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SelectingInjection Molds
Herbert Rees
Bruce Catoen
Weighing Cost vs Productivity
Hanser Publishers, Munich Hanser Gardner Publications,
Cincinnati
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IV
The Authors:
Herbert Rees, 248386-5 Side Road (Mono), RR#5, Orangeville,
Ontario, Kanada L9W 2Z2Bruce Catoen , 21 Hamilton Crescent,
Ontario, Kanada L7G 5J4
Distributed in the USA and in Canada byHanser Gardner
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USAFax: (513) 527-8801Phone: (513) 527-8977 or
1-800-950-8977www.hansergardner.com
Distributed in all other countries byCarl Hanser VerlagPostfach
86 04 20, 81631 Mnchen, GermanyFax: +49 (89) 98 48
09www.hanser.de
The use of general descriptive names, trademarks, etc., in this
publication, even if the former are not especially identified, is
not to be taken as a
sign that such names, as understood by the Trade Marks and
Merchandise Marks Act, may accordingly be used freely by
anyone.While the advice and information in this book are believed
to be true and accurate at the date of going to press, neither the
authors nor the editorsnor the publisher can accept any legal
responsibility for any errors or omissions that may be made. The
publisher makes no warranty, express orimplied, with respect to the
material contained herein.
Library of Congress Cataloging-in-Publication Data
Rees, Herbert, 1915- Selecting injection molds : weighing cost
versus productivity / HerbertRees, Bruce Catoen. p. cm. ISBN-10:
1-56990-389-1 (hardcover) ISBN-13: 978-1-56990-389-61. Injection
molding of plastics. 2. Injection molding ofplasticsEquipment and
supplies. I. Catoen, Bruce. II. Title. TP1150.R446 2005 668.412dc22
2005023027
Bibliografische Information Der Deutschen Bibliothek:
Die Deutsche Bibliothek verzeichnet diese Publikation in der
Deutschen Nationalbibliografie;detaillierte bibliografische Daten
sind im Internet ber abrufbar.
ISBN-10: 3-446-40308-6ISBN-13: 978-3-446-40308-6
All rights reserved. No part of this book may be reproduced or
transmitted in any form or by any means, electronic or mechanical,
includingphotocopying or by any information storage and retrieval
system, without permission in wirting from the publisher.
Carl Hanser Verlag, Munich 2006Production Management: Oswald
ImmelTypeset by Manuela Treindl, Laaber, GermanyCoverconcept: Marc
Mller-Bremer, Rebranding, Mnchen, GermanyCoverdesign: MCP Susanne
Kraus GbR, Holzkirchen, GermanyPrinted and bound by Appl,
Germany
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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 ofThermosets, and then
worked through the gradual shift to Injection Molding, after the
Second World War. In 1985, I wasasked by a Canadian
non-governmental organization that supplies technical assistance to
industries in developing countriesto join them. At their request, I
traveled to countries in East Asia, North Africa, South and Central
America, and workedwith a number molders and mold designers of
small and medium sized operations in the plastics industry. The
time spentthere was very rewarding, and I was able to help them to
improve their designs, methods and, ultimately, their
productivity.
These experiences abroad, but also many previous events
throughout my career pointed out a general need for
easilyunderstood technical (theoretical and practical) education.
As a result, I started putting my thoughts and experience firstinto
a book Understanding injection Molding technology (1988) and
followed it up by other books on Injection MoldDesign and
Engineering, as well as on Product Design for Injection Molding.
But still missing was an easily understoodbook about the
relationship between Productivity, Production and Mold Costs.
I was fortunate that my friend Bruce Catoen, who joined Husky in
1987 as development engineer and who is, at this time,VP of
Packaging and Systems at Husky accepted my invitation to co-author
the book I had in mind. The purpose was,partly, that Bruce should
review what I had written so far, but mainly, to update it where
necessary and add the latest
developments, where they are germane to the subject of this
book.
Injection molds are always expensive to make, but unfortunately,
without a mold there cannot be a molded product. Everymold designer
will have his or her own approach to the design of a new mold, and
there are many different ways a moldcan be designed and built.
A frequently asked question is then how to get the lowest cost
mold. But this is the wrong question. The question to askmust
always be:
How can I get the best molded product at the lowest cost, for
the expected production?
Whenever talking with molders, mold makers and mold designers I
have been asked many times how to decide whichfeatures a mold
should have. (Number of cavities, methods of injection, type of
runners, methods of gating, methods ofejection, machine selection,
etc). I have also been frequently asked how one can reasonably
estimate the mold cost.
As will be shown, mold cost, mold quality and cost of product
are inseparable. The often-quoted saying: The devil is in
thedetailsapplies clearly to molds, and the effect of many such
details are illustrated and discussed. Productivity and Cost
ofInjection Molds is not a design manual, although there are a
number of suggestions for the mold and product designershow to
select certain design features to build the most suitable mold for
the job. The authors highlight some of the criticaldecision areas
for the construction and the operating details for the most
economical mold for the job on hand in an easilyunderstood
language, with a minimum of theory or complicated formulae.
The book tries to explain to the decision makers, i.e. the
persons given the responsibility of deciding what kind of moldto
design and build, (or to purchase, if the mold is to be built
elsewhere,) how they should examine the product design andits
specifications, and to highlight the significance of some of the
features of the product design on the expected productivity.Such
examinations often result in suggestions for practical product
design changes that will make it easier to build the
best-suited mold at the lowest cost. I have used some examples
of molds I have been involved with, and tried to show howeven
little details can significantly affect the mold cost, the cost of
the product, and the productivity of the mold. For theactual mold
engineering process I have referred occasionally to my earlier
books Mold Engineering (ME) andUnderstanding Product Design for
Injection Molding (UPDIM.)
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VI
An event (from the early 1960s) will illustrate the importance
of getting the right mold for the purpose. A friend, startingup as
a custom molder with a few small machines, came one day, and told
me of a prospective customer, requiring 100,000each of three very
similar, simple, round containers, who had approached him with the
request to quote 3 single cavitymolds, to be used on his 100 ton
machines. We quoted these molds at $3,000 each, (runnerless, fully
automatic,) andestimated a cycle time of 1012 s. Based on these
figures, the molder submitted a quotation to his customer who liked
theprice of the products, but objected to the high price of the
molds. He said he could get these molds for $1,000 each. Themolder,
glad to get an order for his machines, accepted that the customer
would supply the molds. When they weredelivered, they were of very
poor quality, with a sprue to be cut, with only token cooling, and
the mold ran no better thanat a 60 s cycle, or 60 pieces per hour,
also, it needed an operator to cut the sprue and to scrape flash
where the stripper
joined the core. The molder had based his pricing on a 12 s
cycle, or 300 pieces per hour. At a machine hour cost of
$25.00,this would be $0.08 per piece machine hour time. However,
the machine hour cost with the supplied molds would be
$0.42 per piece. The mold cost per piece based on our proposal
was $0.03, for the 100,000 pieces, and with the cheapermolds only
$0.01, so there was little difference ($0.02) in mold costs per
piece, but a huge difference in the machine hourcosts. In order not
to lose his shirt on this deal, the molder thereupon asked us to
supply the molds, and paid for them outof his own pocket. He would
have lost 0.42 0.08 = $0.34 per piece shipped, and his total loss
would have been in the orderof $100,000.00 for machine time and the
unforeseen labor!
The example above shows that a mold is not just a mold! When
ordering a mold it must be clearly specified what isexpected from
it. The cheap molds would have been all right for very small
requirements, but were very expensive for theexpected
production.
Bruce and I would like to thank all those companies that
contributed illustrations and photos to the book. We would alsolike
to give special thanks to Elaine Lafontaine for her administrative
assistance during the writing of this book.
H. Rees
Preface
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VII
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . V
1 Introduction and Planning . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 11.2 Oversimplification . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 3
1.2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 31.3 Is Injection Molding the Right Choice for this Product?
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 The
Injection Molding Machine . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.1 The Right Machine for the Mold . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.5 The
Injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.5.1 What Is an Injection Mold? . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5.2
Elements of an Injection Mold . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 9
2 The Plastic Product . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 112.1 The Product Design . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 122.2 Product Drawings . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 13
2.2.1 Product Shape: How Can the Product Best Be Molded? . . . .
. . . . . . . . . . . . . . . . . . 132.2.2 Parting Line (P/L) . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 13
2.2.3 Side Cores . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 152.3 Accuracy and Tolerances Required . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
182.3.1 General and Specific Tolerances . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.2 Are
Special Fits with Matching Products Required? . . . . . . . . . . .
. . . . . . . . . . . . . . 212.3.3 Tolerances for the Filling
Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 212.3.4 Stacking of Products and Free
Dispensing . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 222.3.5 Mismatch (Deliberate) . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 23
2.4 Tolerances, Mold Alignment, and Mold Costs . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5 Heat
Expansion, Alignment, and Mold Cost . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 282.6 Surface Finish . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.6.1 Finish of Molding Surfaces . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.6.2
Texturing of Surfaces . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.6.3
Fitting Surfaces of Mold Parts . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 31
2.7 Engravings . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 31
2.7.1 Engravings Versus Applied Labels . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 312.7.2
Two-Color and Two-Material Engraving . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 322.7.3 Depth of Engravings .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 32
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VIII Contents
2.7.4 Font Style and Size of Artwork . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.7.5
Polarity of Engraving . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.7.6 Are the Locations Selected for Engraving Practical? . . .
. . . . . . . . . . . . . . . . . . . . . . 342.7.7 Engravings in
the Walls and Bottoms of Products . . . . . . . . . . . . . . . . .
. . . . . . . . . 34
2.8 General Appearance of the Product . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
372.8.1 Flatness . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 372.8.2 Sinks and Voids . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 392.8.3 Witness Lines . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 402.8.4 Weld Lines . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 43
2.8.5 Surface Defects (Flow Marks, Splay, Record Grooves, Haze,
Jetting, Hooks,and Ripples) . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 43
2.8.6 Identification of the Molded Piece . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 442.9 Product
Strength Requirements . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 45
2.9.1 Gate Location to Increase Product Strength . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 462.10 Special
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
2.10.1 Holes and Counter Bores for Assembly Screws or Rivets . .
. . . . . . . . . . . . . . . . . . . 472.10.2 Hinges and Snaps for
Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 47
3 Cost Factors Affecting Productivity . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
493.1 Where Will the Mold Be Operated? . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1.1 Condition of Ambient (Shop) Air . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 Coolant
Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? .
. . . . . . . . . . . . . . . . . 513.2.2 Is the Cooling Water
Clean? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 513.3 Power Supply . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 523.4 Will the Mold Run in
a Variety of Machines or a Single Machine? . . . . . . . . . . . .
. . . . . . . 533.5 Is the Mold Planned to Run in a Newly Created
Operation? . . . . . . . . . . . . . . . . . . . . . . . . 543.6
Projected Requirements . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3.6.1 Making Prototype or Experimental Molds . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 55
3.6.2 Production Molds . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
573.7 Forecasting the Cycle Time . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.7.1 Type of Plastic Molded . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
603.7.2 Wall Thickness of Product . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.7.3
Mold Materials . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.7.4
Efficiency of Cooling . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.7.5
Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.7.6 Effect of Molding Machine on Cycle Time . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 673.7.7 Ejection .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 853.7.8
Ambient Temperatures and Humidity . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 94
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3.7.9 Comparing Molding Cycles of the Same Product in New Molds
. . . . . . . . . . . . . . 943.8 Number of Cavities Required . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 95
3.8.1 Available Operating Time . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 953.8.2
The Minimum Number of Cavities . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 963.8.3 Machine Hour Cost
per Unit Molded . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 973.8.4 Mold Cost per Unit Molded . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 993.8.5 Calculation of the Required Clamp Size . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003.8.6
Shot Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1023.8.7 Plasticizing Capacity . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
102
3.8.8 Preferred (Practical) Number of Cavities . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 1023.8.9 Business
Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 1043.8.10 Preliminary
Estimate of Product Cost . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 105
4 Mold Selection . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 1134.1 Selection of an Appropriate Mold . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 113
4.1.1 Dedicated Mold, Universal Mold Shoe . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 113
4.1.2 One-Product Molds or Family Molds? . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 1144.1.3 Where to Gate . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 1164.1.4 Gate Size and Runner
Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 1234.1.5 Hot Runner Molds . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 1254.1.6 Single Cavity Molds . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 1294.1.7 Two and More Cavities, Cold or Hot Runner
Molds . . . . . . . . . . . . . . . . . . . . . . . 1354.1.8
Single- or Multi-Level Molds? . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 1424.1.9 Semi or
Fully Automatic Operation? . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 1544.1.10 Insert Molding . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 155
4.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 158
5 Mold Cost, Mold Price and Delivery . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1595.1 Mold Cost and Records . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
159
5.1.1 Spare Parts for the Mold . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1605.1.2 The Basic Elements of the Mold Cost . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 1615.1.3 Cost of
the Preparation of a Mold Manual . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 170
5.2 Overhead . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 1705.3 Mold Price . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 171
5.3.1 Risk Factor . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 1735.4 Mold Cost Is Absorbed by the Molder . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
5.5 Arriving at Mold Cost and Delivery Time . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1755.5.1
Calculating the Mold Cost . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 1765.5.2 Estimating
the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 176
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X Contents
5.5.3 Guesstimating the Mold Cost . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 1805.5.4 Ball
Parking the Mold Price . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 180
5.5.5 Mold Price Catalogue . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1805.6
The Quotation . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
181
5.6.1 Delivery Time . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1815.6.2 Confirmation . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
182
5.7 In-House Mold Making Operations . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
6 Warranties, Patents, and Ethical Considerations . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
6.1 Warranties and Guaranties . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1856.1.1 Guaranteed Quality . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1856.1.2 Guaranteed Shrinkage . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1856.1.3 Guaranteed Cycle Time . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1866.1.4 Guaranteed Delivery . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
186
6.2 Patents and Ethical Considerations . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1866.2.1 Patents . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 187
6.2.2 Ethical Considerations . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
References . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 191
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 193Appendix 1: Suggested Contents of a Mold
Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 193Appendix 2: Mold Set-up Guide Blank . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
194Appendix 3: Example of Light-Weighting a Product and Increasing
Productivity . . . . . . . . 197Appendix 4: Buying a Mold . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 199Appendix 5: Suggested Format of a
Confirmation of Order . . . . . . . . . . . . . . . . . . . . . . .
. . . 204Appendix 6: Molding Properties of Injection-Grade Plastics
. . . . . . . . . . . . . . . . . . . . . . . . . 208Appendix 7:
General Properties . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 210Appendix 8:
Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 212
Appendix 9: Thermal Properties . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
216Appendix 10: Typical Mold Materials . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
220Appendix 11: What Characterizes a Good, High-production Mold? .
. . . . . . . . . . . . . . . . . . . 222Appendix 12: Advice for
the Mold Designer . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 225Appendix 13: Surface Finishes . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 228
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 229
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191
References
[1] Rees, Herbert, Understanding Product Design for Injection
Molding(1996) Carl Hanser Publishers, Munich, Section 4.4.3.2.5
[2] Malloy, Robert A., Plastic Part Design for Injection
Molding, 1994, CarlHanser Publishers, Munich
[3] Tres, Paul A., Designing Plastic Parts for Assembly, 5thed.,
2003, CarlHanser Publishers, Munich
[4] Belofsky, Harold, Plastics: Product Design and Process
Engineering, 1995,Carl Hanser Publishers, Munich
[5] Rees, Herbert, Mold Engineering, 2nded. Chapter 8, 2002,
Carl HanserPublishers, Munich
[6] Rees, Herbert, Mold Engineering, 2nded. Chapter 14, 2002,
Carl HanserPublishers, Munich
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1
1 Introduction and Planning
1.1 Introduction
Today, injection molding is probably the most important method
of pro-cessing plastics in the production of consumer and
industrial goods, and isperformed everywhere in the world.
Once the decision has been made to use injection moldingfor a
new product,a number of difficult choices are ahead which will be
addressed later in moredetail:
Number of cavities
Stack design, which is the purely technical aspect of how to
mold the pro-duct. It is important to understand the design of that
portion of the moldthat is actually in contact with the plastic
(the stack), i.e., the cavity, core,and any other mold components,
which determine how the final product
will be shaped and how the plastic will enter the cavity space
Method of ejection, i.e., how the product will be ejected from the
mold
What machine should be used?
Automation will it be required?
With new, possibly difficult shapes, these decisions are usually
left to theingenuity of a mold designer. More
frequently,precedentsfrom earlier molds
are used and reapplied. However, the mold designer must be aware
of andevaluate new ideas, new methods, and developments, which when
appliedwould lead to better quality, higher productivity, simpler
molds, and savingsin the cost of the molded products.
After the design of the basic stackand beforeproceeding with any
mold design,the mold designer must understand what kind of mold
should be selected;in other words, which features will be most
suitable for the application toachieve the most economic
overallmanufacturing method for the product.This means not just to
specify the number of cavities that will be requiredfor the
expected output, but also the selection of mold materials and the
degreeof sophisticationof the mold. Any planned automation,
especially in producthandling after molding,can affect the mold
layout, particularly spacing andorientation of the stacks. The mold
designer must never lose sight of theultimate goal: The cost of
theproductmust be the lowest possible, while stillachieving
allspecified requirements. The most important information is toknow
beforehand the quantities to be molded, a piece of information,
particularly with newproducts, often very difficult to
obtain.
It should also be pointed out that of the total cost of almost
all plastic pro-ducts, the cost of the plastic material alone
constitutes the greatest component.
Mold designers must not be stuck ina comfortable rut
The ultimate goal of a mold is toproduce an acceptable
qualityproduct at the lowest possible cost
Often, the most important informa-tion is the most difficult to
obtain
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2 1 Introduction and Planning
The most sophisticated, best designed mold will not lower the
cost of theproduct by as much as the reduction of just a few
percent of the amount ofplastic material, if it could be removed
from the product without affecting
its quality or serviceability. Most often, unnecessarily heavy
wall thicknessand ribbing affects the cost more than anything else.
Chances are that thelowest weight will be achieved with the highest
quality molds.
In my long experience, I have had numerous occasions when the
client insistedon having his way. When I strongly believed it was
the wrong thing to do, Isuggested to them to have this mold built
somewhere else. Almost all cameback sooner or later for other
business, and acknowledged that they shouldhave listened to me.
The foremost intent of this book is to present various
alternatives availableto the mold designer or decision maker when
planning a mold for a newproduct or planning to increase the
productivity for a product for which amold exists. It raises many
of the questions that must be asked by anybodywho needs a mold
built. Some of these questions may appear obvious andnot worth
mentioning, or their pursuit may be thought a waste of time, butI
like to point out that any input could significantly affect the
productivity as
well as the cost of a mold. For an experienced mold designer,
the answers formany of these questions often come automatically,
without him or her beingaware of the fact that a decision has been
made. But even the most experiencedmold designer can gain important
information by systematically investigatingall areas that can
affect the design and the complexity of the mold and eventhe most
experienced designers overlook some obvious facts.
In this book an attempt has been made to explain why certain
mold featuresshould be selected, considering the planned
productivity and expected costs.
There will also be occasionally references to other books on
this subject, suchas Mold Engineering [5] and Understanding Product
Design for InjectionMolding [1].
Since in many mold shops the mold designer is also involved in
estimatingthe cost of the mold to be quoted, the book also intends
to discuss variousways of how to estimate mold costs. Properly
estimating mold cost is probablythe most difficult part of running
a successful mold making operation.Regardless of how well a shop is
equipped with machine tools and other
mold making equipment, and how high the level of experience is
of themachinists and mold technicians (mechanics), if the mold cost
is notadequately quoted it will be impossible to stay in business.
We must neverforget that the primary purpose of any business is to
make money, and thereis nothing easier than to lose money by poorly
estimating and quoting. Thereis no magic formula to estimate a mold
cost, but good understanding of theprinciples will lead to better
cost estimates.
We must not forget in dealing withthe customers who require a
newmold that it is not what they wantbut what they need
Figure 1.1 Typical mold-making factoryusing automated
equipment
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3
1.2 Oversimplification
In the early 1950s, when I was an R&D engineer at a large
electrical manu-facturer, I had just submitted a request of
appropriation for a mold for a newproduct when the vice-president
of sales stormed into my office, and said:Why do you always need so
much money for a mold? What is a mold? Isntit just an upper and a
lower half?
This was in the heydays of compressionmolding, beforethe
injection moldingtechnology gained importance dramatically. A
compression mold was exactlywhat the VP implied: a lower half with
one or more cavities, and an upper
half with the matching cores (see Fig. 1.2). The plastic was
hand-fedinto thelower (open) cavity; there was no or little
sophistication with heating (thesemolds were processing thermosets
and therefore needed to be heated, notcooled). Often, there was no
ejection mechanism at all, or it was relativelysimple.
Of course, what the VP failed to understand was the complexity
and accuracyof the work required to build the various components of
these halves, thestrength required to resist the high molding
forces, the time required for
machining and good polishing, and other features required for
even a simplemold. Unfortunately, even today, many years later,
this attitude of oversimpli-fication is frequently encountered when
discussing a required mold and itscost.
Since that time, thermosets (compression) molding has become
quitesophisticated, and is using injection molding technology
occasionally, but isstill mostly using the verticalmachine
arrangement, because of the originalloading method of the plastic
material by gravity. This was also the time
when injection molding took over the molding market from small
beginnings.But for a number of reasons it soon became more
convenient to use horizontalmachines, although today again, some
vertical injection molding machinesare used for certain
applications. But regardless of the type machine used,the most
important part of the molding system is still the mold.
1.2.1 DefinitionsBefore continuing, here is a list the various
terms used:
Product: an injection molded plastic piece
End product: an assembly, of which the product is a part
User (end user): persons using the product or end product
Customer: the person or company interested in buying the
injection mold
Mold maker: the person or company engaged in making injection
molds
Mold designer: the person responsible for designing the mold
1.2 Oversimplification
a
b
c
d
e
fg
h
a Upper platen (stationary or moving)b Heating platenc Upper
mold halfd Coree Cavityf Lower mold halfg Heating platen
h Lower platen (stationary or moving)
Figure 1.2 Schematic of a compressionmold for a plate
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4 1 Introduction and Planning
Product designer: the person responsible for designing the
product to bemolded
Molder: the person or company engaged in injection molding
plasticproducts
1.3 Is Injection Molding the Right Choice
for this Product?
Before proceeding, we must ask: why was injection molding
selected for thejob?
The molder may have a financial or other interest in preferring
to have theproduct made by injection molding, but we must keep an
open mind.
Have alternative methods or product designs been considered or
investigated,employing other manufacturing processes using the same
or a similarmaterials, or using other materials which may permit a
similar end product,
possibly even with better quality, and/or at lower cost?A few
typical examples of possible manufacturing alternatives for
injectionmolding:
Thermoforming
Foam molding
Coining and die stamping (blanking)
Extrusion blow molding
Machining, forming of sheets
Some other, maybe yet to be developed methods and materials
Another possibility is not to use plastics at all, but rather
use:
Paper (cardboard), wood, cloth
Metals (steel, aluminum, etc.)
Injection molding has many advantages, particularly low mass,
achievableaccuracy, good strength-to-weight ratio, good appearance
and surfacedefinition, and numerous specific physical properties.
But injection moldedproducts always suffer from the fact that the
initial capital outlay for moldsand machines can be very high.But
we must never forget that on a per unitbasis, especially whenever
large quantities are considered, the contribution
of the cost of the equipment (mold, machine, etc.) to the cost
of the productis small and often almost negligible.
Figure 1.3 Typical injection molded parts
The relatively high capital cost of amold is often almost
negligible whenevaluated on a per-molded-partbasis
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5
1.4 The Injection Molding Machine
We will not discuss the advantages and disadvantages of the
various injectionmolding machines that are on the market, but
rather introduce the readerwho is not too familiar with this
industry to the various terms that will beused from time to time if
a subject under discussion will have special referenceto a machine
element or feature. The accuracy of molding, and especiallywhen
molding so-called thin-walled products, is very dependent on
thequality of the molding machine, its mechanical rigidity,
accuracy of alignment,parallelism of platens, the quality of its
controls, and the state of maintenance.
Every good injection-molding machine consists of these basic
elements
1. A rigid base
2. A rigid clamping unit, consisting of two platens, for the
mounting of themold halves and provisions for guiding the platens
(tie bars or ways)
3. Provision for moving the platens, preferably fast, relative
to each other,for opening and closing the mold; the speed of motion
is usually
adjustable
4. Provision for clamping, i.e., holding the mold shut against
the force ofthe injection pressures within the mold (in some
machines, provisions 3and 4 are combined)
5. Provision for ejecting the molded product(s) from the
mold
6. Provision to transform the raw plastic (pellets, etc.) into
an injectablemelt (the plasticizing unit)
7. Provision for injecting the melt into the mold (in most
machines,provisions 6 and 7 are combined in one unit)
8. Provision for heating the plastic in the plasticizing
unit
9. Cycle controls (sequencing logic, timers, etc.) and a command
post formanual operation and for mold setup
10. Heat controls for all heaters in machine and molds. Some
machines have
a limited number of heat controls and additional controls could
berequired for the molds, especially with larger hot runner
systems. Thispoint must be considered when estimating the mold
cost.
11. Safety gates to protect operators and bystanders from all
hazards whenoperating the machine
12. Mechanical safety drop bar(s) to prevent closing the machine
when gatesare open, in case of failures of the other (electric and
hydraulic) safetymeasures.
13. Provision for cooling water distribution to the mold
14. Provision for compressed air, for auxiliary actions required
in the mold
Even the best machine if poorlymaintained will not perform as
itshould
1.4 The Injection Molding Machine
The mold designer who believes thatthe product considered could
bemade better by other methods has aduty to discuss this with
thecustomer, even if it could mean lostbusiness, this time
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6 1 Introduction and Planning
There are other features available, e.g., for the convenience of
quick moldinstallation, setting up and operation of the mold and
machine; these featuresare often offered as options which can be
bought with the machine or added
on later.
1.4.1 The Right Machine for the Mold
Often, the mold cost will surpass the cost of the machine. It
does not matterhow ell a mold is built if the machine cannot meet
the molding requirementsto produce quality products. While
considering the purchase of an injection
mold, it is always important to make sure the machine can do the
job. Someof the basic considerations are:
Tie bar spacing
Stroke and shut height
Injection speed (average and peak)
Available injection pressure
Recovery rate capability (throughput)
Platen rigidity (are the platens rigid/robust enough to carry
the moldweight?)
Available clamp tonnage
Platen parallelism
Clamp speed requirements
Shut-off nozzles
Screw design
Accuracy and repeatability of controls
Operator access
Mold protection capability
As the machine and mold act together as a system, it is fair to
say that thesystem will perform only as well as its weakest
component. If an existingmachine is to be used, the machine should
match the machine's capability.
The mismatched machine can easily destroy the new mold in a
matter ofmonths, resulting in costly rework.
To determine the right machine, the following information on the
mold isrequired:
Mold width, length, and height
Opening stroke required (usually 2.5 part height)
There is no point in buying apremium priced mold to run it in
anout-dated machine
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7
Ejector rod locations
Locating ring size
Part dimensions, including wall thickness
Flow length (length of flow from gate to longest flow path)
Part weight
Runner weight (if cold runner)
Cavitation
Nozzle radius
Material (including color and additives, viscosity)
1.5 The Injection Mold
To the customer or entrepreneur not familiar with the problems
of moldingand mold making who wants to make a new product, the
price of a moldmay seem to be high, occasionally even outrageous;
it is often difficult toconvey that the mold priceconstitutes only
a very small portion of the product(piece) cost, and depends much
on the expected production of the mold.
1.5.1 What Is an Injection Mold?
A (plastics) injection mold is a permanent tool, i.e., a tool
that, if properly
designed, constructed, and maintained will have a life
expectancy (usefullife) well beyond the time where the product
itself becomes obsolete. Thisdifferentiates it from a one-time use
mold such as a sand-casting mold, asused in metal foundries. A mold
can be used to make products in a virtuallyinfinite variety of
shapes, made from injectable plastics. Common to all moldsis the
condition that it must be possible to remove the product after
molding,without the need to destroy the mold (as is the case in
sand-castings).
There is an exception to this, the so-called lost-core molding:
There are
injection molds for intricate products, such as intake manifolds
for internalcombustion engines, previously made from cast iron,
which have an outsideshape that canbe molded with conventional
(permanent, open and close)molds but where the intricate inside
shapeis made from a molded, low meltingpoint metal composite which
is inserted into the mold before injection, andthen ejected
together with the molded product; the metal is then removedby heat
at a temperature above the melting point of the insert, but of
coursebelow the melting point of the plastic used for this product;
the molded
metal insert is thereby destroyed, but the metal will be
reused.
A basic mold consists of two mold halves, with at least one
cavity in onemold half, and a matching core in the other mold half.
These two halves
1.5 The Injection Mold
It is important to understand that it
is not the mold cost but the piece(unit) cost of the product,
which isimportant
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8 1 Introduction and Planning
meet at a partingplane(parting line). As the mold opens after
the injectedplastic (now in the shape of the desired product) is
sufficiently cooled andrigid the product can be removed by hand or
be automatically ejected.
Because injection-molding machines are mostly built with the
injection onthe stationary platen side, there is, typically, no
built-in ejection mechanismon this side. If ejection from the
injection side should be required alwaysthe case in stack molds,
and occasionally required in single level molds anyrequired
mechanism must be added to the mold, and occasionally to
themachine; in either case, this adds complexity and increases
costs. Only moldsdesigned for using only air ejection do not
require any external ejectionmechanism.
Most products are removed (ejected) from the core. There are
also manymolds, which need special provisions to allow the products
to be removedfrom either the cavity or the core. This is the case
with products having severeundercuts or recesses on the inside
and/or the outside of the product, suchas screw threads, holes,
ribs or openings in the sides of the product, etc., ormolds for
insert molding.
Some of these design features of the product may require moving
side cores,
which are either inserts or whole sections of the cavity that
move at an anglewhich is 90 to the natural opening path of the
mold. Others may requirespecial unscrewing mechanisms, either in
the core or in the cavity side. Themold may require split cavities
(or splits), i.e., the cavity consists of two ormore sections,
which are mechanically or hydraulically moved in and out
ofposition, and then clamped together during injection. In some
cases, themold may require collapsible cores, or retractable
inserts, which are all quitecomplicated (and expensive)
methods.
Any of the above special features can add considerably to the
mold cost whencompared to a simple up and down mold where the
products can be readilyejected with the machine ejectors during the
mold opening stroke or afterthe mold is open, without the need for
any of these complicated mold features.
Note that in this book, the term (simple) up and down molding is
used,which comes from the earlier vertical molding machines, even
though, today,most general-purpose injection molding machines are
horizontal and themold opens and closes in a horizontal motion.
Example 1.1
To illustrate how different mold features affect the mold cost,
we assumethat a single face mold with air ejection of the products
costs X dollars.A similar mold, but with mechanical ejection, costs
about 1.2 times X.A similar, air- ejected 2-level stack mold will
be about 1.8 times X.An unscrewing mold for a similar size mold and
product will cost about
2 times X.
Almost any shape can be molded
but at what cost?
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9
1.5.2 Elements of an Injection Mold
Most readers will expect to see some illustrations (photos or
schematics) of
injection molds at this point. However, we must not forget that
this is not abook about mold design, but about the relationship
between productivityand cost of molds, as well as the cost of the
products to be made. There willbe, however, a number of photos of
molds accompanying the text wheredeemed useful.
There are books that show designs of numerous, specific molds
but it isvirtually impossible to show every possible configuration
that may berequired. It is more important for the designer, and any
person requesting a
new mold, to understand that a mold consists essentially of a
number ofelements from which to choose for the most appropriate
design for thepurpose.
Every injection mold consists of the following basic
elements:
1. One or more matching cavities and cores, defining the cavity
space(s)(today, there are molds with anywhere between 1 and 144
cavities).
2. A method, or element, to duct the (hot) plastic from the
machine nozzleto the cavity spaces:
There is a choice between Cold runners (2-plate or 3-plate
systems) Hot runners (various systems) Insulated runners, through
shooting Sprue gating (cold or hot)
3. Provision to evacuate air from the mold (venting):
There is a choice between Natural venting Vacuum venting
4. Provision to cool the injected hot plastic sufficiently to
allow ejection ofthe molded product
5. Provision to eject the molded product:
There is a choice between Manual product removal Ejector pins
and sleeves Stripper s (stripper rings or bars) Air ejection Random
ejection Various methods of in-mold product removal methods Robotic
product removal
6. Provision to attach (interface) the mold to the molding
machine:
There are several methods to consider Mold is for one machine
only
1.5 The Injection Mold
Product quality, productivity, andmold cost depend heavily on
theproper selection of the runnersystem
10
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10 1 Introduction and Planning
Mold to be used on several, different machines Quick mold change
methods (various designs)
7. Method of alignments of cavities and cores:There are several
methods to consider No alignment feature provided in the mold
Leader pins and bushings (2, 3, or 4) Leader pins and bushings
between individual cavities and cores Taper fits between individual
cavities and cores Taper fits between plates Any combination of the
above
8. Any number of (mold) plates to provide the necessary for
carrying andbacking the above elements
But molds could have additional features, which will also be
discussed in thefollowing. Each of these features can add (often
considerable) costs to themold but in many cases can increase the
productivity of the mold and reducethe cost of the product. They
may or may not all be necessary and must becarefully considered
when deciding on the type of mold most suitable (and
most economical) for the job on hand.
Features such as serviceability of the mold may affect the mold
cost; forexample, the access to hot runners for cleaning plugged
gates or makingminor repairs, such as changing a nozzle, a
burned-out heater, or a faultythermocouple at a hot runner drop
will cost more in the initial mold, butthis will be easily recouped
by reducing the down time necessary to accom-plish such repairs. By
designing easy access to these components in themachine (without
the need to remove the whole mold, or part of it, to the
bench), such repairs can be made in less than an hour, instead
of takingseveral hours. This work can also be done by the mold
setup staff rather thangetting the (expensive) mold makers
involved.
Another area where valuable maintenance time can be saved is to
design andprovide easy access from the parting line to screws
holding modular moldparts to their mounting plates, while the mold
is in the machine.
On the other hand, in my experience, many molds, particularly
molds for
lower production quantities, have been vastly over-designed and
much moneyhas been wasted.
The main purpose of this book is to discuss the various elements
or featureslisted above and to facilitate the selection and the
decision making. Definingwhat is reallyrequired considering the
shape and complexity of the productand the required production
quantities will enhance mold productivity. Inaddition, the book
should facilitate investigating if, even minor, changes tothe
product shape could lower the mold cost and improve the
productivity
of the mold or the whole system.
Figure 1.4 Mold maintenance in the pressis important. Here, the
operator is changinga nozzle tip while the mold is in the
press(Courtesy: Husky)
Easy serviceability of the mold isimportant but often
overlooked. Itadds some mold cost, but savesmuch more in future
servicing costsand downtime
Even minor changes to the part candramatically lower or increase
moldcosts
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11
2 The Plastic Product
Plastics have evolved to be a very useful material. Today,
plastics are used inalmost every area, from small bottle caps,
disposable cutlery, and packagesfor dairy products, to large
containers, such as laundry baskets and garbagepails.
Plastics have transitioned from a cheap substitute for metal and
glass tothe material of choice providing almost unlimited design
freedom, uniqueproperties, and significant cost savings.
Figure 2.1 shows various industrial containers and house wares
that createdurable products in cycles from 1030 seconds.
Figure 2.2 shows various thin-walled containers are typically
used in the dairyindustry and are molded with wall sections
typically less than 0.7 mm withcycles of 20 shots per minute.
Figure 2.3 shows a collection of PET bottles for water, soft
drinks, etc. andsome of the preforms used for blowing these
bottles. Today, more than 500,000
tonnes annually of plastic are converted into bottles. Cycle
times for moldingthese parts have been reduced from 35 to 8 s in
the last 20 years. In addition,cavitations have increased from 8 to
144 cavities, resulting in significantlylower product costs.
Figure 2.4 shows a sampling of stadium cups with printed or
in-moldlabelled decorations.
Figure 2.6 shows samples of small, thin-walled technical
products made from
engineering plastics such as ABS, Acrylic, and PC.
Figure 2.1 Molded products of various sizes(Courtesy: Husky)
Figure 2.2 Various thin-walled containers(Courtesy: Husky)
Figure 2.3 PET bottles for water, soft drinks,etc. and some of
their preforms(Courtesy: Husky)
Figure 2.4 Stadium cupsFigure 2.5 Small and large technical
(engineering) products, heavy-walled jarsfor cosmetics, and tubular
containers with integral, hinged lids (Courtesy: Husky)
12 2 The Plastic Product
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12 2 The Plastic Product
2.1 The Product Design
The following contains suggestions for the product design and
how it mayimpact the mold design and the productivity of the
mold
A new mold is usually required
For a new product
After the redesign of an existing product
To increase the productivity and the output of the production
facilitiesalready in place. This usually provides a good
opportunity to reevaluateand improve the product, and to reduce
manufacturing costs, particu-larly through the reduction of the
plastic mass of the product.
The mass of the plastic accounts for a significant portion of
the cost of everyproduct. Reducing wall thickness and reduction of
unnecessarily heavy crosssections will not only reduce the cost of
plastic material for the product, butwill also result in sometimes
significantly faster molding cycles. The result
is that more of the products can be made per hour at lower cost
than waspossible with the preceding design.
In such a case, important considerations are
The output of the plasticizing unit and the dry cycle of the
machinemanufacturing the product before the planned changes
If there was special handling equipment (product removal,
stacking,
printing, etc.) with the old mold, will it be able to handle the
greateroutput, or will it need improvements as well
The above will be discussed in more detail later in this
book.
Figure 2.6 Small, thin-walled technicalproducts made from
engineering plastics
132 2 Product Drawings
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132.2 Product Drawings
2.2 Product Drawings
Occasionally, only samples or CAD models of a new product are
available.This may be of some advantage to better visualize the
product, but it isabsolutely necessary, to minimize risk for all
parties involved in the finaldecision, to have a complete detail
drawing of the product, showing allfeatures, tolerances, and
specifications.
This is also the moment when the designer has the greatest
opportunity todecide on the most suitable design for the mold,
and/or to make suggestionson how the product design might be
modified to improve the productivity,
to simplify the mold design, and to reduce mold costs. This is
also the timeto consider any ancillary equipment required for this
production. Anopportunity graph (Fig. 2.7) shows symbolically the
value of planning aproject. At the outset of the project, the
opportunity to make improvements,revisions, and selections is
highest to affect the final outcome of the project,while the costs
are lowest. After concept analysis, once the elements of theproject
have been agreed upon and as engineering of the mold progresses,the
opportunity to make conceptual changes or improvements
diminishes,and any costs associated with it will increase. By the
time the project reachescompletion and gets into testing and
production, the opportunity to makechanges is low, and any costs
could be very high.
To confirm that the part drawing is acceptable to all parties it
should alwaysbe signed off in writing as acceptable. Appendix 12
provides some generaladvice for the designer on how to critique a
part drawing.
2.2.1 Product Shape:How Can the Product Best Be Molded?
Here, even an experienced, conscientious designer may want to
consult withanother (knowledgeable) colleague, and/or with anyone
else who is familiarwith the type of product for which the mold is
to be built, and discussproblems of making andof operating such a
mold, to get their input regardingthe proposed product design. In
the following, some of the most important
areas to be contemplated are discussed.
2.2.2 Parting Line (P/L)
Is There an Obvious Location for the (Main) Parting Line?
In many products, the location of the parting plane (parting
line, P/L) isobvious. It is along the largest cross-sectional
dimension of the product, atright angles to the motion of the
opening and closing of the mold, and shouldpreferably be in
oneplane. This is the least expensive, and fortunately, themost
frequent case. However, there are many cases where the P/L cannot
be
It is critical that complete productdrawings are available for
the molddesigner before any mold design isstarted
Opportunity
C
osts
Costs
Time
Period of evaluation of product,opportunity for changes is
high,changes are easy to obtain,and low in cost.
During engineering, opportunityfor revisions is still fairly
high.Changes are still relatively inexpensive
During manufactoring, there islittle opportunity to make
revisions.Changes can be quite costly.
Mold tests and roduction:
Figure 2.7 Opportunity graph
The old proverb a stitch in timesaves nine applies here too:
Spendmore time at the beginning of theproject, to save much time
later on
14 2 The Plastic Product
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14 2 The Plastic Product
located there, and requires special consideration. A few
examples are listedbelow:
Simple parting lines (Fig. 2.8)
Sometimes, the P/L mustbe offset because of the shape of the
product(Fig. 2.9).
It may be of advantage to place the P/L at a level, which is
notat thelargest cross section, to force the product to stay on the
side from whereit will be ejected, as can be the case with
flatproducts. This would notaffect the mold cost; however, flat
products often cause trouble at ejection,
because they do not always stay reliably with the side from
where theyare ejected. Additional mold features, such as sucker
pins, or grooving inthe side of the product (pull rings) may be
required to hold the producton the ejection side to make sure that
the mold can operate automatically,without interruptions (Fig.
2.10).
The P/L is curved. This is sometimes unavoidable because the
productshape will not permit a straight P/L; for example in some
toys, butoccasionally also in technical products. A typical example
is the P/L for
plastic forks or spoons. In all these cases, the matching of the
P/L is difficultand expensive. It may need special, costly grinding
equipment or expen-sive fitting by hand (bluing) (Fig. 2.11).
Figure 2.9 Example of simple mug handle,using offset P/L
Figure 2.11 Typical mold profilefor cutlery
Figure 2.10Typical flat piece withundercut below parting
line
Figure 2.8 Examples of straight,simple parting lines (top: at
the opening;bottom: at the largest diameter)
152.2 Product Drawings
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15
2.2.3 Side Cores
Is There a Need for Side Cores, Splits, or for Other Methods to
Release Severe
Undercuts or Threads?
Any of these features will add considerable cost to the mold
(and to the costof the product), not only because of the added
complexity of the stack butalso because each stack requires much
more spacethan a simple stack withoutside cores. For the same
number of cavities, a much larger mold and thereforeoften also a
larger machine size may be required just to accommodate themold in
the available platen area, even though the clamping forces
requiredwould be little more than for the mold without side cores
or splits.
Such side cores, splits, etc will lengthen the cycle time and
reduce productivitycompared to molds that do not have such
features.
Could a Redesign of the Product Avoid the Need for Side
Cores?
In some cases, round holes or odd shape openings generated by
using sidecores or split cavities could be redesigned without
sacrificing the usefulnessof the product, and possibly without
significantly changing the appearance,
by creating such holes or openings in the side walls (or even in
ribs inside theproduct) with a design method where core and cavity
meet on a shutoff.This may require the use of special inserts in
either or both of cavity andcore, which may necessitate a change in
the shape (or in the draft angle) ofthe side wall of the product,
or require an opening in the bottom of it. Inmany cases, this could
be acceptable for the end use of the product and allowa much
simpler, less costly mold [1]. By just giving a bit more thought to
theproduct design before planning and designing a mold, and by
understanding
the application for which the product is used, a little redesign
can often resultin spectacular savings in mold and product
costs.
Selecting Other than the Conventional Parting Line
Occasionally, the choice of the obvious placing of the parting
line wouldrequire a side core, while by slanting the P/L, the
product could be moldedwith a simple up-and-down mold. An example
is a simple louver (Fig. 2.13),but the principle applies to any
similar case. The cost of a mold with a slanted
P/L is somewhat higher than that of a mold with an ordinary P/L,
but muchlower than a mold with a side core.
Investigate Shape of Threads and Undercuts
Often, a design specifies threads or undercuts, on the insideof
the product(Fig. 2.14). Is the specified shape of thread or
undercut designed with moldingin mind? Many such threads or
undercuts could be molded without un-screwing, or the need for
collapsible cores, by changing the shape of the
undercut so that the product can be stripped off the core, i.e.,
the undercutscan easily slip out of the grooves that created them
when pushed by ejectorsor a stripper.
Figure 2.13 Example of louver; top: needsside core; bottom:
tilted it becomes anup-and down mold
Figure 2.14Typical bottle cap with
tamper-proof ring and stripped threadfor simpler ejection (no
unscrewing moldrequired). This product is outside-gated,using a hot
runner hot tip gate
g
Figure 2.12 4-cavity handle mold with 3 sideactions per cavity
(Courtesy: Topgrade Molds)
16 2 The Plastic Product
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Figure 2.15 shows the difficulties of a typical unscrewing mold.
The coremust rotate out of the cap before it can be ejected. This
makes core coolingmore difficult and results in 30% longer cycle
times than a stationary core.Unscrewing molds are much more
complicated than bump-off (stripped)closure molds.
Figure 2.16 shows a schematic of a much simpler mold, where the
thread(and the cap) can be stripped. Here, core cooling can be very
efficient. The
cycle time for a typical (28 mm) bottle cap made from HDPE MFI
19,weighing less than 3 g, molded in a 24-cavity mold running in a
90 t(1,000 kN) machine is in the order of 4.0 s, equaling a
productivity of 21,600caps per hour.
Figure 2.17 exemplifies of how a small change in the angle of
the flank of thethread can allow a thread to be stripped from the
core, rather than requiringan unscrewing mold. Small changes like
this can have a major impact onproduct cost because mold cycle,
cost, and maintenance will be significantly
improved with a stripped product.
Need for Two-Stage Ejection or Moving Cavity
This applies to a shape or design feature of a product
consisting of
Deep ribs on the cavityside, as is often the case with
containers withfalse bottoms. Such ribs could also be specified on
technical enclosures,etc., as illustrated in Fig. 2.20. The depth
of the rib Fand the ratio of the
thickness of the rib t, as well as the draft angles of the rib
are criticalconsiderations, or
Deep ribs (often circular) on the coreside; typically, the
underside of anover-cap, as illustrated in Fig. 2.21 (even without
the thickening at theend of the rib as shown in this
illustration).
In both cases, if the ratio of F/t> 2, or if there is any
thickening at the end ofthe rib (as in Fig. 2.21), either a
two-stage ejection or a moving cavity arenecessary, which will
increase the mold cost by about 1520%. In both cases,it is
important to provide especially good ventingat the end of the ribs
toensure proper filling. Failure to use these methods will make it
very difficult
Figure 2.18 72-cavity unscrewing mold(Courtesy: Stackteck)
Rachets
Rotatingcore
Stationaryratchetring
Figure 2.15 Schematic of difficultiesof a typical unscrewing
mold.
Stripperring
Core
Figure 2.16 Mold where threadcan be stripped
Types of closures
Top of threadalmost flat, lessthan 15.If stripped will begreatly
deformed.
Angle on top ofthread allowsthread to bestripped offthe
core.
Unscrewed thread Stripped thread
Figure 2.17 Change in flank angleallows thread to be
stripped
Figure 2.19 Stripped closure mold
172.2 Product Drawings
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to withdraw (eject) the products, and increases the risk of
breaking portionsof the rib in the mold.
A2-stage moldwill cost about 1520% more than a comparable mold
withoutthis feature. Also, because the sleeve is usually rather
thin, it is very difficultto get cooling into it; the mold will
cycle much slowerthan a similar productwithout this complication,
and the maintenance cost of such molds is muchhigher.
Moving cavitiesare more complicated and cost about 10% more than
a moldwithout this feature. Some molders use it despite its higher
cost for productseven without a false bottom, because they can
cycle even faster than a mold
with a conventional cavity.
Post-Molding Operations
Sometimes, molds can be much simplified by doing additional work
to theproduct after molding. Post-molding operations are of
particular importancewhenever relatively small quantitiesare to be
made. For example, one or afew simple holes or openings in the side
wall of a product would require aside core in the mold, but such
holes or openings could also be drilled or
die-stamped aftermolding. Such additional operations may require
a drillingfixture or a stamping die. The actual time (direct labor)
for such post-moldingoperations and any costs for tools or fixtures
would have to be added to the
Always keep in mind:It is possible to mold almost anyshape, but
at what cost?
Figure 2.21 A product with deep ribs and(with or without)
thickening at the end isejected in two stages; 1: Sleeve and
stripperlift product off the core; 2: Strippercontinues to push
product off the sleeve
Figure 2.20 Schematic of a moving cavityin two halves; top: mold
is closed; bottom:mold opens and follows core for a limiteddistance
to ensure that the rib becomes free
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total cost of the product. But such post-molding operations
could also takeplace later at the assembly line, where the product
is assembled or packed,without any additional labor cost if
properly integrated in the process. Again,
it is the overall costof the end product that is important, not
just the cost ofthe mold or the molded piece itself. In many cases,
the savings in the moldcost achieved by eliminating a side core (or
some other complications of themold) can be substantially greater
than the combined additional cost forfixtures or tools, plus the
cost of the additional direct labor to finish theproduct.
A typical example for this would be the need for small holes for
a hinge pin(for a hinged lid), located in two lugs projecting from
the bottom of a product
(see Fig. 2.22). The plastic melt is injected into the bottom of
the product,near the lugs. It is of course feasible to mold these
holes, but it could be quitedifficult to arrange the side cores
required for such holes as well as theactuation for such side
cores, without interfering with the gating and thecooling layout in
this area. It would be, however, quite easy to just mold thelugs as
projections from the container bottom, and then drill the holes,
usinga simple drilling fixture.
2.3 Accuracy and Tolerances Required
Next, the mold designer should look at the specifications
relating to accuracyand tolerances.
Unfortunately, often, after a product has been conceived, the
design has beeneither just sketched by the inventor or an artist,
or a model has been created.
This information has then been passed on to a draftsman to be
put on paper(by computer or pencil drawing). This may result in a
good visual descriptionof the new product, but to be practical for
manufacturing, any drawing mustbe fully dimensioned, and
intelligently toleranced. To design a product forinjection molding
requires certain knowledge of this technology. A designwhich may be
suitable for one method of processing plastics (or othermaterials)
may be unsuitable or impractical for another process, even
thoughthe end use is the same.
For example, a disposable drinking cup of a specified capacity
could be madefrom paper, styrofoam, be thermoformed from sheets, be
injection molded,or made by another, entirely different, new method
or material. The finalproduct design for each of the above cited
materials and methods wouldmost likely look different to suit the
method of manufacturing and theselected material.
Also, while the dimensional accuracyof the product for its final
use (i.e., as adrinking cup) may be of little importance, its
actual dimensions will require
high accuracy because of demands not related to its use, such as
stackingheight (e.g., for packaging), ease of releasing of the
individual cups from thestack as required in automatic vending
machines, and mainly because even
Figure 2.22 Lugs with holes
How is the product to be used?What is really required?
192.3 Accuracy and Tolerances Required
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small variations in wall thickness may have a great effect on
the mass ofplastic used for each unit and on the molding cycle.
A design for a metal product is different from the design for a
similar productmade by injection molding, even though the products
may be fully inter-changeable in their use. This applies especially
for design features such as
Radii and sharp corners,
Flow path for injection (if applicable),
Wall thickness,
Ribbing and reinforcements,
Openings (round or shaped),
Others.
These features, by their presence or absence, not only affect
the making ofthe mold (and its cost) but also affect the speed of
the molding operationitself. I refer the reader to the many books
on product design for injection
molding, which go into much detail on this subject [2, 3, 4].It
is very important to understand that it is relatively easy to
achieve closetolerances for the mold partsusually made from metal;
however, the plasticproducts made by the mold do not solely depend
on the mold dimensions.The designer must be aware that the final
size of the product is greatly affectedby variations in the
shrinkage of the plastic (see Appendix), which in turn iscaused by
variations in molding conditions (pressures, temperatures,
andtiming) and by variations in the composition of the plastic not
only from
batch to batch, but also from manufacturer to manufacturer. All
this makesit very difficult to mold products dimensioned within
close tolerances.
But even the above statement relatively easy to produce the mold
parts toclose tolerances must be qualified. Using unsuitable, old,
and/or poorlymaintained machine tools makes it more difficult to
make mold componentsto close tolerances; the accuracy of the work
depends much on the skill ofthe machinists, and even with good
checking equipment can become timeconsuming, because it requires
frequent measuring of the closely toleranced
dimensions. The alternative is to use good machine tools, or
even machinesspecially designed or adapted for certain steps in the
manufacture of themold parts, requiring much higher investments by
the mold maker. Eitherone of these conditions affect the cost of
machining and explain why closetolerances can be expensive too
achieve.
Note also that dimensions are affected by the ambient
temperature of themachine shop and that even when cooled by cutting
fluids, the work piecesheat up during machining; they will measure
larger when warm immediately
after cutting than after cooling to room temperature. Of course,
the largerthe dimension, the larger the dimensional differences
caused by heat expan-sion.
Many millions of dollars aresquandered annually because
ofdemands for unnecessary tight
tolerances
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As can be seen in Fig. 2.23, the mold cost increases
exponentially with thetightness of the tolerance.
Without giving actual cost figures, the curve just shows how
costs can increase,as the tolerances get tighter. The cost to
achieve a 0.005 mm (0.0002)tolerance can be 3 times the cost of a
0.03 mm (0.0012) tolerance.
Other points that should be clarified when looking at product
dimensionswith close tolerances: how will these dimensions (or the
entire product) bechecked (measured) on the finished product? With
Vernier, micrometer, gages,measuring machines, fits with other
products? Also, when will they bechecked? Immediately after
ejection, one hour later, 24 hours later? Will there
be 100% inspection or statistical (random) inspection?To clarify
all this ahead of time can avoid much future unpleasantness
orarguments.
2.3.1 General and Specific Tolerances
Alltolerances must be specified on the product drawing and must
be looked
at by the mold estimator or designer when starting the project
to see if theyare reasonable. The Society of Plastics Industry
(SPI) has a suggested list ofpracticalgeneral tolerances for
injection-molded products. For more informa-tion, go to the SPI
website www.socplas.org.
In most cases, these tolerances are satisfactory and achievable.
Specific, closertolerances may require that experiments be made
with cavity and core sizes,and under various molding conditions, to
achieve the required sizes. Thiscan mean considerable added costs
for the mold maker and a higher mold
cost.The following tolerances are suggested to be used on
plastic product drawings(radii are not toleranced):
Product weight: 10% on projected weight (range 2%)
Wall thickness: 0.03 mm (in special cases 0.013 mm)
Fit diameter: up to 75 mm 0.20 mm
up to 106 mm 0.25 mmup to 160 mm 0.30 mmup to 300 mm 0.64 mm
Overall height: 0.5% or 0.13 mm minimum
Stack height: 0.5% or 0.13 mm minimum
Note that the steel size requirements, and thus the difficulty
of manufacture,
are dependent on the plastic tolerances on the product
drawing.
Figure 2.23 Relationship betweentolerances and mold cost
Always remember that tighter
tolerances mean higher mold costs,maintenance, and
inspection
212.3 Accuracy and Tolerances Required
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2.3.2 Are Special Fits with Matching ProductsRequired?
Often, certain dimensions of a product are specified with
unnecessary closetolerances, when all the designer wanted to convey
is that the product shouldfit suitably on another product (tightly
or loosely), typically, a container anda matching lid. This
requirement must be clear. Especially, when moldingplastics with
high shrinkage factors (e.g., PP or PE), it can be difficult
toarrive at the proper steel dimensions, and some experimenting may
berequired to achieve the required fit. Specifying the matching
diameters withstandard, loose tolerances may yield pieces correct
in size, but wrong because
the fit is not as desired. The alternative providing closer
tolerances couldbe unreasonable, because the dimension of the
molded product depend notsolely on the steel dimensions of the
stack parts but also on the moldingparameters. In such cases, it is
of advantage to complete the more complicatedmold first and test it
in actual molding conditions until the best cycle time
isestablished. The critical mold parts of the matching product
(e.g., the lid)should be finish-machined only after having
established what the actualmolded container dimensionsare. This
could require completing the lid moldwith only one cavity, using
assumed suitable dimensions, testing the un-finished mold until the
best cycle is achieved, and then adjusting the assumeddimensions so
that the proper fit can be achieved. All lid mold parts can thenbe
finished. For more information on this subject see [5].
2.3.3 Tolerances for the Filling Volume
This applies specifically but is not restricted to containers
into which a
more or less viscous product will be filled by volumeto within
closely specifiedlimits (typically, containers for margarine,
paint, etc.). In their end use, it isimportant for the seller that
a minimum amount must be filled into thepackage without
shortchanging the buyer, but also they should not be over-filled,
which would mean a loss for the seller. There should be clearly
definedfill lines (usually inside the container) to mark the
minimum and maximumvolumes. This can be a problem with plastics
with large shrinkage factorssuch as PE and PP. It requires special
consideration when dimensioning the
cavity and core because of the unavoidable variations in
shrinkage values, asthe plastic flows away from the gate and slowly
cools and as the injectionpressure within the mold decreases. The
same considerations apply tomeasuringcups or vials which have the
various levels (or volumes) indicatedby lines on the sides of the
product. It may be necessary to first test the moldto find the best
cycle times, and then establish the location of the
measuringlines.
Prototyping is often used to verifythe required dimensions or
fits of apart after shrinkage
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2.3.4 Stacking of Products and Free Dispensing
Any product stacked for shipping musthave a clearly defined
stacking height,
which is usually created by resting the outside or the bottom of
one piece onthe inside stacking provision of the following piece.
These provisions forstacking can be stacking lugs, or clearly
defined steps in the product. Thepurpose of these lugs (or steps)
is
The products must notjam when pushed together, which would make
itdifficult to separate them where required by the user, and
They will ensure a total stack heightof a certain, specified
number (e.g.,
20, 25, 40, etc.) of the pro