Injection Mold Design Engineering D. Kazmar

David O. KazmerInjection Mold Design Engineering

InjectionMold DesignEngineering

David O. Kazmer

Hanser Publishers, Munich Hanser Gardner Publications, Cincinnati

The Author:David O.Kazmer, P.E., Ph.D.Department of Plastics Engineering, 1 University Avenue, Lowell,MA 01854, USADistributed in the USA and in Canada byHanser Gardner Publications, Inc.6915Valley 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 HanserVerlagPostfach 86 04 20, 81631 Mnchen, GermanyFax: +49 (89) 98 48 09www.hanser.de

Your use of the information provided herein is conditioned upon your agreement, and the agreement of your employer orany third party to whom you provide information, to make use of these materials only in accordance with and subject tothe following terms and conditions.The information provided herein is made availableas iswithout warranty of any kind, either express or implied, includingbut not limited to the implied warranties of merchantability, fitness for a particular purpose, satisfactory quality, or non-infringement. We may in the future modify, improve or make other changes to the information made available. All theincluded information may include technical or typographical errors and we will not be responsible for any such errors.Any pricing and other information about products and services contained herein is not an offer to provide such goods orservices.You agree not to bring any legal action against the author or publisher based on your use of the provided information.Youagree to indemnify and hold the copyright holder and its affiliates, officers, agents, and employees harmless from any claimor demand, including reasonable attorneys fees,made by any third party due to or arising out of your use of the providedinformation. The sole and maximum liability of the copyright holder, its affiliates and subsidiaries for any reason, and yourexclusive remedy for any cause whatsoever, shall be limited to the amount paid, if any, for the provided information.

Library of Congress Cataloging-in-Publication DataKazmer, David.Injection mold design engineering / David O. Kazmer.

p. cm.ISBN-13: 978-1-56990-417-6 (hardcover)ISBN-10: 1-56990-417-0 (hardcover)1. Injection molding of plastics. I. Title.TP1150.K39 2007668.412--dc22

2007018765

Bibliografische Information Der Deutschen BibliothekDie Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie;detaillierte bibliografische Daten sind im Internet ber abrufbar.

ISBN 978-3-446-41266-8

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic ormechanical, including photocopying or by any information storage and retrieval system, without permission in wirtingfrom the publisher.

Carl HanserVerlag,Munich 2007Production Management: Oswald ImmelTypeset by Manuela Treindl, Laaber, GermanyCoverconcept: Marc Mller-Bremer, Rebranding,Mnchen, GermanyCoverillustration by David O.Kazmer, Lowell, USACoverdesign: MCP Susanne Kraus GbR,Holzkirchen, GermanyPrinted and bound by Druckhaus Thomas MntzerGmbH, Bad Langensalza, Germany

Preface

Mold design has been more of a technical trade than an engineering process. Traditionally,practitioners have shared standard practices and learned tricks of the trade to developsophisticated molds that often exceed customer expectations.

However, the lack of fundamental engineering analysis duringmold design frequently resultsin molds that may fail and require extensive rework, produce moldings of inferior quality,or are less cost effective than may have been possible. Indeed, it has been estimated that onaverage 49 out of 50 molds require some modifications during the mold start-up process.Many times, mold designers and end-users may not know how much money was left onthe table.

Thewordengineeringin the title of this book implies amethodical and analytical approach tomold design.The engineer who understands the causality between design decisions andmoldperformance has the ability tomake better andmore informed decisions on an application byapplication basis. Such decision making competence is a competitive enabler by supportingthe development of custom mold designs that outperform molds developed according tostandard practices. The proficient engineer also avoids the cost and time needed to delegatedecision to other parties, who are not necessarily more competent.

The book has been written as a teaching text, but is geared towards professionals working ina tightly integrated supply chain including product designers, mold designers, and injectionmolders. Compared to most handbooks, this textbook provides worked examples withrigorous analysis and detailed discussion of vital mold engineering concepts. It should beunderstood that this textbook purposefully investigates the prevalent and fundamental aspectsof injection mold engineering.

I hope that Injection Mold Design Engineering is accessible and useful to all who read it. Iwelcome your feedback and partnership for future improvements.

Best wishes,

David Kazmer, P. E., Ph. D.

Lowell, MassachusettsJune 1, 2007

Contents

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

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XV

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Overview of the Injection Molding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Mold Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Mold Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 External View of Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2 View of Mold during Part Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3.3 Mold Section and Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Other Common Mold Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.4.1 Three Plate, Multi-Cavity Family Mold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.4.2 Hot Runner, Multi-Gated, Single Cavity Mold . . . . . . . . . . . . . . . . . . . . . 111.4.3 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.5 The Mold Development Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.6 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Plastic Part Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1 The Product Development Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.1 Product Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.1.2 Product Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.1.3 Business and Production Development . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1.4 Scale-Up and Launch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1.5 Role of Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2 Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.1 Application Engineering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.2 Production Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2.3 End Use Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.4 Product Design Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.2.5 Plastic Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.3 Design for Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3.1 UniformWall Thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3.2 Rib Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.3 Boss Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.4 Corner Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.3.5 Surface Finish and Textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.3.6 Draft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.7 Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

VIII Contents

3 Mold Cost Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1 The Mold Quoting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2 Cost Drivers for Molded Parts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2.1 Effect of Production Quantity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.2.2 Break-Even Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.3 Mold Cost Estimation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.3.1 Cavity Cost Estimation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.3.1.1 Cavity Set Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3.1.2 Cavity Materials Cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3.1.3 Cavity Machining Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.3.1.4 Cavity Discount Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.3.1.5 Cavity Finishing Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3.2 Mold Base Cost Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.3.3 Mold Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.4 Part Cost Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.4.1 Mold Cost per Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.4.2 Material Cost per Part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.4.3 Processing Cost per Part. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4.4 Defect Cost per Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.5 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4 Mold Layout Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.1 Parting Plane Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.1.1 Determine Mold Opening Direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.1.2 Determine Parting Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704.1.3 Parting Plane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.1.4 Shut-Offs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.2 Cavity and Core Insert Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.2.1 Height Dimension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.2.2 Length andWidth Dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2.3 Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.3 Mold Base Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.3.1 Cavity Layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.3.2 Mold Base Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.3.3 Molding Machine Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.3.4 Mold Base Suppliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.4 Mold Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.4.1 Strength vs. Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.4.2 Hardness vs. Machinability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 854.4.3 Mold-Makers Cost vs. Molders Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.4.4 Material Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.5 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

IXContents

5 Cavity Filling Analysis and Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915.2 Objectives in Cavity Filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.2.1 Complete Filling of Mold Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.2.2 Avoid Uneven Filling or Over-Packing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.2.3 Control the Melt Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.3 Viscous Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.1 Shear Stress, Shear Rate, and Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.2 Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.3.3 Rheological Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.3.4 Newtonian Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.3.5 Power Law Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

5.4 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.5 Cavity Filling Analyses and Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

5.5.1 Estimating the Processing Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.5.2 Estimating the Filling Pressure and MinimumWall Thickness . . . . . . 1075.5.3 Estimating Clamp Tonnage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.5.4 Predicting Filling Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.5.5 Designing Flow Leaders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.6 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

6 Feed SystemDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196.2 Objectives in Feed System Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

6.2.1 Conveying the Polymer Melt from Machine to Cavities . . . . . . . . . . . . 1196.2.2 Impose Minimal Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1206.2.3 Consume Minimal Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1216.2.4 Control Flow Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

6.3 Feed System Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236.3.1 Two-Plate Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236.3.2 Three-Plate Mold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.3.3 Hot Runner Molds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

6.4 Feed System Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.4.1 Determine Type of Feed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1336.4.2 Determine Feed System Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346.4.3 Estimate Pressure Drops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386.4.4 Calculate Runner Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1406.4.5 Optimize Runner Diameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.4.6 Balance Flow Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1456.4.7 Estimate Runner Cooling Times. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.4.8 Estimate Residence Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

6.5 Practical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506.5.1 Runner Cross-Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506.5.2 Sucker Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

X Contents

6.5.3 Runner Shut-Offs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1556.5.4 Standard Runner Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1576.5.5 Steel Safe Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

6.6 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

7 Gating Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1617.1 Objectives of Gating Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

7.1.1 Connecting the Runner to the Mold Cavity . . . . . . . . . . . . . . . . . . . . . . . 1617.1.2 Provide Automatic De-Gating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1617.1.3 Provide Aesthetic De-Gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1627.1.4 Avoid Excessive Shear or Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . 1627.1.5 Control Pack Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

7.2 Common Gate Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1637.2.1 Sprue Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1637.2.2 Pin-Point Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1647.2.3 Edge Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1657.2.4 Tab Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1667.2.5 Fan Gate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1677.2.6 Flash/Diaphragm Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1687.2.7 Tunnel/Submarine Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1697.2.8 Thermal Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1727.2.9 Valve Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

7.3 The Gating Design Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1757.3.1 Determine Type of Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1757.3.2 Calculate Shear Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1767.3.3 Calculate Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1787.3.4 Calculate Gate Freeze Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1797.3.5 Adjust Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

7.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

8 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1858.1 Venting Design Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

8.1.1 Release Compressed Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1858.1.2 Contain Plastic Melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1858.1.3 Minimize Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

8.2 Venting Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1868.2.1 Estimate Air Displacement and Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1868.2.2 Identify Number and Location of Vents . . . . . . . . . . . . . . . . . . . . . . . . . . 1868.2.3 Specify Vent Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

8.3 Venting Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1928.3.1 Vents on Parting Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1928.3.2 Vents around Ejector Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1948.3.3 Vents in Dead Pockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

8.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

XI

9 Cooling SystemDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1999.1 Objectives in Cooling System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

9.1.1 Maximize Heat Transfer Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1999.1.2 Maintain UniformWall Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1999.1.3 Minimize Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2009.1.4 Minimize Volume and Complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2009.1.5 Minimize Stress and Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2009.1.6 Facilitate Mold Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

9.2 The Cooling System Design Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2019.2.1 Calculate the Required Cooling Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2019.2.2 Evaluate Required Heat Transfer Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2069.2.3 Assess Coolant Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2089.2.4 Assess Cooling Line Diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2099.2.5 Select Cooling Line Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2119.2.6 Select Cooling Line Pitch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2139.2.7 Cooling Line Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

9.3 Cooling System Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2199.3.1 Cooling Line Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2199.3.2 Cooling Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2229.3.3 Conformal Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2229.3.4 Highly Conductive Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2239.3.5 Cooling of Slender Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

9.3.5.1 Cooling Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2259.3.5.2 Baffles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2269.3.5.3 Bubblers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2279.3.5.4 Heat Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2279.3.5.5 Conductive Pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2299.3.5.6 Interlocking Core with Air Channel . . . . . . . . . . . . . . . . . . . . . . 229

9.3.6 One-Sided Heat Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2309.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

10 Shrinkage andWarpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23310.1 The Shrinkage Analysis Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

10.1.1 Estimate Process Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23510.1.2 Model Compressibility Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23510.1.3 Assess Volumetric Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23710.1.4 Evaluate Isotropic Linear Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24110.1.5 Evaluate Anisotropic Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24210.1.6 Assess Shrinkage Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24410.1.7 Establishing Final Shrinkage Recommendations . . . . . . . . . . . . . . . . . . 245

10.2 Shrinkage Analysis and Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24710.2.1 Numerical Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24710.2.2 Steel SafeMold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24910.2.3 Processing Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

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10.2.4 Semi-Crystalline Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25110.2.5 Effect of Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

10.3 Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25210.3.1 Sources of Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25210.3.2 Warpage Avoidance Strategies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

10.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

11 Ejection SystemDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25911.1 Objectives in Ejection System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

11.1.1 Allow Mold to Open . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26111.1.2 Transmit Ejection Forces to Moldings. . . . . . . . . . . . . . . . . . . . . . . . . . . . 26211.1.3 Minimize Distortion of Moldings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26211.1.4 Actuate Quickly and Reliably . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26211.1.5 Minimize Cooling Interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26311.1.6 Minimize Impact on Part Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26411.1.7 Minimize Complexity and Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

11.2 The Ejector System Design Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26511.2.1 Identify Mold Parting Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26511.2.2 Estimate Ejection Forces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26511.2.3 Determine Ejector Push Area and Perimeter . . . . . . . . . . . . . . . . . . . . . . 26911.2.4 Specify Type, Number, and Size of Ejectors . . . . . . . . . . . . . . . . . . . . . . . 27111.2.5 Layout Ejectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27311.2.6 Detail Ejectors and Related Components . . . . . . . . . . . . . . . . . . . . . . . . . 276

11.3 Ejector System Analyses and Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27811.3.1 Ejector Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27811.3.2 Ejector Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28011.3.3 Ejector Sleeves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28211.3.4 Stripper Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28311.3.5 Elastic Deformation around Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . 28511.3.6 Core Pulls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28711.3.7 Slides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29111.3.8 Early Ejector Return Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29411.3.9 Advanced Ejection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

11.4 Chapter Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

12 Structural SystemDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29912.1 Objectives in Structural System Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

12.1.1 Minimize Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30012.1.2 Minimize Mold Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30412.1.3 Minimize Mold Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

12.2 Analysis and Design of Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30612.2.1 Plate Compression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30612.2.2 Plate Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30912.2.3 Support Pillars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

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12.2.4 Shear Stress in Side Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31712.2.5 Interlocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31912.2.6 Stress Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

12.3 Analysis and Design of Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32512.3.1 Axial Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32612.3.2 Compressive Hoop Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32712.3.3 Core Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

12.4 Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33212.4.1 Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33212.4.2 Socket Head Cap Screws. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33612.4.3 Dowels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

12.5 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

13 Mold Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34313.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34313.2 Coinjection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

13.2.1 Coinjection Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34513.2.2 Coinjection Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34613.2.3 Gas Assist/Water Assist Molding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

13.3 Insert Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35013.3.1 Low Pressure Compression Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35013.3.2 Insert Mold with Wall Temperature Control . . . . . . . . . . . . . . . . . . . . . . 35113.3.3 Lost Core Molding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

13.4 Injection Blow Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35513.4.1 Injection Blow Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35513.4.2 Multilayer Injection Blow Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

13.5 Multi-Shot Molds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35813.5.1 Overmolding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35913.5.2 Core-Back Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36013.5.3 Multi-Station Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362

13.6 Feed Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36413.6.1 Insulated Runner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36413.6.2 Stack Molds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36513.6.3 Branched Runners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36813.6.4 Dynamic Melt Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

13.7 MoldWall Temperature Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37213.7.1 Pulsed Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37213.7.2 Conduction Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37313.7.3 Induction Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37513.7.4 Managed Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

13.8 In-Mold Labeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37813.8.1 Statically Charged Film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37913.8.2 Indexed Film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

13.9 Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

Contents

XIV

13.9.1 Split Cavity Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38113.9.2 Collapsible Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38313.9.3 Rotating Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38413.9.4 Reverse Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387

13.10 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389Appendix A: Plastic Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390Appendix B: Mold Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394

B.1 Non-Ferrous Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394B.2 Common Mold Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395B.3 Other Mold Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396B.4 Notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

Appendix C: Properties of Coolants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398Appendix D: Statistical Labor Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

D.1 United States Labor Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399D.2 International Labor Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399D.3 Trends in International Manufacturing Costs . . . . . . . . . . . . . . . . . . . . . 401

Appendix E: Unit Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402E.1 Length Conversions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403E.2 Mass/Force Conversions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403E.3 Pressure Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403E.4 Flow Rate Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404E.5 Viscosity Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404E.6 Energy Conversions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404

Appendix F: Advanced Derivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

Contents

Nomenclature

Mold engineering requires analysis, and so an extensive nomenclature has been developed.L,W, and H refer to the length, width, and height dimensions as shown in Figure 1.

Figure 1: Length,width, and height nomenclature

Variable names have been selected and consistently used as expected (e.g.,T for temperature,C for cost, P for pressure, etc.). R refers to rate-related constants (with time dependence)and refers to monetary constants (with cost dependence). To provide clarity, subscripts areunabbreviated throughout most of the book. The nomenclature for many of the variablesand their units are as follows.

Table 1: Nomenclature

Variable Meaning

Thermal diffusivity [m2/s]

Compressibility [1/MPa]

Deflection [m]

bending Deflection due to bending [m]

compression Deflection due to compression [m]

total Deflection due to bending and compression [m]

Strain [m/m]

XVI Nomenclature

Variable Meaning

plastic Plastics strain to failure [%]

Shear rate [1/s]

max Maximum allowable shear rate for a plastic melt being molded [1/s]

Viscosity [Pa s]

Thermal conductivity [W/mC]

insert Cost per unit volume of core and cavity insert materials [$/m3]

mold Cost of mold metal per kilogram [$/kg]

plastic Cost of plastic per kilogram [$/kg]

Tolerance limit [m]

Apparent viscosity for Newtonian model [Pa]

static Coefficient of static friction []

Density [kg/m3]

insert Density of core and cavity insert materials [kg/m3]

plastic Density of plastic [kg/m3]

Draft angle []

Draft angle []

buckling Stress level at which column buckles [MPa]

cyclic Imposed cyclic stress [MPa]

endurance Maximum allowable stress given cyclic loading [MPa]

hoop Hoop stress [MPa]

limit Maximum allowable stress given cyclic loading or yielding [MPa]

yield Maximum allowable stress given yield failure [MPa]

Shear stress [Pa]

Specific volume []

ejectors Total perimeter of all ejectors [m]

Acavity_Projected Projected area of the mold cavity [m2]

Acompression Area exposed to compressive stress [m2]

Aeff Effective area under stress [m2]

Aejectors Total area of all ejectors [m2]

Apart Total surface area of the molded part

Ashear Area exposed to shear stress [m2]

C Tolerance coefficient for a standard fit [m2/3]

Cauxiliaries Total cost of all auxiliaries [$]

XVIINomenclature

Variable Meaning

Cinserts Total cost of all cavities [$]

Cinsert_finishing Cost of finishing one set of core and cavity inserts [$]

Cinsert_machining Cost of machining one set of core and cavity inserts [$]

Cinsert_materials Cost of materials for one set of core and cavity inserts [$]

Cmold Total cost of purchasing mold [$]

Cmold/part Cost of purchasing mold amortized across total production quantity [$]

Cmold_base Total cost of mold base and modifications [$]

Cmold_customization Total cost of all customizations of mold base [$]

Cmold_steel Initial purchase cost of mold base or steel [$]

Cpart Total cost per molded part [$]

Cplastic/part Cost of material used in molding one part [$]

Cprocess/part Cost of machinery and labor used to mold one part [$]

CPplastic Plastics specific heat [J/kg C]

CTE Coefficient of thermal expansion [1/C]

CVTE Coefficient of volumetric thermal expansion [1/C]

D Diameter [m]

Dhydraulic Hydraulic diameter of runner segment [m]

Dpin Diameter of ejector pin [m]

E Elastic modulus [GPa]

f Factory of safety []

fcavity_complexity Factor related to the complexity of the cavity

fcavity_discount Discount factor related to production of multiple sets of core and cavity inserts

fcycle_efficiency Factor associated with the efficiency of operating the molding machine

fefficiency Factor related to the overall efficiency of all machining operations

ffeed_waste Factor associated with material wasted in molding the feed system

f icavity_customizing Factor associated with customization of one set of core and cavity inserts

f ifinishing Percentage of the molded parts surface area to be finished in the manner i

f imold_customizing Factor associated with customizing mold base

fmachine Factor associated with cost of operating different molding machines andauxiliaries

fmachining Factor related to the average material removal rate of all machining processesrelative to standard milling

fmaintenance Mold lifetime maintenance factor

fwear Factor associated with maintenance due to mold wear

XVIII Nomenclature

Variable Meaning

fyield Fraction of molded parts that are good

F Force [N]

Fbuckling Critical load at which column buckles [N]

Fclamp Mold force tonnage [metric tons, t]

Feject Ejection force [N]

Finsertion Insertion force for interference fit [N]

Ftensile Maximum tensile force for a socket head cap screw [N]

h Nominal cavity wall thickness [m]

h Heat transfer coefficient [W/C]

Hcavity Height of cavity inserts [m]

Hcore Height of core inserts [m]

Hinserts Combined height of core and cavity inserts [m]

HLine Distance from cavity surface to the center of cooling line [m]

Hmold Total stack height of mold [m]

Hpart Maximum height of molded part [m]

I Moment of inertia [m4]

K Stress concentration factor []

k, n Reference viscosity and power law index per the power law model [Pan, ]

kplastic Plastics thermal conductivity [W/m C]

Linserts Length of core and cavity inserts [m]

Lmold Length of mold [m]

Lpart Maximum length of molded part [m]

MFI Plastics melt flow index [g/min]

Mmold Total mass of mold base [kg]

ncycles Number of molding cycles []

n, *,D1,D2,D3,A1, A3

WLF model coefficients

ncavities Number of cavities in mold []

ncavities_length Number of cavities in the length direction []

ncavities_width Number of cavities in the width direction []

ncycles Total number of mold cycles that a mold is operated []

nj Number of j-th portions of mold cavity in mold []

nlines Number of cooling lines []

XIX

Variable Meaning

nparts Total production quantity of parts to be molded []

P Pressure [Pa]

Pinject Pressure required to fill the cavity [Pa]

Qmolding Total thermal energy of moldings [J]

lineQ Cooling power per cooling line [W]

moldingQ Cooling power [W]

rv Relative change in the specific volume []

R Radius [m]

Re Reynolds number []

Rfinishing_cost Hourly cost of finishing [$/h]

Rifinishing Rate of finishing the parts surface in the manner i [m2/h]

Rmachining_cost Hourly rate of machining [$/h]

Rmachining_area Rate of machining per unit area [m2/h]

Rmachining_volume Rate of machining per unit volume [m3/h]

Rmolding_cost Hourly cost of operating molding machine and operator if required [$/h]

RW Radius of curvature due to warpage [m]

s Linear shrinkage rate [m/m]

s Shrinkage rate perpendicular to flow [m/m]

s// Shrinkage rate parallel to flow [m/m]

save Average shrinkage rate [m/m]

tc Cooling time [s]

tcycle Cycle time of molding machine [s]

tinsert_area Time required to machine the cavity surface area for one set of core and cavityinserts [h]

tinsert_finishing Time required to completely finish one set of core and cavity inserts [h]

tinsert_machining Time required to perform all machining for one set of core and cavity inserts [h]

tinsert_volume Time required to machine the cavity volume for one set of core and cavity inserts[h]

tp Packing time before gate solidification [s]

tresidence Residence time of the polymer melt [s]

Tc Mold coolant temperature [C]

Te Plastics ejection temperature [C]

Tg Plastics glass transition temperature [C]

Nomenclature

XX

Variable Meaning

THDT Plastics heat deflection temperature [C]

Tmelt Melt temperature [C]

Twall Mold wall temperature [C]

v Linear melt velocity [m/s]

Vinserts Combined volume of one set of core and cavity inserts [m3]

Vj Volume of the j-th portion of mold cavity [m3]

Vpart Volume of molded part [m3]

V Volumetric flow rate [m3/s]

Wcavity Width of core and cavity inserts [m]

Wcheek Distance from cavity side wall to side of mold [m]

Wmold Width of mold [m]

Wpart Maximum width of molded part [m]

Wpitch Distance between parallel cooling lines [m]

Nomenclature

1 Introduction

Injection molding is a fantastic process, capable of economically making extremely complexparts to tight tolerances. Before any parts can be molded, however, a suitable injection moldmust be designed, manufactured, and commissioned. The injection mold is itself a verycomplex system comprised of multiple components that are subjected to many cycles oftemperatures and stresses.

Engineers should design injectionmolds that arefit for purpose,whichmeans that themoldshould produce parts of maximal quality atminimal cost while taking aminimumamount oftime and money to develop. Accordingly, this chapter proceeds as follows. First, an overviewof the injection molding process is provided so that the mold design engineer can estimatethe operating conditions of the mold during mold design. Next, the layout and componentsin a few of the mold common mold designs are presented; this book assumes that the molddesign engineer is familiar with both injectionmolding and the structure and basic functionof these molds. Finally, the mold engineering design methodology is discussed.

1.1 Overview of the InjectionMolding Process

An operating injectionmoldingmachine is depicted in Figure 1.1. Injectionmolding is calleda net shapemanufacturing process because it forces the polymermelt into an evacuatedmoldcavity, after which it cools to the final desired shape.

While molding processes can differ substantially in design and operation, most injectionmolding processes generally include plastication, injection, packing, cooling, and moldresetting stages. During the plastication stage, the polymer melt is plasticized from solidgranules or pellets through the combined affect of heat conduction from the heated barreland the internal viscous heating caused by molecular deformation with the rotation of aninternal screw. During the filling stage, the polymer melt is forced from the barrel of themolding machine and into the mold. The molten resin travels down a feed system, throughone ormore gates, and throughout one ormoremold cavities where it will form one ormoredesired products.

After the mold cavity is filled with the polymer melt, the packing stage provides additionalmaterial into the mold cavity as the molten plastic melt cools and contracts. The plasticsvolumetric shrinkage varies with the material properties and application requirements, butthe molding machine typically forces 1 to 10% additional melt into the mold cavity duringthe packing stage.After the polymermelt ceases to flow, the cooling stage provides additionaltime for the resin in the cavity to solidify and become sufficiently rigid for ejection. Then, themoldingmachine actuates the necessary cores, slides, and pins to open the mold and removethe molded part(s) during the mold resetting stage.

2 1 Introduction

A chart plotting the approximate duration of the molding process is shown in Figure 1.2 foramolded part approximately 2 mm thick. The filling time is a small part of the cycle and so isoften optimized tominimize the injection pressure andmolded-in stresses. The packing timeis of moderate duration, and is often minimized through shot weight stability studies to endwith freeze-off of the polymer melt in the gate. In general, the cooling stage of the moldingprocess dominates the cycle time since the rate of heat flow from the polymer melt to thecolder mold steel is limited by the low thermal diffusivity of the plastic melt. However, the

Figure 1.1: Depiction of the injection molding process

Filling

Packing

Cooling

Plastication

Mold opening

Part ejection

Mold closing

0 10 20 30Time (s)

Filling

Packing

Cooling

Plastication

Mold opening

Part ejection

Mold closing

0 10 20 30Time (s)

Figure 1.2: Injection molding process timings

3

plastication timemay exceed the cooling time for very large shot volumeswith lowplasticationrates. Mold reset time is also very important to minimize since it provides negligible addedvalue to the molded product. To minimize the molding cycle time and costs, molders striveto operate fully automatic processes with minimum mold opening and ejector strokes.

Variants of the molding process (such as gas assist molding, water assist molding, insertmolding, two shotmolding, coinjectionmolding, injection compressionmolding, and othersdiscussed inChapter 13) are used to provide significant product differentiationwith respect topart properties, butmay increase risk and limit the number of qualified suppliers. In any case,the molding processes are generally similar in that each includes the injection, cooling, andejection of the plastic part. The cost estimation and mold design of these different processesis also very similar; significant differences in the mold design and molding processes will belater discussed.

1.2 Mold Functions

The injection mold is a complex system that must simultaneously meet many demandsimposed by the injection molding process. The primary function of the mold is to containthe polymer melt within the mold cavity so that the mold cavity can be completely filled toform a plastic component whose shape replicates themold cavity.A second primary functionof the mold is to efficiently transfer heat from the hot polymer melt to the cooler mold steel,such that injection molded products may be produced as uniformly and economically aspossible. A third primary function of the mold is to eject the part from the mold in a rapidbut repeatable manner, so that subsequent moldings may be produced efficiently.

These three primary functions contain the melt, transfer the heat, and eject the moldedpart(s) also place secondary requirements on the injectionmold.Figure 1.3 provides a partialhierarchy of the functions of an injection mold. For example, the function of containing themelt within the mold requires that

the mold resist the enormous forces that will tend to cause the mold to open or deflect,and

the mold contain a feed system connecting the nozzle of the molding machine to one ormore cavities in the mold for the transfer of the polymer melt.

These secondary functions may also give rise to tertiary functions that are fulfilled with theuse of specific mold components or features.

It should be understood that Figure 1.3 does not provide a comprehensive list of all functionsof an injection mold, but just some of the essential primary and secondary functions thatmust be considered during the engineering design of injection molds. Even so, a skilleddesigner might recognize that conflicting requirements are placed on the mold design byvarious functions. For instance, the desire for efficient cooling may be satisfied by the use of

1.2 Mold Functions

4 1 Introduction

multiple, tightly spaced cooling lines that conform to the mold cavity. However, the need forpart removal may require the use of multiple ejector pins at locations that conflict with thedesired cooling line placement. It is up to themold designer to consider the relative importanceof the conflicting requirements, and ultimately deliver a mold design that is satisfactory. Thetendency among novice designers, when in doubt, is to over design. This tendency should beavoided since it tends to lead to large, costly, and inefficient molds.

1.3 Mold Structures

An injectionmold hasmany structures to accomplish the functions required by the injectionmolding process. Since there are many different types of molds, the structure of a simpletwo plate mold is first discussed. It is important for the mold designer to know the namesand functions of the mold components since later chapters will assume this knowledge.The design of these components and more complex molds will be analyzed and designed insubsequent chapters.

1.3.1 External View of Mold

An isometric view of a two platemold is provided in Figure 1.4. From this view, it is observedthat amold is constructed of a number of plates, bolted together with socket head cap screws.

Injection Mold

Core pulls

Many cooling lines

Contain melt Transfer heat Eject part(s)

Resist displacement

Guide melt

Open mold

Parting plane

Multiple interlocks

Feed system

Lead heat from part

Large support pillars

Thick plates

Flow leaders

Slides & lifters

Ejector pins

Remove part(s)

Robotic assist

Conductive inserts

Tight pitch & depth

Lead heat from mold

High coolant flow rate

Large diameter lines

Figure 1.3: Function hierarchy for injection molds

5

These plates commonly include the top clamp plate, the cavity insert retainer plate or Aplate, the core insert retainer plate or B plate, a support plate, and a rear clamp plate orejector housing. Some mold components are referred to with multiple names. For instance,the Aplate is sometimes referred to as the cavity insert retainer plate since this plate retainsthe cavity inserts. As another example, the ejector housing is sometimes referred to as therear clamp plate since it clamps to themoving platen located towards the rear of themoldingmachine.

This type of mold is called a two plate mold since it uses only two plates to contain thepolymermelt.Mold designsmay vary significantly while performing the same functions. Forexample, somemold designs integrate theBplate and the support plate into one extra thickplate. As another example, some mold designs may split up the ejector housing, which hasa U shaped profile to house the ejection mechanism and clamping slots, into a rear clampplate and tall rails (also known as risers). The use of an integrated ejector housing as shownin Figure 1.4 provides for a compact mold design, while the use of separate rear clamp plateand rails provides for greater design flexibility.

To hold the mold in the injection molding machine, toe clamps are inserted in slots milledin the top and rear clamp plates and bolted to the stationary and moving platens of the

Figure 1.4: View of a closed two-plate mold

1.3 Mold Structures

6 1 Introduction

moldingmachine.A locating ring, usually found at the center of themold, closelymates withan opening in the molding machines stationary platen to fully orient the mold. The use ofthe locating ring is necessary for at least two reasons. First, the inlet of the melt to the mold(at the sprue bushing) must mate with the outlet of the melt from the nozzle of the moldingmachine. Second, the ejector knockout bar(s) actuated from behind themoving platen of themoldingmachinemustmatewith the ejector systemof themold.Moldingmachine andmoldsuppliers have developed standard locating ring specifications to facilitate mold to machinecompatibility, with the most common locating ring diameter being 100 mm (4 in).

When the molding machines moving platen is actuated, all plates attached to the rear clampplates will be similarly actuated and cause the mold to separate at the parting plane. Whenthe mold is closed, guide pins and bushings are used to closely locate the A and the Bplates on separate sides of the parting plane, which is crucial to the primary mold functionof containing themelt. Improper construction of themold componentsmay cause improperalignment of the A and B plates, poor quality of the molded parts, and accelerated wearof the injection mold.

1.3.2 View of Mold during Part Ejection

Another isometric view of the mold is shown in Figure 1.5, oriented from left to right foroperation in a horizontal injection molding machine. The plastic melt has been injected andcooled in themold, such that themoldings are now ready for ejection.To perform ejection, themold is opened by at least the height of the moldings. Then, the ejector plate and associatedpins are moved forward to push the moldings off the core. From this view,many of the moldcomponents are observed including the B or core insert retainer plate, two different coreinserts, feed system, ejector pins, and the guide pins and bushings.

Thismold is called a twoplate, two cavity familymold.The termfamilymoldrefers to amoldin which multiple components, either in an assembly a mold in which multiple componentsof varying shapes and/or sizes are produced at the same time. The term two cavity refersto the fact that the mold has two cavities to produce two moldings in each molding cycle.Such multi-cavity molds are used to rapidly and economically produce high quantities ofmolded products. Molds with eight or more cavities are common. The number of moldcavities is a critical design decision that impacts the technology, cost, size, and complexityof the mold; a cost estimation method will be provided in Chapter 3 to provide a guidelinefor mold design.

In a multi-cavity mold, the cavities are placed across the parting plane to provide roombetween the mold cavities for the feed system, cooling lines, and other components. It isgenerally desired to place the mold cavities as close together as possible while not sacrificingother functions such as cooling, ejection, etc. This usually results in a smaller mold that isnot only less expensive, but is also easier for the molder and can be used in more moldingmachines. The number of mold cavities in a mold can be significantly increased by not onlyusing a larger mold, but also by using different types of molds such as a hot runner mold,three plate mold, or stack mold as later discussed.

7

1.3.3 Mold Section and Function

Figure 1.6 shows the top view of the mold, along with the view that would result if the moldwas physically cut along the section lineA-A and viewed in the direction of the arrows.Varioushatch patterns have been applied to different components to facilitate identification of thecomponents. It is very important to understand these components and how they interactwith each other and the molding process.

Consider now the stages of the molding process relative to the mold components. Duringthe filling stage, the polymermelt flows from the nozzle of themoldingmachine through theorifice of the sprue bushing. The melt flows down the length of the sprue bushing and intothe runners located on the parting plane.The flow then traverses across the parting plane andenters the mold cavities through small gates. The melt flow continues until all mold cavitiesare completely filled.

After the polymer melt flows to the end of the cavity, additional material is packed into thecavity at high pressure to compensate for volumetric shrinkage. Themold plates and supportpillars must be designed to resist deflection when subjected to high melt pressures. Theduration of the packing phase is controlled by the size and freeze-off of the gate between therunner and the cavity.During the packing and cooling stages, heat from the hot polymermelt

Figure 1.5: View of molding ejected from injection mold

1.3 Mold Structures

8 1 Introduction

Figure 1.6: Top and cross section views of a two-plate mold

9

is transferred to the coolant circulating in the cooling lines. The heat transfer properties ofthe mold components, together with the size and placement of the cooling lines, determinesthe rate of heat transfer and the cooling time required to solidify the plastic.

After the part has cooled, the molding machines moving platen is actuated and the movinghalf of the mold (consisting of the B plate, the core inserts the support plate, the ejectorhousing, and related components)moves away from the stationary half (consisting of the topclamp plate, the Aplate, the cavity inserts, and other components). Typically, the moldingsstay with the moving half since they have shrunken onto the core.

After themold opens, the ejector plate is pushed forward by themoldingmachine.The ejectorpins are driven forward and push themoldings off the core. Themoldingsmay then drop outof themold or be picked up by an operator or robot.Afterwards, the ejector plate is retractedand the mold closes to receive the melt during the next molding cycle.

1.4 Other CommonMold Types

A simple two-plate mold has been used to introduce the basic components and functions ofan injectionmold.About half of all molds closely follow this design, since the mold is simpleto design and economical to produce. However, the two-plate mold has many limitations,including:

restriction of the feed system route to the parting plane;

limited gating options from the feed system into the mold cavity or cavities;

restriction on the tight spacing of cavities;

additional forces imposed on the mold by the melt flowing through the feed system;

increased material waste incurred by the solidification of the melt in the feed system;and

increased cycle time related to the plastication and cooling of the melt in the feedsystem.

For these reasons, molding applications requiring high production quantities often do notuse two-plate mold designs, but may instead utilize mold designs that are more complex yetprovide for lower cost production of the molded components. Such designs include threeplate molds, hot runner molds, stack molds, and others. Three plate molds and hot runnermolds are the next most common types of injection molds, and so are next introduced.

1.4.1 Three Plate,Multi-Cavity Family Mold

The three plate mold is so named since it provides a third plate that floats between the moldcavities and the top clamp plate.

1.4 Other Common Mold Types

10 1 Introduction

Figure 1.7 shows a section of a three plate mold that is fully open with the moldings still onthe core inserts. As shown in Figure 1.7, the addition of the third plate provides a secondparting plane between the A plate assembly and the top clamp plate for the provision of afeed system. During molding, the plastic melt flows out the nozzle of the molding machine,down the sprue bushing, across the primaries, down the sprues, and into the mold cavities.The feed system then freezes in place with the moldings.

When the mold is opened, the molded cold runner will stay on the stripper plate due to theinclusion of sprue pullers that protrude into the primary runner. As the mold continues toopen, the stripper bolt connected to the B plate assembly will pull the A plate assemblyaway from the top clamp plate. Another set of stripper bolts will then pull the stripper plateaway from the top clamp plate, stripping the molded cold runner off the sprue pullers. Thestripper plate may then be actuated to force the moldings off the core.

The three plate mold eliminates two significant limitations of the two plate molds. First, thethree plate mold allows for primary and secondary runners to be located in a plane abovethe mold cavities so that the plastic melt in the cavities can be gated at any location. Suchgating flexibility is vital to improving the cost and quality of the moldings. Second, the threeplate mold provides for the automatic separation of the feed system from the mold cavities.Automatic degating facilitates the operation the molding machine with a fully automaticmolding cycle to reduce the cycle times.

Figure 1.7: Section of an open three plate mold

11

There are at least three significant potential issues with three plate molds, however. First andmost significantly, the cold runner is molded and ejected with each molding cycle. If thecold runner is large compared to the molded parts, then the molding of the cold runner mayincrease the material consumption and cycle time, thereby increasing the molded part cost.Second, the three plate mold requires additional plates and components for the formationand ejection of the cold runner, which increases the cost of the mold. Third, a large moldopening stroke is needed to eject the cold runner. The large mold opening height (from thetop of the top clamp plate to the back of the rear clamp) may be problematic and requirea molding machine with larger daylight between the machine platens than required for atwo plate or hot runner mold.

1.4.2 Hot Runner,Multi-Gated, Single Cavity Mold

Hot runner molds provide the benefits of three plate molds without their disadvantages,yet give rise to other issues. The term hot runner is used since the feed system remainsin a molten state throughout the entire molding cycle. As a result, the hot runner does notconsume any material or cycle time associated with conveying the melt from the moldingmachine to the mold cavities.

A section of a multi-gated, single cavity mold is provided in Figure 1.8. This mold contains asingle cavity,which is designed to produce the front housing orbezel for a laptop computer.The hot runner system includes a hot sprue bushing, a hot manifold, two hot runner nozzlesas well as heaters, cabling, and other components related for heating. The hot runner systemis carefully designed to minimize the heat transfer between the hot runner system and thesurrounding mold through the use of air gaps andminimal contact area. Like the three platemold design, the primary and secondary runners are routed in the manifold above the moldcavities to achieve flexibility in gating locations. Since the polymer melt stays molten, hotrunners can be designed to provide larger flow bores and excellent pressure transmissionfrom the molding machine to the mold cavities. As such, hot runner system can facilitate themolding of thinner parts with faster cycle times than either two plate or three plate molds,while also avoiding the scrap associated with cold runners.

During themolding process, thematerial injected from themachine nozzle into the hot spruebushing pushes the existing material in the hot runner system into the mold cavity. Whenthe mold cavities fill, the thermal gates are designed to solidify and prevent the leakage ofthe hot polymer melt from inside the hot runner system to the outside of the mold when themold is opened. The melt pressure developed inside the hot runner system will cause thesethermal gates to rupture at the start of the next molding cycle.

There aremany different hot runner and gating designs.While they providemany advantages,including gating flexibility, improved pressure transmission, reducedmaterial consumption,and increased molding productivity, there are also two significant disadvantages. First, hotrunner systems require added investment for the provision and control of temperature. Theadded investment can be a significant portion of the total mold cost, and not all moldershave the auxiliary equipment or expertise to operate and maintain hot runner molds. The

1.4 Other Common Mold Types

12 1 Introduction

second disadvantage of hot runner systems is extended change-over times associated withthe purging of the contained plastic melt. In short run production applications havingaesthetic requirements, the number of cycles required to start-up or change resins may beunacceptable.

1.4.3 Comparison

The type of feed system is a critical decision that is made early in the development of themold design. From amold designers perspective, the choice of feed system has a critical rolein the design of themold, the procurement of materials, and themoldmaking, assembly, andtesting processes. From themolders perspective, the choice of feed system largely determinesthe purchase cost, molding productivity, and operating cost of the mold.

Figure 1.8: Section of hot runner mold

13

Table 1.1 compares the different types of moldswith respect to several performancemeasures.In general, hot runner molds are excellent with respect to molding cycle performance, butpoor with respect to initial investment, start-up, and on-goingmaintenance. By comparison,two plate molds have lower costs, but provided limited in-cycle performance.

The evaluation of three platemolds in Table 1.1warrants some further discussion. Specifically,three plate molds do not provide as high a level of in-cycle performance compared to hotrunner molds, and at the same time have higher costs than two plate molds. For this reason,there has been a trend away from three plate molds with the penetration of lower cost hotrunner systems.

1.5 TheMold Development Process

Given that there is substantial interplay between the product design, mold design, and theinjectionmolding process, an iterativemold development process is frequently used as shownin Figure 1.9. To reduce the product development time, the product design and mold designare often performed concurrently. In fact, a product designer may receive a reasonable costestimate for a preliminary part design given only the parts overall dimensions, thickness,material, and production quantity. Given this information, the mold designer develops apreliminary mold design and provides a preliminary quote as discussed in Chapter 3. Thispreliminary quote requires the molder and mold maker to not only develop a rough molddesign but also to estimate important processing variables such as the required clamp tonnage,machine hourly rate, and cycle times.

Once a quote is accepted, the engineering design of themold can begin in earnest as indicatedby the listed steps on the right side of Figure 1.9. First, themold designer will layout themolddesign by specifying the type of mold, the number and position of the mold cavities, andthe size and thickness of the mold. Afterwards, each of the required sub-systems of the moldis designed, which sometimes requires the redesign of previously designed subsystems. Forexample, the placement of ejector(s) may require a redesign of the cooling system.

Table 1.1: Feed system comparison

Performance measure Two plate Three plate Hot runner

Gating flexibility Poor Excellent Excellent

Material consumption Good Poor Excellent

Cycle times Good Poor Excellent

Initial investment Excellent Good Poor

Start-up times Excellent Good Poor

Maintenance cost Excellent Good Poor

1.5 The Mold Development Process

14 1 Introduction

To reduce the development time, the mold base and other materials may be ordered andcustomized as the mold design is being fully detailed. Such concurrent engineering shouldnot be applied to fuzzy aspects of the design. However, many mold-makers do order themold base and plates upon confirmation of the order. As a result of concurrent engineeringpractices,mold development times are now typically measured in weeks rather thanmonths[1].Customers have traditionally placed a premiumonquickmold delivery, andmold-makershave traditionally charged more for faster service. With competition, however, customersare increasingly requiring guarantees on mold delivery and quality, with penalties applied tomissed delivery times or poor quality levels.

After themold is designed,machined, polished, and assembled,molding trials are performedto verify the basic functionality of the mold. If no significant deficiencies are present, themoldings are sampled and their quality assessed relative to specifications. Usually, the moldand molding process are sound but must be tweaked to improve the product quality andreduce the product cost. However, sometimes molds include fatal flaws that are not easilycorrectable and may necessitate the scrapping of the mold and a complete redesign.

Initial design

Develop preliminarymold design & quote

Project OK?

Layout design

Feed system design

Cooling system design

Ejector system design

Structural system design

Machining, polishing,assembly, & trials

Moldings OK?

Close project

No

No

Review part designand specifications

Figure 1.9: Amold development process

15

1.6 Chapter Review

After reading this chapter, you should understand:

The basic stages of the injection molding process,

The primary functions of an injection mold,

The most common types of injection molds(two plate, three plate, hot runner, single cavity, multi-cavity, and multi-gated mold),

The key components in an injection mold, and

The mold development process.

In the next chapter, the typical requirements of a molded part are described along withguidelines for design. Afterwards, the mold layout design and detailed design of the varioussystems of a mold are presented.

1.6 Chapter Review

2 Plastic Part Design

2.1 The Product Development Process

Mold design is one significant activity in a much larger product development process. Sinceproduct and mold design are inter-dependent, it is useful for both product and mold designengineers to understand the plastic part development process and the role of mold designand mold making. A typical product development process is presented in Figure 2.1, whichincludes different stages for product definition, product design, business and productiondevelopment, ramp-up, and launch.

Market analysis

Team assembled

Detailed design,performance analyses,rapid prototyping, &preliminary testing

Production planning

Approval?

Tooling fabrication

Concept development Sales forecasting Budgeting

Design for assembly

Fits & tolerances

Design for manufacturing

Geometry & material

Approval?

Production release

Business development

Alpha test

Pilot production

Approval?

Quality & training plans

Beta install & test

Approval?

Mold

quoting

Mold

mak

ing

Product

definition

Product

des

ign

Dev

elopmen

tSca

le-u

pLa

unch

Mold

des

ign

Figure 2.1: A product development process

18 2 Plastic Part Design

While there are many product development processes, most share two critical attributes:

a structured development plan to ensure tracking and completeness of the design andmanufacturing, and

a toll-gate process tomitigate risk by allocating larger budgets only after significant reviewsat project milestones.

The product development process shown in Figure 2.1 is split into multiple stages separatedby approval toll-gates. An overview of each stage is next provided.

2.1.1 Product Definition

The product development process frequently begins with an analysis of the market, bench-marking of competitors, definition of the product specifications, and assessment of potentialprofitability. If management agrees that a new product is to be developed, then an appropriateteam is assembled to perform the early concept design and business development.During thisfirst stage, the approximate size, properties, and cost of the product are estimated. Sketches,mock-ups, and prototypes may be produced to communicate and assess the viability of theconcept.

With respect to profitability, market studies during the early product development stagewill strive to predict the potential sales at varying price points. At the same time, manpowerand project cost estimates will establish the budget required to develop and bring theproduct to market. A management review of the concept design, sales forecast, and budgetis usually performed to assess the likelihood of the commercial success of continued productdevelopment. At this time, the proposed product development project may be declined,shelved, or modified accordingly.

2.1.2 Product Design

If the project is approved and a budget is allocated, then the product development processcontinues, usually with additional resources to perform further analysis and design. Duringthis second stage, each component in the product is designed in detail. The design of plasticcomponents may include the consideration of aesthetic, structural, thermal,manufacturing,and other issues.Design formanufacturingmethodsmay be used to identify issues that wouldinhibit the effective manufacturing of the components. Design for assembly methods maybe used to reduce the number of components, specify tolerances on critical dimensions, andensure the economic assembly of the finished product.

The outcome of the initial product design stage (through the second management approvalin Figure 2.1) is a detailed and validated product design. The term detailed design impliesthat every component is fully specified with respect to material, geometric form, surfacefinish, tolerances, supplier, and cost. If a custom plastic component is required, then quotesfor these molded parts are often requested during this stage. These costs are presented to

19

management along with the detailed design for approval. If the product design and costs areacceptable, then the required budget is allocated and the product development now focuseson manufacturing.

2.1.3 Business and Production Development

While mold design and mold making are a focus of those in the plastics industry, all theseactivities are encompassed by the single activity titled Tooling Fabrication in Figure 2.1. Atthe same time, important business development and production planning is being performed.Specifically, business development is required to fully define the supply chain and establishinitial orders to support the product launch.Production planning is required to layout assem-bly lines, define manpower requirements, and develop the manufacturing infrastructure.

When the mold tooling is completed,alphaparts are produced, tested, and assembled. Theresulting alpha product undergoes a battery of tests to verify performance levels, regulatorycompliance, and user satisfaction. If the assembled alpha product is not satisfactory, then themanufacturing processes, associated tooling, and detailed component designs are adjustedas appropriate. Concurrently, the operations staff develops detailed plans governing qualitycontrol and worker training.

2.1.4 Scale-Up and Launch

Amanagement review is often used to verify that the developed product designs and produc-tion plans are satisfactory. If so, a pilot production runmay be used tomanufacture amoderatequantity of products according to the standardmanufacturing conditions.Themanufacturedbeta products are frequently provided to the marketing department, sales force, and keycustomers to ensure product acceptability. As before, the design and manufacturing of theproductmay be revised to address significant issues.When all stakeholders (marketing, sales,manufacturing, critical suppliers, and critical customers) are ready, the pilot productionprocesses are ramped up to build an initial inventory of the product after which the productis released for sale.

2.1.5 Role of Mold Design

Mold quoting, mold design, and mold making support the larger product developmentprocess. Requests for mold and/or part cost quotes are usually made towards the end of theconcept design stage or near the beginning of the detailed design stage. It is somewhat unusualfor themolder or themold-maker to be given fully detailed designs at this time, since 1)muchof the mold design could have been performed concurrently with a less developed productdesign, and 2) the mold engineering process may suggest significant changes in the designrelated to manufacturability or part performance.

2.1 The Product Development Process

20 2 Plastic Part Design

Themold development process (first introduced in Figure 1.9) often beginswith a preliminarydesign that is lacking in detail and would result in an unsatisfactory product if used directly.The critical part design information required to begin a mold design includes just the partsize, wall thickness, and expected production quantity. Given just this information, the molddesigner can develop initial mold layouts, cost estimates, and product design improvements.To accelerate the product development process,mold design can be performed concurrentlywith the procurement and customization of the mold components.

For better or for worse,moldmaking and commissioning occurs near the end of the productdevelopment process. For this reason, there can be significant pressure on mold suppliersandmolders to provide high quality moldings as soon as possible. This task can be extremelychallenging given potentialmistakesmade earlier in the product design process.As such,molddesignersmay be required to redesign and change portions of themold andwork closely withmolders to qualify the mold for production.

2.2 Design Requirements

There aremany requirements of an injectionmolded part which need to be considered duringthe mold design. The following sections provide some useful tables to gather the requiredinformation, along with some relevant discussion.

The information in this section is largely motivated for two reasons. First, detailed andavailable documentationwill improve the design and reduce the cost of themold engineering.Second, ISO and other regulatory agencies often require formal documentation and approvalof product development. Accordingly, the mold design en