x
PREFACE TO THIRD EDITION
data, and reminders to keep them aware of some critical items at just the
right time to prevent error. Thus, an extensive checklist is presented. It will
insure consideration of the potential hazards, weaknesses, and misunderstandings that face mold designers, engineers, and builders.
There are, of course, many variations of molds, whatever their general
classifications. Naturally, it is not possible in a presentation on mold fundamentals 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 methodseven those that are rarely used. Our purpose here is to stimulate interest and
to encourage original study.
We wish to thank the many users of the previous editions for their helpful
suggestions for changes and improvements in the text. Since many pieces
of equipment that are obsolete by present standards continue to be used, we
have described mold types for some of them. For instance, this text is used
in parts of the world where very primitive equipment is employed. There,
the people need data on molds for simple processing equipment, and to use
the supply of moldmaking materials, which may be available in these localities but far removed from suppliers of standard mold parts.
We are indeed grateful for the widespread acceptance and distribution of
this text since it was first published in 1946 by the American Technical
Society. We appreciate the obligation this places on us to be accurate, precise, and factual. In preparing this new edition, we have carefully researched the intervening developments and have made every effort to provide serious readers with a body of knowledge that they can carry confidently
J. HARRYDUBOIS
Morris Plains, New Jersey
WAYNEI. PRIBBLE
New Haven, Indiana
,; t,.w
•
Contents
vii
ix
2
.
15
18
.
,
I
h%WkA~
f iq - : 1
id
I
+i
37
)CLm713a
@7
.Xi1
:,
2
&&
1 , .
%
P R ~ E S S E S , EQUIPMENT AND
Pawl E. Ferland
64
65
82
86
CONTENTS
I$',
f.
S
SLrface Finish, Molds and Parts
r'.:,
xi11
%rink Fit Allowances
Mold ?%ts
W e ~ M d Parts
d
T e m m r e Control Media and Methods
Wbtt$@Cavities and Plungers
;1 D p W Cavities and Balanced Molds
Burfab@ Phishes and Textured Molds
Refe
iCX)MPRESSION MOLDS, Wayne I. Pribble
Bgign of 12-Cavity Semiautomatic Mold
i#@ng-Box Molds
W i n g Shoe and Stripper Plate Molds
M t i v e Mol&
&&$positive
Molds
kbmtvity Gang Molds
Bracket Mold
pression Mold Considerations
INJECTION MOLDS FOR
S, S. E. Tinkham and Wayne I. Pribble
299
300
vr- d z ( q
.I
7 -
j'l~\,'v
' 3 ~ : :
- 7 .
.,.,,t\
9; QtXDMom DBSIGN, Leon R* Egg
c
1:
,
'j'
,,,< i.!{-b:d&iI{-I
,a!~r:aM
9$2 ,*it.nrl'i: TO^ &'
'
,!?
I
Chapter Introduction to
1
I
Plastics Processing
Revised by Wayne I. Pribble
Wqty of applications in d i v a rnmufacturing fields (Fig. 1.1). These ma-
quality of the tool-make& wark. The molds and dies used are the
of dies or moklo bwe used
ile it "sets" or hardens to
2
PLASTICS MOLD ENGlNEERlNO HANPBOOK
8
INTRODUCTION TO PLASTICS PROCESSING
PLASTICS MOLD ENOlNEERlNO HANDBOOK
9
Continuous Extrusion
Plastics materials are extruded in continuous strips of regular section, Fig.
1.7. This is done by a machine which operates much like a sausage stuffer.
The raw material is placed in a hopper, where it is moved into and through
a heating chamber by a screw feed. At the die end of the heating cylinder
the material (which has been heated and compressed to a plastie mass) is
forced through a die which shapes the extruded section. A moving belt carries
the section away from the die, and the final dimension of the part is governed
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.
1
rl
I'
Plasti~izi
reciprae:
screw
L
k" ddh
Blow
olding
Botties and other hollow articles are extrusion blow molded of thermoplastic 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 pressure is i m w d k t f ~ l applied
y
in the clamped tube to expand it and fill out the
.
mold ntour ( ~ i g 119).
1'
I
C
Basic extrusion blowhgpdnoiple for blow molding. The parison is a tube of molten
i& i s clamped bet-n the die bEves and expanded to tlie die shape by air pressure.
te process, c d k d
-&law molding forms the parison in
n and then 1 n r 4 - m ~quiokly into the blow mold position for
ternal air pressure w &@wn by Fig. 1.la. In the final position,
OE~
is stripped from the.~otep;fn while the msld is open.
lding machines are cqdppwl to rotate the molds continuously
in the horizontal axis during the molding cycle. This pro-dwre facilitates the p r d e o n of i n r q d hoflow parts of almost
,aqp-open or closed-rigid or flexible. Ip
process, a mead liquid or powdered
&@dd is p l w d in each mold
@
@
i-n3 heated area while
-ale
. mold halves closed, they art3
~pslyin the- t w.~ ' ~- h n until
e s the mtirqiassr mdd surface
and
:nin
thesmophstics has formed in the mold su&a&, the,
J@@ 'a mter spray or air blast while rotation continued h a IF&$
faIzn:ated by aluminum casting, rnacM-*
A* 1.41- - a n %rbp.E~mmd
mbkel. The molds are vented byb%
QLW&.@I&
Wzcils on the &tpk mold designs
*w
-
t
**-ST
An extrusion die E&3
.
bm ,%&
mmd&nnc ma GP-
14
PLASTICS MOLD ENGINEERING H
A
N
~
~
INTRODUCTION TO PLASTICS PROCESSiNG
~
15
old then opens slowly under proper controls to pern. In the low pressure process, conventional injecwith resins containing preblended foaming agents or
ded by.a "piped in" gas.
the mold-maker. Good tools
,
. . ,-.
..{'
I
18
PLASTICS MOLD .EMGINEERIKT MAWDBQW
jbs, and fixtures out of plasThe metal vqs&a$3~wsO;iesmak6ptmy
tics, and thew m&are identifed by thc snmsp*urira rooling. The plasies refer to their tcl& as molds, dies, and fixtures.
tics pr
Basic Mold Types
and Features
REFEREMBaq, ~ f i c~, @ m & n g Design fir Flasrtcs, Mew YD&. Van Nostrand Reinhold,
Van Nostrand Reinhold,
~ ~ c Ronald
k ,
D.,P M i c s Prodird Deaig~n,2nd &., New
1980.
&mhdt, E.C ,f i ~ s s i n gof T h e m p k W c Ma%*, New York: Van N0s-d
Reinhold,
1974.
, E@&W td., ~ k Pwb& M-F*rM+
!W.
~ f * m d Ph9da, pew York: Yap Mwtrend Reinhold,
'
Revised by Wayne I. Pribble
.A mold is only one item in a series of
material. The vast majority of molds are
which open and close. One half of the
s 0 . 1 w t @ ~Mign),
-~
in Mod-
Yerk; !JWbw-Hill, 1984.
half forms the inside of a part,
contour of a part.
1984.
For further reading, we suggest:
to lasers for practical way to do the tough fab&atiagjobs~Modwu,a w eR.,
em Pfastics Magazim, p. 61, May 1984.
,
,
r
cialize in resins, fillers,
ther components that
terial. The final matefrom many different
BASIC MOLD T m S AND FEATURES
by the
extremely stmple molds of wood and m t e r are usually
hobbyist who wants to experiment with p b t k s . Some readers will quation this a a e h o f i M u s e break-away pbSt€%
b Very COmmon in &&? -0xY
and fi&+s
lg@p and in the molding Of a* d m and intriwlte'shap
for the a h d t idustry.
ableto cover eventhin&. This erePla3tjcs old Engineering will not
lude to basic &$M tgrpes and feartum,&jB
&&90~
Bn idtial undwBnding
of the molds only. We urge you to co~lect3tZdog and house o r w s which
describe and promote n w meth& Qf-OwF h 6 h @ t i o mof old methods
have put togetlfer t o make a f a h a t i n g device
that
that probably g l l do a job previou~lyconsidered impossible. Our point
here is-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
21
--
I
BASIC MOLD TYPES AND FEATURES
23
1 SPRUE BUSHING
2 LOCATING RING
3 TOP CLAMPING
PLATE
4 FRONT CAVITY PLATE
5 R M R CAVITY PLATE
6 SOPPORT PLATE
7 EJECTOR HOUSING
8 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.
I-
pro. 2.2. Various cornpo&p&&~d~1e
twgplate iqjection mold used for injection molding. (Courtesy Dow Chernic@&C~;,
4f&#EBnd, MI)
Ci
i
-
,..
this type. Since hyh c r i p t i o n is given
press, Two general
aed the t ~ w m t r o k epress.
&,~ 1 i r d 90
e ~that the ram
p m u r e to the mold. This
molds and operating t
draulic presses are mo
of the operation of a mo
types of presses are u
The downstroke press
and top platen are m
t p e of press is wide
such parts as truck
vantage of this press
and allows the operat
10 foot square are not uncommon
b r g e Molds in Cha
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
.
h
v-
-
-7 - z - 7
A
.
A
~
.
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;
of two h t i n c t types- A plunger or auxiliary ram transS the 6'F most often used. It has a built-in transfer pot
GENERAL 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. Historically, 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 ~use in the
1930s. For a history of the development of the industry, ref^^^ should
be made to Plastics History, U.S.A.*
Compression Molds
BASIC MOLr 'YPES
AND FEATURES
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 at one or 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 mateis 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
rt 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
'flowing into the cavity." This'bavity means the space between the male
n and the female section into which the molten plastics will eventually
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 or gates in various locations.
ch location has its advantage and disadvantages. The proper choice of
ting is one of the essential fundamentals of mold engineering thatmust
mastered by the mold designer.
Injection molds are used for molding either thermosetting or thermoplastic
.
,
,'
1
-
I
@wineclassifiwtion of materials. T
and manufacturing technique b o ~
r of tk raw material tha
34
PLASTICS MOLD ENGINEERING HANDBOOK
usually split lengthwise, and the inflated object retains the size and shape
of the inside of the mold. The blow mold is a variation of the split-wedge or
split-cavity mold. The major difference is that in this case the entire mold
is split, and temperature control is provided within each half of the mold. The
clamping of the split halves is accomplished by action of the blow molding
machine. The design and construction are quite different from those of
conventional molds. This subject is covered in Chapter 11 as a separate and
special problem.
The expanded-plastics, reaction injection and foam molds are treated
separately in Chapter 13. Structural foam is an element of the expanded
plastics and requires unique mold designs, as well as specially adapted
injection machines. Here, again, the serious designer must pay close atten. , tion to the trade literature available for the many speciiic materials that can
be foam molded. Essentially, the molds used for foam molding confine the
charge while it is expanded by heat, or gas pressure liberated in the heating
.process, filling the space between the cavity and core to form the desired
.molded part.
'
SPECIFIC TYPES OF MOLDS
The second manner in which molds are described is by the manner in which
they are to be operated. Molds can be classXH as a hand molds, sedautomatic molds or automatic molds.
lifts for opening a hi..,
.nold.
BASIC MOLD TYPES AND FEATURES
39
PLUNGER (OR FORCE) OR CORE
m-!\F@i. 2.14. Cross section.of a simple flash mold.
'b
m l o w c e other than that which closes the mold and keeps
I
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 d h bholdlu in Fig. 2.14. The depth of the mold
, f1'
.
y this constricted section
mold. This does not per-
mi. %FRThis
tb h d . If the $mold
and keep it closed during the curing or lret t
m&tlon .or harelrshg time .r&&MECl fovr
harden sufficiently to be ejected from the mo
in other publications the- pke-nomaan of p
&sig&@f4imold and Is dM@gg.dst' t b
imdw&nt athim maximmi d&u&b
BASIC MOLD TYPES AND FEATURES
and a loading shoe. The
of mold, Fig. 2.16, consists of a m
ed midway between the
loading shw'&mekly a floatin
forms or powder may
plunger and cavity when the mol
a loading shoe with
be loaded ia W4type of mold.
molded p.kp partially ejected.
The loding shoe mold offers mom %&lwnh@s for certain types of
compression molding. The cavity iar mqe acxxs~iblethan is tbat of the
landed plunger mold, and inserts m y lk loaded easily in it. The height
of the cavity well is lower in this classification, but the mold will nevertheless cost about the same as a landed plunger mold bemuse of the added
shoe. High-impact materials may be molded'in this type a f mold, therefore
used for work which specifies these materials, although
next described may provide even greater advantage.
loading shoe molds are not mommendad because
may cause binding of the loading shoe. f i s h
--
sha mold is Mat thb ship&?*Brt (or 'soxpewhmy ~ ! m t&:&$iP
n
~ " % t s i ddimant$&
e
of &
s TRIPPER
PLATE
'
FIG.2.
b pusha
from plu
I
the load& shoe arrivesat its normal position.
43
h
onl:
binc
spel
7
2.1E
PUS
.
%h$ I- ai W bmkie %ftho w16@
@ad w W j a t never larger t$
acti
&W
the
by i
the
F
plat
the
usec
the
U
, the
,
a pmk1
in a
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
strips the molded pieces off the mold plunger. The area of free
ie stripper plate is limited, as indicated at 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 be in
~dat (A), and in the left side at (0. ,,
ejection of the molded piece is at all times important. Much of
b n a l 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 piece can always be made
. ~ettingthe part out of the mold in one piece is another
.
4
BASIC MOLD TYPES AND FEATURES
45
DEPTH OF CAb'lrY
b
a piston in a cylinderiston. It is used chiefly
a long draw, such as
und must be weighed
y excess compound.
closely, with approximately 0.003 in. per side
d is preferable to any type of landed mold for
materials. In attempting to mold cloth-filled
ill be seen that the small pieces of cloth will
and so will absorb pressure and prevent
t and has the advanhere is no land in a
he compound. The
plunger rubbing the side wall of the
Considerable di
-
Ive'8aold to allow wall thickness
L '.
BASIC MOLD TYPES AND FEATURES
47
elarnine and urea compounds should be semipositive
olds are not required. These compounds require
bring them to the plastic state, and good molded
without causing the compound to flow under
ithout keeping it sealed in the cavity during the
olds described in the foregoing pages are the
which, with various modifications, are used for dfl 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 contate the molding of complex shapes which may not be
the more simple molds.
tEo mold consists of a group, or "gang," of cavities
a loading well, and it is used in compression moldcavities may be contained in each gang of a mold
Sueh molds are frequently built with from three
$c!kt%ining fifty to one hundred cavities. The caviM@m d a loading space, as shown at A in Fig. 2.22.
&Wig.
wtly work required to
ahc Gast of rnindi
i
usring a p d m uf the
~~dthekndsho
fix many medim-ski
aruitko 4s dose t
bp.damebdthQk.jssdingareilE08
b~4@~5h~91rillpn,~aaaupsl
h*s sbm~ltb.b#,-w wbtisfacto~: .
*
,, ' a a
..
1
'
4
.
i
"
,. >,*sI
8
BASIC MOLD TYPES AND FEATURES
&@ PbASTlCS MOLD ENGINEERING HANDBOOK
53
0
To KMoCKoUT
WEDGE
MOT
(4
m WEDGE
KMOCKOUT BAR AND W E D M
(8)
FIG.2.27. Wedge-type mold used 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
Removable Plate 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
must be threaded into the plat-. The use of the extra plate will, in 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, although fifteen pounds is considered better. When heavier plates are required, they should be designed to slide from the mold onto a track W the
of deep parts from the plunger.
BASIC MOLD TYPES AND FEATURES
55
FIG. 2.30. Pulling out
Co., Pittsfield, MA)
fork used in spring box mold. (Courtesy General Electric
provides the extra pressure needed to insure full density after the normal
flow takes place.
Double-Ejector Molds
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 a
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. In like manner,
the bottom ejector pins extend up out of the cavity when the mold is open
to permit easy loading of the inserts.
BASIC MOLD TYPES AND FEATURES
61
etimes formed in these mold
from sheet stock. This type
re will not be described in
am Wmnted with
h n s w d r blocks
age set ,up in chzs manner
, or s-ia~k-eaultymolds
for larger par&. .
gegeral practice which is predicated on the type
g m t and l a b o ~available, and the experience
of the various methods. In all cases the mold
ts at minimum cost. Many
dures may be determined
For some jobs there is
the mold a n t the tool-maker must weigh the
~ssibledesign before he decides which is best
Ids is shown in Fig. 2.35.
F ~ n ~ a l2nd
s Ed.,
,
New
ark; McGraw-Hill, 1986.
Press, 1967.
PuIxlWons, 1944.
: Pm-n
:
'
*
-. .
I
I
k
.
J
-
'
J
1
?
I
representdin one &&,
Specialization by proawes and relattd ski& k t q s , - , kcut& of modem
mold making at a masonable h
1
,
Fer examp&
a s shops that b*
only standardized or cu~taanmaid b m s a d
mld-making industry (see-CWpter 8). Ths
invest in the brp equipment
as W
R;rdMm needed to bum p&&@
m
p
d
fe- i*of ern&& bases fa.$
frm 8;
~ b o mf i b, Wl met%$hap a h
m&ng
inserts that cannot be machined wanomkdy. Haah# % ri
supp~&sare cmpemtiv
equipment and s W to
lk
64
PLASTICS MOLD ENGINEERING HANDBOOK
CUT-OFF EQUIPMENT
A power hacksaw (Fig. 3.1) or band cut-off saw is most commonly Used
for cutting bar st&k to the desired rough size. A mold maker may or may
not invest in a power cutsff saw depending upon the availability of steel
stock in a 11
metal supply warehouse, ~he.supplierwill certainly have
one (qg. 3.2). These machines are motordriven and apply a metal-cutting
blade or band to the bar stock. Some of the newer machines feature an automatic fwd between s ~ s s i v cuts
e of the same length, with a pre-set number
of cuts.
An abrasive cut-off machine is found in some shops. In this machine a
narrow motordriven abrasive wheel moves into the stationary bar, making
a very smooth cut with little waste. This is a most efficient way to cut small
pre-hardened steel bars, certain alloys, or hardened standard ejector pins
to approximate length before final machining and fitting in a mold assembly
(Fig. 3.3).
.x%xtrical
- .. . spark as in EDM.Equipment
.. ..for
66
PLASTICS MOLD ENGINEERING HANDBOOK
bevel cut from one edge of a steel plate.
Shapers are built in A 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. A 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 tool room equipment. A standard
AKlNG PROCESSES, EQUIPMENT AND METHODS
67
68
PLASTICS MOLD ENGINEERING HANDBOOK
70
I
PLASTICS MOLD ENGINEERING HANDBOOK
me
t@@L MAKING PROCESSES, EQUIPMENT AND METHODS
2
PLASTICS I#OLD ENGINEERING HANDBOOK
73
rk to be ground is placed on a round magnetic
t opposite in direction
of the work can be coned. 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 finishfaces will be parallel.
so that angles or radii
3.14, use a magnetic chuck to
the grinding wheel. Micrommay be controlled dmely.
"wet (lr+diy"grinders.
1 cylindrical grinder (Fig.
ay be rotated on centers.
amhments. The universal grinder will
Y
Fro. 3.1 1. Mold maker using a j i g d f o r the precision location of if hole in a mold section.
IG.3.12. In the rotary sunace gnnaer, the work 1s 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.
1:
TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
15
$. The uuivemal cvlindrical grinder used to grind outside diameters.
isus accessories or attachments, are the most versatile machine
making.
w h i n e s are often called die-sinking machines, because
#@&
supporting the cutting tool, or end mill, will move along
[B$IWI law.er the cutter into the work piece. In horizontal milling
@#@dJe axis is parallel to the plane of the work table. A univerbme is a horizontal type with an additional swivel movement
q horizontal plane.
L
1 Ma
@i?&&i~spntaJ
Milling Machines. These perform some of the
%%%%amthe swEace grinder, shaper or planer. They use milling
@ 8! & ohular saw with a wide face. One or more cutters
i@%W'b$.ox
WWis mounted into, and driven by, the horizontal
f&~ttpp0:rkdagainst excessive deflection by a heavy overas &own in Fig. 3.17.
$hell cutter mounted directly in the spindle for
-1 block is shown in Fig. 3.18. Other stub arbor
milk, and[in these instances the arbor and over-
w@
F$
PLRSTI-
MOLD ENOINEERING HAtdDSOOK
ulm pockets in a mold plate using a jig-mill. (Courtesy D M E
Fro. 3.18. Squaring block with a stub-arbor, shell cutter in a plain horizontal milling machine. (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. A tracing stylus of proper shape and size is mounted in the tracH
spindle and centered ~ ~ thee r
'J:k
pattern must be the m e size@
the Finished wwk ,pie+ Qw
pr$(y .ap I: I, but reduction far w i d
A
1
,s, Inc.,
82
PLASTICS MOLD ENGINEERING HANDBOOK
FIG.3.24. 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 coordinate movements are used for mounting and positioning the pattern and the work. These machines are used for mechanical engraving, 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
84
PLASTICS MOLD ENGINEERING HANDBOOK
Frc. 3.26. Pantograph mill. The pattern at left is ten times the size of the work at right.
(Courtesy Ethyl-Marland Mold, Pittsfield, MA)
in the conventional manner and hardened and polished. The cavity block is
a prepared block of S.A.E. 3110 steel or the equivalent, and the impression
is made cold. The press must exert very high pressure. Some hobbing presses
develop pressures as high as 3000 tons. Many mold makers send their hobbing to outside specialists who have the large presses required for this work.
The "Cavaform"* process may be used to advantage for deep, small diameter 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 for this process is large and expensive 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.
86
TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
PLASTICS MOLD ENGINEERING HANDBOOK
87
METAL EROSION PROCESSES
Electrical Erosion (Electrical Discharge Machining-EDM)
Again, in this metal removal process, a master pattern is required; however,
it is used as an electrode and must be electrically conductive. Hardness is
not a requirement so copper alloys are generally used to make the master.
Cast zinc and machined graphite are also used in some instances. Figure 3.29
shows the principle of spark erosion applied to mold making. The gap
between the master and the work is quite uniform and small. As the master descends, small intense sparks are generated wherever the gap is reduced. Erosion occurs on both master and work, but is a negative polarity, and the mas-
,
at all times to remove the minute particles that are formed between the master
and the work piece. Electrical erosion is slow compared to mechanical
i
3 30 Electrical erosion machine with power
Courtesy ELOX Corp., Troy, MI)
at right and dielectric fluid pump
produce textures such as fabrics and fine leather on molded
the reverse detail into a hard or soft metal cavity. In the
the chemically resistant coating is produced in a film
contours and surfaces. The film is then exposed to a
washed away to expose the metal for erosion. Detail
Y chemical erosion or displacement is of quite uniform
PLASTICS MOLD ENGINEERING HANQBOOK
depth and dll app?ar.as raised markings or decorations QQ the molded
part.
CASTING PRO
sses include casting and i
can be seen (in any
molds rarely provides
I
.
.
&&a had plaster, &&!or mt pl&$~,.-oran~ golid m e t . .
wbo'amibthese kr&&j.yi%h i s b Qwipdata and infor-
&%lop$ adequate pressure in the assembled ceramic nrolelas molten m e d is
poured in and fills the molds. This pressure and the uniform fine porosifji
af the entire mold produce excellent castings which are dense and sound.
ted tool steel alloy for mold making can be cast to shape by th$
Shaw mold can make only one reprduction or cast, thys a s e p q
I S made for each insert ordered. In the dsht appl@$~n, it
ornical compared to machining a ca%tYw'&re. ~ k k i b ~ n ,
produced by other specialists, usingceetdfu&l easthg
r.am-pieck:semi-ceramic mold or investment. This method may be ca]
isrt ~ . ~ p&
,*a; * ~>-::,.~ r~
mmt casting, centrifugal
7
ure on all surfaces
86
PLASTICS MOLD ENGINEERING HAND-OK
and $m&&mpfeswre castings. Note flutes and ribs which an
R P O ~3.a.
form& In tb8 cavity sedon8. (Churtesy Tooling Specialties, Inc.. Denver, CO)
TOOL MAKING PROCESSES, EQUIPMENT A.ND METHODS
91
process which differs from the <casting
?@"
mmend the proper alloy steel for various
Pmskre castingepcialists will
h o b or m9stefs. Canfiguration f the hob detail and the number of castings
to be produced determine the selection of the proper heat-resisting steel.
Some, with highly alloy content rue expmwive and difficult to machine. C a t
of the master is offset by the advantap obtained by the process which can
provide cavity detail impossible by machining or cold hobbing. Figure 3.32
shows examples of master hobs and castings produced from them.
Molded Cavities
"Custom molded cavities" is a 1982 development ~f the 3M Center, St+
Paul, MN, and data concerning this process should be in the designer's file
for ready reference where the process is applicable. The principd 1imitatioQ
is physical size af the fietool, which cannot exceed 3 in. X 3 in. X 3 in*
3M custom cavities are molded from a STEZLITEmalloy
rigid model (male or female) supplied by the customer. Any
tical cavity inserts can be made from a single model (as oppo* mb t$e
Process, which is destructive to the model). The applicatio
the same manner as machined steel, hobbed steel, or b
The indicated use is for cavities with decorative detafl
complexity. Cavities with functional detail, such as c
STELLITEe is registered tra-k
of the 6:-
Gw.
thkhms or build-up
OT Wrical energy to
h a porous structure
4 shafts in mbrs, engines
a t h w : ~ ~ e - p o ~ tpr
ion
3~ anakd mddng. The designer m g
d n g or "mechanical plating" in consitlerhg possible appliations.
MISCELLANEOUS PROCESSES
Mald-Maklng Procedure
TIw sequence of operations ill the making of various mold members should
& d e r s t o o d by the mold designer so that he may more accurately visualize
f a a e is delivered. Delivery commitments determine the choice. In either
am,the disassembled plain plate members, ground square and parallel on
a'surfaces, are taken to the bench for layout work. Screw holes, water or
m, . m lines, and other holes are laid out for drilling. Pockets may be laid
mt for rough sawing to shape, mill, bore or lathe-turn to finished size.
Cevlty and Core Inserts
'Them @a @sllallyprodueed from anpealed tool steel alloy bars or forgings
d sdt;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
*SeeCbpWn 5 , 7 and 8.
•
%h~=r,vertical mill, or lathe as required.
v a t a p ~are used for machining rectangular
surfaces are often finished on a grinder
.h&rance machining. External machining profag iw* p r o d u ~ dby metaldisplacement or metal-
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.
2
4
Measurement and Layout
M
Y mf the conventional hand tools are used by mold makers. The vernier
h@bt gauge (Fig. 3.37) vernier calipers, micrometers, clamps, indicators,
V-bhks, parallels, surface plates, angle plates, sine bars, and gauge blocks
invaluable for layout and dimension operations, such as checking work
in 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
by hand, working out machine marks and imperfections in molding surfaces
96
PLASTICS MOLD ENGINEERING HANDROOK
FIG.3.37. A froished mold ~ d o isn arefully checked with a vernier height gauge before
assembly.
I
FIG.3.38. Hand tools are frequently used for the engraviw of malds.
w ~ r kis essmcral.
are produced by careful
TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
101
also used as a hand grinder, but it has ~ p w ~ p o w eand
r less
t machine. This unit, fitted with a polishing wheel, is shown
int of the functioning and performance re-
Fig. 3.49, preplating treatment and the plating
ler after he has the work will, in many cases, eliminate
, '.
2,
L MAKING PROCESSES, EQUIPMENT AWD METHODS
109
plating. (Courtesy Nutmeg Ckrome Corp., West Hart-
' (Courtesy Nutmeg Chrome Corp., West Hartford, CT)
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 ovens equipped
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
'
104
PLASTICS MOLD ENGINEERING HANDBOOK
106
PLASTICS MOLD ENGINEERING HANDBOOK
TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
107
be a complex and academic subject, the basic principles are
the growing importance of achieving, measuring and interquality 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 assembly
h
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 requirements has frequently determined the surface finish values; indeed, a surface
finish governed by visual appearance could very well be over-specified,
leading to unnecessarily high production costs (Fig. 3.51). While surface
Re, 3.51. TkEg &mphindiafea
iWsh Ss mtq#s.
rally results from the condition of the production tool
or grinding wheel (Fig. 3.53).
bf a surfPce am: R, mughaerrs (primary tcxt~m);A,, wav-
Was swin$, and W + IQ*
waviness &IS-mghnes9.
jd TOOL MAKING PROCESSES, EQUIPMENT AND METHODS
109
1
E-v
s
Meter Cut-off (mm)
Traverse Length (mm)
Minimum
Maximum
R t l + R t z + Rt3+ Rt4 + R t s
'bpR1 R2
I #4
the grit and its size used in a grinding wheel (Fig. 3.53).
' ,
Root-Mean-Square (RMS) is an average geometric roughness and was an,
American standard. In 1955, it became obsolete, but naturally enough will
,!&I1 be encountered occasionally. It is sufficient to say that its numerical
value is some 11 percent higher than that of R, (CLA, AA) (see Fig. 3.53).
The foregoing standards, with the exception of RMS, are in common use.
'Other terms will be encountered in the study of surface measurement such
as k,
R,, tp, R.
It is worth enlarging on the parameters R, and R,, since increasing reference is being made to them by manufacturers of surface measuring devices
and Standards Institutions. Both R,, and R, are parameters that give a measumment of the average peak-to-valley height, the former being intend*
for measurement by a machine whereas the latter lends itself to graphical
determination and cannot yet be reproduced by a machine. In some cases
I;&!; &
,, and R, can be used as alternative or supplementary parameters to Rt.
,
' (For determination of these parameters see Fig. 3.54).
In order to simplify the illustration of surface measurement principles?
reference will be confined in the remainder of this article to R,, Rp, and Rt,
as it is felt that these parameters will be sufficient to develop the basic
3
1
I
I
R3
R5
R7
R4 R G R ~Ra
Rlo
( R i + R j + . . R9)-(R2+R4+...R10)
5
.
Frc. 3.54. Definitions.
i4 .
Secondary TeMwe. Secondary texture is that part of the surface texture
underlies t s r m g h n e s s . All types of machine vibrations, for-instance
spindle deflectiln Bnd imbalance, can be the cause; it is generally described
as waviness or more simply W (Fig. 3.52).
The p r o d i t i o n process used will form patterns on the surface.
9rhe Pred0minankpattern direction is known as the lay.
Y
CoC-off(Sampling kngth). Cut-off is a facility that is built in to most
measurin$devices.
3
Its function is to suppress waviness (secondary
"XhUe)
whatever d e g is~ required within the limitations of the cut-off
this facility is of great importance as it allows the effects of
Process to be studied gparately from the effects of machine
lkfficieficies. Cut-off is a filtering operation that is performed by a frequencydcpnqent
electro?$,&filter. The cut-off values according to the British
Unltl
m O L MAKING PROCESSES, EQUIPMENT AND METHODS
.
We h v e chosen to discuss two points in stppe.deptb, ,because they am
constantly giving rise to doubt and argument.
111
tw s a q values mice. The length of sfroke should
40 traverse a partsular feature. It shouM be borne
since the surface appeaqnce varies, it is desirable
reasons previously highlighted. Likewise use of
tures two or mofe times.
is equivalent to. 39.37 mi-
not used elsewhere. Unlike R,,
far ease of specification and
py a sliding skid (Fig. 3.55). The
PLASTICS MOLD ENGINEERING HANDBOOK
112
TOOL MAKINQ PWCESIES, EQkllPMENT M D METHOD8
118
---
iI
t'
dFR$hKering, p. 45.
It i
q i
.
a; Equipment, Oct.
!Z
1
--- .
----
-
h a permit the use of mate
. The mold desipet. has many
vestment that
bg lost if;.
MATERIALS FOR MOLD MAKING
&eel, &&lting
Steel, water Wdening
Steel, oil hardening'
Steel, low dlay & carbon
Kirksite (zbioallay)
Mulainam aUoy
Silicone rubber
P stress. The premme in a
117
PLASTICS MOLD ENGINEERIN.&)IANDB80K
arc melting process, is remelted by using it as an electrode. Figure
rates the Electroslag Remelting Process. The (ingot) electrode,
the cathode, is slowly melted away in small droplets. These dropthrough a molten slag bath which has a purifying action and proe liquid metal from oxidation. It solidifies quickly in the watermold, which also tends to form it in a more uniform structure. In
a s s , no special atmosphere or vacuum is used to protect the metal.
tion is rendered entirely by the layer of liquid slag.
Vacuum Arc Remelting process, no liquid slag is used to protect
g& metal, because the process is contained in a vacuum. It is more costly
; : + ~ u $the
e equipment is more sophisticated.
paring the charge for the electric furnace, pure iron or carefully
scrap is used, together with alloy materials in the percentages
. Inasmuch as the chemical composition of every constituent
steels is known, various elem nts can be brought into association
per ratio to achieve a desir result. "Cold melt" electric furnace
made from a charge of cold ma rials, the name serving to differit from production steels produ
by charging the electric furtb molten steel from an open-hearth race.
the steel has been melted and refined it is poured into iron molds
ingots. Tool steel ingots may range in size from 6 in. up to 70 in.
I .
\
MATERIALS FOR MOLD MAKING
119
t the liquid metal from the electric
degassing process which removes most of the hydrogen and
ually done by means of a vacuum
ing Argon into the liquid metal. The removal of most of
gases will result in a cleaner steel which is very desirable in
steels. There are several methods of degassing. The most p o p
(A), the ladle of liquid steel is put into a sealed vessel from
WIllW
ladle
v
'has been pumped out to create a vacuum. This is called
(B) of the sketch, the ladle is sitting on top of the vacuum
uid metal is poured into the ingot mold which has been
w
ti
tl
r of the ingot being the last to become
SOIKI.
i~
P
least,
tainix
0
, the center of the ingot may be left hollow or, at
ensity. The defect resulting is called pipe. Ingots conf the manufacture
ss known as cogging. In the cogging
81 methods. A Ladle degassing process. B. Vacuum ingot degassing pro-
6IOO
PLASTICS MOLD ENQINEEWQ HANDBOOK
MATERIALS FOR MOLD MAKING
process, the ingot is heated to the proper temperature and then worked to
the desired size and shape by a process of hammering, pressing, or roll$R$. Some types of steel, such as high-speed steel, should always be hamm e d or pressed in the forming of billets. These billets may be round or
quare. The cogging operation is used to form the billet to the shape desired and to reduce the cross-sectional area of the ingot and increase its
length.
Surface defects may develop during cogging, such as seams, laps, or
$packs. These must be removed before further work is done on the billets,
Removal of the defects is accomplished by one of three methods, namely:
'&hipping, grinding, or rough turning. Billets are sometimes pickled in
lacid to remove scale and to "point up" defects. The chipping process
makes use of air-hammers and gouge chisels to remove seams. Grinding
i s usually done with swing-frame grinders. High-speed steel billets usually
. m i r e grinding, and they are
&en removed in a milling machin
o m types are milled all
away the entire surfa
W s lathe.
- f t W t s prepared in the manner just described are then rolled or hamrnered to a specified size. In the rolliag of tool steel, the cross section of
She biilet must be reduced slowly, and this is accomplished in repeated
+piaxes through the rolls, a small reduction occurring with each pass. Care
is exercised t o do the rolling while the temperature of the steel is between
maximum and minimum temperature limits. After being rolled, the bars
must be annealed to remove the stresses.and to make them soft enough for
machining.
Annealing is done by heating the steel to the correct temperature and
fbm allowing it to cool slowly. Scale will form on the surface if this is
do@ in air, and to prevent this, the bars are annealed in an atmosphere
bmtrolled furnace or are pipe-annealed. In the pipe-annealing p r o w ,
bars are placed in large pipes and surrounded with a baterial t b t re%&
oxidation. (Cast iron chips are generally used for this purpose). Tht?
pipes are then sealed and the entire charge is heated to the.annealing te&
p a t t u n . On reaching this .temperature, the charge
slowly as required for the type of steel being prod
18 to 24 hours. Today much tool steel is being an
controlled furnaces. n e atmosphere in the furnace
oxidation or scafing 09 the metal.
In the various heating cycles required for cogghg, rolling a+ a
ihg, some carbon will be oxidi- %$yair fn wcontact Mth: tb ~ttiYit&
LE 4.3.
121
Minimum Allowances for Machining and Maximum
Decarburization Limits.
produces a condition known as surface decarburizace skin must be removed, and steel producers uniar stock for tool-making purposes. The allowances given
pared as cold-drawn bars for certain uses. In the coldthe preparation of drill rod, these bars are ground in
to remove all decarburized surface and provide close
STEEL FORGINGS
Many
ing the
desired
forging
eration
112
PLASTICS MOLD ENGINEERINQ HANDBOOK
MATERIALS FOR MOLD MAKING
123
'the twhnical data furnished by
surface wrist ~s$&tiop.This
t iron chips or pit&'coke, by
, or by heating in a w u u m
MACHINABtLITY
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
m a u n t of machinability, steel stock from which mold plungers arc cut on
&duplicatormay require a different annealing process than a block of steel
ing to which the stock has been
ar machinist's experience.
(such as nickel and man-
HEAT TREATMENT
.
Aftel ,ing froq
stre&
from
des&nated temperature ranghardness and to relieve the
,while relieving the major
stresses I(
cess resu
by a sea
sses inherent to the pro-
Annealin
the meta
ting it tn
bartines of steel, is accomplished by heating
just a b v e ule critical point and then permit-
k r 4 'aTf&ESS
RELIEVING
idud stresses. Steels that have been sub-
ons nlust,be relieved or distor-
124
PLASTICS MOLD ~NGINEERINGHANDBOOK
MATERIALS FOR MOLD MAKING
125
HARDNESS PENETRATION
STRENGTH OF STEEL
wnching solution, cools very quickly. The inner core cools relatively
y. Steels that must be quenched rapidly t o give them proper hardwill have an outer shell that is hard and an inner core that is relatively
owing to delayed action of the quenching. This outer shell may be
to %-in. deep in a water-hardening tool steel. The use of certain al-
MACH1"IYG STRESSES
hing. The oil-hardening steels show greatsr hardwater-hardeping steels, while the air-hardening
the greatest degree of hardness penetration. Molds subject
ection should not have a high degree of penetration.
MOLD
STEEL REQUIREMENTS
. .
heating during heat treat-
ing rapid heating when the thin sections reach the critical temperature
first and start contracting while the thick sections aR still expanding.
Figure 4.4 illustrates the advantage of slow heating, During slow heating, the combined stresses aie below'the yield strength of the mold and
no distortion occurs. During fast heating, the combined stresses are greater
than the yield strength and the m@ldd l s t m , 'The mold cracks if corn-
hardness. They are easily machined and polished to a fine
for most injection molding applicatiohs. It Is advisabt to
nrqlds made from prehardened steel if the mold is comp1h"EedBt
radii and corners.
A 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.
~n#9*.
It must be uniform in structure and. relatively free
.
Steels which machine @ly
y are needed for wonomical mold comtpction.
and the low alloy steels are
ired hardness in
and polishing.
126
MATERIALS FOR MOLD MAKING
PLASTICS MOLD ENGINEERING HANDBQOKr
Stre~gthand 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
is its ability to be hardened satisfactorily in a wide range of sections by
a variety of methods while producing uniform results.
F M . All 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.
127
~btainedby using the cheaper grades makes a negligible difotal mold cost, as it amounts to only a few cents on 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
,machinery steel would be indicated.
1class as the SAE 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
SELECTING THE STEEL*
As the plastics industry developed and presented new materials and 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
it is not suitable for hobbing.
of tool steel has nearly the same hardness
, and 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
Rm Steel
Rgte steel is a low carbon steel such as SAE 1020produced by the openhearth
or &her inexpensive processes, wherein cjeanliness is a less important factor
WRvolume. This material is used almost exclusively for the frames of
molds. Plate steel can be carburized and hardened or casehardened. It k3
sometimes used to make cavities and plungers, but this application is not
recommended because of the low core strength of 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 several qualities of plate steel available and if any press&reis @
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 (something like SAE 4140) for the backly, plates, steam plates, or other members on which stresses may be c o p
centrated. This practice requires that a large inventory of stock bre carried,
thmfure it will be found wiser to use the better grades of plate ftii-~ugho*
*See a h ASM Met& Handbook, 8 Ed., Yd. 1, gage 768 for additional data.
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
128
MATERIALS FOR MOLD MAKING
PLASTICS MOLD ENGINEERING HANDBOOK
the principal suppliers of standard mold parts assures the mold builder of
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
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
various grades of steel available in standard &kames and plates. Besides the
SAE 1020plate steel mentioned above, generally supplied in an analysis up to
SAE 1040, as' the lowest grade of steel, m m suppliers offer better choices
for more severe service, of SAE 4130 to 4150, prehardened to 26 to 29 Rockwell C, and even P-20, prehardened to 28 to 32 Rockwell C.
Besides mold bases, and individual plates ground and sized to close tolerances, there are !eader pins, bushings, ejector pins, sprue bushings, premachined 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-
mechanisms themselves. As the result of previous wide application of these
devices, he can-be assured that they are largely fcrolproof when installed
by experienced personnel.
ALLOY STEELS
Nickel
Chromium
Vanadium
Molybdenum
Tungsten
Toughness and strength.
Hardness. Adds to abrasion resistance in high carbon compositions.
Purifier-also adds fatigue resistance.
Widens heat-treating range and adds heat resistance.
Hardness and heat resistance.
129
king the various types of alloy steels and gives the properties
STAINLESS STEEL
w
co:
ab
thc
many alloys classed as stainless steels, only a few nepd be
use in high-pressure molds. Because of the necessity for
the most commonly used. It contains 12 to 14%chromium
(s
ha
n iqieetion molds for t m r o h ma-
/'
PLASTICS MOLD ENGINEERING HANDBOOK
I
MATAltRLS FOR MOLD MAKiNQ
&3
w,
18% 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
131
finished mold is then reheated to about 900°F,held for about 5hr, and cooled
in air. This treatment, called aging,results in an average hardness of 50 Rc,
depending on the type of maraging steel (Table 4.6) which has been used.
These steels shrink during aging and allowance must be made for this by the
mold maker.
STEEL FOR. MACHINED MOLDS
rcial low carbon machinerysteels are nearlyideal when machinability
ne important consideration. These steels are usually high in phos-1
For 1
depth c
of the n
Rc.
The
binatio
and transfer molds, the P20 mold is carburized to a
P6) type must be carburiwl, resulting in a good comhardness and core toughness.
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 b t experienced with SMC. If a higher hardness is req&ed.,
b deep hardened to RC 45-50.
and some -sf their prope&ies is shown
132
A IS[
PLASTICS MOLD ENGINEERING HANDBOOK
Machinability
Raring
Wear
Resistance
Compressive
Strength
MATERIALS FOR MOLD MAKING
--EDGE
Toarghness
Checking
133
or LAND
r p ~L ED
i
BECAUSE
OF POOR FLOW
OF COMPOUND
I,FIG.4.7. Lines of cleavage resulting from hobbing of land area.
1 thing, any landed plunger or similar mold should be hobbed
The reason for this becomes apparent in a study of Fig. 4.7,
HOB DESIGN
While the actual design of hobs is usually left to the indiddual hobber, the
to sink the hob to the full depth. Generally the hob is pushed % to 5/2 in., after
which it is removed and the hobbing is furnace-annealed. This is followed
! to $4 in, and subsequent annealing, the process being
by another push of %
repeated as many times as necessary to complete the job.
Where a particular dimension on a hob is very important, it is the usulll
practice to make one push on all blanks and then to check the hob and make
the necessary adjustments in size before completing the hobbing. Them
adjustments usually consist of correcting dimensions which have been enlarged by the "upsetting" force on the hob.
FIG.4.6. Group ohfamali master bobs usad in the d i n g of tw*.
finish-machine the hobbings.
d mn the hobs so that they may be removed from the hob-
The draft should be approximately Goper side, although
be much better. It is easier to hob cavities when adequate
been made. The reverse is true when cavities are machine
difficult to machine a straight surface. The nearly ideal
~~on to allow for rolling of land.
MATERIALS FOR MOLD MAKING
cal
Th
of
bla
nza
137
m p k , lnng thought to be impossible, is W-production of
@
t
dundercuts on the side walls, such as for dewrated closures.
i &lease the hob after sinking in such a situation is the result
)!ad experimentatioh. It depends on the expansion of the
W m from radial precbmpression applied prior to hobbing and
ingenious design of the retainerring.-
HOBBING STEELS
pp ,* ':
designations, one is able to select the best potential hobbinn
*ih-&
.
su_sds eo tile! d
~ dtM.
d
after cold-holabing."
, .,
than for bobbing.
ntly B M for
~ injection molds are listed inTable 4.8.
Wms of steel available, some mold makers and
--
T4
I
n s t i s r injection uaa
olds.
35. lhis
Usually
should
used
be
Iled,plasticsand for
aM a h @fQ000 F, but not higher than
nw&?idimtbetilgfietthan60Rc.
MATERIALS FOR M
W MAKING
139
it difficult to select the proper steel. It is e s t w that about
plastics molds are made from only six types of
1 if the industry could standardize cm fewer steels.
more economically in larger qua&@ inventories
ed because of fewer catem*,
inereaskg
maker. The heat treater's task wuld also be
treatment and loner mold life.
for compressian and transfer m0Ms are
predominant
lenable steels,
In molds.
pk&fgs get larger, and lower prduction, in most cases,
#&R
&. : - 18rger moldings, alloys other than steel can often
rtous castable alloys used in molds, including protombIds 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 infor shape and
ting was consp~ifledfor
&,kith F e early machines from Europe (see
& 8lld &terials for blow molds.)
the "pressure
ct the molten
to a hob for
to push out
ing.
~dredpounds,
he process in
i
:f,y'
PLASTICS MOLD EWOINEERINO HANDBOOK
I
:
-
ceramic pattern is used in place of the heat treated steel hob; ise.
c
Casting."
I
.later, improved by the application of a vacuum during the castiq
I
8, because the ceramic is unable to withstand the compactine
i
used in the earlier technique, very reliable castings, virtuah
,.I ..
fw,
and with excellent reproductisn of the surfaces cast against,
1 0 , ~ ~
available for much larger molds. When the conversion of furniture
,
w h h the late 1960s occurred, beryllium casters were able to mold up
,
, 1,
%aW - p o u n d castings.
, .
, .,
,
Waay alloys of copper with beryllium are available. Table 4.10 lists the
'
,&we alloys most commonly used for molds.
&%kction depends on the desired degree of fluidity and the mold-makina
'
to be used. Certain alloys are for making cores and mandrels rathe1
I
8
.(
.=
t i
I'
ture. And the foundry can cast them at lower temperat
@, W F). This is important when using a ceramic mold-ma
8'i+ecjing on available melting equipment. At lower tempera1
s, is
metal-mold reaction. Also, simpler foundry equipmer
As
the class of materials having 1,.7% or more beryllium, hi,
contents give higher fidelity of reproduction. But the hi&er
Eontent, the higher the cost. The 20Cand 245Calloys (see Table
Gammon. But the choice can depend on pattern quality. V
~txck~ent
pattern 2OC will be satisfactory, alloy 245C can corn
&&*kt for less pattern precision.
ie m& important mold manufacturing m Q n s for u ~ h gthe %
4
'
F
1,
-3, 4
' f ,&I
142
PLASTICS M 6 L D ENGINEE~INQ
HANDBOOK
MATERIALS FOR MOLD MAKING
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
of BeCu from pouring temperature to room temperature. Shrinkage is
predictable and consistent.
hardness and strength. Besides machining a hob, the mold maker can sometimes cast one via a ceramic casting process.
When heated, lower the chase or mold casing
and aove the two onto a hydraulic press.
the hob assembly
143
sting of the BeCu around the ceramic, resulting in a sound, dense
1
gaming the ceramic slurry into the rubber mold, the mold maker
I solidify and then fues it in an oven. He then lutes gates and risers
b c . Melting and pouring of the beryllium copper around the
aern 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.
;4
)&%#oys. Where the cavity or core shape is not complex and a cast-
g-$adicated, high strength wrought alloys of beryllium copper are
p ' , tube, bar and plate 'form. These may be ordered from stock
&&ledor precipitation hardened state. Brush AIloy #25 is one of
P and most available wrought alloys.
-2'
h a t . The mold maker can heat treat a beryllium copper mold
b s that the user desires. The hardenabilitv of bervllium CODD-
A deflector will insure this pattern of flow. When the beryllium copper hm
very important. Move a&quickly 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.
4
Ceramic Casting. Ceramic casting follows any one of a number of paten
around the pattern. The elastic quality allows stripping from t b ptq
designed part. The special ceramics u d preserve the
A proper ceramic mixture will combine g o d surfam rn
relatively high permeabilgy. This bst property allows %flag
a befyllEitw wpper (or aluminum) mold by the ceramic castine
ative (righi which is then used as a
cavitv (lefi IS cast from the ceramic-.
en^ Corp., Pleasantville, NY)
w
lrlrl
MAVERtALS FOR Yam MAKlNB
PLA874CO MUIOLD ENBlIIEEERING~HA4WWOK
m&mri'%;
from B80 m high #rts W
o n &e particular a l k y used
1.46
.daoft surfaces far-avity and core co
ng pttoperties, if loading b kept within
at the higher end- d the r a w .
&st# w1ut.b~
hBf trmfing and th
M@chining. After casting and h a t trmtmmt, a machining mge begins,
Tfiis removes gates and risers, produces smooth shut-off surfaces betweeq
Zinc Al
,
Several
cially f~
have tht
and ye1
to or be
In mc
which a
can be L
and inj
Cu 3.54
gation 3
Aluminum
@a c@ fiekpjl frrc~~eds
at a rate
"
H.~,'Simd;rrtEs,W i York, V d NIB-
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 have a 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.
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 11 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
approximately 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 t o remove the "trees" and
~ o i n t sare 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 cavities are 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 stainless steel is the most commonly used material at this writing. For gears,
probably the most common material is brass. Many masters are made of
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
or molded various plastic materials. Many shapes can be molded with
almost no loss of dimension. Epoxy resins can also be used to cast into
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.
w
MATERIALS FOR MOLD MAKINQ
I
147
lications of electroformed cavities are numerous, but it is difficult
ral rule as to when they are indicated. A slight change in
will indicate that a cavity should be made by electroforming
machining, hobbing or casting. A frequent reason for electropresence of delicate detail in a cavity. Since the electroformed
reprodues the finish on the master no polishing o r other
. Electrofonned cavities are frequently used because of
can achieve. A brass master is simple to make and easy
ensions. Electroforming cavities will exactly reproduce the
on the master. When mbdels are available, very often a n
cavity is cheaper because the model can be used for the
and no metal form is required such as for casting or hobbing.
~
I
I
I'
]
w
K'JS- EIeBrnformed insert of large mold for clear plastics drafting instrument. All
1
hv
mmbers and letters are raised on the mold surface. This mold insert would be
k m o u l t to make by any other process. (Courtesy Electromold Corp., Trenton, NJ)
148
PLASTICS MOLD ENQINEERINQ HANDBOOK
Electr~formingis an economical way to make cavities with raised details,
Rg.4.15, 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,
ead the eleetroforming performed over this. The result is eledroformed
avities which have the numbers raised from a highly polished surface,
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
b s iwnimq, to make spur gear cavities, and even helical gars, but when
MATERIALS FOR MOLD MAKING
349
Solution
.006-.010 "/in.
mink cast)
,008-,010
-008
Some alloys
.000-.002
No
Before mach'g.
-
TOOL STEEL CASTINGS*
wka %$j@img$0wae it shs
pt(pk@d method. Tlpes
for "b#mce, with hobbhg.
cate makd components in multiples are economically
1 steels on a custom cast basis, and are therefore
and thermoplastic materials. The suppliers use
pmess to reproduce the original master patterns, similar
& Wt used for the production of beryllium copper casts,
142. In preparing the master, the mold marker must
of a e ceramic cast, the tool steel, which has a difat gf beryllium copper, and of course the molding
c material to be used.
me:
vailable, which are so far only suitable for ther"molded" using powdered metal technology.
y the 3M Company, and while some details of the
W l i u g Green OH.
-
150
MATERIALS FOR MOLB MAKING
PLAITICS M a B ENGlMEERiNG HAbYDIBOOK
of c-tiags i&Otellitea, which is incsrpomd in a matrix of a copper alloy,
persion of carbides. The wW% combbation, in the form of
very fm pow
a
er is compacted around or in the "replication" of a master,
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 limitations are: (1) size-generally not m r e than 9 square inches at
the parting line of the cavity itself; (2) proportions-not more than a 4: 1
ratio of depth to minimum cross-section dimension; (3) surface f ~ s h - 2 0
to 25 microw as furnished, with the possibility of improving this to 4 mim n s by polishing; (4) apparent hardness-Rockwell C-41, which is act u d y deceptive on the conservative side, since the composite includes Stellit# d carbide particles, both of which by themselves have a hardness of
Rockwell C-58 or m
idweat in t
b
test mvhy is @@&I
&&ar ~ e e d
be allowed for.
aiMyds of the &mposite:
'I
CaWt
35%
%
161
the same way a& would be used for the s t e l alIoy involved in
. The dimensions normally increase slightly on heat-treating
PLASTICS MOLD CAVITIES
Physical properties:
Tensile strength
'Compression strength
Modulus of elasdcity
Apparent hardness
,
,
Another type of P/M mold co
pa& of J m w t c m n p l ~ l ydi
@f
Feasa-Tic*. Originally
W rounds and
110,NlQ to 140,009 psi
220,000 tct ~ , 0 0 psi
9
9%
isls are used for "molds" made of plastics materials. One
3f tl
rsnd widely used is the room temperature.vulcanizab1e
RTF
mad
tech
beca
Ci
in tk
mad
prot
can
ast
oun'd reproduces fine details, very accurately and,
undercuts will be no problei.
s epoxy glass, urethane, polyester, etc., are then cast
i
TREATMENT
Stellitee is a registered Trademark of the C
variously as "Custom Cavities," "Replication
Perm-Tic* is a registered trademark of Alloy
NY.
Upon cc
cumstar
Protecti,
not, shc
many a
152
-
PLASTICS MOLD ENGINEERING HANDBOOK
Any rpctnl
marks.
~ ~ 2 n ~ceoraie
d
or *texnrJCTmAid suflaces
Fhoi
every instance, a procedure lower in the table can be followed by one pre
ously mentioned, if applicable to the same metal.
Parn. to -i
mck Q R ~
mistana.
Peem cp, FWYCf k q ~ j ~ ~ t c h&e stool
Nitn
Tungsten Disulfide'
!
Ahmrinum
permanent, as for the graphite process described below.
bricative mating*developed for metal
P r d d ~ t itlcmses
y
of 10 to
*Diversified Drilube, Inc., ~ r h s a ,OK.
Co., Mauntain View, CA.
154
PLASTICS MOLD ENGINEERING HANDBOOK
are reported from thermoplastic molds treated by this process. Mold surfaces to be treated 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
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 specify chrome 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 electroplating generator and facilitiesfor building up 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 a 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.
MATERIALS FOR MOLD MAKING
155
156
PLASTICS MOLD ENGINEERING HANDBOOK
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 as 3% is particularly
. suitable. Otherwise, steel containing carbon of at least .4%, together with
chromium, vanadium or molybdenum will respond to the treatment. Molybdenum is especially beneficial in the alloy since it reduces the characteristic
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 Rockwell 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, glassbonded mica,** a machinable ceramic is used because of its low thermal
conductivity and its absolute dimensional stability. Glass-bonded mica can be
lapped to an optical flat and will hold it indefinitely. For Situations where
thermal barriers must be of minimal thickness, Nomex*** sheeting is proportionally 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.
MATERIALS FOR MOLD MAKING
157
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 temperature.
Don't expect plating to cover surface defects 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 all 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) Slow down final EDM operation 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 375O F for an hour,
before putting in service or applying stress.
Take advantage of the maraging 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 steel to 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 surfaces having hardnesses separated
at least six or eight points on the Rockwell C scale.
158
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 Handbook, Vol. 1, Metals Park, Cleveland,
OH: American Society for Metals.
Heat treating, cleaning and finishing, The Metals Handbook. Vol. 2 , Metals Park, Cleveland,
OH: American Society for Metals, Cleveland OH.
Revere Copper & Brass Publication, New York: Revere Copper and Brass.
Shimel, John F., Prototyping: How and why, Plastics Design Forum, p. 7 5 , Jan./Feb. 1984.
Stahlschlussel (The Key to Steel), Metals Park, Cleveland, OH: American Society for Metals.
Stainless Tool Steels for Molds, Uddeholm Steel Corp., 1984.
Tool Steel, Simplified, Philadelphia, PA: Chilton Publishing.
Worbye, John, Polishing Mold Steel, Plastics Machinery & Equipment, Feb. 1984.
Design
Drafting
Enginee
Practice
I
Revised by Wayne I. Pribble
designers and tool draftsmen follow many general rules which exhas 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 unry 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 conexisting 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 necesbiafomation accurately, legibly and neatly. The tool-maker's first
160
PLASTICS MOLD ENGINEERING HANDBOOK
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
damage done by that one wrong dimension can far outweigh the good of
499 that are correct. It is impossible to emphasize too strongly the necessity for accuracy in all dimensions and for clear presentation as well.
There is a common and very correct tendency among draftsmen who
have worked for a period of time in one place to leave some items to "shop
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 do 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.
At several points in this text, the use of standard mold bases is detailed.
Wherever possible, we recommend the use of these highly specialized
components, and once again, we recommend that the designer keep a complete file of'catalogs for these standard mold bases and standard mold
components. (See Chapters 3 and 8.) Duplication of these catalogs in this
text would serve no useful purpose. However, we do show designs based
upon standard mold bases. Bear in mind that cost is an important factor
in today's economy where the wages of the mold-maker represent a significant part of the overall cost of a mold. The 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 controlled mills, jig borers, and similar equipment, all of which is referenced
in Chapter 3. Figures 8.92, 8.93A, 8.93B and 8.94 cover the utilization of
standard mold drawings to simplify and shorten mold designing time, as
applied to injection mold design.
Tracings
A tracing is a form of drawing used for making prints. Prints are the copies
of the drawings (or tracings) used in the shop as a guide in the construction
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 manufacturing and should not be used for reference purposes. The cost of retracing 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:
PLASTICS MOLD ENGINEERING HANDBOOK
161
instead, have a print
output of a computer program. The size and type plotter used
available for drawselected by the dein advance, to oplar plotter. The usual choices available are: 1) plot what
nitor screen; 2) plot to size; or 3) plot to scale. Item 1) is
used in a reference
ws for each screen
ause of variations
se here is to alert
of their software package. A manual will (or
n on plotter configurations and operation. Of
at you dould also ask, "Who else is using this sys-
i& on a screen made up of rectangular pixel
162
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
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 onetime 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 measurements are taken directly from a plot or drawing. This scaling practice, while
common in automotive applications, should only be done when it is clear
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.
PLASTICS MOLD ENGINEERING HANDBOOK
183
should follow the rules of Orthographic Projection. In the
sent ;
ti
h
0
C
U
fl
tl
omitted except'where it is considered necessary to clarify
s
tions or to ask questions. Tabulation of dimensions
in the actual preparation of drawings, as the possibility
reading is greatly increased thereby. The complete part and
uld be drawn before starting to dimension. This practice
erasing and give cleaner and neater prints.
ntle
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 drawings a ~ the
d 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.
-
4 CAV/ T Y SEM/AU TOMAT/C MOL D
CASSEMBLYJ
FOR SW/TCH COVER
;.sad
~r
mw @/4&l
I~...
C-/023
BMc B d f ~
T-402381
FICA5.1 Typical title form.
i
IY
164
PLASTICS MOLD ENGINEERING HANDBOOK
General Rules of Drafting Practice
The 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. Do not try to second guess the product designer concerning his actual
needs in the final molded part. Refer all questions concerning insufficient
detail or information on 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 manufacturing cost, or prevent an actual error. The best procedure is: to mark
up three identical drawings showing everything that you have used or assumed in designing the mold-this includes suspected errors, unclear information, ejector pin locations, gate locations, drafts, tolerances and
requested or suggested changes, then send two marked copies to the purchasing agent who handled the buying of the mold, and ask him to return
one marked print with the design engineer's approval or comments. Always retain file copies of these negotiations, including the final approved
print to which the mold is designed. Correcting or 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 begun. Redesign the product completely when necessary to make sure that
the piece can be molded consetently and satisfactorily with the production 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, do not overlook the use
of zero draft or negative draft when their employment may be helpful.
6. Be sure that connections for temperature control media, and the
thermostat locations do not interfere with clamps, clamp bo:ts, strain
rods, ejector rods, or 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 ternperature control media and the adjacent holes for ejector pins, screws,
guide pins, bushings, etc. One-fourth inch is the minimum with which moldmakers like to work (carry a special note if it has to 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, or water is
PLASTICS MOLD ENGINEERING HANDBOOK
165
(electric heaters can be close to adjacent holes with no probkage of media). Look up other references in this text for inforheating and cooling channels.
8. Rbmember that most thermoplastic materials require large degrees
of b l i n g . However, many of the engineering thermoplastics require
heating the mold. All the thermosets require heating the mold.
~hermosettingmaterials are not quite so critical in relation to tempera1 of the mold. However, urea and melamine materials require
for best results in molding. Give special
nneling in all molds where maximum production is renot difficult to calculate temperature needs and the heat transneeded in a mold. Time spent in a calculation will pay dividends. As a
empirically state. that it is almost impossible to over
a mold. In any case, over channel is to be preferred to under
of rapid conductive metals, such as beryllium copper,
hodd also be considered. Channels in long slender core pins, is called
Considsf also the use of heat pipes* which will either heat or cool
urce. Air jet cooling is also frequently used, where other
cult or impossible to use.
se of standard lengths of screws, dowel pins, and guide pins
ible. Small deviations from these standards cost money.
10. Specify the type or kind of steel for all hardened mold parts. Call
ame or type of steel t o be stamped on the back of the mold
tice will give the heat treater essential information if it
to anneal or rework the piece.
ntion to any unusual features or important dimensions
meam of notes, so that the tool-maker's attention will be focused on
nts. Tangent radii, negative draft, or special, sharp corardeniqg or tempering to be done, must be plainly indi-
.
not deviate from standard design practice unless at least one
r ewrienced 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
buil t SQ lhat the mold can be dimensioned in the manner best suited to
the equ@ment available.
I(1:
designer should, when possible, indicate the method of setup
for ma&hing by the manner in which dimensions are placed on the
drab
@W the important dimensions in three-place decimals. Show the
nces snly where required by close tolerances on the product draw-
-
eatbank. Hughes Thermal Products Div., Torrance, CA (and others).
166
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
ing. Be sure all close tolerances are actually needed, and that those spwified can be met. Give the tool-maker no more than 50% of the tolerance
allowed by the product drawing.
16. Where involved calculations are required to determine the centers
of radii, hole location, contours, etc., preserve your figures and record
them in such a manner that you can recalculate the dimensions easily a I
few weeks later when changes or checking may be required.
17. When checking dimensions, do a thorough job; assume that all
dimensions are wrong until you personally prove that the calculations arc
correct.
18. If an error is discovered in a dimension, fmd out, if possible, wha
faulty reasoning produced the error.
19. Expect to make mistakes, and check every detail to find them; avoid 1
making the same mistake twice. A mistake on a drawing is only a potential
loss, but it becomes a real loss if it goes into the toolroom undiscovered,
thus causing faulty construction.
20. Check the daylight opening in the press to be sure the molded part
'
can be removed from the mold. Warning: some daylight figures given by
press manufacturers include maximum stroke. Others use maximum daylight plus stroke. Be sure you known which is meant.
Shop Standards
Each design section should compile all of the data which define its shop
practice and any other standards that are followed consistently. These
standards will include such items as:
1. Molding press data showing capacity, mold-size limitations, d a y
light opening, auxiliary rams, ejector operating mechanism, clamp
ing bolts, pressures available, and the location of holes in platens.
2. Material stock lists showing steel sizes in stock or readily available.
3. Drill sizes and tapped hole specifications.
4. Standard insert design and sizes.
5. Technical data on plastic materials showing shrinkage, bulk factaf3
density, draft angles, etc.
6. Spring charts showing sizes and capacities of springs commonl~
used in mold construction.
7. Mathematical tables and formulas.
8. Factual data on shrinkage (transverse, longitudinally, dkmet*
a
167
ter Aided Design (CAD)
osen should use a minimum number of operating comich can be used from on-screen prompting as opposed to constant
to a manual. For the experienced mold designer, a CAD program
e same general technique of drafting as is followed using pencil, Tand triangle will be most quickly learned. Look for and select a
which is also compatible with a digitizer tablet and
hich allows maximum speed of selection and operation.
should be restricted to specific dimensions or specific text.
ljrograms in current use allow for automatic dimensioning, thus
ntry of dimensions should be rarely needed. Every keyboard
ial for error, as is proven by the number of retypings needed
ct copy for this book text.
cases where the designer is fortunate enough to have a CAD
ay become fiart of his duties to predraw many of the
hop standards (above). We have also encouraged the
ulations, checking dimensions, developing standards,
e following text are three check lists. We particularly direct your
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 data base which is part of developing your own
system. The CAD system allows drawing once, checking once, then
over and over as component parts of a total design using a copy or
ge command. Currently, much of the data for mold bases and compads, such as plates, guide pins, bushings, hot nozzles, etc., are part
le from the vendors of these items.
t item 4, of the designer check list, arid you are perith a CAD system, you will note that with a CAD
paper size is made at plot time. (Refer to plots earlier
D system, page size (do not confuse with paper
me. A rule of thumb is to use the smallest page
show the overall of the design to be drawn. It is
er he shows all necessary views on one page, or
arate page for each view. In the latter case, the pages
d to one page, then plotted on the desired paper size,
be plotted on the same paper by specifying size and
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 viewmonitor will be available, otherwise the 200 to 2000
168
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
layer capability of the CAD program will be almost useless. Color monitors
also assist in visualizing depth and shape just as colored pencils were fiequently 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
available for copying from the source disk (remember-draw once and use
it over and over?) For example, assume a 64 cavity mold design for a Tshaped 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 an 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 locations 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 ~roiectedto 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 offer the detailed analysis of any p 8 d
ular part as a service to end umF,'lllbl(l designer, mold maker or ~ 1 d
169
ral colleges and universities have already installed quite sophisticated
,systems (the million dollar type) and offer courses of instruction. They
~ f f e connect-time
r
to local users who only occasionally need such ser8s finite element analysis to determine the adequacy of such items as
~ t h ,impact resistance, and flexibility. Material flow analysis, thermamics (heat exchange) in the mold, or 3-D modeling for aesthetic
f,l
are other services available through these on-line or walk-in services.
p n t the desirability of having all these "goodies" at your fingertips,
& cost of an infrequently used feature is seldom justifiable to manage-
i
k!:
ENGINEERING AND DESIGN PROCEDURES
'neers and designers follow some kind of orderly routine in the
of a mold. We recommend this practice and offer the followists to assist you in theprocedure. Obviously, there must be a
ween the eng eer
designer vho
design. The lgil
:sponsibl~ for
~ldand the n rted
For this :eae check lists are supplied. The first list covers the preliminary
usually made by the responsible engineer. The second list covoment-to-moment decisions to be made by the mold designer,
third list covers the final answers and follow-up usually performed
that the mo
lgner ,ho aspirc
ze himself w
reasc ling that
of these decisions.
1
1 p
ING CHECK LIST (preliminary to design)
w all correspondence, qvotations, orders and other data
may have any bearing on the part application or mold design.
E
To Do
Done
.
- -
Vise customer of any changes needed to bring the part into
- -
ortnance with quotation.
r and number.
b h heating or cooling system. ,
- -
- - rial chosen to be sure it is satisfactory and useable
- - -
- , two-stage ejection, double ejec-
170
PLASTICS MOLD ENGINEERING HANDBOOK
ENGINEERING CHECK LIST (Continued)
PLASTICS MOLD ENGINEERING HANDBOOK
To Do
12. If transfer or injection, establish gating areas and specify type of
gate.
13. Establish mold venting points.
14. Establish mold finish required by customer, by material chosen,
or by method of molding.
15. Establish draft angles to be applied. How much and where? (don't
forget negative draft is useful).
-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?
DDIGNER CHECK LIST
?V!
f,
.&; '
.a7 .
,;!I
.A<
*.
+.,
..
,
,-
, '%
at ('
:I
nents to select the most economical group of components for
the proposed mold design. Can you use a complete mold base?
Will you have to build up from standard plates? Can yo\] use
,
standard components? Will your supplier 'start from scratch"
with raw steer! (See also Chapter 8.)
3. Answer the following questions:
A. Where can "pickups" be placed if
,'
is a deliberate undercut.)
B. Are inserts to be molded-in or assembled after molding? In
any case, get a copy of the insert drawing. lns~stthat 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? Do they butt in center? Do 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 machining or reducing cost? Will customer permit it?
Will the cavity be hobbed, machined, cast or electroplated?
*'&.
Can or should the cavity (or core) be made in one piece?
Where are inserted sections needed?
]t. Where are the high wear areas in the mold? Should they be
inserted or backed up with hard plate
cation needed or provid
mpc
''7
ER CHECK LIST (Continued)
-
ge enough to show all needed views without crowding?
-
IW you are ready to select drawing paper size. Is size selected
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, do so now-layout horizontal and vertical
iter lines. Be sure to allow ample space for all views and details.
yout cavity arrangement prescribed (circles-square-rectanlar). Will spacin allow temperature control media channels?
tablish ejector s stem 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 (or 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
kt runner, runnerless, hot manifold, hot tip, etc.).
,.,ion
mold, establish land a r e a , loading well depth and
wall
thickness.
-.
$ty
;fer mold, establish pot and plunger size (transfer chamber)
e (if needed), runner size and path, and gate size.
matic compression mold, establish land areas, loading
ipecifications and part removal board specifications.
the center line and size of the temperature control media
f
To DO
1. Review the preliminary engineering check list (with the engineer,
. .if possible). Do you understand everything? If not-ASK.
4: ::
2. Review the catalog data on Standard Mold Bases and Compo-
r
Done
-1
Done
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 rplate 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, cavil slides (see 3(L)).
~pportpillars 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: this.)
a steel trade number or identification on a non-working
of all hardened parts (in event of later modifications rehkat treating or annealing).
tolerances'where needed to assure compliance with prod-
nents.
heck all drawings for dimensional errors or reversal
To Do
171
Done
172
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
DESIGNER CHECK LIST (Continued)
173
DIMENSIONING MOLD DRAWINGS
27. Review the engineering check list (preliminary). Have you done
everything as specified? (Vents are most often overlooked.)
28. Be sure your supervisor, checker, or tool engineer knows all the
httle assumptions, changes, additions or.deletions you have made.
If necessary, mark another product drawing to show all these
things and dellver to cognizant engineer.
29. Check all drawings just once more to be sure:
(a) that mold can be assembled.
(b) That, when assembled, all parts are in proper orientation with
each other-right hand to right hand and left hand to left
hand-and
(c) that mold willjir and operate in the equipment specified or
~ntended.
30. File your notes. File all information obtained from others. File
all unusual calculations for future reference (or for checker).
31. Deliver final prints to mold-maker.
--
ENGINEERING CHECK LIST (Jnal)
To Do
1. Review the preliminary check fist (engineers).
2. Review the designer's check list and the tool drawings. Has he
done everything according to your instructions (or given good
reason for not doing so)?
Has he told you (and supplied mab ed product drawing) about
dif~ons?
his assumptions, changes, deletions
3. Get final approwl of customer on final co @ration.
5. Review projected areas or mold, material volume capacity of
equipment and molding material limitations.
6. Issue special instructions for mold set-up and operation if design
warrants.
7. Review entire project with production, tool department, quality
control, set-up, and management personnel.
8. Plan for fixtures or special installation of mold well ahead of
scheduled mold delivery.
9. Follow mold progress at regular intervals of mold construction.
10. When problem arises, take necessary action, and repeat as needed.
I I . Open the mold and visually inspect same before mounting in press.
12. Use thin paper and ask for a paper impression of the mold under
pressure, and before attempting sampling o r production.
13. Review upon completion of sampling and advise interested parties
of adequacy or inadequacy of design and engineering. Note specifically any areas of trouble.
14. Finally, ask what can we learn from this project that will be useful
for future mold designs?
desirable to follow the rules of good practice when dimension-
a center line indi-
Done
- -*
',
05%
-
315 (SEE SECTION A-A
J'
NO DRAFT ON
TW/S SECT/ON
15 (SEE PLAN V I E W )
174
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
6. Enter all esiwntial dimensions on every drawing. It is bad practice
to permit scaling of prints, since the printing and drying process may introduce considpble distortion.
7. Keep all related dimensions together so the tool-maker will not need
to "hunt" for the dimensions he needs.
8. Try to keep the dimensions between the views as much as possible.
9. Dimensions given for length of thread, depth of tapped hole, etc., are
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 dimensions, four-place decimals are used only where the extra accuracy is
essential, or where it is necessary to make the component dimensions "add
up." Use fractions or two-place decimals wherever ordinary scale dimensions will 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
dimensioning. Allow space so that it will not be necessary to crowd dimensions. Decimal points must be distinct so they will print well.
12. Dimensions should be given at points from which it will be easiest
175
ts. Tolerances on mold dimensions are required because of the
nal variations that occur in machining, hardening, polishing,
Allocation. There ale two systems of showing permissible
from a b@ic gauge dimension. One is the bilateral system, and
is the ungateral system.
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
six, nine, and one often cause trouble when read upside down, therefore
the last figure in a group co~tainingany 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:
.125::g,
.2124$%, .314
+ .002
This is done because it is common practice to mention the plus toleran
first when speaking.
Tolerances and Allowances
Allowances are the intentional differences in dimensions on two
which fit together. Toleran~esare the allowances made for unintent
n exact dimension.
ne of these two systems of tolerance allocation may be used in
tolerances for each of the four sets of gauges which may be
be unr
wi
ion. A 1.500 t .005 dimension would require a maximum
176
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
7r
REFER W C E
GAUGE
TOLERANCE
PRECISION
GAUGE
TOLERANCE
ELOCR-,
INSPECTION
GAUGE
TOLERANCE
WORKIING
/
,
TOLGAUdO;E
ERMNCE
Of
REFERENCE
GAUGE -
T O N C E
PRECISION
GAUGE BLOCK
INSPECTION
I3hlJsf
~ E R A F
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 alloas 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 at present for most general commercial work. Ordinance engineers contend that the unilateral system is more scientific than
h e bilateral and more effective in precision work.
d
(A )
,,
I
WORhKlHG
T O ~ ~ . C E
alculating Mold Dimensions 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 an "add-on" factor. Every molding material has
a shrinkage factor specified by the manufacturer. ( Wamina-Some
recentlv
developed materials actually "grow" when taken from the mold, therefore
shrinkage factor is a negative value.) The factors furnished by the manufacturer may be a narrow range such as .003 to .004 in./in. for mica-filled
phenolic. They may also be a wide range, such as .005 to .Win./in. for
Nylon. In any case, the designer always adds shrinkage (except in the
aforementioned warning), Space does not permit disposal of the argument
that shrinkage is added to some parts of the mold and subtracted from other
Paas of the mold. Ifpa ~ d shrinkage
d
to any part of the mold, add it to all
, 7
1
'
gauge sf 1.5Uand a minimum,gtue
~ l sayd that the .005
.OQa5 (onehalf of ~ a teb u
would be dimensioned 1.5
dimensioned 1.495 .OW5
made to the extremes, a
1.5055 'and still pass the
be .Ol 1 instead of .010.
The unilateral sys
either above or below
the gauge is for a maxim
gauge is for a maximum
177
F
any part could be only .010 and still pass the gauge inspection if
+
*
The designer should s&ze every opportunity to obtain and record specific
lrinkage data from his own shop. This is done by checking molded parts
and mold at room tgmperature. Subtract the smaller dimension from the
lPrger dimension, then divide the result 93 the dimension of the molded
Frt.The results of the division is the shlir&@~eallowance in inches per
knch, and should compare with t h ~ , v g , l given
g ~ by the manufacturer. Your
Own
on specific materials &&@ixes used under your shop conditio9s will be far more reliable akd reproducible than the manufacturer's
d a t 8 . 1 ~ tthis point. let us mention the phenomena of different rates of
shrinkage in the same part. Shrinkage parallel to flow may differ from
age transverse to flow. Shrinkage in thin sections may djffer from
age in thick sections.
"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
kage a t e specification is used is different. In thermosetting molding,
F3i
178
PLASTICS MOLD ENGINEERING HANDBOOK
shrinkage rate will be one value when compression molding and another
value when transfer or injection molding an identical material. Thermo,
plastic materials attain different rates of shrinkage depending upon (1)
cylinder temperatures at time of injection, (2) nozzle temperatures, and (3)
temperature of the mold. Some molders select an average shrink rate, apply
this to the mold, then set up manufacturing conditions to obtain the allowed
shrinkage rate.
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,
a dimension of 1.250 :g for the location of a hole, and using a shrinkage
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
Projections, pins, or other male parts of the mold are calculated by
subtracting 1//4 of the total allowable tolerance from the maximum dimension permissible. Then add material shrinkage. For example, ,500 f .010
would become .505 plus shrinkage. Tolerance on mold is given in minus
direction.
\
Cavities, depressionA grooves, and other female parts of the mold are
calculated by adding IJ4 of the total allowable tolerance to the m
dimension permissible.
en add material shrinkage. For exa
.500 .010 wide groove wohld become .495 in. plus shrinkage. Tolera
on the mold is given in plus direction.
Dimensional tolerances, as given on a tool drawing, should amount
to no more than 1J2 the desired tolerance for the molded part because the
mold variation is only one of the factors influencing the final dimension
of the molded part. Other factors affecting the final part dimensions are:
+
+
i
1. variable material shrinkage from batch to batch
2. heat
3. pressure
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
questions than it is to guess wrong.
PLASTICS MOLD ENGINEERING HANDBOOK
179
MOLD STAMPING
letters and numerals when
it is essential that proper depth be allowed for painting, good appearance,
,tc. Many times, the depth of such lettering is left to the mold designer
and, in such case, he should submit his specifications and recommendations
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.
unpainted will be plainly
aracters are large, however,
Characters are often specified by reference to some standard type specimen book such as The Book of American Types, published by American
Type Founders, of Elizabeth, New Jersey. The height of the letter and its
weight and depth should be specified, as shown in Fig. 5.5. Lettering
that isilo be painted will be raised in the mold. The elevation of such letters
should be at least one half the weight of the line. All characters must be
stamped in the m6ld left hand. A typical designation would read: "Stamp
in %-in. L.H.chiracters .005-in. deep." Full information must be given
for special characters.
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
length of the pad for stamped characters (letters or figures) can be calcuhted by using the following formula:
length = number of characters X height of characters
+ the height of one character
n. letters would be caleight) 1/8 in. (height) =
+
t the letters are to be raised or
.
180
PLASTICS MOLD ENGINEERING HANDBOOK
TOOL STRENGTH
PLASTICS MOLD ENGINEERING HANDBOOK
E 5.2.
181
hickness for Mold Cavities
Recommen
Molds are designed to give maximum life and low maintenance expense,
Small, fragile seitions should be designed for easy removal and low-cost
replacement. Designing for adequate tool strength is always a problem,
and no definite formulas can be given. Several fundamental considerations
will serve as a guide in the solution of many problems. The strength r e
quired must be adequate to resist the compressive, bending, or shearing
stresses set up by the highly compressed molding compound as it moves
into position and hardens. Some of these stresses may be calculated when
necessary, but the mechanical construction of the mold is such that most
designers do not calculate the actual total stress loads on all mold-sections.
-
mold sections must be adequate.
3. The wall section of mold cavities, loading pots or transfer chambers,
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
to resist distortion and breakage.
The strength of the ejector bar increases in direct proportion to t k
width of the bar and as the square of the thickness. This means that the
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 considerable increase in width. A desirable average minimum width for the ejectw
bar is 2 in.
for the basic wall thickness of mold sections under 2-in.
to use 60 per cent of the depth of the cavity, but never less than
s where the depth is greater than twice the basic wall thickness,
adc&$ional '/s in. to the wall thickness. For example, a cavity with
. d i a h t e r loading space of 1-in. depth would require a 13116-in. wall
ess, This is calculated as follows:
(1 X .60)
+ 3/16 = 131 16 in.
n the clean-up size of &hestock.
from the formula for beam stresses. In this formula the stress is
strength of the bar is doubled, but if D is doubled, the stren@h-Qf the
is quadrupled.
Table 5.2 shows values which have been found to be satidmtbw f w
wall thickness of mold sections.. An approximate general fOmuYa
.
182
PLASTICS MOLD ENGINEERING HANDBOOK
..>*'.
.\.
.
?
,, j,
Notewol
in the mo
be stressec
sidewall, s
DECIMAL DldlENSfON
FOR THIS FLAT SAME
AS DECIMAL DIMENSION
OF RETAINER
SHRINK FIT ALLOWANCES
hould, be subtracted fram the dze of the mold
of the hole in the retainer plate, Siaple press
HEEL (ORFLANGE)
extreme stresses are antidgated.
necessitated the making of the split cavities. (It would be difficult to machine
such deep barriers in a solid block of steel.) The mold is shown in open
position with the ejector pins raised. Six ejector pins are used, and four
movable pins hold the inserts. This makes a total of ten pins for a piece
approximately 2% by 3% in.
MOLD PINS
Many kin1
to locate i
entering tl.
tiate them
maximum allow2
..,... .
FIG.5.7. Each cavity of this four-cavity semiautomatic landed p h n makd
~ is made
pieces and all are shrink-fitted to the retainer. Malded part is shown at ri$ht.
t
1W
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
which form part of the surface of the molded piece should be chromeple;ted when the rest of the mold is plated. Ejector pins should be fitted
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 misalignment which must be compensated.
%ns with an integral cam to produce sidewall undercuts are called jiggler
pins. Molds should be designed to make use of ejector pins whenever
passible. The number and placement of the pins is entirely dependent
qn the size and shape of the molded piece. The basic function of knockout
pin* is to remove the molded part from the cavity or core with no distortion
c~xarring,ancr 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 at the left shows the gates and runners still attached. Any comMnaPisrn of inserts ma be needed for this part. Six ejector pin marks may
tre wen at the outside edge. An ejector pin is also located at 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 be dimensioned so they will come ,005 in. above
the mold surface unless otherwise indicated on the product drawing. If
~Wmpingssuch 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 above the surface of the piece.
d
185
q ~ pins
~ must
r
be as short as possible but must raise the piece 3/s in.
g 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
hay 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
1 t~ 1/16 of the pin radius are used. Shown in Fig. 5.9 at ( A ) are the
p s vents often used where the ejector pin is inserted at the bottom
1eep cavity, or where it is necessary to fill out a thin-walled section
@&videconsiderable gas relief. Such gas vents are generally flats from
$ .005 in. deep that permit trapped air to flow through and escape.
+R gap provided will allow only a small amount of the molding comK 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
en of 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
e head shown at (A) is formed by heating the rod to a red heat
hen peening or swaging the head. This method is used where the
2
b
'
ik
-
GAS VENT
4 FLATS
- ,003 TO .OO:
DEEP
.
-DEPTH OF
RELIEF
MOLD
SECTION
$0 AND
T O R PIN
. EJECTOR
PIN PLATE
EJECTOR
BAR
Fro. 5.8. Uniform ejection of the parts molded in this single~avitytransfer 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 and pin relief. (B) Gas vents and n:lief applied
gle for pin strength to increase depth of hole.
11dpin.
I&@ PLASTICS MOLD ENGINEERING HANDBOOK
[A
FLABTICS MOLD ENQINEERING MAcglDBOOK
187
(8.3
FIG.5.10. Conventional methods of forming heads on ejector pins. (A) Peened head; (B)
turned head.
f i u m s bins for mold construction. (A) Small pin; (B)large pin; (C)pin
of the pin (or butt w
There are a number
widely used.
In designing the riveted-head type of ejector pin, consideration must be
given 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 baek 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 to g.diame&r. Pins larger than
M in. should have turned heads. Pins which are less than 2 in. in length
generally have turned heads, as shown in F i1 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
hoier in the molded part should not project above the
ore than the amount indicated in Table 5.4, since the
188
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
189
FIG.5.12. Illustrating use of butt pins for long holes.
C
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 bmgth than those shownI in the table are required, they may
be formed b y two pins which butt \t the center, as shown in Fig. 5.12. Butt
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 incr ses 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 misalignment of the mold cavity and pins. Longer holes may be molded by
the use of entering 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 pin should be solid, rather than movable, as sh
The bearing surface b should Inot be more than 14! to 2 times the
a. For pins 3/s in. or larger in (iiametc
1 to Ilk time:s the diameter of a. The
be calculated the same as for 6.
In compression molds, the length of the entering pins must be
to permit enltry to the force bcefore it enters the loading sp
The flow of compound which starts when the force enters
deflect these pins so they could not f
at least ' / 4 O if possible on the molding surface. Shrinkage:)
be added.
"hr,
1
VG.5.i3. Cleaqnces and allowances for entering pins in compression mold.
5114 vhows good types of construction for long holes. Two pins
f each pi@is 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.
mold holes to the minimum to insure that
190
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
191
COUNTERBORE
I
MOLDED PART
IMATERIAL- WOOD FLOUR PHENOLIC.
SHRINKAGE -.008 PER INCH)
OPTIONAL
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.
FIG.5.14. 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
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 50 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
s 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
FIG.5.15. Design for small mold pins.
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 $4 of the allowable tolerance (plus).
PLASTICS MOLD ENGINEERING HANDBOOK
193
*ADD. SHR/N;AGE AS REQUIRED
8 V COMPOUND BEING USED
b. Female threaded mold section for I in. -8NC-I screw produces male thread on
nued tliread. For the tolerance on the mold, use 1/8 of the pitch
binus).
m i n e the basic pitch diameter of the mold (for %-in. thread
b, or less), use the basic pitch diameter of the molded thread
ye allowable tolerance plus the material shrinkage. For the
the basic mold dimension, use '/s of the allowable tolerance
r %-in. engagement, make use of all the allowable tolerance
pllowable tolerance (minus) for tool error. If more than %-in.
!!is re.quired, compensate for the shrinkage in the lead also.
basic minor mold diameter, use the basic minor diameter of the
plus % of the allowable tolerance plus material shrinkage.
ces are taken as '/a of the tolerance (minus).
:xample is shown in Fig. 5.19 for a male threaded mold secn.-8NC-1 nut having the following dimensions:
@a the b*~
I
I
W m the t b r d
.
.!# I t ' !
?
j,l!
3 1
>
allswatile tolerance
W ?A 4 tbe aUswa
~b J Major diameter, 1.0000
W)ig,i6LUs~kreBlhrtatolmrar.
Pitch diameter, 0.9188 :Ei
*'
'.-'. ..,JI:',j-111
Major diamet&, (g.9W
Pitch diameter, 0.9154 2%
, rrc;
Minor diameter, 0.8432 qaximm
,
-.I
I
Minimum
. !. Ir, .cw=~;-.t.rb
3
Minor diameter, 0.8647 3%
et iaahknpiwly used in mold.clonsuuction and typical
applications
---
boaring or'm ~ i d sand return of eject1
?is,$@ @t(A) and (B), may be used
$&#&
To d e t e d m ti&
diameter tolemwe
all of E&
1'1.
r
)ars. The side
eithlei tqp or
!.
i,
@
l
k
tPilw1 is mall, and where 1pace rmitsi, internal
&hamin Fig. 5.21. In such applications, the ejector
194
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS WOL
ENGINEERING HANDBOOK
t30/1!$:7!
MAJOR DIA. 7
(FOR
ENGAGEMENT
-
*ADD SHRINXdGE AS REQUIRED
BY COMPOUND BEING USED
FIG.5.19. Male threaded mold section for I in. -8NC-I
molded piece.
nut produces female thr
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
(A)
(8 )
FIG.5.20. Application of side springs in plastics molds.
-head screws are best for mold work because they are easily disled. They also act as dowels in mold assembly, since usually they
only &/a-in. clearance. Small 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 socketw is the same as the body diameter. Thus a 51 16-in. screw requires
. minimum depth of counterbore. The right and wrong way to use
ws is shown in Fig. 5.22, which also indicates the proper clearances.
pth to which a screw should enter (dimension F) is the same as the
diameer of the screw. At this depth the screw would break a t about
@bia% time the threads would strip. A rule commonly applied in deterk (he c m c t length of screw is: length of screw shall be the same
196
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
-
-SUPPORT
197
PIN
- PARALLEL
(A)
(B)
FIG.5.22. (A) Improper application of socket-head cap screws; (B) proper application.
as the thickness of plate into which the screw head is recessed. Where greater
holding power is required, more
Appendix.)
Parallels
The parallels should be as close together as possible under the cavity;
allowing 1/ 16 to 31 16 in. clearance on each side of the ejector bar. The height
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.
The maximum width of ce
ejector bar. Center parallels
cannot be used, the additional s
These center support pins are usually
and they pass through the ejector bar. Support pins are best located m
way between center of the cavi
lowance for one pin is 1132-in. for the
pins use 1132-in. clearance for the en
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
ropped. Slots are cut in the parallels, as shown in Fig. 5.23
e blowing operation, and they are often cut in both front and
IPERATURE CONTROL MEDIA AND METHODS
@vianusly stated that temperature is an essential ingredient in
a?g operation or at some point in the process. Generally, this!$&with controlling mold temperatures. Uniformity of heating
k'w objective to be gained, and the problem justifies considerable
bnning. The bibliography lists texts dealing with heat transfer
6 used in calculating it. Heat transfer is the name of the game,
sferring heat out of the material and into the mold surface,
t out of the mold surface and into the material. In either
,
198
PLASTICS MOLD ENQINEERINQ HANDWOK
PLASTICS MOLD ENGINEERING HANDBOOK
needed for the very difficult molding where flow controlled by heating or
cooling is critical to success in manufacturing. The following simplified system may be useato calculate heating and cooling loads. You will note that
the simplification advises that 20% be added for a safety margin. This means
that the approximations are only accurate within 20%, which will probably
be satisfactory in 80% of the cases. The authors subscribe to the 80120 mle
which, in essence, states that 80% of the benefit can be obtained for 20%
of the cost (or effort). Certainly this is true in heat calculation unless thete
is a computer program available to perform the complicated calculations.
The quantity of heat or cooling required is determined by the larger of the
following two factors;
1. Quantity of heat or cold required to bring the equipment to operating
temperature in a given time period.
the desired temperature
2. Quantity of heat or cold required to m
during operation.
When calculations are completed, add
margin of safety.
an error factor and as a
To calculate initial heat requirements:
A + B + C + D
Heat-up time (hr)
A = weight (lb) of mild or platen X specific heat
Mfg.Corp., C E e w b d ~OH)
Mer to Hchamelr"This &uld be interpreted
chafhels* h 2 oil channeb, hot water channels, cold water
ow ever,
X temperature rise, O F
C = weight (lb) of liquid X specific heat
f
X temperature rise, OF
heat loss/sq ft
at final temp.
2
time allowed
for heat up (he
Total Btu/hr = Btu absorbed by material added per hour
.
,
.-..
+ Btu/ hr lost through radiation from moIds and' pipi
.
v
v
I
.
&&
a@
-I
to 30 or 43 in. of depth.
15- td 20 int should be drilled from owpo@itesidm.
, p m y spaced about 2 to 3 in. .apafi for ordinary.
Openings
ordinadly tapped
te fit the cundpits used in your molding
ble f ~ water
r
or refrigmanta, S&al
r stearn or ho-t nil, but should be tw.4
rupture at accidental disconnect
have set-in cavities, as ~ h o m
nQ clmx than % in. for holes less th&n
larger clearanas between the cbart,ael
To calmlate heat required for operation:
.. ..
- Btu/hr losm t h r ~ u g hradktiorl fraw molds and piping
the chameh shouId increase in size or number.
B = weight (lb) of system piping X specific heat
D =
I,Z
X specific heat X temp. change O F
9/16 in. chmneh are adequate: for the average mold.
X temperature rise, O F
exposed surface
area (sq ft)
'
directed flow with baffles, rrr n o n 4 m t e d fl~w.
b surrounded as weil as possible by channeb [see
plates .are channeled so temperature &ritrol m i be
and b i ~ the
w malded pam. Various sizes of channels
hop practice and the availability of sp&c types
,
1 media h a given molding shop.
Total Btu/ hr
X 1.20 safety factor
KW hr =
3412
Total Btu/ hr =
.
199
200
PLASTICS M O W ENGINEERING HANDBOOK
PLASTICS MOLD ENWMEGRIWI Hm-
201
STA T/ONIRY
AWTY BLOCK
no. 5.24. (A) Proper steam channeling: r (greatex than H y) represents shortest distam
from channel to aavity; y represents gr%at& dbt8a~efrom channel to cavity. (B)ImpropeF
staam channelinn: x (less than 'h vl reoresents shortest distance from channel to cavitv w
R
UNGER PLATE
the same as the reit of Qt: m~#d.'
Lf t.bg h is laqg .&oy$h to permit sq
of the media to be chanjxded 'throp@ it;, by ah means make that provish
This ap&es particularly to ,iqjw$ion molds, w b r e even very small
pins must be channoSed to g e d i fast &&b trawier out of she mold[
material. Long or large makt pluqers must be cham14 or cored as shot
by Fig. 5.25 to get uniform tempedture'hth the femaEe section of the mo
We emphasize t ' h t molding of urea or tgekminta materials
uniform heating which means direct chonneb in the a l e section of 1
mold, a s well as, direct channels in the r&rnt~
seo.tim of the mold. k
general rule for all molds, it is wise to use dkM o b n n e b whenever4
male seetion is longer t h a twice
~
if$ diameter. Referenw tm vendor's c a w
will show several standard baffle types 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 A
. M htdd
crt wires on the operator side if there is any other wag d mtkir&g
saq con~k~tions.
All other champ1 openings should
p q g #
to prevent leakage of the media.
i v,iL
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
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. In 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 warning
xder here. When steam channels enter the male section and the conte must return to a higher level for discharge, be sure to use directed
A generkl rule, provide a channeling so that temperature variation
rectiok b &tion will not egceed 20° F.
4
s Pins and Guide Bushings
* pins and bushings are used on all except the very simplest and 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. Threeour-pin
z mpy use the same size pins if an unsymmetrical
202
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
203
CLEARANCE HOLE=
DIAAfCrER 06 - GUIDE PIN + 7
,
(C)
and guide bushings. (A) Guide pin; (B) guide bushing;
I
FIG.5.26. Assembly of plungers, steam or water plate, guide pins and stop blocks. D
lines indicate channels for directed flow of media.
spacing is used. Guide pins are located as far a
effect of the clearance between the. pin and bus
The diameter of guide pins varies from '/5: in, for small molds U
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
molds. When considerable side thrust is expe
metrical flow conditions, the larger size pins should be selected.
The length of guide'pins should be such t
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 be used
plates are not hardened. When hardened plates are used, guide
are not absolutely necessary but they are preferred, espeeirrfly if
will be a long running mold.
Guide pins and bushings must be press
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 :
,
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
s the plate thickness, allowance should be made
peenin8 each end of the hole or use a shoulder bushing. Set
often used when the length is very short. A set screw, as shown
e guide pin is not backed up by a plate. The
uld rest against a flat on the side of the pin. There are several
standard guide pins and bushings. It is recommended that
pins are seldom lubrimted and frequently are used in a rusted
ses them to stick. Thus considerable damage
out of the plates. These factors necessitate
pins. (Also improved shop
).
HOBBED CAVITIES AND PLUNGERS
ed by the methods used for
ob comes within any of
economical for the job.
esigns are required in the mold.
- -w
204
PLASTICS MOLD ENGINEERING HANDBOOK
PLASTICS MOLD ENGINEERING HANDBOOK
205
being smaller cross section as the runner branches out. The
match at the parting line.
OPPOSED CAVITIES AND BALANCED MOLDS
When the shape of the cavity or core is such that the molding press
will create side pressure (bending pressures) the cavity arrangement
usually "back-to-back,* i.e., so that distorting pressures in adjacent
offset each other. This is indicated in Fig. 5.28 which also shows
"balancing a mold." However, the balanced mold is most often a ter
in describing a particular arrangement of cavities in an injection
is the same length. Further, runner sizes (diameters or cross sectio
gradually in exactly the .%me fashion in each of the paths. The
RFACE FINISHES AND TEXTURED MOLDS
of the Plastics Industry and the Society of Plastics Engineers
master blocks with surface finishes clearly defined, specifmble
atld capable of duplication by any knowledgeable moldmaker.
houses have also created textured designs that can be specified
resumably to be applied by them). Specialized businesses
making the finish requirements known at the time you
REFERENCES
FIG. 5.28. Opposed cavities equalize s i d e - p r e s s ~ ~The
. I!
for sinking cavities.
e PA: Robo Systems, 1984.
of plastics part design, Plastics Design Forum, Nov./Dec.
206
PLASTICS MOLD ENGINEERING HANDBOOK
DuBois, J. H. and W. I. Pribble, Plast~csMold Engineering, 1st Ed., Chicago: American
Technical Society, 1984.
Fine, Arthur and McGonical, Charles, Design of complex connector mold, Plastics Machinery & 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 Engineering, Jan. 1984.
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 nForum.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 1984.
.
S o n , Laszlo, Designing for producibility-A roundtable forum, Plastics Design Forur
p. 23, Jan.lFeb. 1984.
\
Suggested for Further Reading
Krouse, John K., Automation revolutionizes mechanical design, High Technology, Mar. 1984.
Levy, Sidney, What CAD/CAM programs may not do for the designer (yet). Plastics Design
F o r m , p. 62, Nov./Dec. 1984.
Levy, Sidney, Complete CAD/CAM moldmaking software, Modern Plastics, July 1984.
pression Molds
Revised by Wayne I. Pribble
of the molds used for thermosetting plastics are the
ression type. It is the oMer molding method, and later developments
prior arb. In developing this text on mold design, it is
he compression molds and follow with the other
of the design of a single-cavity hand mold that is
atic 12cavity mold will be used to introduce the
1 calculations and basic design procedure. This
undamental mold type is followed by discussion
considerations that arise in the design of other
Hand molds are being eliminated for many applications because of the
and molds continue to offer better answers for
rt assemblies in many applications. Hand molds
nsfer and injection molding. Shown 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 or 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 to 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
207
.
COMPRESSlON MOLDS
PLASTICS MOLD ENGlNEERlNO HANDBOOK
208
209
gligible. The use of preforms also reduces the loading space ree mold.
uction requirements of the user of the molded parts will determine
m numb& of cavities to be used. The fact that 100 cavities are
intain the rate of d 'very does not mean that all must be in one
r bottle caps hav contained as many as 150 cavities; molds
may contain 500 ca ties in a single mold. These large molds are
e average mold will be found to contain from 5 to 15cavities
-sized parts. The use of a low number of cavities has manyadvanthe smaller molds remain open a shorter period for loading, etc.
the smaller molds, pressures are more uniformly distributed
s minimized, as repair work on a small mold means production
number of cavities. For example, if a 36-cavity mold were
press for repair of a broken pin, all production would be
instead,&hree 12cavity molds were used and one had to be reone-third'the total productign would be lost.
4
1
W111g
~
E
.airktftpmfot single cavity c ~ m p m t ~ bmid*.
n
vany hand moLd9are
(Comesy MMMF UnJt Die Frud-
w
bD 28misRt~a~~d~
W
by the usc ~f these u&.
Inn., 8csnwillc, HI]
,assume that a mold for a lever is desired. See Fig. 6.2. Specifhthis part to be: molded from one of two materials: black wood
ienolic or gray urea-formaldehyde compound. The user wants a
built quickly for test, for sample to be followed by consttuc;duction mold capable of producing 7500 pieces a week. The
kast costly mcsld that can be built will be a singlecavity hand
that the wmpaund and inserts may be intduced w i l y and the part ejected
without distarrion. Since the mold is idle while it ie being loaded and unloaded, the efficiwcy of theise operations, the quality of the pbce, and the
w ~d
t the flnishixl.g opmttic~swill be a tme measure of the quality of the
mold.
a m 1 molding prm~3simolw the pmblern of forcing a bulky mat*
rid intb a given shape and spa= by the use of pressurnsranging from 2,500
upward, accompanied by the applimtim of heat ,m-3804
F) far the purpo*
af plastbiaing the compound and causing it ter flow and fill out to the mold
contour. 'Ilris action may ppsdwce highly l o c a l i d stresms in various p M
af the mold, and thenby caw serious mold breakage if the w l d p i fra*
are not properly designid.
The ww materials mag be charged into the mold by at least three d 8 f e n t
nxthods. Tkme are li5:ted in the order af tbeir preference and gem&
DESIGN OF HAND MOLD
the design of such a mold will be the tabulation of informadata' card, and this involves the determination of the bulk
I
olic wood flour
Bulk Factor
. 3
3
Shrinkage
.006-.009
.008
lations may be made either by laying out the piece in sections
I. Pmfoms, or "pills."
2. Frolumetrie loading by loading b a r d or measurn cup,
3, Weighed c b q e af powder or preforms.
.
?rgthe volume of each section or by computationfrom the weight
obtained from the volume, using formula (I),
formula is also used to calculate the volume when the
The preforming d a r not change the miterial itself, but @I%
to VO*?
a loadiryl unit of pndnermiocd weigh. Preforms an ODW
b.ndW
I
210
COMPRESSION MOLDS
PLASTICS MOLD ENGINEERING HANDBOOK
21 1
(1)
V = total volume of part
Wu = unit weight of material
WT f total weight of part
&me of the lever has been found to be 0.70 cu in. We must add to
kh factor of 10%to include the material required for the flash. Flash
@&l that will be squeezed out around the plunger and through
bw slots as the mold closes. This allowance must be made in comholding to prevent precure a t the parting line and to enable that
@ich lies on the land to escape outward and provide a good pinchb r f l o w also takes care of any variation in the load by permitting
~d to escave.
additipn of the i ~ foro f l ~ the
h total volume of compound
b..
-.70,
+ 10% = 0.77 cu in.
L'h
@attion of formula (1) we obtain for the gross weight of the lever:
'6 = .77 cu in.
P.
,FJr;= .76 ozlcu in. for phenolic or .85 ozlcu in. for urea
,,76 = .58 oz gross weight for phenolic
.&51= .66 oz gross weight for urea
$.
pie
I
of 1
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,
WXBF
w,x loo = y
Gross weight of iolded part per 100 pieces
Bulk factor of compound
- Weight per cu in. of compouna
.@54lb/cu in. for urea or .048 lb/cu in. for phenolic
" Total volume of compound required
( W a d WImust both be expressed in lb or oz)
(2)