HARRY DUBOIS Plastics Mold Engineering Handbook 1

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x PREFACE TO THIRD EDITION

data, and reminders to keep them aware of some

critical items

at just the

right time to prevent error. T hus, 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, m any variations of m olds, 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.

W e 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-

calities but far removed from suppliers of standard mold parts.

15

W e 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. W e 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

J . H A RRY U OIS h W k A ~ i

q - : id

Morris Plains, New Jersey

i

7

W A Y N E . PRIBBLE

: CLm713a

@

New Haven, Indiana 1 . .Xi1

,;

t .w

PR~ESSES EQUIPMENT AND

Pawl

E

Ferland

64

65

82

8

vii

ix



S

SLrface Finish, Molds and Parts

CONTENTS

xi11

I ',

.

.

rink

Fit Allowances

Mold ? ts

W e ~ M d darts

T e m m r e Control Media and Methods

Wbtt @Cavities and Plungers

D p W

Cavities and Balanced Molds

Burfab@Phishes and Textured Molds

Refe

iCX)MPRESSION MOLDS, Wayne I Pribble

B g i g n 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

99

3



v r 7

- d z q j l ~ \ , v

3 ~ : :

7 . . . t \

<

. {-b:d&iI{-I

, ?

I 9; QtX om DBSIGN, eon R

Egg

,a ~r:aM

: 2 ,*it.nrl'i: T ^ &'

j



hapter

Introduction to

Plastics Processing

Revised by Wayne I Pribble

W q ty o f 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 we use

ile it

sets

or hardens to



PL STICS MOLD ENGlNEERlNO HANPBOOK







8 PLASTICS MOLD ENOlNEERlNO HANDBOOK

Continuous Extrusion

Plastics materials are extruded in continuous strips of regular section, Fig.

I

l

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

movingbelt 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

low

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-

sure is imwdktf~lypplied in the clamped tube to expand it and fill out the

mold

ntour (~i g.

19).

An extrusiondieE&3

INTRODUCTION TO PLASTICS PROCESSING

9

Plasti~izi

reciprae:

screw

Basic extrusion blowhgpdnoiple for blowmolding.

The

parison

is

a

tube

of molten

i

i s clampedbet n the die bEves and expanded

to

tliedie shape by

air

pressure

te process, cdkd

- law

molding forms the parison in

n and then 1 n r 4 m ~

uiokly

into the blow

mold

position for

ternal air pressure

w @wn

by Fig. la. In thefinal position,

O E ~s stripped from the.~ote ;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 dhoflow parts ofalmost

,aqp-open or closed-rigid or flexible. Ip

process, a mea-

d

iquid or powdered

w @dd

is p l w d in each mold

@

@

mold halves closed, they art3

ale .

i-n

3

eated area while

~ p s l yn the

.

w ~ ' ~ h n e suntil the mtirqiassrmdd 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& .41

a n

aIzn:ated by aluminum casting, rnacM-*

A -ST - - .

rbp.E~mmd

bkel.

The

molds are vented byb

QLW

zcils

on the &tpk mold designs

bm &

mmd nnc

m aGP







4 PL STICS MOLD ENGINEERING H N ~ ~ ~

INTRODUCTION TO PL STICS PROCESSiNG 5

old then

opens

slowly under proper controls to per-

n. In the low pressure process, conventional injec-

with

resins containing preblended foaming agents or

ded

by.a piped

in gas

the

mold-maker. Good tools

. -.

. . {





18 PL STICS

MOLD .EMGINEERIKT

MAWDBQW

hemetal

vqs&a 3~wsO;ies

mak6ptmy

jbs, and fixtures out of plas-

tics

and

thew

m -

are

identifed

by thc

snmsp urira rooling

he

plas-

tics

pr

ies refer to their tcl

as

molds dies and fixtures.

REFEREM

Basic old Types

Baq, ~ f i c ,

@ m n g

Design

ir Flasrtcs, Mew

YD&. Van Nostrand Reinhold,

~ ~ c k ,onald D., M i c s Prodird Deaig~n, nd ., New

Van Nostrand Reinhold,

and

Features

1980.

&mhdt, E.

C

i ~ s s i n g

f

T h e m p k W c

Ma *,

New York: Van N0s-d Reinhold,

1974.

Revised by Wayne I Pribble

A

mold is only one item in a series of

,

E@ W

td.

~ k w

b

M-F*rM+

W .

material. The vast majority of molds are

~ f * m d h9da p w York: Yap Mwtrend Reinhold,

which open and close. One half of the

s 0 . 1 w t @ ~ ~

ign), in Mod-

Yerk;

JWbw-Hill, 1984.

half forms the inside of a part

1984.

contour of a part.

For further reading, we suggest:

wu a w e

.,

to lasers for practical way todothe tough fab&atiagjobs~

Mod-

cialize in resins fillers

em Pfastics

Ma g a z i m ,

p. 61, May 1984.

ther components that

terial.

he

final mate-

, from many different



BASIC MOLD T m S AND FEATURES

2

extremely stmple molds of wood and m t e r are usually

by

the

hobbyist who wants to experiment with pb t k s . Some readers will qua

tion this a ae ho f i M u s e break-away pbSt€ Very COmmon n ? 0xY

and fi +s

lg@p and

in the molding

f a* d m

nd in t r iwl t e ' shap

for the h d t dustry.

Pla tjcs

old ngineering

will not

ab l e to cover eventhin .This ere-

lude to basic

& M

tgrpes and

feartum,&jB

90~n idtial undwBnding

of

the molds only. We urge you to co~lect

tZdog

and house

o rws

which

describe and promote n w meth Qf Ow h 6 h @ t i o m

of old

methods

that

have put togetlfer to make a f a h a t i n g device

that probably

g l l

d o

a

job previou~ly onsidered impossible. Our point

here i s -ne most compliqted mold ever built

Was

made UP of the simple

cmponentg and

.Ct'&

'described in this text.

bfosI

inventions are simply

6. Vacuum formi



I

-

BASIC

MOLD TYPES AND

FE TURES

3

1 SPRUE BUSHING

2

LOCATING

RING

3 TOP

CLAMPING

PLATE

4 FRONT CAVITY PLATE

5 R M R

CAVITY PL TE

6

SOPPORT PLATE

7

EJECTOR HOUSING

EJECTOR

RETAINER

PLATE

9 EJECTOR PLATE

l Q

WECToR

PINS

TI CORE INSERT (male

section)

la

CAVITY I N E R T

ternale

section)

1 SPRUE

WLLER

PIN

14 WATER LINES

1

pro.

2.2. Various

cornpo p ~d~1e

wgplate iqjection

mold

use or njectionmold

ing (Courtesy Dow Chernic@ C~;,

f& EBnd,

MI

i

-

i ..

molds and operating t

this

type. Since hy-

draulic presses are mo

h c r i p t i o n is given

of the operation of a mo

press, Tw o general

types of presses are u

a e d

the

t ~ w m t r o k e ress.

The downstroke press

,~ 1 i r d e ~0

that the ram

and top platen are

m

p m u r e t o the mold. This

t p e of press is wide

such parts as truck

vantage of this press

and allows the operat

1

foot square are not uncommon

b r g e Mo ld s in Ch a

the main ram to its fully open position) or double-acting (on&

.

uses pressure in one direction

to

Uclose and pressure in the othab

compression molding, another cylinder is frequently used to operate

-.- r

.

v z 7



on the press platens with clamp bolts at

t-hand press

in Fig. 2.3 is

a double ejector

features shown are:

2 . air cylinder 2-way);

3 . U-washer;





o two htin ct types-

A

plunger or auxiliary ram trans-

S

the

6 F

most often used. It has a built-in transfer pot

GENER L MOLD TYPES

The variety of molding materials and molding methods has necessitated

the development of many mold types in order that full advantage of the

material possibilities might be secured. Three general types of molds are

used and these may be subdivided into several classes. The three general

types are compression molds transfer molds and injection molds.

These three systems described in Chapter 1 will be reviewed here. There

is no particular significance to the order in which they are presented. His-

torically compression molds were the very first types to be used in the

middle 1800s. The injection molds came into being in the

19

for the

thermoplastics processing and the transfer molds came i n t ~ se

n

the

1930s

For a history of the development of the industry ref^^^ shoul

be made to

Plastics History U.S.A.

ompressionMolds





B SIC MOLr YPES ND FE TURES 33

ession molded parts. In most cases which involve molding proble

h as those itemized above, the lower final cost of the part, after all

;

A

variation of the full size transfer mold is the hand-transfer illustra

, in Fig. 2 8 These molds usually have a loose plate and are relatively s

in size. They are used where inserts must be held a t one o r both ends

Inption Molds

material in it). After the application of pressure to close the mold and

it tightly clamped against injection pressure; the molten plastics mate-

is forced into the closed cavity by a source of pressure other than that

caused the mold to close. The melting of the plastics material in the

ve machine cylinder is calledplasticizing. Figure

2.9

shows a molded

t as it comes from the injection mold. The runner clearly shows as the

ss-bar in front of the operators left arm. One gate is indicated by his left

mb. The molten material passes through the runner and gates (2) on its

y into the cavity. The point at which the molten plastics material passes

m the runner into the cavity is called the gate. You will note that we refer

winelassifiwtion of materials.

'flowing into the cavity. This'bavity means the space between the male

and manufacturing technique b o ~

n and the female section into which the molten plastics will eventually

of t raw material tha

into the desired shape and detail. The point at which the core and

vity separate or move apart when the mold is opened is called the parting

e

Chapters

7

and

8

detail the different manners in which the material can

introduced into the cavity through a gate o r gates in various locations.

c h location has its advantage and disadvantages. The proper choice of

ting is one of the essential fundamentals of mold engineering tha tmus t

mastered by the mold designer.

Injection molds are used for molding either thermosetting or thermoplastic







f1

.

mi

FR

This

and keep it closed during the

curing

or lret t

m&tlon

.or harelrshg time MEC fovr

harden sufficiently to be ejected from the mo

in other publications

the-

pke-nomaan ofp

BASIC MOL TYPES ND FE TURES

9

PLUNGER

OR

FORCE

R CORE

m- \F@i. 2.14. Cross

section. f a

simple flash mold.

'b

m l o w c e o th er t ha n that

which

closes the mold and keeps

r armprbssion molding is designed in a manner that

@ t t w . w p e asi ly as the pressure is applied. A cross

w bholdlu

in Fig. 2 14 The depth of the mold

y

this constricted section

mold. This does not per

tb

h d . If

th

mold

&sig&@f4imold and [email protected]'t b

imdw nt

thim

maximmi

d u b



BASIC MOLD TYPES AND FEATURES

43



of mold, Fig.

2.16

consists of a m

and a loading shoe. The

loading s h w ' & m e k l y

a floatin

ed midway between the

plunger and cavity when

the

mol

forms or powder may

be loaded ia

W

ype of mold.

a loading shoe with

molded

p.kp

partially ejected.

The l o d i n g

shoe

mold offers

mom

%&lwnh@s for certain types of

compression molding. The cavity i r mqe acxxs~ible han is tbat of the

landed plunger mold, and inserts m y

l

loaded easily in it. The height

of the cavity well is lower in this

classification,

but the mold will never-

theless cost about the same as a landed plunger mold bemuse of the added

shoe. High-impact materials may be molded'in this type a f mold, therefore

used for work which specifies these materials, although

next described may provide even greater advantage.

loading shoe molds are not mommendad because

may cause binding of the loading shoe. f i s h

the load& shoe arrivesat

its

normal position.

- - .

sh mold is Mat thb ship&?*- %h I ai

W

bmkie

f

ho w16@

Brt (or soxpewhmy ~ m n&:& iP @adw W j a t ever larger t

~ % t s i d e

imant$&

of W

FIG

b

pusha

from

plu

s TRIPP R

P L T E

ing

by this meihod should

be

confined to units which contain

small number of iavities, as temperature differentials may cause

of the plates. Conversely, large numbers of cavities would require

onl:

binc

spel

7

2 1E

PUS

acti

the

by

i

the

F

plat

the

usec

the

U

the

a pmk1

in

strips the molded pieces off the mold plunger. The area of free

ie stripper plate is limited, as indicated a t

A.

This control prevents

.I9 shows a stripper plate injection mold in which the stripper

erated by the opening and closing of the press. B ) shows how

r plate fits around the mald parts;

(A)

shows the mechanism

oving the stripper plate. Note that the molded part would

e

in

~d at

(A),

and in the left side a t 0.

,,

ejection of the molded piece is at

all

times important. Much of

bnal accuracy of the piece may depend o n uniformity of the

e.

Proper ejection

&.&de'

'from the mold always presents

and it has been said wisely, One

pie e

can always

be

made

.

.

~etting he part out of the mold in one piece is another



B SIC MOLD TYPES ND FE TURES

47



elarnine and urea compounds should be semipositive

olds are not required. These compounds require

bring them to the plastic state, and good molded

without causing the compound to flow under

ithout keeping it sealed in the cavity during the

olds described in the foregoing pages are the

which, with various modifications, are used fordfl mold

ns have been devised to meet special problems.

d to meet the compression molding problems

compress the material and at the same time

as the mold closes. Transfer and injection molds are

pound is introduced into the cavity, therefore

iscussion of these jpecial problems. As stated

sually adopt designs similar

no extra loading space is required and the

'ECIAL MOLD CLASSIFICATIONS

types has been developed for special classes of work.

were devised to reduce costs or improve operating con-

tate the molding of complex shapes which may not be

the more simple molds.

t o

mold consists of a group, or gang, of cavities

loading

well,

and it is used in compression mold-

cavities may be contained in each gang of a mold

Wig. Sueh molds are frequently built with from three

$c kt%ining fifty to one hundred cavities. The cavi-

M m

d

oading space, as shown at

A

in Fig. 2 22



wt y work required

to

ahc ast o

rn-

indi

usrin

p d m f

the

~ ~ d t h e k n d s h o

fix

many

m e d i m - s k i

* r u i t k o

s ose

t

bp damebdthQk jssdingareilE08

b ~ 4 @ ~ 5 h ~ 9 1 r i l l p n ~ a a a u p s l

h*s

s m~l

b.b ,-w wbtisfacto~: i > , s I

a

4



@ Pb STlCS MOLD ENGINEERING H NDBOOK


5



WEDGE

o KMoCKoUT

KMOCKOUT B R

ND W E D M

MOT WEDGE

4

8)

FIG. .27.

Wedge-type mold

use

for producing side holes in molded pieces.

A

removable

wedge

is

shown at

A); a fixed

wedge,

at B).

The knockout pin

raises

wedge out of cavity for

removal of part.

For most applications, the construction shown at A) is slightly better

than that of B), as it is less difficult for the operator to make certain that

RemovablePlate Mdd

cia1 ejector fixtures. To facilitate production, two plates are used in most

cases, as the parts may

be

removed from one plate during the curing period

of the other. These extra plates are used extensively when several inserts

of deep parts from the plunger.

must be threaded into the plat-. The use of the extra plate

will

n most

instances, give a fifty per cent increase in production.

Molds which

use

this construction must not be top large as excessive

weight

make the plate unwieldy and overheavy to,handle. Twenty

pounds is a desirable maximum weight for the removable plate mold, al-

though fifteen pounds is considered better. When heavier

plates are

re

quired, they should be designed to slide from the mold onto a track W the






FIG

2 30 Pulling out

fork used in spring box mold Courtesy General Electric

Co. ,

Pittsfield,

MA)

provides the extra pressure needed to insure full density after the normal

flow takes place.

Double E jector

olds

It is generally possible to design a mold so that the molded part will stay

on the plunger or in the cavity. In some cases it is desirable to provide

double-ejector arrangment in order that the piece may be ejected from the

cavity or plunger. The design of the piece may not permit the use of pickups

and therefore the piece may stick to either part of the mold.

Double-ejector designs are also desirable when inserts are to be molded

in the top and bottom of a piece and the length of the plunger will interfere

with the loading of inserts in the top. This is illustrated in Fig. 2.31. The

top ejector pins extend down to the bottom of the plunger when the mold

is open so that inserts may

be

loaded readily on the pins. n like manner

the bottom ejector pins extend up out of the cavity when the mold is open

to permit easy loading of the inserts.



BASIC

MOLD

TYPES

AND

FEATURES

61



etimes formed in these mold

from sheet stock. This type

re will not be described in

m

Wmnted

with

h n

w d r blocks

gegeral practice which is predicated on the type

g m t and l abo~ vailable and the experience

age set

up

n hzs manner

of the various methods. In

ll

cases the mold

or s-ia~k-eaultymolds

ts

at minimum cost. Many

for larger

par .

.

dures may be determined

For some jobs there is

the mold ant the tool-maker must weigh the

~ssible esign before he decides which is best

Ids is shown in Fig. 2.35.

F ~ n ~ a l snd

Ed. New

ark;

McGraw-Hill

1986

Pmn Press 1967

PuIxlWons 1944

:

I .

k

I

1

?



m ng representdin one

,

Specialization by

proawes

and relattd ski& k t q s , - , k ut& of modem

mold

making t a

masonable

h1

er

examp

s hops that b*

only standardized or cu~taan

maid

b m s

a d

mld-making industry (see-CWpter 8). hs

invest in the rp equipment

as W

R;rdMmneeded to um p @

p

fe-

i

f

ern bases

fa.

f rm ;

~ b o m ,

i b l met $hapa h

inserts that cannotbe machined

wanomkdy.

Haah

i

l

supp~&sare

cmpemtiv

equipment and s

o



PLASTICS MOLD

ENGINEERING

HANDBOOK

AKlNG PROCESSES EQUIPMENT

AND

METHODS 7



bevel cut from one edge of steel plate

Shapers are built in variety of sizes from small high-speed units to large

machines that take 36-inch blocks.

A planer does the same job as a shaper but uses fixed cutting tools. The

work is placed on a moving table that passes under the cutting tools as shown

in Fig.

3 4

This is a powerful machine that takes large cuts from one or more

surfaces at every stroke. In most shops a shaper is used for the finishing

of blocks and plates requiring a work stroke for /2 to about 20 in. planer

is commonly used where the work stroke varies from 1 or 2 feet up to several

feet. A planer may be used for finishing several plates of the same size and

setup just as the shaper.

Generally a shaper is used in preference to a planer for work within its

capacity. The shaper operates more rapidly than the planer and is more

efficient for the jobs that it can handle. In recent years the shaper and planer

have been replaced by milling machines with carbide tooling which are

much more productive.

The lathe is the most common

piece

of too room equipment.

A

standard

8 PL STICS MOLD ENGINEERING H NDBOOK






2

PLASTICS

I OLD

ENGINEERING HANDBOOK

@@LMAKING PROCESSES, EQUIPMENT AND METHODS

73



Fro

3.1 1 Mold maker using

a

j i g d f o r he pre ci sion loc at io n of i hole in a mold section.

IG

.12.

In the rotary sunace gnnaer, the

work s

placed on magnetic chuck so that

it

may

be

rotated

under

the

horizontal grinding wheel as the chuck moves into grinding position.

This

grinder

is

used for rough grinding and fast removal of stock.

rk to

be

ground is placed on a round magnetic

t opposite in direction

of the work can be con-

ed Several pieces, to

be

the magnetic chuck and ground

is usually operated w i h a spray

sing 8@r

the work.

ce grinder

is

used for grinding soft or hardened

an inexpensive means of finish-

faces will be parallel.

so that angles or radii

3.14

use a magnetic chuck t

the grindingwheel. Microm-

may be controlleddmely.

wet (lr+diy rinders.

1 cylindrical grinder Fig.

ay be rotated on centers.

amhmen t s . The universal grinder will

:

TOOL MAKING PROCESSES EQUIPMENT AND METHODS

5



. The

uuivemal

cvlindrical grinder used to

grind

outside diameters.

isus accessories or attachments, are the most versatilemachine

making.

w h i n e s are often called die sinking machines, because

@& supporting the cutting tool, or end mill, will move along

[B IWI

law.er the cutter into the work piece. In horizontal milling

@ @dJe axis is parallel to the plane of the work table. univer-

me

s a horizontal type with an additional swivelmovement

horizontal plane.

L

1 Ma

@i? i~spntaJ

illing

Machines.

These perform some of the

%%%%

mthe

swEacegrinder, shaper or planer. Theyuse milling

8

& ohular saw with a wide face. One or more cutters

i@%Wb .ox

smounted into, and drivenby, the horizontal

f ~ttpp0:rkd

gainst excessivedeflectionby a heavy over-

as own in Fig. 3 17

hell cutter mounted directly in the spindle for

-1

block is shown in Fig.

3.18.

Other stub arbor

w

milk and[ n these instances the arbor and over-



F

PLRSTI MOLD ENOINEERING

H tdDSOOK



ulm pockets in a mold plate using a jig-mill.

Courtesy

Fro

3.18 .

Squaring block with a stub-arbor shell cutter in a plain horizontal milling

ma

chine. Courtesy Tooling Specialties, Inc., Denver, CO

dimension on a drawing, can be generated from suitable model or pattern.

Figure 3.22 shows a duplicator setup machining a cavity for a blow mold.

The tracing head on the right operates a servo-control valve, controlling.

hydraulic circuits to cylinders which power the three coordinate movemenu

of the work table and cutter spindle. Both the master pattern and the work

piece are fastened. securely to the movable work table. The cutter usmlly

a ball nose end mill) is mounted

in

the power spindle and centered over the

work. tracing stylus

o

proper shape and size is mounted in the tracH

spindle and centered ~ ~ e r

he J:k

attern must be the m e

iz

the

Finished

wwk

pie

w pr (y.ap : I but reduction far w id

D M E

,s, Inc.,



82 PLASTICS MOLD ENGINEERING HANDBOOK



FIG 24

Bridgeport vertical miller with

rotary

table and special angle milling head

Courtesy Ethyl-Marland Mold, PittsJeld,

MA)

Pantograph Milling Machines. These are similar in function to duplicating

machines. However the ratio is larger than 1:l and may be as high as 20:1

so the pattern must be appropriately larger than the work piece. Independent

tables with three coordinatemovements are used for mounting and position-

ing the pattern and the work. These machines are used for mechanical en-

graving and when set for large ratio reduction will cut very delicate detail

from a large pattern. Realistic models for hobbyists are machined in this

manner. Small letters or numbers are cut from large master types. Figures

3 26

and 3 27 show pantographs that are used in mold making.

METAL-DISPLACEMENT PROCESSES

These processes are more commonly called hobbing and cavaforming.Since

each method regardless of the name involves the displacement of the metal

by some means other than machining and the use of master patterns to

determine the final dimensions of the work piece we shall consider these

processes as similar. They are most frequently used in making cavities or

8

PLASTICS MOLD ENGINEERING HANDBOOK



Frc. 3 26

Pantograph mill. The pattern at left is ten times

the

size of the work

at

right.

Courtesy Ethyl-Marland Mold, Pittsfield, MA)

in the conventional manner and hardened and polished. The cavity block is

a prepared block of S.A .E. 3110 steel or the equivalent, and the impression

is made cold. The press m ust exert very high pressure. Some hobbing presses

develop pressures as high a s

3000

tons. Many mold makers send their hob-

bing to outside specialists who have the larg e presses required fo r this work.

The Cavaform * process may be used to advantage for deep, small di-

ameter cavities having draft and other internal configuration instead of

straight round holes. The pencil barrel cavity is a typical application. A

highly accurate hardened and polished male master is made. Annular mold

inserts are then gun drilled to the desired depth, hardened and polished to a

4 8

microinch finish. The mold insert is then placed over the male master

and reduced to its configuration by a swaging-extrusion process. Fifteen

hundred cavities have been made over a single mandrel by this process. The

machinery required f or this process is large and expen sive and such work is

done on a job basis by the owners of the Cavaform trade name.

Massie Tool and Mold,

I n c . St

Petersburg,

FL.









hafts

in

mbrs,

engines

a t h w : ~ ~ e - p o ~ t i o nr

~ n kd

mddng. The designer

m g

d n g r

mechanical

plating in consitlerhg possible appliations.

MISCELLANEOUS PROCESSES

Mald Maklng Procedure

T w sequence of operations ill the making of various mold members should

d e r s t o o d by the mold designerso that hemay more accurately visualize

%h~=r,vertical

mill,

or lathe as required.

v a t a p ~re used for machining rectangular

surfaces are often finished on a grinder

.h&rance machining. External machining pro-

f g

iw

p r o d u ~ d

y

metaldisplacement or metal-

f a a e

is delivered. Delivery commitments determine the choice. In either

am,

he disassembled plain plate members, ground square and parallel on

a urfaces are taken to the bench for layout work. Screw holes, water or

m m

ines, and other holes are laid out for drilling. Pockets may be

laid

t for rough sawing to shape, mill, bore or lathe-turn to finished size.

evlty and Core Inserts

Them@a@sllallyprodueed from anpealed tool steel alloy bars or forgings

dt;able stmla

sim

After rough cutting to size with a cut off saw, round

twembers

ore

turned to approximate size on the lathe; rectangcskr parts

are

tough

sized on the millers. Round or circular internal.openings are

SeeCbpWn5 7

and 8.



Pra.

3.35.

Jig mill boring mold base. (Courtesy hyl-Marland Mold, Pitrsfield, MA)

deposition processes. Hobs and master patterns are machined in the same

manner as a core insert for a mold. Letters and numbers are generally

stamped or engraved in the mold members after other operations are

completed, and prior to polishing.

Measurement and ayout

Y m the conventional hand tools are used by mold makers. The vernier

h@bt

gauge (Fig.

3.37

vernier calipers, micrometers, clamps, indicators,

V - b h k s , parallels, surface plates, angle plates, sine bars, and gauge blocks

invaluable for layout and dimension operations, such as checking work

n Pe.t ss. (See also measurement of surface finish Page 105).

nd.

tbols necessae for bench finishing are diemaker's files and riffle

fikes

chisels, scrapers, and engraving tools (Fig.

3.38 .

Abrasive materials

are coated cloth and paper, graded stones, lapping compounds,and diamond

paste. The entire finishing and polishing technique is one of metal cutting

y hand, working out machine marks and imperfectionsin molding surfaces

9 PLASTICS MOLD ENGINEERING HANDROOK



FIG 3 37 A

froishe

mold ~ d o ns arefully checked with a vernier height gauge before

assembly

FIG

38

Hand tools are frequently used for the engravi w

o

malds

are produced by careful

w ~ r ks essmcral



TOOL MAKING PROCESSES EQUIPMENT AND METHODS 101



lso

used

as

hand grinder but it

has ~ p w ~ p o w e r

nd

less

t

machine.

This

unit fitted with polishing wheel is shown

int of the functioning and performance re-

Fig. 3.4 9 preplating treatmentand the plating

ler after he has the work will in many cases eliminate

.

2

MAKING PROCESSES EQUIPMENT

WD

METHODS 109



plating Courtesy Nutmeg Ckrome Corp., West Hart-

Courtesy Nutmeg Chrome Corp., West Hartford, C T)

Heat Treating Equipment

Most plastics molds use hardened cavities, plungers and pins. Other parts

of the mold are also hardened. Mold steels are generally annealed before

work is begun, and they are often annealed or normalized during the mold

making process. Most small mold parts are made from forgings. Both of

these materials must be annealed so that they will machine easilf. Mold

parts are heat-treated after machining or hobbing

to

obtain strength,

wearing qualities and distortion resistance. The equipment most frequently

used Figs.

3.47

and

3.48

consists of an annealing furnace, a tempering

furnace, a carburizing furnace, a large burner and suitable quenching baths.

Oil, gas and electricity are the heating media most frequently used in the

heat treating of steel. The furnaces are merely fire-brick lined ovensequipped

with a heating unit. Liquid baths of lead or salt serve special needs; the

lead bath to draw and temper steel parts, and the salt baths to minimize

1 4 PL STICS MOLD ENGINEERING H NDBOOK



106 PLASTICS MOLD ENGINEERING HANDBOOK TOOL MAKING PROCESSES, EQUIPMENT AND METHODS

107

be a complex and academic subject, the basic principles are



the growing importance of achieving, measuring and inter-

quality surface finishes, this basic study of surface measure-

booklet, much of what is said can be applied to other

any study of surface measurement be preceded by a clear

the terms in general use.

or roughness is that part of surface texture best defined

FIG.3.50.

~ o l hssembly area

at by a process of trial and error. It is important to bear in mind that the

best finish attainable by a skilled operator, within the limits of his machine,

is not necessarily the best finish for a component.

In the past, visual appearance rather than mechanical design require-

ments has frequently determined the surface finish values; indeed, a surface

finish governed by visual appearance could very well be over-specified,

r lly results from the condition of the production tool

leading to unnecessarily high production costs Fig. 3.51). While surface

or grinding wheel

Fig. 3.53).

Re,

3 51

Tk g

mph

ndiafea

iWsh

s mtq s.

f a

surfPce am:

R,

mughaerrs(primary

t cx t~m); ,,

wav-

Was swin ,

and

W IQ

aviness

&IS-mghnes9.

j

TOOL MAKING PROCESSES, EQUIPMENT AND METHODS

109

1



s E v

Meter

Cut off

(mm) Traverse

Length

(mm)

inimum aximum R t l + Rtz + Rt3+ Rt4 +R t s

bpR1 R2 R3 R5 R7 R R G R ~

a

Rlo

4

( R i +R j + . . R9) - (R2+R4+. . .R10)

the grit and its size used in a grinding wheel (Fig. 3.53) .

5 .

' , Root-Mean-Square

RMS) is an average geometric roughness and was an ,

American standard. In 1955 it became obsolete, but naturally enough will

I

Frc. 3.54.

Definitions

, &I1

e

encountered occasionally. It is sufficient to say that its numerical

value is some 1

1

percent higher than that of R CLA, AA) (see Fig. 3.53).

i4

The foregoing standards, with the exception of RMS, are in common use.

Secondary TeMwe.

Secondary texture is that part of the surface texture

'Other terms will be encountered

in

the study of surface measurement such

underlies t s r m g h n e s s . All types of machine vibrations, for-instance

as

k ,, tp, R.

spindle deflectiln Bnd imbalance, can be the cause; it is generally described

It is worth enlarging on the parameters

R

and R since increasing ref-

as

waviness or mor e simply Fig.

3.52).

erence is being made to them by manufacturers of surface measuring devices

and Standards Institutions. Both R,, and R are parameters that give a mea-

The p r o d i t i o n process used will form patterns on the surface.

summent of the average peak-to-valley height, the former being intend*

rhe Pred0minankpattern direction is known as the lay.

for measurement by a machine whereas the latter lends itself to graphical

determination and cannot yet be reproduced by a machine. In some cases

CoC-offSampling kng th ) .

Cut-off is a facility that is built in to most

,

I&

,,,

and R can be used as alternative or supplementary parameters to Rt.

measurin$devices. Its function is t o suppress waviness (secondary

(For determination of these parameters see Fig.

3 .54 ) .

"XhUe)

whatever d e g ~s required within the limitations of the cut-off

In

order to simplify the illustration of surface measurement principles?

Unltl

this facility is of great importance a s it allows the effects of

reference will be confined in the remainder of this article to R,,

Rp,

and

Rt

Process to be studied gpar a t e ly f rom the effects of machine

as it is felt that these parameters will be sufficient to develop the basic

lkfficieficies. Cut-off is a filteringoperation that is performed by a frequency-

d pnqentlectro?$,&filter. The cut-off values according to the British

m O L

MAKING PROCESSES EQUIPMENT AND METHODS



We h v e chosen to discuss two points

in

stppe.deptb, because they

am

w

s a q

values mice. The length of sfroke should

constantly giving rise to doubt and argument.

4

traverse a partsular

feature

It shouM be borne

since the surface appeaqnce varies, it is desirable

reasons previously highlighted. Likewise use of

tures two or mofe times.

is equivalent to.

39 37 mi

ot

used elsewhere. Unlike

R,,

f r

ease of specification and

py a sliding skid Fig.

3.55 .

The

2

PLASTICS MOLD ENGINEERING HANDBOOK TOOL MAKINQ P WC E S I E S

EQkllPMENT M D

METHOD8

8



t

t i dFR hKering, p 45.

q i

. Z

a;Equipment

Oct.



h ermit th use of m te

The mold desipet has many

v stm nt th t g lost if;

M TERI LS FOR MOLD M KING

7



eel, lting

Steel, water Wdening

Steel,

oil hardening

Steel,

low

dlay carbon

Kirksite zbioallay)

Mulainam

a oy

Silicone

rubber

stress The premme in

a

PLASTICS MOLD ENGINEERIN. )IANDB80K MATERIALS FOR MOLD MAKING

9

arc melting process, is remelted by using it as an electrode. Figure

t the liquid metal from the electric

rates the Electroslag Remelting Process. The (ingot) electrode,

degassing process which removes most of the hydrogen and

the cathode, is slowly melted away in small droplets. These drop-

ually done by means of a vacuum

through a molten slag bath which has a purifyingaction and pro-



ing Argon into the liquid metal. The removal of most of

e liquid metal from oxidation. It solidifies quickly in the water-

gases

w ll

result in a cleaner steel which is very desirable in

mold, which also tends to form it in a more uniform structure.

n

steels. There are severalmethods of degassing. The most p o p

a s s , no specialatmosphere or vacuum is used to protect the metal.

tion is rendered entirely by the layer of liquid slag.

A),

the ladle of liquid steel

is

put into a sealed vessel from

Vacuum Arc Remelting process, no liquid slag is used to protect

has

been pumped out to create a vacuum. This is called

I g metal, because the process

is

contained in a vacuum. It is more costly

; : + ~ u e

he equipment

is

more sophisticated.

B)

of the sketch, the ladle

is

sitting on top of the vacuum

paring the charge for the electric furnace, pure iron or carefully

uid metal is poured into the ingot mold which has been

scrap is used, together with alloy materials in the percentages

Inasmuch as the chemical composition of every constituent

steels is known, various elem nts can be brought into association

er ratio to achieve a desir

result. Cold melt electric furnace

r of the ingot being the last to become

made from a charge of cold

m

rials, the name serving to differ-

it from production steels produ by charging the electric fur-

tb molten steel from an open-hearth

race.

the steel has been melted and refined it is poured into iron molds

he center of the ingot may be left hollow or, at

ingots. Tool steel ingots may range in size from 6 in. up to 70 in.

ensity. The defect resulting is called pipe. Ingots con-

f

the manufacture

ss known as cogging In the cogging

8 methods.

A

Ladle degassing process.

B.

Vacuum ingot degassing pro-

WIllW

l dle

v

w

t

tl

SOIKI.

i~

P

least,

tainix

IOO PLASTICS MOLD ENQINEEWQ HANDBOOK MATERIALS FOR MOLD MAKING

121

process, the ingot is heated to the proper temperature and then worked to

LE 4 3 MinimumAllowances

for

Machining and Maximum

the desired size and shape by a process of hammering, pressing, or roll-

DecarburizationLimits

R .

Some types of steel, such as high-speed steel, should always

e

ham-

m e d o r pressed in the forming of billets.

These

billets may

be

round or



q u a r e . The cogging operation is used to form the billet to the shape de-

sired and to reduce the cross-sectional area of the ingot and increase its

length.

Surface defects may develop during cogging, such s seams, laps, or

packs.

These

must

be

removed before further work is done on the billets,

Removal of the defects

is

accomplished by one of three methods, namely:

'&hipping, grinding, or rough turning. Billets are sometimes pickled in

lacid

to remove scale and to point up defects. The chipping process

makes use of air-hammers and gouge chisels to remove seams. Grinding

s usually done with swing-frame grinders. High-speed steel billets usually

. m i r e grinding, and they are

en removed in a milling machin

o m ypes are milled all

away the entire surfa

s athe.

- f t W t s prepared in the manner just described are then rolled or ham-

produces a condition known as surface decarburiza-

rnered to a specified size. In the rolliag of tool steel, the cross section

of

ce skin must e removed, and steel producers uni-

he

biilet must

be

reduced slowly, and this is accomplished in repeated

+piaxes through the rolls, a small reduction occurring with each pass. Care

a r stock for tool-making purposes. The allowances given

is exercised t o do the rolling while the temperature of the steel is between

maximum and minimum temperature limits. After being rolled, the bars

must

be

annealed to remove the stresses.and to make them soft enough for

pared as cold-drawn bars for certain uses. In the cold-

machining.

Annealing is done by heating the steel to the correct temperature and

fbm allowing it to cool slowly. Scale will form on the surface if this is

the preparation of drill rod, these bars are ground in

do@

in

air, and to prevent this, the bars are annealed in a n atmosphere

to remove all decarburized surface and provide close

bmtrolled furnace or are pipe-annealed. In the pipe-annealing p r o w ,

bars

are placed in large pipes and surrounded with a baterial t b t re-

& oxidation. (Cast iron chips are generally used for this purpose). Tht STEEL

FORGINGS

pipes

are then sealed and the entire charge is

heated

to the.annealing te&

p a t t u n . On reaching

t is

.temperature, the charge

slowly as required for the type of steel being prod

18 to 24 hours. Today much tool steel is being an

controlled furnaces. n e tmosphere in the furnace

oxidation or scafing

09

the metal.

In the various heating cycles required for cogghg, rolling a + a

ihg some carbon will be oxidi

% y

ir fn

wcontact

Mth t

~ttiYit&

Many

ing the

desired

forging

eration

2

PLASTICS MOLD ENGINEERINQ HANDBOOK

MATERIALS FOR

MOL

MAKING

23

'the

twhnical d t furnished by

surface wrist ~s &tiop.This



MACHINABtLITY

t iron chips or pit 'coke, by

or by heating in a w u u m

Borne grades of steel will undergo certain machining operations with

eater ease than others. The machinability of steels may vary with the

w a l i n g process. It is possible to anneal specially a piece of steel to give

better machinability for a given process. For obtaining the maximum

maunt of machinability, steel stock from which mold plungers arc cut on

duplicatormay require a different annealingprocess than a block of steel

ing to which the stock has been

des nated temper ture rang

hardness and to relieve the

ar machinist's experience.

(such as nickel and man-

while relieving the major

sses inherent to the pro-

bartines of steel, is accomplished by heating

just a b v e

ule

critical point and then permit-

HEAT TREATMENT

k r 'aTf ESS

ELIEVING

idud stresses. Steels that have been sub-

ons nlust,be relieved or distor-

Aftel -

ing froq

stre

from

stresses I

cess resu

by se

Annealin

the meta

ting it tn

24

PLASTICS MOLD ~NG INEERINGHANDBOOK

MATERIALS

OR

MOLD MAKING

25

STR NGTH OF ST L

HARDNESS PENETRATION

wnching solution, cools very quickly. The inner core cools relatively



MACH1 YG STR SS S

y. Steels that must be quenched rapidly to giv them proper hard-

will have an outer shell that is hard and an inner core that is relatively

owing to delayed action of the quenching.

This

outer shell

m y be

to -in.deep in a water-hardening tool steel. The use of certain al-

hing. The oil-hardening steels show greatsr hard-

water-hardeping steels, while the air-hardening

the greatest degree of hardness penetration. Molds subject

ection should not have a high degree of penetration.

MOLDTEEL REQUIREMENTS

heating during heat treat-

p o d mold steel must be clean; it should not contain

clusions which will cause pitting during polishing.

e and free from voids and porosity.

ing rapid heating when the thin sections reach the critical temperature

~n 9*. It must be uniform in structure and.relatively

fr

first and start contracting while

the

thick sections aR still expanding.

Figure 4.4 illustrates the advantage of slow heating, During slow heat-

ing, the combined stresses aie below'the yield strength of the mold and

no distortion occurs. During fast heating, the combinedstressesare greater

than the yield strength and the

m@ld

d l s t m , The mold cracks

if

corn-

Steels

which machine @ly

y are needed for wonomical mold comtpction.

hardness. They are easily machined and

polished

to a fine

for most injection molding applicatiohs. It

s

advisabt to

and the low alloy steels are

nrqlds made from prehardened steel

if

the mold is comp1h Eed

Bt

radii and corners.

ired

hardness in

and polishing.

126

PLASTICS MOLD ENGINEERING HANDBQOKr

Stre~gth nd Toughness.

Molds require

a

hard surface and

a

very

tough core-the larger the mold, the greater the core strength needed for

resisting distortion or cracking.

Heat treatrirg SSqfety. An important characteristic of a good mold steel

MATERIALS FOR MOLD MAKING

127

~btained y using the cheaper grades makes

a

negligible dif-

otal mold cost, as it amounts to only a fewcentson the pound.

e

usually made from plate steel, while knockout bars and

made from machinery steel bars or cold-rolled steel. It is pos-

to use cold-rolled steel, unless these



is its ability to be hardened satisfactorily in a wide range of sections

by

a variety of methods while producing uniform results.

F M . ll mold steels must be able to take a mirror-like finish easily,

although a dull surface is often used as the desirable final finish.

Wear Resistance.

Wear resistance is a fundamental requirement of

a

p o d general-purpose mold steel. Some of the plastics cause little tool

wmr, others, such as the glass and asbestos-filled compounds, require

the

maximum amount of wear resistance.

SELECTING THE STEEL

As the plastics industry developed and presented new materialsand molding

methods, larger and larger moldings were developed and the steel makers

cooperated by building the larger

They also provided stronger and

impurities available in all the

demand for mold

and abrasive plastics in

Rm

Steel

Rgte steel is a low carbon steel such as

S E

1020produced by the openhearth

or &her inexpensive processes, wherein cjeanlinessis a less importantfactor

W

olume. This material is used almost exclusively for the frames of

molds. Plate steel can be carburized and hardened or casehardened. It k

sometimes used to make cavities and plungers, but this application is not

recommended because of the low core strengthof triis stecl,and also becaub

structural faults, such as pipe, seams, pits, and other defect&,are comrnoa

@

it.

Blate steel should not be used for cavities or plunger on any byt the

&heapestof molds.

There are severalqualities of plate steel available and if any press&re

s

concentrated on the plate, the better grades should

be

w k t d . s o d

mold builders use the cheaper grades.of boiler plate for ckmping plat&

parallels, etc., and the better grades (somethinglike SAE4140 for the back-

ly

plates, steam plates, or other members on which stresses may

be

c o p

centrated. This practice requires that a large inventory of stock re

carried

thmfure it will

be

found wiser to use the better grades of plate ftii-~ugho*

See a h SM

Met Handbook Ed. Yd

g ge

768

for

additional data

machinery steelwould be indicated.

1class

as

the S E 1020 plate steel.

hot-rolled into flat or square bars

these bars can be used without any

pt a surface grinding on both sides to produce flatness.

which of these

it isnot suitablefor hobbing.

of tool steel has nearly the same hardness

nd may lack toughness. As a result, the mold may

r

than

distort when excess pressure is applied.The initial

high.

It is frequently used for injection molds because

ure

as

easily as other steels.

rdened all the way through.

when properly applied.

tool steel may he

used

when maximum hardness is

hen hardened, therefore ample allowance

distortion must be held to a minimum and are recom-

STMIPARD MOLD COMPONENTS

e mold designer

can

save hours of decision-making

selection if he will use standard mold frames where

mold parts, which are inevitably indicated

mold he may build. (See Chapter 8.)

mass fabrication facilities afforded by

28

PLASTICS MOLD ENGINEERING HANDBOOK MATERIALS FOR MOLD MAKING

29

the principal suppliers of standard mold parts assures the mold builder of

kin

he various types of alloy steelsand gives the properties

economy and reliability he can not otherwise achieve, except in unusual

circumstances. He can expect that the quality of the materials, and the heat

treatment, if any, employed for these products are better controlled than

it

ST INLESS STEEL



is

possible to do in a shop with less demand and supervision.

He must, however, be aware of the differences and intended use of the

many alloys classed as stainless steels, only a few nepd

be

various grades of steel available in standard &kamesand plates. Besides the

use in high-pressure molds. Because of the necessity for

SAE 1020plate steel mentioned above, generallysuppliedinan analysisup

t

SAE

1040, as' the lowest grade of steel, m m suppliers offer better choices

the most commonly used. It contains 12to 14%chromium

for more severe service of SAE 4130 to 4150, prehardened to 26to

9

Rock-

well C, and even

P-20

prehardened to

28

to 32 Rockwell C.

Besides mold bases, and individual plates ground and sized to close toler-

ances, there are eader pins, bushings, ejector pins, sprue bushings, pre-

machined cores and tool steel cavity blocks.

Other items which have been added to the ever-growing list, as demand

increases, are such more complicated, heat-treated and assembled devices

as;

early returns (for the.knockout system), latch-lock mechanisms to deter-

mechanismsthemselves. As the result of previous wide application of these

devices, he can be assured that they are largely fcrolproof when installed

n

qieetion

molds

for

t m r o ma

by experienced personnel.

LLOY STEELS

Nickel Toughness and strength.

Chromium

Hardness. Adds to abrasion resistance in high carbon compositions.

Vanadium Purifier also adds fatigue resistance.

Molybdenum

Widens heat treating range and adds heat resistance.

Tungsten Hardness and heat resistance.

w

co:

ab

thc

s

ha

MATAltRLS

FOR

MOL

MAKiNQ

131

/

PLASTICS MOLD

ENGINEERING

HANDBOOK



For

depth

of the

n

Rc.

The

binatio

3

w

8 Nickel hfar~ging el

0.03

18 25 7 75 4 80

&

me 18 Nioeel Mirragbtg Steel 0.03 18 25

7 50 4 25

finishedmold is then reheated to about

900°F

eld forabout

5

hr, and cooled

in

air

This treatment, called aging esults

in

an average hardness of

50

Rc,

depending on the type of maraging steel (Table

4.6)

which has been

used.

These steelsshrink during aging and allowance must be made for this bythe

mold maker.

and transfer molds, the

P20

mold is carburized to a

STEEL FOR MACHINED MOLDS

P6

type must be carburiwl, resulting in a good com-

hardness and core toughness.

rcial low carbon machinerysteels

re

nearlyideal when machinability

ne important consideration. These steels are usually high in phos-1

HOBS AND HOBBING

Re 30-35 where it is readily machinable. No hwt

It can be chrome-plated .when corrosion

mdd lrt:

r fmhg the mold

frame.

Selection

as

t t

experienced with

SMC

f a higher hardness is req ed.,

and some

sf

their prope ies isshown

b eep hardened to RC45-50.







M TERI LS FOR M W KING

139

it difficult to select the proper steel. It is e s t w hat about

plastics molds are made from only six

types

of

if the industry could standardize

m

fewer steels.

more economicallyin larger qua @ inventories

ed because of fewer catem*, inere skg



maker.

he

heat treater's

task wuld

also be

treatment and loner

mold

life.

for compressian and transfer

m Ms

are

predominant

lenable steels,

In molds.

pk fgs

get larger, and lower prduction,

in

most

cases

&R 8rger

moldings, alloys other than steel can often

.

rtous castable alloys used in molds, includingproto-

mbIds are: alloys of copper and beryllium, alloys

alloys.

cX,bper alloys, with a basic composition of 2.5%

mdoer

has been used for aver 40.vears for in-

for shape and

ting

was

con-

sp~if ledor

,kith

F e

early

machines from Europe

see

& lld &terials for blow molds.)

the pressure

ct the molten

to a hob for

to push out

ing.

~dred ounds,

he process

in

:f,y

PLASTICS

MOL

EWOINEERINO

HANDBOOK

ceramic pattern is used in place o the heat treated steel hob; ise.

c Casting.

. la ter,

improved by the application of a vacuum during the castiq

I

8

because the ceramic is unable to withstand the compactine

i

8

.

,.

used in the earlier technique, v ry reliable castings,

virtu h



I

1 0 , ~ ~

fw and with excellent reproductisn of the surfaces cast against,

.= vailable for much larger molds. When

the

conversion of furniture

,

w he late 1960s occurred, beryllium casters were able to mold up

, t i

a

W - p o u n d castings.

. .

Waay alloys of copper with beryllium are available. Table 4.10 lists the

'

,&we

lloys

most commonly used for molds.

I '

&%kction depends on the desired degree of fluidity and the mold-makina

to e used. Certain alloys are for making cores and mandrels rathe1

ture. And the foundry can cast them a t lower temperat

@, F .

This

is

important when using a ceramic mold-ma

8'i+ecjing on available melting equipment. At lower tempera1

s is

As

metal-mold reaction. Also, simpler foundry equipmer

the class of materials having

1,.7

or more beryllium,

hi

contents give higher fidelity of reproduction. But the hi&er

1,

' - 3

Eontent, the higher the cost. The 20Cand 245Calloys (see Table

'

f , I

Gammon. But the choice can depend on pattern quality.

V

~ t x c k ~ e n tattern 2OC will be satisfactory, alloy 245C can corn

&&*kt for less pattern precision.

ie

m

important mold manufacturing m Q n s or

u ~ h g

he

42

PLASTICS M 6 L D ENGINEE~INQ HANDBOOK

pressure while the metal is molten. The metal flows to conform to the shape

and surface finish of the hob

very

precisely.

When making a hob, the mold maker must include fillets wherever


43

sting of the BeCu around the ceramic, resulting in a sound, dense

gaming the ceramic slurry into the rubber m old, the mold maker

I solidify and then fues it in a n oven. He then lutes gates and risers

b c . Melting and pouring of the beryllium copper around the



of BeCu from pouring temperature to room temperature. Shrinkage s

predictable and consistent.

hardness and strength. Besides machining a hob, the mold maker can some-

times cast one

via

a ceramic casting process.

When heated, lower the chase o r mold casing the hob assembly

and a o v e the two onto a hydraulic press.

A deflector will insure this pattern o flow. When the beryllium copper hm

very

important. Move

a

uickly

as

safety allows.

the hob from the mold before too much cooling occurs.

If

cooling prow

too far, it will be necessary to heat the hob and mold to about 1000°F

facilitate separating them.

Ceramic Casting.

Ceramic casting follows any one of a number of paten

4

around the pattern. The elastic quality allows stripping from t b tq

designed part. The special ceramics u d preserve the

A proper ceramic mixture will combine g o d surfam r

relatively high permeabilgy.

his

bst property allows flag

ae rn follows the steps covered under hot hobbing.

blidifiition of the metal, the caster breaks away the ceramic.

xefully controls the cooling of the metal to achieve good dimen-

*ces in the casting . See Fig. 4.14.

;

)&% oys.

Where the cavity or core shape is not complex and a cast-

g- adicated, high strength wrought alloys of beryllium copper are

p , ube, b ar and plate 'form. These may be ordered from stock

led

or precipitation hardened state. Brush AIloy

25 is

one of

P

nd most available wrought alloys.

2

h a t The mold maker can heat treat a beryllium copper mold

b s hat the user desires. The hardenabilitv of bervllium CODD-

a befyllEitw wpper

or

aluminum)mold by the ceramic castine

ative righi

cavitv lefi

which is then

used

as a

IS

cast from the ceramic

.

en^

orp. Pleasantville, NY



146


dependent upon the shape of the master. For simple regular shapes, such

as pen barrels, the plating can go very fast, and a wall thickness of

3 32

in.

and over can be built up in a day or so. On plating masters that havea number

of recesses, such as those for gears, the plating rate will generally be slower

and sometimes as little as .010 in. is put on each day.

MATERIALS FOR MOLD MAKINQ 47

lications of electroformed cavities are numerous, but it

is

difficult

ral rule

as

to when they are indicated. A slight change in

w ll indicate that a cavity should bemade by electroforming

machining, hobbing or casting.

A

frequent reason for electro-

presence of delicate detail in a cavity. Since the electroformed



There are two methods used in the making of electroformed cavities.

In one method a relatively thin layer of hard nickel is put on, generally

less than 16 in., and the balance of the build-up is a softer nickel. The

hard nickel runs around

500

Brinell and the softer nickel about

150

Brinell.

Another method of electroforming, developed in England, builds

UD

I

pproximately

3 s

in. of nickel having a hardness of

450

Brinell. To buili

up the main mass of the cavity, copper of 220 Brinell is plated over the

nickel. The copper is used because it is somewhat haFer than the soft nickel

generallv used. it builds at a much faster rate. and Gt builds much more

evenly so that the many machining~during the

build-up which

are usually necessary to remove the trees and

~ o i n t sre not reauired.

At some point during the electroforming process the master is pulled from

the cavity which has been formed. In some instances, such as formation

of cavities for pen barrels, the masters are pulled when the electroformed

shells are about 1 16 in. thick. These masters are invariably of metal and

they are started over in the cycle while the first shells formed are returned

to the plating baths for continuation of the build-up. In this way a number

of cavities can be made from one master in a reasonable time. On such

things as gears with relatively delicate teeth, it is seldom practical to reuse

the master, so the master is left in until the cavitiesare ready for machining.

In such instances a master is required for every cavity desired.

The plating masters are made in numerous ways. A very common method

is to machine them of metal. In the case of pen barrels, heat-treated stain-

less steel is the most commonly used material at this writing. For gears,

probably the most common material is brass. Many masters are made o

plastic materials. A common method of making multiple masters is to

machine them first, and from this make a cavity into which can be cast

almost no loss of dimension. Epoxy resins can also be used to cast into

or molded various plastic materials. Many shapes can be molded with

I

such a cavity, reproducing the original master with considerable accuracY.

When oriknal masters are made out of wood. leather, or other substances

which cannot be put into the plating bath, they can be reproduced by making

a cast over the original master and casting back to reproduce the original

master in the desired plastic material. If the proper materials and techniques

are used, no discernible loss of detail will result.

1

reprodues the finish on the master no polishing or other

Electrofonned cavities are frequently used because of

can achieve. A brass master is simple to make a nd easy

ensions. Electroforming cavities will exactly reproduce the

on the master. When mbdels are available, very often an

cavity is cheaper

because

the model can

be

used for the

and no metal fo rm is required suchas for casting o r hobbing.

K JS- EIeBrnformed insert of large mold for clear plastics drafting instrument All

vmbers and letters are raised on the mold surface This mold insert would be

k

m o u l t o make by any other process Courtesy Electromold Corp. , Trenton, NJ

148 PLASTICS MOLD ENQINEERINQ H NDBOOK MATERIALS FOR

MOLD

M KING 349

Electr~formings an economicalway tomake cavities with raised details,

Rg

.15 For example, in the making of molds for speedometer or clock

dbb a piece af bram can

be

polished and then engraved with the numbrs,

olution

e d

the eleetroforming performed over this. The result

is

eledroformed

avities

which

have the numbers raised from

a

highly polished surface,

.006-.010 /in.



and no further work is necessary.Cavities with delicate and undercut detail

em side walls can be made.

0 t h methods and materials are available, usually with the result of

m nk cast

-008

,008-,010

Some alloys Before

mach g.

b s wnimq,

to make spur gear cavities, and even helical

gars,

but when

.000 .002 No

TOOL

STEEL CASTINGS

cate

makd

components

in

multiples are economically

1steels on a custom cast basis, and are therefore

and thermoplastic materials. The suppliers use

pmess to reproduce the originalmaster patterns, similar

& t used for the production of beryllium copper casts,

wka

% j@img 0wae it shs

me:42. In preparing the master, the mold marker must

pt(pk@d method. Tlpes

of a e ceramic cast, the tool steel, which has a dif-

for

b#mce,

with hobbhg.

at gf beryllium copper, and of course the molding

c material to be used.

vailable, which are so far only suitable for ther-

molded using powdered metal technology.

y the

M

Company, and while some details of the

Wliug

Green

OH

-

15 PLAITICS M a B

ENGlMEERiNG

HAbYDIBOOK MATERIALS

FOR

MOLB

M KING

161

of c-tiags i&Otellitea, which is i n c s r p o m d in a

matrix

of a copper alloy,

persion of carbides.

he

wW combbation, in the form of

the

same way a would be used for the s t e l alIoy involved in

very m pow r is compacted around o r in the replication of a master,

The dimensions normally increase slightly on heat-treating

which the mold maker prepares and f u r n h w , Only one master is necessary,

regardless of the number of cavities

needed

The imitations are: (1) size-generally not mr than 9 square inches at



the parting line of the cavity itself; (2) proportions-not more than a 4 : 1

ratio of depth to minimum cross-section dimension; 3) surface f ~ s h - 2 0

to 25 microw s furnished, with the possibility of improving this to 4 mi-

m n s by polishing; (4) apparent hardness-Rockwell C-41, which is ac-

t u d y deceptive on the conservative side, since the composite includes Stel-

lit# arbide particles,

both

of which by themselves have a hardness of

Rockwell

C-58

orm

i dwea t

int

test mvhy is @@

&&ar

~ e e de

allowed

for.

aiMyds of

the

&mposite:

'I CaWt

35

PL STICS MOLD C VITIES

sls

are used for molds made of plastics materials. One

rsn widely used is the room

temperature.vulcanizab1e

Physical properties:

Tensile strength 110,NlQto 140,009 psi

oun'd reproduces fine details, very accurately and,

'Compression strength

220,000 t

~ , 0 0 9si undercuts will be no prob le i .

Modulus of elasdcity

s epoxy glass, urethane, polyester, etc., are then cast

Apparent hardness

Another

type

of P/M mold

co

pa of J m w t c m n p l ~ l y i

feasa-Tic*.

Originally

rounds and

TREATMENT

Stellitee is a registered Trademark of the C

variously as Custom Cavities, Replication

Perm-Tic* is a registered trademark of Alloy

NY.

9

3f tl

RTF

mad

tech

beca

Ci

in tk

mad

prot

can

a s t

Upon cc

cumstar

Protecti,

not, shc

many a

152 PLASTICS

MOLD ENGINEERING HANDBOOK



-

Parn.

to

i

mck

Q R ~

mistana.

Any rp tnl

Peem

cp FWY k q ~ j ~ ~ t c h e s

tool

marks.

~ ~ n dceora i e r

texnrJCTmAid

suflaces

every instance, a procedure lower in the table can

be

followed by one pre

ously mentioned, if applicable to the same metal.

Tungsten Disulfide

Ahmrinum

permanent, as for the graphite process described below.

bricativemating developed formetal

P r d d ~ t ytl mses of 10 to

Diversified

Drilube, Inc.,

~ r h s a ,

OK

Co.

Mauntain

View, CA

Fhoi

Nitn

54


are reported from thermoplastic molds treated by this process. Mold sur-

faces to be t reated by this process are treated with a binder and then exposed

to a high pressure 120-130 psi) spray of ultrafine graphite particles. This

pressure spray impinges the graphite onto the tool surface to a depth of

0.0002 to 0.0004 in. A surface coat of .00008 in. builds up on the surface

of the mold component. Lubricative plating may beapplied tochrome-plated


55



surfaces. However, to restore size and polish, surfaces must be buffed.

Chrome Plating

Many molders have found that chromium plated molds are a great asset

and specifychrome plating on all mold cavities and plungers. Chrome plating

is also used in mold repair work for building up worn sections.This requires

a

cleaning tank, an etching tank, a plating tank and a final cleaning tank.

An electroplatinggeneratorandfacilitiesfor buildingup the anodes, as shown

in Fig. 4.18 are also needed. Chrome plating equipment is very useful in the

hands of an experienced workman. Plating specialists do this work for the

small tool shops. See also Chapter 3 . Only few platers* are equipped to

chrome plate

nitrided surfaces.

Electroless Plating

As used on plastics mold components, electroless plating is nickel plating.

Electroless simply means that no electric potential is applied to the bath. The

result is a much more even deposit-no undesirable extra build-up on sharp

corners, and nearly perfect penetration into recesses. Furthermore, if one

of several patented baths is used, the deposit may be hardened after plating

by baking at 750° F.

The softer nickel deposits and even electroforms which average Rockwell

C-50 can be protected against scratching and wear by hard chrome plating,

if desired. Chrome plating bonds better to nickel thandirectly to steel. Nickel

bonds better to steel than chrome does.)

Besides the obvious situation where electroless nickel is used for molding

surface protection, it is extremely valuable and unique in use for protecting

the surfaces of the mold frame itself, including the drilled water lines, from

corrosion resulting from acid conditions in the water and condensation on

other surfaces from humid atmospheres combined with refrigerated water.

Accordingly, the rear surfaces of cavities and cores, including 0 Ring

grooves and cooling holes, are improved by electroless nickel plating.

Nutmeg Chrome C o p . , W . Hartford, CT.

Armoloy Cop.--Various Locations.

56

PL STICS MOLD ENGINEERING H NDBOOK

M TERI LS

FOR

MOLD M KING 57

of the steel, and if suitable elements are present, combines with them to form

the very hard nitrides.

Steel containing aluminum in small percentages, such as3 is particularly

suitable. Otherwise, steel containing carbon of at least .4%, together with

chromium, vanadium or molybdenum will respond to the treatment. Molyb-

denum is especially beneficial in the alloy since it reduces the characteristic

Don't specify a hardness in excess of that recommended for the steel

used for the particular application. In general, more molds are lost by

:racking than are worn out by use.

Always double temper after hardening any steel, and if the hardness

is too high after the first tempering, double temper at a lower tem-

perature.



brittleness of nitriding.

The normal depth of penetration of

.003 to .005 in. is obtained in 12 hours,

but it takes 72 hours to get penetration of .015 in.

Of the steels listed in Table

4.8

(for injection molds) the following can

be

nitrided: P-20, H-13, and 420 stainless; of the steels listed in Table 4.9

(for compression molds) H-13 and S-7 are nitridable.

An excellent steel, besides the through hardening Nitralloy Series, which

yields optimum nitriding, is P-21. It contains aluminum and if it has been

solution heat-treated as is normally done for use for molds it will age harden

as it is being nitrided, to a core hardness of 38 while the surface is 70 Rock-

well C.

THERMAL BARRIERS FOR MOLDS

As molds go up in temperature, a point is reached where the loss of heat

going from the mold to the pre&''platen cannot be tolerated. At ordinary

mold temperatures, this problem is often minimized by multiple channels

in the clamping plates, giving the effect of minimum heat transfer areas.

Transite* asbestos sheet is commonly used in the intermediate temperature

zones where dimensional control across the parting line is not critical. For

highly accurate molding with absolutely flat and parallel press platens, glass-

bonded

mica,**

a machinable ceramic is used because of its low thermal

conductivity and its absolute dimensionalstability. Glass-bondedmicacanbe

lapped to an optical flat and will hold it indefinitely. For Situations where

thermal barriers must be of minimal thickness, Nomex*** sheeting is pro-

portionally effective. For localized areas where high physical properties are

needed, alloys of titanium may give some relief.

POINTERS

In case of doubt, use a type of steel which is better than the one You

might select but are not sure it will be satisfactory.

Transite, Johns Manville, Greenwood Plaza, Denver, CO.

Mykroy, Mykroy Ceramics Company, Ledgewood, NJ.

Nomex-E. I. DuPont de Nemours and Co. Inc., Wilmington,

DE

Don't expect plating to coversurfacedefects or to improve polish; plating

exaggerates pits, scratches and blemishes.

Don't,try to cover up cracks by welding. If the crack is not too extensive,

cut it ll away, and build up the weld from sound structure.

Do not use nickel plating in contact with rubbers containing sulfur,

nor chrome plating in contact kith chloride or fluoride plastics.

Go over the sharp corners of cores and cavities after chrome plating

and check for excessive build-up which may interfere on fitting and on

sliding surfaces. Excessive compressive loads can result at the parting

b e s when clamped. Failure to do this may result in chipping of the

chrome.

Watch for,''white layer embrittlement from EDM operations on molds.

There are ways to avoid this problem: (1) Slowdown final EDM opera-

tion at the end, using low amperage.

2)

Inspect for white layer and

polish away. (It is seldom over .0001 or .0002 in. thick.)

Check with your chrome plater to be sure he takes precautions against

hydrogen embrittlement. Bake chrome plated parts 375OF for an hour,

before putting in service or applying stress.

Takeadvantage of themaraging and precipitation hardening steels;while

being nitrided, these age harden to improve interior structure and

hardness.

Remember that nitriding is theoretically an irreversible process while

through-hardened and pack-hardened steels can be annealed; chrome

and nickel plating can be stripped.

DOnot subject a steelto a surface treatment that involves a temperature

higher than that at which it has been tempered.

Often, molds are tried-out before they are plated. Do not try out

molds for corrosive or highly abrasive materials, unless they have been

plated. It is better to repolish lightly a mold that has had to be stripped

of its plating in order to make corrections, than to have to remove a

substantial amount of metal because of corrosion.

Where metal slides on metal, select materials and heat treatments for

the two components so as to obtain surfaceshaving hardnessesseparated

at least six or eight points on the Rockwell C scale.


REFERENCES

Alcoa Aluminium Handbook Pittsburgh PA: Aluminium Co. of America.

Bengtsson Kjell and Worbye John Choosing mold steel for efficient heat transfer Plastics

Machinery Equipment, Aug. 1984.

Hoffman M. What you should know about mold steels Plastics Tech., p.

67

Apr.

1982.

Properties and selection of metals in The Metals Handb ook, Vol. 1 Metals Park Cleveland

Design



OH: A merican Society fo r Metals.

Heat treating cleaning and finishing The Metals Handb ook. Vol. 2 , Metals Park Cleveland

OH: American Society for Metals Cleveland OH.

Revere Copp er Brass Publication, New York: R evere Copper and Brass.

Shimel John F. Prototyping: How and why Plastics Design Forum, p. 7 5 , Jan./Feb. 1984.

Stahlschlussel The ey to S teel) , Metals Park Cleveland OH: American Society for Metals.

Stainless Tool Steels orMolds, Uddeholm Steel Corp. 1984.

Tool Steel, Simplified, Philadelphia PA: Chilton Publishing.

Worbye John Polishing Mold Steel Plastics Machinery Equipment, Feb.

1984.

Drafting

Enginee

Practice

Revised by Wayne

I.

Pribble

designers and tool draftsmen follow many general rules which ex-

has shown are both practical and desirable. Some of these rules

n established as standards for the preparation of mold drawings;

who follow these rules avoid many of the troublesome and un-

y

mold designs which result from neglect of fundamentals. This

was prepared to detail the principles and rules of design which,

ny years, have been found to give the best results. Understanding

rules and intelligent application of them will help the draftsman

uce drawings that will convey his design to the toolmaker in such

that he may interpret it readily with no possibility of misunder-

It must be understood that the rules given here are general in

ation and are to be interpreted with regard for the special con-

existing practices of the shop where the tools will be designed,

usfd. The mold designer must familiarize himself with his own

ice and learn what limitations will modify the application of

are the permanent record of a design from which many copies

mdamental requirement of a drawing is that it shall give the neces-

biafomation

accurately, legibly

and

neatly.

The tool-maker s first

16


PL STICS MOLD ENGINEERING H NDBOOK 161

measure of a draftsman's ability is based on the neatness and legibility

of the print which is furnished him. His final evaluation is made on the

basis of the accuracy of the drawing. A drawing may carry as many as

500

dimensions and, of this number, only one may be inaccurate, but the

instead, have a print

damage done by that one wrong dimension can far outweigh the good of

499 that are correct. It is impossible t o emphasize too strongly the neces-

sity for accuracy in all dimensions and for clear presentation as well.



There is a common and very correct tendency among draftsmen who

have worked fo r a period of time in one place to leave some items to shop

output of a computer program. The size and type plotter used

practice. This may include such things as clearances, tap drill sizes, tapped

holes, etc. The consulting designer and the designer of molds which may

be built in any one of several tool shops cannot d o this to any large extent

because plastics practice is not standardized. Shop practice varies widely

among molders, and the molding shops where the molds are designed and

the men who build the molds may be several hundred miles apart .

available for draw-

At several points in this text, the use of standard mold bases is detailed.

selected by the de-

Wherever possible, we recommend the use of these highly specialized

in advance, to op-

components, and once again, we recommend that the designer keep a com-

lar plotter. The usual choices available are:

1)

plot what

plete file of'catalogs fo r these standard mold bases and standard mold

nitor screen; 2) plot

to

size; or

3)

plot to scale. Item

1)

is

components. (See Chapters

3

and

8.

Duplication of these catalogs in this

text would serve n o useful purpose. However, we d o show designs based

used in a reference

upon standard mold bases. Bear in mind that cost is a n important factor

ws for each screen

in today's economy where the wages of the mold-maker represent a sig-

nificant part of the overall cost of a mold. Th e mold bases are built as

complete units by tool shops that have the varied equipment needed to

fabricate these units. This equipment includes large grinders, tape con-

trolled mills, jig borers, and similar equipment, all of which is referenced

ause of variations

in Chapter 3 . Figures 8 . 9 2 , 8 .93A, 8.93B and 8 . 9 4 cover the utilization of

se here is to alert

standard mold drawings to simplify and shorten mold designing time, as

applied

to

injection mold design.

racings

of

their software package.

A

manual will (or

A tracing is a form of drawing used for making prints. rints are the copies on plotter configurations and operation. Of

of the drawings (o r tracings) used in the shop as a guide in the construction

at you dould also ask, Who else is using this sys-

of the mold or product. Most draftsmen make the drawing directly on

tracing paper or tracing cloth. Others make the drawing complete and

then prepare a tracing from the original.

A

tracing is never used for manu-

facturing and should not be used for reference purposes. The cost of re-

tracing is high and the draftsman must see that the tracings receive proper

care and treatment. Tracings are easily damaged by careless handling,

therefore the following rules should be observed:

i on a screen made up of rectangular pixel


shapes. However, a plotter is designed to plot a straight line from

A

to B,

or to plot a circle in very small increments so a true circle results. In any

case, we recommend the selection of a plotter based on the usual size of

should follow the rules of Orthographic Projection. In the

your output drawings.

Plotting is the final step for which a design is created. Thuspaper,

vellum

or transparent film are choices for the final plot. The choice of plotting

medium will also determine the type of pen needed for the plotting. If a one-


83



4

CA

V /TY SEM/AU TOMAT/C MOLD

CASSEMBLYJ

FOR SW/TCH COVER C /023

;sad ~r w@/4 l~

Mc d f ~

T 402381

time use

in

the tool shop is all that is required, paper will

be

satisfactory.

Plots to vellum are usually used when it is desired to have a permanent plot

from which prints can be made as needed. A plot to a transparent vellum or

film should be made because the cost of making a print from a transparency

is low compared to the time and cost required to make multiple copies of an

individual plot. The choice of transparent film is dictated when accuracy of

the plot is essential, such as in automotive panels, where scale measure-

ments are taken directly from a plot or drawing. This scaling practice, while

common in automotive applications, should only be done when it is clear omitted except where it is considered necessary to clarify

that the designer intended for the final plot to be used in that manner. In

other words, a print should never-repeat-never be scaled because handling

and humidity conditions can distort paper images.

tions or to ask questions. Tabulation of dimensions

ntle

in the actual preparation of drawings, as the possibility

reading is greatly increased thereby. The complete part and

i

uld be drawn before starting to dimension. This practice

I

erasing and give cleaner and neater prints.

1. Size of mold (number of cavities).

2. Type of mold.

3

What the mold will produce.

4 A

serial drawing number.

5 The names of people who worked on the dr wings a ~ dhe dates on

which the work was done. Names must be written as signatures.

Standard abbreviations may

be used

for the months, but the months

should not be designated by number.

FIC

1 Typical title form

sent ;

ti

C

fl

tl

s


General Rules of rafting Practice

Th e following suggestions are given for the purpose of presenting those

fundamentals considered essential to good practice. Observance of these

rules will serve to avoid errors commonly made in the design of molds.

1.

D o not try to second guess the product designer concerning his actual

needs in the final molded part . Refer all questions concerning insufficient


65

(electric heaters can be close to adjacent holes with n o prob-

kage of media). Look u p other references in this text for infor-

heating and cooling channels.

8. Rbmember that most thermoplastic materials require large degrees

o

b l i n g . However, many of the engineering thermoplastics require

heating the mold. All the thermosets require heating the mold.

~ h e r m o s e t t i n gmaterials are not quite so critical in relation to tempera-



detail or information o n the product drawing to the responsible design

engineer. Always secure authorization in writing for any changes that you

believe will improve the product, reduce the cost of tooling, reduce manu-

facturing cost, o r prevent an actual error. The best procedure is: to mark

up three identical drawings showing everything that you have used or as-

sumed

in designing the mold-this includes suspected errors, unclear in-

formation, ejector pin locations, gate locations, drafts, tolerances and

requested o r suggested changes, then send two marked copies to the pur-

chasing agent who handled the buying of the mold, and ask him to return

one marked print with the design engineer's approval o r comments. Al-

ways

retain file copies of these negotiations, including the final approved

print to which the mold is designed. Correcting o r rebuilding a mold built

to unauthorized deviations can be

very

expensive, time consuming, and

frustrating to the customer.

2.

Check the product drawing very carefully before mold design is be-

gun. Redesign the product completely when necessary to make sure that

the piece can be molded consetently and satisfactorily with the produc-

tion methods and materials ava~lable .

3.

In cases where the estimator specifies the mold design that was used

as the basis for his quotation, make sure that this design is followed unless

approval is given for deviation.

4.

Long slender cores and mold sections should be designed as mold

inserts when they cannot be eliminated by a change in the product design.

5.

While positive draft is the usual practice, d o not overlook the use

of zero draft o r negative draft when their employment may be helpful.

6. Be sure that connections for temperature control media, and the

thermostat locations d o not interfere with clamps, clamp bo:ts, strain

rods, ejector rods, o r other parts of the machine or press for which the

mold is being designed. Make a note on your assembly drawing specifying

the machine or machines for which the mold is designed.

7. Be sure to allow ample clearance between drilled holes for the tern-

perature control media and the adjacent holes for ejector pins, screws,

guide pins, bushings, etc. One-fourth inch is the minimum with which mold-

makers like to work (carry a special note if it

has t be

less than in.). For

holes in the 12- to 20-in. range, use in. clearance. Use proportion all^

greater clearance for longer or deeper holes where steam, oil, o r water is

buil

the

I

for

drab

1 of the mold. However, urea and melamine materials require

for best results in molding. Give special

nneling in all molds where maximum production is re-

not difficult to calculate temperature needs and the heat trans-

needed in a mold. Time spent in a calculation will pay dividends. As a

empirically state. that it is almost impossible to

over

mold. In an y case, over channel is to be preferred to under

of rapid conductive metals, such as beryllium copper,

h o d d also be considered. Channels in long slender core pins, is called

Considsf also the use of

heat pipes

which will either heat o r cool

urce. Air jet cooling is also frequently used, where other

cult or impossible to use.

se of standard lengths of screws, dowel pins, an d guide pins

ible. Small deviations from these standards cost money.

10. Specify the type o r kind of steel for all hardened mold parts. Call

ame o r type of steel t o be stamped o n the back of the mold

tice will give the heat treater essential information if it

to anneal o r rework the piece.

ntion to any unusual features o r importan t dimensions

m e a m of notes, so that the tool-maker's attention will be focused on

nts. Tangent radii, negative draft , o r special, sharp cor-

ardeniqg o r tempering to be done, must be plainly indi-

not deviate f ro m standard design practice unless a t least one

e w r i e n c e d designer has agreed that the changes will improve the

ation @f the mold.

methods used in the tool shop where the mold is to be

SQ

lha t the mold can be dimensioned in the manner best suited to

equ@ment available.

designer should, when possible, indicate the method of setup

ma h i n g by the manner in which dimensions are placed o n the

W the important dimensions in three-place decimals. Sh o w the

nces

snly

where required by close tolerances on the product draw-

eatbank. Hughes Thermal Products

Div. ,

Torrance,

C

and others).



167

ing. Be sure all close tolerances are actually needed, and that those spwi-

ter

Aided Design CAD)

fied

can be met. Give the tool-maker

no

mor than

50

of the tolerance

allowed by the product drawing.

osen should use a minimum number of operating com-

16. Where involved calculations are required to determine the centers

ich can be used from on-screen prompting as opposed to constant

of radii, hole location, contours, etc., preserve your figures and record

to a manual. For the experienced mold designer, a CAD program

them in such a manner that you can recalculate the dimensions easily a

e same general technique of drafting as is followed using pencil,

T

few weeks later when changes or checking may be required.

and triangle will be most quickly learned. Look for and select a



17. When checking dimensions, do a thorough job; assume that all

which is also compatible with a digitizer tablet and

dimensions are wrong until you personally prove that the calculations arc

hich allows maximum speed of selection and operation.

correct.

should be restricted to specific dimensions or specific text.

18. If an error is discovered in a dimension, fmd out, if possible, wha

ljrograms in current use allow for automatic dimensioning, thus

faulty reasoning produced the error.

ntry of dimensions should be rarely needed. Every keyboard

19

Expect to make mistakes, and check every detail to find them; avoid

ial for error, as is proven by the number of retypings needed

making the same mistake twice. A mistake on a drawing is only a potential

ct copy for this book text.

loss, but it becomes a real loss if it goes into the toolroom undiscovered,

cases where the designer is fortunate enough to have a CAD

thus causing faulty construction.

ay become fiart of his duties to predraw many of the

20 Check the daylight opening in the press to be sure the molded part

hop standards above). We have also encouraged the

can be removed from the mold.

Warning:

some daylight figures given by

ulations, checking dimensions, developing standards,

press manufacturers include maximum stroke. Others use maximum day

e following text are three check lists. We particularly direct your

light plus stroke. Be sure you known which is meant.

to the designer check list covering moment-by-moment decisions

of the designer. As you become familiar with CAD systems, it will

ecorhe evident that many of the cautions and choices will actually

me selections from a

d t b se

which is part of developing your own

Shop Standards

system. The CAD system allows drawing once, checking once, then

Each design section should compile all of the data which define its shop

over and over as component parts of a total design using a

copy

or

practice and any other standards that are followed consistently. These

ge

command. Currently, much of the data for mold bases and com-

pads, such as plates, guide pins, bushings, hot nozzles, etc., are part

standards will include such items as:

le from the vendors of these items.

1. Molding press data showing capacity, mold-size limitations, da y

t item

4,

of the designer check list, arid you are per-

light opening, auxiliary rams, ejector operating mechanism, clamp

ith a CAD system, you will note that with a CAD

ing bolts, pressures available, and the location of holes in platens.

p per size is made at plot time. Refer to plots earlier

2.

Material stock lists showing steel sizes in stock or readily available. D system,

p ge size

do not confuse with paper

3. Drill sizes and tapped hole specifications.

me. A rule of thumb is to use the smallest page

4

Standard insert design and sizes.

show the overall of the design to be drawn. It is

5. Technical data on plastic materials showing shrinkage, bulk factaf3 er he shows all necessary views on one page, or

density, draft angles, etc. arate page for each view. In the latter case, the pages

6. Spring charts showing sizes and capacities of springs commonl~ d to one page, then plotted on the desired paper size,

used in

mold construction.

be plotted on the same paper by specifying size and

7. Mathematical tables and formulas.

8. Factual data on shrinkage transverse, longitudinally, dkmet*

hat a 12 to 19 in.

monitor

screen allows only a certain

nt of observation at any one time. Using the

zoom

feature found in

all detail can be enlarged for easy drawing or view-

monitor will be available, otherwise the 2 to

2000


PLASTICS MOLD ENGINEERING HANDBOOK 69

layer capability of the CAD program will

be

almost useless. Color monitors

also assist in visualizing depth and shape just as colored pencils were

fie-

quently used to color-code a complicated part drawing to effect a 3-D image

for the mind.

Well designed and documented CAD software will follow the sequence

of items as called out in the designer check list. However, many of the

required items will have been predrawn, either for the current design or

ral colleges and universities have already installed quite sophisticated

,systems (the million dollar type) and offer courses of instruction. They

~ f f e ronnect-time to local users who only occasionally need such ser-

8s finite element analysis to determine the adequacy of such items as

~ t h ,mpact resistance, and flexibility. Material flow analysis, ther-

mamics (heat exchange) in the mold, or 3-D modeling for aesthetic

f lre other services available through these on-line or walk-in services.



available for copying from the source isk (remember-draw once and use

it over and over?) For example, assume a

64

cavity mold design for a T-

shaped part. Orientation for gating is four groups of

16

cavities each with

the gate at the bottom of the T. By predrawing the T-shape and

filing

or

saving it as a pictorial item, the CAD system allows copying the predrawn

item, orienting in any direction, mirror imaging scaling to any size, and

placing the image in

n

exact position on the page as many times as desired.

compare the

one at a time

indicating one each of 4 different orientations-and that may

be

adequate.

One tendency when using CAD is to overdraw by adding more detail than

is necessary for any given item. For example, let us assume ten screw lo-

cations are shown in a plan vie Only one screw need be shown in a front

or end elevation, but CAD plants a picture of a screw length. Thu

the unwary dl

suffice. Alwa

Hopefully,

made on the basis of ease of operation, simplicity of command structure and

speed of execution. Some software uses 150 or more commands, whereas

other software may use only 25 or 30 commands as on-screen selectable

choices. Most of those CAD programs using a large number of command8

will be run with a digitizer tablet with cursor control. Regeneration to the

monitor screen is a function of hardware, but it should be quite rapid m

avoid operator waiting time. Finally, we recommend a CAD program whid

uses precision to

6

decimal places. Most NC (Numerical Control) equipment

requires accuracy to four decimal places, or else it will reject the wmPUb

tation or entry.

The use of CAD is ~roiectedo grow at an ever-increasing rate. currently

any particular

useable much

cence of the old technology. Thus, we encourage selection of a CAD

gram which will be periodically

upgraded

by the supplier at iittle orno

to the user.

We forsee greater use of microcomputers replacing the drafting boa&.''

the 1980s. Several compaoid now off r the detailed analysis of ny p d

ular part as a service to end umF, lllbl(ldesigner, mold m ker or ~ d

p n t the desirability of having all these "goodies" at your fingertips,

cost of an infrequently used feature is seldom justifiable to manage-

k :

ENGINEERING ND DESIGN PROCEDURES

'neers and designers follow some kind of orderly routine in the

of a mold. We recommend this practice and offer the follow-

ists to assist you in theprocedure. Obviously, there must be a

ween the eng

design. The

~ld nd the n

eer

lgil

rted

designer

:sponsibl~

For this

vho

for

:ea-

e check lists are supplied. The first list covers the preliminary

usually made by the responsible engineer. The second list cov-

oment-to-moment decisions to be made by the mold designer,

third list covers the final answers and follow-up usually performed

that the mo

ze himself w

of these decisions.

ING CHECK LIST preliminary to design)

lgner

reasc

,ho aspirc

ling that

To

o one

p

all correspondence qvotations orders and other data

.

may have any bearing on the part application o r mold design.

ise

customer of any changes needed to bring the part into

ortnance with quotation.

and number.

b h heating o r cooling system.

ial chosen to be sure it is satisfactory and useable

wo-stage ejection double ejec-


ENGINEERING CHECK L I S T (Continued)

To Do Done

12. If transfer or injection, establish gating areas and specify type of

gate.

3.

Establish mold venting points.

4. Establish mold finish required by customer, by material chosen,

or by method of molding.

5. Establish draft angles to be applied. How much and where? (don't

forget negative draft is useful).


ER CHECK L I S T (Continued)

To Do Done

IW you are ready to select drawing paper size. Is size selected

ge enough to show all needed views without crowding?

using a standard mold base (or plates) draw in the complete

)Id base outline including location of guide pins, screws, return

IS, etc.

not done in Item 5, d o so now-layout horizontal and vertical

iter lines.

Be

sure to allow ample space for all views and details.



6 .

Engineer review items 1 through 15. Secure customer's approval

where needed.

7

Engineer discuss with designer.

18. Establish shrinkage factor (transverse-parallel). If, thermoset, is

post-baking a requirement? Have you allowed extra shrinkage?

D D I G N E R C HEC K

LIST

?V

r

.-

1. Review the preliminary engineering check list (with the engineer,

f .

::

if

possible). D o you understand everything? If not-ASK.

. ; '

2. Review the catalog data on Standard Mold Bases and Compo-

.a

nents to select the most economical group of components for

7

the proposed mold design. Can you use a complete mold base?

,; I

,

Will you have to build up from standard plates? Can yo\] use

A <

standard components? Will your supplier 'start from scratch"

* .

+., ..

with raw steer (See also Chapter 8.)

,- 3.

Answer the following questions:

,

at

A.

Where can "pickups"

be

placed

if

: , '

s a deliberate undercut.)

B. Are inserts to

be

molded-in or assembled after molding? In

any case, get a copy of the insert drawing. l n s~s that inserts not

t

be

made until after the mold is deslgned (when the inserts are

to

be

molded in.)

C

Are side inserts necessary and,

if

so, how are they to be

J'

supported?

D. Are wedges or side cores required? removable or captive?

'

E. Where will wedge split line (parting line) be located? How

operate wedge or side cores?

F. What type of insert pins are to be used and how will inserts

be held on the pins?

G .

Do mold pins spot holes? D o they butt in center? D o they

enter the matching section of the mold?

Where will mold-maker want radil for ease of machining?

Will customer permit it?

Where will mold-maker want sharp corners for ease of ma-

chining or reducing cost? Will customer permit it?

Will the cavity be

hobbed, machined, cast or electroplated?

*'&. Can o r should the cavity (or core) be made in one piece?

mpc

Where are inserted sections needed?

7 t. Where are the high wear areas in the mold? Should they be

-

inserted or backed up with hard plate

cation needed or provid

To DO Done

1

yout cavity arrangement prescribed (circles-square-rectan-

lar). Will spacin allow temperature control media channels?

tablish ejector s

f

tem to be ready for item 10.

yout one molded part of each configuration in plan view and

in force and cavity outlines.

M

ejector pins (o r system established in item 8).

injection or transfer mold, establish sprue; runner and gate

es

and the material route from nozzle or pot to cavity (do not

k runner, runnerless, hot manifold, hot tip, etc.).

,on mold, establish land a r e a , loading well depth and

ty

wall thickness. -

.

;fer mold, establish pot and plunger size (transfer chamber)

e (if needed), runner size and path, and gate size.

mat ic compression mold, establish land areas, loading

ipecifications and part removal board specifications.

the center line and size of the temperature control media

Is.

n guide pins, return pins, screws, and stop pins. Use ample

r

of screws with calculated holding power to resist stresses.

:miner plates, clamp plates, width and length of ejector

rallels and stop pins.

the top layout (plan view) to front and side views to de-

~ t a i n e r late thickness, length of screws, etc.

NOTE:

One

late i i better than two thin ones. If a long running mold,

d

or prehardened plates to back-up core pins, forces, cavi-

l slides see 3(L)).

~pport illars or parallels.

U

adequate support to prevent

rgging under pressure.

n sprues, runners, gates and ejector pins.

lion all views and parts as required.

trt numabrs, material list and general assembly notes in-

,

mold number, part number, operating press data, etc.

cavities, forces, core pins, wedges, slides, side cores and

:r

parts requiring detail drawings. (Shop practice will de-

:

his.)

a

steel trade number or identification on

a

non-working

of all hardened parts (in event of later modifications re-

hkat treating or annealing).

tolerances'where needed to assure compliance with prod-

nents.

heck all drawings for dimensional errors or reversal



74 PLASTICS

MOLD ENGINEERING H NDBOOK PL STICS MOLD ENGINEERING H NDBOOK

75

6.

Enter all esiwntial dimensions on every drawing. It is bad practice

to permit scaling of prints, since the printing and drying process may in-

troduce considpble distortion.

7.

Keep

all

related dimensions together s the tool-maker will not need

to hunt for the dimensions he needs.

8

ry to keep the dimensions between the views as much as possible.

ts. Tolerances on mold dimensions are required because of the

9 Dimensions given for length of thread, depth of tapped hole, etc., are

nal variations that occur in machining, hardening, polishing,



generally understood to indicate the minimum length of the full thread,

Tool-makers will make the required allowance for the thread ending.

10 Use three-place decimals for all ordinary cavity and plunger di-

mensions, four-place decimals are used only where the extra accuracy

is

essential, o r where it is necessary to make the component dimensions add

up. Use fractions o r two-place decimals wherever ordinary scale dimen-

sions w ill suffice.

I I

Numerals and figures on tracings must be heavy enough to print

well. Most designers use 3H pencils for layout work and 2H pencils for

Allocation. There ale two systems of showing permissible

dimensioning. Allow space so that it will not be necessary to crowd di-

from a b@ic gauge dimension. One is the bilateral system, and

mensions. Decimal points must be distinct so they will print well.

is the ungateral system.

12.

Dimensions should be given at points from which it will be easiest

r to understand the unilateral and bilateral systems of tolerance

lerance is a measurable extent of magnitude, it, like any other

can be accurate only within specified limits. There is no such

n exact dimension.

ne of these two systems of tolerance allocation may be used in

six, nine, and one often cause trouble when read upside down, therefore

tolerances for each of the four sets of gauges which may be

the last figure in a group co~ta in ing ny of these numerals should

be

changed. For example, 0bl may be read as .190 when inverted, wher

.062 obviously would be upside down if the print were turned.

14. Where holes are designated for assembly purposes, as shown in

5.3,

use a

double letter such as aa,

bb, cc

etc.; this will avoid confusin

designation with a letter drill size.

15. The plus tolerance is always placed above the minus tolerance w

they are added to the drawing. For example:

ion. A

1 500 005

dimension would require a maximum

.125::g, .2124$%, .314 .002

This

is

done because it is common practice to mention the plus toleran

first when speaking.

Tolerancesand Allowances

Allowances are the intentional differences in dimensions on two

which fit together. Toleran~es re the allowances made for unintent

be

unr

wi


REFERWC E INSPECTION

GAUGE GAUGE

PRECISION TOLERANCE TOLERANCE WORKIING

GAUGEOLERANCELOCR-

TOLGAUdO;ERMNCE


F

Of any part could be only .010 and still pass the gauge inspection if

gauges were made to the extremes.

In the bilateral system, the high and low limits would

bisect

the gauge

talerance z o m , and the tolerance for each gauge would be allocated as

or minu from its respective basic dimension, as shown in Fig. 5.4.

In the unihteral system, the high and low limits would

encompass

all

the gauge tolerances, so that the tolerance on the gauges would be allo-



A

REFERENCE INSPECTION

GAUGE I hlJsf

PRECISION

WORhKlHG

T O N C E

~ E R A F

GAUGE BLOCK T O ~ ~ . C E

\

as minus from the high limit and as plus from the low limit.

bilateral system has been in use in this country for a long time and

probably is adhered to a t present for most general commercial work. Ordi-

nance engineers contend that the unilateral system is more scientific than

h e bilateral and more effective in precision work.

alcula ting Mold imensions and Tolerances

imensions are compund from the following general rules, which

sate for the variables. Nevertheless, remember that good judgement

s better than following a rule.

Shrinkage allowance is a n "add-on" factor. Every molding material has

a shrinkage fac tor specified by the manufacturer. Wamina-Some recentlv

1

developed materials actually "grow" when taken from the mold, therefore

shrinkage factor is a negative value.) The factors furnished by the manu-

facturer may be a narrow range such a s 003 to

004

in./in. for mica-filled

phenolic. They may also be a wide range, such a s .005 to

W

n./in. for

gauge

s

1 5U n d a minimum,gtue

Nylon. In any case, the designer

always adds

shrinkage (except in the

~ l day that

the

005

aforementioned warning , Space does not permit disposal of the argument

OQa5 onehalf of ~ a eb u

that shrinkage is added to some parts of the mold and subtracted from other

would be dimensioned

1 5

Paas of the mold.

Ifpa

~ d d

hrinkage t o any part of the mold , a d d it to all

dimensioned

1 495 .OW5

made to

the extremes a

The designer should s&ze every opportunity to obtain and record specific

1.5055 and still pass the

l r inkag e da t a f rom his own shop. This is done by checking molded parts

be .Ol 1 instead of .010.

and mold at room tgmperature. Subtract the smaller dimension from the

The unilateral sys

lPrger dimension, then divide the result 93 the dimension of the molded

either

above

o r

below

Frt

he results of the division is the shl i r&@~e llowance in inches per

the gauge is for a maxim

knch, and

should compare with t h ~ , v g , l g ~iven by the manufacturer. Your

gauge is for a maximum

Own

on specific materials &&@ixes used under your shop con-ditio9s will be far mo re reliable akd reproducible than the manufacturer's

d a t 8 . 1 ~ t h is point. let us mention the phenomena of different rates of

shrink ge in the same part. Shrinkage

parallel

to flow may differ f rom

age

transverse

to flow. Shrinkage in thin sections may djffer f rom

age in thick sections. F3 "differ," we mean that the rate of shrinkage

ent Or

in thousands of inches per inch or v k t q v e r other method

k ag e a t e specification is used is different. In thermosetting molding,

78 PLASTICS MOLD ENGINEERING HANDBOOK PLASTICS MOLD ENGINEERING HANDBOOK 79

shrinkage rate will

be

one value when compression molding and another

MOLD STAMPING

value when transfer or injection molding an identical material. Thermo,

plastic materials attain different rates of shrinkage depending upon (1)

letters and numerals when

cylinder temperatures at time of injection, (2) nozzle temperatures, and

3)

it is essential that proper depth be allowed for painting, good appearance,

temperature of the mold. Some molders select an

average shrink rate

apply

,tc. Many times, the depth of such lettering is left to the mold designer

this to the mold, then set up manufacturing conditions to obtain the allowed

and, in such case, he should submit his specificationsand recommendations

shrinkage rate.

to the product designer for approval, thus making certain that the height

of raised letters or lines will not complicate the assembly of the device.



The dimensions which locate

holes and bosses

in the plan view of a mold

should use the

nominal

dimension plus the shrinkage factor. For example,

unpainted will be plainly

a dimension of 1.250 g or the location of a hole, and using a shrinkage

aractersare large, however,

factor of

008

in. per in. would

be

specified

as

1.265[1.255 (nominal) times

1 008

equals 1.2651 or [1.255 (.008

X

1.255) 1.2651

Characters are often specified by reference to some standard type speci-

Projections pins

or other male parts of the mold are calculated by

men book such as

The Book of American Types

published by American

subtracting 1 4 of the

total allowable

tolerance from the

maximum dimen-

Type Founders, of Elizabeth, New Jersey. The height of the letter and its

sion

permissible. Then add material shrinkage. For example, ,500 010

weight and depth should be specified, as shown in Fig. 5.5. Lettering

would become .505 plus shrinkage. Tolerance on mold is given in minus

that isilo be painted will be raised

in

the mold. The elevation of such letters

direction. \

should be at least one half the weight of the line. All characters must be

Cavities depressionA grooves

and other female parts of the mold are

stamped in the m6ld left hand. A typical designation would read: Stamp

calculated by

adding IJ

of the

total allowable

tolerance to the

m

in %-in. L H hiracters .005-in. deep. Full information must be given

.

dimension

permissible. en add material shrinkage. For exa

for special characters.

500 .010 wide groove wohld become .495 in. plus shrinkage. Tolera

on the mold is given in plus direction.

Pad Length. Lettering is often placed on a removable pad in order to

facilitate stamping and to permit a change in lettering when required. The

Dimensional tolerances, as given on a tool drawing, should amount

length of the pad for stamped characters (letters or figures) can

be

calcu-

to no more than 1J2 the desired tolerance for the molded part because the

hted by using the following formula:

mold variation is only

one

of the factors influencing the final dimension

of the molded part. Other factors affecting the final part dimensions are:

length number of characters X height of characters

the height of one character

1.

variable material shrinkage from batch to batch

2.

heat

n. letters would be cal-

3 pressure

eight)

1 8

in. (height)

4 cure or chill time

The previous rules are used because a hole maybe made la

boss may be made smaller to achieve the desired results after t

mold indicates the actual shrinkage and the accuracy of the tool

W

Drafls

and

Taper.

Wherever possible, draft should be al~owed

the tolerance given by the part drawing. However, this is not alwa

sible. Then the designer must be very careful in using draft. The c

should determine just where a dimension is to be taken and in which d

tion draft should be allowed.

In case of doubt, dimension the mold so metal can be removed to

the correction at a later date. Don't forget, it is always cheaper to

t the letters

re

to e

r ised

or

questions than it is to guess wrong.

18 PLASTICS MOLD ENGINEERING HANDBOOK PLASTICS MOLD ENGINEERING HANDBOOK 181

TOOL STRENGTH E 5 2 Recommen hickness for Mold Cavities

Molds are designed to give maximum life and low maintenance expense,

Small, fragile seitions should be designed for easy removal and low-cost

replacement. Designing for adequate tool strength is always a problem,

and no definite formulas can be given. Several fundamental considerations

will serve as a guide in the solution of many problems. The strength r e

quired must be adequate to resist the compressive, bending, or shearing



stresses set up by the highly compressed molding compound as it moves

into position and hardens. Some of these stresses may be calculated when

necessary, but the mechanical construction of the mold is such that most

designers do not calculate the actual total stress loads on all mold-sections.

mold sections must be adequate.

for the basic wall thickness of mold sections under 2-in.

3.

The wall section of mold cavities, loading pots or transfer chambers,

to use

60

per cent of the depth of the cavity, but never less than

must be sufficient to resist the spreading force resulting from the mold

pressure.

4. The thickness of the bottom area of mold cavities must be sufficient s

wh r

the depth

is

greater than twice the basic wall thickness,

to resist distortion and breakage.

adc& ional /s in. to the wall thickness. For example, a cavity with

The strength of the ejector bar increases in direct proportion to

t

.

d iah te r loading space of 1-in. depth would require a

131

16-in. wall

width of the bar and as the square of the thickness. This means that the

ess,

his is

calculated as follows:

bar should

be

kept at the minimum width required for the ejector pins

since a small increase in thickness is much more effective than a consider-

(1 X .60 3 16

131

16 in.

able increase in width. A desirable average minimum width for the ejectw

bar is 2 in.

n the clean-up size of &he tock.

from the formula for beam stresses.

In

this formula the stress is

strength of the bar

is

doubled, but

if D

is doubled, the stren@h-Qfthe

is quadrupled.

Table

5.2

shows values which have been found to

be

satidmtbw f w

wall thickness of mold sections.. An approximate general fOmuYa


..>* . \

?

,,

j

DECIMAL DldlENSfON

FOR THIS FLAT SAME

AS DECIMAL DIMENSION

OF RETAINER

SHRINK FIT ALLOWANCES

Notewol

in the mo

be stressec

sidewall, s



hould, be subtracted fram the dze of the mold

of the hole in the retainer plate, Siaple press

HEEL OR-

FLANGE)

extreme stresses are antidgated.

necessitated the making of the split

cavities

It would be difficult to machine

such deep barriers

in

a solid block of steel.) The mold is shown in open

position with the ejector pins raised. Six ejector pins are used and four

MOLD PINS

movable pins hold the inserts.

his

makes a total of ten pins for a piece

approximately

2

by

3%

in.

FIG.

5.7.

Each

cavity

of

this

four-cavity semiautomatic landed

p h n ~akd is

made

t

pieces and all are shrink-fitted to the retainer. Malded p rt

is

shown t ri ht.

Many kin1

to locate

entering tl.

tiate them ..,...

maximum allow2

1W

PLASTICS MOLD

ENGINEERING

HANDBOOK


which form part of the surface of the molded piece should be chrome-

ple;ted when the rest of the mold is plated. Ejector pins should be fitted

q ~ ~ rins must be as short as possible but must raise the piece

3 s

in.

the

top of the cavity for production convenience. If wedges are raised

ejector pins, they should be raised 6 in. above the retainer so that

h y be picked up easily.

designers specify that ejector pins

3/16

in. and larger be given flat

a e f s . Pins up to %-in. diameter have flats whose depth is equal to

p

radius of the pin. For pins from % - to %-in. diameter, four flats

t~ 1/16

of the pin radius are used. Shown in Fig. 5.9 a t A ) ar e the

imosely in the ejector pin plate. This allows the pins to align themselves

k r h the holes in the mold sections. The ejector bar does not expand as

much as the heated retainers, and this differential introduces some mis-

alignment which must be compensated.

%ns with a n integral cam to produce sidewall undercuts are called jiggler

pins. Molds should be designed to make use of ejector pins whenever



p s vents often used where the ejector pin is inserted a t the bot tom

1eep cavity, or where it is necessary to fill out a thin-walled section

@&vide onsiderable gas relief. Such gas vents are generally flats f rom

005

in. deep that permit trapped air to flow through and escape.

R

ap provided will allow only a small amount of the molding com-

K

to pass, since the plastic material will set up quickly in this thin

a n thereby block the flow. The addition of two grooves near the

e n

2

f the pin, as shown in Eig. 5.10

B)

will help keep ejector pin

of flash. These grooves will carry the flash forward out of the

each stroke. A .blast of air dislodges the flash from the pin.

plethods, as shown in Fig. 5.10, are used to form the heads on ejector

b head shown a t (A) is formed by heating the rod to a red heat

h n peening or swaging the head. This method is used where the

k

passible. The number and placement of the pins is entirely dependent

qn he size and shape of the molded piece. The basic function of knockout

pin s to remove the molded part from the cavity o r core with no distortion

c~xarring,

ncr it is better to have too many rather than too few pins to

wxximplish the desired result (see Fig. 5.8).

Figure 5.8 shows a single-cavity transfer mold and two molded parts.

% part a t the left shows the gates and runners still attached. Any com-

MnaPisrn of inserts ma be needed for this part. Six ejector pin marks may

tr w n a t the outside edge. An ejector pin is also located a t each insert

@ &a

and under e ch runner. This makes a total of 26 movable pins

m Igci for this one c ity.

Ejector pins shou e dimensioned so they will come 005 in. above

the mol surface unless otherwise indicated on the product drawing. If

~Wmpings uch as the cavity number or trade mark are desired on the

'

ej& orpin, the letters should be 005 in. deep and the pin should project

blQ

in,

above the mold surface to make sure that the lettering does nat

project abo ve the surface of the piece.

GAS VENT

FLATS

-

003 TO

.OO:

DEEP

EPTH OF

RELIEF

MOLD

SECTION

0 ND

T O R PIN

EJECTOR

PIN PLATE

EJECTOR

BAR

Fro.

5.8. Uniform ejection of the parts molded in this s ing l e ~ a v i tyransfer mold is assured

by adequate number of well-located ejector pins. Molded part at left still has runners attachedb

Note ejector pin marks near outer edge.

rd

gas vents

gle for pin s

and pin relief.

B)

Gas vents and n

trength to increase depth of hole.

:lief applied 11dpin.

I @ PL STICS MOLD ENGINEERING H NDBOOK FL BTICS MOLD ENQINEERING M cglDBOOK 87



[A

8.3

FIG.

5.10.

Conventional methods of formingheads on ejector

pins.

A) Peened head;

(B)

turned head.

f i um s bins for mold construction

A)

Small pin;

B)

rge pin; C) in

of the pin (or butt w

There are a number

widely used.

hoier in the molded part should not project above the

In designing the riveted-head type of ejector pin, consideration must

be

ore than the amount indicated in Table 5.4 since the

giv n

to the fact that the principal stress is tensile. When the pin is pulled

back, flash causes the end of the pin to bind, and considerable force may

be needed to pull these pins b ek into place after they have been lifted ta

eject the molded part. The hdad must be strong enough for this stress.

A

good general rule to follow is to have the height of the riveted head equal

to one-half of the pin diameter in sizes

up

t o g.diame r. Pins larger than

M

in. should have turned heads. Pins which are less han 2

in.

in length

gener lly have turned heads, a s shown in

i

1 a't A) . The center

fix

turning

or

grinding is a great help to the tool-maker, who will make go

Pins

lxrhich are

to be prevented from turning make use of a flat t


FIG 5 12 Illustrating use of butt pins for long holes




lengths indicated have been found to be the maximum that will stand up

under average conditions. Certain special conditions and molding care

may permit use of longer mold pins. Holes which are formed by solid

sections of the mold and which cannot be replaced in the event of breakage

should be only one-half the height shown in the table. Where molded holes

of greater

be formed b

mgth than those

y two pins which

shown

butt \t

pins should be backed up by harde ed steel plates or have oversize heads

to prevent them from sinking in a so late, as sinking causes a heavy flash

to form between the pins and thus incrhr, es finishing costs.

It will be noted from Fig. 5.12 that the diameter of one pin is greater

than that of the other. This variance serves to compensate for slight mis-

alignment of the mold cavity and pins. Longer holes may be molded by

the use of

nt ring pins

which enter the opposite half of the mold, as

shown in Fig. 5.13. Entering pins should always have turned heads, since

the flash sometimes causes them to stick badly. The taper ream in the

clearance holes, as shown in Fig. 5.13 allows the flash that enters around

'

diameter

a

to move up freely when the mold is blown out, or when it filr

pushed up by the entering pin.

If

this flash is not given an easy exit, it

will build up a solid plug in the hole and, in a short time, cayse the pin to

stick, bend or buckle.

Where dimension c (Fig. 5.13) must be held to a tolerance closer t h d ~ a

010 to ,015,

The bearing

a. For pins

1 to lk time

the pin should b

surface b should

/s in. or larger in

:s the diameter of

e solid

not be

iiametc

a. The

be calculated the same as for

6

In compression molds, the length of the entering pins must

be

to permit en

The flow of

deflect these

ltry to the force bc

compound which

pins so they coul

efore it

starts

d not

f

at least /4O if possible on the molding surface. Shrinkage:)

be added.

in the table are required, they may

the center, as shown in Fig. 5.12. Butt

VG

i3 Cleaqnces

and

allowances

or entering pins in compression mold

5114 vhows good types of construction for long holes. Two pins

f each pi@ s ysually the nominal size plus shrinkage,,plus

002

a sli8htly oversize hole. This small amount of oversize play

be

th; *final molded pie& shows too much clearance, whereas

of

be

h d e

larger if it is made too small.

ather than movable, as sh

more than

4

to 2 times the

enters the loading sp

when the force enters

mold

holes to the minimum to insure that


COUNTERBORE

MOLDED PART

IMATERIAL- WOO

FLOUR

PHENOLIC.

SHRINKAGE -.008 PER INCH


191

OPTIONAL



FIG.

.14. Mold pin construction for deep holes with shrinkage allowance added.

as shown in Fig. 5.15, since it is quite difficult to drill a hole straight in

these small sizes. It is also difficult to hold the diameter closely and obtain

a good fit with the pin. This construction permits the tool-maker to lap

or polish the short bearing area after hardening in order to fit the pin.

Small ejector pins ( 4 in. diameter or less) are often designed to

be

made

FIG

.15. Design for small mold pins.

nd back-up plates

rence. This con-

*

OTHER MOLD PARTS

ty Pins or Push backs

nsively used where small ejector pins are required.

derives from their function, which is to protect the ejector pins.

along with it, and, as the mold closes, these heavy

bar back to its proper position and thus the force is

gh the ejector pins.

e diameter and number of these pins vary with the size of the mold,

I iih. is usually the minimum size. For average molds using up to

5

or

in. pin-will be satisfactory. Two pins may be used

but three or four are needed where the bar is wide.

ngement should be used so the mold cannot be

plates should be 1 16 in. larger than the pin. In

s are usbally slip fitted so they will support the

of the ejector assembly in a horizontal position.

e Melded Threads

bask major diameter of the mold section is determined by subtracting

the allowable tolerance from the basic major diameter of the molded

d, then adding the proper material shrinkage. For the tolerance on

asia mold dimension, use 4 of the allowable tolerance plus).


193



a the b ~

.

. #

t ' j , l

3

>

m he t b r d

allswatile tolerance

W

?A4 tbe aUswa

W)

g i6LUs~kreBlhratolmrar

*' '.-'. ..,JI: ,j

111

Major diamet&, g.9W

Pitch diameter, 0 9154 2

rc;

- . I I

Minor diameter,

0 8432

qaximm

. Ir, .cw=~;-.t.rb

T o d e t e d m

ti

all

of

E

diameter to lemwe

-

ADD. SHR/N;AGE AS REQUIRED

8 V COMPOUND BEING USED

b. Female threaded mold section for

I

in.

-8NC-I screw

produces

male thread

on

n u e d tliread. F o r the tolerance on the mold, use 1 8 of the pitch

binus) .

m i n e the basic pitch diameter of the mold (for -in. thread

b, o r less), use the basic pitch diameter of the molded thread

ye allowable tolerance plus the material shrinkage. For the

the basic mold dimension, use /s of the allowable tolerance

r -in. engagement, make use of all the allowable tolerance

pllowable tolerance (minus) for tool error. If more than -in.

s re.quired, compensate for the shrinkage in the lead also.

basic minor mold diameter, use the basic minor diameter of the

plus

%

of the allowable tolerance plus material shrinkage.

ces are taken as /a of the tolerance (minus).

:xample is shown in Fig. 5.19 for a male threaded mold sec-

n.-8NC-1 nut having the following dimensions:

~b J Major diameter, 1.0000 Minimum

Pitch diameter, 0.9188 :Ei

Minor diameter, 0.8647

3

et

iaahknpiwly used

in

mold.clonsuuction and typical applications

boaring

or'

m ~ i d s n d return of

?is, @ @tA) and (B), may be

$& & '

r

i .

lk

Pilw1

is

m a l l , and where

1

- -

eject1

used

p ace

-

-

)ars.

eith

rmits

The side

lei tqp or

i internal

&ham

n

Fig 5.21.

In such applications, the ejector


t30/1 $:7AJOR DIA.

PLASTICS WOL

ENGINEERING HANDBOOK



FOR

ENG GEMENT

*ADD

SHRINXdGE AS REQUIRED

BY COMPOUND BEING USED

FIG

.19. Male threaded mold section for I in. -8NC-I nut produces female thr

molded piece.

A )

8

FIG 5 20

Application of side springs in plastics molds.

b v e l G should

be

limited to 1 or

2

in. This arrangement is also

here no auxiliary cylinders are available to operate the ejector bars.

number of springs

needed

for any given mold

is

dependent upon the

the mold and the size of the press being used. The minimum number

in spring boxing is two on each side of the mold. Generally,

top1 of six are used except in unusual applications.

ejector operating springs (Fig. 5.21 , two or three springs usually will

-head screws are best for mold work because they are easily dis-

led

They also act as dowels in mold assembly, since usually they

only &/a-in. clearance.

mall

screws are used in the small molds

er schws in the larger molds. The thickness of plate has definite

hip to the size of the screks. The thickness of the head on a socket-

w is the same as the body diameter. Thus a 5 16-in. screw requires

.

minimum depth of counterbore. The right and wrong way to use

ws is shown in Fig. 5.22, which also indicatesthe proper clearances.

pth to which a screw should enter (dimension

F

s the same as the

d iameer of the screw. At this depth the screw would break a t about

@bia time the threads would strip. A rule commonly applied in deter-

he c m c t length of screw is: length of screw shall be the same



SUPPORT PIN

- PARALLEL



A) B)

FIG.5.22. A) Improper application of socket-head cap screws; B)

proper application.

as the thickness of plate into which the screw head is recessed. Where greater

holding power is required, more

Appendix.

Parallels

The parallels should be as close together as possible under the cavity;

ropped. Slots are cut in the parallels,

as

shown in Fig 5 23

allowing 1

16

to 3116 in. clearance on each side of the ejector bar. The height

e

blowing operation, and they are often cut in both front and

of parallels is calculated to allow the ejector pins to push the molded piece

in. or more above the cavity. Most designers calculate this height and

then add

A

in. for the additional clearance that may be needed in the press

set up.

IPERATURE CONTROL MEDIA AND METHODS

The maximum width of ce

@vianusly stated that temperature

is

an essential ingredient in

ejector bar. Center parallels

a g

operation or at some point in the process. Generally, this-

cannot be used, the additional s

$&with controlling mold temperatures. Uniformity of heating

These center support pins are usually

k'w

objective to

be

gained, and the problemjustifies considerable

and they pass through the ejector bar. Support pins are best located bnning. The bibliography lists texts dealing with heat transfer

way between center of the cavi

used in calculating it. Heat transfer is the name of the game,

lowance for one pin is 1132-in. for the

sferring heat out of the material and into the mold surface,

pins use 1132-in. clearance for the en

t out of the mold surface and into the material. In either

These ends holes with the small

bar and minimize the horizontal t

clearance for the central pins gives

ance in his mold construction. Support pins are not often hardened

used in molds which operate verti

often combined in molds whi

that hardened guide pins and bus

injection molds, particularly if sm



200 PLASTICS

MOLD

ENWMEGRIWIHm-

PLASTICS

MO W ENGINEERING H NDBOOK 201

STAT ONIRY

AWTY

BLOCK



no 5.24. (A) Proper steam channeling:r (greatex

than

H

y

representsshortest d i s tam

from channel to aavity; y represents gr%at& dbt8a~erom channel to cavity. B)mpropeF

staam channelinn:

x

less

han

h

vl reoresents shortest

distance

from

channel to cavitv

w

the same as the reit of Qt:

m~#d.'

t bg h s laqg .&oy h to permit sq

of the

medi

to

be

chanjxded 'throp@ it;, by

ah

means make that provish

This ap&es particularly to ,iqjw ion molds, w b r e even very small

pins must be channoSed to

g e d i

fast &&btrawier out of she mold[

material. Long or large makt pluqers must

be cham14

or cored as shot

by Fig. 5.25 to get uniform tempedture'hth the femaEe section of

the

mo

e

emphasize t ' h t molding of urea or tgekminta materials

uniform heating which means direct chonneb in th a l e section of

mold, a s well as, direct channels in the

r&rnt~

eo.tim of the mold.

general rule for all molds, it is wise to

us dkM

obnneb whenever4

male seetion

is

longer t h a ~wice if diameter. Referenw

tm

vendor's c a w

will show several standard baffle typ s

as

rvahble off the shelf.*?

M a t molders want the temperature centro1 media &f~xtiong~odl

ba& side of the mold, that is, away from .the o p e a t ~ r . M

t

rt wires on the operator side if there is any other

wag

d

mtkir&g

saq

con~k~tions.ll other champ1 openings should

p q g

to prevent leakage of the media. i

v i L r.d;;.,

I

The assembly shown in Fig. 5.26

gives

the

meld half using directed flow of media.

The

W ~ M

to mold designs in which there is anly

in passing from the inlet to the outiet.

T

in a zig-zag fashion from inlet to t

R

U N G E R PL TE

mold plunger has been cored out for circulation of heating or cooling

Chemical Products Inc. Kingsport TN

b-

rfled to pgovide a one-way channel. We definitely recommend directed

n all molds yquiring cooling or using hot oil or hot water, for heating.

npressioq transfer or injection molds for thermosets, that use steam

ieating medium, make good use of undirected flow.

n

this case, all

els in the

sdme

plate are interconnected to allow free.access of high-

[re steam The only requirement is that the inlet be at the highest

in the rnqld, and the outlet at the lowest point in the mold. A w rning

xder here.

When

steam channels enter the male section and the con-

te

must return to a higher level for discharge, be sure to use

directed

A

generkl rule, provide a channeling so that temperature variation

rectiokb &tion will not egceed 20°F.

s

Pins

and Guide Bushings

pins and bushingsare used on all except the very simplestand cheapest

ld molds. At least two pins will be used and as many as four may be

ed. Where only two guide pins are used, one should always be '/s in.

than the,:pQer

so

the mold cannot

be

assembled incorrectly. Three-

our-pin

z

mpy use the same size pins i an unsymmetrical

2 2 PLASTICS MOLD ENGINEERING HANDBOOK

PLASTICS MOLD ENGINEERING HANDBOOK 2 3



CLEARANCE HOLE

DIAAfCrER

6

- -

GUIDE PIN 7

C)

and guide bushings.

A)

Guide pin; B) guide bushing;

I ,

cilitates entry perpendicular to the plate. The

the holb in the retainer plate, shown at

x

in (C), is usually peened

sure that the bushing does not pull out. In cases where the length

FIG.

5.26. Assembly of plungers, steam or water plate, guide pins and stop blocks.

s the plate thickness, allowance should

be

made

lines indicate channels for directed flow of media.

peenin8 each end of the hole or use a shoulder bushing. Set

often used when the length is very short. set screw, as shown

spacing is used. Guide pins are located as far a

e guide pin is not backed up by a plate. The

effect of the clearance between the. pin and bus

uld rest against a flat on the side of the pin. There are several

The diameter of guide pins varies from /5: in, for small molds

standard guide pins and bushings. It is recommended that

5- to 10-ton presses up to 3-in. diameter for 1500-ton presses. A

practical size is 3 4 to 1 -in. diameter for the

pins are seldom lubrimted and frequently are used in a rusted

molds. When considerable side thrust is expe

ses them to stick. Thus considerable damage

metrical flow conditions, the larger size pins should be selected.

out of the plates. These factors necessitate

The length of guide'pins should be such t

pins. (Also improved shop

pin will enter the bushing to a depth equal to

plunger enters the loading space. Guide bus

long as the diameter of the pin and they must always e used

.

plates are not hardened. When hardened plates are used, guide

HOBBED

CAVITIES

AND PLUNGERS

are not absolutely necessary but they are preferred,

espeeirrfly

i

ed by the methods used for

will be a long running mold.

ob comes within any of

Guide pins and bushings must

be

press

economical for the job.

talerances and construction details for the

at A) and (B). The section dimensioned 1.248

at

(B),

is helpful to the tool-maker because

bushing in the plate a short distance before the p r e ~ ~ - f i t ~ d :

esigns are required in the mold.

-

w

2 4 PL STICS MOLD ENGINEERING H NDBOOK

PL STICS MOLD ENGINEERING H NDBOOK 2 5

being smaller cross section as the runner branches out. The

match at the parting line.

OPPOSED CAVITIES AND BALANCED MOLDS

When the shape of the cavity or core is such that the molding press



will create side pressure (bending pressures) the cavity arrangement

usually back-to-back,* i.e., so that distorting pressures in adjacent

offset each other. This is indicated in Fig.

5 28

which also shows

balancing a mold. However, the balanced mold is most often a ter

in describing a particular arrangement of cavities in an injection

is the same length. Further, runner sizes (diameters or cross sectio

gradually in exactly the .%me fashion in each of the paths. The

RFACE FINISHES AND TEXTURED MOLDS

of the Plastics Industry and the Society of Plastics Engineers

master blocks with surface finishes clearly defined, specifmble

atld capable of duplication by any knowledgeable moldmaker.

houses have also created textured designs that can be specified

resumably to be applied by them). Specialized businesses

making the finish requirements known at the time you

REFERENCES

e

PA

Robo Systems,

1984.

of plastics part

design,

lastics esign Forum Nov./Dec.

IG.

5.28.

Opposed

cavities equalize

s i d e - p r e s s ~ ~ .

he I

for

sinking

cavities.


DuBois, J. H and W.

I.

Pribble,

Pl a s t ~ c sMold Engineering,

1st Ed. , Chicago: American

Technical Society, 1984.

Fine, Arthur and McGonical, Charles, Design of complex connector mold, Plastics Machin-

ery Equipment, Mar. 1984.

Heat Pipes, Torrance, CA: Hughes Thermal Products, 1975.

Leonard, Laverne, Window profiles raise designers sights, Plastics Design Forum, p. 62,

JulyIAug. 1984.

Mafilios, Emanuel P . , Designing molds to cut thermoset scrap,

Plastics Engineering,

p. 35,

Oct. 1984.

Mock, John A , , Mold design, manufacture and control-An integrated concept, Plastics

En-

pression Molds

gineering,

Jan. 1984.



Revised by Wayne

I.

ribble

Nelson, J. D.. Shrinkage patterns for molded phenolics, Plastics Engineering, July 1975.

Pixley, David and Richards, Peter, Thermoset or thermoplastic for electrical/electronic E/E,

Plastics Design Forum, p. 28, Apr. 1981.

Pribble, Wayne 1 Galley of goofs (phenolic part d ~lastics ~ e s i g norum Nov /Dc~

1984.

Sors, Laszlo, Plastics Mould Engineering, Oxford: Pergamon Press, 1967.

Sors, Laszlo, General Electric launches potent design info system, Plastics World, p.

32,

~ u g . 984.

of the molds used fo r thermosetting plastics are the

S o n , Laszlo, Designing for producibility-A roundtable forum, Plastics Design Forur

ression type. It is the oMer molding method, and later developments

p. 23, Jan.lFeb. 1984.

prior arb. In developing this text o n mold design, it is

he compression molds and follow with the other

Suggested

for

FurtherReading

of the design of a single-cavity hand mold that is

atic 12cavity mold will be used t o introduce the

Krouse, John

K.,

Automation revolutionizes mechanical design, High Technology, Mar. 1984

1 calculations and basic design procedure. This

Levy, Sidney, What CAD/CAM programs may not do for the designer (yet). Plastics Design

F o r m , p. 62, Nov./Dec. 1984.

undamental mold type is followed by discussion

Levy, Sidney, Complete CAD/CAM moldmaking software, Modern Plastics, July 1984.

considerations that arise in the design of other

Hand molds are being eliminated for many applications because of the

and molds continue t o offer better answers for

rt assemblies in many applications. Hand molds

nsfer and injection molding. Show n in Fig. 6.1

set that may be used for the conversion of hand molds

semiautomatic. Standard mold bases and units should be considered

r all new hand o r single cavity molds.

I

COMPRESSION MOLDING

reader should review the data on compression molds and compression

t this time so that the forms of the various types

s and the operation of compression molding presses

The basic molding problem calls for a mold that will

the compound t o the desired shape, and hold it under compression

while the chemical action which hardens it takes place. This must

t and least costly manner, the mold being designed so

2 7

208

PLASTICS MOLD ENGlNEERlNO HANDBOOK

COMPRESSlON MOLDS 209

gligible. The use of preforms also reduces the loading space re-

e mold.

uctionrequirements of the user of the molded

partswill

determine

m numb& of cavities to be used. The fact that 100 cavities are

intain the rate

of

d 'very doesnot mean that all must be in one

r bottle caps hav contained as many as 150cavities;molds

may contain 500 ca

1

ies in

a

single mold. These large moldsare

e averagemold will be found to contain from5to 15 cavities

-sized

parts. The use ofa low number of cavitieshas manyadvan-



contour. 'Ilris action may ppsdwce highly local id stresms in various

p

I

af the mold,

and

thenby

c w

serious mold breakage if the w l d

p i ra

the smaller molds remain open a shorter period for loading, etc.

the smaller molds, pressures are more uniformly distributed

s minimized, as repairwork on a smallmold means production

number of cavities. For example, if

a

36-cavitymold were

press for repair of a broken pin, all production would be

instead,&hree 12cavity molds were used and one had to be re-

one-third'the total productign would be lost.

airkt

ftpm

ot

singlecavity c ~ m pm t ~ b nid . vany hand

moLd9are

W111g~ wD

28misRt~a~~d~y the

usc ~f

these u .

Comesy

MMMF

UnJt

Die

Frud-

Inn. 8csnwil lc I]

that the

wmpaund and inserts

may

be

i n tduced

w i l y and

the part

ejected

without distarrion.

Since

the

mold

is

idle

while

t

ie being loaded

and

un-

loaded, the efficiwcy of theise operations,

the

quality of the pbce, and the

w ~ t

he flnishixl.gopmttic~s ill

be

a tme

measure

of

the

quality

o

he

mold.

a m 1molding prm~3smolw the pmblern of forcinga bulky

mat

ri intba givenshapeandspa= bythe use of pressurnsranging from2,500

upward, accompanied by the applimtim of heat

m-3804

) farthe purpo*

af plastbiaing the compound and causing it ter flow

and

fill

out

to the mold

are not properly

designid.

The

ww

materials

mag

be charged into

the

mold byat

least threed 8 f e n t

I

Pmfoms

or pills.

2.

Frolumetrie loading by loading b a r d or measurn cup,

The preforming d a r not change the

miterial

itself,

but

@ to

VO*?

a

loadiryl

unit

o

pndnermiocd weigh. Preforms

an O W

ndW

nxthods.

Tkme

are

li5:ted

in the order af tbeir preferenceand

gem

3 Weighed

c b q e af powder or preforms.

E

DESIGN OF HAND MOLD

ssumethat a mold for a lever isdesired.See Fig. 6.2. Specifh-

this part to e

ienolic or gray

molded from one of two materials:

urea-formaldehyde compound. The

black wood

user wantsa

built quickly for test, for sample to be followed by consttuc-

;duction mold

kast costly mc

capable of producing

7500

pieces

sld that can be built will

be

a single

a week.

The

cavity hand

the designof sucha mold will

be

the tabulationof informa-

data'card, and this involves the determination of the bulk

Bulk Factor Shrinkage

olic wood flour 3 .006-.009

3 .008

l tions

may be made eitherby layingout the piece in sections

?rg he volume ofeach sectionorbycomputation rom theweight

obtained from the volume, using formula

I) ,

formula

is

also

used

to calculate the volume when the

I


COMPRESSION MOLDS 2

1

(1)

V

total volume of part

u unit weight of material

WT

total weight of part

&me of the lever has been found to be 0.70 cu in. We must ad d to

kh factor of 10 o include the material required for the flash. Flash

@&l tha t will e squeezed out around the plunger and through



pie

of

bw slots as the mold closes. This allowance must e made in com-

hold ing to prevent precure a t the parting line and to enable that

@ich lies on the land t o escape outward and provide a good pinch-

b r f l o w also takes care of any variation in the load by permitting

~d

to esc

ave

additipn of the i ~ oor f l ~ hhe total volume of compound

b.

-.70 10 0.77 cu in.

Lh

@attion of formula (1) we obtain for the gross weight of the lever:

6

.77 cu in

P

FJr; 76 ozlcu

in

for phenolic o r 85 ozlcu in. for urea

76

.58

oz gross weight for phenolic

. 51 .66 oz gross weight for urea

$.

tical calculations will convert these weights to 3 6 lb per hundred

hendic and 4.1 lb per hundred pieces for urea compound.

a general forinula used for calculating the total volume

r preforms. Thus,

W X B F

y

w

loo

2)

Gross weight of i o l d e d part per 100 pieces

Bulk factor of compound

- Weight per cu in. of compouna

@54 b/cu in. for urea o r .048 lb /cu in. for phenolic

Total volume of compound required

( W a d W must

oth

be expressed in lb or oz)

Documents

HARRY DUBOIS Plastics Mold Engineering Handbook 1