Herbert Rees Bruce Catoen Selecting Injection Molds Weighing Cost vs Productivity Hanser Publishers, Munich • Hanser Gardner Publications, Cincinnati IV The Authors: Herbert Rees, 248386-5 Side Road (Mono), RR#5, Orangeville, Ontario, Kanada L9W 2Z2 Bruce Catoen , 21 Hamilton Crescent, Ontario, Kanada L7G 5J4 Distributed in the USA and in Canada by Hanser Gardner Publications, Inc 6915 Valley Avenue, Cincinnati, Ohio 45244-3029, USA Fax: (513) 527-8801 Phone: (513) 527-8977 or 1-800-950-8977 www.hansergardner.com Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20, 81631 München, Germany Fax: +49 (89) 98 48 09 www.hanser.de The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made The publisher makes no warranty, express or implied, with respect to the material contained herein Library of Congress Cataloging-in-Publication Data Rees, Herbert, 1915Selecting injection molds : weighing cost versus productivity / Herbert Rees, Bruce Catoen p cm ISBN-10: 1-56990-389-1 (hardcover) ISBN-13: 978-1-56990-389-6 Injection molding of plastics Injection molding of plastics—Equipment and supplies I Catoen, Bruce II Title TP1150.R446 2005 668.4‘12—dc22 2005023027 Bibliografische Information Der Deutschen Bibliothek: Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über abrufbar ISBN-10: 3-446-40308-6 ISBN-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, including photocopying or by any information storage and retrieval system, without permission in wirting from the publisher © Carl Hanser Verlag, Munich 2006 Production Management: Oswald Immel Typeset by Manuela Treindl, Laaber, Germany Coverconcept: Marc Müller-Bremer, Rebranding, München, Germany Coverdesign: MCP • Susanne Kraus GbR, Holzkirchen, Germany Printed and bound by Appl, Germany V 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 of Thermosets, and then worked through the gradual shift to Injection Molding, after the Second World War In 1985, I was asked by a Canadian non-governmental organization that supplies technical assistance to industries in developing countries to join them At their request, I traveled to countries in East Asia, North Africa, South and Central America, and worked with a number molders and mold designers of small and medium sized operations in the plastics industry The time spent there 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 easily understood technical (theoretical and practical) education As a result, I started putting my thoughts and experience first into a book “Understanding injection Molding technology” (1988) and followed it up by other books on Injection Mold Design and Engineering, as well as on Product Design for Injection Molding But still missing was an easily understood book 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 Every mold designer will have his or her own approach to the design of a new mold, and there are many different ways a mold can 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 ask must 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 which features a mold should have (Number of cavities, methods of injection, type of runners, methods of gating, methods of ejection, 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 the details” applies clearly to molds, and the effect of many such details are illustrated and discussed “Productivity and Cost of Injection Molds” is not a design manual, although there are a number of suggestions for the mold and product designers how to select certain design features to build the most suitable mold for the job The authors highlight some of the critical decision areas for the construction and the operating details for the most economical mold for the job on hand in an easily understood 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 mold to design and build, (or to purchase, if the mold is to be built elsewhere,) how they should examine the product design and its 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 how even little details can significantly affect the mold cost, the cost of the product, and the productivity of the mold For the actual mold engineering process I have referred occasionally to my earlier books “Mold Engineering” (ME) and “Understanding Product Design for Injection Molding” (UPDIM.) VI Preface An event (from the early 1960s) will illustrate the importance of getting the right mold for the purpose A friend, starting up as a custom molder with a few small machines, came one day, and told me of a prospective customer, requiring 100,000 each of three very similar, simple, round containers, who had approached him with the request to quote single cavity molds, to be used on his 100 ton machines We quoted these molds at $3,000 each, (runnerless, fully automatic,) and estimated a cycle time of 10–12 s Based on these figures, the molder submitted a quotation to his customer who liked the price of the products, but objected to the “high” price of the molds He said he could get these molds for $1,000 each The molder, glad to get an order for his machines, accepted that the customer would supply the molds When they were delivered, they were of very poor quality, with a sprue to be cut, with only token cooling, and the mold ran no better than at 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 cheaper molds only $0.01, so there was little difference ($0.02) in mold costs per piece, but a huge difference in the machine hour costs In order not to lose his shirt on this deal, the molder thereupon asked us to supply the molds, and paid for them out of 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 order of $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 is expected from it The “cheap” molds would have been all right for very small requirements, but were very expensive for the expected production Bruce and I would like to thank all those companies that contributed illustrations and photos to the book We would also like to give special thanks to Elaine Lafontaine for her administrative assistance during the writing of this book H Rees