HARRY DUBOIS Plastics Mold Engineering Handbook 1
Transcript of 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
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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
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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
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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
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PL STICS MOLD ENGlNEERlNO HANPBOOK
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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
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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
. -.
. . {
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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
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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
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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
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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;
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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
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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
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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
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BASIC MOLD TYPES AND FEATURES
43
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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
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B SIC MOLD TYPES ND FE TURES
47
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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
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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
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@ Pb STlCS MOLD ENGINEERING H NDBOOK
B SIC MOLD TYPES ND FE TURES
5
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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
B SIC MOLD TYPES ND FE TURES
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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.
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BASIC
MOLD
TYPES
AND
FEATURES
61
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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
?
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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
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PLASTICS MOLD
ENGINEERING
HANDBOOK
AKlNG PROCESSES EQUIPMENT
AND
METHODS 7
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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
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7 PL STICS MOLD ENGINEERING H NDBOOK
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2
PLASTICS
I OLD
ENGINEERING HANDBOOK
@@LMAKING PROCESSES, EQUIPMENT AND METHODS
73
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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
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. 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-
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F
PLRSTI MOLD ENOINEERING
H tdDSOOK
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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.,
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82 PLASTICS MOLD ENGINEERING HANDBOOK
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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
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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.
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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.
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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
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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
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TOOL MAKING PROCESSES EQUIPMENT AND METHODS 101
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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
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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
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106 PLASTICS MOLD ENGINEERING HANDBOOK TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
107
be a complex and academic subject, the basic principles are
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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
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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
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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
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t
t i dFR hKering, p 45.
q i
. Z
a;Equipment
Oct.
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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
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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-
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
MATERIALS FOR MOLD MAKING
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
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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
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146
PLASTICS MOLD ENGINEERING HANDBOOK
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
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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.
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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
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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
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-
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
PLASTICS MOLD ENGINEERING HANDBOOK
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
MATERIALS FOR MOLD MAKING
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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.
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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.
58 PLASTICS MOLD ENGINEERING HANDBOOK
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
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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
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.
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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
62 PLASTICS MOLD ENGINEERING HANDBOOK
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-
PLASTICS MOLD ENGINEERING HANDBOOK
83
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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
64 PL STICS MOLD ENGINEERING H NDBOOK
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
PL STICS MOLD ENGINEERING H NDBOOK
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-
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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).
166 PL STICS MOLD ENGINEERING H NDBOOK
PL STICS MOLD ENGINEERING H NDBOOK
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
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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
68 PLASTICS MOLD ENGINEERING HANDBOOK
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.
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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-
170 PLASTICS MOLD ENGINEERING HANDBOOK
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).
PLASTICS MOLD ENGINEERING HANDBOOK 171
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.
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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
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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,
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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
176 PLASTICS MOLD ENGINEERING HANDBOOK
REFERWC E INSPECTION
GAUGE GAUGE
PRECISION TOLERANCE TOLERANCE WORKIING
GAUGEOLERANCELOCR-
TOLGAUdO;ERMNCE
PLASTICS MOLD ENGINEERING HANDBOOK 177
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-
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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.
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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
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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
82 PL STICS MOLD ENGINEERING H NDBOOK
..>* . \
?
,,
j
DECIMAL DldlENSfON
FOR THIS FLAT SAME
AS DECIMAL DIMENSION
OF RETAINER
SHRINK FIT ALLOWANCES
Notewol
in the mo
be stressec
sidewall, s
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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
PLASTICS MOLD ENGINEERING HANDBOOK 185
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
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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
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[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
88 PLASTICS MOLD ENGINEERING HANDBOOK
FIG 5 12 Illustrating use of butt pins for long holes
PLASTICS MOLD ENGINEERING HANDBOOK 89
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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
190 PLASTICS MOLD ENGINEERING HANDBOOK
COUNTERBORE
MOLDED PART
IMATERIAL- WOO
FLOUR
PHENOLIC.
SHRINKAGE -.008 PER INCH
PLASTICS MOLD ENGINEERING HANDBOOK
191
OPTIONAL
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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).
PLASTICS MOLD ENGINEERING HANDBOOK
193
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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
94 PLASTICS MOLD ENGINEERING HANDBOOK
t30/1 $:7AJOR DIA.
PLASTICS WOL
ENGINEERING HANDBOOK
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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
96 PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK 97
SUPPORT PIN
- PARALLEL
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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
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200 PLASTICS
MOLD
ENWMEGRIWIHm-
PLASTICS
MO W ENGINEERING H NDBOOK 201
STAT ONIRY
AWTY
BLOCK
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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
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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
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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.
206 PL STICS MOLD ENGINEERING H NDBOOK
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.
8/12/2019 HARRY DUBOIS Plastics Mold Engineering Handbook 1
http://slidepdf.com/reader/full/harry-dubois-plastics-mold-engineering-handbook-1 108/110
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-
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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
210 PLASTICS MOLD ENGINEERING HANDBOOK
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
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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)