Selecting Injection Molds

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SelectingInjection Molds

Herbert Rees

Bruce Catoen

Weighing Cost vs Productivity

Hanser Publishers, Munich Hanser Gardner Publications, Cincinnati


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The Authors:

Herbert Rees, 248386-5 Side Road (Mono), RR#5, Orangeville, Ontario, Kanada L9W 2Z2Bruce Catoen , 21 Hamilton Crescent, Ontario, Kanada L7G 5J4

Distributed in the USA and in Canada byHanser Gardner Publications, Inc.6915 Valley Avenue, Cincinnati, Ohio 45244-3029, USAFax: (513) 527-8801Phone: (513) 527-8977 or 1-800-950-8977www.hansergardner.com

Distributed in all other countries byCarl Hanser VerlagPostfach 86 04 20, 81631 Mnchen, GermanyFax: +49 (89) 98 48 09www.hanser.de

The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a

sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone.While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editorsnor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express orimplied, with respect to the material contained herein.

Library of Congress Cataloging-in-Publication Data

Rees, Herbert, 1915- Selecting injection molds : weighing cost versus productivity / HerbertRees, Bruce Catoen. p. cm. ISBN-10: 1-56990-389-1 (hardcover) ISBN-13: 978-1-56990-389-61. Injection molding of plastics. 2. Injection molding ofplasticsEquipment and supplies. I. Catoen, Bruce. II. Title. TP1150.R446 2005 668.412dc22 2005023027

Bibliografische Information Der Deutschen Bibliothek:

Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie;detaillierte bibliografische Daten sind im Internet ber abrufbar.

ISBN-10: 3-446-40308-6ISBN-13: 978-3-446-40308-6

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopying or by any information storage and retrieval system, without permission in wirting from the publisher.

Carl Hanser Verlag, Munich 2006Production Management: Oswald ImmelTypeset by Manuela Treindl, Laaber, GermanyCoverconcept: Marc Mller-Bremer, Rebranding, Mnchen, GermanyCoverdesign: MCP Susanne Kraus GbR, Holzkirchen, GermanyPrinted and bound by Appl, Germany


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Preface

When I retired in the early 1980s, from my position as VP of R&D and Engineering at Husky Injection Molding Systems,Ltd., I had been in the plastics field since almost from the beginning of the technology, from Compression Molding ofThermosets, and then worked through the gradual shift to Injection Molding, after the Second World War. In 1985, I wasasked by a Canadian non-governmental organization that supplies technical assistance to industries in developing countriesto join them. At their request, I traveled to countries in East Asia, North Africa, South and Central America, and workedwith a number molders and mold designers of small and medium sized operations in the plastics industry. The time spentthere was very rewarding, and I was able to help them to improve their designs, methods and, ultimately, their productivity.

These experiences abroad, but also many previous events throughout my career pointed out a general need for easilyunderstood technical (theoretical and practical) education. As a result, I started putting my thoughts and experience firstinto a book Understanding injection Molding technology (1988) and followed it up by other books on Injection MoldDesign and Engineering, as well as on Product Design for Injection Molding. But still missing was an easily understoodbook about the relationship between Productivity, Production and Mold Costs.

I was fortunate that my friend Bruce Catoen, who joined Husky in 1987 as development engineer and who is, at this time,VP of Packaging and Systems at Husky accepted my invitation to co-author the book I had in mind. The purpose was,partly, that Bruce should review what I had written so far, but mainly, to update it where necessary and add the latest

developments, where they are germane to the subject of this book.

Injection molds are always expensive to make, but unfortunately, without a mold there cannot be a molded product. Everymold designer will have his or her own approach to the design of a new mold, and there are many different ways a moldcan be designed and built.

A frequently asked question is then how to get the lowest cost mold. But this is the wrong question. The question to askmust always be:

How can I get the best molded product at the lowest cost, for the expected production?

Whenever talking with molders, mold makers and mold designers I have been asked many times how to decide whichfeatures a mold should have. (Number of cavities, methods of injection, type of runners, methods of gating, methods ofejection, machine selection, etc). I have also been frequently asked how one can reasonably estimate the mold cost.

As will be shown, mold cost, mold quality and cost of product are inseparable. The often-quoted saying: The devil is in thedetailsapplies clearly to molds, and the effect of many such details are illustrated and discussed. Productivity and Cost ofInjection Molds is not a design manual, although there are a number of suggestions for the mold and product designershow to select certain design features to build the most suitable mold for the job. The authors highlight some of the criticaldecision areas for the construction and the operating details for the most economical mold for the job on hand in an easilyunderstood language, with a minimum of theory or complicated formulae.

The book tries to explain to the decision makers, i.e. the persons given the responsibility of deciding what kind of moldto design and build, (or to purchase, if the mold is to be built elsewhere,) how they should examine the product design andits specifications, and to highlight the significance of some of the features of the product design on the expected productivity.Such examinations often result in suggestions for practical product design changes that will make it easier to build the

best-suited mold at the lowest cost. I have used some examples of molds I have been involved with, and tried to show howeven little details can significantly affect the mold cost, the cost of the product, and the productivity of the mold. For theactual mold engineering process I have referred occasionally to my earlier books Mold Engineering (ME) andUnderstanding Product Design for Injection Molding (UPDIM.)


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An event (from the early 1960s) will illustrate the importance of getting the right mold for the purpose. A friend, startingup as a custom molder with a few small machines, came one day, and told me of a prospective customer, requiring 100,000each of three very similar, simple, round containers, who had approached him with the request to quote 3 single cavitymolds, to be used on his 100 ton machines. We quoted these molds at $3,000 each, (runnerless, fully automatic,) andestimated a cycle time of 1012 s. Based on these figures, the molder submitted a quotation to his customer who liked theprice of the products, but objected to the high price of the molds. He said he could get these molds for $1,000 each. Themolder, glad to get an order for his machines, accepted that the customer would supply the molds. When they weredelivered, they were of very poor quality, with a sprue to be cut, with only token cooling, and the mold ran no better thanat a 60 s cycle, or 60 pieces per hour, also, it needed an operator to cut the sprue and to scrape flash where the stripper

joined the core. The molder had based his pricing on a 12 s cycle, or 300 pieces per hour. At a machine hour cost of $25.00,this would be $0.08 per piece machine hour time. However, the machine hour cost with the supplied molds would be

$0.42 per piece. The mold cost per piece based on our proposal was $0.03, for the 100,000 pieces, and with the cheapermolds only $0.01, so there was little difference ($0.02) in mold costs per piece, but a huge difference in the machine hourcosts. In order not to lose his shirt on this deal, the molder thereupon asked us to supply the molds, and paid for them outof his own pocket. He would have lost 0.42 0.08 = $0.34 per piece shipped, and his total loss would have been in the orderof $100,000.00 for machine time and the unforeseen labor!

The example above shows that a mold is not just a mold! When ordering a mold it must be clearly specified what isexpected from it. The cheap molds would have been all right for very small requirements, but were very expensive for theexpected production.

Bruce and I would like to thank all those companies that contributed illustrations and photos to the book. We would alsolike to give special thanks to Elaine Lafontaine for her administrative assistance during the writing of this book.

H. Rees

Preface


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Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

1 Introduction and Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Oversimplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Is Injection Molding the Right Choice for this Product? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 The Injection Molding Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4.1 The Right Machine for the Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.5 The Injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.5.1 What Is an Injection Mold? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5.2 Elements of an Injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 The Plastic Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1 The Product Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2 Product Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.1 Product Shape: How Can the Product Best Be Molded? . . . . . . . . . . . . . . . . . . . . . . 132.2.2 Parting Line (P/L) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.3 Side Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.3 Accuracy and Tolerances Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.1 General and Specific Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.2 Are Special Fits with Matching Products Required? . . . . . . . . . . . . . . . . . . . . . . . . . 212.3.3 Tolerances for the Filling Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.3.4 Stacking of Products and Free Dispensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.3.5 Mismatch (Deliberate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.4 Tolerances, Mold Alignment, and Mold Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5 Heat Expansion, Alignment, and Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.6 Surface Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.6.1 Finish of Molding Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.6.2 Texturing of Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.6.3 Fitting Surfaces of Mold Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.7 Engravings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.7.1 Engravings Versus Applied Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.7.2 Two-Color and Two-Material Engraving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.7.3 Depth of Engravings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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2.7.4 Font Style and Size of Artwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.7.5 Polarity of Engraving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.7.6 Are the Locations Selected for Engraving Practical? . . . . . . . . . . . . . . . . . . . . . . . . . 342.7.7 Engravings in the Walls and Bottoms of Products . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.8 General Appearance of the Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.8.1 Flatness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.8.2 Sinks and Voids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.8.3 Witness Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.8.4 Weld Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.8.5 Surface Defects (Flow Marks, Splay, Record Grooves, Haze, Jetting, Hooks,and Ripples) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.8.6 Identification of the Molded Piece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.9 Product Strength Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.9.1 Gate Location to Increase Product Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.10 Special Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.10.1 Holes and Counter Bores for Assembly Screws or Rivets . . . . . . . . . . . . . . . . . . . . . 472.10.2 Hinges and Snaps for Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3 Cost Factors Affecting Productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.1 Where Will the Mold Be Operated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.1.1 Condition of Ambient (Shop) Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 Coolant Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? . . . . . . . . . . . . . . . . . . 513.2.2 Is the Cooling Water Clean? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.3 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.4 Will the Mold Run in a Variety of Machines or a Single Machine? . . . . . . . . . . . . . . . . . . . 533.5 Is the Mold Planned to Run in a Newly Created Operation? . . . . . . . . . . . . . . . . . . . . . . . . 543.6 Projected Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.6.1 Making Prototype or Experimental Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.6.2 Production Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573.7 Forecasting the Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.7.1 Type of Plastic Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.7.2 Wall Thickness of Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.7.3 Mold Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.7.4 Efficiency of Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.7.5 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.7.6 Effect of Molding Machine on Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.7.7 Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.7.8 Ambient Temperatures and Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
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3.7.9 Comparing Molding Cycles of the Same Product in New Molds . . . . . . . . . . . . . . 943.8 Number of Cavities Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.8.1 Available Operating Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953.8.2 The Minimum Number of Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 963.8.3 Machine Hour Cost per Unit Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 973.8.4 Mold Cost per Unit Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993.8.5 Calculation of the Required Clamp Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003.8.6 Shot Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1023.8.7 Plasticizing Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

3.8.8 Preferred (Practical) Number of Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1023.8.9 Business Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043.8.10 Preliminary Estimate of Product Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

4 Mold Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.1 Selection of an Appropriate Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.1.1 Dedicated Mold, Universal Mold Shoe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.1.2 One-Product Molds or Family Molds? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1144.1.3 Where to Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1164.1.4 Gate Size and Runner Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1234.1.5 Hot Runner Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1254.1.6 Single Cavity Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.1.7 Two and More Cavities, Cold or Hot Runner Molds . . . . . . . . . . . . . . . . . . . . . . . 1354.1.8 Single- or Multi-Level Molds? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424.1.9 Semi or Fully Automatic Operation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1544.1.10 Insert Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

4.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5 Mold Cost, Mold Price and Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595.1 Mold Cost and Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

5.1.1 Spare Parts for the Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1605.1.2 The Basic Elements of the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615.1.3 Cost of the Preparation of a Mold Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

5.2 Overhead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1705.3 Mold Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

5.3.1 Risk Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1735.4 Mold Cost Is Absorbed by the Molder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

5.5 Arriving at Mold Cost and Delivery Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1755.5.1 Calculating the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1765.5.2 Estimating the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
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5.5.3 Guesstimating the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1805.5.4 Ball Parking the Mold Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

5.5.5 Mold Price Catalogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1805.6 The Quotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

5.6.1 Delivery Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815.6.2 Confirmation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

5.7 In-House Mold Making Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

6 Warranties, Patents, and Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

6.1 Warranties and Guaranties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1856.1.1 Guaranteed Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1856.1.2 Guaranteed Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1856.1.3 Guaranteed Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1866.1.4 Guaranteed Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

6.2 Patents and Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1866.2.1 Patents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

6.2.2 Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Appendix 1: Suggested Contents of a Mold Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Appendix 2: Mold Set-up Guide Blank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194Appendix 3: Example of Light-Weighting a Product and Increasing Productivity . . . . . . . . 197Appendix 4: Buying a Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199Appendix 5: Suggested Format of a Confirmation of Order . . . . . . . . . . . . . . . . . . . . . . . . . . 204Appendix 6: Molding Properties of Injection-Grade Plastics . . . . . . . . . . . . . . . . . . . . . . . . . 208Appendix 7: General Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210Appendix 8: Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Appendix 9: Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216Appendix 10: Typical Mold Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220Appendix 11: What Characterizes a Good, High-production Mold? . . . . . . . . . . . . . . . . . . . . 222Appendix 12: Advice for the Mold Designer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225Appendix 13: Surface Finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
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191

References

[1] Rees, Herbert, Understanding Product Design for Injection Molding(1996) Carl Hanser Publishers, Munich, Section 4.4.3.2.5

[2] Malloy, Robert A., Plastic Part Design for Injection Molding, 1994, CarlHanser Publishers, Munich

[3] Tres, Paul A., Designing Plastic Parts for Assembly, 5thed., 2003, CarlHanser Publishers, Munich

[4] Belofsky, Harold, Plastics: Product Design and Process Engineering, 1995,Carl Hanser Publishers, Munich

[5] Rees, Herbert, Mold Engineering, 2nded. Chapter 8, 2002, Carl HanserPublishers, Munich

[6] Rees, Herbert, Mold Engineering, 2nded. Chapter 14, 2002, Carl HanserPublishers, Munich


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1

1 Introduction and Planning

1.1 Introduction

Today, injection molding is probably the most important method of pro-cessing plastics in the production of consumer and industrial goods, and isperformed everywhere in the world.

Once the decision has been made to use injection moldingfor a new product,a number of difficult choices are ahead which will be addressed later in moredetail:

Number of cavities

Stack design, which is the purely technical aspect of how to mold the pro-duct. It is important to understand the design of that portion of the moldthat is actually in contact with the plastic (the stack), i.e., the cavity, core,and any other mold components, which determine how the final product

will be shaped and how the plastic will enter the cavity space Method of ejection, i.e., how the product will be ejected from the mold

What machine should be used?

Automation will it be required?

With new, possibly difficult shapes, these decisions are usually left to theingenuity of a mold designer. More frequently,precedentsfrom earlier molds

are used and reapplied. However, the mold designer must be aware of andevaluate new ideas, new methods, and developments, which when appliedwould lead to better quality, higher productivity, simpler molds, and savingsin the cost of the molded products.

After the design of the basic stackand beforeproceeding with any mold design,the mold designer must understand what kind of mold should be selected;in other words, which features will be most suitable for the application toachieve the most economic overallmanufacturing method for the product.This means not just to specify the number of cavities that will be requiredfor the expected output, but also the selection of mold materials and the degreeof sophisticationof the mold. Any planned automation, especially in producthandling after molding,can affect the mold layout, particularly spacing andorientation of the stacks. The mold designer must never lose sight of theultimate goal: The cost of theproductmust be the lowest possible, while stillachieving allspecified requirements. The most important information is toknow beforehand the quantities to be molded, a piece of information,

particularly with newproducts, often very difficult to obtain.

It should also be pointed out that of the total cost of almost all plastic pro-ducts, the cost of the plastic material alone constitutes the greatest component.

Mold designers must not be stuck ina comfortable rut

The ultimate goal of a mold is toproduce an acceptable qualityproduct at the lowest possible cost

Often, the most important informa-tion is the most difficult to obtain


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2 1 Introduction and Planning

The most sophisticated, best designed mold will not lower the cost of theproduct by as much as the reduction of just a few percent of the amount ofplastic material, if it could be removed from the product without affecting

its quality or serviceability. Most often, unnecessarily heavy wall thicknessand ribbing affects the cost more than anything else. Chances are that thelowest weight will be achieved with the highest quality molds.

In my long experience, I have had numerous occasions when the client insistedon having his way. When I strongly believed it was the wrong thing to do, Isuggested to them to have this mold built somewhere else. Almost all cameback sooner or later for other business, and acknowledged that they shouldhave listened to me.

The foremost intent of this book is to present various alternatives availableto the mold designer or decision maker when planning a mold for a newproduct or planning to increase the productivity for a product for which amold exists. It raises many of the questions that must be asked by anybodywho needs a mold built. Some of these questions may appear obvious andnot worth mentioning, or their pursuit may be thought a waste of time, butI like to point out that any input could significantly affect the productivity as

well as the cost of a mold. For an experienced mold designer, the answers formany of these questions often come automatically, without him or her beingaware of the fact that a decision has been made. But even the most experiencedmold designer can gain important information by systematically investigatingall areas that can affect the design and the complexity of the mold and eventhe most experienced designers overlook some obvious facts.

In this book an attempt has been made to explain why certain mold featuresshould be selected, considering the planned productivity and expected costs.

There will also be occasionally references to other books on this subject, suchas Mold Engineering [5] and Understanding Product Design for InjectionMolding [1].

Since in many mold shops the mold designer is also involved in estimatingthe cost of the mold to be quoted, the book also intends to discuss variousways of how to estimate mold costs. Properly estimating mold cost is probablythe most difficult part of running a successful mold making operation.Regardless of how well a shop is equipped with machine tools and other

mold making equipment, and how high the level of experience is of themachinists and mold technicians (mechanics), if the mold cost is notadequately quoted it will be impossible to stay in business. We must neverforget that the primary purpose of any business is to make money, and thereis nothing easier than to lose money by poorly estimating and quoting. Thereis no magic formula to estimate a mold cost, but good understanding of theprinciples will lead to better cost estimates.

We must not forget in dealing withthe customers who require a newmold that it is not what they wantbut what they need

Figure 1.1 Typical mold-making factoryusing automated equipment


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3

1.2 Oversimplification

In the early 1950s, when I was an R&D engineer at a large electrical manu-facturer, I had just submitted a request of appropriation for a mold for a newproduct when the vice-president of sales stormed into my office, and said:Why do you always need so much money for a mold? What is a mold? Isntit just an upper and a lower half?

This was in the heydays of compressionmolding, beforethe injection moldingtechnology gained importance dramatically. A compression mold was exactlywhat the VP implied: a lower half with one or more cavities, and an upper

half with the matching cores (see Fig. 1.2). The plastic was hand-fedinto thelower (open) cavity; there was no or little sophistication with heating (thesemolds were processing thermosets and therefore needed to be heated, notcooled). Often, there was no ejection mechanism at all, or it was relativelysimple.

Of course, what the VP failed to understand was the complexity and accuracyof the work required to build the various components of these halves, thestrength required to resist the high molding forces, the time required for

machining and good polishing, and other features required for even a simplemold. Unfortunately, even today, many years later, this attitude of oversimpli-fication is frequently encountered when discussing a required mold and itscost.

Since that time, thermosets (compression) molding has become quitesophisticated, and is using injection molding technology occasionally, but isstill mostly using the verticalmachine arrangement, because of the originalloading method of the plastic material by gravity. This was also the time

when injection molding took over the molding market from small beginnings.But for a number of reasons it soon became more convenient to use horizontalmachines, although today again, some vertical injection molding machinesare used for certain applications. But regardless of the type machine used,the most important part of the molding system is still the mold.

1.2.1 DefinitionsBefore continuing, here is a list the various terms used:

Product: an injection molded plastic piece

End product: an assembly, of which the product is a part

User (end user): persons using the product or end product

Customer: the person or company interested in buying the injection mold

Mold maker: the person or company engaged in making injection molds

Mold designer: the person responsible for designing the mold

1.2 Oversimplification

a

b

c

d

e

fg

h

a Upper platen (stationary or moving)b Heating platenc Upper mold halfd Coree Cavityf Lower mold halfg Heating platen

h Lower platen (stationary or moving)

Figure 1.2 Schematic of a compressionmold for a plate


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Product designer: the person responsible for designing the product to bemolded

Molder: the person or company engaged in injection molding plasticproducts

1.3 Is Injection Molding the Right Choice

for this Product?

Before proceeding, we must ask: why was injection molding selected for thejob?

The molder may have a financial or other interest in preferring to have theproduct made by injection molding, but we must keep an open mind.

Have alternative methods or product designs been considered or investigated,employing other manufacturing processes using the same or a similarmaterials, or using other materials which may permit a similar end product,

possibly even with better quality, and/or at lower cost?A few typical examples of possible manufacturing alternatives for injectionmolding:

Thermoforming

Foam molding

Coining and die stamping (blanking)

Extrusion blow molding

Machining, forming of sheets

Some other, maybe yet to be developed methods and materials

Another possibility is not to use plastics at all, but rather use:

Paper (cardboard), wood, cloth

Metals (steel, aluminum, etc.)

Injection molding has many advantages, particularly low mass, achievableaccuracy, good strength-to-weight ratio, good appearance and surfacedefinition, and numerous specific physical properties. But injection moldedproducts always suffer from the fact that the initial capital outlay for moldsand machines can be very high.But we must never forget that on a per unitbasis, especially whenever large quantities are considered, the contribution

of the cost of the equipment (mold, machine, etc.) to the cost of the productis small and often almost negligible.

Figure 1.3 Typical injection molded parts

The relatively high capital cost of amold is often almost negligible whenevaluated on a per-molded-partbasis


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5

1.4 The Injection Molding Machine

We will not discuss the advantages and disadvantages of the various injectionmolding machines that are on the market, but rather introduce the readerwho is not too familiar with this industry to the various terms that will beused from time to time if a subject under discussion will have special referenceto a machine element or feature. The accuracy of molding, and especiallywhen molding so-called thin-walled products, is very dependent on thequality of the molding machine, its mechanical rigidity, accuracy of alignment,parallelism of platens, the quality of its controls, and the state of maintenance.

Every good injection-molding machine consists of these basic elements

1. A rigid base

2. A rigid clamping unit, consisting of two platens, for the mounting of themold halves and provisions for guiding the platens (tie bars or ways)

3. Provision for moving the platens, preferably fast, relative to each other,for opening and closing the mold; the speed of motion is usually

adjustable

4. Provision for clamping, i.e., holding the mold shut against the force ofthe injection pressures within the mold (in some machines, provisions 3and 4 are combined)

5. Provision for ejecting the molded product(s) from the mold

6. Provision to transform the raw plastic (pellets, etc.) into an injectablemelt (the plasticizing unit)

7. Provision for injecting the melt into the mold (in most machines,provisions 6 and 7 are combined in one unit)

8. Provision for heating the plastic in the plasticizing unit

9. Cycle controls (sequencing logic, timers, etc.) and a command post formanual operation and for mold setup

10. Heat controls for all heaters in machine and molds. Some machines have

a limited number of heat controls and additional controls could berequired for the molds, especially with larger hot runner systems. Thispoint must be considered when estimating the mold cost.

11. Safety gates to protect operators and bystanders from all hazards whenoperating the machine

12. Mechanical safety drop bar(s) to prevent closing the machine when gatesare open, in case of failures of the other (electric and hydraulic) safetymeasures.

13. Provision for cooling water distribution to the mold

14. Provision for compressed air, for auxiliary actions required in the mold

Even the best machine if poorlymaintained will not perform as itshould

1.4 The Injection Molding Machine

The mold designer who believes thatthe product considered could bemade better by other methods has aduty to discuss this with thecustomer, even if it could mean lostbusiness, this time


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There are other features available, e.g., for the convenience of quick moldinstallation, setting up and operation of the mold and machine; these featuresare often offered as options which can be bought with the machine or added

on later.

1.4.1 The Right Machine for the Mold

Often, the mold cost will surpass the cost of the machine. It does not matterhow ell a mold is built if the machine cannot meet the molding requirementsto produce quality products. While considering the purchase of an injection

mold, it is always important to make sure the machine can do the job. Someof the basic considerations are:

Tie bar spacing

Stroke and shut height

Injection speed (average and peak)

Available injection pressure

Recovery rate capability (throughput)

Platen rigidity (are the platens rigid/robust enough to carry the moldweight?)

Available clamp tonnage

Platen parallelism

Clamp speed requirements

Shut-off nozzles

Screw design

Accuracy and repeatability of controls

Operator access

Mold protection capability

As the machine and mold act together as a system, it is fair to say that thesystem will perform only as well as its weakest component. If an existingmachine is to be used, the machine should match the machine's capability.

The mismatched machine can easily destroy the new mold in a matter ofmonths, resulting in costly rework.

To determine the right machine, the following information on the mold isrequired:

Mold width, length, and height

Opening stroke required (usually 2.5 part height)

There is no point in buying apremium priced mold to run it in anout-dated machine


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7

Ejector rod locations

Locating ring size

Part dimensions, including wall thickness

Flow length (length of flow from gate to longest flow path)

Part weight

Runner weight (if cold runner)

Cavitation

Nozzle radius

Material (including color and additives, viscosity)

1.5 The Injection Mold

To the customer or entrepreneur not familiar with the problems of moldingand mold making who wants to make a new product, the price of a moldmay seem to be high, occasionally even outrageous; it is often difficult toconvey that the mold priceconstitutes only a very small portion of the product(piece) cost, and depends much on the expected production of the mold.

1.5.1 What Is an Injection Mold?

A (plastics) injection mold is a permanent tool, i.e., a tool that, if properly

designed, constructed, and maintained will have a life expectancy (usefullife) well beyond the time where the product itself becomes obsolete. Thisdifferentiates it from a one-time use mold such as a sand-casting mold, asused in metal foundries. A mold can be used to make products in a virtuallyinfinite variety of shapes, made from injectable plastics. Common to all moldsis the condition that it must be possible to remove the product after molding,without the need to destroy the mold (as is the case in sand-castings).

There is an exception to this, the so-called lost-core molding: There are

injection molds for intricate products, such as intake manifolds for internalcombustion engines, previously made from cast iron, which have an outsideshape that canbe molded with conventional (permanent, open and close)molds but where the intricate inside shapeis made from a molded, low meltingpoint metal composite which is inserted into the mold before injection, andthen ejected together with the molded product; the metal is then removedby heat at a temperature above the melting point of the insert, but of coursebelow the melting point of the plastic used for this product; the molded

metal insert is thereby destroyed, but the metal will be reused.

A basic mold consists of two mold halves, with at least one cavity in onemold half, and a matching core in the other mold half. These two halves


It is important to understand that it

is not the mold cost but the piece(unit) cost of the product, which isimportant


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meet at a partingplane(parting line). As the mold opens after the injectedplastic (now in the shape of the desired product) is sufficiently cooled andrigid the product can be removed by hand or be automatically ejected.

Because injection-molding machines are mostly built with the injection onthe stationary platen side, there is, typically, no built-in ejection mechanismon this side. If ejection from the injection side should be required alwaysthe case in stack molds, and occasionally required in single level molds anyrequired mechanism must be added to the mold, and occasionally to themachine; in either case, this adds complexity and increases costs. Only moldsdesigned for using only air ejection do not require any external ejectionmechanism.

Most products are removed (ejected) from the core. There are also manymolds, which need special provisions to allow the products to be removedfrom either the cavity or the core. This is the case with products having severeundercuts or recesses on the inside and/or the outside of the product, suchas screw threads, holes, ribs or openings in the sides of the product, etc., ormolds for insert molding.

Some of these design features of the product may require moving side cores,

which are either inserts or whole sections of the cavity that move at an anglewhich is 90 to the natural opening path of the mold. Others may requirespecial unscrewing mechanisms, either in the core or in the cavity side. Themold may require split cavities (or splits), i.e., the cavity consists of two ormore sections, which are mechanically or hydraulically moved in and out ofposition, and then clamped together during injection. In some cases, themold may require collapsible cores, or retractable inserts, which are all quitecomplicated (and expensive) methods.

Any of the above special features can add considerably to the mold cost whencompared to a simple up and down mold where the products can be readilyejected with the machine ejectors during the mold opening stroke or afterthe mold is open, without the need for any of these complicated mold features.

Note that in this book, the term (simple) up and down molding is used,which comes from the earlier vertical molding machines, even though, today,most general-purpose injection molding machines are horizontal and themold opens and closes in a horizontal motion.

Example 1.1

To illustrate how different mold features affect the mold cost, we assumethat a single face mold with air ejection of the products costs X dollars.A similar mold, but with mechanical ejection, costs about 1.2 times X.A similar, air- ejected 2-level stack mold will be about 1.8 times X.An unscrewing mold for a similar size mold and product will cost about

2 times X.

Almost any shape can be molded

but at what cost?


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9

1.5.2 Elements of an Injection Mold

Most readers will expect to see some illustrations (photos or schematics) of

injection molds at this point. However, we must not forget that this is not abook about mold design, but about the relationship between productivityand cost of molds, as well as the cost of the products to be made. There willbe, however, a number of photos of molds accompanying the text wheredeemed useful.

There are books that show designs of numerous, specific molds but it isvirtually impossible to show every possible configuration that may berequired. It is more important for the designer, and any person requesting a

new mold, to understand that a mold consists essentially of a number ofelements from which to choose for the most appropriate design for thepurpose.

Every injection mold consists of the following basic elements:

1. One or more matching cavities and cores, defining the cavity space(s)(today, there are molds with anywhere between 1 and 144 cavities).

2. A method, or element, to duct the (hot) plastic from the machine nozzleto the cavity spaces:

There is a choice between Cold runners (2-plate or 3-plate systems) Hot runners (various systems) Insulated runners, through shooting Sprue gating (cold or hot)

3. Provision to evacuate air from the mold (venting):

There is a choice between Natural venting Vacuum venting

4. Provision to cool the injected hot plastic sufficiently to allow ejection ofthe molded product

5. Provision to eject the molded product:

There is a choice between Manual product removal Ejector pins and sleeves Stripper s (stripper rings or bars) Air ejection Random ejection Various methods of in-mold product removal methods Robotic product removal

6. Provision to attach (interface) the mold to the molding machine:

There are several methods to consider Mold is for one machine only


Product quality, productivity, andmold cost depend heavily on theproper selection of the runnersystem

10


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Mold to be used on several, different machines Quick mold change methods (various designs)

7. Method of alignments of cavities and cores:There are several methods to consider No alignment feature provided in the mold Leader pins and bushings (2, 3, or 4) Leader pins and bushings between individual cavities and cores Taper fits between individual cavities and cores Taper fits between plates Any combination of the above

8. Any number of (mold) plates to provide the necessary for carrying andbacking the above elements

But molds could have additional features, which will also be discussed in thefollowing. Each of these features can add (often considerable) costs to themold but in many cases can increase the productivity of the mold and reducethe cost of the product. They may or may not all be necessary and must becarefully considered when deciding on the type of mold most suitable (and

most economical) for the job on hand.

Features such as serviceability of the mold may affect the mold cost; forexample, the access to hot runners for cleaning plugged gates or makingminor repairs, such as changing a nozzle, a burned-out heater, or a faultythermocouple at a hot runner drop will cost more in the initial mold, butthis will be easily recouped by reducing the down time necessary to accom-plish such repairs. By designing easy access to these components in themachine (without the need to remove the whole mold, or part of it, to the

bench), such repairs can be made in less than an hour, instead of takingseveral hours. This work can also be done by the mold setup staff rather thangetting the (expensive) mold makers involved.

Another area where valuable maintenance time can be saved is to design andprovide easy access from the parting line to screws holding modular moldparts to their mounting plates, while the mold is in the machine.

On the other hand, in my experience, many molds, particularly molds for

lower production quantities, have been vastly over-designed and much moneyhas been wasted.

The main purpose of this book is to discuss the various elements or featureslisted above and to facilitate the selection and the decision making. Definingwhat is reallyrequired considering the shape and complexity of the productand the required production quantities will enhance mold productivity. Inaddition, the book should facilitate investigating if, even minor, changes tothe product shape could lower the mold cost and improve the productivity

of the mold or the whole system.

Figure 1.4 Mold maintenance in the pressis important. Here, the operator is changinga nozzle tip while the mold is in the press(Courtesy: Husky)

Easy serviceability of the mold isimportant but often overlooked. Itadds some mold cost, but savesmuch more in future servicing costsand downtime

Even minor changes to the part candramatically lower or increase moldcosts

11


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11

2 The Plastic Product

Plastics have evolved to be a very useful material. Today, plastics are used inalmost every area, from small bottle caps, disposable cutlery, and packagesfor dairy products, to large containers, such as laundry baskets and garbagepails.

Plastics have transitioned from a cheap substitute for metal and glass tothe material of choice providing almost unlimited design freedom, uniqueproperties, and significant cost savings.

Figure 2.1 shows various industrial containers and house wares that createdurable products in cycles from 1030 seconds.

Figure 2.2 shows various thin-walled containers are typically used in the dairyindustry and are molded with wall sections typically less than 0.7 mm withcycles of 20 shots per minute.

Figure 2.3 shows a collection of PET bottles for water, soft drinks, etc. andsome of the preforms used for blowing these bottles. Today, more than 500,000

tonnes annually of plastic are converted into bottles. Cycle times for moldingthese parts have been reduced from 35 to 8 s in the last 20 years. In addition,cavitations have increased from 8 to 144 cavities, resulting in significantlylower product costs.

Figure 2.4 shows a sampling of stadium cups with printed or in-moldlabelled decorations.

Figure 2.6 shows samples of small, thin-walled technical products made from

engineering plastics such as ABS, Acrylic, and PC.

Figure 2.1 Molded products of various sizes(Courtesy: Husky)

Figure 2.2 Various thin-walled containers(Courtesy: Husky)

Figure 2.3 PET bottles for water, soft drinks,etc. and some of their preforms(Courtesy: Husky)

Figure 2.4 Stadium cupsFigure 2.5 Small and large technical (engineering) products, heavy-walled jarsfor cosmetics, and tubular containers with integral, hinged lids (Courtesy: Husky)

12 2 The Plastic Product


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2.1 The Product Design

The following contains suggestions for the product design and how it mayimpact the mold design and the productivity of the mold

A new mold is usually required

For a new product

After the redesign of an existing product

To increase the productivity and the output of the production facilitiesalready in place. This usually provides a good opportunity to reevaluateand improve the product, and to reduce manufacturing costs, particu-larly through the reduction of the plastic mass of the product.

The mass of the plastic accounts for a significant portion of the cost of everyproduct. Reducing wall thickness and reduction of unnecessarily heavy crosssections will not only reduce the cost of plastic material for the product, butwill also result in sometimes significantly faster molding cycles. The result

is that more of the products can be made per hour at lower cost than waspossible with the preceding design.

In such a case, important considerations are

The output of the plasticizing unit and the dry cycle of the machinemanufacturing the product before the planned changes

If there was special handling equipment (product removal, stacking,

printing, etc.) with the old mold, will it be able to handle the greateroutput, or will it need improvements as well

The above will be discussed in more detail later in this book.

Figure 2.6 Small, thin-walled technicalproducts made from engineering plastics

132 2 Product Drawings


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132.2 Product Drawings


Occasionally, only samples or CAD models of a new product are available.This may be of some advantage to better visualize the product, but it isabsolutely necessary, to minimize risk for all parties involved in the finaldecision, to have a complete detail drawing of the product, showing allfeatures, tolerances, and specifications.

This is also the moment when the designer has the greatest opportunity todecide on the most suitable design for the mold, and/or to make suggestionson how the product design might be modified to improve the productivity,

to simplify the mold design, and to reduce mold costs. This is also the timeto consider any ancillary equipment required for this production. Anopportunity graph (Fig. 2.7) shows symbolically the value of planning aproject. At the outset of the project, the opportunity to make improvements,revisions, and selections is highest to affect the final outcome of the project,while the costs are lowest. After concept analysis, once the elements of theproject have been agreed upon and as engineering of the mold progresses,the opportunity to make conceptual changes or improvements diminishes,and any costs associated with it will increase. By the time the project reachescompletion and gets into testing and production, the opportunity to makechanges is low, and any costs could be very high.

To confirm that the part drawing is acceptable to all parties it should alwaysbe signed off in writing as acceptable. Appendix 12 provides some generaladvice for the designer on how to critique a part drawing.

2.2.1 Product Shape:How Can the Product Best Be Molded?

Here, even an experienced, conscientious designer may want to consult withanother (knowledgeable) colleague, and/or with anyone else who is familiarwith the type of product for which the mold is to be built, and discussproblems of making andof operating such a mold, to get their input regardingthe proposed product design. In the following, some of the most important

areas to be contemplated are discussed.

2.2.2 Parting Line (P/L)

Is There an Obvious Location for the (Main) Parting Line?

In many products, the location of the parting plane (parting line, P/L) isobvious. It is along the largest cross-sectional dimension of the product, atright angles to the motion of the opening and closing of the mold, and shouldpreferably be in oneplane. This is the least expensive, and fortunately, themost frequent case. However, there are many cases where the P/L cannot be

It is critical that complete productdrawings are available for the molddesigner before any mold design isstarted

Opportunity

C

osts

Costs

Time

Period of evaluation of product,opportunity for changes is high,changes are easy to obtain,and low in cost.

During engineering, opportunityfor revisions is still fairly high.Changes are still relatively inexpensive

During manufactoring, there islittle opportunity to make revisions.Changes can be quite costly.

Mold tests and roduction:

Figure 2.7 Opportunity graph

The old proverb a stitch in timesaves nine applies here too: Spendmore time at the beginning of theproject, to save much time later on



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located there, and requires special consideration. A few examples are listedbelow:

Simple parting lines (Fig. 2.8)

Sometimes, the P/L mustbe offset because of the shape of the product(Fig. 2.9).

It may be of advantage to place the P/L at a level, which is notat thelargest cross section, to force the product to stay on the side from whereit will be ejected, as can be the case with flatproducts. This would notaffect the mold cost; however, flat products often cause trouble at ejection,

because they do not always stay reliably with the side from where theyare ejected. Additional mold features, such as sucker pins, or grooving inthe side of the product (pull rings) may be required to hold the producton the ejection side to make sure that the mold can operate automatically,without interruptions (Fig. 2.10).

The P/L is curved. This is sometimes unavoidable because the productshape will not permit a straight P/L; for example in some toys, butoccasionally also in technical products. A typical example is the P/L for

plastic forks or spoons. In all these cases, the matching of the P/L is difficultand expensive. It may need special, costly grinding equipment or expen-sive fitting by hand (bluing) (Fig. 2.11).

Figure 2.9 Example of simple mug handle,using offset P/L

Figure 2.11 Typical mold profilefor cutlery

Figure 2.10Typical flat piece withundercut below parting line

Figure 2.8 Examples of straight,simple parting lines (top: at the opening;bottom: at the largest diameter)



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15

2.2.3 Side Cores

Is There a Need for Side Cores, Splits, or for Other Methods to Release Severe

Undercuts or Threads?

Any of these features will add considerable cost to the mold (and to the costof the product), not only because of the added complexity of the stack butalso because each stack requires much more spacethan a simple stack withoutside cores. For the same number of cavities, a much larger mold and thereforeoften also a larger machine size may be required just to accommodate themold in the available platen area, even though the clamping forces requiredwould be little more than for the mold without side cores or splits.

Such side cores, splits, etc will lengthen the cycle time and reduce productivitycompared to molds that do not have such features.

Could a Redesign of the Product Avoid the Need for Side Cores?

In some cases, round holes or odd shape openings generated by using sidecores or split cavities could be redesigned without sacrificing the usefulnessof the product, and possibly without significantly changing the appearance,

by creating such holes or openings in the side walls (or even in ribs inside theproduct) with a design method where core and cavity meet on a shutoff.This may require the use of special inserts in either or both of cavity andcore, which may necessitate a change in the shape (or in the draft angle) ofthe side wall of the product, or require an opening in the bottom of it. Inmany cases, this could be acceptable for the end use of the product and allowa much simpler, less costly mold [1]. By just giving a bit more thought to theproduct design before planning and designing a mold, and by understanding

the application for which the product is used, a little redesign can often resultin spectacular savings in mold and product costs.

Selecting Other than the Conventional Parting Line

Occasionally, the choice of the obvious placing of the parting line wouldrequire a side core, while by slanting the P/L, the product could be moldedwith a simple up-and-down mold. An example is a simple louver (Fig. 2.13),but the principle applies to any similar case. The cost of a mold with a slanted

P/L is somewhat higher than that of a mold with an ordinary P/L, but muchlower than a mold with a side core.

Investigate Shape of Threads and Undercuts

Often, a design specifies threads or undercuts, on the insideof the product(Fig. 2.14). Is the specified shape of thread or undercut designed with moldingin mind? Many such threads or undercuts could be molded without un-screwing, or the need for collapsible cores, by changing the shape of the

undercut so that the product can be stripped off the core, i.e., the undercutscan easily slip out of the grooves that created them when pushed by ejectorsor a stripper.

Figure 2.13 Example of louver; top: needsside core; bottom: tilted it becomes anup-and down mold

Figure 2.14Typical bottle cap with

tamper-proof ring and stripped threadfor simpler ejection (no unscrewing moldrequired). This product is outside-gated,using a hot runner hot tip gate

g

Figure 2.12 4-cavity handle mold with 3 sideactions per cavity (Courtesy: Topgrade Molds)



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Figure 2.15 shows the difficulties of a typical unscrewing mold. The coremust rotate out of the cap before it can be ejected. This makes core coolingmore difficult and results in 30% longer cycle times than a stationary core.Unscrewing molds are much more complicated than bump-off (stripped)closure molds.

Figure 2.16 shows a schematic of a much simpler mold, where the thread(and the cap) can be stripped. Here, core cooling can be very efficient. The

cycle time for a typical (28 mm) bottle cap made from HDPE MFI 19,weighing less than 3 g, molded in a 24-cavity mold running in a 90 t(1,000 kN) machine is in the order of 4.0 s, equaling a productivity of 21,600caps per hour.

Figure 2.17 exemplifies of how a small change in the angle of the flank of thethread can allow a thread to be stripped from the core, rather than requiringan unscrewing mold. Small changes like this can have a major impact onproduct cost because mold cycle, cost, and maintenance will be significantly

improved with a stripped product.

Need for Two-Stage Ejection or Moving Cavity

This applies to a shape or design feature of a product consisting of

Deep ribs on the cavityside, as is often the case with containers withfalse bottoms. Such ribs could also be specified on technical enclosures,etc., as illustrated in Fig. 2.20. The depth of the rib Fand the ratio of the

thickness of the rib t, as well as the draft angles of the rib are criticalconsiderations, or

Deep ribs (often circular) on the coreside; typically, the underside of anover-cap, as illustrated in Fig. 2.21 (even without the thickening at theend of the rib as shown in this illustration).

In both cases, if the ratio of F/t> 2, or if there is any thickening at the end ofthe rib (as in Fig. 2.21), either a two-stage ejection or a moving cavity arenecessary, which will increase the mold cost by about 1520%. In both cases,it is important to provide especially good ventingat the end of the ribs toensure proper filling. Failure to use these methods will make it very difficult

Figure 2.18 72-cavity unscrewing mold(Courtesy: Stackteck)

Rachets

Rotatingcore

Stationaryratchetring

Figure 2.15 Schematic of difficultiesof a typical unscrewing mold.

Stripperring

Core

Figure 2.16 Mold where threadcan be stripped

Types of closures

Top of threadalmost flat, lessthan 15.If stripped will begreatly deformed.

Angle on top ofthread allowsthread to bestripped offthe core.

Unscrewed thread Stripped thread

Figure 2.17 Change in flank angleallows thread to be stripped

Figure 2.19 Stripped closure mold



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to withdraw (eject) the products, and increases the risk of breaking portionsof the rib in the mold.

A2-stage moldwill cost about 1520% more than a comparable mold withoutthis feature. Also, because the sleeve is usually rather thin, it is very difficultto get cooling into it; the mold will cycle much slowerthan a similar productwithout this complication, and the maintenance cost of such molds is muchhigher.

Moving cavitiesare more complicated and cost about 10% more than a moldwithout this feature. Some molders use it despite its higher cost for productseven without a false bottom, because they can cycle even faster than a mold

with a conventional cavity.

Post-Molding Operations

Sometimes, molds can be much simplified by doing additional work to theproduct after molding. Post-molding operations are of particular importancewhenever relatively small quantitiesare to be made. For example, one or afew simple holes or openings in the side wall of a product would require aside core in the mold, but such holes or openings could also be drilled or

die-stamped aftermolding. Such additional operations may require a drillingfixture or a stamping die. The actual time (direct labor) for such post-moldingoperations and any costs for tools or fixtures would have to be added to the

Always keep in mind:It is possible to mold almost anyshape, but at what cost?

Figure 2.21 A product with deep ribs and(with or without) thickening at the end isejected in two stages; 1: Sleeve and stripperlift product off the core; 2: Strippercontinues to push product off the sleeve

Figure 2.20 Schematic of a moving cavityin two halves; top: mold is closed; bottom:mold opens and follows core for a limiteddistance to ensure that the rib becomes free



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total cost of the product. But such post-molding operations could also takeplace later at the assembly line, where the product is assembled or packed,without any additional labor cost if properly integrated in the process. Again,

it is the overall costof the end product that is important, not just the cost ofthe mold or the molded piece itself. In many cases, the savings in the moldcost achieved by eliminating a side core (or some other complications of themold) can be substantially greater than the combined additional cost forfixtures or tools, plus the cost of the additional direct labor to finish theproduct.

A typical example for this would be the need for small holes for a hinge pin(for a hinged lid), located in two lugs projecting from the bottom of a product

(see Fig. 2.22). The plastic melt is injected into the bottom of the product,near the lugs. It is of course feasible to mold these holes, but it could be quitedifficult to arrange the side cores required for such holes as well as theactuation for such side cores, without interfering with the gating and thecooling layout in this area. It would be, however, quite easy to just mold thelugs as projections from the container bottom, and then drill the holes, usinga simple drilling fixture.

2.3 Accuracy and Tolerances Required

Next, the mold designer should look at the specifications relating to accuracyand tolerances.

Unfortunately, often, after a product has been conceived, the design has beeneither just sketched by the inventor or an artist, or a model has been created.

This information has then been passed on to a draftsman to be put on paper(by computer or pencil drawing). This may result in a good visual descriptionof the new product, but to be practical for manufacturing, any drawing mustbe fully dimensioned, and intelligently toleranced. To design a product forinjection molding requires certain knowledge of this technology. A designwhich may be suitable for one method of processing plastics (or othermaterials) may be unsuitable or impractical for another process, even thoughthe end use is the same.

For example, a disposable drinking cup of a specified capacity could be madefrom paper, styrofoam, be thermoformed from sheets, be injection molded,or made by another, entirely different, new method or material. The finalproduct design for each of the above cited materials and methods wouldmost likely look different to suit the method of manufacturing and theselected material.

Also, while the dimensional accuracyof the product for its final use (i.e., as adrinking cup) may be of little importance, its actual dimensions will require

high accuracy because of demands not related to its use, such as stackingheight (e.g., for packaging), ease of releasing of the individual cups from thestack as required in automatic vending machines, and mainly because even

Figure 2.22 Lugs with holes

How is the product to be used?What is really required?



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small variations in wall thickness may have a great effect on the mass ofplastic used for each unit and on the molding cycle.

A design for a metal product is different from the design for a similar productmade by injection molding, even though the products may be fully inter-changeable in their use. This applies especially for design features such as

Radii and sharp corners,

Flow path for injection (if applicable),

Wall thickness,

Ribbing and reinforcements,

Openings (round or shaped),

Others.

These features, by their presence or absence, not only affect the making ofthe mold (and its cost) but also affect the speed of the molding operationitself. I refer the reader to the many books on product design for injection

molding, which go into much detail on this subject [2, 3, 4].It is very important to understand that it is relatively easy to achieve closetolerances for the mold partsusually made from metal; however, the plasticproducts made by the mold do not solely depend on the mold dimensions.The designer must be aware that the final size of the product is greatly affectedby variations in the shrinkage of the plastic (see Appendix), which in turn iscaused by variations in molding conditions (pressures, temperatures, andtiming) and by variations in the composition of the plastic not only from

batch to batch, but also from manufacturer to manufacturer. All this makesit very difficult to mold products dimensioned within close tolerances.

But even the above statement relatively easy to produce the mold parts toclose tolerances must be qualified. Using unsuitable, old, and/or poorlymaintained machine tools makes it more difficult to make mold componentsto close tolerances; the accuracy of the work depends much on the skill ofthe machinists, and even with good checking equipment can become timeconsuming, because it requires frequent measuring of the closely toleranced

dimensions. The alternative is to use good machine tools, or even machinesspecially designed or adapted for certain steps in the manufacture of themold parts, requiring much higher investments by the mold maker. Eitherone of these conditions affect the cost of machining and explain why closetolerances can be expensive too achieve.

Note also that dimensions are affected by the ambient temperature of themachine shop and that even when cooled by cutting fluids, the work piecesheat up during machining; they will measure larger when warm immediately

after cutting than after cooling to room temperature. Of course, the largerthe dimension, the larger the dimensional differences caused by heat expan-sion.

Many millions of dollars aresquandered annually because ofdemands for unnecessary tight

tolerances


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As can be seen in Fig. 2.23, the mold cost increases exponentially with thetightness of the tolerance.

Without giving actual cost figures, the curve just shows how costs can increase,as the tolerances get tighter. The cost to achieve a 0.005 mm (0.0002)tolerance can be 3 times the cost of a 0.03 mm (0.0012) tolerance.

Other points that should be clarified when looking at product dimensionswith close tolerances: how will these dimensions (or the entire product) bechecked (measured) on the finished product? With Vernier, micrometer, gages,measuring machines, fits with other products? Also, when will they bechecked? Immediately after ejection, one hour later, 24 hours later? Will there

be 100% inspection or statistical (random) inspection?To clarify all this ahead of time can avoid much future unpleasantness orarguments.

2.3.1 General and Specific Tolerances

Alltolerances must be specified on the product drawing and must be looked

at by the mold estimator or designer when starting the project to see if theyare reasonable. The Society of Plastics Industry (SPI) has a suggested list ofpracticalgeneral tolerances for injection-molded products. For more informa-tion, go to the SPI website www.socplas.org.

In most cases, these tolerances are satisfactory and achievable. Specific, closertolerances may require that experiments be made with cavity and core sizes,and under various molding conditions, to achieve the required sizes. Thiscan mean considerable added costs for the mold maker and a higher mold

cost.The following tolerances are suggested to be used on plastic product drawings(radii are not toleranced):

Product weight: 10% on projected weight (range 2%)

Wall thickness: 0.03 mm (in special cases 0.013 mm)

Fit diameter: up to 75 mm 0.20 mm

up to 106 mm 0.25 mmup to 160 mm 0.30 mmup to 300 mm 0.64 mm

Overall height: 0.5% or 0.13 mm minimum

Stack height: 0.5% or 0.13 mm minimum

Note that the steel size requirements, and thus the difficulty of manufacture,

are dependent on the plastic tolerances on the product drawing.

Figure 2.23 Relationship betweentolerances and mold cost

Always remember that tighter

tolerances mean higher mold costs,maintenance, and inspection


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2.3.2 Are Special Fits with Matching ProductsRequired?

Often, certain dimensions of a product are specified with unnecessary closetolerances, when all the designer wanted to convey is that the product shouldfit suitably on another product (tightly or loosely), typically, a container anda matching lid. This requirement must be clear. Especially, when moldingplastics with high shrinkage factors (e.g., PP or PE), it can be difficult toarrive at the proper steel dimensions, and some experimenting may berequired to achieve the required fit. Specifying the matching diameters withstandard, loose tolerances may yield pieces correct in size, but wrong because

the fit is not as desired. The alternative providing closer tolerances couldbe unreasonable, because the dimension of the molded product depend notsolely on the steel dimensions of the stack parts but also on the moldingparameters. In such cases, it is of advantage to complete the more complicatedmold first and test it in actual molding conditions until the best cycle time isestablished. The critical mold parts of the matching product (e.g., the lid)should be finish-machined only after having established what the actualmolded container dimensionsare. This could require completing the lid moldwith only one cavity, using assumed suitable dimensions, testing the un-finished mold until the best cycle is achieved, and then adjusting the assumeddimensions so that the proper fit can be achieved. All lid mold parts can thenbe finished. For more information on this subject see [5].

2.3.3 Tolerances for the Filling Volume

This applies specifically but is not restricted to containers into which a

more or less viscous product will be filled by volumeto within closely specifiedlimits (typically, containers for margarine, paint, etc.). In their end use, it isimportant for the seller that a minimum amount must be filled into thepackage without shortchanging the buyer, but also they should not be over-filled, which would mean a loss for the seller. There should be clearly definedfill lines (usually inside the container) to mark the minimum and maximumvolumes. This can be a problem with plastics with large shrinkage factorssuch as PE and PP. It requires special consideration when dimensioning the

cavity and core because of the unavoidable variations in shrinkage values, asthe plastic flows away from the gate and slowly cools and as the injectionpressure within the mold decreases. The same considerations apply tomeasuringcups or vials which have the various levels (or volumes) indicatedby lines on the sides of the product. It may be necessary to first test the moldto find the best cycle times, and then establish the location of the measuringlines.

Prototyping is often used to verifythe required dimensions or fits of apart after shrinkage



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2.3.4 Stacking of Products and Free Dispensing

Any product stacked for shipping musthave a clearly defined stacking height,

which is usually created by resting the outside or the bottom of one piece onthe inside stacking provision of the following piece. These provisions forstacking can be stacking lugs, or clearly defined steps in the product. Thepurpose of these lugs (or steps) is

The products must notjam when pushed together, which would make itdifficult to separate them where required by the user, and

They will ensure a total stack heightof a certain, specified number (e.g.,

20, 25, 40, etc.) of the pro

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Selecting Injection Molds