating, or it could be reduced to increase residence time), a change in temperature profile (generally an overall increase), or perhaps a different screw design (barrier screws are designed specifically to inhibit breakup).

7.1.2

High Melt Temperature

As a rule, it is best to minimize the temperature of the extrudate. One reason is simply for economy: It costs money to put heat into the polymer, and it costs money to take it out. Therefore, it is best not to generate any more heat than is necessary. Another reason is that the removal of heat from the extrudate (bubble) as it travels through the cooling system is usually the rate-limiting factor. If the extrudate is cooler when it exits the die, then the line can be run faster, increasing productivity. Of course, this discussion must include the presupposition that the polymer has enough heat to protect both it and the equipment from damage during processing.

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The problem of high melt temperature can be separated into two categories: an original condition and a condition that developed over time. When the problem is an original condition (i.e., it has always been a problem), it is most likely due to an incorrect temperature profile or an inadequate screw design. All polymers require a certain minimum melt temperature for processing. When a barrel zone is set below this point, the shear energy from screw rotation may raise the temperature up to this value. In such cases, it is usually best, from a control standpoint, to place the temperature controller set points at those “natural” conditions. However, it is often the case that the screw design is not matched adequately to the processing conditions (material, head pressure, throughput). Then, it is possible to use a screw that will run the polymer at a lower melt temperature, often with the added benefit of increased throughput. A deeper pumping channel on the screw usually results in reduced shear heating and increased throughput. This works when the pressure gradient over the pump length is not too high, as is often the case with a grooved feed throat. When a high melt temperature develops over time, it is usually because of either a dirty screen pack or screw wear. As the screens do their job and filter contaminants out of the melt stream, they create a greater flow restriction that corresponds with increased head pressure. As a result, throughput decreases and the average residence time of polymer in the extruder increases. The increased residence time creates additional heating of the polymer. Replacing the contaminated screen pack should alleviate the problem. When it is not practical to perform many screen changes in a short time, such as when the change requires production shutdown, a different filtration system should be considered. Slide plate and continuous screen changers are used in many operations to minimize production disruptions. Screw wear is another common cause of high melt temperature. As the screw flight becomes worn and the clearance between the flight tip and barrel wall increases, specific throughput (throughput per RPM) decreases. This is because leakage of melt can take place over the screw flight, reducing its pumping capability. Also, a reduction in melting capability can occur from decreased shear rate between the barrel wall and solid bed. As clearance increases over time, operators tend to compensate for decreased throughput with upward adjustments in screw speed. The effect may be very gradual over long time periods. The result, however, is that more shear heating is applied to the material in the effort to gain the same output, leading to higher melt temperature. The best way to diagnose this situation is to maintain a regular production log of all measured process conditions, including throughput and screw speed. It is also important to perform periodic measurements of screw and barrel dimensions at positions along the screw length. These measurements can be made less frequently as the extrusion staff better understand normal screw wear behavior.

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Material change is another cause of high melt temperature development over time. Any change of material type or of composition ratio, even perhaps something as subtle as a lot change, can cause differences in the flow behavior inside the extruder. This may readily lead to a difference in shear heating or pressure generation that will increase melt temperature. Finally, high melt temperature may result simply from an incorrect temperature profile. Occasionally, one operator will make set point adjustments that are later adjusted by another operator, and so on. After time, the temperature profile in place may not be best for the present combination of material, hardware, and throughput. This is another reason that a regular log should be maintained of all process conditions. It provides the ability to diagnose problems by re-establishing conditions that performed acceptably.

7.1.3

Excessive Cooling

Practically all extruders have a barrel cooling system, such as air fans or water circulators, to remove excess heat from the polymer. However, it is best that the unwanted heat not be added to the polymer in the first place. Therefore, when the temperature controller in an extruder cooling system (or zone) calls for cooling often, or all the time, it indicates inefficiency in the system. In an efficient system the extruder will provide just the amount of energy necessary to heat the polymer to the desired melt temperature. This occurs through a combination of mechanical energy (screw friction) and conductive heat (heater bands, cartridges, etc.). It is most efficient if the screw provides the vast majority of this energy while it rotates to convey material, and the heaters – powered by an accurate temperature controller – provide the remaining amount. If the screw generates more than enough energy, then the controller will call for cooling. The causes of excessive cooling are essentially the same that cause high melt temperature (see previous section). The difference here is that overriding the set point in a particular barrel zone (thereby creating excessive cooling in that zone) may not necessarily lead to excessive temperature of the melt as it exits the die. For example, a zone in the middle of the barrel may override, yet the melt still exits the die at the desired temperature. This is still a problem, however, because of wasted energy generated in that particular zone. The solution will then depend on what is generating the heat there, such as an improper screw design.

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7.1.4

Low Output

The problem of low output generally refers to a reduction in specific throughput (throughput per RPM). In most cases, it is possible to increase the output simply by increasing the extruder screw speed. This may solve the problem at hand; however, increasing screw speed may have other detrimental consequences associated with it. So, the real problem here is when a process has deteriorated as measured by a reduction of material extruded per hour at a specific screw speed. The main causes of reduced output are increased flow restriction, commonly a result of clogged screens and screw wear. As screens perform their function properly and capture contaminants in the melt stream, they create an increased restriction to flow through the system. This increased restriction will result in higher head pressure. Additionally, there will be more recirculation of melt in the screw channel and less throughput. As discussed above in the section on high melt temperature, changing the screens should alleviate this problem. It is possible that other sources of flow restriction could exist, such as screens with an incorrect mesh size or a valve in the extruder head. Screw wear, most often resulting in increased flight clearance, reduces output in two ways. First, leakage of melt over the flight in the screw’s pumping section decreases the amount of polymer conveyed forward with each rotation. Second, a reduction in shear rate over the solid bed decreases the amount of solids that are converted to melt with each screw rotation. As discussed above, it is important to monitor and document screw wear over time. One potential way to differentiate between a contaminated screen problem and a screw wear problem is to check for notable increases in head pressure, which is associated with clogged screens. Low output can be caused by a few other sources as well. Any material, hardware, or process set point changes could readily result in decreased throughput. It is good practice to perform periodic throughput checks. One can easily collect output over a short time interval, such as two minutes, and record this value for four or five different screw speeds. By dividing throughput (weight per time) by screw speed (RPM), a specific throughput is obtained. This value can be used as a benchmark when process changes are made.

7.2 Film Problems

7.2

125

Film Problems

The problems discussed in this section are those observed in the film either visually or by a property measurement. In the case of visual problems (defects), it is good practice to maintain samples for extrusion personnel to refer to when judging the existence or severity of a problem and for training new personnel. For example, a notebook can be kept in the plant quality assurance area or supervisor’s office that contains samples of film with gels, die lines, melt fracture, etc. When a problem exists with a measured film property, personnel can perform a standardized test and compare the results to documented specifications for that product.

7.2.1

Melt Fracture

Melt fracture is an aesthetic defect that appears as roughness on the film surface. It has names such as orange peel and sharkskin. It may also appear as wavy lines in the film. Most researchers identify the source of these problems as excessive shear stress on the melt as it passes through the die so reducing this stress can eliminate this type of defect. Shear stress is the product of polymer viscosity and shear rate. So, any process change that will reduce either viscosity or shear rate is a potential solution for melt fracture. The simplest process modification is to increase the temperature of the melt as it flows through the die, thus lowering viscosity. Generally, increasing the die temperature is the best approach. Another common solution is to add an internal lubricant to the material composition, which may help in two ways. First, it may allow the polymer to flow with less internal resistance (viscosity); second, it will often leave a thin coating on the inside die surface, causing the material to flow through the die with less sticking. Reducing the shear rate to eliminate melt fracture can be accomplished a couple of ways, though neither is very easily implemented. The first method is to increase the die gap. Of course, this change would require an increase in line speed (increased take-up ratio) to achieve the same film thickness and bubble diameter. The other method is to decrease the flow rate through the die by decreasing the screw speed. This is not a popular solution with those responsible for meeting production goals!

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7.2.2

Thickness Variation

Perhaps the single most important objective for any blown film extrusion operator is to produce film that meets thickness specification. Many resources are devoted to ensuring that thickness measurements are in-specification across the film web and remain so throughout a production run. Variations in thickness can lead to product failure, downtime of conversion machinery, and large amounts of film scrap. Also, large thickness variations can significantly reduce profits through excessive material consumption. Thickness variations can be categorized as either time-dependent or position-dependent. For the case of time-dependency, thickness varies in the machine direction (MD) of the film, which may be caused by extruder surging (as described above). Also, temperature drifts on the extruder can lead to longer-term changes in the flow rate of polymer, hence thickness variations. It may be beneficial to document melt temperature on a chart recorder over several hours if this cause is suspected. Finally, bubble instabilities can result in MD thickness variability. Bubble instability is a condition where the shape of the bubble changes with time [49]. This can occur as steady oscillation in diameter or frost-line height, or as random movement of the bubble, sometimes called “snaking”. These shape changes result in variations in melt stretching below the frost line, which directly affects thickness. In some cases, the frost line is too high and providing additional cooling through increased blower air speed or decreased air temperature can stabilize the condition. In other cases, particularly when the proximity of the air ring to the bubble is very close, decreasing the cooling rate or increasing the polymer throughput can stabilize the condition. A comprehensive, practical overview of bubble instabilities is given by Waller in an article entitled “What to Do When the Bubble Won’t Behave” [50]. In it he identifies and describes seven different types of instabilities, shown in Fig. 7.1, and recommends remedies for each type. These will be briefly summarized here. “Draw Resonance” appears as a continuous variation in bubble diameter. As in other extrusion processes, such as profile, it occurs when the melt is stretched too quickly (i.e., a high take-up ratio). Solutions act to reduce the take-up ratio, for example increasing the melt (screw) speed. A “Helical Instability” (also known as “Snaking”) occurs when a bulge in the bubble appears to rotate around the circumference as it exits from the air ring. This is generally caused by the frost line being too low and not allowing adequate escape of cooling air around the bubble. Adjustments that increase the frost line, such as increased throughput, are used to solve this problem.

7.2 Film Problems

Draw resonance

Bubble tears

Helical instability

Frost line oscillation

Bubble flutter

127

Bubble sag

Bubble breathing

Figure 7.1 Seven different types of bubble instabilities according to Waller [50]

“Frost Line Oscillation” is seen as an up and down movement of the point where the bubble first reaches its maximum diameter. It can result from a few causes, such as varying extruder output (surging) and changes in the ambient condition around the bubble, such as drafts. Surging, as discussed earlier in this chapter, can be stabilized by improving solids feeding and melting. “Bubble Sag” is seen as the bubble expanding to its maximum diameter over a very short height. It results from inadequate cooling and solutions act to lower the frost line. “Bubble Tears” take place at the die lips and result when the stretching rate on the film is excessively high. When the film is drawn too fast or cooled too quickly, tears may occur. Potential solutions include increasing the die lip temperature and reducing the take-up ratio.

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“Bubble Flutter” generally occurs when the frost line height is low, causing the flowing air from the air ring to set-up vibrations in the base of the bubble. Solutions involve raising the frost line, for example, by decreasing blower speed. “Bubble Breathing” occurs when the air volume inside the bubble increases and decreases periodically. This is primarily a problem with internal bubble cooling (IBC) systems, since there is an internal air exchange occurring continuously. Solutions generally focus on checking IBC valves, blowers, and sensors. Finally, with regard to bubble instabilities, Jung and Hyun have developed models of the film blowing process in order to analyze bubble stability [51]. Perhaps their most interesting conclusions are concerned with the “multiplicity of steady states”. That is, as one variable in the process is changed, such as blow-up ratio, the bubble may move through states of being stable and unstable. As an example, they cite a case where an unstable bubble is stabilized by increasing the cooling rate. However, increases in the cooling rate above the value yielding stability lead to yet another unstable condition. Position-dependent thickness variation is when the thickness varies from one point to another on the circumference of the bubble (i.e., in the transverse direction, TD). This also may be caused by bubble instability, but more likely causes are a noncentered die, uneven cooling, uneven die lip temperature, and nonuniform exit velocity of the melt. A noncentered die is one in which the die ring (or bushing) that forms the outside surface of the bubble is not centered on the pin (or tip, or mandrel) that forms the inside surface of the bubble. This means that the die gap through which polymer exits has a large opening and a small opening. The large opening provides less flow restriction, so more polymer exits there than through the small opening in any given time. Consequently, the final film web will be thicker at the location corresponding to the large opening and thinner at the location corresponding to the small opening. All dies include an adjustment mechanism for centering. This should be set up with a brass feeler gauge as close to center as possible prior to a run, but fine adjustment is often required soon after beginning a run. A centered die will produce a bubble that is symmetrical around the vertical centerline. This assumes that other process variables, such as cooling air speed, are also equal around the bubble. Interestingly, there seems to be some confusion in industry about whether the large opening side in a noncentered die corresponds with the thick or thin side of the web. Figure 7.2 shows a noncentered die on top with the large opening at the 3 o’clock position (extruder feed is at 12 o’clock). This die has a nominal die gap of 0.040 in and is positioned here with a large opening of 0.060 in and a small opening of 0.020 in. The bottom of Fig. 7.2 shows how the bubble bulges at the position (right side of bubble) corresponding to the large opening in the die. This is expected since there is more heat

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129

at that position, and the film will stretch longer before it solidifies compared to the opposite side. Final film thickness measurements at these conditions showed the web in the figure to be approximately two times thicker (∼ 0.0030 in vs. ∼ 0.0015 in) at the position corresponding to the large die opening.

Figure 7.2 Shown on the left is a non-centered die with the large opening at the three o’clock position, while shown on the right is a bubble that bulges at the position corresponding to the large die opening, yielding thicker film at that position

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Uneven cooling around the base of the bubble can also cause TD thickness variations. When one position on the bubble has greater than average cooling, melt in that area will solidify sooner, hence not stretch as much and be thicker in the final film. This condition can be caused a few ways. Unbalanced cooling can occur when the hoses feeding the air ring do not all have the same flow restriction (e.g., length and diameter) or the same heat transfer (such as one hose near a heater band). Also, the internal baffling of the air ring should ensure an equal delivery at all points on the circumference. Some sophisticated thickness control systems can automatically adjust cooling air speed or temperature at many positions around the base of the bubble to ensure thickness uniformity. Uneven die lip temperature produces similar results to uneven bubble cooling. If one position at the die gap is hotter than other positions, the polymer will flow more easily through that area, yielding thicker film in that region. Again, this effect is exploited by some automatic thickness control systems that employ heaters embedded near the die lips to adjust flow rates by position in an effort to ensure thickness uniformity. A final cause for TD thickness variation is nonuniform exit velocity around the die gap circumference. If the melt exits the die slower than average at one position, the final film will tend to be thinner at the corresponding position in the web. This is because the pull of the nip rollers is the same at all positions. Some die designs, particularly older ones, do not lead to a uniform die exit velocity. Also, a die that contains material buildup in the lips may cause nonuniformity.

7.2.3

Die Lines

Die lines can be seen on the surface of the bubble running continuously in the machine direction. They are a problem because they reduce the aesthetic appeal of the film, diminish measurable optical properties such as gloss and transparency, and weaken mechanical properties such as tear strength. The most common cause of these lines is a dirty die. Over time, degraded material can stick to the inside surfaces of the die and build up as charred particles. As polymer flows through the die gap, it must flow over and around these particles. The flow pattern of splitting and rejoining creates weld lines in the flow field, leading to die lines in the film. Periodic disassembly and cleaning of the tip and ring will alleviate this problem. Another source of die lines is scratched or roughened die flow surfaces, particularly in the lips. A machinist can usually refinish flow surfaces.

7.2 Film Problems

7.2.4

131

Gels

Gels, also known as fisheyes, are small, hard globules encapsulated in the film or stuck on the film surface (Fig. 7.3). They can have the appearance of unmelted solid feed, but they may actually have been formed from completely melted material. (Sometimes unmelted solids do exit through the die.) A gel is comprised of degraded polymer of very high molecular weight, perhaps even crosslinked. Two main problems arise from the presence of gels. First, they are aesthetically unpleasing. Second, they may act as stress concentration points that result in premature product failure. Gels either originate from the incoming raw material or are produced during the extrusion process. On occasion, pellets supplied by a resin producer contain gels when shipped. However, this is unusual and can be quickly corrected. The resin supplier should be notified immediately if gels are identified in first generation pellets. More often, gels are the result of overheating (degradation) of the polymer during extrusion. This can take place due to an excessively high temperature profile, but the more common causes are excessive residence time and excessive mechanical energy input. When polymer hangs up on the screw or in a die, it may remain under high temperature for hours, days, or longer. Eventually, the degraded particles may break loose and exit with the melt as a gel. The term “gel shower” is used for an event where many gels have broken loose and exited at the same time. It is often possible to identify the hardware source of the hang up (such as a barrier flight on the screw or a nonradiused corner in a die) during disassembly and cleaning. Gels may also be associated with high mechanical energy input in the extruder. Grooved feed throats and some types of mixing elements impart a lot of energy on the melt. In those sections very high local temperatures may be generated quickly, leading to the formation of gels. In all cases of gels forming by polymer degradation, a potential solution is to include a processing stabilizer, such as an antioxidant, with the raw material. This additive acts to inhibit degradation from starting and propagating.

Figure 7.3 Gel or fish eye [48]

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7.2.5

Low Mechanical Properties

The most commonly measured mechanical properties for films are tensile strength, tear strength, and impact resistance. The first two are generally measured in both the machine and transverse directions (MD and TD). The last one is a biaxial test, examining MD and TD simultaneously. The magnitude of these properties in any film depends on several factors. The primary factor is the type of polymer used to make the film. All polymers can be ranked by their inherent strength. Some, such as nylon, are generally very high, and others, such as lowdensity polyethylene, are relatively low. Of course, one must always keep in mind that there are property trade-offs to make when evaluating materials to perform a job, and strength is not always the most important quality characteristic. Nevertheless, the base polymer is what primarily determines film strength. The next determining factor in film strength is composition of the material mix entering the extruder. While the base polymer establishes foundational properties, additives can be used to significantly modify them. Although some additives, such as antistat, have negligible influence on mechanical properties, many additives have a major impact on strength, impact resistance, and more. Reinforcements, for example glass fibers, are used specifically to increase properties such as tensile strength. Rubber additives improve impact resistance. On the other hand, some additives cause a deterioration of mechanical properties. Incorporating large concentrations of reprocessed material can significantly reduce strength due to the degradation of previously processed polymer. Although the material mix that the film is comprised of generally remains fixed through the majority of a product’s life, mechanical properties may still measure low on occasion. Assuming no change in material has occurred, the cause for this is process related. There are several ways in which the process influences mechanical properties, but perhaps the most significant influence comes from the molecular orientation imparted to the bubble by process stretching. As detailed in an earlier chapter, MD stretching via take-up and TD stretching via blow-up have a profound effect on the orientation of polymer chains and, as a result, on the strength and impact resistance of the film. It is possible, with practically any material, to significantly increase or decrease a given mechanical property by changing process stretching. Generally, as stretching in a given direction increases, tensile strength increases and tear strength decreases in that direction. High, balanced stretching in both directions maximizes impact resistance. There are several other process variables that affect mechanical properties. The frostline height, which is a measure of polymer cooling rate, has a large influence on crystal structure for semicrystalline materials such as polyethylene. (Process stretching also

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133

influences the degree and orientation of crystallinity.) Crystalline regions are denser and stiffer than amorphous regions, so the amount of crystallinity strongly affects tensile strength and impact resistance. Interestingly, in a production facility the extrusion operator generally can only adjust the frost-line height as a bubble geometry variable. This is because product specifications require that layflat width and film thickness remain fixed; therefore, blow-up ratio (BUR) and take-up ratio (TUR) must remain fixed, assuming that the die diameter and die gap remain constant. Of course, there are several methods for changing frost-line height, each having a different affect on mechanical properties: cooling air temperature and speed can be adjusted through the chiller and blower; nip speed and screw speed can be adjusted together to maintain a constant TUR; and melt temperature can be adjusted through the temperature profile. Additional process-related factors that affect mechanical properties are extruder residence time and die maintenance. Residence time refers to the length of time a polymer molecule remains at high temperature in the extrusion line. It is primarily related to screw speed, but depends on other factors such as screw design, die design, and head pressure. For fixed screw and die design, residence time decreases with increasing screw speed and increases with increasing head pressure. As the average residence time increases, the amount of polymer degradation also increases. This results in a reduction of mechanical properties. Therefore, optimizing a system includes consideration for minimizing residence time. Proper die maintenance is critical to ensuring acceptable mechanical properties. A common occurrence is to have degraded material buildup and stick to the inside of the die lips. As melt flows around these charred particles, the flow stream can divide and then rejoin. Although the film may appear intact, the resulting die line leaves a weak spot in the film. Because tears can begin and easily propagate along these lines, the film is sometimes called “splitty”. The best way to avoid this is to regularly clean the flow surfaces of the die.

7.2.6

Poor Optical Properties

Poor optical properties result from many causes that can be divided into two problem groups: material and processing. Two material problems that cause poor optical properties are contamination and phase separation of incompatible materials. Contamination occurs when foreign matter (such as dirt, degraded particles, or water) is in the extrudate, creating specks or streaks in the film. Phase separation of incompatible materials from improper blending of polymer types or including an incorrect additive can lead to a reduction in clarity.

134

7 Troubleshooting

Processing problems that result in poor optical properties include die lines, low cooling rates, melt fracture, and, for coextrusion, interfacial instabilities. Die lines, as discussed previously, are bothersome because they can reduce both mechanical and optical properties. The cooling rate has a significant effect on polymer crystallization. Because crystallinity in polymers generally reduces optical properties, cooling rates are often increased to improve film clarity. Melt fracture will usually have an effect similar to die lines with respect to optical properties. Any occurrence that creates texture on the film surface will reduce film transparency. Similarly, if the texture occurs at the boundary of two layers in a coextruded film, as happens with interfacial instabilities, the same reduction in optical properties will result.

Appendix A: The Blown Film Extrusion Simulator

A.1

Introduction

Included with this book is a companion compact disc (CD) for installation of The Blown Film Extrusion Simulator software on your computer. This software was developed to teach personnel about operating and controlling a blown film extrusion line [52]. Using realistic graphics and computer metaphors that allow most actual operating procedures to be modeled with mouse movements, the easy-to-use software is helpful for both training employees new to blown film extrusion and extending the system knowledge of experienced operators, technicians, and engineers. Throughout this book, there are many computer-based exercises to supplement the text by practicing on the virtual extrusion line in The Blown Film Extrusion Simulator. The reader is encouraged to take a break from the text when encountering these exercises to spend time practicing a skill or better comprehending a concept. These exercises are identified with the symbol •. In addition to the hands-on exercises provided throughout the book, this appendix includes a worksheet developed to successfully integrate the simulator into college-level plastics courses. By asking questions that move from basic principles to more difficult concepts, the worksheet will help readers use the simulator to develop a deeper level of comprehension of the system and its behavior. The Blown Film Extrusion Simulator is one of many reusable learning objects (RLOs) developed under the Plastics Resources for Educators Program (PREP, www.pct.edu/ prep). PREP is part of the Plastics and Polymer Engineering Technology program at the Pennsylvania College of Technology in Williamsport, PA, funded by the National Science Foundation under the Advanced Technological Education grant program.

136

Appendix

A.2

Installation

The Blown Film Extrusion Simulator runs on all Windows-based computers. To install the program, simply follow these steps: 1. Place the CD in the CD-ROM drive. 2. Locate and run the setup file (setup.exe) on the CD. 3. Follow the directions in the setup program. 4. Once setup is complete, run the program by going to Start → Programs → Blown Film Extrusion Simulator → Blown Film Extrusion Simulator. (See Fig. A.1) Note: with some computers, the system may need to be re-started after installation in order for the program to work properly.

Figure A.1 The Blown Film Extrusion Simulator program is shown installed in the simulator folder

Appendix A: The Blown Film Extrusion Simulator

137

The program is best used on a large color monitor set to a screen resolution of 800 x 600 pixels so the simulator will fill the entire screen. The program, however, will operate at any screen resolution.

A.3

Running the Program

An introductory splash screen appears (Fig. A.2) when the program is started. Clear the screen by clicking anywhere. The simulator is designed to closely model actual machine and operating conditions. Specifications for the simulated extrusion line are as follows: • 2-inch diameter extruder screw • 1 lb/hr/RPM specific throughput • 3-inch diameter annular die • 0.040-inch die gap • 0.91 g/cc solid density polyethylene

Figure A.2 The introductory splash screen

138

Appendix

When the program first begins, the conditions are similar to entering a plant prior to equipment start-up; therefore, all power to the system is off. Even with the power off the learner can learn to identify various machine components. Pointing the cursor at a component for two seconds will bring up an identification tag (Fig. A.3). Some tags include operational information in addition to the component name. Once the main power has been turned on by clicking the appropriate machine switch, the objective is to successfully create a blown film bubble from low-density polyethylene pellets (this polymer type is identified in the hopper). The learner must zoom-in on the extruder control panel and turn on all temperature controllers (Fig. A.4). Next, a valid temperature profile must be established for the polymer type being extruded by increasing the set point with the up button or decreasing the set point with the down button. If the drive motor is started and the potentiometer (screw speed control knob) is turned up with an invalid temperature profile, the system may shut down or even cause component damage! Note: All knobs, including the potentiometer, are turned up by left clicking and down by right clicking. Several extruder operating features are built into the simulator to help learners understand fundamental extrusion concepts. For example, with the extruder control panel visible the learner will observe that head pressure depends on both melt temperature and screw speed. By noting this dependence on melt temperature, the learner can make the connection between polymer viscosity, which is temperaturedependent, and head pressure. At a given set of operating conditions, the learner can increase the melt temperature (controlled by Barrel Zone 3) and see the head pressure decrease. With respect to screw speed, a nonlinear (shear thinning) relationship between viscosity and screw speed is designed into the program. As screw speed increases, the polymer experiences a higher shear rate, so its viscosity decreases. This is observed by doubling the screw speed and noting a less than double increase in head pressure, which is viscosity-dependent. Additionally, a less than double increase in motor load (drive amperage) is observed during the same adjustment. Another observation the learner will make about extruder operating features is that the melt temperature overrides the controller set point (of Barrel Zone 3 in this simulator) at very high screw speeds. This may be caused when the cooling capability is inadequate (Fig. A.5); this example may help the learner understand the concept of shear heating through screw-generated friction. After the reader has started the extruder screw with a valid temperature profile, extrudate begins to flow from the top of the die (Fig. A.6). The molten, high melt–strength polymer can then be dragged up the tower to the nip rollers. Dropping the melt onto the nip rollers will string the bubble, if the nip rollers have been turned on and set to a speed

Appendix A: The Blown Film Extrusion Simulator

139

Figure A.3 An identification tag pops up when the reader allows the cursor to pause over a component for two seconds

Figure A.4 Shown is the extruder control panel zoomed open with temperature controllers powered on

140

Appendix

Figure A.5 The melt temperature (376 °F) is shown here overriding the barrel zone temperature (370 °F) because of the excessive frictional heat generated at high screw speed (90 RPM)

Figure A.6 Extrudate is shown flowing from the top of the die to be stretched up the tower while stringing the bubble

Appendix A: The Blown Film Extrusion Simulator

141

greater than 0 ft/min. The nip rollers are operated by zooming in on the tower control panel (Fig. A.7). Note: both the extruder and tower control panels can be moved by dragging from the top center and will retain their settings when closed. The winder collects film on the wind-up roll (Fig. A.8). If the power to the winder is not turned on, film will pile up at the base of the tower.

Figure A.7 Shown is the tower control panel zoomed open

Figure A.8 The winder is shown with power off, so extruded film is piling up on the floor next to the tower

142

Appendix

Figure A.9 The internal bubble air valve can be opened to increase bubble diameter

Figure A.10 By cutting a hole in (clicking on) the bubble, air is removed and the diameter decreases

Once the bubble has been strung and film is being collected on the winder, there are four primary controls that establish bubble geometry: nip speed, cooling (blower) speed, screw speed, and internal bubble volume. Nip speed and blower speed are adjusted on the tower control panel. Screw speed is adjusted on the extruder control panel. Internal bubble volume is increased by opening the air valve mounted on the tower (Fig. A.9) and is decreased by cutting a hole in the bubble (Fig. A.10). All controls have reasonable minimum and maximum setting values and the resulting bubble dimensions have reasonable minimum and maximum limitations. For example, the bubble diameter cannot be larger than the width of the collapsing frame. For any set of processing conditions, the resulting bubble geometry is quantified in the measurements panel (Fig. A.11). Shown are film thickness in mil (1 mil = 0.001 in), bubble diameter (BD) in inches, layflat width (LF) in inches, and frost-line height in inches. Because of the strong interdependence of process variables, adjustment of any of the four primary controls will affect all three of the bubble geometry quantities: film thickness, bubble diameter, and frost-line height. Layflat width correlates with bubble diameter through the relationship 2 · LF = π · BD.

Appendix A: The Blown Film Extrusion Simulator

143

Figure A.11 The measurements panel shows values for the key bubble geometry characteristics

Each of the three geometric characteristics depends differently on each of the four control variables. For example, bubble diameter increases at a low rate with increases in nip speed, but at a much higher rate with increases in internal bubble air volume. By tracking values in the measurements panel with changes in the control variables, the various geometric dependencies can be identified. Once the reader has a good understanding of the interdependence of process variables, a specified set of bubble characteristics at a specified production rate can be efficiently established. This is an essential skill gained from the use of the simulator. The ability to produce film within specification at the desired production rate is perhaps the most important capability for those involved in film manufacturing. The worksheet in this section provides an example of how to use the simulator to learn to establish specified production conditions.

144

Appendix

A.4

Worksheet

Use The Blown Film Extrusion Simulator software to perform the tasks listed below. Record your responses on a separate piece of paper. Basic Questions (1–11)

1.

List 10 components in the system.

2.

Identify the polymer in the hopper.

3.

State the polymer’s melting point (see a polymer text or other reference.)

4.

State the mathematical relationship between layflat (LF) width and bubble diameter (BD).

5.

State the purpose of the blower motor. • Enter the following temperature profile (°F) and allow the system to equilibrate. Barrel

6.

Nozzle

Zone 1

Zone 2

Zone 3

200

200

200

200

Adaptor 200

Die Zone 1

Zone 2

200

200

What is the melt temperature? Try to turn the screw. What happens? Why? • Enter the following temperature profile (°F) and allow the system to equilibrate. Barrel

7.

Nozzle

Zone 1

Zone 2

Zone 3

330

350

350

350

Adaptor 350

Die Zone 1

Zone 2

230

230

What is the melt temperature? Try to turn the screw. What happens? Why? • Enter the following temperature profile (°F) and allow the system to equilibrate. Barrel Zone 1

Zone 2

Zone 3

330

350

350

Nozzle

Adaptor

350

350

Die Zone 1

Zone 2

350

350

145

Appendix A: The Blown Film Extrusion Simulator

• Turn the screw speed up to 10 RPM. (Note: left click to increase, right click to decrease) 8.

Record the melt temperature, head pressure, and motor current. What else do you observe? • String the bubble through the tower and be sure to collect the film on the winder. • Inflate the bubble with internal air.

9.

Record the film thickness, bubble diameter, layflat width, and frost-line height. (Note: control panels can be dragged from their top center) • Double the nip speed.

10. Record the film thickness, bubble diameter, layflat width, and frost-line height. Explain why each geometric variable changed from step 9 in the direction (increased or decreased) that it did. 11. Change any controls to produce film with the following specifications: film thickness = 1 (±0.1) mil, bubble diameter = 14 (±0.1) in, frost-line height = 7 (±0.1) in. For these conditions, record the nip speed, screw speed, and blower motor speed. Advanced Questions (12–20)

• Restart the drive motor, enter the following temperature profile (°F), and allow the system to equilibrate. Barrel

Nozzle

Zone 1

Zone 2

Zone 3

330

350

350

350

Adaptor 350

Die Zone 1

Zone 2

350

350

• Set the screw speed to 20 RPM. 12. Record the melt temperature, head pressure, and motor current. • Increase Barrel Zone 3 temperature to 400 °F. 13. Record the melt temperature, head pressure, and motor current. Describe why each of these values changed (increased or decreased) as they did from their values in step 12. • Return Barrel Zone 3 temperature to 350 °F. Double the screw speed to 40 RPM.

146

Appendix

14. Record the head pressure. Did it exactly double from step 12? Why or why not? • Increase the screw speed to 100 RPM and observe the melt temperature as the screw speed climbs. Repeat, if necessary. 15. What happens to the melt temperature as the screw speed increases above 70 RPM? Why? • Return the screw speed to 20 RPM. Set the nip roller speed to 35 feet/minute and the blower speed to 110 cubic feet/minute. String the bubble onto the winder. Establish an 8.0 in layflat. 16. Record the film thickness, bubble diameter, layflat width, and frost-line height. • The polyethylene solid density is 0.91 g/cc. 17. Using the conservation of mass at a point above the frost line (mass flow = solid density · bubble area · film velocity), determine the throughput in lb/hr. • The melt density is approximately 80% of the solid density. The die diameter is 3 in and the die gap is 0.040 in. 18. Using the conservation of mass at the die face, determine the melt velocity and the take-up ratio. 19. Determine the blow-up ratio and the forming ratio. 20. Compare the molecular orientation and the film tensile and tear strengths in the machine and transverse directions.

Appendix B: Useful Equations

147

Appendix B: Useful Equations

The following equations are useful for characterizing and troubleshooting a blown film process. Extruder Flow Rate (Newtonian)

This equation is used to calculate the volumetric flow rate from an extruder for a Newtonian material. It assumes that the head pressure is generated over the metering length of the screw. Q =αN −

where Q α β D H φ N ΔP μ Lm

β ΔP μ Lm

≡ volumetric flow rate [in3/s], [cm3/s] ≡ screw geometry constant = (π2/ 2) D2 H (sin φ) (cos φ) [in3], [cm3] ≡ screw geometry constant = (π /12) D H3 (sin φ)2 [in4], [cm4] ≡ screw diameter [in], [cm] ≡ metering channel depth [in], [cm] ≡ screw helix angle [°] ≡ screw speed [rev/s] ≡ head pressure [psi], [Pa] ≡ polymer viscosity [psi·s], [Pa·s] ≡ metering channel length [in], [cm]

Extruder Flow Rate (Power Law)

This equation is analogous to the previous equation, but includes correction for shear thinning materials. ⎛ 3 ⎞ β ΔP ⎛8 + 2 n⎞ Q=⎜ αN −⎜ ⎝ 10 ⎟⎠ ⎝ 1 + 2 n ⎟⎠ μ Lm where n ≡ polymer power law index μ ≡ polymer viscosity at screw speed N [psi · s], [Pa · s]

148

Appendix

Annular Die Flow Rate (Newtonian)

This equation is used to calculate the volumetric flow rate through an annular die gap for a Newtonian material. Q= where κ Ri Ro ΔP μ L

π Ro4 ΔP 8μL

⎡ (1 − κ 2 )2 ⎤ 4 1 − κ − ⎢ ⎥ ln(1/ κ) ⎦ ⎣

= Ri / Ro < 1 ≡ die tip radius [in], [cm] ≡ die ring radius [in], [cm] ≡ head pressure [psi], [Pa] ≡ polymer viscosity [psi·s], [Pa·s] ≡ die land length [in], [cm]

Annular Die Flow Rate (Power Law)

This equation is also used to calculate volumetric flow rate through an annular die gap, but includes correction for shear thinning materials. The correction factor is obtained graphically from Fig. B.1 [53] below. 1/ n

⎛ ΔP ⎞ n π Ro (Ro − Ri )2 +(1/ n) ⎜ Q= 1+ 2n ⎝ 2 K L ⎟⎠

F (n, κ)

where n ≡ polymer power law index K ≡ polymer consistency index [psi·sn], [Pa · sn] F(n,κ) ≡ correction factor found in Figure B.1 below

Figure B.1 Correction factor F(n,κ) for flow rate through an annular die using a power law material

Appendix B: Useful Equations

149

Melt Density (Estimate)

This equation provides an estimate of the polymer melt density based on the solid density. It can be used for calculations of mass flow rate. ρm ≈ 0.8 ρs where ρm ≡ polymer melt density ρs ≡ polymer solid density Mass Flow Rate (Volume Flow-based)

This equation is used to obtain the mass flow rate (throughput) when the volumetric flow rate is known. m = ρm Q where m ≡ mass flow rate [lb/hr], [kg/hr] ρm ≡ polymer melt density [lb/in3], [kg/cm3] Q ≡ volumetric flow rate [in3/hr], [cm3/hr] Layflat Width

This equation is used to calculate the layflat width if the bubble diameter is known. More often, however, the layflat width is measured, then the bubble diameter can be calculated by rearranging this equation. LF =

π (BD) 2

where LF ≡ layflat width [in], [cm] BD ≡ bubble diameter [in], [cm] Die Gap Area

This equation is used to calculate the annular open area through which polymer exits the die. 2 2 Ad = π (Rdo − Rdi )

where Ad ≡ die gap area [in2], [cm2] Rdo ≡ die ring radius [in], [cm] Rdi ≡ die tip radius [in], [cm]

150

Appendix

Bubble Cross-Sectional Area

This equation is used to calculate the annular cross-sectional area comprised of the film thickness in the bubble. 2 2 Ab = π (Rbo − Rbi )

where Ab Rbo Rbi tf

≡ bubble cross-sectional area [in2], [cm2] ≡ bubble outside radius = BD/2 = LF/π [in], [cm] ≡ bubble inside radius = Rbo – tf [in], [cm] ≡ film thickness [in], [cm]

Mass Flow Rate (Line Speed-Based)

This equation is used to calculate the mass flow rate (throughput) based on the line speed. The bubble cross-sectional area can be closely approximated by multiplying twice the layflat width by the film thickness. m = ρs Ab Vf ≈ 2 ρs (LF) tf (NS) where m ρs Ab Vf LF tf NS

≡ mass flow rate [lb/hr], [kg/hr] ≡ polymer solid density [lb/in3], [kg/cm3] ≡ bubble cross-sectional area [in2], [cm2] ≡ film velocity above frost line = NS [in/hr], [cm/hr] ≡ layflat width [in], [cm] ≡ film thickness [in], [cm] ≡ nip speed [in/hr], [cm/hr]

Extruder Throughput (Rough Estimate)

This equation is used to quickly obtain a rough estimate of extruder throughput. It uses values in units convenient to plant floor personnel. However, it neglects certain key variables, such as polymer viscosity and head pressure. The result is shown only in English units [lb/hr]. m ≈ 2 D2 N h do where m D N h do

≡ throughput [lb/hr] ≡ screw diameter [in] ≡ screw speed [RPM] ≡ metering channel depth [in] ≡ polymer solid density [g/cm3]

Appendix B: Useful Equations

151

Specific Throughput (Screw-Based)

This equation is used to calculate specific throughput of the extruder, which is often used as a measure of screw performance. msp =

m N

where msp ≡ specific throughput [lb/hr/RPM], [kg/hr/RPM] m ≡ throughput [lb/hr], [kg/hr] N ≡ screw speed [RPM] Specific Throughput (Die-Based)

This equation is used to calculate specific throughput of the bubble, which is often used as a measure of die and cooling performance. msp = where msp m cd DD

m cd

≡ specific throughput [lb/hr/in], [kg/hr/cm] ≡ throughput [lb/hr], [kg/hr] ≡ die gap circumference = π(DD) [in], [cm] ≡ die diameter [in], [cm]

Take-up Ratio

This equation is used to calculate the ratio of film speed to melt speed (i.e., take-up ratio). It provides quantification of the degree of machine direction (MD) stretching imparted on the melt by the process conditions. MD stretching is related to MD molecular orientation. Melt speed is difficult to measure, but its value is not required in order to calculate take-up ratio as described in the next section (Conservation of Mass). TUR = where TUR Vf Vm NS

Vf NS = Vm Vm

≡ take-up ratio ≡ film velocity above frost line [ft/min], [m/min] ≡ melt velocity through die gap [ft/min], [m/min] ≡ nip speed [ft/min], [m/min]

152

Appendix

Conservation of Mass

This equation states that the mass flow rate at any point in the system, such as through the die exit, is equal to the mass flow rate at any other point in the system, such as in the bubble above the frost line. It is shown in the general form and also rearranged to solve for the take-up ratio. m = (ρm Ad Vm)die = (ρs Ab Vf )bubble TUR = where m ρm Ad Vm ρs Ab Vf

ρ A Vf = m d Vm ρs Ab

≡ mass flow rate [lb/hr], [kg/hr] ≡ polymer melt density [lb/in3], [kg/cm3] ≡ die gap area [in2], [cm2] ≡ melt velocity through die gap [in/hr], [cm/hr] ≡ polymer solid density [lb/in3], [kg/cm3] ≡ bubble cross-sectional area [in2], [cm2] ≡ film velocity above frost line = NS [in/hr], [cm/hr]

Blow-up Ratio

This equation is used to calculate the ratio of bubble diameter to die diameter (i.e., blow-up ratio). It provides quantification of the degree of transverse direction (TD) stretching imparted on the melt by the process conditions. TD stretching is related to TD molecular orientation. BUR =

BD DD

where BUR ≡ blow-up ratio BD ≡ bubble diameter [in], [cm] DD ≡ die diameter [in], [cm] Forming Ratio

This equation is used to calculate the ratio of TUR to BUR. It provides quantification of the degree of balance of process stretching. As FR approaches a value of one, the process approaches equal MD and TD stretching, hence mechanical properties in MD and TD approach equality. However, it should be noted that molecular orientation is affected by other factors in addition to process stretching, and so FR is only a general guideline to the degree of balance.

Appendix B: Useful Equations

FR =

153

TUR BUR

where FR ≡ forming ratio Drawdown Ratio

This equation is sometimes used instead of the take-up ratio to provide an indication of the machine direction stretching. It represents the degree of thickness reduction from the die gap to the final film, both easily obtainable values. Of course, thickness reduction during processing occurs in the transverse direction as well as the machine direction. DDR = where DDR td tf BUR

td (t f ⋅ BUR)

≡ drawdown ratio ≡ die gap thickness [in], [cm] ≡ final film thickness [in], [cm] ≡ blow-up ratio

Film Thickness

By combining several of the above equations, this equation can be obtained to provide a convenient relationship for film thickness based on process stretching. tf ≈ where tf td BUR TUR

0.8 t d (BUR) (TUR) ≡ film thickness [in], [cm] ≡ die gap thickness [in], [cm] ≡ blow-up ratio ≡ take-up ratio

Linear Production

This equation is used to calculate the linear amount of film produced during a run. Lf = tr (NS) where Lf ≡ length of film produced [ft], [m] tr ≡ time of run [min] NS ≡ nip speed [ft/min], [m/min]

154

Appendix

Roll Weight

This equation is used to calculate the weight of film on a roll. It assumes the layflat is wound two layers thick. Wr = 2 ρs (LF) tf Lf where Wr ρs LF tf Lf

≡ film weight on roll [lb], [kg] ≡ polymer solid density [lb/ft3], [kg/m3] ≡ layflat width [ft], [m] ≡ single layer film thickness [ft], [m] ≡ length of film on roll [ft], [m]

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References

157

[38] A. Sukhadia, SPE ANTEC, Tech. Paper (1998) p. 160 [39] R. Castillo, SPE ANTEC, Tech. Paper (2001) p. 64 [40] K. Xiao, SPE ANTEC, Tech. Paper (2001) p. 362 [41] M. Martyn, T. Gough, R. Spares, P. Coates, and M. Zatloukal, SPE ANTEC, Tech. Paper (2003) p. 300 [42] J. Zyrd and J. Dooley, SPE ANTEC, Tech. Paper (1998) p. 217 [43] R. Ramanathan, R. Shanker, T. Rehg, S. Jons, D. Headley, and W. Schrenk, SPE ANTEC, Tech. Paper (1996) p. 224 [44] C. Tzoganakis, M. Zatloukal, J. Perdikoulias, and P. Saha, SPE ANTEC, Tech. Paper (2000) p. 46 [45] J. Vlcek, J. Perdikoulias, and J. Vlachopoulos, SPE ANTEC, Tech. Paper (1993) p. 3365 [46] M. Zatloukal and J. De Witte, SPE ANTEC, Tech. Paper (2004) p. 381 [47] C. Rauwendaal and K. Cantor, SPE ANTEC, Tech. Paper (2000) p. 117 [48] M. Noriega and C. Rauwendaal, Troubleshooting the Extrusion Process, Carl Hanser, Munich (2001) [49] T. Butler, SPE ANTEC, Tech. Paper (2000) p. 156 [50] P. Waller, “What to Do When the Bubble Won’t Behave”, Plastics Technology, (December 2002), p. 36 [51] S. Hatzikiriakos and K. Migler (Eds.), Polymer Processing Instabilities: Control and Understanding, Marcel Dekker, New York (2005) [52] K. Cantor, SPE ANTEC, Tech. Paper (2000) p. 341 [53] S. Middleman, Fundamentals of Polymer Processing, McGraw-Hill, New York, (1977) [54] F. Hensen (Ed.), Plastics Extrusion Technology, Carl Hanser, Munich (1997)

Subject Index

Index Terms

Links

A adapter

31

32

additive

14

50

57

60

82

83

101

106

132

133

adhesion

12

19

98

adhesiveness

19 9

74

75

76

85

87

126

128

11

13

45

air ring

130 air volume ammeter amorphous

2 41 7 133

amp

41

amperage

120

antiblock additive

110

antiblocking agent

15

antioxidant

15

138

60

131

1

12

14

17

30

37

41

77

barrier

13

98

cling

19

color

16

antistat

132

antistatic agent application

15

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

application (Cont.) commodity

1

high temperature

10

mixing

33

outdoor

18

5

ASTM

103

ASTM D1003

113

ASTM D1004

107

ASTM D1238

114

ASTM D1505

113

ASTM D1709

109

ASTM D1746

112

ASTM D1790

111

ASTM D1894

110

ASTM D1922

107

ASTM D1938

107

ASTM D2457

112

ASTM D3351

111

ASTM D3354

110

ASTM D3420

109

ASTM D3596

111

ASTM D3835

116

ASTM D4272

109

ASTM D882

105

107

80

81

axial (or center) winder

B bagmaking

60

balloon

1

bamboo

55

This page has been reformatted by Knovel to provide easier navigation.

Index Terms barrel

zone barrier

Links 25

26

27

29

30

31

32

37

39

40

42

44

62

122

13

98

100

9

13

18

65

121

30 12

flight property screw

131

bed

62

biaxial

89

stress block

109

132

6

15

104

110

70

92

106

128

133

152

74

75

87

126

128

133

142

109 15

blocking blow-up ratio

blower

borescope

30

bottom fed

65

branched

6

brass

35

breaker

33

plate

bridging brittleness bubble

66

31

32

47

120

27

44

33

40

62

104 2

cooling internal diameter

73

77

76

128

71

72

76

This page has been reformatted by Knovel to provide easier navigation.

79

Index Terms

Links

bubble (Cont.) 80

86

90

91

93

125

126

142

143

149

152

2

70

72

89

92

94

133

142

119

126

128

86

90

91

16

29

43

44

45

61

92

93

95

96

133

152

calcium carbonate

17

50

capillary rheometry

104

115

116

carbon black

16

18

19

carbon fiber

18

channel

28

46

47

48

2

5

10

11

13

14

95

100

112

133

134

49

geometry

instability pressure

72

stability

11

volume

72 142

bulk density

BUR

C

65 depth clarity

28

clay

18

clearance

29

47

122

124

cling

20

This page has been reformatted by Knovel to provide easier navigation.

66

Index Terms coextruded

Links 11

12

13

98

100

134

66

68

70

98

101

134

collapsing frame

78

79

142

colorant

16

18

60

compression

28

29

ratio

29

46

63

98

99

151

152

64

119

17

72

43

44

multilayer coextrusion

77

6 97

65

121 concentric dies

68 68

cone angle

26

conservation of mass

92

contaminant

33

control

36

cooling

2

air speed

91

excessive

39

hose

74

rate

2

speed

90

tower

6

unit

123

39

copper

35

corona discharge

82

corrosive

29

crammer feeder

26

crosslinking

15

crystal

8

10

132

crystalline

2

6

45

This page has been reformatted by Knovel to provide easier navigation.

61

107

Index Terms crystallinity

crystallization

Links 7

8

9

10

77

95

112

113

133

134

7

10

11

72

134

D degassing

29

53

degradation

15

19

23

35

49

51

60

64

98

106

131

132

19

52

53

56

64

111

121

130

131

133

133 degraded

delamination density

gradient column

100 6

7

8

9

18

104

113

149

113

devolatilization

53

die

21

23

29

30

31

32

35

36

37

39

40

44

47

54

65

133

9

70

71

93

133

152

drool

17

54

56

57

gap

66

70

71

72

85

93

125

128

bolt design diameter

85 133

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

die (Cont.) 130

133

148

149

125

130

133

127

128

130

133

swell

54

55

dispersed

17

dispersive mixing

49

distributed

17

distributive mixing

49

153 line

119 134

lip

50

51

50

51

44

45

18

downgauging

9

drag-induced

43

draw down ratio

93

dried

13

14

drive

23

24

dryer

26

drying

11

dual lip

75

76

dye

16

17

104

107

E elongation elongational

51

energy

41

45

46

50

51

64

109

121

122

123

131

64

119

ethylene vinyl acetate

12

ethylene vinyl alcohol

12

excessive cooling

39

extensional

100

This page has been reformatted by Knovel to provide easier navigation.

123

Index Terms

Links

extraction section

53

extraction zone

31

F falling dart feed

109 26

27

28

29

43

60

64

84

16

26

27

30

44

62

64

17

18

19

85 feeding

113

instability feed throat

filler

120

60

106 film property

2

filter

31

filtering

11

33

84

fisheye

33

104

111

flight

28

62

29

49

flood fed

27

43

flow-splitting device

99

fluff

43

tip

regrind

60

131

122

61

83

26

fluffy

44

foaming

31

53

forming ratio

92

93

110

152

93

95

96

17

26

44

153 FR

92 152

friction

8

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

friction (Cont.) 45

46

47

56

57

62

104

109

110

120

121

123

25

61

63

2

3

7

9

71

72

74

79

83

86

87

90

91

92

106

126

127

128

132

133

84

138 frictional

23 110

heat generation frost line

45

142 function

29

functional zone

42

funnel

44

flow

43

44

G gas

31

53

gauge

66

80

83

85

86

100

23

24

25

30

11

14

15

33

64

95

104

111

119

125

131

18

106

132

104

112

130

gear box

62 gel

glass

18

fiber gloss

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

gravity-induced

43

grooved feed throat

guide

8

9

27

45

48

62

63

64

65

120

122

131

2

H haul-off

79

80

104

113

head assembly

31

32

head pressure

5

25

31

33

35

37

40

54

61

63

84

98

118

120

122

124

133

138

147

heat-seal ability

66

98

heater

30

haze

band

45

46

47

123

130 heating unit heat seal

39 5

heat shrinkability

13

helix angle

46

high (or long) stalk

73

high-density polyethylene

7

8

73

75

113

homogeneous

21

28

65

83

hopper

26

27

42

43

44

60

61

138

15

74

75

humidity

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

I IBC

76

86

128

impact resistance

104

109

110

132

impact strength

103

83

89

142

134

impact toughness

2

infrared detector

38

instrumentation

36

insulator

75

interdependence

2 143

interface

100

interfacial instability

100

101

internal bubble cooling

76

128

interrelationship

90

isotropic

93

110

isotropy

93

96

L L/D ratio

29

layflat

71

72

80

83

86

89

93

133

142

149

150

154

width leakage

3 122

leakage flow

124

49

linear

6

linearity

8

linear low-density polyethylene

9

8

73

76

72

line control

83

line speed

36

55

125

150

This page has been reformatted by Knovel to provide easier navigation.

79

Index Terms

Links

long stalk

9

low-density polyethylene

7

73

132

138

17

75

113

56

57

125

89

108

109

120

126

130

151

153

mandrel

66

69

70

128

masterbatch

14

18

matrix

18

19

50

51

MD

89

91

92

93

95

105

106

126

132

151

lubricant

M machine and transverse direction direction

orientation melt

105 132

96 64

film fracture

46

47

5

17

54

55

56

101

119

125

114

115

116

134 index

104 117

pool

46

121

pumping

47

84

quality

30

65

66

83

5

7

9

11

13

14

64

65

84 speed strength

72

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

melt (Cont.)

melting

instability

73

138

29

41

42

43

44

45

46

47

49

83

84

120

121

122

120

121

43

48

147

mesh

33

metallocene

10

polyethylene

10

metering

28

mix

21

mixing

26

29

33

42

44

49

60

64

65

66

83

84

51

84

131

33

36

44

74

111

6

65

66

23

24

40

113 element moisture

monolayer motor

53

41

138 current

37

41

power

7

8

multilayer

6

13

66

N nanocomposite

18

nested mandrel

68

nip roller

15

37

72

74

78

79

91

138

This page has been reformatted by Knovel to provide easier navigation.

Index Terms nip speed

Links 2

3

72

79

90

91

93

133

142

143

13

132

30

35

37

46

60

62

66

70

73

122

2

3

32

37

85

90

98

115

122

123

126

133

112

130

133

134

orange peel

55

125

orient

54

55

2

8

89

92

93

95

106

107

108

110

132

133

151

152

1

24

33

37

40

49

63

64

77

83

85

91

119

120

121

122

124

127

nylon

O operation

operator

135 optical

orientation

output

ozone

82

P pancake

68

pellet

16

18

26

29

42

43

44

49

60

61

62

65

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

PID control

37

pigment

16

17

plug flow

26

44

7

9

72

73

1

5

6

9

10

12

13

17

54

63

64

70

74

82

107

111

113

120

132

82

pocket bubble

50

75 polarity

12

polishing

45

polyamide

13

polyethylene

polyisobutylene

19

polypropylene

10

73

polystyrene

7

11

polyurethane

13

polyvinyl chloride

13

19

111

powder

16

26

27

43

power

23

39

51

52

21

24

28

29

30

32

34

35

41

47

48

49

51

54

64

65

84

98

120

40

41

5

41

103

12

13

14

9

12

18

consumption pressure

transducer property

17

120

125 adhesive barrier

This page has been reformatted by Knovel to provide easier navigation.

98

Index Terms

Links

property (Cont.) dependence on processing

2

59

94

95

89

93

die

35

feed

60

63

8

10

11

14

15

17

18

19

64

93

95

98

100

119

132

133

10

11

14

95

100

119

133

134

1

9

89

29

42

51

65

122

124

regrind

44

120

reinforcement

17

106

132

reinforcing

18

mechanical

optical

solid state pumping

54

R

property

18

reprocessed

60

83

85

132

residence time

49

69

70

98

121

131

133

ripple

55

rupture disk

30

40

S safe

36

safety

30

40

82

84

34

35

122

85 screen

124

changer

33

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

screen (Cont.) pack

32

33

34

40

23

24

25

27

28

29

30

32

41

42

43

44

63

64

65

83

122

123

122 screw

147 beat

120

channel

121

124

design

83

121

133 diameter

21

29

flight

120

122

speed

2

11

23

37

40

47

54

56

63

64

72

84

86

90

91

92

106

118

121

122

124

125

133

138

142 segregation shape freedom

44

60

2

shark skin

55

125

shear

51

100

heating rate

64 117

125

138

stress

55

100

117

shrinkage

95

side fed

65

66

This page has been reformatted by Knovel to provide easier navigation.

125

Index Terms simulator

Links 4

22

37

40

41

59

90

96

135

144

47

122

single lip

75

slide plate

122

solid

62

bed

42

46

124 breakup

41

121

43

84

specific heat

6

74

speed reducer

23

24

spider leg

66

spiral mandrel

65

66

68

stabilizer

19

60

78

stack

68

98

99

69

70

stalk

71

72

75

91

starve fed

27

static

15

18

44

81

33

49

51

83

120

conveying

die

mixer

120

84 pressure strain gage pressure transducer

74 41 41

surface winder

80

81

surging

46

63

119

121

126

127

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

T tackifier

19

take-up ratio

92

106

125

126

127

133

151

152

153 talc

18

TD

73

89

91

93

95

96

105

106

128

130

132

152

89

95

96

104

107

108

130

132

21

28

36

37

30

32

36

37

46

75

84

85

2

89

95

96

104

105

106

107

13

16

17

tear

89 strength

temperature

40 control

tensile strength

132 thermal stability

5 64

thermistor

38

thermocouple

38

39

3

37

55

66

70

71

72

78

79

80

83

85

89

90

91

93

100

119

120

125

126

130

133

142

thickness

153

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

throat

27

120

throughput

17

22

42

45

47

49

59

62

73

93

118

121

122

123

124

126

149

150

151

23

24

25

8

9

23

34

41

65

8

14

95

73

74

79

141

142

transfer pipe

32

69

70

transition

28

43

46

47

104

112

130

89

105

108

109

128

152

153

thrust bearing torque

tough

32

89

toughness tower

translucent transparency

80

112 16 134

transverse direction

treatment troubleshooting TUR

82 119

147

92

93

95

133

151

152

turret winder

81

two-stage screw

31

53

U unmelt

65

This page has been reformatted by Knovel to provide easier navigation.

96

Index Terms

Links

V valve

40

vented barrel

31

53

vent flow

31

54

viscosity

7

11

14

17

23

33

40

41

47

50

51

54

55

56

65

84

85

97

104

115

116

117

118

125

138

14

17

18

29

30

33

122

124

141

142

W wear

weld line

66

winder

80

wood flour

18

This page has been reformatted by Knovel to provide easier navigation.