Plastics Mold Engineering Handbook

x PREFACE TO THIRD EDITION

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

There are, of course, many variations of molds, whatever their general classifications. Naturally, it is not possible in a presentation on mold 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 methods- even those that are rarely used. Our purpose here is to stimulate interest and

Contents

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

suggestions for changes and improvements in the text. Since many pieces of equipment that are obsolete by present standards continue to be used, we have described mold types for some of them. For instance, this text is used in parts of the world where very primitive equipment is employed. There, the people need data on molds for simple processing equipment, and to use the supply of moldmaking materials, which may be available in these lo- 2 calities but far removed from suppliers of standard mold parts. 15

We are indeed grateful for the widespread acceptance and distribution of . . 18 this text since it was first published in 1946 by the American Technical Society. We appreciate the obligation this places on us to be accurate, pre- cise, and factual. In preparing this new edition, we have carefully re- searched the intervening developments and have made every effort to pro- , vide serious readers with a body of knowledge that they can carry confidently I

J. HARRY DUBOIS h % W k A ~ i f q-: 1 id Morris Plains, New Jersey %

I + i 37 WAYNE I. PRIBBLE :, 2 )CLm713a @7

New Haven, Indiana && 1 , . .Xi1 ,; t,.w PR~ESSES, EQUIPMENT AND

Pawl E. Ferland 64 65 82 86

vii

ix

S

SLrface Finish, Molds and Parts

CONTENTS xi11 I$', f . ..'r:,,,, %rink Fit Allowances

Mold ?%ts W e ~ M d d Parts T e m m r e Control Media and Methods Wbtt$@ Cavities and Plungers ; 1 D p W Cavities and Balanced Molds

Burfab@ Phishes and Textured Molds Refe

iCX)MPRESSION MOLDS, Wayne I. Pribble

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

Bracket Mold

pression Mold Considerations

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

300

v r - 7 - - d z ( q j ' l ~ \ , ' v ' 3 ~ : : - 7 . . , . , , t \ ,,,< i.!{-b:d&iI{-I ,!?

.I c 9; QtXD Mom DBSIGN, Leon R* Egg ,a!~r:aM -9-

1: $2 ,*it.nrl'i: TO^ &' ' , 'j'

Chapter Introduction to 1 Plastics Processing I

I Revised by Wayne I . Pribble

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

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

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

. - -J

2 PLASTICS MOLD ENGlNEERlNO HANPBOOK

8 PLASTICS MOLD ENOlNEERlNO HANDBOOK

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

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

Blow olding Botties and other hollow articles are extrusion blow molded of 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 pres- 1 ' sure is imwdktf~ly applied in the clamped tube to expand it and fill out the I mold ntour ( ~ i g . 119).

An extrusion die E&3

INTRODUCTION TO PLASTICS PROCESSING 9

Plasti~izi reciprae: screw

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

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

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

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

--

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

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

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

- thesmophstics has formed in the mold su&a&, the, t J@@ 'a mter spray or air blast while rotation continued ha- IF&$ 1.41 a n faIzn:ated by aluminum casting, rnacM-* A* **-ST - - . %rbp.E~mmd mbkel. The molds are vented byb%

QLW& &I@. Wzcils on the &tpk mold designs bm ,%& mmd&nnc ma GP-

14 PLASTICS MOLD ENGINEERING H A N ~ ~ ~ INTRODUCTION TO PLASTICS PROCESSiNG 15

old then opens slowly under proper controls to per- n. In the low pressure process, conventional injec-

with resins containing preblended foaming agents or ded by.a "piped in" gas.

the mold-maker. Good tools

, . . ,-. ..{' I

18 PLASTICS MOLD .EMGINEERIKT MAWDBQW

The metal vqs&a$3~wsO;ies mak6ptmy jbs, and fixtures out of plastics, and thew m&- are identifed by thc snmsp*urira rooling. The plastics pr ies refer to their tcl& as molds, dies, and fixtures.

REFEREM- Basic Mold Types Baq, ~ f i c , ~ @ m & n g Design fir Flasrtcs, Mew YD&. Van Nostrand Reinhold, ~ ~ c k , Ronald D., P M i c s Prodird Deaig~n, 2nd &., New Van Nostrand Reinhold, and Features

1980. &mhdt, E. C, f i ~ s s i n g of T h e m p k W c Ma%*, New York: Van N0s-d Reinhold,

1974. ' Revised by Wayne I. Pribble

. A mold is only one item in a series of , E@&W td., ~ k w Pb& M-F*rM+ !W. material. The vast majority of molds are ~ f * m d Ph9da, pew York: Yap Mwtrend Reinhold, 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, 1984. contour of a part.

For further reading, we suggest: wu, a w e R., to lasers for practical way to do the tough fab&atiagjobs~ Mod- cialize in resins, fillers,

em Pfastics Magaz im, p. 61, May 1984. ther components that terial. The final mate-

, , r from many different

BASIC MOLD T m S AND FEATURES 21

extremely stmple molds of wood and m t e r are usually by the hobbyist who wants to experiment with pbtks . Some readers will qua- tion this aaehof 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.

Pla3tjcs old Engineering will not ableto cover eventhin&. This ere- lude to basic &$M tgrpes and feartum,&jB &&90~ Bn idtial undwBnding of the molds only. We urge you to co~lect 3tZdog and house orws which describe and promote n w meth& Qf-Ow Fh6h@tiom of old methods that have put togetlfer to make a fahat ing device that probably g l l do a job previou~ly considered 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

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

I 1. pro. 2.2. Various cornpo&p&&~d~1e twgplate iqjection mold used for injection molding. (Courtesy Dow Chernic@&C~;, 4fEBnd, MI) Ci - i ,.. molds and operating t this type. Since hydraulic presses are mo hcr ip t ion is given of the operation of a mo press, Two general types of presses are u aed the t ~ w m t r o k e press. The downstroke press &, ~ 1 i r d e ~ 90 that the ram and top platen are m p m u r e to the mold. This 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 - . v - - - 7 - z - 7

A . A ~ h

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 inc t types- A plunger or auxiliary ram trans- S 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. His- torically, compression molds were the very first types to be used in the middle 1800s. The injection molds came into being in the 19% for the thermoplastics processing and the transfer molds came i n t ~ 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 mate- , ' is forced into the closed cavity by a source of pressure other than that 1

caused the mold to close. The melting of the plastics material in the I 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 @wine classifiwtion of materials. T 'flowing into the cavity." This'bavity means the space between the male and manufacturing technique b o ~

n and the female section into which the molten plastics will eventually r of tk raw material tha into the desired shape and detail. The point at which the core and

vity separate or move apart when the mold is opened is called the parting e. Chapters 7 and 8 detail the different manners in which the material can introduced into the cavity through a gate 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

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, sedauto- matic molds or automatic molds. lifts for opening a hi.., .nold.

, f1' .

mi. %FR This

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

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 wd h bholdlu in Fig. 2.14. The depth of the mold

y this constricted section mold. This does not per-

tb h d . If the $mold

&sig&@f4i mold and Is [email protected] st' t b imdw&nt athim maximmi d&u&b

of mold, Fig. 2.16, consists of a m and a loading shoe. The loading shw'&mekly a floatin ed midway between the plunger and cavity when the mol forms or powder may be loaded ia W4 type of mold. a loading shoe with molded p.kp partially ejected.

The loding shoe mold offers mom %&lwnh@s for certain types of compression molding. The cavity iar mqe acxxs~ible than 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 never- theless cost about the same as a landed plunger mold bemuse of the added shoe. High-impact materials may be molded'in this type a f mold, therefore

used for work which specifies these materials, although next described may provide even greater advantage. loading shoe molds are not mommendad because '

may cause binding of the loading shoe. f i sh

the load& shoe arrivesat its normal position. I - - .

sha mold is Mat thb ship&?*- %h$ I- ai W bmkie %f tho w16@ Brt (or 'soxpewhmy ~ ! m n t&:&$iP @ad w W j a t never larger t$ ~ " % t s i d e dimant$& of & &W

FIG. 2. b pusha from plu


s TRIPPER PLATE

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

h onl: binc spel 7

2.1E PUS acti the by i the

F plat the usec the U

, the

, a pmk1 in a

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 ~d at (A), and in the left side at (0.

,,

ejection of the molded piece is at all times important. Much of bnal accuracy of the piece may depend on uniformity of the %e. Proper ejection 4 #&.&de' 'from the mold always presents and it has been said wisely, "One piece can always be made . . ~etting the part out of the mold in one piece is another


DEPTH OF CAb'lrY

b

a piston in a cylinder- iston. 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 advan- here is no land in a he compound. The

plunger rubbing the side wall of the

Considerable di

Ive'8aold to allow wall thickness

L '.

-


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 con- tate 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 mold-

cavities may be contained in each gang of a mold &Wig. Sueh molds are frequently built with from three

$c!kt%ining fifty to one hundred cavities. The cavi- M@m d a loading space, as shown at A in Fig. 2.22.

wtly work required to ahc Gast of rn- indi i

usring a p d m uf the ~ ~ d t h e k n d s h o fix many medim-ski * a r u i t k o 4s dose t bp.damebdthQk.jssdingareilE08 b ~ 4 @ ~ 5 h ~ 9 1 r i l l p n , ~ a a a u p s l h*s sbm~l tb.b#,-w wbtisfacto~: . i " , . 8 > , *s I

, a , ' a . . 1 ' 4 .

&@ PbASTlCS MOLD ENGINEERING HANDBOOK

WEDGE To KMoCKoUT KMOCKOUT BAR AND WEDM MOT m WEDGE (4 (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.


0

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 of deep parts from the plunger. 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

FIG. 2.30. Pulling out fork used in spring box mold. (Courtesy General Electric Co., Pittsfield, MA)

provides the extra pressure needed to insure full density after the normal flow takes place.

Double-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.


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

gegeral practice which is predicated on the type g m t and l a b o ~ available, and the experience

age set ,up in chzs manner of the various methods. In all cases the mold , or s-ia~k-eaulty molds ts at minimum cost. Many

for larger par&. . dures may be determined For some jobs there is

the mold an t the tool-maker must weigh the ~ssible design before he decides which is best

Ids is shown in Fig. 2.35.

F ~ n ~ a l s , 2nd Ed., New

ark; McGraw-Hill, 1986. : Pm-n Press, 1967.

PuIxlWons, 1944.

: '

*

I -. . k

I .

J

' J

- 1

?

I

m&ng representdin one &&, Specialization by proawes and relattd ski& k t q s , - , k cut& of modem

mold making at a masonable h1, Fer examp& a s shops that b* only standardized or cu~taan maid 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&&@ mpd fe- i* of ern&& bases fa.$ frm 8;

~ b o m , f i b Wl met% $hap a h

inserts that cannot be machined wanomkdy. Haah# % ri lk

supp~&s are cmpemtiv equipment and s W to


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 1-1 metal supply warehouse, ~he.supplier will 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 e cuts 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 AKlNG PROCESSES, EQUIPMENT AND METHODS 67

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


I

2 PLASTICS I#OLD ENGINEERING HANDBOOK

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.

me t@@L MAKING PROCESSES, EQUIPMENT AND METHODS 73 rk to be ground is placed on a round magnetic

t opposite in direction of the work can be con- ed. Several pieces, to be

the magnetic chuck and ground is usually operated w i h a spray

sing 8@r the work.

ce grinder is used for grinding soft or hardened an inexpensive means of finish-

faces will be parallel. so that angles or radii

3.14, use a magnetic chuck to the grinding wheel. Microm-

may be controlled dmely. "wet (lr+diy" grinders.

1 cylindrical grinder (Fig. ay be rotated on centers.

amhments. The universal grinder will

Y

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 univer- bme 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 WW is mounted into, and driven by, the horizontal

f& ~ttpp0:rkd against excessive deflection by a heavy over- as &own in Fig. 3.17. $hell cutter mounted directly in the spindle for

-1 block is shown in Fig. 3.18. Other stub arbor w@ milk, and[ in these instances the arbor and over-

F$ PLRSTI- MOLD ENOINEERING HAtdDSOOK

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

Fro. 3.18. Squaring block with a stub-arbor, shell cutter in a plain horizontal milling 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 ~ ~ e r the A 'J:k pattern must be the m e size @ the Finished wwk ,pie+ Qw pr$(y .ap I: I, but reduction far wid

1

D M E

,s, Inc.,


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 position- ing 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


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 di- ameter cavities having draft and other internal configuration instead of straight round holes. The pencil barrel cavity is a typical application. A highly accurate hardened and polished male master is made. Annular mold inserts are then gun drilled to the desired depth, hardened and polished to a 4-8 microinch finish. The mold insert is then placed over the male master and reduced to its configuration by a swaging-extrusion process. Fifteen hundred cavities have been made over a single mandrel by this process. The machinery required 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, Inc. , St. Petersburg, FL.


METAL EROSION PROCESSES

TOOL MAKING PROCESSES, EQUIPMENT AND METHODS 87

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 de- scends, small intense sparks are generated wherever the gap is reduced. Ero- , sion 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 at right and dielectric fluid pump Courtesy ELOX Corp., Troy, MI)

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

&&a had plaster, &&!or mt pl&$~,.-oran~ golid me t . . wbo'amib these kr&&j.yi% h i s b Qwip data and infor-

&%lop$ adequate pressure in the assembled ceramic nrolelas molten m e d i s 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%tY w'&re. ~ k k i b ~ n , produced by other specialists, using ceetdfu&l easthg

r. am-pieck: semi-ceramic mold or investment. This method may be ca] mmt casting, centrifugal isr t ~ . ~ & * ~ ~ ~ p,*a; >- .,:: r 7

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 . .

ure on all surfaces

86 PLASTICS MOLD ENGINEERING HAND-OK TOOL MAKING PROCESSES, EQUIPMENT A.ND METHODS 91

R P O ~ 3.a. and $m&&m- pfeswre castings. Note flutes and ribs which an form& In tb8 cavity sedon8. (Churtesy Tooling Specialties, Inc.. Denver, CO) process which differs from the

4 shafts in mbrs, engines a t h w : ~ ~ e - p o ~ t i o n pr

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

%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 pro- fag iw* p r o d u ~ d by metaldisplacement or metal-

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 @sllally produeed 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.

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 MY 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 n is arefully checked with a vernier height gauge before assembly.

FIG. 3.38. Hand tools are frequently used for the engraviw of malds. I are produced by careful

w ~ r k is essmcral.

TOOL MAKING PROCESSES, EQUIPMENT AND METHODS 101

also used as a hand grinder, but it has ~ p w ~ p o w e r and 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

106 PLASTICS MOLD ENGINEERING HANDBOOK TOOL MAKING PROCESSES, EQUIPMENT AND METHODS 107

be a complex and academic subject, the basic principles are

the growing importance of achieving, measuring and inter- quality surface finishes, this basic study of surface measure-

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

any study of surface measurement be preceded by a clear the terms in general use.

or roughness is that part of surface texture best defined

\ FIG. 3.50. ~ o l h assembly area.

a t 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,

rally results from the condition of the production tool leading to unnecessarily high production costs (Fig. 3.51). While surface or grinding wheel (Fig. 3.53).

Re, 3.51. TkEg &mph indiafea iWsh Ss mtq#s. bf a surfPce am: R, mughaerrs (primary tcxt~m); A,, wav- Was swin$, and W + IQ* waviness &IS-mghnes9.

d j TOOL MAKING PROCESSES, EQUIPMENT AND METHODS 109 1

s E-v

Meter Cut-off (mm) Traverse Length (mm) Minimum Maximum R t l + Rtz + Rt3+ Rt4 + R t s

'bpR1 R2 R3 R5 R7 R4 R G R ~ Ra Rlo I #4 ( R i + R j + . . R9) - (R2+R4+. . .R10)

the grit and its size used in a grinding wheel (Fig. 3.53). 5 . ' 3 , 1 Root-Mean-Square (RMS) is an average geometric roughness and was an,

American standard. In 1955, it became obsolete, but naturally enough will I Frc. 3.54. Definitions. ,!&I1 be encountered occasionally. It is sufficient to say that its numerical value is some 1 1 percent higher than that of R, (CLA, AA) (see Fig. 3.53). i4 . The foregoing standards, with the exception of RMS, are in common use. Secondary TeMwe. Secondary texture is that part of the surface texture

'Other terms will be encountered in the study of surface measurement such underlies t s rmghness . All types of machine vibrations, for-instance as k, R,, tp, R. spindle deflectiln Bnd imbalance, can be the cause; it is generally described

It is worth enlarging on the parameters R, and R,, since increasing ref- I as waviness or more simply W (Fig. 3.52). erence is being made to them by manufacturers of surface measuring devices and Standards Institutions. Both R,, and R, are parameters that give a mea- The prodi t ion process used will form patterns on the surface. summent of the average peak-to-valley height, the former being intend* 9rhe Pred0minankpattern direction is known as the lay. for measurement by a machine whereas the latter lends itself to graphical Y determination and cannot yet be reproduced by a machine. In some cases CoC-off (Sampling kngth). Cut-off is a facility that is built in to most

, I;&!; &,,, and R, can be used as alternative or supplementary parameters to Rt. measurin$devices. 3 Its function is to suppress waviness (secondary ' (For determination of these parameters see Fig. 3.54). "XhUe) whatever d e g ~ is required within the limitations of the cut-off

In order to simplify the illustration of surface measurement principles? Unltl this facility is of great importance as it allows the effects of

reference will be confined in the remainder of this article to R,, Rp, and Rt, Process to be studied gparately from the effects of machine as it is felt that these parameters will be sufficient to develop the basic lkfficieficies. Cut-off is a filtering operation that is performed by a frequency- dcpnqent electro?$,&filter. The cut-off values according to the British

m O L MAKING PROCESSES, EQUIPMENT AND METHODS 111

We h v e chosen to discuss two points in stppe.deptb, ,because they am tw s a q values mice. The length of sfroke should . constantly giving rise to doubt and argument. 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

112 PLASTICS MOLD ENGINEERING HANDBOOK TOOL MAKINQ PWCESIES, EQkllPMENT M D METHOD8 118

---

i I

t' I t i dFR$hKering, p. 45. q i

. !Z a; Equipment, Oct.

1 --- . ---- -

h a permit the use of mate

. The mold desipet. has many

vestment that bg lost if;.

MATERIALS FOR MOLD MAKING 117

&eel, &&lting Steel, water Wdening Steel, oil hardening'

Steel, low dlay & carbon Kirksite (zbioallay) Mulainam aUoy

Silicone rubber

P stress. The premme in a

PLASTICS MOLD ENGINEERIN.&)IANDB80K MATERIALS FOR MOLD MAKING 119

arc melting process, is remelted by using it as an electrode. Figure t the liquid metal from the electric rates the Electroslag Remelting Process. The (ingot) electrode, degassing process which removes most of the hydrogen and the cathode, is slowly melted away in small droplets. These drop- ually done by means of a vacuum through a molten slag bath which has a purifying action and pro- ing Argon into the liquid metal. The removal of most of

e liquid metal from oxidation. It solidifies quickly in the water- gases will result in a cleaner steel which is very desirable in mold, which also tends to form it in a more uniform structure. In steels. There are several methods of degassing. The most p o p

a s s , no special atmosphere or vacuum is used to protect the metal. tion is rendered entirely by the layer of liquid slag. (A), the ladle of liquid steel is put into a sealed vessel from

Vacuum Arc Remelting process, no liquid slag is used to protect 'has been pumped out to create a vacuum. This is called I . g& metal, because the process is contained in a vacuum. It is more costly ; : + ~ u $ e the equipment is more sophisticated. (B) of the sketch, the ladle is sitting on top of the vacuum

paring the charge for the electric furnace, pure iron or carefully uid metal is poured into the ingot mold which has been scrap is used, together with alloy materials in the percentages . Inasmuch as the chemical composition of every constituent

steels is known, various elem nts can be brought into association \ per ratio to achieve a desir result. "Cold melt" electric furnace r of the ingot being the last to become made from a charge of cold ma rials, the name serving to differ- it from production steels produ by charging the electric fur- tb molten steel from an open-hearth race. the steel has been melted and refined it is poured into iron molds , the center of the ingot may be left hollow or, at ingots. Tool steel ingots may range in size from 6 in. up to 70 in. ensity. The defect resulting is called pipe. Ingots con-

f the manufacture ss known as cogging. In the cogging

81 methods. A Ladle degassing process. B. Vacuum ingot degassing pro-

WIl lW

ladle

v

w ti tl SOIKI.

i~ P least, tainix

0

6IOO PLASTICS MOLD ENQINEEWQ HANDBOOK MATERIALS FOR MOLD MAKING 121

process, the ingot is heated to the proper temperature and then worked to LE 4.3. Minimum Allowances for Machining and Maximum the desired size and shape by a process of hammering, pressing, or roll- Decarburization Limits. $R$. Some types of steel, such as high-speed steel, should always be ham- m 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 ham- produces a condition known as surface decarburiza- rnered to a specified size. In the rolliag of tool steel, the cross section of ce skin must be removed, and steel producers uni- She biilet must be reduced slowly, and this is accomplished in repeated +piaxes through the rolls, a small reduction occurring with each pass. Care ar stock for tool-making purposes. The allowances given is exercised to do the rolling while the temperature of the steel is between maximum and minimum temperature limits. After being rolled, the bars must be annealed to remove the stresses.and to make them soft enough for pared as cold-drawn bars for certain uses. In the cold- machining.

Annealing is done by heating the steel to the correct temperature and fbm allowing it to cool slowly. Scale will form on the surface if this is the preparation of drill rod, these bars are ground in do@ in air, and to prevent this, the bars are annealed in an atmosphere to remove all decarburized surface and provide close bmtrolled furnace or are pipe-annealed. In the pipe-annealing p r o w ,

bars are placed in large pipes and surrounded with a baterial t b t re- %& oxidation. (Cast iron chips are generally used for this purpose). Tht? STEEL FORGINGS pipes are then sealed and the entire charge is heated to the.annealing te& pa t tun . 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- %$y air fn wcontact Mth: tb ~ttiYit&

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 MACHINABtLITY t iron chips or pit&'coke, by

, or by heating in a wuum Borne grades of steel will undergo certain machining operations with &eater ease than others. The machinability of steels may vary with the w a l i n g process. It is possible to anneal specially a piece of steel to give $$ better machinability for a given process. For obtaining the maximum maunt of machinability, steel stock from which mold plungers arc cut on &duplicator may require a different annealing process than a block of steel

.

ing to which the stock has been des&nated temperature rang- hardness and to relieve the

ar machinist's experience. (such as nickel and man- , while relieving the major

sses inherent to the pro-

bartines of steel, is accomplished by heating just a b v e ule critical point and then permit-

HEAT TREATMENT kr4 'aTf&ESS RELIEVING

idud stresses. Steels that have been sub-

ons nlust,be relieved or distor-

Aftel ,- ing froq stre& from

stresses I( cess resu by a sea

Annealin the meta ting it tn

124 PLASTICS MOLD ~NGINEERING HANDBOOK MATERIALS FOR MOLD MAKING 125

STRENGTH OF STEEL

MACH1" IYG STRESSES

HARDNESS PENETRATION

wnching solution, cools very quickly. The inner core cools relatively y. Steels that must be quenched rapidly t o give them proper hard- will have an outer shell that is hard and an inner core that is relatively owing to delayed action of the quenching. This outer shell may be to %-in. deep in a water-hardening tool steel. The use of certain al-

hing. The oil-hardening steels show greatsr hard- water-hardeping steels, while the air-hardening

the greatest degree of hardness penetration. Molds subject ection should not have a high degree of penetration.

MOLD STEEL REQUIREMENTS . .

heating during heat treat-

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. ing rapid heating when the thin sections reach the critical temperature ~n#9*. It must be uniform in structure and. relatively free 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@ld d l s t m , 'The mold cracks if corn- . Steels which machine @ly

y are needed for wonomical mold comtpction.

hardness. They are easily machined and polished to a fine for most injection molding applicatiohs. It Is advisabt to and the low alloy steels are nrqlds made from prehardened steel if the mold is comp1h"Eed Bt radii and corners.

ired hardness in

and polishing.

126 PLASTICS MOLD ENGINEERING HANDBQOKr

Stre~gth and 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.

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

Rm Steel Rgte steel is a low carbon steel such as SAE 1020 produced by the openhearth or &her inexpensive processes, wherein cjeanliness is a less important factor WR volume. 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 &heapest of molds.

There are several qualities of plate steel available and if any press&re is @ 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 back- ly, plates, steam plates, or other members on which stresses may be c o p centrated. This practice requires that a large inventory of stock 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.


~btained by using the cheaper grades makes a negligible dif- otal 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.

1 class 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

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

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 PLASTICS MOLD ENGINEERING HANDBOOK MATERIALS FOR MOLD MAKING 129

the principal suppliers of standard mold parts assures the mold builder of king the various types of alloy steels and gives the properties economy and reliability he can not otherwise achieve, except in unusual circumstances. He can expect that the quality of the materials, and the heat treatment, if any, employed for these products are better controlled than it is possible to do in a shop with less demand and supervision. STAINLESS STEEL

He must, however, be aware of the differences and intended use of the many alloys classed as stainless steels, only a few nepd be various grades of steel available in standard &kames and plates. Besides the use in high-pressure molds. Because of the necessity for SAE 1020 plate steel mentioned above, generally supplied in an analysis up to SAE 1040, as' the lowest grade of steel, m m suppliers offer better choices the most commonly used. It contains 12 to 14% chromium for more severe service, of SAE 4130 to 4150, prehardened to 26 to 29 Rock- well C, and even P-20, prehardened to 28 to 32 Rockwell C.

Besides mold bases, and individual plates ground and sized to close toler- ances, there are !eader pins, bushings, ejector pins, sprue bushings, pre- machined cores and tool steel cavity blocks.

Other items which have been added to the ever-growing list, as demand increases, are such more complicated, heat-treated and assembled devices as; early returns (for the.knockout system), latch-lock mechanisms to deter-

mechanisms themselves. As the result of previous wide application of these devices, he can-be assured that they are largely fcrolproof when installed n iqieetion molds for t m r o h ma- by experienced personnel.

ALLOY STEELS

Nickel Toughness and strength. Chromium Hardness. Adds to abrasion resistance in high carbon compositions. Vanadium Purifier-also adds fatigue resistance. Molybdenum Widens heat-treating range and adds heat resistance. Tungsten Hardness and heat resistance.

w co: ab thc (s ha

MATAltRLS FOR MOLD MAKiNQ 131

For 1 depth c of the n Rc .

The binatio

/'

PLASTICS MOLD ENGINEERING HANDBOOK

&3 w, 18% Nickel hfar~ging &el 0.03 18.25 7.75 4.80 I &% me, 18% Nioeel Mirragbtg Steel 0.03 18.25 7.50 4.25

finished mold is then reheated to about 900F, held for about 5 hr, 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.

and transfer molds, the P20 mold is carburized to a

STEEL FOR. MACHINED MOLDS P6) type must be carburiwl, resulting in a good com- hardness and core toughness.

rcial low carbon machinerysteels are nearlyideal when machinability ne important consideration. These steels are usually high in phos-1 HOBS AND HOBBING

Re 30-35, where it is readily machinable. No hwt . It can be chrome-plated .when corrosion mdd'lrt:

r fmhg the mold frame. Selection

as tbt experienced with SMC. If a higher hardness is req&ed., and some -sf their prope&ies is shown b deep hardened to RC 45-50.


A IS[ Machinability Wear Compressive --EDGE or LAND Raring Resistance Strength Toarghness Checking r p ~ i L ED 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 finish-machine the hobbings. which it is removed and the hobbing is furnace-annealed. This is followed

by another push of !% to $4 in, and subsequent annealing, the process being d mn the hobs so that they may be removed from the hob- repeated as many times as necessary to complete the job. The draft should be approximately Go per side, although

Where a particular dimension on a hob is very important, it is the usulll be much better. It is easier to hob cavities when adequate practice to make one push on all blanks and then to check the hob and make been made. The reverse is true when cavities are machine the necessary adjustments in size before completing the hobbing. Them difficult to machine a straight surface. The nearly ideal adjustments usually consist of correcting dimensions which have been en- larged by the "upsetting" force on the hob.

FIG. 4.6. Group ohfamali master bobs usad in the d i n g of tw*. ~~on to allow for rolling of land.

su_sds eo tile! d ~ d dtM. after cold-holabing." , .,

cal Th of bla nza


m p k , lnng thought to be impossible, is W-production of @td undercuts 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

&-hi* than for bobbing. ntly B M ~ for injection molds are listed inTable 4.8.

Wms of steel available, some mold makers and . I - -

T4 n s t i s r injection uaa olds. 35. lhis Usually should used be

Iled,plastics and 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 proto- mbIds are: alloys of copper and beryllium, alloys alloys.

cX,bper alloys, with a basic composition of 2.5% mdoer, has been used for aver 40.vears for in-

for shape and

ting was con- i sp~ifled for

&,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. ~dred pounds, he process in

:f,y' PLASTICS MOLD EWOINEERINO HANDBOOK I

: - ceramic pattern is used in place of the heat treated steel hob; ise.

I c Casting."

. later, improved by the application of a vacuum during the castiq I I 8, because the ceramic is unable to withstand the compactine i 8 .( ,. .

used in the earlier technique, very reliable castings, virtuah I . 1 0 , ~ ~ fw, and with excellent reproductisn of the surfaces cast against, , .= 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 , t i %a W - 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. I ' '

&%kction depends on the desired degree of fluidity and the mold-makina to be used. Certain alloys are for making cores and mandrels rathe1

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 As metal-mold reaction. Also, simpler foundry equipmer

the class of materials having 1,.7% or more beryllium, hi, contents give higher fidelity of reproduction. But the hi&er 4

F 1, ' -3 , 4 Eontent, the higher the cost. The 20Cand 245Calloys (see Table ' f ,&I 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 g the %

142 PLASTICS M6LD ENGINEE~INQ HANDBOOK

pressure while the metal is molten. The metal flows to conform to the shape and surface finish of the hob very precisely.

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

of BeCu from pouring temperature to room temperature. Shrinkage is predictable and consistent. - 1 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 the hob assembly and aove the two onto a hydraulic press.

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 1000F facilitate separating them.

Ceramic Casting. Ceramic casting follows any one of a number of paten 4

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


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

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 &&led or 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 befyllEitw wpper (or aluminum) mold by the ceramic castine ative (righi cavitv (lefi

w

which is then used as a IS cast from the ceramic -.

en^ Corp., Pleasantville, NY)

lrlrl PLA874CO MUIOLD ENBlIIEEERING~ HA4WWOK MAVERtALS FOR Yam MAKlNB 1.46

m&mri'%; from B80 m high #rts W on &e particular a l k y used .d aoft 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

Aluminum

@a c@ fiekpjl frrc~~eds at a rate " H. ~,'Simd;rrtEs, W i York, V d NIB-

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


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 I 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 w ~ - - ---- evenly so that the many machining~ during the build-up which are usually necessary to remove the "trees" and ~ o i n t s are 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

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

I I

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

such a cavity, reproducing the original master with considerable accuracY. ] When oriknal masters are made out of wood. leather, or other substances w which cannot be put into the plating bath, they can be reproduced by making a cast over the original master and casting back to reproduce the original master in the desired plastic material. If the proper materials and techniques are used, no discernible loss of detail will result. 1

MATERIALS FOR MOLD MAKINQ 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 electro- presence of delicate detail in a cavity. Since the electroformed reprodues the finish on the master no polishing or other

. Electrofonned cavities are frequently used because of can achieve. A brass master is simple to make and easy

ensions. Electroforming cavities will exactly reproduce the on the master. When mbdels are available, very often an

cavity is cheaper because the model can be used for the and no metal form is required such as for casting or hobbing.

K'JS- EIeBrnformed insert of large mold for clear plastics drafting instrument. All 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 MATERIALS FOR MOLD MAKING 349

Electr~forming is 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, Solution ead the eleetroforming performed over this. The result is eledroformed avities which have the numbers raised from a highly polished surface,

.006-.010 "/in. and no further work is necessary. Cavities with delicate and undercut detail em side walls can be made.

0 t h methods and materials are available, usually with the result of mink cast) -008 ,008-,010 Some alloys Before mach'g. b s iwnimq, to make spur gear cavities, and even helical gars, but when .000-.002 No -

TOOL STEEL CASTINGS* 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, wka %$j@img $0 wae it shs me: 142. In preparing the master, the mold marker must pt(pk@d method. Tlpes of a e ceramic cast, the tool steel, which has a dif- for "b#mce, with hobbhg. at gf beryllium copper, and of course the molding

c material to be used.

vailable, which are so far only suitable for ther- "molded" using powdered metal technology.

y the 3M Company, and while some details of the

Wliug Green OH.

-

150 PLAITICS M a B ENGlMEERiNG HAbYDIBOOK MATERIALS FOR MOLB MAKING 161

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

very fm pow a % er is compacted around or in the "replication" of a master, . The dimensions normally increase slightly on heat-treating which the mold maker prepares and fu rnhw, Only one master is necessary, regardless of the number of cavities needed. The limitations are: (1) size-generally not mre 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 mi- m n s by polishing; (4) apparent hardness-Rockwell C-41, which is ac- t u d y deceptive on the conservative side, since the composite includes Stel- lit# d carbide particles, both of which by themselves have a hardness of Rockwell C-58 or m

idweat in tb test mvhy is @@&I

&&ar ~ e e d be allowed for. aiMyds of the &mposite:

'I CaWt 35% PLASTICS MOLD CAVITIES

isls are used for "molds" made of plastics materials. One rsnd widely used is the room temperature.vulcanizab1e

Physical properties: Tensile strength , 110,NlQ to 140,009 psi oun'd reproduces fine details, very accurately and,

'Compression strength , 220,000 tct ~ , 0 0 9 psi undercuts will be no problei. Modulus of elasdcity s epoxy glass, urethane, polyester, etc., are then cast Apparent hardness

Another type of P/M mold co pa& of J m w t c m n p l ~ l y di @f Feasa-Tic*. Originally W rounds and

i

TREATMENT

Stellitee is a registered Trademark of the C variously as "Custom Cavities," "Replication

Perm-Tic* is a registered trademark of Alloy NY.

9% 3f tl RTF mad tech beca

Ci in tk mad prot can as t

Upon cc cumstar Protecti, not, shc many a


-

Parn. to i- mck Q R ~ mistana.

Any rpctnl Peem cp, FWYC f k q ~ j ~ ~ t c h e s & tool marks.

~ ~ 2 n d ~ceoraie or *texnrJCTmAid suflaces every instance, a procedure lower in the table can be followed by one pre ously mentioned, if applicable to the same metal.

Tungsten Disulfide' ! Ahmrinum

permanent, as for the graphite process described below.

bricative mating* developed for metal P r d d ~ t y itlcmses of 10 to

*Diversified Drilube, Inc., ~ r h s a , OK. Co., Mauntain View, CA.

Fhoi

Nitn


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 facilities for 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.



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. Molyb- denum 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 Rock- well C.

THERMAL BARRIERS FOR MOLDS

As molds go up in temperature, a point is reached where the loss of heat going from the mold to the pre&''platen cannot be tolerated. At ordinary mold temperatures, this problem is often minimized by multiple channels in the clamping plates, giving the effect of minimum heat transfer areas. Transite* asbestos sheet is commonly used in the intermediate temperature zones where dimensional control across the parting line is not critical. For highly accurate molding with absolutely flat and parallel press platens, glass- bonded mica,** a machinable ceramic is used because of its low thermal conductivity and its absolute 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 pro- portionally effective. For localized areas where high physical properties are needed, alloys of titanium may give some relief.

POINTERS

In case of doubt, use a type of steel which is better than the one You might select but are not sure it will be satisfactory.

*Transite, Johns Manville, Greenwood Plaza, Denver, CO. **Mykroy, Mykroy Ceramics Company, Ledgewood, NJ.

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

Don't 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. DO not 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.


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 ex- has shown are both practical and desirable. Some of these rules n established as standards for the preparation of mold drawings; who follow these rules avoid many of the troublesome and un- ry mold designs which result from neglect of fundamentals. This

was prepared to detail the principles and rules of design which, ny years, have been found to give the best results. Understanding rules and intelligent application of them will help the draftsman

uce drawings that will convey his design to the toolmaker in such that he may interpret it readily with no possibility of misunder- . It must be understood that the rules given here are general in

ation and are to be interpreted with regard for the special con- existing practices of the shop where the tools will be designed,

usfd. The mold designer must familiarize himself with his own ice and learn what limitations will modify the application of

are the permanent record of a design from which many copies

mdamental requirement of a drawing is that it shall give the neces- biafomation accurately, legibly and neatly. The tool-maker's first

160 PLASTICS MOLD ENGINEERING HANDBOOK PLASTICS MOLD ENGINEERING HANDBOOK 161

measure of a draftsman's ability is based on the neatness and legibility of the print which is furnished him. His final evaluation is made on the basis of the accuracy of the drawing. A drawing may carry as many as 500 dimensions and, of this number, only one may be inaccurate, but the instead, have a print damage done by that one wrong dimension can far outweigh the good of 499 that are correct. It is impossible 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 output of a computer program. The size and type plotter used practice." This may include such things as clearances, tap drill sizes, tapped holes, etc. The consulting designer and the designer of molds which may be built in any one of several tool shops cannot 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. available for draw-

At several points in this text, the use of standard mold bases is detailed. selected by the de- Wherever possible, we recommend the use of these highly specialized in advance, to op- components, and once again, we recommend that the designer keep a com- lar plotter. The usual choices available are: 1) plot what plete file of'catalogs for these standard mold bases and standard mold nitor screen; 2) plot to size; or 3) plot to scale. Item 1) is components. (See Chapters 3 and 8.) Duplication of these catalogs in this text would serve no useful purpose. However, we do show designs based used in a reference upon standard mold bases. Bear in mind that cost is an important factor ws for each screen in today's economy where the wages of the mold-maker represent a sig- nificant part of the overall cost of a mold. 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 ause of variations in Chapter 3. Figures 8.92, 8.93A, 8.93B and 8.94 co

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