02. mold making techniques

8/13/2019 02. Mold Making Techniques

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Figure 2 1 Com parison of costs: production methods in mold making [2.1]

Cavities made by electrolytic deposition as well as other procedures, which cannot bedone in-house, call for additional working hours until the mold is finally available. Thismay be rather inconvenient. The making of an electrolytically deposited insert takesweeks or even months. A cavity made of heat-treated steel can be used for samplingwithout problems and still be finished afterwards. The high production costs also justifythe application of a superior material because its costs are generally only 10 to 20% ofthe total mold costs.

In spite of all the modern procedures in planning, design, and production, mold

making calls for highly qualified and trained craftsmen and such personnel are in shortsupply n owad ays. Thus, the production of m olds always poses a bottleneck.

2 M o l d M a k i n g T e c h n i q u e s

Injection molds are made by a highly varied number of processes and combinationsthereof.

Figure 2.1 demonstrates the relative costs for cavities made from various materials.Accordingly, steel cavities appear to be many times more expensive than those made ofother materials. In spite of this, a cavity made of steel is normally the preferred choice.This apparent contradiction is explained with the consideration that the service life of asteel mold is the longest, and the additional costs for a cavity represent only a fractionof those for the whole mold.

Miscelaneous

Manua labor

Machnng costs

Material costs

100

90

80

70

60

50

40

30

20

10

0

Machinedsteel

Zinccasting

Electrolyticdepositionof nickel

Syntheticresn castng

Metal-sprayingprocess: MCP alloy



It is clear, therefore, that only up-to-date equipment is found in modern mold-makingfacilities such as numerically-controlled machine tools. With their help one tries toreduce the chances of rejects or to automate the working process without human operator(e.g. EDM ).

2 . 1 P r o d u c t i o n o f M e t a l l i c I n j e c t i o n M o l d s

a n d M o l d I n s e r t s b y C a s t i n g

The production of mold inserts, or whole mold halves by casting, attained a certainpreeminence in some application areas for a time. The reason was that the castingprocess offers suitable alloys for nearly every type of application and that there arehardly any limits concerning geometry. Molds requiring extensive machining couldtherefore be made economically by casting. Another application area is the simple, morecost effective production of injection molds for low production runs and samples,particularly of non-ferrous metals. Only a brief account of the casting methods forproducing mold inserts is provided below. Readers requiring more detailed informationare referred to the literature at the end of the chapter.

2 .1 .1 C a s t i n g M e t h o d s a n d C a s t A l l o y s

Of the numerous casting methods available [2.2, 2.3], variants of sand casting andprecision casting are used to make mold inserts. The choice of casting method dependson the dimensions of the mold, the specified dimensional tolerances, the desiredfaithfulness of reproduction and the requisite surface quality.

After casting, the mold essentially has the contours necessary for producing themolding. For large molds cast in one piece, the heat-exchange system can be integrateddirectly by casting a tubing system or by means of a special arrangement of recesses atthe rear through which the temperature-control medium can flow freely.

Generally, the inner contours of the mold (the mold recesses) are cast slightly largerand so require only a minimal amount of additional machining. Another critical factor isthe requirements imposed on the surface quality of the molding. Any posttreatment ofthe surfaces (e.g. polishing) that may be necessary is performed by the same methods asin conventional mold making. Grained and textured surfaces such as can be produced byprecision casting mostly do not require posttreatment. With cast molds, just as inconventional mold making, incorporation of holes for ejector pins, sprue bushings andinserts as well as the fitting of slide bars and the application of wear-resistant protective

layers are all performed on the cast blank.The metallic casting materials suitable for mold making fall into two groups:

- ferrous m aterials (cast steel alloys, cast iron m aterials), and- non-ferrous ma terials (alum inum , copper, zinc and tin-bismuth alloys).

Only cast steel will generally satisfy the mechanical demands of mold inserts that arerequired for more than just experimental and low-production runs. Furthermore, onlysteel has an adequate degree of polishability. Many of the steel grades successfullyemployed in mold making are amenable to casting. However, it must be borne in mind

that castings always have a coarse structure that is not comparable to the transformationstructure of forged or rolled steels. At the macroscopic level, castings have different



primary grain sizes between the edge and core zones. There is limited scope for usingsubsequent heat treatment to eliminate the primary phases that settle out on the grainsurfaces during solidification. For these reasons, when making cast molds, it is best touse steel grades that have little tendency to form coarse crystals or to separate by

liquation [2.4]. Some common cast steel grades are shown in Table 1.3.Not only does thermal posttreatment bring about the improvement in structure

mentioned above but it also enhances the mechanical properties, and the necessary notchresistance and stress relief are obtained. The strength, which depends on the carboncontent, is lower than that of rolled or forged steel, and so too are the toughness andductility [2.5]. How ever, they m eet the major demand s imposed on them. The service lifeof cast steel molds depends on the wear resistance and, under thermal load, on thethermal shock resistance. Given comparable steel grades, the thermal shock resistance ofcast steels is generally lower than that of worked steels.

M old inserts of copper and aluminum alloys are made both by casting and m achining.Refined-zinc cast alloys for injection molding are used only for making mold inserts forexperimental injection, for the production of low runs and for blow molding molds.Refined-zinc cast alloys, like copper alloys, have excellent thermal conductivity of100 W/(m • K). The mo ld-filling characteristics of zinc alloys so ou tstanding thatsmooth, pore-free surfaces are even obtained in the case of pronounced contours withstructured surfaces [2.6]. The most common refined-zinc alloys, sold under the namesZam ak, K irksite and Kay em, are sum marized in Table 1.5.

Tin-bismuth alloys, also called Cerro alloys, are comparatively soft, heavy, low-melting metals (melting point varying according to composition between 47 and 170

0C)

[2.7]. Particularly suitable for mold making are the Cerro alloys that neither shrink norgrow during solidification. Due to their moderate mechanical properties, Cerro alloys ininjection molding are only used for molds for trial runs or for blow molds. Moreover,they serve as material for fusible cores. The physical and mechanical properties of someCerro alloys are shown in Table 1.8.

2 . 1 .2 S a n d C a s t i n g

This process is used to produce medium -to-large m olds weighing several tons per mold

half. It consists of three major production steps:

- produc tion of a negative pattern (wood, plastic, metal),- produc tion of the sand mold with the aid of the negative pattern, and- casting the sand mold and rem oving the cooled casting.

The negative pattern is made either direct or from an original or positive master pattern.To an extent depending on the shape, dimensions, alloy and sand-casting method,allowance must be made for machining and necessary drafts of 1° to 5°. When makingthe pattern, allowance for shrinkage has to be made. To determine the shrinkage, thedimensional change of the cast metal from solidification and cooling and the shrinkageof the plastics to be processed in the mold have to be taken into account (does not applyto wooden p atterns) [2.8]. Typical allowances for a num ber of cast metals in sand castingare listed in Figure 2.2. The exact measurements in each case depend on the castingmethod, part size and part shape. These should be set down in the design phase after

consultation with the foundry.



The casting mold is made by applying the mold material to the pattern and solidifying iteither by compaction (physically) or by hardening (chemically).

A wide range of synthetic mold materials of varied composition is available [2.2].

Washed, classified quartz sand is the predominant refractory base substance. For special

needs, e.g. to prevent high-alloy casting materials (cast-steel alloys) from reacting withthe melt, chromite, zirconium or olivine sand may be used. The binders used for moldsands are organic and inorganic. The inorganic binders may be divided into natural andsynthetic types. Natural inorganic binders are clays such as montmorillonite, glauconite,kaolinite and illite. Synthetic, inorganic binders include waterglass, cement and gypsum.

Organic binders are synthetic resins such as phenol, urea, furan and epoxy resins. Inpractice, the molds are made predominantly of bentonite-bound (a natural inorganicbinder) mold materials (classified quartz sand) that have to be mechanically compactedin order that adequate sag resistance may be obtained.

After the mold has been p roduced, the pattern is remo ved. To an extent depending onthe requirements imposed on surface texture and alloy, the finished sand mold may ormay not be smoothed with a facing. After casting, the finished mold is more or lessready. The sand mold is destroyed when the mold is removed.

T s Sodus temperature

T L Lqudus temperature

Lqud shrnkage

Solidification shrnkage

Sod shrnkage

Temperature [0C]

Volume

ompensaton

through

f

r

ompensaton

through degree

ofshrnkage

Figure 2 2 Shrinkage

on solidification and

shrinkage for differentcasting alloys

Material

Cast iron:With lamellar graphiteWith spheroidal graphite

Malleable cast ironCast steel

Aluminum baseCopper base

Solidification

shrinkage in

-1 to 41 to 65.5 to 65 to 6

5 to 64 to 8

Shrinkage in

0.9 to 1.10.8 to 0.90.5 to 1.91.5 to 2.8

0.9 to 1.40.8 to 2.4



A somewhat different procedure is employed in the lost foam method. In this, apolystyrene foam pattern is embedded in sand, remains in the casting mold, and isgasified by the casting heat only when the liquid metal is poured into the casting mold.Polystyrene patterns may be milled from slab material (once-off production) or foamed

in mold devices (mass production). Since the pattern is generally only used a few timesto make inserts for injection molds, the use of CAD interfaces can allow a polystyrenepattern to be milled quickly and cost effectively.

The major advantage of cast molds is the fact that the mold is ready for use almostimmediately after casting. Posttreatment is limited, especially if a heat-exchange systemhas already been integrated by embedding a prefabricated tubing system before casting.

2 .1 .3 P r e c i s i o n C a s t i n g T e c h n i q u e s

Precision casting is used for mold inserts that must satisfy particularly high demands onreproducibility. The techniques are eminently suitable for fine con tours and, ow ing to thevery high reproducibility, for the faithful reproduction of surface structures, such as thatof wood, leather, fabrics, etc.

A number of different types of precision casting process exist [2.9] that vary in thesequence of processes, the ceramic molding material and the binders employed. Moldinserts are usually made by the Shaw process (Figure 2.3) or variants thereof. For

molding, a pattern is required that already contains the shrinkage allowance (see alsoSection 2.1.2). The patterns are reusable and so further castings can be made forreplacement parts. This pattern forms the basis for producing the ceramic casting mold(entailing one, two or more intermediate steps, depending on method chosen). The liquidceramic molding compound usually consists of very finely ground zirconium sand mixedwith a liquid binder. After the mold has been produced, it is baked for several hours atelevated temperatures. It is then ready to be used for casting. After casting, the ceramicmold is broken and the part removed.

Precision-cast parts may be made from the same molding steels employed for making

injection molds, but all other casting alloys may also be used. Posttreatment of prec ision-cast parts is generally restricted to the mounting and mating surfaces, as well as allregions that comprise the mold parting surface.

2 . 2 R a p i d T o o l i n g f o r I n j e c t i o n M o l d s

Time and costs are becoming more and more important factors in the development of

new products. It is therefore extremely important for the injection molding industry toproduce prototypes that can go into production as quickly as possible. To be sure, rapidprototyping is being employed more and more often but such prototypes frequentlycannot fully match the imposed requirements. Where there is a need for molds that areas close to going into production as possible, rapid tooling (RT) is the only process bywhich the molds can be made that will enable the production of injection moldedprototypes from the same material that will eventually be used for the mass-producedpart. Rapid tooling allows properties such as orientation, distortion, strength and long-term characteristics to be determined at an early stage in product development. Such

proximity to series production, however, also entails greater outlay on time and costs.For this reason, RT will only be used where the specifications require it.



1 2

Mount top box Pour in slurry Surry sets

3

6 5 4

Remove modery (Skin-dry)

Assembe

mod haves

7 8 9

Knock outastire

1 0

Fig ure 2.3 Steps in the Shaw process Clean



RT also covers conventional processes for removal or coating. These include high-speed

cutting (HSC) [2.10] with direct control through the processing program generated bythe computer from the CAD mo del; erosion with rapidly m achining graphite electrodes;and metal spraying, which has been used for decades in mold making.

Master molding techniques like precision and resin casting may be considered asbelonging to RT. These process chains become rapid tooling techniques when an RPtechnique is used to produce the necessary m aster mold.

Whereas, in the techniques mentioned so far, the prototype mold is produced eitherdirectly by means of a material additive method or in several processing stages, the so-

called hybrid techniques integrate several such stages in one item of equipment. Theseprocessing stages are a combination of processes from the other three groups(conventional, master-molding, and material additive techniques). Because hybridtechniques combine sequential processes in unit, they can be just as fast as the materialadditive techniques. All these techniques are still in one development, however.

For a better understanding of the diverse processes and combinations involved, thedifferent RT techniques will be presented and discussed in this chapter.

Figure 2.5 illustrates the general procedure for RT. All methods rely on the ex istenceof consistent 3D CAD data that can be converted into closed volume elements. These

data are processed and sliced into layers. 2D horizon tally stacked, parallel layers are thusgenerated inside the computer that, with the aid of a technique such as laser sintering,

Figure 2.4 Classification of RT processes

2 . 2 . 1 S t a t e o f t h e A r t

A breakdown of all RT techniques is shown in Figure 2.4. The material additive pro-cesses lead fastest to moldings and are therefore the most promising. These also include

new and further developments in RP. Examples of such techniques are laser sintering,laser-generated RP and stereolithography, which enable mold inserts to be made directlyfrom a three-dimensional CAD model of the desired mold.

Conventional removaland coating processes

Removal:

Coatng:

Turning, milling (HSC, SOM)Eroding

Meta sprayngElectroforming, nickel plating

Material-additive processes

Metallic:

Nonmetalc:

Seectve laser sinteringLaser generaton

3D printing (metal)Stereolthography3D printing (ceramc)

RapidTooling

Master mold processes

Gravty casting:

Centrfuga casting:

Precson castngSand castngMeta castng

Resn castngVacuum castng

Spin/roto castng

Hybrid processes

Controed meta bud-upShape melting



Figure 2 5 Basic approaches to RT

can be successively created within a few hours without the use of tools or a mold [2.11].Furthermore, there is generally no need for supervision by an operative.

To an extent depending on the principle underlying the chosen RT method, either apositive pattern, i.e. the molding to be fabricated later, or a negative pattern (the requi-site mold geometry) is produced. Once a physical positive pattern has been produced,usually any number of moldings may be made by master-molding and coating tech-niques, which will ultimately lead to a prototype mold after one or more stages.Examples of such process chains are resin casting and metal spraying.

The systematic use of 3D CAD systems during design affords a simple means of

generating mold cavities. Most 3D CAD systems already contain modules that canlargely perform a conversion from positive to negative automatically.

Once the data have been prepared thus, there are two possibilities to choose from. Oneis to create a physical model of the moving and fixed mold halves (negative patterns) soas to make a certain number of moldings that will lead to a mold. Metal casting is anexample of this. Alternatively, the negatives, perhaps made by stereolithography, may beelectrostatically coated. This process chain is shorter than the molding chain justmentioned.

Because the possibilities presented so far involve a sequence of different processes,they are known as indirect tooling. By contrast, direct tooling involves using thegenerated negative data without intervening steps to produce on an RP/RT system, as isthe case with selective laser sintering. Although this is undoubtedly a particularly fastoption, the boundary conditions need closer examination. Some of the resultant moldsentail laborious postmachining, w hich is more time consum ing.

A major criterion other than subdivision into direct and indirect RT is the choice oftooling material. These are either metallic or so-called substitute materials, the latterusually being filled epoxy resins, two-component polyurethane systems, silicone rubber

[2.12] or ceramics.The m ore important and prom ising RT methods are presented below.

o lTme

PrepaeCAD daa

Negave daaosve daa

Make negavemode

indrec RTMake posve

mode

Demodng

• Resn casng• Mea sprayng Demodng•Mea casng Coang• Eecoormng

Drect RT



2 . 2 . 2 D i r e c t R a p i d T o o l i n g

The goal of all developments in the field of RT is automated, direct fabrication ofprototype molds, whose properties approach those of production parts, from 3D CAD

data describing the mold geometry. This data set mu st already allow for technical aspectsof molds, such as drafts, allowance for dimensional shrinkage and shrinkage parametersfor the RT process.

Processes for the direct fabrication of metallic and nonmetallic molds are presentedbelow. In either case, the mold may be made from the CAD data direct.

2.2.2.1 Dire ct Fa brica t ion of M etal l ic M old s

Direct fabrication of prototype molds encompasses conventional methods that allowrapid processing (machining) of, e.g. aluminum.

2 2 2 Ll Generative Methods

A common feature of generative methods for making metallic molds is that theworkpiece is formed by ad dition of m aterial or the transition of a material from the liquidor powder state into the solid state, and not by removal of material as is the case withconventional production methods.

All the processe s involved h ere have been developed out of RP methods (e.g. selective

laser sintering, 3D printing, metal LOM (Laminated Object Manufacturing), shapemelting, and multiphase jet solidification) or utilize conventional techniques augmentedby layered structuring (laser-generated RP, controlled metal build-up).

In selective laser sintering (SLS) of metals, a laser beam melts powder startingmaterials layer by layer, with the layer thickness varying from 0.1 to 0.4 m m in line withthe particle size of the metal powder [2.13, 2.14]. The mold is thus generated layer bylayer.

Sintering may be performed indirectly and directly. In the indirect method (DTMprocess), metal powder coated with binder is sintered in an inert work chamber (e.g.

flooded with nitrogen). Heated to a temperature just below the melting point of thebinder, the powder is applied thinly by a roller and melted at selected sites. The geom etryof the desired mold inserts is thus ob tained by m elting the polym er coa ting. The resultantgreen part, which has low mechanical strength, is then heat-treated. The polymer binderis burned out at elevated temperatures to produce the brown part, which is then sinteredat a higher tem perature. At an even high er temperature again, the brown p art is infiltratedwith copper (at approx. 1120

0C), solder alloy or epoxy resin [2.15], this serving to seal

the open pores that were formed when the polymer binder was removed (Figure 2.6).

In direct laser sintering manufacture, metals are sintered in the absence of binder(EOS process). The advantage of not using coated pow ders is that the laborious removalof binder, and the possibility of introducing inaccuracies into the processing stage, canbe dispensed with. Nevertheless, the part must be infiltrated since it has only provedpossible so far to sinter parts to 70% of the theoretical density [2.16]. After infiltration,posttreatment is necessary and generally takes the form of polishing.

Aside from pure metal powders and powders treated with binder, multicomponentmetallic powders are used. These consist of a powder mixture containing at least twometals that can also be used in the direct sintering process. The lower-melting

component provides the cohesion in the SLS preform and the higher-melting comp onentmelts in the furnace to imbue the mold with its ultimate strength. Candidate metals and



Figure 2 6 Indirect sintering followed by infiltration

metal alloys for direct and indirect sintering are, according to [2.17]: aluminum, alu-minum bronze, copper, nickel, steel, nickel-bronze powder and stainless steel. Themaximum size capable of being made by laser sintering is currently 250 • 250 • 150 m m

3.

Another way to apply metal is by laser-generated RP. Powder is continually added tothe melt in a m ovable process head [2.18]. The added m aterial com bines with the m eltedmaterial on the preceding layer. The layers can be added in thicknesses of 0.5 to 3 mm.M etal pow der is blown in and melted in a focused laser beam. As the process head m ovesrelative to the work surface, fine beads of metal are formed. The materials used arechrom e and nickel alloys, copper and steel. Laser-generated RP is not as accurate as laser

sintering and can only generate less complex geometries due to the process setup.A further development of laser-generated RP is that of controlled metal build-up

[2.19]. This is a combination of laser-generated R P and HSC milling (Figure 2.7). Oncea layer 0.1 to 0.15 mm thick has been generated by laser, it is then milled. This resultsin high contour accuracy of a level not previously possible with laser-generated RRThe maximum part size is currently 200 mm

3 for medium complexity. No undercuts are

possible.

Other processes still undergoing development are shape melting and multiphase jet

solidification [2.20]. Both processes are similar to fused deposition modeling, which isan RP process [2.21]. In shape melting, a metal filament is melted in an arc anddeposited while, in multiphase jet solidification, melt-like material is applied layer bylayer via a nozzle system. Low-melting alloys and binders filled with stainless steel,ceramic or titanium powder are employed. As in SLS, the binder is burned out, and theworkpiece is infiltrated and polished. However, the two processes are still not asaccurate as SLS.

3D printing of metals is now being used to fabricate prototype molds for injectionmolding, but it is not yet commercially available. Figure 2.8 illustrates how the 3D

printing process w orks. After a layer of metal powd er has been app lied, binde r is appliedselectively by means of a traversing jet that is similar to an ink jet. This occurs at low

Pror to sinteringin the unit

Sinteringin the unit

Purgng the bnderin the furnace

Sintering the partin the furnace

Infiltrationin the furnace

Infiltration materialntered bnder

Bnder film

Metal



temperatures because only the binder has to be melted. Whereas local heating in direct

laser sintering can cause severe distortion, this effect does not occur in 3D prin ting. O ncea layer has been printed, an elevator lowers the platform so that more powder can beapplied and the next layer generated. The coating is 0.1 mm thick. When steel powder isused in 3D printing, bronze is used for infiltration. Shrinkage is predictable to ± 0.2%.Part size is still severely restricted by the equipm ent and currently cannot exceed an edgelength of 150 mm [2.21].

Another process currently being develop ed for the fabrication of metallic molds is thatof metal LOM in which m etal sheets of the same thickness are drawn from a roll, cut outby laser and then joine d together. The join ing metho d is simply that of bolting, a ccording

Profiles and surface millingaser generation

BiId 2.7 Con trolled metal build-u p after [2.19]

Hgh-speed cutter

Laser generating headaser beam

Focusng lens

Metal pow ei

Protectve gas

Layer build-up

Appy powder Appy bnder Start next level

Part Bnder feed

PlatformMeta or ceramc

powder

Fi gu re 2.8 Steps in 3D printing



to [2.22]. So far, molds made in this way have only been used for metal shaping and forinjection molding wax patterns for precision casting. The advantage of molds joined bybolts is that the geometry can be modified simply by swapping individual metal sheets.

The variant developed by [2.23] is a combination of laser cutting and diffusion

welding. Unlike metal LOM and most other RT processes, which grow the layers atconstant thickness, this process variant allows sheets of any thickness to be used. As aresult, simple geometric sections of a mold may be used as a compact segment, a factwhich allows RT only to be used where it is necessary and expedient. Possibledimensional accuracy is in the order of 0.1%. Due to the process itself, it is never lowerthan ± 0 . 1 mm in the build direction. The tolerances of laser cutting are from 0 .001 to0.1 mm. Unlike most of the processes mentioned so far, this process imposes virtuallyno restrictions on part size [2.24].

Direct RT processes are still in their infancy. Apart from selective laser sintering and3D printing of metals, all the processes discussed in this section are still being developedand so are not yet available on the market. This explains why stereolithography, despitethe fact that it is a direct fabrication process involving nonm etallic m aterials, is virtuallythe only one used for these purposes. Because it has constantly evolved over the last 10years and is offered by many service providers, it is readily available. Moreover, manylarge companies are in possession of stereolithographic equipment and still elect to useit for m aking prototype m olds.

2.2.2.7.2 Direct Fabrication of Nonm etallic Mo ldsWhile most direct methods for making metallic molds require posttreatment (infiltrationand mechanical finishing), the production of molds from auxiliary materials largelydispenses with this need.

Stereolithography (STL) is based on the curing of liquid, UV-curing polym ers throughthe action of a computer-controlled laser. The laser beam traverses predeterminedcontours on the surface of a UV-curable photopolymer bath point by point, therebycuring the polymer. An elevator lowers the part so that the next layer can be cured. Oncethe whole part has been generated, it is postcured by UV radiation in a postcuring

furnace [2.24].STL's potential lies in its accuracy, which is as yet unsurpassed. Because it was the

first RP process to com e onto the m arket, at the end of the 1980s, it has a head-start o verother technologies. Ongoing improvements to the resins and the process have broughtabout the current accuracies of 0.04 mm in the x- and y-axes and 0.05 mm in the z-axis.

The process was originally developed for RP purposes but is also used for rapidtooling of injection molds because of its accuracy and the resultant good surfaces whichit produces. When STL is used to make a mold cavity, the mold halves are generated on

the machine and then mounted in a frame. Usually, however, the shell technique isemployed. In this, a shell of the mold contour is built by STL and then back-filled withfilled epoxy resin [2.25]. The use of the shell technique to produce such an RT mold isillustrated in Figure 2.9.

Parts made by stereolithography feature high precision and outstanding surfaceproperties. Unlike all other direct methods for making metallic molds, no furthertreatment is necessary other than posttreatment of the typical step-like structurestemming from the layered build-up by the RP/RT processes. This translates to aconsiderable advantage time-wise, particularly when the mold surfaces must be glossy

and planar. The downside is the poor thermal and mechanical properties of the availableresins (acrylate, vinyl ether, epoxy), which cause the molds to have very short service



Figure 2 9 The shell technique for generating an STL mold

lives. The best dimensional and surface properties are obtained with epoxy resins; theuse of particularly powerful lasers makes for faster, more extensive curing of the resineven during the stereolithography process, and this in turn minimizes distortion [2.26].

Although STL has primarily been used for RP, the number of RT applications is onthe increase. It is used to make molds for casting wax patterns as well as for injectionmolding thermoplastics. Such molds serve in the production of parts for a pilot series,which can yield important information about the filling characteristics of the cavities.

Moreover, it is even possible to identify fabrication problems at this very early stage.Ceram ics are other materials used for direct rapid tooling . Bettany [2.27] has reported

on the use of ceramic molds for injection molding. They are employed in the 3D printingprocess described in the previous section as well as in ballistic particle manufacturing(droplets of the melted material are deposited by means of piezoelectric ink-jet nozzle).The advantage over metallic molds is the high strength of ceramic molds. This comesparticularly to the fore when abrasive, filled polymers are processed.

2 . 2 . 3 I n d i r e c t R a p i d T o o l i n g M u l t i s t a g e P r o c e s s C h a i n s )

An RT chain is defined here as a succession of individual molding stages. The use ofsuch a molding chain leads from a master pattern to a cavity that may be used forinjection molding. In the sense of this definition, intermediate stages such as machiningor simple assembly of already finished cavity modules do not count as individual linksin this chain. A good RT chain is notable on the one hand for having a minimum num berof molding stages (chain links). The lower the number of molding stages, the more

accurately the part matches the master pattern and the faster a prototype mold can bemade. Every intermediate pattern can only be as good as the pattern from which it

Support frame

Mod insert

3D CADmodel

Slicing

Backing

Build processStereolthography

Rosfcyrng

UV radiationackng resn

Cavity

Support

Eevator platform

s e r Scanner

Resn bath



proceeds. Consequently, the attainable tolerances become greater and the surface qualitydiminishes from casting to casting. On the other hand, each RT chain must finish with acavity that withstands the high mechanical and thermal loads that occur in the injectionmolding process. The bottom line for all RT molding chains is therefore to have as few

links as possible so as to end up with an injection mold whose strength and qualitysomewhat exceed requirements.

Indirect rapid tooling may be effected with a positive or a negative pattern. Whilethese patterns serve as the master patterns for casting processes, using the virtualnegative pattern and new RT process chains can dispense with the master pattern andenable a cavity to be made directly in sand or ceramic slip for casting metals.

2.2.3.1 Process Chains Involving a Positive Pattern

Rapid manufacture of prototype molds using the shortest possible process chainfrequently involves using RP to make a positive pattern. These patterns can be made byany means, i.e. also conventionally.

Casting is frequently employed in the production of prototype molds. This master-pattern process entails observing the ground rules for designing cast parts. These include:

- avoidance of accumulation of material,- avoidance of major changes in cross-section, of thin flanges (1.5 mm minimum) and

of sharp edges (minimum radius of 0.5),

- avoidance of vertical walls (1% min. conicity) [2.28].

The simplest, and at the same time a very common process, is that of making a siliconerubber mold, starting from an RP pattern. The pattern is equipped with gate and risersand fixed in a frame. Once the parting line has been prepared, liquid silicone resin ispoured over the pattern in a vacuum chamber. It is not possible to use this type of moldto injection mold prototypes in produc tion m aterial. This process is known as soft toolingsince prototypes with certain heat or mechanical resistance can be cast in two-componentpolyurethane resins [2.24]. Not only can the Shore A hardness be adjusted to the range

47-90, heat resistance of up to 140 0C and resin strengths of up to 85 Shore D arepossible. This is the only casting method in which the ground rules mentioned above canbe largely ignored, due to the use of yielding silicone rubber. Nor is any shrinkageallowance required (1:1 reprodu ction).

Resin casting is illustrated in Figure 2.10. The RP pattern is embedded as far as theparting line and fixed in a frame. After delineating, a thixotropic surface resin (gelcoat)is usually applied and cooling coils are incorporated. An aluminum-filled epoxy mold-casting resin is then used for back-filling. W hen the molds are being designed, allowance

must be made not only for a design suitable for plastics but particularly for shrinkage bythe resin (range: 0.5-1.5%).

The service life of the mold depends greatly on the injected material and theprocessing parameters during injection molding.

The process allows metal inserts and slide bars to be embedded (Figure 2.11).

The coating processes employed are familiar from conventional mold making. Theyinclude flame spraying, arc spraying, laser coating, and plasma and metal spraying.Because both flame spraying and plasma spraying entail temperatures of 3,000

0C and

above, it is necessary to create a heat-resistant positive pattern. For this reason, we shall

in the context of rapid production only discuss metal spraying with a metal-sprayingpistol. In this process, two spray wires are melted in an arc and atomized into small



particles in the presence of compressed air (Figure 2.12). When the particles impinge onthe surface of an RP positive pattern, a liquid film forms that solidifies instantaneously.

The homogeneity of the 1.5-5 mm thick layer depends on the temperature and thedistance of the nozzle from the pattern. Since the particles are cooled immediately on

Figure 2 11 sliding splitmold cast in resin

Figure 2 10 Resin casting technique for making a mold1st mod halfst mod half

2nd mod half

Mod inserts

Master pattern Molding

Castng2nd mod half

Deneatng andappyng gecoaf

Delneatingand appyng gecoat

Castng1st mod half

Removngbedding

Castng resnGelcoatartng lineastng frame

Pastcne beddng



Figure 2.12 Principle underlying metal spraying

contacting the pattern from approx. 2000 0

C to 60 0

C , woo den patterns, for instance, maybe used in addition to RP materials [2.29]. A shell made in this way only needs to beback-filled with, e.g. casting resin.

Another coating method is the long-established electroforming, which has frequentlybeen used in the past [2.30] for high-quality injection molds (Figure 2.13).Electroforming is the most accurate method of reproducing surface texture in metal[2.31].

The R P pattern is first coated w ith silver or graphite to render it electrically conducting[2.32]. In an electroplating bath, individual metals are successively or simultaneouslyelectrolytically deposited, the pattern being coated with the corresponding material. Theresult is shells 4-5 mm thick that may be built up of different alloys or metal layers.Electroforming with nickel yields the best results due to such good material propertiesas high strength, rigidity and hardness (e.g. NiCo alloy, up 50 RC hardness), itscompatibility with the base material and its good corrosion resistance.

For large parts, RP master patterns are made and then coated. It is essential

beforehand to make a heat-resistant mold of this pattern as the thermal expansion of thestereolithographic resin could cause excessive distortion.

Electroforming reproduces the finest of details, but the part frequently has to remainin the electroplating bath for several days. The layer thickness is much more homo-geneous than that produced by manual metal spraying. The maximum part size isrestricted by the size of the electroplating bath.

2.2.3.2 Process Chains Involving a Negative Pattern

All of the process chains below begin with the creation of an RP pattern of the mold(negative pattern). To produce a purely metallic prototype mold, several casting

Metallic cavity

Castng resn

Master pattern

Mod insert

Molding

Bock-filling

Matat sprayng

Bedding

Auxiliary frameParting line Metal wire feed

Compressed air

kArc



Figure 2 13 Principle underlying electroforming

techniques may be used in addition to RT. This will often considerably shorten theprocess chains, as will be demonstrated below.

Of greatest economic imp ortance and hence the most w idespread casting technique isthat of investment casting, which normally employs patterns of investment wax [2.33].

The range of possible processes for creating these patterns has been extended by theadvent of RT.

Thus, it is possible w ith the aid of selective laser sintering, fused deposition mo deling

and ballistic particle manufacturing to fabricate patterns from investment wax direct[2.12]. The stereolithography technique, given suitable software, makes it possible toproduce hollow-structure patterns, e.g. by means of Quick-Cast (a 3D system) or theshell-core technique (EOS) [2.34]. In conjunction with a special illumination technique,these epoxy resin STL parts can be employed as expendable patterns for investmentcasting. To this end, only the molding shell is built up from the resin; the innerconstruction consists of a large number of honeycomb-shaped chambers all joined toeach other. The density of the mold part is now only 20% that of the solid part, but has

excellent strength values and a very good surface finish [2.35]. Very low internal stressesoccur so that the mold is extraordinarily accurate and dimensionally stable. Forinvestment casting, the vent holes of the STL part are sealed with investment wax.During burning out of the STL part, the part gasifies almost residue-free (residual ashcontent approx. 2 mg/g). A further possibility is direct production of a gasifiable patternto produce sintered patterns of polycarbonate. These are sturdier and less heat-sensitivethan wax patterns.

All the patterns made like this are surrounded with a ceramic coating. This is achievedby immersing the pattern into a ceramic slip bath and subsequently covering it with sand.

This process is repeated until the desired coating thickn ess of the refractory ceram ic shellis achieved. After this, the mold part must dry before it is burned in excess oxygen at

Metallic cavityAuxiliary frame

Castng resn

Electrolyte

Part

Master pattern

Mod insert

Preparaton

Castng with resn

Electroforming Me

Power sourceeta cathode

Bedding

Partng line

Electrically conductngsurface coatng



1,100 0C. During firing, the master pattern gasifies and so the corresponding materialscan then be cast in the resultant ceram ic mold. After drying, the ceramic body is sm ashedto yield the desired part. It is important for the quality of the cast part that the wax patternbe totally and uniformly wetted when first immersed in the ceramic bath.

Since the cast material shrinks on cooling down in the ceramic mold, the masterpattern must be correspondingly larger than the original. Additionally, shrinkage in eachRP process employed, as well as of the ceramic shell, must be considered. Distortion ofthe mold shells must also be expected, and must be rectified. Prototypes made byinvestment casting can accommodate high loads, have a high workpiece accuracy andgood surface quality. A serious disadvan tage of the process is the long drying time of theceramic shell of up to one week. The lowest wall thickness that can be produced is 1.5mm. Attainable surface roughness quoted in the literature ranges from mean values of5.9 to 23 urn [2.36].

Investment casting is used for making metallic prototype molds, inserts and metallicpilot parts. The process is particularly suitable for cylindrical cores. Figure 2.14 showsan example of an investment-cast mold of complex geometry that was successfullyinjection molded.

The process chain of investment casting can be shortened considerably by theapplication of 3D printing. In this case, a virtual negative pattern is required instead of aphysical negative one. The 3D printing process described above thus allows directproduction of the ceramic shell. This is referred to as direct shell production casting

(DSPC) [2.37]. Pouring in the metal and deforming are the only steps necessary.As in investment casting, evaporative pattern casting employs expendable patterns

that remain in the mold and evaporate without residue when the hot metal is poured in[2.38]. Very high accuracy can be achieved with this technique.

A one-part, positive master pattern of a readily evaporative material (EPS foam) ismodeled in sand. After compaction of the sand, high-melting metal can be poured

Figure 2 14 Example of a mold made by investment casting



directly into the mold. The gas produced by decomposition of the pattern can escapereadily because the sand is porous. A material frequently employed in the field of RT isthe light metal Zamak, a zinc-aluminum-copper alloy that is easy to posttreat [2.28].

An RT operation based on evaporative pattern casting is that of expandable pattern

casting (EPC). To enable fast production of the desired component, an auxiliary mold isinstead constructed which m ay be generated by any R P method. In the EPC process, firstpolystyrene beads are prefoamed to a specified density by slow heating to 110

0C. The

beads are then foamed in a mold. The individual polystyrene beads are thermally bondedand welded together. Finally, the part is covered with a ceramic coating. Hot metal isthen poured into the m old, causing the polystyrene to evaporate at the same time. A gooddimensional stability and reusability of the ceramic sand are notable features of this process.

An even simpler and therefore shorter process chain is the direct production of thepolystyrene master pattern via RP by means of the Sparx process. Foamed polystyrenefilm in a so-called hot-plot machine is cut with the aid of a plotter and bonded onto thepreceding layer/The material is gasifiable and therefore suitable for evaporative patterncasting [2.25].

A sinter process which still requires upstream molding steps is the Keltool technique.In this process, unlike the process chains described above, a higher strength copy of amold pattern is made. The goal here is to convert patterns made of a low-strength RPmaterial into metal parts.

To prepare an injection mold, a pattern of the mold half is generated first by any RP

process.As in the silicone casting process, a highly heat-resistant RTV silicone that can be

demolded after curing is poured around the master pattern (Figure 2.15). An epoxy binderthat is very highly filled with a metal alloy in powder form is then poured into this m old.

The result is a stellite green part of the cavity to be made. As with selective lasersintering, the binder is thermally desorbed, and infiltration performed, with the poly-meric binder being replaced by a copper/zinc (Cu/Zn) alloy (Figure 2.16). The surfacescan then be machined [2.39].

Cavty pattern

Cavty of meta

Creatng a silicone

castng mod

Infiltrating

Pourng in a meta

powder-resn mxture

Drvng out the binderand sintering

Figure 2 15 Principle underlying the Keltool process



Figure 2 16 Processes occurring in the Keltool process

2 2 4 Outlook

Many of the direct RT operations proposed here are still in development and are subjectto size limitations, for example. Nevertheless, all process chains presented here aregenerally available on the market and may be used for rapid prototyping of molds.

Finally, an overview of all the processes discussed here is presented for comparisonpurposes (Figure 2.17). Many of the metallic processes are not yet commerciallyavailable on the market. Only selective laser sintering, metal spraying and

electroforming are available. The Keltool process is also available but it is onlywidespread in the American market at this time. Aside from complexity and stability,however, major criteria are attainable surface quality, the availability and the price,which can vary extensively according to geometry.

Therefore, despite their speed, when using all these operations, it is always best toestimate whether it is more economical to avail of RT or whether it might not be betterto emp loy a conventional process instead.

2 . 3 H o b b i n g

Robbing is used for producing accurate hollow molds. It comes in two variants, cold-hobbing, the more common, and hot-hobbing.

Cold-hobbing is a technique for producing molds or cavities without removingmaterial. A hardened and polished hob, which has the external contour of the molding,is forced into a blank of soft-annealed steel at a low speed (between 0.1 and 10 mm /min ).The hob is reproduced as a negative pattern in the blank. Figure 2.18 demonstrates the

process schematically. The technique is limited by the m aximum permissible pressure onthe hob of ca. 3,000 M Pa and the yield strength of the blank m aterial after annealing . The

Infiltratingin the furnace

Infiltration material

Sintering the partin the furnace

Drvng out the bnderin the furnacefter castng

Binder

Metal



Commerciallyavailable

onmetallicetallicrocess

Selective laser sintering

Controlled metal build-up

Shape melting/multiphase jet

solidification

3D-metal printing

Metal laminated objectmanufacturing

Laser cutting/diffuse welding

Direct stereolithography

Resin casting

Metal spraying

Electroforming

Investment casting

Keltool

igur 2 17 Survey of rapid tooling processes



best conditions for cold-hobbing are provided by steels annealed to a low strength of

600 MPa.

The yield strength after annealing depends primarily on the content of alloysdissolved in ferrite and on the quantity and distribution of embedded carbides [2.42,2.43]. In accordance with their hardness after annealing (Brinell hardness) and theirchemical composition, materials commonly used for cold-hobbing fall into threecategories (Section 1.1.2 . Figure 2.19 shows the attainable relative hobbing depth as afunction of hobbing pressure and hardness after annealing. The nondimensional hobbingdepth t/d is the ratio between the depth t and the hob diameter d of a cylindrical hob. If

the hob has a different cross section, e.g. is square or rectangular, then t/l, 1 3 V A , whereA is the cross-sectional area [2.43]. The hobbing depth can be increased beyond thedimensions shown in Figure 2.19 by certain steps. Strain-hardening of the blank materialoccurs with increasing depth. This strain-hardening is neutralized by intermediateannealing (recrystallization). Thereafter, the hobbing process can continue until the

Figure 2 18 Schem atic presentation

of hobbing [2.43]

Hob•Locatng ring•Blank-Extenson ring

Holder

Relative hobbing depth t/d

Figure 2 19 Pressure for hobbing commo n tool steels with a cylindrical hob (d = 30 mm jz

and cylindrical blank (D = 67 mm 0, h = 60 mm), hob velocity v = 0.03 mm/s, hob is coppe

plated, lubrication w ith cylinder o il

a Steel with 300 to 400 BHN,

b Steel with 500 to 600 BHN ,c Steel with 600 to 700 BHN [2.43]

-^2

Hobbing pressure p

bc

a



maximum load on the hob is reached again. Care must be taken to avoid the formationof scale during annealing, because only clean surfaces permit optimum hobbing results.The hobbing depth may also be increased by preheating the blank. Depending on materialand preheating temperature, 20 to 50% more hobbing depth can be attained. Finally,recesses can ease the flow of the material and result in increased hobbing depth [2.43].

During h obbing, the rim of the blank is pulled in. This indentation has to be machinedoff afterwards and must be considered when the hobbing depth is determined. Figure

2.20 shows the correlation between hob travel, usable depth and indentation. A hobbedmold insert, not yet machined, is shown in Figure 2.2 1.

The surface quality of hob and blank is of special significance for hobbing. Onlyimpeccably polished surfaces do not impede the flow of the material and they preventsticking and welding. For the same reason, attention has to be paid to sufficient lubrication.Molybdenum disulfide has proved to be an effective lubricant, while oil usually does nothave adeq uate pressure resistance. To reduce friction, the hob is frequently copper platedin a solution of copper sulfate after having been polished [2.42-2.44].

Besides surface quality of hob and blank, the dimensions of the blank are alsoimportant for flawless flow of the material. For cold-hobbing into solid material, theoriginal height of the blank should not be less than 1.5 to 2.5 times the diameter of thehob [2.42, 2.44]. The diameter of the blank, which has to correspond to the size of theopening in the cavity retainer plate, should be double the diameter of the hob.

Cold-hobbing is generally used for low cavities with little height. It offers severaladvantages over other techniques. The hob, which constitutes the positive pattern of thefinal molding, can often be made more economically than a negative pattern. With a hob,several equal mold inserts can be m ade in a short time. Because the fibers of the m aterial

are not cut, unlike the case for machining operations, the mold has a better surfacequality and a long service life.

Figure 2 20 Relationship between

hob travel, usable depth and depth of

displaced material [2.42]

Figure 2 21 Mold insert mad e by

hobbing (left) and matching hob

(right) [2.41]

Hob travel = Usabe depth + displacement9 = f

+ e

Spacer

Blank

Hob



Limitations on cold-hobbing result from the mechanical properties of hob and blank

and therefore the size of a cavity.

2 . 4 M a c h i n i n g a n d O t h e r M a t e r i a l R e m o v i n g

O p e r a t i o n s

2 .4 .1 M a c h i n i n g P r o d u c t i o n M e t h o d s

Machining production methods may be divided into processes with geometricallydefined cutter (turning, milling, drilling, sawing) and geometrically undefined cutter(grinding, honing, lapping). The machinery, frequently special equipment, has to finishthe object to the extent that only little postoperation, mostly manual in nature (polishing,lapping, and finishing), is left.

M odern tooling m achines for mold m aking generally feature m ultiaxial CNC controlsand highly accurate positioning systems. The result is higher accuracy and greaterefficiency against rejects. The result of a survey [2.45] shows NC machining as havingjust a 25% share compared to 7 5 % for the copying technique, but this does not hold truefor modern tool shops and the fabrication of large molds.

Nowadays, heat-treated workpieces may be finished to final strength by milling (e.g.Rm up to 2000 MPa). Various operations, e.g. cavity sinking by EDM, can be replacedby complete milling operations and the process chain thus shortened. Furthermore, thethermal damage to the outer zone that would otherwise result from erosion does notoccur. Hard milling can be used both with conventional cutting-tool materials, such ashard metals, and with cubic boron nitride (CBN). For plastic injection molds, hardmetals or coated hard m etals should prove to be optimum cutting-tool m aterials.

Machining frees existing residual stresses. This can cause distortion eitherimmediately or during later heat treatment. It is advisable, therefore, to relieve stresses

by annealing after roughing. Any occurring distortion can be compensated by ensuingfinishing, which usually does not generate any further stresses.

After heat treatment, the machined inserts are smoothed, ground and polished toobtain a good surface quality, because the surface conditions of a cavity are, in the end,responsible for the surface quality of a molding and its ease of release.

Defects in the surface of the cavity are reproduced to different extents depending onthe molding material and processing conditions. Deviations from the ideal geometricalcontour of the cavity surface, such as ripples and roughness, diminish the appearance in

particular and form underc uts , wh ich increase the necessary release forces.There are three milling variants:

- three-axis milling,

- three-plus-two-axis milling and

- five-axis milling (simultaneous).

Competition has recently developed between high-speed cutting (HSC) andsimultaneous five-axis milling. HSC is characterized by high cutting speeds and highspindle rotation speeds. Steel materials with hardness values of up to 62 HRC can also

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02. mold making techniques

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