x PREFACE TO THIRD EDITION

data, and reminders to keep them aware of some critical items at just the right time to prevent error. Thus, an extensive checklist is presented. It will insure consideration of the potential hazards, weaknesses, and misunder- standings that face mold designers, engineers, and builders.

There are, of course, many variations of molds, whatever their general classifications. Naturally, it is not possible in a presentation on mold fun- damentals to describe in detail the very complex designs that sometimes evolve. However, you can be sure that any complex design can be broken down into its simplistic fundamentals as outlined in this text.

We have tried to mention all mold-design and moldmaking methods- even those that are rarely used. Our purpose here is to stimulate interest and

Contents

to encourage original study. We wish to thank the many users of the previous editions for their helpful

suggestions for changes and improvements in the text. Since many pieces of equipment that are obsolete by present standards continue to be used, we have described mold types for some of them. For instance, this text is used in parts of the world where very primitive equipment is employed. There, the people need data on molds for simple processing equipment, and to use the supply of moldmaking materials, which may be available in these lo- 2

calities but far removed from suppliers of standard mold parts. 15 We are indeed grateful for the widespread acceptance and distribution of . . 18

this text since it was first published in 1946 by the American Technical Society. We appreciate the obligation this places on us to be accurate, pre- cise, and factual. In preparing this new edition, we have carefully re- searched the intervening developments and have made every effort to pro- ,

vide serious readers with a body of knowledge that they can carry confidently I

J. HARRY DUBOIS h % W k A ~ i f q-: 1 id

Morris Plains, New Jersey %

I + i 37 WAYNE I. PRIBBLE

:, 2 )CLm713a @7 New Haven, Indiana && 1 , . .Xi1

,; t,.w • PR~ESSES, EQUIPMENT AND Pawl E. Ferland

64

65

82 86

vii

ix

S

SLrface Finish, Molds and Parts

CONTENTS xi11

I$', f . ..'r:,,,, %rink Fit Allowances

Mold ?%ts W e ~ M d d Parts T e m m r e Control Media and Methods Wbtt$@ Cavities and Plungers

; 1 D p W Cavities and Balanced Molds Burfab@ Phishes and Textured Molds Refe

iCX)MPRESSION MOLDS, Wayne I. Pribble

Bgign of 12-Cavity Semiautomatic Mold i#@ng-Box Molds W i n g Shoe and Stripper Plate Molds M t i v e Mol& &&$positive Molds kbmtvity Gang Molds

Bracket Mold

pression Mold Considerations

INJECTION MOLDS FOR S, S. E. Tinkham and Wayne I. Pribble 299

300

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Chapter Introduction to 1 Plastics Processing I

I Revised by Wayne I . Pribble

Wqty of applications in d i v a rnmufacturing fields (Fig. 1 .1 ) . These ma-

quality of the tool-make& wark. The molds and dies used are the

of dies or moklo bwe used ile it "sets" or hardens to

. - -J

2 PLASTICS MOLD ENGlNEERlNO HANPBOOK

8 PLASTICS MOLD ENOlNEERlNO HANDBOOK

Continuous Extrusion

Plastics materials are extruded in continuous strips of regular section, Fig. I' rl 1 1.7. This is done by a machine which operates much like a sausage stuffer. The raw material is placed in a hopper, where it is moved into and through a heating chamber by a screw feed. At the die end of the heating cylinder the material (which has been heated and compressed to a plastie mass) is forced through a die which shapes the extruded section. A moving belt carries the section away from the die, and the final dimension of the part is governed L by the speed of this take-off belt. The extruded piece is stretched to a reduced sec-tion area by the take-off belt.

The extrusion dies are relatively simple and inexpensive and are quite similar to extrusion dies used for the low-melting-point metals. Figure 1.8 shows the rear or screw side of an extrusion die used to make a rectangular strip. Note the tapered entry. k" ddh

Blow olding

Botties and other hollow articles are extrusion blow molded of thermoplas- tic materials. For this, a tube, called a parison, may be extruded and this hot thermoplastic tube is clamped between the fttces of a blow mold. Air pres- 1 ' sure is imwdktf~ly applied in the clamped tube to expand it and fill out the I mold ntour ( ~ i g . 119).

An extrusion die E&3

INTRODUCTION TO PLASTICS PROCESSING 9

Plasti~izi reciprae: screw

Basic extrusion blowhgpdnoiple for blow molding. The parison is a tube of molten i& i s clamped bet-n the die bEves and expanded to tlie die shape by air pressure. C

te process, cdkd -&law molding forms the parison in n and then 1nr4-m~ quiokly into the blow mold position for ternal air pressure w &@wn by Fig. 1 .la. In the final position,

O E ~ is stripped from the.~ote p;fn while the msld is open.

lding machines are cqdppwl to rotate the molds continuously in the horizontal axis during the molding cycle. This pro- --

dwre facilitates the p r d e o n of i n r q d hoflow parts of almost ,aqp-open or closed-rigid or flexible. Ip process, a mea- d liquid or powdered *w &@dd is p l w d in each mold

@ @ mold halves closed, they art3 -ale . i-n 3 heated area while

~psly in the - . t w ~ ' ~ h n e s - until the mtirqiassr mdd surface

and :nin -

thesmophstics has formed in the mold su&a&, the, t J@@ 'a mter spray or air blast while rotation continued ha- IF&$ 1.41 a n faIzn:ated by aluminum casting, rnacM-* A* **-ST - - .

%rbp.E~mmd mbkel. The molds are vented byb% QLW& &I@. Wzcils on the &tpk mold designs bm ,%& mmd&nnc ma GP-