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Plastic Forming Processes

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Plastic Forming Processes

Maurice Reyne

First published in France in 2006 by Hermes Science/Lavoisier entitled: “Transformations, assemblages et traitements des plastiques” First published in Great Britain and the United States in 2008 by ISTE Ltd and John Wiley & Sons, Inc. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address: ISTE Ltd 27-37 St George’s Road London SW19 4EU UK

John Wiley & Sons, Inc. 111 River Street Hoboken, NJ 07030 USA

www.iste.co.uk

www.wiley.com

© ISTE Ltd, 2008 © LAVOISIER, 2006 The rights of Maurice Reyne to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. Library of Congress Cataloging-in-Publication Data Reyne, Maurice. [Transformations, assemblages et traitements des plastiques. English] Plastic forming processes / Maurice Reyne. p. cm. ISBN 978-1-84821-066-0 1. Plastics--Molding I. Title. TP1150.R48 2008 668.4'12--dc22 2008033359 British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library ISBN: 978-1-84821-066-0 Printed and bound in Great Britain by CPI Antony Rowe Ltd, Chippenham, Wiltshire.

Table of Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chapter 2. Polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Synthetic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Thermoplastics and thermosets . . . . . . . . . . . . . . . . . . . . . 2.1.3. Abbreviations for plastics . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Plastics classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Classification by price/quality . . . . . . . . . . . . . . . . . . . . . 2.2.2. Classification by molecular structure . . . . . . . . . . . . . . . . . 2.2.3. Division between amorphous and crystalline structures . . . . . . 2.3. General properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Average mechanical, thermal and chemical properties for virgin polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Main qualitative characteristics. . . . . . . . . . . . . . . . . . . . . 2.4. Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 3. Converting Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.1. Manufacture of molded parts in 3D. . . . . 3.1.1. Standard injection molding . . . . . . 3.1.2. Specific injection molding processes 3.1.3. Compression and transfer . . . . . . . 3.1.4. Pressing between hot plates . . . . . . 3.1.5. Reaction injection molding (RIM) . .

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24 24 36 59 62 64

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3.1.6. Casting and inclusion. . . . . . . . 3.2. Manufacture of long products . . . . . . 3.2.1. Standard extrusion . . . . . . . . . 3.2.2. Extrusion with shaped die . . . . . 3.2.3. Specificities of extrusion. . . . . . 3.2.4. Calendering . . . . . . . . . . . . . 3.2.5. Coating (flexible PVC or PUR). . 3.3. Manufacture of hollow products . . . . 3.3.1. Blow molding . . . . . . . . . . . . 3.3.2. Specificities of blow molding . . . 3.3.3. Injection-blow molding . . . . . . 3.3.4. Rotomolding . . . . . . . . . . . . . 3.3.5. Dip molding . . . . . . . . . . . . . 3.4. Manufacture of thermoformed parts . . 3.4.1. Standard thermoforming . . . . . . 3.4.2. Specificities of thermoforming . . 3.5. Manufacture of foamed products . . . . 3.5.1. Expandable polystyrene molding. 3.5.2. Polyurethane molding . . . . . . . 3.5.3. Other types of foams . . . . . . . . 3.6. Machining and cutting . . . . . . . . . . 3.6.1. Operation . . . . . . . . . . . . . . . 3.6.2. Cutting . . . . . . . . . . . . . . . . 3.6.3. Sanding and polishing . . . . . . . 3.6.4. Applications . . . . . . . . . . . . .

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67 70 70 75 92 110 114 117 117 123 128 132 141 143 143 154 159 159 166 173 174 175 175 175 175

Chapter 4. Assembly and Fixations . . . . . . . . . . . . . . . . . . . . . . . . . . 177 4.1. Undemountable processes . . . . . . 4.1.1. Adhesive bonding. . . . . . . . 4.1.2. Welding. . . . . . . . . . . . . . 4.1.3. Riveting. . . . . . . . . . . . . . 4.2. Demountable assemblies . . . . . . . 4.2.1. Ratchet assembly . . . . . . . . 4.2.2. Screwing . . . . . . . . . . . . . 4.2.3. Assembly with flexible hinge . 4.2.4. Insert . . . . . . . . . . . . . . .

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177 177 178 193 195 195 197 197 198

Chapter 5. Finishing Treatments. . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 5.1. Plastics deposition on metal (or metal coating) . 5.1.1. Torch gun spray . . . . . . . . . . . . . . . . 5.1.2. Fluidized bed . . . . . . . . . . . . . . . . . 5.1.3. Electrostatic powder coating . . . . . . . .

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199 200 201 203

Table of Contents vii

5.1.4. Dip coating, suspension or aerosol . . . . . . . . . . . . . 5.1.5. Powder selection . . . . . . . . . . . . . . . . . . . . . . . 5.2. Metal deposition on plastics . . . . . . . . . . . . . . . . . . . . 5.2.1. Vacuum metallizing . . . . . . . . . . . . . . . . . . . . . 5.2.2. Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3. Electroplating . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4. Advantages and disadvantages of the various processes 5.3. Printing and decorating . . . . . . . . . . . . . . . . . . . . . . . 5.3.1. Preliminary treatments . . . . . . . . . . . . . . . . . . . . 5.3.2. Printing or decoration on a rigid substrate . . . . . . . .

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Chapter 6. Ecology and Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 6.1. Nuisance and pollution . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Ecological appearances (waste built-up) . . . . . . . . . . . 6.1.2. Biological appearances (contamination of the atmosphere) 6.1.3. Positive appearances . . . . . . . . . . . . . . . . . . . . . . . 6.2. Solid waste treatment . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Regenerating plastics . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Energy enrichment . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Planned degradation . . . . . . . . . . . . . . . . . . . . . . . 6.2.4. Conditions for success . . . . . . . . . . . . . . . . . . . . . .

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231 231 232 233 233 234 236 237 238

Chapter 7. Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 7.1. Standard molds. . . . . . . . . . . . . . . . 7.1.1. Base components . . . . . . . . . . . 7.1.2. Materials and heat transfer systems 7.1.3. Fabrication processes. . . . . . . . . 7.1.4. Calculation of mold costs . . . . . . 7.2. New mold concepts . . . . . . . . . . . . . 7.2.1. Shorter mold making time . . . . . . 7.2.2. Thermal appearances of molding. .

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239 239 241 242 243 244 244 247

Chapter 8. Economic Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 8.1. Costs and prices . . . . . . . . . . 8.1.1. Polymer prices. . . . . . . . 8.1.2. Conversion costs . . . . . . 8.1.3. Productivity . . . . . . . . . 8.1.4. Cost and sales price. . . . . 8.2. Structure of the plastics industry 8.3. Markets . . . . . . . . . . . . . . .

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251 251 253 255 256 257 257

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Chapter 9. Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.1. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.2. Conversion processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 Appendix. Symbols Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Preface

Most books about plastics are written by professors who explain the behavior of these materials through their chemical contents and their mechanical properties from mathematical concepts. These approaches are of interest for research, but they are far removed from the daily problems of converters or users who need practical advice. There are few books offering a complete technical analysis of converting processes. The books that exist generally mention only the standard processes of injection molding or extrusion. There are actually more than 20 basic techniques, most of them with specific derivative developments, which are increasingly functional. Unfortunately, the trade experts are too busy and they fail to describe their practices, or they do so only for their own techniques. Therefore, many processes are simply ignored by potential users. The purpose of this book is thus to analyze in an almost fully exhaustive way, the many processes now practiced, or under development, covering both the assembling and the specific treatments. In order to do so, this book covers, for each of the major converting process techniques: – the polymers used; – the process principle, with its advantages and limitations; – the description of the manufacturing equipment, molds, machines and all accompanying devices; – characteristics of the converting process, pressure, temperature, vacuum;

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Plastic Forming Processes

– current operational characteristics, dimensions, output rate, waste, runs; – typical or specific applications; – and development trends. In order to describe this in a practical way, the book contains 400 drawings and pictures to describe the various processes.

Chapter 1

Introduction

The volume of all the items and components made of synthetic materials in the world today is larger than the volume of metal parts. The manufacture of industrial products is not measured in weight, because the objective is not to make masses but to make parts and/or functions. Future history books will thus say that the Iron Age gave way to the Manmade Age in the 1980s.

Figure 1.1. Trend of world consumption of plastics polymers

Today electricity and electronics would not exist without these materials which, thanks to their insulating properties, have made their development and miniaturization possible; a result that ceramics could not achieve.

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Plastic Forming Processes

Plastics materials can be found in all industries, just like steel: automobiles, railroads, boats, aircraft, electricity/electronics, household appliances, sports and leisure, health, building construction and civil engineering, textiles, agriculture, packaging, etc. Other materials generally focus on some specific end-uses: paper and cardboard are used 50% in packaging and 50% in writing, glass is used two thirds in bottles and one third in building construction, two thirds of rubber is used in tires, 100% of concrete and cement are used in building and civil engineering. The success and development of plastics materials was boosted by a number of causes: – the lower prices that could be achieved in mass-produced articles, with the possibility of obtaining a finished product with function integration, in one single operation, and quickly, while reducing the number of parts, thus making a single material product. With the non-plastics materials previously used, the process took several steps, melting or forging, then machining, then assembling; – lighter products, particularly in packaging, handling, transportation, for which this is a major issue, plus the miniaturization that could be obtained; – the new physical and chemical properties which widened the range of possible applications, and, among others, have largely contributed to the development of electricity/electronics. Moreover, the full energy balance of the components made of plastics remains low compared to that of other materials. All plastics actually consume only 5% of oil, a very marginal share compared to the bulk of oil used in transportation and heating.

Figure 1.2. Split of oil consumption in Europe

Chapter 2

Polymers

Many books have been written on plastics polymers, including those by the author. Most books are about the chemical origin of plastics. On the contrary, the purpose of this book is to describe in a comprehensive way all the converting processes, assembling and finishing processes of plastics. There are many of these processes, often of an electromechanical nature, and they are relatively little known. This chapter summarizes their basic data, classifications, physical and chemical characteristics, and specific properties. For more information, there are a number of books mentioned in section 2.4. 2.1. Definitions A plastic is a material that can change its shape under the application of an outside force, and that keeps its new shape when the outside force is stopped. Thus, from this point of view, clay and glass can be considered as natural plastics. However, the title plastics or polymers is used for synthetic products obtained by joining several molecules of a monomer together to make a macromolecule, a polymer, whose structure generally contains mostly carbon atoms.

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Plastic Forming Processes

2.1.1. Synthetic materials The large family of synthetic materials includes plastics, or “plastomers”, most often marketed as molded or extruded products, and also as textile fibers and elastomers, excluding natural rubber from hevea or latex. Plastomers include: – in solid form: powders, pellets or resins which, after conversion, give solid 2D or 3D products; – in emulsion or dissolution: for the manufacture of paints, varnishes, binders, adhesives; – in heterogenous form, which includes: - multilayers made of the layer association of films or sheet of different plastics, in order to associate the properties of each. The main applications of multilayers are in packaging, - composites, which are plastics reinforced with textile fibers to impart higher properties, allowing large area rigid structures1, - sandwiches, thicker shapes, which are both multilayers and composites. Synthetic materials 1D Textile fibers

Plastomers

Homogenous Rigids

Elastomers

Heterogenous

Solutions

Multilayers

Composites

2D, 3D Sandwiches Figure 2.1. Tree structure of synthetic materials

1. This topic is the subject of specific books. It is not discussed here; see books by the author on this topic.

Polymers

5

2.1.2. Thermoplastics and thermosets Firstly, plastics can be divided into two groups: thermoplastics (TP) and thermosets (TS) after the different conversion methods. When heating a macromolecular material with linear structure, the material softens. Conversely, its shape is set when cooled, and the operation is reversible. This type of material is called thermoplastic, TP (plastic due to thermal action) or a plastomer. Converting this type of material will require it, after heating, to be set into shape in a cold, or cooled mold. Any resulting waste will be collected and recoverable, meaning the operation is reversible. On the other hand, if a macromolecular network material is heated, the material is set directly, in a non-reversible way, in its heated shape, as the heat makes the ramified network rigid. This type of material is a thermoset, TS, hardening with heat or a “duromer”. The conversion of thermosets is performed in a heated mold, and the production scrap is lost, since the operation not reversible and the material is not recoverable after heating. TP

TS

Tools or mold

Cooled

Heated

Shaping or molding without chemical reaction

Reversible state

Non-reversible state

Production cycle

Fast

Slower (about twice as slow)

Scrap

Recyclable

Lost

The fast conversion rate of thermoplastics, in injection molding and extrusion, with a lower energy expense, and the possibility of recycling scrap, has favored their use at the expense of thermosets. As a rule, plastics are rigid, except for low density polyethylene PE-LD (naturally flexible), plasticized PVC (made flexible by modification of the homopolymer), some variants of polyurethanes and silicones that can be considered both as plastics and as elastomers.

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Plastic Forming Processes

2.1.3. Abbreviations for plastics All thermoplastics are internationally abbreviated with two or three capital letters. The first letter is most often a P, meaning polymerization, the second and third letters referring to the monomer content. For instance, polyethylene is abbreviated as PE, meaning polymerization of ethylene. For the sake of simplicity, these abbreviations are used throughout the book, and their alphabetical list is shown in the Appendix. There also are abbreviations for thermosets, but they are used much less often. 2.2. Plastics classification The potential users of plastics often find it difficult to understand the various terms used, in order to make the right choice. Generic plastics are sometimes mixed up with brand names, which further increases the non-specialist confusion. As a rule, there are only about 20 major families of thermoplastics and less than 10 families of thermosets on the market, but they include hundreds of brands and grades. In order to stay neutral, only generic polymers are mentioned in the book. Any classification system supposes the choice of a criterion that can be technical, such as physical, chemical, thermal properties, or economical, such as prices, enduses, or by application types, or simply alphabetical. It is difficult to sort out plastics according to their properties. The range of properties is much too wide to arrive at any homogenous grouping. It is not useful either to make any choice according to the alphabetical order. Therefore, there are two kinds of more operative classifications. 2.2.1. Classification by price/quality The economic criterion gives consistent groups of polymers, from the point of view of costs and uses, allowing a first choice when having to select a polymer. This approach is very practical. The following table gives the names of current polymers, with abbreviated names and its relative rank.

Polymers

Figure 2.2. Relative ranking, price and end-uses classification for TP and TS2

2. Order of magnitude of average prices beginning 2008.

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Plastic Forming Processes

2.2.2. Classification by molecular structure Depending upon the type of molecular structure, the structure can be amorphous (unorganized) or crystalline (organized) and the resulting properties can be summarized as follows. Structures

Molecules

Characteristics (trends)

Very ramified chains, random pellets Amorphous Softening range, high (unorganized) fluidity Fast cycle

Crystalline (organized)

In a straight line, symmetric Low friction coefficient

Dimensional stability Creep resistance Transparency Dynamic fatigue resistance Less heat distortion Chemical resistance (hydrocarbons, solvents) Precise fusion point Good flow properties (fibers, films) Opacity Shrinkage

2.2.3. Division between amorphous and crystalline structures Other types of classifications can be carried out: – by temperature resistance: - < 100°C

commodity plastics,

- 100 to 150°C

engineering plastics,

- 150 to 300°C

specialty high performance polymers;

– by market: - > 100,000 tons/year

commodity plastics,

- 1,000 to 10,000 tons/year engineering plastics, - about 100 tons/year

high performance plastics, with temperature resistance.

These various classifications enable a first selection of plastics. Literature by the plastics producers, either in print or on line, provides many details on the properties. There also are CDs with properly detailed descriptions, such as, in France: “Polymères”, CD, available from DUNOD, or at www.eplasturgy.com or www.omnexus.org on the Internet.

Figure 2.3. Division between amorphous and crystalline structures

Polymers 9

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However, plastics producers often choose the properties that are best value to their products. Thus, wide disparities can be found among the properties as claimed for the properties indicated for the same polymer. Moreover, the many possible formulations further limit the possible comparisons. 2.3. General properties 2.3.1. Average mechanical, thermal, and chemical properties for virgin polymers 2.3.1.1. Thermoplastics

Polymers

2.3.1.2. Thermosets, foams and TP elastomers

2.3.1.3. Elastomers Properties

TPS

TPO

PE-BA

Hardness Shore A

30

65

70

Hardness Shore D

50

50

85

40-70

80

25-30

Tear strength (MPa)

TPET

TPU 65

15-35

5-25

20-40

20-60

-90 to -50

-60

-50

-40

Upper temperature (°C)

60-120

140

140

80

Resistance: O2 and O3

M to B

B

B

B

B

Resistance oils and solvents

M

M to B

E

E

B

Resistance acids and bases

B

E

B

B

B

Fragility temperature (°C)

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2.3.2. Main qualitative characteristics 2.3.2.1. Polyolefins Advantages

Limitations Low heat resistance

Flexibility (without plasticizer) Impact resistance (unbreakable) PE LD

Chemically inert Electrical insulation Moisture barrier Easy to convert

Non-flammable (with drop) Stress cracking Poor UV resistance Destructible by rodents Poor gas barrier (O2, CO2) Need for treatment before printing Difficult to glue, HF welding impossible

Improved characteristics: – better rigidity PE HD

– cold resistance (-100°C), and possible sterilization (+100°C) – non-adhesive

Same disadvantages: – combustible – poor UV resistance

– chemical stability, radiation resistance Rigidity, good mechanical and abrasion resistance Bending resistance (thin hinges possible) Heat resistance (110 to 130°C) Electrical insulation PP

Same as PE HD, and also:

Chemical resistance

– poor cold resistance (to 0°C, improved by copolymer)

Stress cracking resistance

– difficult to paint and glue

Glossy appearance, transparency (films) No nail scratching Low density (0.90)

Polymers

13

2.3.2.2. Copolymers and olefin derivatives Advantages

EVA

Limitations

Elasticity

Low mechanical resistance

Cold resistance (-60°C)

Poor heat resistance

O3 and UV resistances

Very permeable

High barrier EVOH

Taste keeping Oil and solvents resistance

Reduced by O2, CO2, and moisture

Gloss Tear resistance IO

Strong welding and resistance to stress corrosion

Poor UV resistance

Easy to print

2.3.2.3. Vinylics Advantages Rigidity, abrasion resistance Dimensional stability (no water absorption) Self-extinguishing (M 1) Rigid PVC

Electrical insulation Barrier to gases Relative water barrier Good chemical resistance (oils, fats, ozone) Easy extrusion, forming, machining, adhesive bonding

Flexible PVC

Flexibility HF welding

Limitations Relatively high density (1.4) Poor low temperature resistance (-10°C) Poor irradiation resistance Low heat resistance (70°C) Poor fuel resistance Chlorine vapor exhaust when decaying or burning Difficult for injection molding (low temperature steps) Lower chemical resistance with plasticizers Need for blocking agents to avoid migration

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Plastic Forming Processes

2.3.2.4. Copolymers and vinylic derivatives Advantages PVAc

PVA

PVdC

Helps in PVC converting Provides a good surface finish

Limitations Water sensitive

Gas barrier Water soluble Barrier to smells, water, gases, vapors Chemical inertia

Poor resistance to ammoniac

2.3.2.5. Styrenics Advantages

Limitations

Rigidity and gloss Possible transparency Shape and dimension stability

PS

Brittleness

HF electrical isolation

Low thermal resistance (60-70°C)

Tropical climate compatibility

Combustible (with black smoke)

Easy to injection mold (wide temperature steps and low shrinkage (precision))

Very electrostatic

Easy to glue and decorate

High solvent sensitivity Difficult to blow mold

Easy electronic welding

2.3.2.6. Copolymers and styrenic derivatives Advantages PSC

Improved impact resistance Impact and scratch resistance

SAN

Gloss Hydrocarbon resistance

Limitations Lower rigidity and gloss Opacity Only color transparency Brittle

Polymers

Advantages

MBS

Limitations

Transparency and improved mechanical properties Good rigidity Dimensional stability Hard and glossy finish

ABS

Opacity

Improved impact and scratch resistance Electrostatic Fairly good thermal, moisture and fat resistance Easy to mold and form

Poor chemical resistance Poor weather resistance

Easy to print, to metalize under vacuum, to electroplate

2.3.2.7. Acrylics Advantages Transparency (transmission = 92%) Easy to color (clear or opaque) PMM

Excellent UV resistance, good aging (10-year guarantee) Hard, smooth and glossy surfaces Light conductivity (optical fibers) Easy to machine Transparency (< PMMA)

PAN

Impact resistance Good solvent resistance Barrier to gases and smells

Limitations Brittleness (crazing) moisture regain Poor scratch resistance, can be repolished Poor thermal resistance Combustibility (without fumes) Poor chemical resistance

15

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Plastic Forming Processes

2.3.2.8. Cellulosics Advantages Nitrocellulose Toughness

Limitations Non-flammable Steam permeable

Regenerated cellulose

Barrier to gases, smells, fats

Short life

Good transparency and gloss

Water absorption Attacked by rodents

Cellulose triacetate

Dimensional stability Hardness, impact resistance

Cellulose propionate

Plasticity Fogging absorption Toughness, impact resistance Non-electrostatic

CA

Glossy surface and transparent coloring

Water absorption Aging outside

Barrier to O2, CO2

Solvent sensitivity

Light and gasoline resistance

Differential shrinkage

Easy forming, machining, adhesive bonding, printing and decorating

CAB

Better properties than CA:

Limited chemical resistance

– mechanical properties

Poor hydrolysis behavior

– good tropical and weather resistance

Smell of rancid butter when converting

Polymers

17

2.3.2.9. Polyamides Advantages

PA 6-6

Impact and fatigue resistance Abrasion and scratch resistance Can be translucent Good mechanical and thermal resistance (120°C), low creep Low electrostatic behavior Self-extinguishing Good hydrocarbon and solvent resistance and fairly good gas barrier

PA 11

Dimensional stability Can be transparent

PA 4-6

Improved thermal resistance

PA A

Little affected by moisture Rigidity, stability Improved heat resistance (140°C on a continuous basis) Reduced creep Coefficient of dilatation close to steel No need for pre-drying

Limitations

Poor resistance to moist heat (steam, boiling water) and in dried air (brittle) Sensitivity to electric arc Need to pre-dry pellets before converting

2.3.2.10. Polyacetal Advantages

POM

Hard and smooth surface Rigidity, resiliency, fatigue resistance (maintained over life) Excellent dimensional stability Low friction coefficient Spring effect (ease of snapping on) Dielectric quality Good resistance to hot water, gases, fuels, fats, soaps, solvents Low creep

Limitations Relatively high density (1.4) Opacity Sensitivity to UV (possible protection) Attacked by acids and detergents Combustible and low resistance to continued heat Converting temperature close to decomposition temperature (formaldehyde vapor in case of over heating) Differential shrinkage, difficult to blow mold

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Plastic Forming Processes

2.3.2.11. Polyphenylene oxide Advantages

Limitations

Stiffness, impact resistance, scratch resistance, gloss

PPO

Dimensional stability (low water absorption)

Opacity

Good electrical properties

Relatively high friction coefficient

Wide range of temperature resistance (standing repeated sterilizations) and good mechanical resistance

Brittleness of large parts Poor UV resistance

Self-extinguishing

Poor behavior with aromatics, fuels and solvents

Hydrolysis resistance (hot water and detergents)

Limited coloring range

Easy gluing, printing and metallizing Ultrasonic welding

2.3.2.12. Polycarbonate Advantages Good mechanical and electrical properties High impact resistance, toughness surface hardness PC

Limitations

Poor resistance to detergents and solvents, decomposed by bases Poor behavior with hydrolysis

Dimensional stability under heat (-100 to +130°C)

Sensitivity to cracking, scratching and abrasion

Barrier to water vapor

Need for pre-drying pellets before converting

Good urine resistance Transparency

Difficult to paint

Polymers

19

2.3.2.13. Saturated polyesters Advantages

Limitations

Rigidity, toughness Abrasion and scratch resistance Good mechanical and electrical properties Fatigue resistance Dimensional stability PBT+PET

Low friction coefficient Barrier to water and gases Chemical stability (gasoline, solvents) Resistance to J rays

Relatively high density (1.3) Sensitivity to hydrolysis Attacked by strong acids and bases Need to pre-dry pellets before converting

Self-extinguishing Wide range of temperature resistance (-60 to +100°C, then creep, but can be used up to 200°C)

Specificities PBT: higher hydrocarbon resistance, good stress cracking resistance, good impact resistance, easier to mold. PET: transparent, more rigid, higher mechanical resistance, higher heat deflection temperature, good surface finish, possibility to blow mold, to be metallized. 2.3.2.14. Fluorinated plastics Advantages

Limitations

Very high chemical inertia Wide range of temperature resistance (-270 to +300°C, -80 to +250°C continuous)

TFE

High density (2.1 to 2.2) Creep

Decomposition at 327°C with exhaust of fluorinated fumes (dangerous)

Electrostatic

Very low friction coefficient (f = 0.04)

Impossible to weld, difficult for adhesive bonding (no solvent)

Water repellent (water absorption = 0) not UV sensitive High arc tracking resistance Non-burning (M-0) Without toxic effects on human body

Differential shrinkage

Sintering or/and machining only possible High price

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Plastic Forming Processes

Specificities PCFE: easier converting but lower solvent resistance. EPF: moldable and weldable. PVDF: higher rigidity, piezoelectricity. 2.3.2.15. Sulfones Advantages

Limitations

Excellent creep resistance Dimensional stability

PSU

Wide range of temperature resistance (100 to +150°C) without sagging

Sensitive to high stress cracking

Self-extinguishing Transparency (yellow)

Difficult to convert, need to pre-dry pellets

Constant shrinkage

High prices

Poor behavior with fuels and solvents

Easy to metalize and to electroplate (> ABS) Rigidity, dimensional stability High temperature resistance (260°C), particularly on welding bath PPS

Self-extinguishing Good behavior creep, hydrolysis, UV Chemical resistance (solvents up to 120°C)

Breaking on impact Difficult coloring Need for high pressure injection molding

Dimensional stability, dielectric PES

Low flame spread Heat, cold and UV resistance

2.3.2.16. Arylates Advantages Dimensional stability, dielectric PAR + PAS

Low flame spread Heat, cold and UV resistance

Polymers

21

2.3.2.17. Ketones Advantages Continuous heat resistance: 250°C and peak temperature 300-350°C Good mechanical, chemical (solvents) and electrical properties Good fatigue and abrasion resistance

PEEK

2.3.2.18. Standard foam Advantages

Limitations

PSE

Rigidity Good thermal insulation Damping capacity Barrier to water Insensitivity to cold Very low density Easy to mold Machinable with hot wire

Non-flammable Sensitive to hydrocarbons and solvents Product life before use limited (pentane exhausting) Electrostatic Too large volume after converting

PU

Wide range of uses: rigid, flexible or intermediate (15 to 800 kg/m3) Good resistance alternating bending Abrasion resistance Excellent thermal insulant and good sound insulant Chemical behavior and imputrescibility, resistance to seawater, to fuel, to gasoline Possible to mold in situ

Combustibility (except variant or polyisocyanurate) Yellowing with light exposure Difficult to color (except black) Unwanted gluing

2.3.2.19. Other foams Advantages Olefins

Vibration protection

PE and PP

Damping

Vinylics

Flexible, HF welding Rigid: thermal insulation

Acrylics

Dimensional stability at high temperature Good solvent resistance Easy machining and adhesive bonding

Phenolics

Fire protection insulation

Aminoplasts

Fire protection insulation

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Plastic Forming Processes

2.3.2.20. TP Elastomers Advantages Excellent flexibility at high and low TPS

Possibility of low hardness Oil absorption at high temperature Tear resistance

TPO

Resistance to chemical fluids Good fatigue resistance

Limitations Poor resistance to hydrocarbons and under temperature (60°C)

Property loss at high temperature Low elastic memory

Good vibration dampening Flexibility PE-BA

Resistance to impact and to repeated bending

Possible deterioration with water

Resistance to low temperature Colorability TPES

Tear abrasion

High hardness

Chemical fluid resistance

2.4. Further reading M. REYNE, Les Composites, PUF, France, 1995. M. REYNE, Technologie des composites, 3rd edition, Hermes, 1998. M. REYNE, Solutions composites, JEC, 2006.

Chapter 3

Converting Processes

The following table summarizes the various processes, showing the shape of the base polymer and the type of the resulting finished and semi-finished products.

Figure 3.1a. Main converting processes

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Plastic Forming Processes

Figure 3.1b. Main converting processes

3.1. Manufacture of molded parts in 3D 3.1.1. Standard injection molding 3.1.1.1. Principle Pellets of a thermoplastic are heat softened in a cylinder. The plastic melts, or plasticizes, and it is pushed (injected) under high pressure into a cold mold in which the plastics freezes into shape and is then ejected. Advantages Fast production runs Consistency and precision of the molded parts

Limitations Expensive presses and molds

3.1.1.2. Breakeven The high cost of the injection molds and tools requires large volume production. As a rule, 10,000 parts are a minimum, although there may be exceptions. Currently, required volumes are runs of 100,000 to 1 million units, or more.

Converting Processes

25

Figure 3.2. Injection molding press (Netstal)

3.1.1.3. Basic plastics As a rule, all thermoplastics pellets can be injection molded: – PE-HD, PP, ABS, generally to make large parts; – PE-LD, flexible PVC, PS, PMMA and engineering plastics, more often for smaller size parts, and relatively little rigid PVC that is difficult to mold. Thermoset plastics and vulcanizing elastomers can also be injection molded, by heating the mold above the plasticization temperature to achieve polymerization or vulcanization. 3.1.1.4. Production equipment 3.1.1.4.1. Machine A standard injection molding press includes: 1. a feed hopper for plastics pellets. It can be filled by hand, from bags, or mechanically, from a container or a bin; 2. a cylinder, or a plasticizing barrel, heated by electric resistance; 3. a piston screw with a non-return valve, for 2 functions: – by rotation, the screw plasticizes the plastic, backing after injection, the melted polymer passes to the front of the screw, – by translation, the head of the screw serves as a plunger to fill the mold.

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Plastic Forming Processes

Figure 3.3. Principle of piston screw injection molding operation

The screw is defined by its length/diameter ratio, comprised between 15 and 25.

Cross-section AA

Figure 3.4. Screw and non-return valve

Converting Processes

27

In the case of small presses, for micro-injection molding, just a piston is sometimes used, instead of the piston screw. This was the case of the very first presses, imitating the light alloy pressure molding. This is also sometimes used with colorants which, instead of being blended by the screw, will be distributed at random in the mold, giving a stained effect to the finished molded part. 4. a metal structure, molded or mechanically cast, which receives: – a fixed and a mobile platform on which to fix the two parts of the mold; – a bearer supporting the 4 columns linked to the fixed platform and driving the mobile platform; – a device to handle and to hold the mold.

Figure 3.5. Main parts of an injection molding press

3.1.1.4.2. Operation The heating of the cylinder (200 to 300°C) is achieved by heating collars which raise the internal plasticizing temperature to 150 to 250°C. Actually, the heat comes both from the electrical resistance and from friction in the plasticizing barrel. The mold is cooled by cold water tubes. It reaches temperatures of 40 to 60°C, through the operation, for easy to mold plastics. For some polymers, the mold may be heated to more than 100°C, but it still remains relatively cold, compared to these polymers, so that they can solidify.

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Plastic Forming Processes

The injection pressure may reach up to 1,000 bars. The pressure in the mold is about 50% of this value, because of losses. Practically, and for commodity plastics, the injection pressure is about 300 bars. It is important to close the mold securely. This is often done with toggle or hydraulically driven system, with the advantages of strength, and fast opening and closing. Corner locking is used on very large injection molding presses (no flexibility over a large open-scale range).

Figure 3.6. Closure toggle system

The clamping force, pressure/projected area of the mold, is generally more than several hundred tons.

Converting Processes

29

Thus, an injection molding press can be described by the following parameters: – clamping force in tons, from a few tens of tons for a small press to several thousand tons for a large press; – injection molding capacity in cm3 or in kg, under 1,000 bars, and plasticizing capacity in kg/h, depending upon the type of polymer and the geometry of the final part, particularly the cooling down time: - Euromap definition: for instance 900 H 260, - 900 = clamping force (kN) or (t), - H = horizontal press, - 260 = volume that can be injected in cm3, at 1,000 bars. There are other criteria: the range of opening-closing, the space between columns, the size of the platform, the distance between platforms, etc. There are two possible configurations, depending upon the direction of the injection molding: – horizontal press, by far the most frequent: the axis of the screw is horizontal and the separation plan of the mold is vertical. The opening results in the immediate exit of the part by gravity, meaning a saving in time and easy automation; – vertical press, less frequently found: the axis of the screw is in a vertical position, and the mold opening is on a horizontal plane. This type of press is used to mold small parts with metal inserts, overmolding. The demolding then requires a transfer.

Figure 3.7. Horizontal press and vertical press

30

Plastic Forming Processes

There are also hybrid presses, playing on the changing of the injection cylinder, and barrel presses, for small capacities, as well as presses with a turn platform, used for shoe manufacture. 3.1.1.4.3. Drive Hydraulic presses (pump and accumulator) Standard presses are driven by servo valves and the injection cylinder is driven in closed loop. The speed and pressure are constantly adjusted according to given values. The settings for each mold are stored as tapes or disks, insuring time saving and good reproducibility.

Process Actuator dimmer

Sensor Programmable logic controller

Whenever the process is disturbed (pressure, temperature, etc.) this is detected by a sensor, and then forwarded to an actuator through a programmable logic controller, PLC. Thus the process is operated as closed loop. Figure 3.8. Hydraulic press

Electric presses All electrical presses have started relatively recently with alternative servo motors, with the following advantages: – fast start, no oil to heat; – precision (1/100 mm versus 1/10 in hydraulic presses, sensitivity to dampness, reproducibility (lower reject rate), and constancy settings; – improved performance, as the motors are not permanently operating. Energy saving is 50% and the availability rate is > 90%; – reactivity (faster correction of disturbances; – cycle time saving of 10 to 15% because of limited moves; – cleanliness, making easier the work in clean rooms, with no contamination, no oil leaks, no lubricating, reduced noise level, 60 dB versus 85 dB in hydraulic presses;

Converting Processes

31

– reliability, with fewer breakdowns, hence easier maintenance and wear reduction; – minimal infrastructure, since there are no vibrations and the floor area is less. There are a few disadvantages however: – investment cost is higher, +20 to 30%; – difficult to adapt the molds with kernels (or cores), requiring an independent hydraulic power source; – only available for medium size presses, less than 500 ton clamping force for the moment.

Figure 3.9. Electrical press (source: MCM)

3.1.1.5. Molds The tools in two parts (male and female molds) feature the shape, most often made of high resistance steel (800 to 1,100 Mpa, 33 to 34 HRC). The surface must be perfect, polished mirror. It can be made up of one cavity, for medium or large parts, or several cavities on a platform, for smaller parts. The choice depends upon the number of parts to be produced. A multi-cavity mold is more productive but more expensive. The number of cavities increases the cycle time. A balance must be reached. In all cases, there are cooling runners in the cavity and the ejection systems, and sometimes moving parts (drawers) to ease the ejection of the part, in case of an undercut.

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Plastic Forming Processes

The feeding part of the mold (runner or feeding runners of the polymer) will solidify when cooling. It is thus a scrap part called sprue, or stalk, whose relative share can be small for large parts, 2 to 3%, but which can be much higher in a multicavity molding. It is of interest to reduce or to recycle this sprue, because the mark may stay visible on the molded article, and because it must be blended again to be re-used, and this is an added cost. There are two techniques to separate the parts more easily: – capillary injection which features a very small diameter impact, pin-point, in the injection axis. This system is best for cylindrical parts, for instance packaging closures; – in submarine injection, the polymer is fed under the clamping plane of the mold, in order to achieve an automatic separation when the mold opens. The interest is to separate the parts directly from the sprues; – in order to avoid dropping polymer, the hot sprue (or sprue less) process is used, by heating the sprue which will feed the next injection. This system is particularly interesting with multi cavity molds (this is called hot runners).

Standard (large parts)

Capillary

“Submarine” Figure 3.10. Classic injection, capillary injection and “submarine” injection

Converting Processes

Heating resistance

Cooling runners

Figure 3.11. Hot runners

3.1.1.5.1. Peripherals The following devices can be found around an injection molding press: – handling robots and sprue picks; – conveyors to evacuate parts; – sprue grinders; – thermal controls, hot blocks, control photocells, weighing device. Operation The production cycle is discontinuous, with 4 phases.

Phases

Typical time in seconds (for a 100 g part of PS)

Mold closing

2

Injection

2

Cooling

6

Opening + ejection (combined)

2

33

34

Plastic Forming Processes

The cooling phase is the longest, about 50% of the total cycle time. This time is about proportional to the square of the thickness. As plastics are poor heat conductors, it is important to make relatively thin parts, with ribs to stiffen them. The current thickness limits of molded parts are 3 to 4 mm. The cooling time can be reduced if the part does not require too severe dimension definitions, by dipping into water after ejection. Of course this cannot be done when molding engineering plastics. 3.1.1.6. Operational characteristics Standard capacity: – from a few g to 10 kg Thicknesses: – standard, from 0.4 to 2.5 mm Production rates: 10 to 20 sec/mm thickness – small parts: 50 to 100 g = 5 to 10 cycles/min – large parts: 1.5 to 2 kg = 1 to 1.5 cycles/min Example: handling containers (> 1 kg) 60 parts/hr

Time (minutes)

Scrap: 1 to 4% – high (multiple parts with sprues) – low (hot sprues)

0.5

1.5 Thicknesses (mm)

2.5

Figure 3.12. Injection molding time and molded part thickness

Converting Processes

35

Figure 3.13. Injection molding workshop and mold

3.1.1.7. Applications All the mass produced multifunction molded parts in 3D are made by injection molding. There also are plastics parts overmolded on metal, such as for connectors, sockets, car keys, etc.

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Plastic Forming Processes

Figure 3.14. Typical injection molded parts

3.1.2. Specific injection molding processes In order to optimize molded parts, injection molding has diversified into about 15 different techniques, with specific know-how that is often specific to a small number of companies. There are processes to increase productivity, quality and size of molded products. 3.1.2.1. Fast run injection molding This applies to thin parts and high production rates, such as with lids, caps, closures, pots for the food industry, disposables, CDs, etc. The injection molding presses are fast and well stabilized to avoid vibrations, with multi-cavity molds and hot runner systems. The sprues of a molded part stay hot and pass into the injection molding of the next part. Applications: lids, caps and closures, pots, bottle preforms for the food industry, disposable objects, CDs.

Converting Processes

37

CDs made of polycarbonate, PC, weight 16 g, thickness 1.2 mm, diameter 12 cm, made in 10 to 12 cycles per minute, with single performed cavity mold. The metalizing is made simultaneously in 2 to 3 sec. Closures made of PE and PP, weight about 10 g, thickness 0.5 to 1 mm, made in 6 to 8 cycles per minute, in molds with up to 64 cavities. A double mold is used to make screw caps. One mold is used for injection molding, whilst the other mold achieves cooling and unscrewing in blind time.

Figure 3.15. Hot runner molds

3.1.2.2. Micro-injection molding Light parts of 0.10 g or less, down to 0.001 g, can thus be made. The sprue may be up to 90% of the molded part. In some cases, there must be a vacuum in the micro-cavities of the mold. Due to the precise closing, the evacuation of the air inside would be too slow. Small presses are used, with a 10 ton clamping force, with a plasticizing screw of 10 to 20 mm diameter, and a compressing piston to push the melted plastic into the micro cavity. Sophisticated programming of the presses and air conditioning of the workshops are necessary to insure the best for these parts whose precision is measured with dimension tolerances of plus or minus 0.01 mm, and whose weight spread must be less than 0.02%.

38

Plastic Forming Processes

Metering hopper

Screw

Platform

Valve

Molded part

Piston

Mold

Press

Figure 3.16. Micro-injection molding description

Applications: parts for electronics, computers, clocks, photography, video, medical. This process is bound to develop with growing demand for microsystems.

Figure 3.17. Wristwatch gear (weight 0.008 g)

There are several levels of injection molding, with the various accepted tolerances: – standard = 1%; – engineering = 0.6%; – precision = 0.3%.

Converting Processes

39

3.1.2.3. Injection under neutral atmosphere This type of injection molding is often additional to the micro-injection. In order to mold perfectly clean parts, with zero defects, in conditions of total safety, as required by the aircraft, nuclear, electronic or medical industries, the injection molding must be performed in clean rooms. The operators must wear headgear and gloves to avoid any pollution. About 70% of contaminations come from the human operators. Applications: electronic components, medical disposables, prostheses, CDs and DVDs. These techniques increasingly use injection molding presses with electrical controls for precision and cleanliness. 3.1.2.4. Injection-compression This process uses molds with sunken edges of 5 mm leaving a gap of 0.5 to 3 mm when closed. The cavity is then fully filled, the screw is blocked, and the slow final closing will compress the part with its accepted excess polymer. The pressure is the same throughout all of the surface of the part. 1 to 3 mm

Moveable edges

Mold Closed press Closed mold

Injection Polymer

Edges out molded volume 40 to 80%, more than final volume

Edges in compacting the injected polymer

Figure 3.18. Injection-compression description

Applications: optical lenses made of PMMA which can tolerate neither internal pressure nor flowing.

40

Plastic Forming Processes

Figure 3.19. Injection-compression press

3.1.2.5. Injection liquid crystal polymers In order to optimize the liquid crystal polymers, LCP, in large parts, a two head plasticizing machine must be used, operating in alternative opposition, push-pull. By repeating the operation several times, a satisfactory interlocking of the weld and a good orientation of the whole mass of the molded part are achieved. Otherwise, only the polymer in contact with the wall of the mold would be oriented. However, small parts can accept a standard operation. 2 injection heads

Mold

Platform

Figure 3.20. Description of LCP injection

3.1.2.6. Injection-intrusion on several molds The injection press is mobile, by rotation, in order to feed the molds arranged in a semi-circle (carrousel).

Converting Processes

41

Figure 3.21. Injection-intrusion on several molds

The operation time is thus reduced just by a deposit of polymer in the mold (intrusion). However, since the injection pressure is not maintained, the molded parts may lack cohesion. Typical application: shoe soles made of PVC or PUR. 3.1.2.7. Heavy injection molding Heavy injection molding means using presses of over 2,000 ton clamping force. The largest injection molding presses may have a clamping force of up to 10,000 tons, which is the equivalent volume of two locomotives. Such presses can mold about 100 kg. The split by injection press type is approximately as follows. Injection type Small Medium Large Very large

Injection molding press clamping force in tons < 200 200 to 800 800 to 2,000 > 2,000

Mesh presses are sometimes used. The standard columns are replaced by stationary frames that block the table and the fixed platform. The mold is put on one side of the machine, then rolled up to the fixation platforms. In this way, the mere addition of steel frames makes it possible to customize the presses.

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Plastic Forming Processes

Figure 3.22. Large injection molding press, clamping force over 5,000 tons

It is also possible to join together, in parallel, two large identical presses on the same mold, to produce very large parts. Same mold

Identical presses

Figure 3.23. Double press feeding a single mold (billion)

A very large press with all its peripherals may cost €1 million or more. Therefore the heavy injection molding may avoid off-shoring, because of the cost of the investment and the high transportation costs of very large molded parts.

Converting Processes

43

Applications: large molded parts are needed for cars, such as body parts, and handling equipment, such as containers, pallets, garbage cans, garden furniture, etc. EXAMPLE.– a telephone booth made of PC, 90 kg, molded with 3 presses of 2,500 tons, with a mold of 2.3 x 3.7 x 3.2 m. Sometimes metal parts, or textile parts, can be included in the large part, to enhance it with various decorations. There are several techniques. 3.1.2.8. Injection molding on insert Small metal elements, called inserts, can be put into a molded part. This may be to improve the chemical resistance at places, with pivots, threads, or to create electrical conductivity, in sockets or connectors. In these latter cases, the volume of metal is always much smaller than that of plastic in the molded part.

Figure 3.24. Metal inserts

Types of applications: axis, pivot, bushing or metal threading on a molded plastic part. 3.1.2.9. Outset injection When installing an insert, the volume of the metal is always much smaller than that of plastics in the finished part. There is also the reverse process, called outset, which consists of injecting a polymer on a supporting metal plate, of much larger volume than that of the molded material.

44

Plastic Forming Processes

Figure 3.25. Outsets (plates for appliances)

It is thus possible to obtain, in a single operation, chassis for mechanical or electrical devices that bear all the controls. The metal provides rigidity and the plastics integrate all functions, replacing all the cross frames, bosses and other elements that should have been screwed-on or riveted in a standard mounting operation, thus resulting in very high productivity gains when assembling. Types of applications: knob panel on standard and microwave household ovens, washing machines, dishwashers, sewing machines. 3.1.2.10. Injection with in-mold decoration The principle is to transfer a small decoration (emblem, label). This is often multicolored, (rotogravure) supported on a transparent film, made of the same plastic as the molded part, which fits to the form of the cavity when pushed by the melted material and clings to it. This operation can be carried out in blind time, thanks to the press being programmed and a robot that makes it possible to control the synchronized film in advance.

Converting Processes

45

Decor or label

Supporting film with synchronized feeding

or, programmed robot

Figure 3.26. Description of injection with decoration

This process makes it possible to avoid rerunning deposits and drying colors, as well as pollution, with a gain of 15 to 30%, and it is possible to recycle if needed. Types of applications: decorating molded boxes or blow molded bottles, hubcaps, monog, knobs, CDs, cassettes, mobile phones, knob panels on appliances, perfumes, etc.

Figure 3.27. Molding with film insert

46

Plastic Forming Processes

It is also possible to include a fabric to decorate a low pressure injection molded part. Iron powder is put into the textile so that it can cling to the walls of the mold perfectly, which are also magnetized, with an electromagnet. Otherwise it is possible to insert a large area decoration by injection molding, then compressing in two steps: make the injection molded part and open the mold before cooling, install the decoration, then press it on the hot part before cooling.

Finished part

Injection Mold opening putting fabric in + compression

Figure 3.28. Including decorative fabric

Applications: furniture, car upholstery, etc. 3.1.2.11. Injection with scales and graining effects Compatible polymers are incorporated, with a slightly different melting point and of varying granulometry. The larger the pellets, the more clearly defined the graining, with a small size. On the contrary, the smaller the pellets, the larger and more fogged the graining. The operation is carried out with a piston press or a special screw. It is also possible to inject several colors at the same time, with as many injection heads, grouped into a common runner which controls the split of the colored polymers in the mold cavity, or by spot injecting. This gives sprinkled, stained, marbled, flower petal appearances, etc. that are easy to reproduce.

Converting Processes

47

Figure 3.29. Stained effect

Applications: boxes for cosmetics, brushes, eyeglasses, hair barrettes, combs, artificial flowers. 3.1.2.12. Sequential injection Whilst in standard injection molding, all the injection points are fed at the same time, they can be ordered to feed only gradually, as each is passed over by the flow of material coming from upstream. This process enables control of the mold filling, reduces stress and eliminates the welding lines, or moves them into a less visible or less stressed spot. The process is of interest for visible appearance parts. In this case, hot runner molds are used with up to 20 nozzles with valves. This process also operates at low pressure. It is thus possible to overmold on a textile decoration, without penetrating it and putting it out of shape.

48

Plastic Forming Processes

(a)

(b) Figure 3.30. (a) Injection on fabric (source: Usine Nouvelle); (b) valve system (source: Sise)

Traditional injection molding simultaneous opening of the five nozzles

Nozzles

Sequential injection opening of nozzle 3, then 2 and 4, then 1 and 5

Figure 3.31. Difference between standard and sequential injection

Converting Processes

49

Applications: car upholstery, dashboards, bumpers, furniture (seats), etc. Other processes make it possible give to specific functions to molded parts, such as lighter weight, hollow design, or put together several polymers or colors. 3.1.2.13. Lighter injection The principle is to introduce foaming agents, (azodiocarbonamides = 1 to 3%) in the polymer pellets. After mixing, the heat expands the plastic which is then quickly put into the mold. The foaming agent pushes the plastic against the walls of the mold, on which it becomes denser, whilst the core of the molded part stays as rigid foam. This can be performed on all presses. However, in order to obtain a competitive cost, it is better to use the best adapted machines, with low pressure and a large platform. Foaming agent polymer Mold

Heating

Compact skin Foam score Structure of the finished part

Press

Figure 3.32. Description of lighter injection

It is thus possible to obtain structural foam with the same polymer, featuring a skin 0.8 to 1.5 mm thick, and a foam core of greater thickness, equal to 4 mm or more, of 0.6 to 0.8 densities. Actually, stiffness is achieved with the overall thickness instead of ribs. This is more satisfactory to the general public as it gives a solid material impression. The operation cycle is longer however. Advantages: – thickness giving a quality image, similar to wood; – rigid structure for the same weight, doubling thickness with 4 times higher stiffness; – good noise damping comparable to that of wood (70 DB for 6 mm thickness); – little or no internal constraints;

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Plastic Forming Processes

– possibility to make large parts on machines with limited clamping force, one fourth of what is needed for standard injection, because of the low pressure given by the foaming agent; – the molded parts can be sawn like wood. Limitations: – longer cooling and cycle, more than twice than standard injection molding; – wrinkled surface of the molded parts, with the foaming agent bubbles bursting on the mold walls. It is often necessary to paint to improve the surface. The effect is then that of an oven dried paint. It is also possible to obtain polished surfaces by reheating and or by counter pressure, (15 to 20 bars) in the mold. Applications: furniture parts (easy chairs, drawers), loudspeakers, office machine casings, computers, television receivers, washing machine bases, etc.

Figure 3.33. Cross cut of a part of foamed plastics

3.1.2.14. Hollow injection This is a process which derives from the process discussed above. An inert gas, nitrogen, is introduced into the bulk of the melted polymer just after it has entered the mold. The gas does not blend with the injected plastic, but moves into the middle of the thicker sections of the molded part. It tends to go towards the center, where the material is more fluid, and to stay in the thicker parts. With its internal pressure, it prevents cavity shrinking on the visible surface of the part. This way, hollow parts

Converting Processes

51

can be obtained, as with blow molding, but as injection molded parts, with varying thicknesses, ribs, etc. Nitrogen entry Polymer

Hollow molded part

N2

Mold

Press

Figure 3.34. Description of hollow injection with nitrogen

It is also possible to introduce nitrogen gas directly into the mold, to reduce the shrinkage

(a)

(b) Figure 3.35. (a) Injection of gas into the mold; (b) typical part

It is necessary to have nitrogen bottles, or to extract oxygen from the air, by permeation, with a membrane generator, with pre-compressed dried air, up to 12 bars, and a compressor of about 300 bars.

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Plastic Forming Processes

Figure 3.36. Nitrogen generator using membranes (Air Liquide)

Advantages: – less polymer weight (20 to 50%); – faster cooling (by nitrogen injection); – less clamping force (20 to 30%); – no reinforcements or sunk spots; – simpler tools. Limitations: – higher investment cost; – importance of know-how, each part being a specific case study. A new competitive hollow injection system uses water instead of nitrogen. The water is fed by a piston pump. A pressure accumulator, with pressure up to 300 bars and speed at 50 l per minute acts as piston to push the plastic onto the mold walls with the advantages of: – a reduction in the cycle, by about 30%, thanks to the internal cooling of the plastic, the incompressible cold water, and the cost, with water replacing nitrogen; – a reduction in the cost of the raw material, with thinner walls, and a good stiffening of the molded part thanks to the column effect (possible diameter over 25 mm) brought by the hollow part, which gives a higher moment of inertia; – obtaining very smooth inside surfaces that can be useful. All of this is obtained at the expense of a damp atmosphere and more complex fluid flows.

Converting Processes

Polymer

Polymer

53

Screen

Water

Sprue

Water Measured water

Use of sprue

Measured water or sprue Polymer

Electro valve

Water pressure Injection of the plastic into the mold Injection nozzle for plastic

Mold

Sprue

Water

Injection molded press

Injection of water at 120 bars creating a piston effect Needle for water injection

Water flow

Emptying of the mold by gravity, the part is finished, the sprue can be removed

Mold cavity Plastic

Figure 3.37. Description of the hollow injection under water (source: Usine Nouvelle)

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Plastic Forming Processes

Applications of the two processes (nitrogen and water): hollow parts of variable thicknesses for cars (handles, armrests, spoilers), for furniture (seats, legs, armrests), for casings of TVs and appliances, for shoe heels, etc. 3.1.2.15. Injection molding on meltable core This is a variant of the manufacture of hollow or complex parts, for parts that cannot otherwise be ejected. This is solved by injecting polymer on a meltable core, similar to the lost-core foundry. The technique takes four steps: – shell molding of a metal core, a tin/bismuth alloy, 138°C melting point; – injection of the plastic on this core, for instance, PA 6-6, melting point 250°C; – melting of the core by magnetic induction and oil bath; – retrieving the part and the melted alloy, in order to recycle the core.

Mold Core box

Meltable core Induction oven Melting of the core Finished part Retrieving the core Figure 3.38. Description of injection molding with meltable core

The additional cost of the operation, compared to standard injection, is about 20%, but the finished parts are lighter, with a weight gain of about 50%, and do not need machining. The resulting finish is very good. Applications: massive parts, particularly in-take manifolds for thermal motors, volutes of centrifugal pumps, valve bodies (plus fewer loading losses, etc.).

Converting Processes

55

Figure 3.39. Car in-take manifold made with meltable core

3.1.2.16. Sandwich injection or co-injection molding The process is to inject successively two polymers that are different, but that can be blended, in the same mold. One will be the skin of the part and the other will be the core. The latter pushes the former like a finger in a glove. The molding cycle takes three steps: – injecting an amount of plastic that will be the skin. The injection pressure pushes forward a punch that blocks the polymer which will be the core; – injection of the plastic which is to be the core. The pressure pushes a punch backward. It blocks the skin polymer which has started to solidify on the mold walls. The flow selects the open space and pushes the first plastic against the mold walls, thus making a skin; – re-injection of a small quantity of the skin plastic, to close the part and to clean the runner for the next cycle. The result is a sequential injection.

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Plastic Forming Processes

Surface polymer (skin)

Valve

Mold

Reground (core)

Molded part

Densified skin

Foamed core

Figure 3.40. Description of sandwich co-injection

It is thus possible to make a sandwich in situ, ready-made, generally of the following materials: – skin: “noble” polymer (virgin, compact, rigid, good appearance, colored); – core: filler polymer, less expensive, second choice (recycled, reground, colored scrap).

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57

The raw material is thus optimized, hence the cost. It is possible to select rigid foam for the core, to avoid painting, as in the lighter injection molding described earlier. Advantages: – no need for finishing, the skin is good; – possible saving in material cost, by using recycled plastics for the core; – good sound insulation. Limitations: – the 2 polymers must be compatible; – necessary to require thick parts, to insure proper filling; – limitation to simple shapes, that can be reproduced; – high investment and conversion costs, with a 2-head press, double injection.

Figure 3.41. Part made by co-injection (recycled polymer in the middle)

Applications: car lights, ski bindings, casings for appliances, sewing machines, street garbage cans, furniture, etc. 3.1.2.17. Multi-plastic injection with precise interface The mold design makes it possible to inject first one plastic, then, after moving the drawers internally or after mold rotation, to inject a second plastic on the still hot first plastic, (good clinging), and even a third plastic, after moving another separator, or rotating.

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Plastic Forming Processes

2 head injection press Mobile separator

First injection

Mold

Second injection

Figure 3.42. Description of multi-plastic injection with interface

The advantage of this technique is that it makes it possible to inject different polymers successively, with an exceptional mechanical bonding, even when they are not perfectly compatible, even a flexible polymer on a rigid polymer, for instance PP + EPDM. Of course, polymers of different colors can also be associated with this process.

(a) (b)

Figure 3.43. (a) Multi-plastic pot: rigid container, flexible handle; (b) car rear lights made in multiple colors

Converting Processes

59

Types of applications: car lights, control displays for appliances, keyboards, parts featuring rigid and flexible elements, such as handles for cars and appliances, handles for toothbrushes and razors. 3.1.2.18. Multicolor injection molding This process requires injection units whose flows are grouped into a single runner which splits the colored polymers in the cavity. The injection can be carried out vertically or horizontally on the joint plane. It is thus possible to obtain various effects, stained, marbled, flower petals, with a good reproducibility. Types of applications: perfume and cosmetic boxes, eyeglass frames, brushes, combs, artificial flowers, etc. In this way, the injection molding of plastics has become, in just a few decades, a major technique for the manufacture of ready-made complex parts obtained with a short cycle. There are new developments still to come. 3.1.3. Compression and transfer These two processes were the basic plastics conversion, and are now falling into decline. 3.1.3.1. Basic materials The basic materials are essentially thermoset plastics, phenolics and amino resin powders, with appropriate fillers or vulcanizable elastomers. 3.1.3.2. Description of compression molding A powder thermoset is put into a heated mold. It will shape itself under the twofold effect of pressure and temperature, with the water vapor being eliminated. It can also be eliminated by slightly opening the mold, after closing it. The cycle is fairly long (about 1 min per 1 mm thickness). It can be reduced, by about half, by pre-heating the compressed powder, as a pellet, in a high frequency oven, before putting it into the mold. It is also possible to use a pre-plasticizer to avoid weighing, making into pellets and putting into the oven. However, in all cases, the gap between the two parts of the mold generates a loss of the thermoset at the mold parting line.

60

Plastic Forming Processes

Heated mold Molded part

powder or preform

Open mold

Ejector

Closed mold

Figure 3.44. Description of compression molding

3.1.3.3. Description of transfer molding This process is an improvement over compression molding. The thermoset polymer is put into a transfer chamber, which is integrated into the press. In the first step, the powder is melted. In the second step, the powder is transferred in a viscous state, with a piston, to the mold cavity, where the thermoset solidifies in the heat. The thermoset remaining in the transfer pot will harden as well and will be lost. Closed mold (pressing)

Compression mold

Sprue (lost)

Transfer mold

Molded part Figure 3.45. Description of the transfer

3.1.3.4. Comparative advantages Compression

Transfer

Thick parts

Possibility of thin walls

Minimal losses (mold parting lines)

Ease to install inserts

Cheaper molds

Shorter cycle, but higher losses

Converting Processes

61

As a rule, compression molding is preferred for massive parts, and transfer molding (intermediate between compression and injection molding) is preferred for more complex parts.

Hydraulic press

Mold

Figure 3.46. Description of compression molding press

3.1.3.5. Machines and operation Hydraulic presses are used, ranging between 10 and 400 ton clamping force, semi-automatic and automatic, sometimes with barrel, to reduce the operation cycle by sharing it. Operation pressures range from 150 to 300 bars, and temperatures range from 130° to 190°C for commodity thermosets. 3.1.3.6. Molds Molds are also heated, by steam or by heating fluid. They are relatively difficult to lubricate and to automatize, with limited gaps to avoid material infiltration. Molt types

Advantages

Disadvantages

Cutting

Simple

Losses of about 10% need for deflashing

Positive

Easy deflashing

Expensive need for exact assay

Figure 3.47. Various types of molds

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Plastic Forming Processes

3.1.3.7. Operational characteristics Capacity: weight < 1 kg Thicknesses: minimum 1 mm (for transfer) and 2 mm (for compression) Speed: § 1 min/mm thickness, for powder compression, and from 30 to 40 sec/mm thickness, for transfer with preheated compressed pellet.

3.1.3.8. Applications There are not many parts now made with these processes. Many have shifted to injection molding, with a faster operation cycle. There still are a few typical applications: pan handles for gas stoves, toilet seats, massive electrical parts. 3.1.4. Pressing between hot plates 3.1.4.1. Description The operation first features a continuous impregnation of the supporting reinforcement by a resin in solution, followed by cutting into sheets after having eliminated the solvent. The sheets are then stacked, put between two polished stainless steel sheets and pressed between heating plates.

Figure 3.48. Continuous impregnation

Converting Processes

63

A hydraulic press with several platforms is generally used. Heating plates

Stainless sheet

Clad copper

Kraft paper and PF resin

Reinforcement Resin

Decorative laminates

Printed circuits

Resin Kraft paper and MF resin

Large insulants

Reinforcement

Separating sheet

Electrical insulation

Figure 3.49. Description of pressing

Constituent materials: – reinforcements: Kraft paper, glass fabric; – resins: polyester, phenolics, epoxy, polyimide. 3.1.4.2. Operational characteristics Molding pressure: 80 to 100 bars Molding temperature: 160 to 170°C Plate area: up to 2 m x 1 m 3.1.4.3. Types of products The products can be large flat insulants, sandwich walls, decorative laminates for furniture and substrates for printed circuits. In this latter case, the upper sheet is made of a copper clad sheet, to be finished by electrochemical machining.

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Plastic Forming Processes

Figure 3.50. Press with heated plates

3.1.5. Reaction injection molding (RIM) 3.1.5.1. Definition This process features a di-isocyanate and polyol measuring machine and a press with a mold into which these very reactive liquid pre-polymers are injected. These liquid pre-polymers polymerize into shape directly in the mold (the reaction is exothermic).

Converting Processes

65

Low pressure press Polyisocyanate

Polyol Mold Molded part

High pressure pump

Figure 3.51. Description of reaction injection molding

Advantages Direct use of monomer (cheaper than polymer) Feed filling of the mold (liquid) and ease of obtaining fine details Molding under low pressure allowing large parts without high energy expenses (exothermic reaction) Versatility (rigid, semi-rigid, flexible)

Limitations Need for frequent cleaning of the mold as PUR sticks Need for deflashing limited to compact PUR (limited mechanical parts) Need to paint to obtain a nice appearance

3.1.5.2. Base materials Mainly PUR, polyols (poly-alcohols) and polyisocyanates (polyether and polyesters), in liquid form. 3.1.5.3. Machines and operation The feeding of the components is carried out with a high pressure (150 to 200 bars) piston pump, of the injection type with a Diesel motor to blend them, at counter flow. It then pushes them into the metal mold in which the material expands under low pressure (about 5 bars). The mold is fixed by a closing device that can oscillate to ease the feeding and the ejection (opening in portfolio). 3.1.5.4. Operational characteristics Most frequent capacity: up to 10 kg Optimal thicknesses: rigid = 5 to 10 mm, semi-rigid = 3 to 5 mm Speed: 20 to 40 large parts per hour

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Plastic Forming Processes

3.1.5.5. Applications Automotive (grilles, dashboards, body parts), motorcycle bodies, ski boots, over molding of gas house containers, office machines, medical applications, etc.

Figure 3.52. RIM molding presses and tools

3.1.5.6. Trends New more reactive and self-cleaning formulations of polyureas (polyetherspolyamines/aromatic polyisocyanates), by incorporating a demolding agent, are now being developed: – time saving over the cycle of about 30%; – less frequent mold cleaning, about every 50 parts.

Converting Processes

67

Caprolactam, a precursor of PA is also used with pre-polymer and elastomer activator. This formulation is of interest when making thick parts, or parts with very variable section thicknesses, but it has fabrication constraints, with formulation cooling and mold heating. It is likely that this concept that enables us to make large and precise parts, directly obtained under low pressure, and directly starting from the monomer, will develop in the future. 3.1.6. Casting and inclusion 3.1.6.1. Definition In the casting process, the plastic to be molded is poured, in liquid state, into a mold the shape of which it takes when solidifying. In the inclusion process, a part already placed into the mold is drowned in the plastic. Advantages Simplicity

Limitations

Low investment

Low mechanical strength (not much bonding)

Possibility of making thick parts

Risk including air bubbles

3.1.6.2. Selected polymers The polymers used in these processes are in liquid form, essentially acrylics, polyesters, epoxy and silicones. 3.1.6.3. Operation and molds In order to obtain a good consistency in the molded part, it is necessary to shake the mold and to let the liquid polymer rest to help remove the bubble (manual technique) or to operate under a vacuum (industrial technique). Depending upon the size of the run, the molds can be made of plaster, plastics (PE, silicone), metal or even glass. 3.1.6.4. Applications Applications vary, depending upon the length of the runs. 3.1.6.4.1. Hand-made products Conservation of specimens (anatomical, zoological, geological) or reproduction of artworks.

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Plastic Forming Processes

Model to duplicate Filled and colored resin

Silicone mold Support mold

Figure 3.53. Duplication of a model, from a silicone mold

Figure 3.54. Reproduction of artwork

Converting Processes

69

Museums often use this technique to duplicate artworks for sale. The original is used to make a silicone mold which carries the finest detail and which makes demolding easy. A polyester resin, with mineral fillers (chalk, slate, microspheres, metal) is poured into the mold in order to achieve the same density as the model. The resin also contains colorants. However, the colors are also coated on by paints or varnishes to match the original, even showing the defects it may have. An expert from the museum checks the results and destroys any faulty copies.

Figure 3.55. Inclusions

Industrial production: thick acrylic plates, thicker than 4 mm, by catalytic casting of the monomer between two sheets of glass or coating of electrical and electronic components, called encapsulation. Monomer

Sheet of glass PVC joint

Oven

Plate of PMMA

Figure 3.56. Casting of methyl methacrylate to make plates of PMMA

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Plastic Forming Processes

Figure 3.57. Model duplication, wood imitation

3.2. Manufacture of long products 3.2.1. Standard extrusion Extrusion is still the most important plastics conversion technique, with about 40% of production volume. All the pellets used for many other conversion processes are obtained by extrusion. 3.2.1.1. Description Polymer pellets are softened in a preheated barrel heated by electrical resistance. There is a screw inside that turns and pushes the heat-and-friction-plasticized material in a continuous way, up to the end called die. The shape of the extrusion die makes it possible to obtain a broad range of products (threads, profiles, tubes, films, sheets, plates). Advantages Continuous operation, easy to automatize

Limitations Screw can be adapted to the selected polymer Screw can be adapted to the selected polymer

3.2.1.2. Breakeven The tuning and the balancing of an extruder takes about an hour, hence the need to work for several days of the same type of production, about one week, equivalent to several kilometers of extruded products.

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71

3.2.1.3. Polymers used Mostly PE, PP, PVC, PS, to a lesser extent, ABS, PET, PMMA, PC, PA. Extrusion is possible for all thermoplastics. 3.2.1.4. Installation The basic machine, the extruder, like injection presses, features upstream a plasticizing system, with a feeding hopper. The extruder is essentially a barrel heated by electrical resistances, and a screw (an Archimedes screw) driven by a gear motor.

Extruded profile

Die

Heating resistances

Barrel

Screw Gear motor

Figure 3.58. Extruding machine operation

Figure 3.59. Different components

Powder or pellets

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Plastic Forming Processes

A plastic extrusion unit includes the following parts: – storage silos for the plastics, with measuring and blending devices; – an extruder with filter changer; – a die of adapted selected shape; – a control system; – a stretch station; – a cutter or winder at the end of the cycle. There are three types of extruders: – standard single screw: plasticizing is achieved under heat, with the heating resistance in the barrel, and with the effect of the screw mixing, at a speed of < 150 rpm; – adiabatic single screw: the fast rotation speed of the screw, 250 < v < 1,000 rpm, provides most of the heat; – multi-screw: generally, with 2 screws rotating in opposite directions, there is a pump effect, particularly when preparing PVC or various compound formulations; – screw: the extruder characteristics are the screw diameter, its length and its compression rate. Diameter, in mm: the output rate increases with the diameter, the standard diameter ranging from 50 to 20 mm. Screw diameter (mm)

Theoretical output rate (kg/h)

60

100-150

90

200-300

120

300-600

150

600-1,000

(a)

(b) Figure 3.60. (a) Typical extruder; (b) extrusion shop

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3.2.1.5. Link between screw diameter and output rate The actual output rate depends upon the shape of the die, in general about 50% of the theoretical output rate. Ratio: length/diameter (l/d = 20 to 35). It differs for each polymers and it gives the compression rate. The profile of the screw determines 3 functional zones: feeding, compression, extrusion.

Feeding

Figure 3.61. Various functions of the screw

3.2.1.6. Filter The role of the filter is to transform the flow generated by the helical screw into a straight regular flow, and to stop the unmelted pellets and the recycling impurities. The filter must be often changed for cleaning.

Hydraulic cylinder Filter Extrusion direction

Figure 3.62. Filter changer

3.2.1.7. Tooling There are several types of dies: profile die, tubular die, slot die, for a broad variety of products and activities, as shown below.

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Plastic Forming Processes

Profile

Shaped die Extruder screw

Tubular die

Thick tubular

Tube Sheath

Gap Film

Heating resistances

Sheet

Flat die

Plate

Figure 3.63. Various types of dies and typical products

Pellets, which are the basic elements of thermoplastics, are obtained by extrusion as well. After polymerizing the polymer, the resulting powder is converted into rods in a specific extruder. After cooling, the rods are cut into pellets.

Circular die (extrusion)

Cutting Flat die (extrusion)

Cooling (water)

Cooling (water)

Figure 3.64. Making of plastics pellets

Pellets

Converting Processes

75

3.2.2. Extrusion with shaped die 3.2.2.1. Monofilaments (ø < 0.1 mm) The die features a large number of openings (50 to 100) which make as many filaments that are dipped alternatively into cooling baths and stretching systems, to achieve the final thread characteristics. The fibers are made in the textile industry, by spinning: – wet, for viscose; – dry, for cellulosics and for chlorofibers; – by melting, for polyamides and polyesters. Collodion

Die

Extruder

Hot air Solvent

Die

Die

Glue bath

Serration

Fiber shapes

Tubular

Figure 3.65. Making of manmade fibers

In the first two processes, the coagulation and evaporation of the solvent cause the formation of serrations, creeks or lobes around the thread. In the third process, the thread more or less keeps the tubular shape given by the die. 3.2.2.1.1. Basic plastics Mainly PET and nylon, also PP, vinylics, acrylics, polyurethanes and cellulosics.

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Plastic Forming Processes

3.2.2.1.2. Applications Fibers for the textile industry: – stretched monofilaments: thread, net, brush horsehair, carpet tuft, wall coverings; – non-wovens (sometimes coated): bags and purses, luggage, clothes, car mats; – fibrillation, twisting: twine, cordage. 3.2.2.2. Profiles A shaped die is used, followed by cooling through dipping into water, then a sizing system, then a device to help draw the profile out. The profile is either wound, if it is a flexible profile, or cut at the desired length with a saw, if it is a rigid profile.

Shaped die

Winding (flexible plastic)

Sizing system

Cooling

Drawing Cutting (rigid plastic)

Extruder

Figure 3.66. Profile extrusion with shaped die

(a)

(b) Figure 3.67. (a) Die for a window profile; (b) profile extrusion

Converting Processes

77

3.2.2.2.1. Basic plastics Mainly PVC, also PE, PS, possible for all thermoplastics. 3.2.2.2.2. Applications Rigid and flexible profiles, for automobile, building construction, of which a large part is for windows, furniture, etc. 3.2.2.3. Extrusion with tubular die 3.2.2.3.1. Rigid tubes, thick walls The extrusion for flexible tubes is similar to that of profile extrusion. In the case of rigid tubes, there is a precise sizing system, by depression or pressure, and a stronger drawing device. The rigid tube is then cut to length by a saw. The tube is then taken to be broadened at one end, after re-heating. Die and conformer

Moving saw

Heating Cooling Extruder

Drawing device

Recovery Tulip shaped

Figure 3.68. Extrusion of rigid tubes and end broadening

Figure 3.69. Multiple extrusions of small diameter tubes

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Plastic Forming Processes

Figure 3.70. Extrusion of rigid tube with thick wall tubular die (source: Battenfeld)

Basic plastics Mainly rigid PVC and PE-HD, also flexible PVC and PE-LD for small diameters. Installation At the end of the extruder, there are: – squeeze rollers, essential to obtain a perfect tube, according to specifications. The operation involves vacuum and/or air pressure; – a water cooling container; – a drawing bench; – a device for cutting to length, with a moving saw; – and, possibly for re-runs, as the extruder operates in a continuous process, a device to broaden the end of the tube on a barrel, which will enable us to nest the tubes one into the other to build up the required length. Applications Tubes for building construction and public works, agriculture and industry, etc. up to 400 mm diameter and sometimes more. There are even mobile extruding machines, for specific applications, by bringing the extruder on site, or by extruding the tubes on water to be more easily shipped, in order to make very long tubes in one single piece.

Converting Processes

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Figure 3.71. Various types of conformers

3.2.2.3.2. Thin sheathes A thin sheath (a thin tube of less than 0.2 mm thick) is extruded and pinched between rollers. Air pressure enters into the sheath thus closed, in order to stretch it until the required reduced thickness is reached. The bubble thus made is cooled by air, flattened and wound-up. Installed equipment The equipment includes, besides the extruder: – an angular extruder head, from which the bubble generally rises. Sometimes the bubble is horizontal or sinks, in case of small diameters; – a bubble cooling system with cool air brought by external round nozzles; – a pressure ram with non-adhesive slopes, or a train of rollers to flatten the bubble, followed by pinchers that make it watertight; – a winding device of the flattened bubble on a coil.

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Plastic Forming Processes

Elastomer cylinder (airtight proof) Guiding

Bubble Air pressure

Air cooling

Extruder

Tubular die

Winding

Figure 3.72. Extrusion-blowing of sheath with thin tubular die

Converting Processes

(a)

(b) Figure 3.73. (a) Details of the extrusion head and the die; (b) calibration system of the bubble

81

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Plastic Forming Processes

The height between the die and the pressure ram must enable cooling and avoid self-sticking.

(a)

(b) Figure 3.74. (a) Typical equipment shop; (b) tubular die

Converting Processes

Figure 3.75. Standard bubble, for PE-LD

Figure 3.76. Swan neck shaped bubble for PE-HD

83

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Plastic Forming Processes

The extrusion blowing of PE-HD makes it necessary to first let the swan neck sheath exit before inflating it. Basic plastics Mainly PE-BD or HD, and flexible PVC, yet no rigid PVC or PS. This process (extrusion blowing) enables the production of very wide films, with dies up to 1.80 m diameter, fed by a very large extruding machine, yielding sheaths of 5 and 6 m diameter. The films can thus be up to 20 m wide, whilst calendered films do not exceed 4 m. However, this technique requires a very high shop ceiling, to have room for the bubble to cool.

Figure 3.77. Very large diameter bubble (source: Dolci)

There are two important improvements that have helped the development of extrusion blowing: – improvement of the thickness consistency: in standard films, thickness tolerances are of + or -10%, at best. This cannot be accepted on an automatic packaging machine. Hence in order to cope with this defect, the die, or the table, is turned, so that the film is evenly distributed around the sheath; – productivity increase: with the introduction of a (sintered) porous tube in the bubble, it is possible to extract the hot air inside, and to take cooler air in, thus gaining about 50% speed.

Converting Processes

th1

85

th2

a) Static die (fixed defect: th2 > th1)

b) Turning die or table (moving defect; thickness balanced)

Figure 3.78. Film thickness control

Sintered tube

Cool air intake Hot air exhaust

Figure 3.79. Bubble cooling system

Lengthwise stretching is obtained by the difference of extrusion and winding speeds. Crosswise stretching is obtained by the differences of the diameters of the die and of the bubble. The film is normally single oriented, as the dominant stretch is in the drawing direction. However, it is possible to make a bi-oriented film, by modifying the inflating rate. Then the lengthwise stress and the crosswise stress are about the same.

86

Plastic Forming Processes

Mono-oriented

Bi-oriented

Figure 3.80. Mono-oriented and bi-oriented films

Applications Films made by extrusion blowing are mainly used in packaging, as bags and sacks (volume from small bags to heavy duty bags of more than 50 liter contents) for pallet wrap as stretch and shrink films, for bundling collation and for automatic packaging, such as Form/Fill/Seal (FFS). The extruded sheath can be used as such, to make bags by sealing on the side, or it can be opened as film, by slitting it with a guide line. A system of wedges sunken into the forming sheath makes a bellows shape, if required.

Figure 3.81. Various bags that can be made

Formed hood Pallet load to wrap

Oven

Pallet

Shrink sheath

Pallet wrap with shrink hood

Figure 3.82. With shrink film

Converting Processes

87

Figure 3.83. With stretch film

3.2.2.4. Flat die extrusion This type of die is made of a narrow and adjustable exit slot, of rectangular cross-section, through which the hot film or sheet passes. The film or sheet is then formed into shape on cooled cylinders, or by dipping into water in the case of thick sheets. Flexible films are wound directly, whilst rigid sheets are cut to the required length with a mobile saw. 3.2.2.4.1. Installation The die used here is a lip die, of narrow rectangular cross-section, to allow better thickness adjustment, followed by a chilled roll cylinder, to make a cast film, or by a calender to cool and perfect the thickness, and to finish up the surface when extruding sheets or plates. Then, in both cases, there is a winding device for small thicknesses (< 1 mm) or a cutting machine for thick plates (> 2 mm).

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Plastic Forming Processes

Goods to wrap

Straight film wrap

Film slip on

(a)

(b) Figure 3.84. (a) Pallet wrap with shrink or stretch film; (b) automatic packaging

Figure 3.85. Flat dies with lip adjustment

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3.2.2.4.2. Basic plastics Pellets, PE, PP, flexible PVC, and, to a lesser extent, PMMA, PC, rigid PVC. 3.2.2.4.3. Applications: films (thickness < 0.2 mm) Films thus obtained have better thickness tolerances than films made by extrusion blowing. This is why these films are used in industrial applications, or for automatic packaging, or as multilayer laminated film support. These films can also be grained after exiting the die, to give a textile appearance. There is a variant for PP. The film is split into thin tapes, to be woven, to make the backing of tufted carpets or for large carrying bags. Pellets Flat die

Extruder

Cooling cylinder (chilled roll)

Film on reels

Figure 3.86. Films

Applications: magnetic tapes, video tapes, capacitor dielectrics, diapers, disposable towels, automatic packaging, films for multilayer laminates.

(a)

(b) Figure 3.87. (a) Flat die film extrusion; (b) flat die

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Plastic Forming Processes

Applications: all films requiring precise thicknesses, multilayers and for tapes. Tapes and strips are made from cut films (PP, PET).

Figure 3.88. Manufacture of fibers and tapes

Applications: – weaving: bags, carpet backing; – stretching: gift-wrap tapes, magnetic tapes, strapping strips. 3.2.2.4.4. Sheets and plates (thickness > 0.2 mm) The fabrication of sheets and plates is more complex, since in order to obtain a thick shape of consistent profile it is necessary to use counter-blades or narrowing bars, before the material moves through the die, to force the material to reach the ends of the die, then to readjust, by proper pinching of the die lips. The adjustment is thus more difficult. The major application is for sheets for thermoforming. It is also possible to take advantage of the time when the sheet is still hot to give it a grained appearance, or a rippled effect.

Converting Processes

Pellets

91

Polish cylinder

Cooling Sheet wound-up in reels

Rough profile not corrected

Blade to perfect plastic distribution

Finished profile once corrected

Finish hinge die

Figure 3.89. Flat die sheet extrusion

Figure 3.90. Typical flat die extrusion equipment

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Plastic Forming Processes

Applications: sheet for thermoforming and various building applications. Ray gauge Sweep in 60 sec Sweep in 30 sec

Frequency

30 sec 60 sec Manual

Calculator The faster the sweep speed, the most precise and well controlled the adjustment

Figure 3.91. Thickness control

Operational characteristics Dies

Products Threads

Shaped

Tubular thick Tubular thin Flat die

Dimensions th < 0.1 mm

Speed, m/min 150 to 250 m/min

Small profiles

10 to 50 m/min

Large profiles

1 to 10 m/min

Example: windows

3 to 5 m/min

Small tubes

= 10 mm PE

60 m/min

Large tubes

= 50 mm PVC

5 m/min

Films

th < 0.2 mm, l < 4 m

20 to 40 m/min

Films

th = 0.2 to 0.8 mm

60 to 80 m/min and over

Sheet

th = 1 to 2 mm

§ 10 m/min

3.2.3. Specificities of extrusion A first specific extrusion route is to protect or set apart a metal or plastic support by the following operations.

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3.2.3.1. Extrusion-covering This is the standard insulation of electrical wires and cables. The metal core moves through a larger diameter die and comes out covered with a plastic sheath. The difference of the radius of the wire and of the sheath is the thickness of the insulation. This operation is performed in a single step for low voltage electrical wire, but it requires many stepped insulation passes for high voltage or underwater cables. Metal wire

Insulated wire

Angular extruded head Polymer

Figure 3.92. Extrusion covering of small diameter wires

Figure 3.93. Extrusion covering of cables

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Plastic Forming Processes

3.2.3.2. Extrusion-embedding of metal tubes The purpose here is to protect tubes against oxidation by covering them with a plastic sheath, essentially made of PE. This can be performed in two ways: – by longitudinal extrusion, for diameters less than 300 mm. The tube moves inside the L designed extruder head; – by transverse extrusion, for diameters over 300 mm. A flat die extruder is used. It puts a sheet of a thickness adapted to the tube around helically. Steel tube diameter < 300 mm

Steel tube diameter > 300 mm

Polymer (PE) Induction heating

Angular extruder head

Flat die extruder

Polymer

Figure 3.94. Extrusion-embedding of metal tubes

Typical application: surface protection of oil or gas pipelines, buried or overhead. Similarly, it is possible to cover a metal profile with a polymer to protect it from weather conditions.

Figure 3.95. Profile protection with plastics extrusion

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3.2.3.3. Extrusion on cylindrical mandrel When flexible tubes or profiles are extruded, and then hot wound on a mandrel, the result when unwinding is: – either a spiral tube when starting from a flexible tube wound on a small diameter mandrel (Figure 3.96; applications: compressed air ducts for workshop pneumatic tools, or truck brakes, made of nylon); – or a tank, if the extruded product is a flat profile wound on a large diameter mandrel, then heated to make a poly-melt when winding (Figure 3.97; application: storage tanks for liquids made of PE-HD); – or a flexible tube if the profile, of adequate shape, enables snapping on when winding (Figure 3.98; applications: flexible conduits used in air conditioning).

Figure 3.96. Compressed air ducts

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Plastic Forming Processes

Figure 3.97. Silos

Figure 3.98. (a) Welded profile; (b) shrunk on profile

3.2.3.4. Extrusion-canvas covering of plastics tubes (also called “knitting”) A first extruder makes a flexible tube (PE or PVC) on which a thread canvas, made of PA or PET, is deposited in a helical way, with parallel equipment. The whole resulting product is put through a second extruder of larger diameter die, to protect the canvas by covering it. The result is a higher resistance flexible tube. Another approach is to modify the structure or the shape of an extruded profile by the following methods.

Converting Processes First tube

97

Final tube (reinforced)

Textile thread bobbins Second extruder

First extruder

Figure 3.99. Extrusion canvas covering of plastics tubes by equipment in parallel

3.2.3.5. Extrusion of cellular profiles By introducing a foaming agent in the sheath of the extruder, upstream of the die, the polymer is expanded. The expansion can be controlled at the conformer after passing through the die. The result is cellular profiles. Polymer

Foaming agent

Compact skin Cellular core

Extruder

Finished structural Expansion profile and calibration

Figure 3.100. Extrusion of cellular profiles

Applications: – rigid profiles made of PVC (0.5 kg/dm3: called structural foam) used in building construction and in furniture. The profiles can be nailed, worked like wood, and used as skirting or frames;

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Plastic Forming Processes

– profiles made of EPS foam (20 to 40 kg/m3) most often cut into small parts that can be used as protective cushioning packaging, as chips. They can also be used as such to make plates for thermal insulation; – flexible profiles of PE or PP (50 to 120 kg/m3) used as anti-vibration in packaging, as body protection in active sports, as cushioning support core in tennis rackets and as anti-shock absorbers in car bumpers.

Figure 3.101. Extrusion of EPS sheets

3.2.3.6. Extrusion of corrugated tubes The tube that is still hot when leaving the die is taken into a tool system in which it is put out of its shape by a set of mobile jointed pads, before cooling to fix the shape. A set of rollers is dedicated to each diameter. This means it takes a fairly long time to change the tools. Mobile jointed parts

Corrugated tube Extruder

Figure 3.102. Extrusion of corrugated tubes

Converting Processes

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Typical applications: flexible insulation sheathing for electric cables, conduits for air conditioning, scaffolding protection in cities. 3.2.3.7. Direct net extrusion (meshing) Starting from tubular dies (oscillating or rotating) it is possible to make meshed sheaths: – by oscillating die, made of a notched piston and a cylindrical part. When put at a low position, the plastic comes out as rods, following the notches. When put at a high position, the plastics comes through as a ring. The combination of the two phases, vertical rods and horizontal ring, results into a sheath with a right angle mesh net; – by rotating die, the moving and the fixed parts have notches, that are alternatively face to face, providing a large through section, or out-of-line, thus reducing the section. It is thus possible to obtain, gradually, a meshed sheath in a lozenge shape. In both cases, dimensions depend upon the die geometry and their oscillating or rotating speed. A toothed wheel device can also be used, which perforates the extruded product, giving a holed sheath.

Low position (rods)

High position (rings)

Oscillating die Notch

Extruder

Notched piston Cross-section view

Resulting mesh sheath

Figure 3.103. Mesh net extrusion with oscillating die

"Trical" die

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Plastic Forming Processes Spin

Resulting meash sheath Extruder Rotating die Fixed die Notches Cross-section view

"Nelton" die

Figure 3.104. Net extrusion with rotating die

Converting Processes

101

Plastic intake

Toothed wheels

Perforated film

Film

Toothed wheels Laminet die Figure 3.105. Extrusion of perforated sheath

Applications: colored nets for public works, trenches for pipes, blue for water, red for electricity, green for telephone, protection of finished parts, covers for sausages and bottles, garden fences, windbreaks, etc.

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Plastic Forming Processes

3.2.3.8. Extrusion with open-close system A system of tight closure, which can be opened and closed by pressure, can be used in bags (Minigrip patent).

Detail of sheath making Sheath 2 Sheath 1

Extruder Figure 3.106. Extrusion with an open-close device

Application: small leakproof bags. It is also possible, always by extrusion, to add new functions by the following means. 3.2.3.9. Bi-oriented extrusion The standard manufacture of films and sheets involves an internal dominating stress, in the drawing direction, that is lengthwise. In order to improve on it, there can be crosswise stretching, with articulated mobile clips. A correct balance of lengthwise and crosswise tensions is thus achieved. The bi-oriented films offer better mechanical properties, and often a good clarity as well, particularly for PP and PET films. This bi-orienting operation is performed immediately after flat die extrusion, or with a double bubble. Generally there is a fixing treatment in an oven.

Converting Processes

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Tunnel oven for stabilizing

Extruder Flat die Stretching dips Mono-oriented film

Bi-oriented film

Figure 3.107. Flat die extrusion with film bi-orientation

Figure 3.108. Flat film bi-orientation equipment

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Plastic Forming Processes

Applications: packaging films for food and hosiery (PP), capacitors (PP and PET), audio and video magnetic tapes (PET), flexible printed circuits (PI). A similar result can be obtained with the Shorko double bubble process.

First bubble

Extruder

Second bubble S2 S1 Re-inflating Re-heating

Extruder

Figure 3.109. Extrusion with double bubble and typical equipment

Converting Processes

105

3.2.3.10. Co-extrusion It is possible to make multilayers of various polymers by using several extruders. The layers are directly extruded together if they are compatible. Tie-layers of various adhesive polymers are extruded in-between. A standard commodity plastic that is not very expensive is generally selected for support (PE, PVC). One or several other plastics will be co-extruded or laminated, up to 7-8 layers, to provide specific properties. Just like standard extrusion, co-extrusion varies with the type of extrusion die. 3.2.3.10.1. Shaped die Profiles of complex cross-section can be created by associating extruded products issued from the same polymer or from compatible polymers. The various extruded products can be of varying profiles, rigidity or color. They are hot welded at the end of the die. Extruder 2 Flexible joint Flexible PVC Rigid PVC

Mixed profile Rigid support

Extruder 1

Figure 3.110. Profile co-extrusion

Applications: windows and doors, lights, colored decorative tapes, etc. 3.2.3.10.2. Tubular die and bubble The flows of several extruders pile up into a common tooling die, without mixing but self-gluing, to create multilayer films in which the preferred support is mainly thick PE-LD. The association of the multilayers includes thin barrier layers, the barrier polymer being put inside.

106

Plastic Forming Processes Pulling

Air cooling

Double die Extruder 1

Extruder 2

Figure 3.111. Film and sheath co-extrusion

Typical application: flexible multilayer films.

Multilayers A-B

Converting Processes

107

3.2.3.10.3. Flat die A common head is used for several extruders. The flow of each plastic follows the direction originally selected for it. The hot extruded products stick together. This process is most often used for three-layer plates, featuring a support, a core of recycled material and a top finished layer that is colored or decorated.

Extruder A

Rat Die

Extruder B Multilayer sheet A-B-C

Extruder C

Figure 3.112. Film or sheet co-extrusion

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Plastic Forming Processes

3.2.3.11. Extrusion lamination (or coating) The various processes described above enable us to make plastics/multilayer plastics. Extrusion lamination makes multilayer featuring plastics associated with non-plastics supports, such as paper, cardboard, aluminum, textiles, the associated polymer always being an olefin. Polymer (polyolefin)

Flat die

Extruder

Paper or aluminum bobbin

Multilayer PE-paper or PE-aluminum

Figure 3.113. Extrusion-lamination for PE-aluminum or cardboard multilayers

A flat die is used. It is placed above the support to be coated, and perpendicular to the general direction axis. The hot film, coming from the die, falls as a curtain on

Converting Processes

109

the hot support, aluminum or paper, and clings to the pinholes of the support that continuously flows underneath. Two or more layers are thus obtained, and many more successive layers can be obtained with as many distributing extruders. Typical applications: cartons for milk, fruit juices, frozen foods, etc. 3.2.3.12. Extrusion-cross-linking In this process, the linear structure of an extruded product made of PE, tube or film, is transformed by cross-linking, to improve performances, mechanical resistance, weld strength, optical transmission, heat shrink, etc. The reticulated PE then behaves in the same way as a thermoset. There are two ways to proceed when extruding: – by chemical reaction, with silanes (PRS): ~ 2%, or silicone oil (PRC) and tin peroxides that cross-link PE with water; – or by alpha beta radiation of the extrudate, with peroxide formation, followed by acrylic acid grafting. An irradiating device is necessary, with adequate radiation protection. PE + silane

Extruder

Tin peroxide + colorant Water Extruder

Figure 3.115. Chemical cross-linking

Acrylic acid

Extruder

Irradiation device

Figure 3.116. Cross-linking by irradiation

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Plastic Forming Processes

The main applications are: – under floor heating, over (40°C) in housing with cross-linked PE, PEr, put into the insulation of the concrete floor layer. This is also used to cool skating rinks (-10°C); – fittings or shrink electrical conduits, for splicing or marking; – multilayer films of PE/aluminum for lids; – PE foams.

Figure 3.116. Floor heating of cross-linked PE

3.2.4. Calendering 3.2.4.1. Definition After blending and mixing PVC and various additives, a dry paste is obtained. It is forced into two heated cylinders, gradually getting closer, until the required sheet thickness is reached. The sheet is then fixed on a cooled cylinder train.

Converting Processes

111

PVC

Working cylinders (hot)

Elliptical profile

Standard profile

Cooling cylinders (cold)

Sheet on reel

Figure 3.117. Description of calendaring

Advantages

Limitations

High production rate

Large investment

Good geometrical product definition

Only for PVC

3.2.4.2. Breakeven The right balancing and adjustment of a calender may take half a day, hence the need to start for large orders (of about 100 tons minimum) only. 3.2.4.3. Installation and operation The fabrication sequence is as follows. 3.2.4.3.1. Upstream Formulation preparation with mixer of the Branbury type, or with a specific filtering extruder, with magnetic separator, to prevent metallic particles on the cylinders. Calender, with 2 to 5 working cylinders, most often 4. Each cylinder is driven by an electric motor, of variable speed, in order to adapt the friction to the selected formulation and to the calendering temperature. The diameter of the cylinders is about 800 mm. Cylinders are made of chill castings, of Brinell hardness 500. They are heated by an internal heating fluid. Any irregularities of the sheet thickness come

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Plastic Forming Processes

from bending of the cylinders from their original profile, cylindrical or slightly bulged. This defect is corrected by the following two devices. By roll bending: this means bending in the clamping direction, in order to correct its flexion from the original profile. The external secondary pads are pushed with hydraulic cylinders. The correction thus obtained is about 0.05 mm.

Figure 3.118. Cylinder bending

By cross-axing: this means to slant one of the cylinders away from its original axis in order to artificially compensate the bending. This is obtained by two motors driving the cylinder pads. The correction thus obtained is 0.2 mm, as a maximum.

Figure 3.119. Cylinder cross-axing

Converting Processes

113

In both cases, as in flat die extrusion, the plastic is pushed towards the outside of the cylinders, where it does not flow naturally, to control the thickness. In general, the two effects are combined. 3.2.4.3.2. Downstream There are finishing systems, particularly: – de-sticking rolls, which stretch the sheet out of the calendar; – graining rolls. Since the sheet is still hot, it can be of interest to decorate it with a grained effect, with a dedicated cylinder; – cooling rolls. These are stainless steel cylinders, cooled by inside circuits of refrigerated water, in order to fix the sheet. The number and size of the cooling cylinders depend upon the selected formulation; – thickness control. The equipment is based of beta ray absorption that is completely automatic. The fully automatic gauge sets the spacing of the cylinders in order to obtain a consistent thickness; – festoon accumulators (possibly); – edge trimming and drawing rolls, to obtain the sheet at its required width. The scrap is automatically removed, then recycled; – winding and cutting machines, with hand cutting (with accumulators) or automatic cutting, feeding in a new reel and removing the finished reel.

Figure 3.120. Calendering equipment (COMERIO)

Calendering equipment varies, depending upon the types of products to be produced. Calendering machines are often an inversed L shape, for rigid PVC and a Z shape for flexible PVC. There are only 2 cylinders to make PVC floor tiles. Calendering and coating can be associated to produce PVC felt floors. While the sheet is still pasty, it can be lined with a textile support: heavy coating.

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Plastic Forming Processes

Production speed: from 1.5 tons per hour for rigid PVC, to 4 tons per hour for flexible PVC, with exit speeds of about 100 to 120 m/min.

(a) For rigid PVC

(b) For flexible PVC Figure 3.121. Calendering with heavy coating

3.2.4.4. Applications Calendering only: – films or sheets for thermoformed packages (for biscuits, candies, fruits, pills), blisters, tablecloths, curtains, car upholstery, etc.; – sheets for high frequency welding: plastic handbags and luggage, office stationery, inflatable articles, etc. Calendering + coating, or heavy coating: floor coverings, 2 kg of PVC per m². Calendering thus described is not to be confused with the 3-cylinder calendering device used at the end of flat die extrusion. In the first case, the cylinders are heated, as part of the operation, in the second case, the cylinders are cooled, as part of finishing. 3.2.5. Coating (flexible PVC or PUR) 3.2.5.1. Definition A textile support is impregnated, on the surface, without going through, by a plastic in a paste or solution state. The plastic is deposited by casting, often as PVC plastisol, sometimes as PUR. The result is a plastic coated textile.

Converting Processes

115

Blade Carried roll Support roll Plastisol

Oven Fabric

Plastic coated fabric

Support roll (elastomer)

Figure 3.122. Coating definition

Advantages High production rate

Limitations Large investment required, as in calendering

3.2.5.2. Equipment The various steps of this process proceed as follows. 3.2.5.2.1. Upstream The preparation of paste by grinding and mixing, for vinylics in plastisol form (PVC + plasticizer + solvent), requires that the pastes can be coated only after 23Day aging. This operation is faster for PUR. Coating machine: standard coating machines feature a metallic scraping blade and use plastics as paste transferred on to the textile support with an intake and a measuring roll. A third roll brings the textile to be coated, a knitted fabric. The three rolls rotate in the same direction. The thickness of the plastic to be coated is controlled jointly by the cylinder air gaps and by the ratio of their respective speeds. The coated fabric is then sent to an oven, to be gelled, with solvent evaporation. In a fast operation rate, there are 2 or 3 successive heads, to deposit, for instance, a first bonding coat, then a microcellular second coat, ending with a finishing coat. Tunnel oven for gelling: this is composed of several temperature zones, with the same heating in the center and on the edges to avoid irregularities. The heating is obtained by gas, electricity or infrared.

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Plastic Forming Processes

There are two main types of coating: – light, by direct transfer or by reverse roll; – heavy, with calendering first.

Figure 3.123. Two coat coating by reverse roll

Figure 3.124. Heavy coating by calendering

In another technique, sprinkling-fusion, the process starts with PE or PVC powder, which is melted on the textile support in the oven. 3.2.5.2.2. Downstream There are, like in calendering, thickness control devices, cooling rolls, edge cutters, accumulators, winding devices, but the graining and printing operations are performed in a second step. The PUR coated fabrics are sometimes weathered to give them a leather appearance. 3.2.5.3. Applications With scraping blade: plastic coated fabrics are used for tarpaulins, sheeting, camping, (800 g/m²), plastic handbags and luggage (400 to 600 g/m²), car seats and furniture (250 to 500 g/m²), wall coverings, tablecloths. With calendering coating: flooring. With sprinkling: battery separators, non-wovens, etc.

Converting Processes

117

Coating and impregnation must not be confused. In coating, the fabric is covered but not fully saturated. With impregnation, the fabric is fully impregnated at the soaking (for instance laminates). The technique described above is standard coating, but there is also a heavy coating, associated with calendering, and a light coating, or thin coating, by extrusion-lamination. Of course, the fabrication of photographic films, audio and video tapes, etc., adhesive tapes, uses similar processes. 3.3. Manufacture of hollow products 3.3.1. Blow molding This technique also uses extrusion to make the first starting part, a parison. 3.3.1.1. Definition A tube, called a parison, is extruded and clipped at both ends by a 2-part hollow mold. One of the parts self welds, and the other one rests on a blower, through which air pressure is taken in. The air pressure is strong enough to push on the hot parison, and to force it to cling to the internal wall of the mold, which is water cooled, against which it solidifies.

Figure 3.125. Parison

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Plastic Forming Processes

Extruder

Parison

Mold

Mold dosing

Air blowing

Finished hollow part + burr cutting

Figure 3.126. Blow molding

Advantages High production speed

Limitations Large amount of scrap when pinching (to be recylced) Difficult to control thickness

Breakeven Typical runs range from 10,000 units, for large parts, to 100,000 units for smaller parts, and sometimes many more in food packaging, up to millions. 3.3.1.2. Basic plastics The plastics used in extrusion blowing are PE, PP, PVC, some PC and PA. There is practically no PS, which is difficult to blow, because it is difficult to weld. 3.3.1.3. Operating equipment The equipment includes one extruder with angle head, flowing downwards, and one or several blowing molds. It is thus possible to combine a continuous operation, extrusion, with a discontinuous operation, blowing, under a < 8 bar pressure. When making large hollow parts (10 to 200 liters and more) the machine has just one mold which moves by translation, taking the parison to blow it. Then, after the hollow part is completed, the mold returns to pick up the next parison.

Converting Processes

119

Die Accumulating head Parison control Extruder

Parison

Figure 3.127. Parison variator

For very large blown parts, the accumulating head must be able to eject a large parison in a very short time. There is then an additional device with a piston.

Figure 3.128. Equipment for large blown parts (BATTENFELD)

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Plastic Forming Processes

When making small hollow parts, (< 2 liters, for bottles and flasks) the cycle is cut with a device featuring several molds running continuously or semi continuously. 6 radial molds

9 tangential molds Extruder

Extruder Ejection Parison Ejection

Parison Opening

Closing

Closing Opening a) Cooling

Cooling

b)

Figure 3.129. (a) Radial molds; (b) tangential molds

These two systems were often used in the past with PVC bottles for drinks. They are now used less often, with the development of PET bottles, blow molded for the same applications. Extruder

Parison

Mold

Mold conveyor

Figure 3.130. Bottle high speed blow molding

3.3.1.4. Molds The 2-half cavity, made of steel, is the outside shape of the final product. The mold is cooled by water ducts, like for injection molding, under lower pressure.

Converting Processes

121

Sometimes, the parison itself can be crushed, which, when removed, will create a gap, to leave room to put a handle.

(a)

(b) Figure 3.131. (a) Mold for industrial parts; (b) hollow handle

3.3.1.5. Operation The scrap resulting from the clipping currently reaches 20 to 25% of a bottle weight. The bottleneck is the thicker part of the bottle. The scrap may be up to 50% in some complex industrial parts. The scrap must thus be removed and has to be recycled.

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Plastic Forming Processes Parison diameter

Type of product

Scrap

a)

b)

Figure 3.132. (a) Example of scrap to be cut out for a watering can; (b) integrated handle

The cutting can be performed by hand for large parts. It is also possible to provide an automatic cutting on the mold, or, even better for very long runs, a cutting mold that cuts the scrap at the end of the clipping, when cooling. There is another problem with the wall thickness differences. In very long runs for packaging, it is most important to favor the speed and to be sure that the lowest thickness, the most drawn area, is sufficient. The result is often that there is useless over-thickness in places, hence not the most optimal plastic weight. This does not apply to industrial parts which require constant thicknesses. In order to achieve this, there is a parison variator that regulates the flow at the end of the extruder. It is thus possible to produce a preformed parison, thicker in the places that will be the most strongly drawn when blowing, resulting into consistent thicknesses, and a weight gain close to 20% (with a slower speed however). Extruder

Program

Controlled nozzle

Standard parison

Variable thickness (45 g)

Preformed parison

Figure 3.133. Parison variator

Preformed parison

Constant thickness (35 g)

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3.3.1.6. Operational characteristics Capacity: – from a few cubic centimeters to 250 liters, possible up to 5,000 liters Typical speeds: – for food and drink bottles (1 liter) = 250 to 500 bottles/hr - with 6 radial molds, § 3,000 bottles/hr - with 10 tangential molds: from 7,000 to 10,000 bottles/hr – for large hollow parts: 20 to 40 units per hour for 20 liter capacity - 10 to 15 units per hour for 100 liter capacity, for instance, § 40 units per hour for gas tanks for cars Scrap: – ~ 20%, possible up to 50%

3.3.1.7. Applications Bottles of all sizes, oil cans, jerry cans, casks, drums. For automotive: gas tanks, water tanks, air conduits, bellows. 3.3.2. Specificities of blow molding There are several processes: – needle blow molding: in order not to leave any opening on a blow molded product, it is possible to inject air, through the mold wall, with a needle put into the mold. There is no visible trace at all. Applications: balls, balloons; – filling during forming: the cooling time can be reduced if the packaged liquid is cold, and the package is then perfectly sterile. Application: pharmaceutical products. To speed up the cooling time, it is also possible to inject CO² in the hollow part, while blow molding; – multilayer blow molding: as for films, it is possible to coextrude several polymers through the same die, each following its pre-selected direction. At the end of the die, they hot bond together. The multilayer thus obtained is then blown into shape. Up to 7 or 8 layers can thus be associated. Applications: table oil bottles, sauces, ketchup, etc. and chemical products; – blow molding of complex parts: this technique enables us to make very large parts by drawing the parison before blowing, so that the shape becomes close to that of the finished part. Machines feature an accumulator head and a parison variator,

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Plastic Forming Processes

and sometimes molds of multiple parts that open and close independently. Molded parts of the same plastic can also be put into the mold. They will stick to the hollow part after blowing. Applications: car gas tanks, front spoilers, surfboards, etc. The gas tank barrier is achieved sulfonation or by fluoration, with a blowing agent, nitrogen with 2% fluorine; – 3D blow molding: the parison can be moved with an articulated arm, a robot, and it can be put in the space before the mold closes, then it is blown into this selected position. Application: air ducts for cars.

Figure 3.134. Typical blown parts

Converting Processes

Figure 3.135. Blow molding and liquid packaging

125

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Plastic Forming Processes

Figure 3.136. Multilayer blow molding head

Die Accumulator head Parison variator Extruder Parison Drawing tongs Mold

Plastic insert Part Figure 3.137. Complex blowing

Converting Processes

Extruder

Top articulated arm Parison Rotation Guide Blowing device Open half mold

Bottom articulated arm

(a)

(b) Figure 3.138. (a) 3D blow molding; (b) complex blow molding

127

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Plastic Forming Processes

Figure 3.139. Molding equipment and mold for 3D

Figure 3.140. Air ducts for cars

3.3.3. Injection-blow molding 3.3.3.1. Definition This is the association of two processes. A preform is first made by injection molding. The bottleneck is completed at this stage. Then the preform, still hot or reheated, is blown into the final required shape in a hollow mold. Often the preform is first stretched/drawn, to even stresses.

Converting Processes Injection molding press

129

Injection molding step Molded preform Injection molding mold

Drawing step Drawing punch

Blowing step Blow molding mold

Air

Blow molding Injection blow molding --> deformed --> precise bottleneck (possibility of thicker bottleneck) bottleneck

Figure 3.141. Description of injection blow molding

Advantages

Limitations

No need for finishing operation (no fin, no scrap) Controlled thickness, possible spot increased thickness, no welding Well calibrated opening (good barrier, better screwing) Better transparency (with bi-drawing)

Need for 2 molds, injection and blowing Limited to containers with a symmetric axis, without handle or protuberance Limited neck/bottle ratio (< 5)

Weight gain compared to extrusion blow molding (§ 20%)

3.3.3.2. Plastics used Essentially PET, also possibly PP and PC. 3.3.3.3. Equipment and operation (injection + blowing) The preform must be heated before blowing, to make hollow parts with thin walls, of thickness d 1 mm, for food and drink bottles. The preform can be blown directly. For thicker bottle walls, between 1 and 2 mm (for smaller bottles).

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Plastic Forming Processes

There are thus 2 types of injection blow molding: – “cold cycle” is used for very large runs. The cooled injected preform is taken on later, or at another site. It is heated, with an intermediate step of bi-orientation to give a better transparency. This involves two machines; – “hot cycle” is used for relatively thick containers. Injection molding press

Preform injection molding Injected preforms Reheating preforms

Blowing

Ejecting

Injection molding + blowing (thicker small bottles) a - Continuous (hot cycle)

Blow molder

(thinner wall bottles: high speed fabrication) b - Discontinuous (cold cycle)

Figure 3.142. Two types of injection blow molding

Figure 3.143. Cold cycle

Converting Processes

131

Figure 3.144. Hot cycle

3.3.3.4. Operational characteristics Capacity: – up to 20 liters Speed for food and drink bottles: – injection = 8,500 to 17,000 preforms/hr (molds from 48 to 96 cavities) – blowing = from 20,000 to 40,000 bottles/hr Scrap: – minimal 1%

3.3.3.5. Applications Bottles for drinks (carbonated and non-carbonated water, fruit juices, cider), small bottles and jars for cosmetics, perfumes, pharmaceuticals, mini spirit bottles for airlines.

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Plastic Forming Processes

3.3.4. Rotomolding The operation starts from powder and not from pellets. Rotomolding is in competition with blow molding for large size hollow parts, but it is not well adapted to long runs and to thin wall small articles. Mold carrying arm Air exhaust Grain route Mold Fine powder or plastisol Double rotation axes

Oven heating

Water cooling

Possible cut

Loading

Unloading

Figure 3.145. Rotomolding description

3.3.4.1. Definition The fine powder polymer is deposited in a closed mold that double rotates around its main two axes. The powder is distributed all over the surface of the mold.

Converting Processes

133

It is then gelled in an oven, this “skin” is then fixed by cooling by sprinkling the mold with water.

Figure 3.146. View of the plastic powder flow in the mold

Advantages

Limitations

Light molds, not expensive (no pressure) Single piece parts, (no weld) with Large size and complex shapes possible

Fairly long cycle

Regular thickness (contrarily to standard blow molding) and no constraints

Need for thick walls, risk to leave inside air

Limited precision and choice of materials Losses when opening (cutting §10%)

Insert and sandwich possible

3.3.4.2. Plastics used Mostly PE-HD and PP, as a powder, from 150 to 500 μ (sometimes metallocene grades) and plastisol PVC. ABS, PC and PA powders are also used to a lesser extent. 3.3.4.3. Equipment Rotomolding machines always feature a mold support that can be double rotated at speeds from 5 to 30 rpm, with one or more hot air heated metal molds. Temperatures range from 200 to 400°C, and the molds are cooled by water sprinkling.

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Plastic Forming Processes

There are several types of machines that differ by the number of mold carrying arms and by their dimensions: – standard, with 3 stations, and sometimes independent mold supporting devices, to allow molding of different parts; – roto-oscillating (for long parts).

1 arm 2 arm carrousel 1. oven; 2. cooling; 3. loading/extraction Figure 3.147. Types of machines

Figure 3.148. Rotomolding machine (Caccia)

Converting Processes

135

3.3.4.4. Molds Molds are generally heated by gas (hot air from 200 to 400°C), and cooled by water sprinkling. For PE and producing large parts, (> 3 m3), molds are often made of steel sheet, 1 to 3 mm thick. Molds are made of cast aluminum of 6 to 10 mm thickness, for medium size parts or complex shapes. For PVC (plastisol), aluminum molds are mainly used, sometimes in one piece, when the part to be demolded is flexible. In all cases, a vent is needed to allow the hot air to escape as well as a tight joint to assemble the parts of the mold. There also are 2-walled molds, heated by a fluid. They enable us to do without an oven, but the tooling investment is then more expensive. 3.3.4.5. Operation In this process, the outside of the part takes on the surface of the mold, but the inside face tends to end up as “orange peel”. In order to avoid this, an additional rotational heating can be carried out, called “glazing”, after rotomolding. Otherwise, the internal pressure can be controlled to avoid the formation of bubbles. Of course, the powder granulometry determines the surface finish, which is smoother, the finer the powder. It is also possible to make an open hollow part, by heat protection of a part of the mold to avoid plastics powder deposits. This process may also make it possible to obtain variable thicknesses or a side opening. 3.3.4.6. Operational characteristics Capacity: – moldable sizes up to 5 m3, possible 50 m3 Thicknesses: – 2 to 20 mm for PE length: up to 4 m – 0.5 to 2 mm for plastisols Cycle speed: – 3 to 4 min/mm thickness for heating – 1 to 2 min/mm thickness for cooling

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Plastic Forming Processes

3.3.4.6.1. Production speed Volume (l)

Weight (kg)

Speed (cycles/hr)

100 to 250

10

3

500

25

2 to 2.5

1,000

50

1.5

3,000

125

1

Example: fabrication of a canoe-kayak § 25 min. Scrap: ~10% (cut when opening). Standard runs: a few thousand parts (from 100 units to 50,000 units/year). 3.3.4.7. Applications 3.3.4.7.1. Hollow parts, without welding PE (§ 90%): – bins, tanks, containers, cisterns, reservoirs, septic tanks, containers for glass bottles; – gas tanks (tractors, motorcycles), water sprinklers; – road signs, markings; – housings, casings, head and armrests, truck dashboards, trailer trunks, bins for crops, post bags, ammunition boxes; – separators on roads and freeways; – children’s games, balloons, canoes-kayaks, buoys, city and street furniture, etc. Plastisols: balls, balloons, animal toys for children, heads of dolls and mannequins, skin for surfboard covering (filled with PUR after rotomolding, slush molding process).

Converting Processes

Figure 3.149. Typical rotomolded parts

137

138

Plastic Forming Processes

Oven PE powder

Heating

Cooling (water)

Rotation

Mold (metal sheet)

Molded kayak

Figure 3.150. Making a canoe or kayak by rotomolding

Converting Processes

139

Figure 3.151. Rotomolding of non-inflatable balloons (plastisol)

3.3.4.8. Skin rotomolding or slush molding This application is mainly for car dashboards. The process is performed in 3 steps. Starting with a mold whose cavity features a leather-like finish, a thin hollow shape is rotomolded on it, then it is filled with expanding foam. 3.3.4.8.1. Mold manufacture A leather model is made first. It is put into an electrolysis tank into which copper, then nickel is deposited, in order to make a thin skin (2 to 3 mm) which will be the print to be reproduced on the rotomolding mold. Since leather is not conductive, the model will first be dipped into a chemical copper bath, which will enable us to attach a conductive copper, then to deposit, by electrolysis, the coat of copper/nickel to provide a satisfactory mechanical stiffness to handle it easily. The time to obtain such a cavity is fairly long, about 6 weeks, but it gives a perfect reproduction of the leather model.

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Plastic Forming Processes

Leather covered model + chemical copper Leather Cathode

Support

Anode Cu or Ni Thin coat of Cu or Ni

Figure 3.152. Mold cavity made by electro-deposition

3.3.4.8.2. Molding The final mold is then placed on the arm of a rotomolding machine, about one third filled with PVC plastisol, then rotated, heated, cooled and after the standard cycle.

Water cooling

Oven heating

Loading PVC powder

Finished part (skin)

Filling PUR foam

Figure 3.153. Rotomolding, ejecting and inside foaming

Converting Processes

141

3.3.4.8.3. Foaming Lastly, to finish the part, to give it its final shape, PUR is injected.

Figure 3.154. Car dashboard

3.3.5. Dip molding 3.3.5.1. Definition A heated mold, with the inside shape of the product to be made, is dipped into a plastisol which is deposited by gelling. The deposit is then fixed by air or water cooling. This process can also be used to protect small glass bottles. Oven

Naked bottle

Oven

Conveyor

Plastisol

Figure 3.155. Coating of glass aerosols

Coated bottle

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Plastic Forming Processes

3.3.5.2. Base material The only material used is paste plastisol (PVC powder of 0.1 to 5 μ + plasticizer + stabilizer). The same process uses rubber latex to make condoms. 3.3.5.3. Equipment and operation A monorail is used to carry the molds (often made of aluminum) from a preheated oven to a plastisol bath into which they are dipped. The thickness of the deposit depends upon the time of the thermal exchanges. After being removed from of the bath, the gelling is finished in an oven and forms a skin surface on the mold. This skin can be left on the mold or ejected by cutting and elasticity removing. A matte or a shiny appearance can be given by air or water cooling. 3.3.5.4. Operational characteristics Deposit thickness = 1 to 3 mm. Cycle time § 5 minutes. 3.3.5.5. Applications Gloves, boots, insulation of electric tools, protective or decorative coatings on glass bottles, etc.

Figure 3.156. Making gloves with dipping molding

Converting Processes

143

3.4. Manufacture of thermoformed parts 3.4.1. Standard thermoforming This process differs from other conversion processes as it requires the use of a semi-product, such as a plate, sheet or film, first obtained by extrusion or calendering.

Plate die

Films Stamped parts

Sheets

Extruder

Thermoforming machine

Calender

Tool investment

Injection

Stamping (metal working)

RIM

Composites (hand lay up) Hollow ware

Thermoforming

Machining

Figure 3.157. Thermoforming versus other processes

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Plastic Forming Processes

3.4.1.1. Definition A rigid sheet is clamped in a frame, then heated. When the sheet is soft enough, it is drawn by depression into a mold of the final shape, or pushed into a mold with a punch or air pressure. The two actions can be combined. IR heating Clamping TP sheet Forming sheet

Shaped part

Vents Air vacuum

Figure 3.158. Thermoforming

Advantages Simple molds (low pressure) and inexpensive Small run production possible Possible use of thin sheet, rib-stiffened when forming Fast air cooling (thin sheet) Possible drawing of up to 5 times the original surface

Limitations Use of semi-products (more expensive than pellets) Trimming of the clamped part longer than forming large scrap loss when not producing plates in-house Limited geometric tolerances Impossible to make over-thickness, and nonuniform thickness (but limited thickness)

Breakeven: the small cost of the molds allows us to use short runs, from a few tens to a few hundred units. It also enables us to make large size products. 3.4.1.2. Preferred plastics Semi-products (plates and sheet), mainly for: – industrial parts, ABS, PVC, PMMA, PC (thicknesses from 2 to 12 mm); – packaging, PS, PVC, PP, PET (most frequent thicknesses from 0.1 to 1 mm). Most often, amorphous plastics are used, because crystalline polymers require higher heat, twice the amount, (120 kcal/kg, versus 60 kcal/kg), and their forming range is narrower.

Converting Processes

145

3.4.1.3. Equipment The equipment includes: – a device to hold the sheet to be formed; – a heater; – a device for the suction or pressure of the softened sheet. Heating: this takes from 50 to 80% of the thermoforming cycle time (from 1.5 to 4 sec for every 0.1 mm thickness). Heating requires at least 15 kW/m of thermoformed area. Infrared ceramic heaters are most often used, that are small, independently controlled, and enable us to adjust the heating, if needed, and to increase or reduce the plastics softening on a selected zone. The sheet can also be heated by contact panels, with a better transfer in the case of small thicknesses, or in a hot air oven, for thicker plates, in the case of PMMA. In the case of very large thicknesses, or of insulating materials, like plastics foams, it may be necessary to heat the sheet as a sandwich, the lower heating plate being removed at the thermoforming stage. System depression by suction: this is carried out with a vacuum pump (~ 600 Torrs) or by overpressure with compressed air (< 8 bars). Cooling: as thicknesses are relatively small, cooling is generally performed by cooling air/water sprinkling. Forming can be achieved in various ways. Forming processes

Mold shape

Product shape

Specifics Average thicknesses Limited speed

Vacuum suction

Negative

Simple

Overpressure

Negative

Simple

Thin thicknesses Fast speed

Suction + counter punch

Negative

Complex

Thicker at the bottom

Air-slip process (lower frame or higher mold)

Positive

Complex

Thicker at the top

First bubbling then sucking (air predrawing)

Positive

Complex

Good even thickness

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Plastic Forming Processes

3.4.1.4. Various thermoforming processes The first three processes are standard thermoforming. Bubbling is used for large symmetric parts. Thicknesses can then be better balanced.

Figure 3.159. Thermoforming after bubbling

For very thick plates, particularly of PMMA and PC, the plate is heated in an oven and it then flows, still hot, by gravity, onto a positive mold. Transfer Thick plate PMMA or PC Special coating Mold

Oven Finished part

Figure 3.160. Drape forming

Converting Processes

147

Depending upon the plastics feeding system and the end-use of the products, there are two types of thermoforming machines. Sheet by sheet feeding: this is the feeding as generally described. The sheets are pre-cut to the dimensions of the clamping frame to make large size parts for industry. The speed can be improved with a carrousel machine, with three steps, in order to split the cycle into its three main actions: loading, heating-sucking-forming, cooling-unloading, trimming. In this case, the trimming is performed outside of the machine.

Figure 3.161. Industrial thermoforming machine

Figure 3.162. Large part molds

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Plastic Forming Processes

Figure 3.163. Thermoforming workshop

Reel feeding: a reel is taken up mechanically with clips. It passes successively in front of a heater, (radiant or contact), a forming system (pressure), and an automatic cutting press. The operation is semi continuous, as it stops at the forming step. The cutting scrap is recycled. This type of machine is used in food packaging. Sometimes, the finished container is filled immediately afterwards, taking advantage of the fact that it is sterile.

Infrared heating

Cutting

Sheet on reels

Scraped reel Sucking

Formed parts

Figure 3.164. Semi-continuous thermoforming

Converting Processes

149

(a)

(b) Figure 3.165. (a)Thermoforming machine for packaging (source: Illig); (b) cup production

Forming

Filling Cutting

Figure 3.166. Form filling by machine

There also are machines that operate completely in a continuous way. The sheet is heated, as described above, and then sucked onto a rotating drum featuring

150

Plastic Forming Processes

cavities linking it to a single common sucking operation. For instance, this is the case for pill nesting packages, put directly into place with a scraper. Formed sheet Wheel with cavities

Vacuum pump

Heating Sheet to be formed

Figure 3.167. Continuous thermoforming

3.4.1.5. Molds A negative mold features a hollow cavity with the shape of the part to be formed, and a positive mold features the relief part to be molded. The general characteristics of the parts obtained by one or the other of these tools are as follows. Negative mold

Positive mold

Outside surface well made

Outside surface polished and glossy

Polish inside

Fine details inside

Thinner bottom and angles

Thick bottom

Few drawn edges

Thin edges

Limited drawing depth

Higher drawing ratio

Easy demolding

More difficult to eject (clamping)

Less material expense

Risk of creases

The molds for small or medium runs (from one unit to a few hundred) are made of epoxy. Since epoxy is not very heat conductive, it is filled with aluminum powder. The molds for long runs (> 10,000 units) are made of cast and machined aluminum or of steel, sometimes water cooled.

Converting Processes

151

The intake vents are bores of about 0.8 mm diameter, put on the lower part of the mold and along the edges or the concave spots, so as not to mark the formed part. The bottom may sometimes feature a thin grid whose pattern is reproduced on the part. In all cases, these vents must be enough to ensure the evacuation of an air volume about ten times that of the air in the mold, in a short time, about a quarter of a second. Sheet before forming

Positive mold

Negative mold Mobile element

Return spring

Joint Mold for non-injected part (for instance, egg tray in a refrigerator)

Figure 3.168. Type of special mold

3.4.1.6. Operation The problem, as in blow molding, is controlling the thickness of the formed part. If the drawn surface doubles, its residual thickness is reduced by half. In order to correct this, it is possible to: – make a pre-drawing by differential heating of the blank; – fix a selected surface with a cold punch or a heat proof mask; – induce a localized air drawing; – make a pre-bubbling to give a “drop” drawing, to help consistent thinning. In the case of several part thermoforming, there may be wrinkles in-between. This can be corrected with a grid.

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Plastic Forming Processes

Blank push Mold

Wrinkle formation

Figure 3.169. Blank push griddle

All this is part of the know-how of the thermoforming specialist. It is important to do the trimming fast (on the machine or right after) and to recycle the scrap, about 30%. The scrap is always significant, as it is the clamped part on its bigger thickness. The trimming is done directly on the machines in case packaging thermoforming (cups, pots) with a hitting cutting device. 3.4.1.7. Operational characteristics Sheet to sheet (industry)

Reels (packaging)

Capacity

Current sizes < 4 m² Up to 25 m² possible Weight < 20 kg

Width < 1 m

Thicknesses

Current < 6 mm Possible up to 20 mm

80 μ to 1 mm

Sizes

Current: 2 u 4 m, up to 3.5 u 8 m

Weight

A few g to several tens of kg

Speed

Luggage shells (thickness= 3): 40 to 80 units/hr Refrigerator (thickness = 4): 20 to 60 units/hr Bath tub (thickness = 5): 7 to 8/hr Boat hull (thickness = 6): 6 to 8/hr

Scrap

Dairy cups: 20 to 40 cycles/min 20,000 to 40,000 cups/hr Small trays: 50/min Pills on blisters: 50 m/min

from 20% (packaging) to 50% and more (industrial part)

Converting Processes

153

3.4.1.8. Applications Industrial parts: refrigerator bodies and doors, bath tubs, luggage shells, truck dashboards, car body parts, small boats, surfboards, propeller bulbs, casings, handling trays (for motors, gears, clutches).

Figure 3.170. Typical large thermoformed parts

Consumer goods: toys, masks, 3D maps, school bags, etc. Packaging: cups for dairy products (yoghurt, desserts) and food products, meat, fish and fruit trays, nesting trays for biscuits and candy, meal trays, tool boxes, etc.

Gas torch

Rotating thermoformed cup

Cup after flaming (drinking edges)

Figure 3.171. Second run to make drink cups

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Plastic Forming Processes

Packaging makes up two thirds of all applications.

Figure 3.172. Small thermoformed parts

3.4.2. Specificities of thermoforming 3.4.2.1. Industrial thermoforming 3.4.2.1.1. Double thermoforming Two plates are thermoformed at the same time, by drawing them in opposite directions, to make a hollow part.

Converting Processes

155

Figure 3.173. Hollow part made by double thermoforming

3.4.2.1.2. Industrial thermoforming under pressure Standard thermoforming does not enable us to reproduce fine details. Shapes are rounded and grained plates are used to get surface effects. By applying air pressure, 5 to 15 bars, to one of the blank faces, with the help of a closed device, and of a depression on the other face, the softened plastics clings better to the details of the mold. The finished part looks slightly like an injected molded part, with a cheap mold for short runs. Bell

Infrared heater

Plate

Air pressure Formed part

Vents

Mold

Vents

Figure 3.174. Industrial thermoforming under pressure

Typical applications: casings for electronic products, luggage, etc. 3.4.2.1.3. Heat compression of plates (also called wood stock) This is not standard thermoforming, but stamping. This requires that the plastics do not creep under the pressure.

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Plastic Forming Processes

Definition: a thermoplastics plate, filled with wood fiber, is heated, and then pressed in cooled tools. In order to give a good appearance to the formed plate, it is covered with a fabric that shapes under pressure, while clinging to the plate. Fabric

Polymer

Oven

Mold

Press Molded part Heating of the material PP fabric filled with sawdust

Compression

Figure 3.175. Thermocompression cycle ~ 1 minute

Operation Wood or fiber filled plates, plus decorative fabric, are mainly used. They are heated in an oven, at about 200°C (the mold is at about 50°C) and formed under 10 to 30 bar pressure, with a hydraulic press and a metal mold. Application: car rear shelves and upholstery of car trunks.

Converting Processes

157

3.4.2.2. Thermoforming for packaging There are two types of blister packaging: – blister: the product to be packed is put into a transparent thermoformed blister of rigid PVC or PET, and fixed on a board, by heat seal or clipping. In general, the blister is bonded to the card, using heat and pressure. The blister card supports the printed message. The card enables easy display on the store shelves, and it somewhat prevents small objects being stolen (see Figure 3.176); – skin pack: the product to be packaged is put on a micro-perforated heat seal cardboard. It is the packaged product that acts as mold for the forming of the package. A transparent PE film is thermoformed on the product and the supporting cardboard. The film self glues on the support, wrapping the product with a thin film and completely fixing it (see Figure 3.177).

Figure 3.176. Blister

Heating PE film Wrapped film Air suction (through the micro-prefored cardboard)

Figure 3.177. Skin pack

Blisters are more attractive, but they require a mold and can only use rigid sheets. Blisters are best adapted for single products. Skin packs do not require a mold and enable us to use inexpensive film that keeps the product well in place. However, the whole support must be covered, and transparency is limited. Skin packs are preferred for larged size parts, or to pack several parts together.

158

Plastic Forming Processes

Applications Blisters: hardware articles, small tools, electrical accessories, stationery and office articles, notions, cosmetics. Skin-pack: more used in industrial packaging (setting into place), various loose parts, electrical equipment. 3.4.2.2.1. Vacuum or neutral atmosphere packaging As with film packaging, there are thermoforming machines which enable filling and vacuum or neutral atmosphere packaging, at speeds, ranging from 500 to 1,500 filled trays per hour.

Cutting

Forming

Filling

Vacuum and sealing

Figure 3.178. Vacuum packaging

Typical applications: fresh and cooked meat packaging, cheese packaging, snacks, nuts, drinks, etc. 3.4.2.2.2. Making bubble films A PE-HD film is heated on a rotating cylinder that carries cavities as a design of a half bubble to be formed. Each is bonded to a central suction device that continuously imparts this shape to the film. A second film is treated the same way, in reverse, so that the half bubbles are face-to-face. At this point, a hot cylinder presses the two films to heat seal the edges of the bubble, thus shutting in some air which will act as protective cushioning.

Converting Processes

159

Vacuum

Pressure

Vacuum

Figure 3.179. Bubble film

Application: packaging of fragile products. 3.5. Manufacture of foamed products 3.5.1. Expandable polystyrene molding 3.5.1.1. Definition Molding is performed by raising the temperature of the EPS beads (PS + pentane) in two steps. First a free dilatation is started, by water vapor at 100°C, making pre-expanded flakes. Then the flakes are matured and fill a mold in which they self glue into shape, with the introduction of 120°C steam.

160

Plastic Forming Processes

Figure 3.180. Beads of expandable polystyrene

Advantages Inexpensive molds and equipment (very little pressure) Inexpensive thermal insulation and impact protection

Limitations Caution for EPS storage: tight barrels, risk of pentane degassing after opening large space taken by final products

Profitability condition: the production site must be close to the end-using site. 3.5.1.2. Equipment The equipment is different depending on whether the operation is for parallelepiped blocks, semi-products, or finished molded products. In both cases, the equipment includes: – a furnace to generate the water vapor needed for the process; – an air compressor to ease the filling of the molds with EPS beads; – a pre-expanding machine, to make the first expansion; – maturating devices for bead maturing and, depending upon the final use, a mold for blocks or a molding press.

Converting Processes

161

3.5.1.2.1. Block molding A large mold is used. After being filled with EPS beads, the mold water vapor is injected into the mold. A block is extracted, and then custom cut into plates, with a hot wire. Expandable PS Water vapor 100o C Steam generator Water vapor 120o C

Pre-expanded PS Double wall mold Expanded PS

Figure 3.181. EPS molding system

(a)

(b) Figure 3.182. (a) Open mold; (b) block extraction

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Plastic Forming Processes

Figure 3.183. Block molding: block cutting with hot wire

3.5.1.3. Molding shaped products The shaped mold is put on a press that supports it, which features a bead feeding system of the Venturi type, to fill the mold. The entry of water vapor then fixes the EPS into shape. Maturing step Steam generator

Expandable PS Expanded PS Pre-expander Double wall mold Compressed air

Press

Figure 3.184. Shaped molding

Converting Processes

163

Figure 3.185. Press for EPS molding

Figure 3.186. Mold detail

3.5.1.4. Fabrication of plates by extrusion The operation is performed with a tubular die, making a sheath which is cut after expansion and rolled in two sheets of 0.3 to 0.8 mm thickness (see also specific extrusion).

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Plastic Forming Processes

Pump for foaming agent

Hopper for EPS Die

Coating fan

Extruder Winder Sheath cutting

Figure 3.187. Extrusion of EPS sheets

3.5.1.5. Molds Molds are made of two walls, with steam circulating in between. The steam is spread into the mass of EPS flakes through many small holes or slits, evenly distributed on the inside wall. Molds may feature several cavities. 3.5.1.6. Operation After expanding and maturing, the EPS beads stick together in the closed mold, under < 0.8 bar pressure and 105°C to 120°C water vapor temperature. This process is called steam shock. The molded part is then cooled and dried. Productivity can be increased by using two molds (the hot part is transferred into a cold mold) or by vacuum cooling.

Converting Processes

165

Figure 3.188. EPS press workshop

3.5.1.7. Operational characteristics Blocks

Finished molded parts 3

Capacity

up to 6 x 1.3 m (< 4 m )

up to 1.4 x 1.2 m (< 0.5 m3)

Thicknesses

§ 0.5 m

1 to 4 cm

Densities

15 to 25 kg/m3

20 to 40 kg/m3

Speed

0.5 to 1 min/cm thickness 10 to 15 blocks of 3 m3/hr

30 cycles/hr

3.5.1.8. Applications Block molding: thermal insulation for buildings, concrete reserves, stage sets (TV, theater), modeling, etc. For the new sound insulation requirements, press crushed plates (EEPS) are now used. They are less stiff, but the acoustic properties are improved. Molding of finished parts: insulated food packaging, shock cushioning industrial packaging, lost models in foundry. The melted metal is poured directly on the EPS, put in a sand frame or in a shell. It is then vaporized and replaced by metal. This is how car metal engine blocks are made. Extruded sheets: building insulation, thermoformed packaging, like egg trays. Packaging made of EPS chip particles which entangle and block the packaged product.

166

Plastic Forming Processes Packaged product Carboard box

EPS chips

Figure 3.189. Cushioning with EPS chips

3.5.1.9. EPS specificity: “exporite” molding An EPS model is put into a sand foundry mold. Melted metal is poured on the EPS which evaporates, leaving room to metal. This is the same as the lost wax foundry, but it is easily moldable.

Casting feeder

Frame

EPS model

Sand

Vent

Figure 3.190. Metal casting with lost EPS model

3.5.2. Polyurethane molding Whilst plastics are currently converted after polymerizing, polyurethanes are made of liquid components that polymerize only after being mixed, according to the following reaction: Polyols + polyisocyanates + foaming agents + catalysts + tensioactive products Æ PUR foam

Converting Processes

167

3.5.2.1. Definition The mixing of polyols and polyisocyanates triggers an expansion (foam). A measuring machine is used, which brings the liquid components to a mixing head that can put them on a conveyor (free foaming) or in situ, into a mold (forced foaming). The expansion reaction is exothermic and develops a low pressure.

Figure 3.191. PUR casting expansion

Advantages

Limitations

Possible to mold in situ

Unwanted gluing and need for frequent equipment cleaning

Possibility for wide product range (rigid, semi-rigid, flexible) just by changing components

Components combustible

are

sticky,

smelly

and

3.5.2.2. Formulations Due to their low cost, the polyols mainly used are polyethers. The most frequently used polyisocyanates are MDI for rigid foams and TDI for flexible foams.

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Plastic Forming Processes

Foam type

Polyols

Polyisocyanates

Rigid

50%

45 to 48%

Semi-rigid

55%

40%

Flexible

60%

35%

In addition to the basic components, there is also water (giving CO2) and fluorinated derivatives for thermal insulation (HFC, HFA, or pentane, which have replaced the Freon type CFCs, now forbidden by law). There can also be foams with special appearances, such as integral skin, obtained by varying the surface tension. 3.5.2.3. Equipment The machines for measuring-mixing are of two types: – low pressure machine: the mixing-measuring is carried out by a stirrer or compressed air and feeding pump gear (pressure = 10 to 20 bars, speed = 5 to 30 kg/min). It is well adapted for use on site, inexpensive and quickly adapted to all formulations but the flow is slow; – high pressure machine: the feeding and the measuring are carried out by piston pumps, of the Diesel injection type, working reverse for a full mix, under 150 to 200 bar pressure, at 300 kg/min rate. This is a typical workshop machine, with fast start, minimal losses at stops and high flow. Isocyanate

Polyol

Stirrer

Figure 3.192. Pump and low pressure machine

Converting Processes

Figure 3.193. Pump and high pressure machine (source: Henneke)

Components are stored in tanks protected from any moisture. Polyisocyanate

Polyol

Head motion

Cutting Silicone paper

Mixing head

Mattress Conveyor

Figure 3.194. Flexible PUR free foaming, for mattresses (source: Hennecke)

169

170

Plastic Forming Processes

Operation: PUR foams can be obtained in four ways: – free foaming: on a slanted conveyor, covered with silicone Kraft paper with raised edges. The machine operates continuously, with a reciprocating motion over the full width of the conveyor. The material on the conveyor is freely foamed to about 20 times the original volume (see Figure 3.194); – forced foaming: this is performed in a mold made of steel sheet or aluminum. The mold must stand a pressure that depends upon the degree of densification, which can reach 5 to 6 bars, hence, the stiffness to cope with this. When a wood or leather imitation is required, the mold surface is coated with a silicone 3D print, itself molded from an original design. The reaction is exothermic. The core of the molded product must reach an optimal temperature, also depending upon formulations and thicknesses. Some products are finished in an oven. Foam fabrication is an exothermic process. However, as the foam core must reach an optimal temperature, it is sometimes useful to heat the molds or to use a heating tunnel (see Figure 3.195); – foaming in situ: this is a variant of the process above. Filler foam is used, generally as an insulant, in a product featuring a cavity that serves as a mold (see Figure 3.198); – spray foaming: a spray gun is used, with a high pressure head, as for paints. The foaming is free.

Vent

Molded pillow

Mixing head

Open mold

Mold closing

Mold ejecting

Figure 3.195. Forced foaming on carrousel for pillows

Converting Processes

Figure 3.196. PUR foam molding carrousel for shoes

Scrap cut

Refrigerator tank

Refrigerator body

Feeding PUR

Figure 3.197. Refrigerator tank insulation

Figure 3.198. Sandwich insulation; in situ foaming

171

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Plastic Forming Processes

3.5.2.4. Operational characteristics Blocks Capacity

Width up to 2 m

Current, up to 1/4 m3

Height § 1.5 m

Thickness > 1 cm

3 to 6 m/min Speed

Finished moldings

200 to 400 kg/min § 1,000 mattresses/day (5 m3/mn)

Cycle 20 to 30 min 6,000 to 8,000 car seats (with 32 stations)

3.5.2.5. Applications Free foaming: mattresses, furniture (flexible), after cutting with vibrating blade saw. Forced foaming: car seat cushions, furniture (flexible), steering wheels, armrests, sun visors, seat backs, car upholstery (semi-rigid), saddles, shoes and soles. In situ foaming: refrigerator and freezer insulation, water heaters (rigid), sandwiches, car seats covers (flexible). Spray foaming: insulation in chemical industry and in building construction (rigid). 3.5.2.6. Specific case for packaging A mix of polyols and polyisocyanates generate an expansion (rigid foam) that can freely expand or be forced into a mold. PUR

Packaged product

PE bag

Cardboard box

Rigid PUR Sealed bag, inflated by PUR expansion and put into shape

Figure 3.199. Packaging of a single part with rigid foam

Presentation packaging can be produced by cutting-removing on flexible foam.

Converting Processes

Hollowed metal plate

Pressing

Shaving

173

Foam block of flexible PUR

Nesting cavity

Figure 3.200. Custom made packaging of flexible PUR foam

3.5.3. Other types of foams 3.5.3.1. PVC foam This is a discontinuous production, with a 3-step cycle: – mixing of components; – pressure molding in mold (60 x 80 mm) with heating (§ 10 min), gelling and freeing the foaming agent (nitrogen); – hot water expansion (§ 1 hr), allowing block dilatation (2 x 2 m). It is possible to make rigid or flexible foams. Typical applications: sandwich of aluminum-PVC foam for trucks, ships, airplanes (for rigid foams), cloth linings of plastic-coated fabrics (for flexible foams). 3.5.3.2. PE or PP foams These foams can be made by extrusion, as indicated here, or made into a finished form, starting with pellets containing a foaming agent, often propane, which will be expanded in damp heat, as with EPS foam, but under 3 to 5 bar pressure, like PUR foam. Applications PE: packaging (electronic devices), protection (sport articles). PP: automobile (door linings, surfboards, sun visors, bumper absorbers).

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Plastic Forming Processes

3.5.3.3. Acrylic foams Acrylic foams are made from acrylic monomer polymerization, with additives, followed by expansion, removal of the resulting crust and then cutting to the required format. Typical applications: ships and aircraft. 3.5.3.4. Phenolic foam Phenolic foams are made continuously, by blending components (phenol, formalin) and foaming agent (pentane), followed by heat induced polymerization. It is thus possible to obtain rigid blocks of 1 x 0.2 m, which can be further cut. Typical application: firewalls, insulating sandwiches for railroads and building construction, high-rise buildings. 3.5.3.5. Urea formalin foam As with PUR, a two component machine is used, allowing foaming in situ. Typical application: methane ship insulation, artificial snow. 3.6. Machining and cutting Even though mainly concerning molding processes, plastics products are sometimes machined in order: – not to complicate the making of the molds too much; – to obtain a better quality in places; – because the expected runs are too short to justify additional tooling. In this last case, the base material is stock shapes, generally extruded as various profile rods, made of PE HD, PP, PMMA, POM, PA, and even PTFE. In order to better enhance thermoforming, machining and cutting are increasingly used, in specific machining centers. Machining is relatively easy, but there must be some precautions: – since plastics are poorly heat conductive, the resulting heating risks softening thermoplastics and causing burning, spot changes and even unwanted gluing; – in the case of thermosets, the “skin” makes a binding which may peel off in contact with the tool, freeing internal tensions which may affect the material;

Converting Processes

175

– with filled or reinforced resins (heterogenous material) the tool may chatter; – finally, plastics chips are very dusty and may clog machines and create a nuisance for personnel, if the compulsory suction systems are not sufficient. 3.6.1. Operation As a rule the machining operation is similar to those that prevail with light alloys. Turning, milling drilling: the angles of the tools are specific to the machined polymer, as well as the cutting speeds which are respectively 2 to 10 times faster than aluminum or steel machining. 3.6.2. Cutting The flexible or thin plastics can be cut with a guillotine, hollow punch or by knives with reciprocating motion. EPS foams are cut with hot wire. PUR foams are cut with a vibrating blade. Disk saws are used for rigid plastics; particularly for cutting extruded products at the desired length. Cutting by laser CO2 or by hyper bar water-jet, can also be used. 3.6.3. Sanding and polishing Slightly abrasive products are generally used or products containing polymer solvents, to eliminate scratches. 3.6.4. Applications Machining: electrical insulants, parts for aerospace, defense, medical, robots. Cutting: secondary conversion (PE films, flexible PVC sheet).

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Chapter 4

Assembly and Fixations

There are several assembly techniques, depending on whether it is possible or not to demount. Functions

Undemountable

Demountable

Type of assembly

Polymers that can be used

Adhesive bonding

TS and some TP

Welding

Only TP

Riveting

Mainly TP

Screwing

TP and TS

Snap catching

Only TP

4.1. Undemountable processes 4.1.1. Adhesive bonding Adhesives, which are true polymers, enable us to avoid riveting that first needs the parts to be drilled, and welding that heats the parts. Adhesives work particularly well to bond complex parts together. However, there are limitations to take into account: it is difficult to disassemble, the life of adhesives is uncertain and the adhesives may often be toxic. Most thermoplastics are easy to bond by adhesives, for instance PS, PVC, PMMA and PU. Some thermoplastics are difficult to bond with adhesives, such as

178

Plastic Forming Processes

PE, PP, POM and PA. Thermosets that are not weldable, by definition, can always be assembled by adhesive bonding. In all cases, the surface to be glued must first be treated by immersion or coating. Then, the parts to be glued are assembled with the help of a solvent of the polymer or a compatible adhesive. Adhesive bonding can be achieved in three ways: – by solvent evaporation; – by chemical reaction; – or by cooling of a hot melt. The adhesives used most often are of the types: vinylics, acrylics (among which are the cyano-acrylates), elastomers, polyurethanes, epoxy and silicones. Polyolefins, which are difficult to glue as they need a preliminary physical treatment (flame, Corona effect, plasma or chemical (acid attack) treatment) can be excellent adhesives to bond different materials with their flowing character (with plastics, paper, aluminum, textiles using hot melts that are 70% EVA based). The heating to polymerize adhesives under pressure causes a needless higher temperature on the parts to be bonded. Adhesives that contain additives such as ferrites are used to allow an induction polymerization that is faster and does not risk deterioration in the parts to be bonded. In some cases, it is important to know the controlled time for the glue to set, making it possible to better handle the parts. The glue timing can be controlled with specific adhesives or with UV flash light reticulation. 4.1.2. Welding The principle of welding is to soften, by heating, the surface of two plastics to be joined, to help their interpenetration and assembling during cooling. There are two types of welding. Types

Most often used thicknesses

Type of heating

Without rod

Small (§ mm)

Joule effect High frequency Ultrasonic welding

With rod

Large (§ cm)

Hot air (gas or electricity)

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179

There is another type of classification of types of welding. Heating Direct Indirect

Types of welding Infrared: impulsion, mirror Hot air: torch High frequency Ultrasonic

4.1.2.1. Welding without support 4.1.2.1.1. Heat welding Principle: the parts to be assembled are conduction heated, up to their softening point, then pressed together and maintained until completely cooled. This type of welding can be achieved in several ways: – hot jaws; – mirror welding; – by friction. Hot jaws (impulse welding): the area to be welded is pinched between two metal rods heated by Joule effect, which leads to a local melting of the polymer, and a welding when maintained under pressure and cooled. The heating moves from the outside to the inside, in an insulating mass, in order to avoid the whole part melting. The operation is performed in a very short time, controlled with a timer, (hence the term “impulse”). The jaws are covered with a PTFE film, to avoid unwanted gluing. It is also possible to perform a continuous weld, by using contact rollers instead of jaws. Operational characteristics: – welded materials: mainly polyolefin films; – thicknesses: from 12 to 250 μ (possible up to 500 μ), average between 30 and 120 μ; – width: up to 3,000 mm (average < 1,600 mm); – heating temperatures = 180 to 200°C, or 120 to 130°C on the welded surface for 1 to 2 sec, at § 1 bar (for PE and PP films) – speed: depending upon thickness, from 30 to 180 welds/min at film strip speeds between 60 and 120 m/min (on automatic bag machines).

180

Plastic Forming Processes Heating jaws PTFE

Films to weld

Heat propagation outside an insulated mass

Figure 4.1. Film welding by impulsion

Welding machines: range from the small home machine to seal freezer bags, to automatic bag lines, working on a continuous basis to carry out bag bottom welding (for heavy duty sacks) or side welding (smaller bags), with the addition of a thread (garbage bags), of handles (shopping bags), or with folding (gusseted bag, hoods) and perforated films.

Figure 4.2. Thermal welding machines for heavy duty sacks (source: Günter)

These welding machines are very efficient, controlled by motor operation, with continuous heating rods and a programmable control enabling a fast shift of sizes. Typical application: the main application is in bags and multilayer films, of PELD, PE-HD and PP (from small bags to large hoods). Mirror: the parts to be assembled are pressed on a heating plate (called a “mirror”), up to softening, then butted together, one over the other, and cooled in situ. Thicknesses can be larger, as only a fraction of the parts is melted. However, the welding creates an edge which must be trimmed for better appearance. Heating mirror

Cover

Pressure

Weld Edge (from crush) Casing

Figure 4.3. Mirror welding (example of a tray accumulator)

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181

The mirror being in contact with the parts to be welded (infrared heating) it is most often coated with PTFE, to avoid unwanted gluing. NOTE.– PTFE is no longer used for polymers that require heating to around 300°C. A mirror made of polished alloys is preferred, not to risk toxic exhausts of fluorinated vapor. Operational characteristics: – mirror temperature: 180 to 260°C, at 2 to 5 bars (for thick parts of PVC, POM, PA); – welding cycle: fairly long, between 10 and 20 sec. Most frequently used applications: average or large thicknesses. Tubes made of PE or PP: this is butt-welding for diameters ranging up to 500 mm, which requires a perfect alignment of the two tubes to be joined. This operation is most often performed on the building site. It is important to control the heating and cooling times (depending on thicknesses) and to protect the weld from exposure (dust, wind, etc.). An inflated small balloon is sometimes used when joining small diameter tubes, to avoid any edges. Tubes to be welded

Centering

Heating mirror

Figure 4.4. Mirror welding of rigid tube butt welding

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Plastic Forming Processes

Heat shrinkable sleeves are sometimes used for tube assembling. Heat shrinkable sheath (PE + electrical resistance) Tube

Figure 4.5. Heat shrinkable sheath (PE + electrical resistance)

Automobile: accumulators made of PP (cover/tray), car intake manifolds made of PA (competing with parts molded with lost-core injection). Building construction: window profiles of rigid PVC (with need for trimming). Profiles to be welded

Pinching and trimming

Heating mirror

Figure 4.6. Mirror welding for window profiles

Friction: heating is generated by the friction of the two parts to be assembled (one is mobile and the other is fixed). The work is performed: – by rotation for a rotating part of average thickness; – by angular or linear vibration for asymmetric, long and thin parts. As above, the weld is set with a pressure, then a cooling time. Pressure Cap rotation Weld

Clamping

Tube

Figure 4.7. Rotation welding (example of a disposable lighter)

Assembly and Fixations

183

Operational characteristics: – rotation speed = 5 to 15 m/sec under 50 bars; – welding cycles: = 2 to 5 sec for rotation (average thickness 1 to 2 mm), = 0.5 to 2 sec by vibration (thickness < 1 mm). Examples of applications: – disposable lighters (PA or POM): rotation; – plastic straps (PP or PA): vibration. Vibrations Sheet to be welded

Figure 4.8. Vibration welding for plastic strapping

All these types of thermal welding are well adapted to polyolefins (PE, PP). 4.1.2.1.2. High frequency welding Principle: a HF generator creates a discharge between two cold electrodes, which maintain the parts to be assembled by pressure. Their fusion by induction is thus achieved. Welding is carried out at the best place, since the heat dissipates from inside to outside. However, only polar polymers can be thus assembled, resulting in dielectric losses (essentially plasticized PVC) and very thin parts (generally a few tenths of a millimeter). Furthermore, the resulting HF waves (frequency = 27.12 MHz or 11.11 m wavelength) may affect radio or TV broadcasts nearby. It is thus necessary to protect the equipment by shielding (electromagnetic compatibility) and to provide anti-flash devices. Induction

Cold electrodes HF

Films or foils to be welded

Heat propagation at the welding spot (from inside to outside) = favorable

Figure 4.9. HF welding on plasticized PVC sheet

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Plastic Forming Processes

Welding machines: there are hand-held, pedal operated, low power (1 to 4 kW) welding machines used for plastics mock leather handbags and small luggage, and automatic plate, high powered (8 to 15 kW) welding machines for large widths with 1 m long electrodes or more, and 1 to 2 ton pressure. There are stabilized generators in all cases.

Figure 4.10. High frequency welding machine with pedal

Figure 4.11. HF plate welding machines (source: TRM)

Operational characteristics: the power of the welding machine is directly linked to the area to be welded, and indirectly linked to the thickness. As an indication, there must be: – § 80 w/cm, for 2 sheets of 100 μ; – § 50 w/cm, for 2 sheets of 300 μ;

Assembly and Fixations

185

– polymers: films or sheets of plasticized PVC; – sheet thickness: up to 500 μ; – average speed: - about 1,000 welds/hr, - about 6 m/min in a continuous operation. Examples of applications: all applications of plasticized PVC sheet (< 500 μ). Plastics bags and luggage, office supplies, file folders, shoes, waterproof clothing, sunshades, seat covers (furniture, cars), inflatable articles (PET fiber reinforced PVC). 4.1.2.1.3. Ultrasonic welding While thermal welding is mainly for polyolefins (PE, PP), and HF welding for PVC, ultrasonic welding can be used with all thermoplastics. Principle of ultrasonic welding: electrical vibrations are converted into mechanical vibrations with an oscillator. The process is based on the shift of sonic waves at the spot where surfaces must be assembled. This provides a hammering effect, with heating and melt of the surfaces. The shape of the parts may help this effect. The two parts are then pressed and joined after cooling. The cycle is as follows: Ultrasonic generator o transducer o amplifier (transmitting ultrasonic vibrations) o sonotrode o part to be welded This process allows us to exactly limit the softening of the area or to assemble and weld massive parts, but it requires a type of sonotrode, most often made of titanium alloys, whose axis must be perpendicular to the area to be welded, and whose shape is specific to the parts to be assembled, as it must be adapted to their geometry. There are two ultrasonic welding methods: – near-field welding: the device fits the shape of the junction area of the parts to be assembled (for thin parts); – far-field welding: the sonotrode touches the part to be assembled at a spot distant from the welding area, for massive parts, for example welding a cover onto an enclosure.

186

Plastic Forming Processes

The part to be welded that touches the sonotrode oscillates in phase with it, and the opposite part is fixed, to avoid fixed resonance. This type of welding is well adapted to rigid and dense polymers, particularly amorphous polymers such as PS and ABS. Conversely, flexible materials, cushioning materials like foams or heterogenous materials, are not advisable. Water absorbing polymers such as PA must first be dried. It is however possible to assemble polymers that are different but with close characteristics. Ultrasound train

Sound wave guide (sonotrode) Cover

Butt

Welding Enclosure

Shapes of the parts to be assembled to help welding and to hide butts

Figure 4.12. Ultrasonic welding (cover/enclosure)

In order to improve welding and to avoid butts, the part to be welded, often an injection molded part, features a designed shape called an “energy guide”, of > 1.5 mm thickness that concentrates energy and provides the plastics mass required for the joining. Welding machines: the machines feature operating frequency ranges between 15 kHz (for large parts) and 40 kHz (for precision parts), up to 70 kHz (sometimes for subminiature welds). An electronic circuit maintains constant welding amplitude (comprised between 0.7 and 1.8 mm).

Assembly and Fixations

Figure 4.13. Ultrasonic welding machines Sonotrodes

187

188

Plastic Forming Processes

Operational characteristics: – polymers: mainly the most rigid, PS, ABS, PMMA, engineering plastics; – cycles: 0.05 to 2 sec, very rapid; – speed: from 40 to 120 parts/min, about 1,000 welds/hr; Standard applications: – automotive: air filters, brake fluid reservoir, air bag envelopes, various lights, rear reflectors, dashboards, etc.; – telephones: handsets; – computers: disks; – medical articles, (catheters, dialysis filters, etc.), disposable lighters (competing with rotation welding); – other ultrasonic applications (see below): deformation allowing insertion and riveting. 4.1.2.1.4. Laser welding Principle: this process can be used to assemble two plastic parts, one transparent or translucent (to let the melt inducing laser ray move through) and the other opaque (absorbing). Therefore, laser welding is limited to products with one of their parts made of PMMA, PC or PET. Transparent part laser ray Opaque part Ray reflection = heat

Melt area

Figure 4.14. Laser welding

The laser or the parts may be mobile, to help welding. The laser used is a CO2 laser (wavelength = 10.6 μ). Typical application: car lights made of PMMA.

Assembly and Fixations

189

4.1.2.1.5. Point welding This is a variant used when a plastic part to be welded is too massive to be put under a press. Then, a head (hot joining part or mainly sonotrode), in the shape of a gun, which links across the two parts to be assembled, is used in order to impregnate the two melts. However, a part of the melted material moves up to the surface, forming an edge, but the bottom layer is unaffected.

Hot joining part or sonotrode

Assembled parts

Figure 4.15. Point welding

Application type: welding of large thermoformed plates (up to 8 mm thick). 4.1.2.2. Welding with a support This is a craft or semi-craft type operation, using mainly thick plate semiproducts. In practice, a rod of the same polymer as the parts to be assembled is melted with a hot air torch (heated by gas or by electricity). The parts to be assembled have been previously chamfered. With this process it is possible to obtain short run, large volume finished products, with thicknesses ranging from a few mm to a few cm. For instance, a plate of thermoplastics can be rolled and butt welded on the generator to obtain a perfect ferrule. The temperature of the torch can be adjusted between 50 and 500°C.

190

Plastic Forming Processes

(a)

(b) Figure 4.16. (a) Hot air torch; (b) hot air torch with rod

Assembly and Fixations

191

Solder Thread solder Hot air Welding device

Pressure Hot air Round nozzle Welding device Pressure

Weld Hot wire torch

Downturn

Oscillation Welding direction Bow wave

Welding direction Downturn Weld

Solder rod

Plates to be welded

Welding examples

Figure 4.17. Types of welding with solder

Applications: containers and tanks for the chemical industry, for surface treatments, for air conditioning, etc. (mainly from PP and PVC plates). The following table summarizes the various assembly possibilities by welding and adhesive bonding of the most frequently used polymers.

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Plastic Forming Processes

Polymers PE PP Rigid PVC Flexible PVC PS ABS PMMA Cellulosics PA POM PC PBT-PET Fluorinated PSE PU Thermosets

Adhesive bonding

Welding Thermal

HF

Ultrasonic

Average Good

Figure 4.18. Plastics welding and adhesive bonding

IMPORTANT NOTES.– The behavior of welds and adhesive bonding depends upon the intrinsic characteristics of polymers (morphological structure, homo- or copolymers, alloys, nature and quantity of fillers, reinforcements, colorants and other additives). The design of the parts to be assembled also plays an important role in the fabrication (shape, dimensions, complexity, bonding area, etc.). The original design must thus be taken into account at the earlier stage. The same is true for the welding parameters, temperature, pressure, etc. The assembly of crystalline polymers requires materials of the same nature. On the contrary, with amorphous polymers, it is sometimes possible to bond plastics of a different nature, for instance ABS/PMMA. Thermosets, which cannot be regenerated, cannot be welded and must be adhesive bonded. The cost of one weld on a welding machine is only a few cents per unit.

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193

4.1.3. Riveting 4.1.3.1. Riveting by crushing It is possible to join two elements of different polymers, one featuring a cylindrical hole and the other a dog point, by heat crushing (thermal or ultrasonic) under pressure, of the excess part after pressure, after nesting one on the other.

Heated tip

Parts to be assembled

Assembled parts

Figure 4.19. Crushing by hot dog point

Crushing with a dog point does not give a very good appearance, the aim is just the simple joining of two elements.

Sonotrode

Parts to be assembled

Assembled parts

Figure 4.20. Ultrasonic riveting

In ultrasonic snap riveting, the cavity of the sonotrode is to absorb the volume of the melted material, the overall appearance of the riveting thus being improved. It is also possible to use several sonotrodes, to carry out several different welds, on the same part, at the same time. Typical applications: to fix brand emblems, to assemble parts of different plastics, on plastics on a metal sheet, etc.

194

Plastic Forming Processes Positioning dog point

Crushing Hot wire

Brand emblem

Fixation dog point

Support

Figure 4.21. Assembly by crushing

4.1.3.2. Blind riveting In order to bind two sheets or two thin plates, with only one visible side, hence the word “blind”, rivets with flexible sleeves of the “Pop” type are used. They cannot be dismantled because of permanent deformation with the sunken dog point.

Hammering sinking-setting

Rivet

Figure 4.22. Blind riveting

Assembly and Fixations

195

The flexible plastic rivet enters into a hole made on the two sheets to be assembled, (generally metal sheets) by pressure on the dog point. Part of the rivet is broken with a hammer, and the rivet is pushed between its flexible parts that press on the opposite face, thus moving the two sheets together. The polymers used with blind riveting are mainly POM, as well as PA and PET. NOTE.– the elasticity of PA is very limited in a dry atmosphere. Applications: blind riveting is used for large runs of thin metal sheet or accessories on metal sheet. 4.2. Demountable assemblies 4.2.1. Ratchet assembly The advantage of this process rests on a system that comes directly from molding and does not require any additional element to produce the assembly. The plastics flexibility helps the locking, using flexible fasteners generally molded as undercuts.

Flexible fasteners

Fixing on a support Flexible fastener

Molded part

Mold detail

Figure 4.23. Economic molding of flexible fasteners

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Plastic Forming Processes

There are three types of flexible nestings: – with flexible clip; – cylindrical; – with round joint.

Distortion

Round or cylindrical nesting

Ratcheted cover

Claw ratchet

Figure 4.24. Types of ratchet assembly

Applications: all types of fast joining in very large run industries (cars, household appliances). 4.2.1.1. “Velcro” strip “Velcro” is another specific case of joining made possible by the interlacing of the hooks of PA on a strip. The laminating of the 2 strips on reverse enables fixing that remains flexible. Supporting fabric Hooks made of PA

Figure 4.25. Principle of Velcro

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197

Velcro is directly a derivative of nature. The manmade material, made of PA or PET, comes directly from the observation of the burdock (Mestral patent). Applications: the main applications are in clothing and shoes, as substitutes for buttons or zippers. 4.2.2. Screwing A direct threading can be used on a molded part, which requires unscrewing the mold shape, as an undercut, and which lengthens the fabrication process. It is also possible to have only one cylindrical or parallelepipedic hole in the part, the threading being achieved at the mounting step, with a metal screw (by selfthreading). Finally, it is also possible to use a threaded metal insert in the mold, or set by ultrasound in a cylindrical part of the mold. NOTE.– this last solution is to be used as a substitute to threading on plastics when the assembly may have to be taken apart (see below). Metal screw Use a screw with reamed head Bad

d1 Plastic d2
Use a screw with cylindrical head Good

a screw Use aUse screw with cylindrical head with cylindrical

(b)

e
Metal insert Optimal

(a)

(c)

Figure 4.26. (a) Standard screwing; (b) self-tapping screw; (c) with intermediate flexible ring

4.2.3. Assembly with flexible hinge It is possible to produce a mobile joint of the parts by playing upon the hinge property of some polymers, particularly PP. The hinge is molded directly, or added to the part to be assembled.

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Plastic Forming Processes

Figure 4.27. Use of PP hinges

Applications: important in household appliances. 4.2.4. Insert In order to facilitate assembly, inserts are often installed. This operation can be performed at the time of assembly. However, to avoid complicating tooling and/or increasing the operation cycle (expensive in both cases), it is sometimes preferred to resume the operation with ultrasounds, the whole operation thus being less expensive. The technique is then to put the insert on a hole made at the molding stage, whose diameter is slightly less than that of the insert. Ultrasonic vibrations are transmitted through it to melt part of the molded part while pressing the insert into place. The melt material fills the space of the insert/part, and makes them one when cooling. It is also possible to make a metal shank (bush, socket) by mechanical pressure, on a larger plastic element that can resist bursting. However, it is necessary to take into account the fact that the pull force decreases with time because of constraint relaxation.

Chapter 5

Finishing Treatments

5.1. Plastics deposition on metal (or metal coating) Plastics deposition on metals or plastics coating, means coatings of polymer powders, generally to be deposited on metal surfaces, to protect the metal, especially for products left outside, and sometimes to provide a better finished appearance. Plastics coating is thus in competition with other surface treatments, but with bigger thicknesses, as shown in the table below. Coating

Metal covering

Paint

Electroplating

Nature

Polymer powder

Coating solution

Metal depositing

Thickness

60 P to 1 mm

20 P to 60 P

1 to 10 P

1 single coat

Several coasts

No solvent

Solvents

Pollution

Effluents to be treated

Table 5.1. Various types of coatings

The thickness depends upon the type of powder, defined by the polymer, granulometry and melt temperature. The operation is often performed without a solvent, without chemicals, hence without pollution. This metal/plastics combination combines the mechanical

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Plastic Forming Processes

performances of the metal and the chemical resistance of the plastic, plus a low friction coefficient. The following table summarizes the main polymers used as powders, as small grains, 40 to 200 μ, depending upon the various application techniques. Powders Polymer

Processes

Metal adhesion

Spray

Fluidization Electrostatic system spary systems

PVC PE PA-11 PBT CAB Polyester Epoxy

Dispersion

Plastisol

Detachable

Deposited thickness (mm) Good

Dipped

0.4 to 1 and +

0.2 to 0.5

< 0.1 to 0.2

0.01 to 0.05

Possible

Figure 5.1. Polymers and application processes

The PVC, PE and PA-11 powders require a primer coat on the part. Epoxy and polyester do not need primer coats, but they must be polymerized after powder application. 5.1.1. Torch gun spray This process comes from the metallizing of metals. A torch with an oxyacetylene flame is used to spray a molten plastic on the surface to be coated, generally a large area. The supporting area is first degreased, then sanded, and sometimes grooved, in order to create a satisfactory bonding. This technique requires the use of polymers whose fusion point is relatively remote from their decomposition temperature. This is the case of PE-HD and PA. The polymer thicknesses currently deposited are 0.5 to 1, up to 2 mm for PE-HD and PA.

Finishing Treatments

201

Spray systems Part to be coated

Plastic powder tank

Oxypropane

Compressed air

Figure 5.2. Spray system with torch

Applications: large parts that are difficult to transport and treated on site, in the chemical industry, ships, etc. The polymer thicknesses currently deposited are 0.5 to 1, up to 2 mm for PE-HD and PA. 5.1.2. Fluidized bed The support is first prepared by degreasing, chemical pickling, possibly coating with a primer bonding to improve adhesion. Then the pre-heated support is put close to a plastic powder in suspension under a low air pressure, 0.1 to 0.2 bars, in a tank with a porous bottom, called a fluidized bed. The part to be coated is dipped into this suspension. A certain amount of the powder melts and clings to the part in 2 to 3 sec. The main powders used are PA 11. The coating is completed with water cooling, if a gloss effect is preferred, or with air cooling for a matte finish. The equipment includes: – a pre-heating oven at the fusion temperature of the powder, plus 20 to 40°C; – a double bottom tank, with the above part of a porous material, with a high blowing fan; – a water or air cooling system for finishing.

202

Plastic Forming Processes

The handling of the parts is performed on a monorail. The dipping is sometimes done by hand, especially if the part to be coated is of a complex shape. In practice, a better coating of the powder is achieved with light hammer blows on the part. The polymer thicknesses are 150 to 300 P (PA–11). Oven

Part to be coated

Monorail

Coated part

Fluidized plastic powder Porous bottom tank

Fan

Figure 5.3. Fluidization

The equipment is entirely automatic in the case of wire metal coating. Speed ranges from 25 to 50 m/min, and the heating is by HF induction. Applications: furniture (for schools, hospitals, city streets), metal windows and doors, dishwasher baskets, dryers, shelves, water heater interiors, battery connector bars, fire extinguishers, electroplating frame and swing, marine hardware, chemical industry, etc. 3

2

1

4

5 1. Surface treatment 2. Priming 3. Pre-heating 4. Dipping 5. Coating

Fluidization operation 1

2

3

4 1. Surface treatment 2. Priming 3. Electrostatic powder coating 4. Melt oven

Electrostatic powder coating operation

Figure 5.4. Fluidized bed and electrostatic powder coating operations

Finishing Treatments

203

5.1.3. Electrostatic powder coating Like electrostatic painting, and always after a primer preparation, an electric field is used to deposit the powder with a difference of electric potential between the surface to be coated and the spray gun. As opposed to the previous process, the prepared support is relatively cold at first. The temperature is raised after powder depositing. The powders used are mainly thermoset powders (usually epoxy). The plastic is polymerized on its support in a drying oven. The equipment includes an electrostatic generator, with one or several spray guns and a polymerization oven, of 180 to 200°C, for 10 to 20 min. The operation can be done by hand or automated. The thickness coating ranges from 60 to 100 μ (epoxy). Oven

Monorail

Coated part

Part to be coated

Electrostatic spray gun

Figure 5.5. Electrostatic powder coating

Advantages: – cheaper, as the coating thickness is lower; – the part size is not limited by the tank; – it is possible to treat only one side, and to correct any errors; – there is no solvent, compared to painting, hence less danger to operators and less risk of fire. Limitations: – 30% of the sprayed powder does not reach the support and must be recycled; – the process is slower than the fluidized bed, powders are finer, hence more expensive; – a full clean-up is needed after changing color.

204

Plastic Forming Processes

Applications: same as the fluidized bed process, as this is a competing process, electrical wires, protection of metal sheet in a continuous coating operation. 5.1.4. Dip coating, suspension or aerosol A coating can be produced on a metal support by dipping the part in a liquid resin (polyester, epoxy), particularly for electrical insulation. This process is currently used for copper wires, or in a suspension of PVC plastisol, CAB, to obtain a temporary protection, that is possibly removable like the type used to protect sharp tools. Coatings from plastics dispersions are thus obtained, for instance fluorinated products that are sprayed as aerosols on a metal support, grooved by a hydrochloric acid treatment, then oven dried at about 400°C. Applications: pots and pans of the Tefal brand, coating with “Tefal”, feeding chutes in food and chemical plants, molds. 5.1.5. Powder selection 5.1.5.1. Thermoplastics Polymers

Advantages

Limitations

PE

Chemical, food inertia HF insulation Impact resistance Low price

Limited temperature Combustibility

PVC

Chemical behavior Abrasion resistance Self-extinguishing Low price

Limited temperature UV browning

PA 11 “Rilsan”

Abrasion and scratch resistance Mechanical and chemical resistance Low friction coefficient

Sensitive to strong concentrated acids

PTFE

Excellent chemical and thermal resistance Very low friction coefficient

High price

Finishing Treatments

205

5.1.5.2. Thermosets Polymers Polyester

Advantages Chemical and electrical resistance Good aging

Limitations Combustibility

Impact and abrasion resistance Epoxy

Chemical and thermal resistance

UV chalking

Dielectric rigidity

5.2. Metal deposition on plastics Metal deposition on plastics is only applied as thin coats, essentially to add a decorative finish and, sometimes, to provide electrical conductivity. There are three main processes. Processes Metallization

Deposit thickness 0.06 to 0.08 μ 0.06 = translucent

Type of deposit Aluminum

Sputtering

Up to 1 μ

Many metals

Electroplating

15 to 35 μ of which: Cu = 15 to 30; Ni = 2 to 3, Cr = 0.3 to 2

General

Table 5.2. Typical nature and thicknesses of deposits

5.2.1. Vacuum metallizing Vacuum metallizing can be carried out on a 3D part or on a film. The principle is to vaporize a thin metal sheet in a vacuum chamber, by discharge of a capacitor. In practice, a pure, 99.99% aluminum foil is put on a support in a vacuum chamber, (10-4 to 10-5 Torr). The aluminum foil is exploded with a high frequency generator, and the metal vapor is deposited onto the plastic support.

206

Plastic Forming Processes

Figure 5.6. Vacuum metallizing chamber

The operation is discontinuous, generally for small molded part sizes, setting the parts on a support in the vacuum chamber, putting the chamber under vacuum, and picking the finished parts. The whole operation is performed by hand. Light sealed chamber

HF generator

Filament Al 99.99%

Vacuum device

Part to metallize

Figure 5.7. Vacuum metallizing of molded parts

Finishing Treatments

207

On the contrary, in the case of film metallizing, the operation is continuous, as the film unrolls automatically in the vacuum chamber, passing close to the oven for aluminum melting. The metal vapor is deposited on the film.

Figure 5.8. Vacuum metallizing of plastics films

The metallized coating is very thin in all cases, § 0.1 P. If the support is transparent, the coating can be translucent and can be easily scratched by a nail. The coating is then performed on the reverse side and protected with a varnish. The silver color of the aluminum deposit can thus be seen through it. Moreover, the part to be decorated is often first coated with a yellow varnish, which gives a final gold colored appearance. Hence the metal coating is sandwiched between two protective varnishes. Protective varnish Metal deposit Primary undercoat varnish Plastic part Figure 5.9. Cutaway view of a metallized part

208

Plastic Forming Processes

The cycle time within the vacuum chamber is about 7 min, and the whole operation, loading and unloading by hand is about 1 hr. Part dimensions

Number of parts by cycle

Cycles/hr (parts/hr)

Number of runs

§ 200 mm

§ 100

2

100 to 200

§ 10

12

200 to 400

6

§ 20 mm

Table 5.3. Production speed of metallizing

Speed rates for film metallization = 6 to 10 m/sec.

Figure 5.10. Examples of metallized products

Supporting polymers: – ABS, PS, PMMA as molded parts; – PET, PP, PVC as films.

§ 2,000

Finishing Treatments

209

Applications: – for 3D parts, reflectors and frames of car and motorcycle lights, flashlights, knobs, perfume caps, eyeglasses, Christmas decorations, brand badges, etc.; – for films or sheet, continuously metallized in two chamber machines, for various food packages, Christmas garlands, flexible mirrors, dress sequins (of PVC), capacitor dielectrics, magnetic tapes, survival covers, tapes for hot stamping (of PET). 5.2.2. Sputtering The principle is similar to that of the former radio diode lamps. In a chamber that contains a neutral gas, argon, an electric discharge is created between two electrodes. The gas is ionized and the ions are directed to a target, the cathode made of the metal to be deposited which issues metallic atoms. The particles thus removed condense on the plastic support, anode, in the same chamber, first put under a vacuum, (10-4 to 10-5 Torr).

Figure 5.11. Sputtering unit

210

Plastic Forming Processes

The advantage, compared to the previous method is to allow using several coating materials, with better adhesion. As the deposited metal thickness is small, about one micron, a bonding varnish and a finishing varnish are used. Cycle: about 15 min in the chamber, but the total cycle takes about 1 hr, for the hand loading and unloading. For instance, CDs take about 1,000 units/hr.

Figure 5.12. Sputtering

Supporting polymers: all are possible, for instance PC for CDs. Applications: CDs, hubcaps and grilles for automobiles, EMI protection film, membrane keyboards, liquid crystal screens, multilayer circuit junctions, etc. 5.2.3. Electroplating In order to treat plastics by electrolysis, the plastic must first be made conductive. First porosity is created on the surface of the plastic support with a chemical treatment that breaks the organic bridges, to create holes which will help anchor the chemical deposit, copper is used in general. The support having thus been made conductive, it will be possible to deposit other layers by classic electroplating, up to the required thickness of a few microns. Degreasing o neutralizing acid o activator o chemical Cu o electrolytic Cu o Cr

Finishing Treatments

211

Steps of electrolytic chromium plating Figure 5.13. Principle of the application on ABS

The advantage of this process is to enable relatively thick deposits, of a few microns, and not require any further polishing or varnishing. The raw molded skin is enough for the treatment. It is also possible to treat large parts, up to the geometrical limit of the tanks. However, it is important not to soil the part with a demolding agent, and to stick to the standard rules of electroplating, such as to avoid sharp angles. Cr

Parts First bath

Bath of chemical Cu

Figure 5.14. Electrolytic plating

Electroforming

212

Plastic Forming Processes

Production speed: cycle from 1 hr to 1 hr 30 min and more. Supporting polymers: mainly ABS. Applications: command buttons, brand monograms for audiovisual equipment, appliances, hubcaps, grilles and handles for automobiles, mobile phones, bathroom accessories, faucets, perfume packaging, costume jewelry, mold prints for rotomolding of dashboards using the slush molding process. 5.2.4. Advantages and disadvantages of the various processes The following table summarizes the advantages and disadvantages of each process. Vacuum metallizing

Sputtering

Electroplating

Several coating materials are possible Good cling

Lower cost

Higher coating thickness

No heating of parts

Larger parts possible

No distortion

No need to varnish (only in exceptional cases)

Possibility of complex shapes Possible to coat only one side (reserve) No pollution Need for primer varnish and protection

Need for neutral gas

Long operation cycle

Vacuum and end vacuum at each cycle

Covering of 2 sides or expensive reserves

Limited metal deposits (Al)

Need for water treatment (pollution)

Need to clean residues Fragile layer, to be protected: Nail scratching, poor alcohol resistance

Simple shape parts

High investments

Preparation is important for all three processes.

Finishing Treatments

Vacuum metallization

Sputtering

213

Electroplating

Average deposit speed (μ/min)

0.1 to 7.5

0.2 to 5

0.2 to 5

Thickness control

Difficult

Easy

Possible

Uniformity of deposit

Facing the spray

Very good

Irregular on angles and complex shapes

Bonding to the support

Poor

Good

Fair

Pollution

None

None

Toxic and corrosive fumes

Table 5.4. Comparative characteristics of the 3 processes

5.3. Printing and decorating 5.3.1. Preliminary treatments First it is necessary to perform an operation to help the adhesion of pigments, inks or decors on plastic supports with a low surface tension. This is particularly the case of polyolefins, PE or PP, with an “oily” touch and which are fairly inert chemically. The purpose of this treatment is to make the surface rougher, or to create polar groups enabling a better bonding. The wettability of the substrate by the colorant or the ink is essential for its adhesion. Like paper, polymers can be marked by similar techniques, with specific colorants or inks however. As a rule, the plastics that can be easily adhesive bonded can also be easily decorated and printed, and conversely. Print or decoration

polymers

Easy

PS, ABS, PVC, PMMA

Preliminary treatment

PE, PP, PET, PA

Difficult or impossible

POM, PTFE, silicones

Table 5.5. Polymer behavior to printing or decoration

214

Plastic Forming Processes

In general, the substrate is first cleaned. Then the surface substrate is oxidized by various treatments, such as torch burning, corona discharge, plasma, or chemical treatment. For rigid and thick parts, the preferred processes are: – flame oxidation achieved by spot burners in front of which the part to be printed is presented, on a conveyor. The flame, up to 2,000°C, causes a superficial oxidation of the substrate. This is the often used process to print bottles; – or an acid bath (oxidant) or a microwave treatment, under partial vacuum, which requires a more expensive investment. These processes are mainly used for injection molded parts. For thin flexible products, these treatments are too hard. An ozonation by Corona discharge is preferred. There is a discharge between an electrode and the substrate to be printed, which passes through a metal cylinder connected to the ground, in a small space of just a few millimeters, so that the nearby atmosphere can be ionized. The ozone (O3) just generated oxidizes the substrate lightly. This is the most frequently used process to prepare plastics film, because it is simple, and it is a continuous process. Gas burner

Bottle to decorate

Conveyor belt Figure 5.15. Flame oxidizing

These operations make it possible to partially open the polymer chains, and to bond the marking on the branches. However, the printing operation must follow immediately, to avoid the polymer chains closing.

Finishing Treatments

215

Ozonation

Electrode

Oxided film HF generator

Metal cylinder Earth

Figure 5.16. Oxidizing with Corona effect

5.3.2. Printing or decoration on a rigid substrate Printing on a small surface: logos, small decors, instructions for use of packaged products, etc. Many processes can be used, depending upon the type of marking. Processes

Tpe of the deposit

Direct molding, hollow, 3D

Distortion

In-mold decorating

Label (see specific injection molding)

2-color overmolding

Double molding

Inkjet

Ink

Laser

Pigment alteration

Serigraphy

Ink

Hot stamping

Metal

Tampography

Ink

5.3.2.1. Printing on small areas 5.3.2.1.1. Direct molding (hollow or 3D) This is the least expensive marking. It can be done at the same time as the injection molding. The mold features as a hollow, the print which will appear as 3D

216

Plastic Forming Processes

on the part, or the reverse. It is also possible to add a paint coat on the 3D, or in the hollow, to better emphasizes the marking. Mold

Mold

a) Engraving on the part

b) Relief engraving in cavity of the part

Figure 5.17. Hollow molding

Types of applications: brand names, code marks. 5.3.2.1.2. 2-color overmolding The marking to display is made with one first molding then overmolded with another molding of the same plastic, of a different color. This is carried out with a 2head machine, or by another pass, thus making the marking permanent.

First molding

Overmolding (color)

Figure 5.18. Overmolding

Types of applications: keyboard keys, competed with laser marking. 5.3.2.1.3. Inkjet (ultrasounds) An inkjet spray, by vibration, is transformed into even size drops, which receive a discharge proportional to the place they have in the print. This operation is done without contact with the substrate.

Finishing Treatments

217

Ink Drop generator

Charging electrode Ink drop Detection

Deflection plates Printing Product to be printed

Figure 5.19. Inkjet

This technique can be applied on a continuous basis on flexible or rigid products for simple marking, for instance for bar codes. Types of applications: marking and putting date on water bottles and food packaging. 5.3.2.1.4. Laser With control of the power, the screening speed, the impulsion frequency, of a laser beam, a spot vaporization of the material is done. This causes a black printing, carbonization or, if the substrate is transparent or painted, a spot evaporation of the paint, changing the color by chemical reaction. It is thus possible to aim the printing through using a mask. The type of the polymer and the presence of fillers and additives play an important role in the print quality. The carbon black improves contrasts; the glass reinforcements reduce the appearance sharpness. There is no use neither for ink nor solvent. The marking is achieved by pigment discoloration without contact. Thus, there is no risk to affect the material. The marking resists water, detergents and sterilization.

218

Plastic Forming Processes

Laser

Part to mark Rear mirror Crystal Nd:Yag Diaphragm Q-switch

Protective glass UV filter

Safety shutter Exit mirror Deflection mirror and galvanometer

Arc lamp

Telescope Focusing aim

Laser input

Marking head

Figure 5.20. Laser

To obtain a light color marking on a black background, a laser Yag is used. To obtain a dark print on a light color background, a laser Excimer is used. The width of the line can range from a few microns to a few millimeters. There is a variant with the marking by foaming, which produces gas bubbles retained on the superficial layers of the polymer. Types of applications: marking or coding for instrument panel and rear-lighted knobs for cars, keyboards of car radios, telephones, computers, contactors, etc. 5.3.2.1.5. Serigraphy or silk screen process This is the principle of a stencil, which lets the marking ink get through the textile weft in specific spots, the rest being protected by a varnish. The print is generally covered by a protection lacquer.

Finishing Treatments

219

Ink Doctor blade Screen Part to mark

Figure 5.21. Silk screening

Types of applications: CDs, bottles, under air pressure during the operation. The blown bottles always show a notch on the bottom, to enable the drive and the start mark, if the operation uses several colors.

Figure 5.22. Notches

220

Plastic Forming Processes

5.3.2.1.6. Hot stamping A plastic film with a colored coating is sandwiched between a heated metal letter stamp, with the printing characters and the part to be printed. The stamping deposits a printing of the color of the film by hot stamping, 80 to 120°C, on the substrate.

Engraved hot stamp

Marking tape

Part to mark

Figure 5.23. Hot stamping

Film reel

film rting o p p u s Polyester ent ng ag Wax, releasi eta l Pigment or m Adhesive primer

Figure 5.24. Detail of tape components

Finishing Treatments

221

Figure 5.25. Machine

The stamping can be performed hollow, with a relief letter stamp, or flat on a molded 3D part. The stamping film is obtained by vacuum metallizing. The print is generally gold, hence the French “gold pressing”, but many colors are available. Applications: monograms for car brand names, dials, decoration on perfume and cosmetic boxes. 5.3.2.1.7. Tampoprint This is a process derived from the offset for rigid parts of odd shapes. A rubber stamp is used which is supported on a metal plate engraved intaglio

222

Plastic Forming Processes

(photochemically) and which receives a color and transfers it onto the part to be marked. There as many rubber stamps as there are colors. Rubber stamp

Engraved print Ink

Part to mark

Figure 5.26. Tampoprint

5.3.2.2. Decorative printing on a large area The processes vary, depending upon the type of the deposit and its thickness. Process

Deposit thickness

Deposit type

Bulk coloring

Colorants

Paint

About 10 μ

Pigments

Graining

Distortion < 1 mm

Directly on substrate

Flocking

100 to 250 g/m²

Textile

Film coating

20 to 50 μ

Plastic film

Transfer

20 to 40 μ

Plastic film

Sublimation

Entering 10 to 30 μ

Colored ink

Table 5.6. Decorative processes

Finishing Treatments

223

5.3.2.2.1. Bulk coloring (master batch colorants and liquids) This is the simplest and cheapest decorative solution. Plastics can be bulk colored, as colored blend before granulating, by the plastics producer, in the case of large tonnages. It is also possible, at the converting step, to use dry blends, in the case of smaller quantities, by adding master batch colors to the virgin pellets. Or, in the case of very large runs, to use liquid colors that can be directly put on the machines. Coloring types

Advantages

Disadvantages

Blending equipment

Powder colorants

Cheap (dry-blend)

Dust cleaning

Closed (barrel out of line)

Dried

Open (master-batch)

(Pellets)

(Type of cement mixer)

Liquid (concentrated)

Excellent dispersion

Solvent cleaning

On machines

Table 5.7. Plastics coloring

5.3.2.2.2. Standard painting Sometimes plastic parts are painted, particularly when they belong to a larger device that features metal parts that are also painted, in order to obtain a total color look, for instance for car bodies. It is important in such cases to have a strong bonding preparation, with corona discharge or chemical treatment, plus possibly a primer, and to dry, at a temperature suitable with the selected polymer. Master batches can bring special effects by including particles: – granite o mineral appearance: for garden decoration and equipment; – mica, titanium and iron oxides o mother-of-pearl effect: sport articles, packaging; – aluminum or bronze o metallized appearance: sport equipment; – glass o frosted glass effect in transparent materials, cosmetic packaging; – colored fibers o vegetal effect: garden furniture, cosmetic packaging. However, painting is a constraint which may alter the properties of the polymer (possible rings around the treated part and chemical interaction). In addition, painting reduces the recycling possibilities.

224

Plastic Forming Processes

5.3.2.2.3. Graining A grain appearance is achieved on a molded plastic part by using a chemically engraved mold, or a mold treated by electroplating, or sometimes sanding. Graining makes it possible to reduce the visible effect of sunken spots on large areas and to avoid unwanted reflection. Grained pattern cylinder

Mold Graining Part

Grained part

Sheet

Figure 5.27. Graining

Types of applications: parts for car interiors and seats, for furniture made of PP or ABS. This process is also used after extruding or calendering. An engraved cylinder is used, with intaglio relief on the still hot sheet. Then there may be a color printing. Applications: diapers and sanitary towels of PE, decorated PVC sheets. 5.3.2.2.4. Flocking With this technique, there is an electrostatic gun spraying fine textile particles on an adhesive pre-coated surface (from 100 g/m² for short hair to 250 g/m² for longer hair). A hairy appearance is thus obtained, giving a velvety impression and some sound reduction.

Textile flocs

Adhesive coated surface Part

Figure 5.28. Flocking

Finishing Treatments

225

Applications: displays, artificial flowers, toys, etc. 5.3.2.2.5. Plating, film coating With this process, a laminate of two plastics is made. The thicker layer acts as the substrate, the thinner layer film or decorated sheet of PVC or PS gives the appearance and/or protection. The thinner film can be deposited on the hot substrate at the time of fabrication, as extruded decorated sheet.

Heating press Sheet flat laminated on the part

Printed film

Figure 5.29. Plate coating

Types of applications: wood decors on extruded profiles or sheets. 5.3.2.2.6. Transfer Here, the starting point is a flat rotogravure printed decor on a transparent film that can either be: – hot transferred like a decal on the plastic part during molding, or afterwards (PP or PET film); – or dissolved into a bath in which the part is dipped and the printing is transferred: water soluble films, polyvinyl alcohol which is eliminated by washing; immersion process, called the “Cubic” process. Rewinding of the empty support

Supporting film

Reel of prints to transfer

Hot transfer cylinder Part to mark

Figure 5.30. Continuous image print transfer

Applications: decorations, on molded and blown parts (oil cans).

226

Plastic Forming Processes Printed APV film

Water Part

Figure 5.31. Transfer by dissolution (Cubic process)

Applications: finishing of car dashboard (giving the appearance of wood or the color of aluminum) competing with the following process. 5.3.2.2.7. Sublimation This process, like the previous one, is similar to decal. It also uses a decorated paper, preprinted with heliogravure, and specific inks, which, by steam heating, sublimate and deeply penetrate the substrate (8 to 30 μ, depending on time) keeping the fineness of the original decoration stamped on the part with a silicone stamp. Heating press Transfer paper (carrying the decorative inks) Decoration on the part (in depth)

Figure 5.32. Decoration by sublimation

Types of applications: decoration on sport articles, particularly ski tops.

Finishing Treatments

227

Anamorphosis is used to print on a substrate that must be thermoformed. 5.3.2.3. Printing and decoration on flexible substrates (in general, films or flexible sheets) The printing is carried out flat, on a continuous basis in flexography and heliogravure, with colored inks, on flexible substrates, films or sheets. 5.3.2.3.1. Flexography Very fluid ink, air-dried, is transferred on the negative holder cylinder using a set of inking cylinders. The rubber negative (3D and in reverse) is printed at the right spot on the part to be printed. It is the most used process for film printing. Negative holder cylinder Transfer cylinder

Printed sheed

Ink cylinder

Sheet to mark Ink

Before ink application

After ink application Typographical negative

Figure 5.33. Flexography

228

Plastic Forming Processes

Type of applications: all kinds of bags. 5.3.2.3.2. Heliogravure The important part is a cylinder engraved with many holes, each like a small inkpot giving a more or less dark print depending upon its depth (the part to be printed is squeezed between the engraving cylinder and a pressing cylinder). A fluid ink is used. Its spread on the engraved cylinder is controlled by a doctor blade that ensures each hollow is correctly filled.

Sheet to print

Pressure cylinder (rubber) Scraper

Printed sheet Hollow engraved cylinger (copper)

Ink Gravure detail of the cylinder

Figure 5.34. Gravure printing

Types of applications: high quality films for food multilayer packaging.

Finishing Treatments

229

5.3.2.3.3. Off-set The plate or negative, put on the top side on a cylinder, is first wetted, and then inked with greasy ink. The ink is deposited only on the non-wetted spots, because of the water repulsion of the greasy ink. After inking, the negative is transferred the other way on a rubber cylinder, the blanket deposits it and rights it up on the product to be printed. Dry offset, without water, is used for plastics. Decorating pot

Blanket cylinder

Negative holder cylinder

Ink

Silicone Photopolymer Aluminum

Water

Ink

Figure 5.35. Offset (principle), detail for dry offset

Types of applications: containers for food or paints. General marking speeds: from a few thousand to more than 10,000 prints/hr. Control of the printing strength: the standard test is simple. A strip of adhesive film is glued on the printing. The printing is good if it resists ungluing, while it is bad if the print does not resist.

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Chapter 6

Ecology and Recycling

There are four main types of pollution: water (consumption and dirt), air (product destruction by combustion), visual appearance (refuse built-up) and sound (noise). The conversion and use of plastics are not noisy and their water consumption is small. However, the environment can be polluted, ecologically and biologically, through the waste of these materials. What is at stake is the non-degradability of plastics, the other face of one of their main qualities, chemical inertia. Due to their chemical inertia, plastics do not soil the aquifers, like fertilizers or detergents do. 6.1. Nuisance and pollution 6.1.1. Ecological appearances (waste built-up) Plastics are not the only items to be blamed. Without plastics, the demand for packaging, linked to demography, would not have decreased. There would instead be a pile-up of glass bottles and metal cans. All countries must protect agricultural crops, and when this is poorly done, the loss may be up to 50%. Polymers are a very positive aid for keeping food and chemical products, with a small packaging volume.

232

Plastic Forming Processes 4

2.6 2 Without plastics With plastics Weight of package

Energy consumption

Waste volume

Figure 6.1. Hypothesis of the removal of plastics in packaging

Quantitatively, plastics are only a small part of the total amount of polluting waste, far behind agricultural and industrial waste. The largest users of plastics per capita, in Northern Europe, have the smallest amount of plastics littering. This shows that the solution to the waste problem rests with the civic-minded population, opposed (which is, alas, in opposition to the casualness of the Latin population). 6.1.2. Biological appearances (contamination of the atmosphere) Here again, it is better to relativize. In the large cities of industrial countries, most plastics waste ends up in urban garbage, mainly in the form of packaging. This waste is often burnt and, like any combustion, it pollutes the air. However, plastics waste in urban garbage is only 10% of the total weight, and half of it is of PE, which is an excellent fuel for incineration plants. Without the PE waste, the fuel consumption to ignite the waste would be higher. The municipal waste incinerators were formerly considered as a pollutant, but today they have been well perfected. The heat energy of these plastics is four times higher than that of the average of urban garbage. Plastics are often accused of emitting harmful gases. Actually, among all the commodity polymers, only PVC releases hydrochloric gas during combustion and it always stays below dangerous limits. It is far from the pollution generated by the fuel or coal thermal plants. Cars are always the number one pollutant in the cities, way ahead of heating, itself ahead of industrial pollution. Finally, the pollution from smokers largely prevails in all public places. Naturally, as shown below, a large part of plastics waste is now recycled.

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233

6.1.2.1. The false problem of fires The result and the cause are often confused. Any fire results in a lack of the oxygen that enables combustion (formation of CO2 + CO + H2O). A fireman never enters a fire without a mask. Of course, most plastics can burn, but 60% of fires are due to the carelessness of smokers. 6.1.2.2. The false problem of natural materials Up to now, all the materials used have come from nature, either directly or indirectly through synthesis. Plastics are often opposed to paper, adding that paper is biodegradable, whilst plastics are not, which is true. However, plastics are no threat to the great natural balance, because they are inert. They are just victims of their qualities at their end of life, as solid waste. The argument should always rest on the final balance. Plastics cause pollution downstream and paper causes pollution upstream. In order to make paper, which are part of the carbon cycle with plants, trees must be cut down. The carbon cycle is part of human life. The water pollution caused by the pulp production is not negligible (50 to 200 kg are needed to produce 1 ton of paper). Therefore, plastics and plastics conversion are not major pollutants, as they are often described. However, considering their fast growth, we must to do everything in order to better recycle plastics. 6.1.3. Positive appearances The shortcomings of manmade materials are largely compensated by the possibilities they have opened, of which the public is not always aware: – plastics have led to drastic cost cuts, lighter products and much better functionality in many applications, in the automotive industry, appliances, audiovideo, telephones, computers, health, packaging, just to mention the main end-uses; – plastics are the only oil-based materials that are not destroyed at first use, like the gasoline of cars. They can return a large part of their heat energy when burnt for winter heating and electricity production in the summer in large cities. 6.2. Solid waste treatment There are two major routes: – re-using (in production), to save matter and energy. This is the best route to develop, particularly for relatively clean industrial waste;

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– eliminating (for ecology), which is still the easiest, and, as of now, the cheapest solution for polluted urban garbage. When re-using, plastics waste is converted into second choice molded products. New volumes are created. When eliminating, on the other hand, plastics waste is destroyed to get rid of it. If possible, the energy contained in the materials is recovered, and bulky volumes are suppressed. 6.2.1. Regenerating plastics 6.2.1.1. In-house recycling (upstream) This is current practice for thermoplastics converters in injection molding and extrusion, blow molding, calendering. The clean waste and scrap is collected and sorted by polymer types. After grinding, the waste and scrap will be blended with virgin plastics, in varying percentages, 10 to 20%. However, the small converters of semi-products, in thermoforming, welding, finishing, printing and laminating, do not have melting equipment, and cannot recycle their waste and scrap. The same is true for the converters of thermosets. 6.2.1.2. Physical regeneration The main use is with waste coming from: – base polymer producers, for non-standard, spoilt runs; – semi-product converters without melting equipment; – large users, large retailers, for shrink bags, heavy duty sacks, and PE; – end-users of general public packaging, bottles made of PVC, PE, PET. The collected waste is washed and ground. Sometimes, improving agents are added and regranulation is carried out. The resulting pellets are used to make second choice products, excluding food packaging. Types of applications: garbage bags, flower pots made of PE, floor joints, cable trays (PVC), disposable cloth hangers, women’s shoe heels (PS). The limitation of the process is economical. The cost of recovering is often close to the price of virgin polymers for commodity plastics. The return is uncertain. In addition, managing the flows of materials of various ages and uses makes the operation even more complex.

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235

Densification (for films) Grinding

Washing

Spin drying

Cooling Granulating Filtration Putting into bags

Extrusion (in rods)

Figure 6.2. Recycling of plastics solid waste for re-use

6.2.1.3. Chemical regeneration It is also possible to recover the bases of a polymer and to purify it by dissolution, hydrolysis, thermolysis, etc. Applications: car lights made of PMMA, PET bottles. Satisfactory tests have been carried out for multi-layers and EPS and PU foams, without yet having demonstrated their profitability. However, there is the risk of replacing one pollution with another. 6.2.1.4. Re-molding (of mixed waste) This is a variant of physical regeneration, from a blend of various types of polymers. Polyolefins, PE, PP, are statistically dominant and act as binders when the blended and melted material is gravity cast in a mold, an operation similar to foundry. The lowering of mechanical properties is compensated with higher thicknesses. The operation cycle is thus fairly long. Types of applications (thick products): stakes, poles, fences, tables, benches, gutter covers, trays, reels. Developments are being made in Japan, mainly from industrial waste. It is also possible to press the mixed plastic waste under temperature. A spot fusion binds the various plastics into inexpensive sound insulation panels, for instance.

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6.2.2. Energy enrichment 6.2.2.1. Incinerating with energy recovery or thermal recycling Plastics are a high energy source that enriches urban solid waste. About 60% of the original energy can thus be recovered. The resulting heat can be produced directly, in large cities, for city heating in winter, and as electric energy in summer: average PCI (kcal/kg): plastics = 8,800, garbage = 1,800 This solution is the only economical one to rid ourselves of dirty packaging, which is the most common form of the packaging (food, maintenance products, perfumes, cosmetics, pharmaceuticals). 6.2.2.2. Pyrolysis Mixed plastic waste is heated at a high temperature (400 to 800°C) under an inert atmosphere. The residual products depend upon the type of polymers treated and the pyrolysis temperature. The result is generally fuel without sulfur and basic monomers. Raw pyrolysis gas

Plastics waste and scraps

Air Soot

Distillation column

Heating gas

Figure 6.3. Principle of pyrolysis

Pyrolysis gas

Raw gas

Condenser

Fuel bitumen

Fluidized bed reactor

Separator

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The difference between the spent energy, for heating, and the recovered energy, for fuel, is uncertain, as it is linked to the oil price. Given this condition, the process would be ideal to treat polymer waste of all types. 6.2.3. Planned degradation The degradation of plastics is not an ideal solution, but it is a lesser evil compared to visual pollution, particularly to destroy litter, which exists despite legislation. There are 3 types of degradability: – light degradability: this plays on the sensitivity of plastics to UV. Chains are broken up and eventually wither, for instance in the case of agricultural films; – water degradability: the main plastic used is PVA which dissolves in water. It is used for hospital supplies, sanitary towels, doses of foaming bath; – biodegradability: the degradation of plastics is caused by micro-organisms. There are very few biodegradable polymers and they are quite expensive. In spite of the apparent interest of biodegradable plastics, their use is still limited. Biodegradability means complete destruction by live micro-organisms. The start of the process cannot be controlled. Thus, this property seems to be incompatible with packaging as the lifetime of the package must be longer than that of the packaged products, but, packaging would be one of the most interesting applications for biodegradable plastics. However, the consumer no longer feels responsible and there is a risk of anti-ecological reactions. There is also a risk of polluting the aquifers, as opposed to standard plastics that are inert. These types of products are thus reserved for specific uses, for instance packaging of medical products with a use-by date or possibly refuse bags. To summarize, the value of plastics at the end of its life is of three types, with three types of waste, as shown in the following table. Value

Types of plastics waste

Mechanical

Pure (if possible of same grade)

Chemical

Mixed

Energy recovery

Mixed and soiled

Plastics enrichment does really raise any technical problems, only economical problems.

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6.2.4. Conditions for success The collection and treatment of plastics waste are now an ecological must, which will influence their development, but the adequate waste treatment and proper value will be developed only if the following essential conditions are met: – having a waste source of consistent quantity, quality and price; – ensuring a first sorting that requires experienced labor, marking the polymers with their respective symbols, or, even better, including a radioactive marker, which supposes an agreement between producers; – making the collection more efficient, and not creating a new pollution with additional energy consumption for handling and transportation; – using an economical treatment process that can be profitable, for the private investor, and for the city or public body. Subsidies are only short-term palliative solutions. – the resulting products and services must have a satisfactory price/performance ratio and be able to supply a market that can be measured.

Chapter 7

Mold Making

Plastics conversion uses molds and dies, with two functions: – putting the products into their desired shape; – insuring heat transfer. There are specific data and notes for each of the various conversion processes. 7.1. Standard molds 7.1.1. Base components Injection and compression molding tools are the most complex. They are made of two half molds, each with an active part, the cavity that is of the product shape, and which includes cooling channels (for TP) or heating (for TS), and a passive part, the carcass, with a fixed part and a mobile part, on which the punch and the matrix of the cavity will be positioned, and which enables us to be fixed onto the press. There may be one cavity, for large parts or for medium runs, or multiple cavities, for small parts or large runs. The trend is to use standard elements for the carcass. In addition to these basic components, a mold also includes guide pins to facilitate the closure of its 2 halves, a device to eject the finished molded part, sometimes systems to demold with slide jacks and a standard injection or hot sprue nozzle.

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Plastic Forming Processes

a)

b) Figure 7.1. (a) Standard injection molding mold; (b) slide mold (source: SVO)

There also are stage molds, allowing two molds to be installed on the same system of an articulated carcass.

a)

b)

Figure 7.2. (a) Multi-cavity mold: 96 (source: Plastisud); (b) staged mold (source: Männer)

The mold weight ranges from a few kg up to 40 tons. Die outlets are of many shapes, more or less complex: – tools with squeeze rollers, with pressure or with suction, to make rigid tubes; – with 2 components, centered on a central core maintained by fins, for tubular dies. They also feature an internal pressure air intake, and cooling outside air, in the case of extrusion dies for bubble films; – with a counter-blade and a pinch control device at the slot outlet, for flat extrusion dies for films and sheets.

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241

Figure 7.3. Flat die with a control system (source: EDI)

These systems are even more complex in co-extrusion processes. In fact, all these tools also feature gauging devices that can be very sophisticated, like in the cases of rigid tubes or of calendered sheets. 7.1.2. Materials and heat transfer systems The various conversion processes require various materials and heat transfer systems. Conversion processes

Types of tools

Mold materials Steel

Heat transfer systems

Injection

Mold

Compression

Mold

Steel

Hot fluid

Extrusion

Die

Steel

Cooled air or cold water (external)

Calendering

Cylinder

Steel

Hot fluid

Blow molding

Mold

Steel

Chilled water

Rotomolding

Mold

Steel or Al sheet

Chilled water (external)

Thermoforming

Mold

Epoxy or aluminum

Cool air

EPS expansion

Mold

Steel

PUR foaming

Mold

Steel or Al sheet

Al (short runs)

Chilled water

Heating Water steam Exothermic reaction

Table 7.1. Materials used in the making of molds and various thermal exchange systems

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Plastic Forming Processes

7.1.3. Fabrication processes Tools can be made using various processes. Machining with chip removal on treated steel, from a 3 to 5 axis CNC machine, fast speed (MHS or UMHS), from 20,000 to 40,000 rpm (with linear motors), providing a very good surface finish (fine chips, few constraints, short polishing time). Cutting speed = 150 to 350 m/min (steel 54 HRC), precision | 6 μ. Machining by electro-erosion, from copper or graphite electrodes and generating a spark flow through a dielectric liquid (oil petroleum). The result is a very high precision, well adapted to making multi-cavities, with very hard steels. Electroerosion is also used to make shaped dies. In this case, electro-erosion is by wire on a contour instead of an electrode. Cold counter sinking, with a hydraulic press of a shaped punch in a lower hardness cavity, made of “Plastom” type metal (steel for long runs, brass for short runs), which shapes itself gradually until fitting the punch shape. This process is also used to obtain small precise multi-cavities.

Punch

Matrix

Metal displaced by the shape of the cavity

Figure 7.4. Cold counter sinking

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243

Finishing is performed by polishing and chrome plating the surface, to help unmolding and to obtain a perfect polish. In order to obtain particular appearances, such as wood grain or leather, a part of the mold may be grained by chemical etching using a reactive acid. Casting, light alloys or resin, particularly: – aluminum for rotomolding or thermoforming molds, for long runs; – epoxy filled with aluminum powder for better heat distribution, for thermoforming (shorter runs); – alloys of low melting point (§ 380°C), of the “Kayem” type (Al, Zn, Mg) for injection molds for small parts in short runs. Sheet metal work and mechanical welding, for mold making (without pressure) for rotomolding or for EPS block foaming. Erosion by sinking

Wire cutting

Axis x y z

Figure 7.5. Electro-erosion (source: Chamille-Agie)

7.1.4. Calculation of mold costs The cost of a standard mold can be split as follows: – design, study

20 to 30%;

– materials, steel and accessories

8 to 12%;

– machining (labor)

60 to 70% (of which > 50% for the cavity).

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Plastic Forming Processes

The relative share of labor is quite high. Examples of average machining time (injection molding molds): – small mold of simple shape (100 x 100 x 100 mm)

40 to 60 hrs;

– more complex shape

60 to 80 hrs;

– multiple cavities (2 to 6), non-technical

§ 80 hrs;

– separate shapes (1 to 4)

§ 120 hrs;

– medium size mold of simple shape (400 x 400 x 400 mm)

§ 100 hrs;

– unusual shape

§ 200 hrs.

7.2. New mold concepts Mold development takes several appearances: – shorter mold making time; – thermal appearances of molding; – required surface appearance. 7.2.1. Shorter mold making time As a rule, molds are made by chip cutting (milling) or by particle removing (electro-erosion). High speed machining has developed in the last few years and reduced fabrication time. Now, there are new emerging techniques. Stereolithography processes enable us to make polymer prototypes in a short time, and even short runs for not very demanding parts, at the expense of the conversion cycle time. Current improvements are not to make polymer parts, but to make metal components to be obtained directly: – either a short run of small metal parts, a few thousand, if the timing is important; – or to make the tool cavity of a mold to injection mold plastics parts in long runs. This is done from metal powders (granulometry ~ 20 μ), with a computer design, in one tenth of the time it takes for machining. The cavity machining of the mold is about 50% of the mold labor cost.

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245

There are two competing approaches: as for stereo lithography, the starting point is a CAD of the part to be made, in 3D. Sintering is performed by PAD with a laser, or an electron beam, with a metal powder that becomes the cavity of the mold. 7.2.1.1. Sintering Bronze powder (50 μ) or steel powder (20 μ) can be used, spread on layers with a scraper, and then sintered point by point by a CO2 laser. In order to avoid porosity caused by sintering, a SnPb alloy, or high temperature epoxy is infiltrated. Sanding is carried out to eliminate the surface roughness due to sintering. Reflective mirror of the beam

Powder reserve carrier by roller

Sintered part layer by layer by laser beam

CO 2 laser

Distributing roller of powder layers

Receiving thank for the part and the non-sintered powder Receiving tank for the part and non-sintered powder Figure 7.6. Metal powder sintering by CAD guided laser

7.2.1.2. Fusion This is a more versatile process, which makes possible the use of stainless steel or steel alloy powders, or titanium powder, but requires fabrication under a vacuum. The process differs both because of the powder feeding that is performed with a nozzle, and by the heating system, by electron beam. It is possible to use many metallic powders, such as high hardness steels, aluminum, titanium, etc.

246

Plastic Forming Processes

Electron gun system with focus and deviation Vacuum chamber Electron beam Powder tank

Part made of successive layers Residual powder

Moving tray

Figure 7.7. Metal powder melting by CAD guided electron beam (source: Usine Nouvelle)

The interest of these two processes is to optimize the place of the cooling runners, which is difficult, even impossible to do by machining curved shapes, hence a faster molding cycle time. However, the essential advantage is the drastic reduction in time and production cost. The full manufacture of a small complex mold requires three weeks, compared to three months by machining. These processes have already made it possible to produce runs of 60,000 small metal parts, directly without molds, for medical or electrical components. 7.2.1.3. Stratoconception Flat components are machined by cutting into single layers, which are then assembled. This process is more suitable for making large-sized parts.

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247

Finished prototype

CAD Stratification Choice of the plane Choice of the inserts

Choice of plate dimensions Choice of material

Z X

Machine drive

Y

Assembly station 3D machining 3D states setting by inserts Micromilling station

Figure 7.8. Stratoconception description (source: CIRTES)

7.2.2. Thermal appearances of molding In plastics molding, the plastic must be heated to take its shape in the mold cavity. Then the plastic must be cooled, in the case of a thermoplastic, or continue being heated up to its polymerization, in the case of a thermoset. The mold features fluid runners for this function. The cycle time depends upon the efficiency of the heating and of the cooling. Nozzles are produced by machining. The complexity of the part to be molded does not always enable the most rational place, the less so as it is difficult to make curved runners, except at much higher cost. Therefore, in most cases, the mold is heated and cooled, indirectly affecting the plastic to be molded, whilst the action should only be focused on the plastic. This is a loss of time and energy. In order to avoid this, and to increase the operational profitability of the plastics converter, there is another solution. 7.2.2.1. Induction heating In this case, the resin mold has a thin metal cavity (2 to 3 mm), made by electroplating or by spraying. An adapted solenoid is sunk into the mold mass to heat the cavity by induction. This takes just a few seconds.

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Plastic Forming Processes

Electromagnetic field

Plastic 1. Setting the plastic in the cold mold

Electrical current

Heating of the cavity surface

2. Closing the mold and heating the mold cavity surface by induction

Water cooling 3. End of the induction and start of the cooling phase

4. Extracting the cooled part

Finished part Figure 7.9. Induction heating of cavity, cage process (source: Roctool)

7.2.2.2. Obtaining surface appearances There are two methods to impart the required surface appearance to a molded part. 7.2.2.2.1. Making the cavity using electroplating The starting point is a material of which the grain is to be reproduced. The most current application for car dashboards will be for a leather grain. The leather is treated by a copper chemical deposit. This will make it possible to plate, by electrolysis, an easy-to-handle thickness of copper and nickel (Cu + Ni), which will be the cavity, carrying the exactly reproduced appearance.

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249

The average time to obtain a cavity thus treated, of for instance a thickness of 2 to 3 mm, is fairly long, about 6 weeks. Mold covered with leather and chemical copper Leather Support

Cathode Anode Cu or Ni

Thin deposit Cu + Ni

Electrolysis tank

Figure 7.10. Cu-Ni plating by electrodeposition

This type of cavity is used to mold car dashboards, with a leather grain appearance, by the slush molding process described in section 3.3.4 on rotomolding.

Figure 7.11. Car dashboard

7.2.2.2.2. Production of a cavity by metal spraying A ceramic mold must be used in order to spray a vaporized metal in a neutral atmosphere. The ceramic skin is fairly well reproduced. The deposition time is minutes whilst it is days in the case of electroplating. However, the investment cost is higher, and the operation is more complicated, under a neutral atmosphere.

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Figure 7.12. Mist spraying vaporized metal on ceramic (source: CETIM)

Chapter 8

Economic Data

8.1. Costs and prices Manmade materials are often compared to metals on a weight basis, in €/kg, which is wrong. Parts, and not kilos, are made in industry. The important measure is the cost per volume. This measure is more in favor of plastics. Commodity plastics compare well to steel and engineering plastics compare even better to aluminum. Moreover, contrarily to general opinion, manmade materials are not great energy users. When reasoning in volume terms, the energy contents of manmade materials are moderate, particularly compared to non-ferrous metals. Plastics conversion is carried out at 200°C, compared to between 800°C and 1,600°C for aluminum and steel, and 800°C for glass. Plastics do not require heat treatment like quenching, hardening or annealing of metals, at 400 or 1,000°C. Moreover, surface treatments, except for decoration, are not the rule, since plastics cannot be altered and are generally bulk colored. 8.1.1. Polymer prices Base materials are sold by weight, in €/kg. The current price ranges were shown in Chapter 2 in the discussion about value classification.

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Plastic Forming Processes

Price lists can also be found in some plastics trade magazines and on various websites. For international prices, www.plasticsnews.com/subscriber/rprices.phtml offers free-access price lists for a wide range of plastics, as well as more detailed historical information for subscribers; for US marketplace prices, see http://www.ides.com/resinprice; whilst for general chemical prices, see www.icispricing.com, a subscription-based service. PE and PP prices have also been quoted at the London Metal Exchange, LME, since 2005. The following figure summarizes the orders of magnitude of market prices for the 4 largest plastics groups. Prices are different depending on whether plastics are purchased in 30 kg bags or in containers of about 1 ton, or in bulk, in 20 ton trucks. Commondity plastics (large parts)

Medium tonnage

Engineering plastics (small parts)

Performance plastics

Figure 8.1. Polymer prices in 2007

Minimum weight of single orders: – commodity plastics = 20 tons; – engineering plastics = 5 tons. The relative share of the raw material price in the total cost depends upon the type of polymer and the conversion processes. As a rough average, the relative shares of the raw material price in the total conversion cost are as follows. Processes

% of raw plastic price/total cost

Injection molding

45 to 55%

Extrusion

55 to 70%

Blow molding

45 to 75%

Rotomolding

35 to 50%

Calendering

45 to 50%

Thermoforming

40 to 65%

EPS foaming

35 to 40%

PUR foaming

40 to 60%

Table 8.1. Order of magnitude of the share of plastics purchase price in total conversion cost

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253

8.1.2. Conversion costs Conversion costs depend on: – the type of equipment and its investment cost. An hourly rate of machine operation is defined, which includes depreciation, utilities, (energy, oil, compressed air) and all other costs that can be directly counted. As an indication, injection molding requires about 1 kWh/converted kg, for energy. The price of the machine is the largest investment for all mechanized processes; – the number of operators required, their hourly rate, including supervision and all related social benefits; – the production speed, which also depends upon thickness, roughly proportional to the square of the thickness, and of the level of automation of the process; – the possible need for finishing, decoration, printing. 0.15

0.3

0.45

0.6

0.75

0.9

1.5 Price

Injection presses < 400 t 400 to 1,000 t > 1,000 t Extruders

Calenders

d < 90 d = 90 d > 120 to 120 Extruder lines Small Average Large for sheets and profiles for large tubes and plates Blowing machines Small

Average

Packaging lines Thermoforming machines

0.5 m 2

0.5 to 0.2 m 2 Figure 8.2. Average prices of machines

In general, in order to ease calculations, a machine hourly cost is determined, which is in line with the average values, in €/hr, shown on the following curve, of the investment for the type of process.

Plastic Forming Processes

Maximum equipment

Machine cost (Euro/hr)

254

Minimum equipment

, Investments (thousands of Euros)

Figure 8.3. Average values of hourly costs per machine in 2007

These values also depend upon the peripheral equipment, which explains the difference between the two curves, maximum and minimum investment taken into account. In the case of injection molding, for a single press, the hourly rate is about 0.2 €/hr and per ton of clamping force of the press. The depreciation of the mold and the tools must be added to these values. This is defined by dividing the total cost by the number of molded parts. To this, the cost for the time for assembling and dismantling is also to be counted when working by batches. The cost naturally depends upon the number of cavities for molded parts, and the more or less complex tools. For instance, prices as of 2005: – screw cap (24 cavities): 70,000 €; – trash can diameter ø 400 mm, height 400 mm (1 cavity): 110,000 €; – car bumper (1 cavity): 300,000 €; – mold for blow molding, from 2,000 € for a 1-liter bottle to 50,000 € and more for fuel tanks for cars. – die for profile, 2,000 to 3,000 €; – circle and flat dies, from 50,000 to 100,000 €. Molds for injection molding are the most expensive.

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255

The tools made of resin (for thermoforming) or the tools made of welded fabricated metal (for rotomolding or foaming) cost about one tenth of the tools used in injection molding, for a similar volume. 8.1.3. Productivity Here are the orders of magnitude of the annual production per worker, in tons, for the various plastics conversion processes. Injection molding Long runs Industrial parts Very diversified (and small converters) Blowing extrusion Extrusion Flat die Large profiles

Film extrusion Shaped die

Small profiles

Films and bags

Annular die

Diversified production Thermoforming

Blow molding

Integrated with filling Packaging Industrial parts Small diversified converters

Rotomolding: 10 to 20 Calendering: 80 to 100 Foaming: PUR

PSE Blocks for insulation Packaging

Continuous foaming Reactive injection molding

Figure 8.4. Plastics process productivity

No machine will ever operate at 100%. There are production stops, to change the tools, for maintenance, breakdowns, etc. As a rule, an average standard is about 80% of the operating coefficient, between 60 to 85% depending upon the plant management. This operating rate is included in the hourly rate of the machine, for the final calculation of the overall cost.

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8.1.4. Cost and sales price A realistic approach of the overall cost includes the following items: – materials: (weight of the part + scrap percentage) u raw material price; – conversion: machine hourly cost u speed (parts/hr/min); – or/and (if needed for finishing) hourly labor cost u number of hours; – technical depreciation of tools: - cost/number of parts in the run = direct cost, - + fixed costs = overall cost. 8.1.4.1. Practical examples Computer housing, total area 30 u 30 cm = 2,500 cm2, overall thickness = 2 mm, made by: – injection molding of PP at 1 €/kg; – thermoforming of ABS, a quite suitable plastics, at 1.5 €/kg. It is necessary to start from a plate of 2.6 mm to compensate for creep. Injection molding Materials

Thermoforming

3

Volume: 500 cm

Volume: 650 cm3

Density PP: 0.9

Density ABS: 1.1

Theoretical weight: 0.45 kg

Theoretical weight: 0.65 kg

Scrap § 2%

Scrap § 50%

Actual weight = 0.46 kg

Actual weight = 0.98 kg

Plastic cost = 0.46 € Converting

Plastic cost = 1.47 €

Press: 2.5 dm u 300 kg/cm ĺ press force = 600 tons

Selling scrap compensates the cutting cost

Speed: 80 parts/hr

Operating unit: 30 €/h Conversion cost: 30/25 = 1.25 €

2

2

Operating unit: 60 €/hr Conversion cost: 60/80 = 0.75 € Tools

Speed: 25 parts/hr

Steel mold: 120,000 €

Plastic mold: 15,000 €

Run: 500,000 parts

Run: 30,000 parts

Mold depreciation = 0.24 €

Mold depreciation = 0.50 €

Operating cost

1.45 €

3.20 €

Overall cost: add § 20% fixed costs

1.74 €

3.84 €

Sales price (if the part is bought to a converter): plus 5 to 10% margin excluding tax.

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257

8.2. Structure of the plastics industry The plastics industry chain includes: – polymer producers, producing the plastics raw materials. They are often part of large oil groups for commodity plastics and large chemical groups for engineering plastics, or both; – compounders who make master batches and blends adapted to specific requirements; – machine manufacturers who make the equipment used by the various plastics conversion processes; – mold and tool makers who supply the required tools for the various conversion processes; – converters who operate the various plastics conversion processes. Their relative share in percent of tonnage is estimated in Table 8.2; – recycling companies that handle and regenerate the waste and scrap of the operators above. Processes

% share of total

Comments

Extrusion

40%

Of which film = 22% and tubes = 8%

Injection molding

24%

Of which commodity = 20% and engineering plastics = 2%

Blow molding

16%

Of which packaging = 12% and industry and consumer goods = 4%

Foaming

10%

Of which PUR = 6%, EPS = 3%

Calendering and coating

4%

Thermoforming*

5%

Other

1%

Of which packaging = 3% and industry = 2%

* Thermoforming uses extruded or calendered sheet that are semi-products.

Table 8.2. Percent tonnage share of plastics conversion by type of process in Europe (excluding composites and long fibers)

8.3. Markets The main end-uses of plastics in Europe are estimated as follows: – packaging

35% (mainly food packaging);

– building construction, public works 20%;

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Plastic Forming Processes

– transportation

10% (mainly automobile);

– electricity, electronics

10% (including appliances and audiovisual);

– consumer goods

10%;

– agriculture

5%;

– all other (unidentified)

10%.

All other (unidentified) 10% Agriculture 5% Packaging 35% Consumer goods 10%

Electricity, electronics 10%

Transportation 10%

Building construction, public works 20%

Figure 8.5. Percent of total plastics tonnage by end-using industries

Chapter 9

Trends

9.1. Polymers Thermosets that were at the origin of the history of plastics have become marginal compared to thermoplastics. However, thermosets are still important in the composite industries.

Years

Thermosets

Thermoplastics

1950

80%

20%

2005

10%

90%

As for all materials, plastics trends show a life curve with the 4 standard phases with varying growth rates Most polymers, except those in solutions, are converted into concrete shapes, with the standard conversion processes (injection molding, extrusion, blow molding, thermoforming, etc.). They can be called structural plastics.

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Plastic Forming Processes

Figure 9.1. Polymer life curve

In the future, the mechanical, thermal and economical characteristics are bound to improve, with: – larger inroads of medium tonnage engineering plastics, for technical parts; – development of high heat resistant plastics; – use of new alloys and blends to better adapt product to function, custom designed plastics (see Figure 9.2); – growing use of thermoplastic elastomers that fill the gap between rubbers and plastics: - olefinics = TPO, - styrenics = TPS, - co-polyamides = PE-BA, - co-polyesters = TPE, - urethanes = TPU; – growth of multilayer packaging, allowing high barrier and control of permeability.

Trends Polymers

Barrier

EVOH

Gases

PVdC

Water vapor

PAN

Solvents

261

Crystalline polymers

Amorphous polymers

Figure 9.2. Plastics alloys

Functional plastics are different from structural plastics in the sense that, with a small volume, they provide a specific answer to a required function. Functional plastics are now emerging, and their applications are only beginning, but they are very promising for this century. Functional plastics development are found in the following table.

262

Plastic Forming Processes Functions

Applications

Specific Polymers

Porosity

Filtration, felt pens, prostheses, implants

Polyolefins, fluorinated plastics polyaniline

Barrier

Canning, beverages cosmetics, pharmaceuticals

EVOH, PVdC, PAN

Retention

Hygiene, insecticides

Polyacrylate, PVdC-carbonate

Agriculture

Polyolefins

Degradability: – Light degradable

– Water degradable Hygiene

Polyvinyl alcohol

– Bio degradable

Biomedical

Polycaprolactone, polyhydroxybutyrate

Bio-compatibility

Prostheses, implants

Polyglactine, PLA, polydiaxanone

Light sensitivity

Optical fibers, display

PMMA

Color change

Benzopyrane

Packaging

Polyolefins

Electrical insulation

Irradiated PE

Magnetization

Magnets

Plasto-ferrites

Piezoelectricity

Sensors, switches

Fluorinated plastics

Conductivity

Batteries, semiconductors

Polypyrrole

EMI protection

Polythyophene

Shape memory

Table 9.1. New developments of functional polymers

9.2. Conversion processes In general, there are sustained efforts in the following fields. Computerization of all functions. Rapid prototyping by stereolithography (see Figure 9.3).

Trends

263

Control

Work spec.

Numerical control

Materials Shapes

CAD

Product design General control

NC

Product

Machine

Invoice

Tools CAD PC

Programmable logic controller

General accounting

Cost accounting

Figure 9.3. General control

Fast production of mold cavities by high speed machinability, and starting development of sintering of metal powders, substituting for chip removal (see the new mold concepts). Research into material and energy economy: – injection molding (hot runners, press programming, co-injection); – extrusion (control of thicknesses and widths, co-extrusion); – blow molding (parison programming, specific stretch); – linking the process to heat pumps; – organized recycling (strong growth of waste treatment). Specific developments of new techniques (new functions or new products). Improvement of equipment productivity and reliability: – high speed injection molding; – fast machines in extrusion and calendering, with large diameters and large width, inside cooling of plastics film sheaths; – continuous systems in blow molding and thermoforming, for filling, measuring control, decoration, etc., on the machines; – reactive molding, self-cleaning formulations.

264

Plastic Forming Processes

Automatic functions: – material continuous feeding, with containers, silos (see Figure 9.4); – process control (programmed controls); – fast mold and tool changing (SMED method; see Figure 9.5); – magnetic fixing of molds (see Figure 9.6); – efficiency of logistics (manipulators, robots, conveyor belts).

Container

Press

Silo

Figure 9.4. Automatic feeding

Pre-heating Mold rack

Press

Figure 9.5. Mold changing equipment

Trends

Figure 9.6. Magnet fixing of molds

265

266

Plastic Forming Processes

Vertical displacement

Release of the plastic part

Unmolding

Figure 9.7. Robotic manipulator (source: SEPRO)

Increasing size of finished products or miniaturization (electronic components, biomedical, etc.). Production management in real time (CAD).

Trends

Design

267

Silo of polymer

Mold making Table x-y CAD NC machine

Mold changing equipment

Coloration

Programmable control Press no.1 Part making

Molded parts

Manipulating robot Computer for production control

Conveyor belt

Accounting Printer

Figure 9.8. Injection molding fabrication steps

Plastics conversion is still a young industry, with 50 years of accumulated industrial experience, compared to over a century and a half for metal industries. Plastics evolution and obsolescence are rapid. In the future, the conversion processes without fusion, such as stamping of semiproducts, or by reaction in the mold, such as RIM, might compete with more standard melting processes.

268

Plastic Forming Processes

Base materials

Conversion process

Types of processes

TP pellets

Melting

Injection

TP plates

Without melting

Stamping

Reactive in the mold

RIM

Solid polymers

Liquid monomers

Figure 9.9. Possible evolution of plastics conversion processes

In the long run, the development of nanotechnologies and their influence on plastics will bring a new mutation of the techniques. There might be transistors mode of plastics. Nanotechnologies will enable us to build up new molecules, atom by atom, and to displace them one by one for assembly. In a relatively near future, nanotechnologies might lead to the production of nanostructures (10-9 mm) of ultra resistant, even self-repairing composites. The world of plastics is far from over.

Appendix

Symbols Used

ABS APV CA CAB CFE EP EPF EPDM EVA EVOH IO MDI MF MP PA PAA PAEK PAES PAI PAN PAR PAS PBT PC PCL-LCP

acrylonitrile-butadiene-styrene polyvinyl alcohol cellulose acetate cellulose acetobutyrate chlorofluoroethylene epoxy fluorinated ethylene-propylene ethylene propylene diene monomer ethylene vinyl acetate ethylene-alcohol polyvinyl copolymer ionomer diphenylmethane isocyanate melamine formol (aminoplastic) melamine phenol polyamide polyarylamide polyarylketone polyarylethersulfone polyamide-imide polyacrylonitrile polyarylate polyarylsulfone polybutadiene terephthalate polycarbonate liquid crystal polymer

270

Plastic Forming Processes

PE-HD PE-LD PE-LLD PEEK PE EPS PEI PEK PEN PES PET PF PI PMMA POM PP PPA PPO or PPE PPS PS PS HI PSU PTFE PU or PUR PVAc PVC PVdC SAN SI TDI TP TPE TPR TS UF UP

high density polyethylene low density polyethylene low density linear polyethylene polyether-ether-ketone expandable polystyrene polyether-imide polyether-ketone polyethylene naphthalate polyether sulfone polyethylene terephthalate phenol-formol or phenol-formaldehyde (or phenolics) polyimide polymethyl methacrylate polyacetal (or polyoxymethylene, polyformaldehyde) polypropylene polyphthalamide polyphenylene oxide polyphenylene sulfide polystyrene (including PS GP, general purpose) impact polystyrene polysulfone polytetrafluoroethylene polyurethane polyvinyl acetate polyvinyl chloride polyvinylidene chloride styrene acrylonitrile silicone toluene di-isocyanate thermoplastic thermoplastic elastomers (TPE, TPO, TPS, TPU) reinforced thermoplastics thermoset urea-formol (amino plastics) unsaturated polyester (polyenester)

Index

A, B Adhesive bonding 13, 16, 19, 21, 177, 213, 229 Amorphous structures 8, 9, 144, 186, 192 Assembly with flexible hinge 197 Blow molding 14, 17, 19, 45, 51, 117, 123, 128, 133, 151, 241, 252, 259 C Calendering 84, 87, 110, 116, 143, 179, 224, 241, 252 Classification 6 Compression 39, 59, 72, 155, 239, 241 Costs 6, 24, 31, 42, 49, 52, 144, 167, 192, 212, 243 Crystalline structures 8, 9, 144, 192

Fluidized bed 201 Foamed products 50, 159, 170 H, I Hollow products 49, 50, 117, 154, 215, 216, 221 Hot plates 62 Injection molding 5, 13, 14, 20, 24, 186, 214, 215, 243, 252 Injection-blow molding 128, 186 Insert 29, 43, 45, 60, 133, 197, 198 L, M Long products 70 Markets 8, 257 Molded parts 4, 24, 124, 155, 160, 164, 165, 170, 182, 186, 197, 198, 206, 208, 214, 221, 224, 225, 248 P, R, S

D, E, F Dip molding 141 Electroplating 15, 20, 199, 202, 205, 210, 247 Electrostatic powder coating 202, 203 Extrusion 5, 13, 70, 113, 117, 129, 143, 163, 173, 240, 252, 259

Pollution 39, 45, 199, 212, 213, 231 Ratchet assembly 195 Reaction injection molding (RIM) 64 Riveting 44, 177, 188, 193 Rotomolding 132, 241, 243 Screwing 129, 177, 197 Sputtering 205, 209, 212

272

Plastic Forming Processes

T

V, W

Thermoforming 90, 114, 143, 165, 174, 189, 227, 241, 252, 259 Thermoplastics 5, 24, 25, 71, 74, 77, 156, 174, 177, 185, 189, 204 Thermosets 5, 25, 59, 60, 109, 174, 178, 192, 203, 205, 247 Torch gun spray 200

Vacuum metallizing 205 Virgin polymers 10, 56 Welding 13, 14, 18, 20, 21, 47, 114, 129, 136, 177, 178, 243