Jewellery Investment Casting MachinesPeter E. GainsburyThe Worshipful Company of Goldsmiths, London

Investment casting is by far the most widely used technique for the pro-

duction of gold jewellery. Each year, large quantities of carat gold alloys

are processed by this method. It is therefore of importance that

machines used for jewellery investment casting should meet the

efficiency and quality standards expected in large scale manufacturing

equipment.

Despite the extensive use of the related lost waxprocess in antiquity, the modern technique of invest-ment casting has been applied in industry for arelatively short period. Its use in the casting of dentalrestorations in 1907 was not followed immediately byother applications. Thus, it was not until the late1930's that it was taken up by manufacturingjewellers and it was only during World World II thatexperience of it as an engineering process was builtup. Curiously, however, the process as used injewellery production today owes little to engineeringinvestment casting and the techniques used in the twoareas have developed almost independently.

At the present time, jewellery casting is passingthrough an era of considerable change; it appearslikely that the period of empirical development ofmaterials and equipment of the past thirty years iscoming to an end and that a process will emerge witha better tehnical basis than has been the case to date.

The Basic ProcessThe starting point for the production of a jewellery

casting is a master pattern of the part, normally madeof metal to the highest standard of detail and finish.From the pattern, a negative die is made to be usedfor the production of expendable patterns. For this,the most usual procedure is to vulcanize a solid blockof rubber around the master using heat and pressure,subsequently parting the die and removing it byjudicious cutting with a surgical scalpel. Two-partdies made in this way can reproduce complex designswith heavy undercuts by low pressure injection ofmolten wax, the flexible rubber permitting the pat-terns to be removed without distortion. Other diematerials used are cast elastomers, cast epoxy resinsand low fusing alloys. Rigid or metal dies may beused with higher wax injection pressures than flexiblematerials to obtain the finest detail and accuracy. Iflarge production runs are called for, patterns may beproduced in metal dies by the injection moulding ofplastics.

Wax patterns are set up in large or small numbersdepending on production requirements and castingmachine capacity. Various techniques are used for set-ting up but the most common for quantity productionis to attach the patterns radially by short moulded-onsprues to a heavy central feeder (Figure 1).

Once set up, the patterns are placed in a mould andembedded in investment. Almost all jewelleryinvestments are based on combinations of plaster andsilica which are mixed to fluid slurries with water andformulated to set in about 10 minutes from the com-

Fig. 1 This wax tree has been built by attaching individuallymade ring patterns to the tentral feeder. The tree is nowready for investing

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Fig. 2 Simple spring-driven cen-trifugal casting machine. Thecrucible for torch melting andan empty investment flask areseen in position on the arti-culated casting arm. An ad-justable counterweight is fittedat the opposite extremity of thearmPhotograph by courtesy of Hoben DavisLtd., Newcastle u. Lyme, Stafes., England

Y"1^!

mencement of mixing. Solid block moulds are thegeneral rule, multiple investing procedures withinner and backing investments are not used injewellery casting practice. Most investment mouldsare formed in cylindrical heat-resisting metal flasks.Air entrapped on the surface of patterns is removedbefore setting commences by subjecting the mould toa vacuum sufficient to boil the water in the mix.

Set moulds are dewaxed by low temperatureheating, either in air or steam, and then fired tobetween 700 and 800°C to cure the investment andburn out carbonaceous residues. Before casting,jewellery moulds are normally cooled to between 300and 700°C depending on the casting temperature ofthe alloy being used and the nature of the patterns inthe mould.

Casting MachinesJewellery-type investment moulds cannot be filled

by simple gravity pouring. The combination ofunvented moulds with low permeability refractory,the need to reproduce fine detail and delicate sectionstogether with the relatively small size of melts and theconsequent low hydrostatic head and low thermalcontent of the metal preclude this possibility. Castingof the metal into the mould is therefore almost in-variably carried out in some form of casting machine.

A basic function of the casting machine is to applypressure to the molten metal so that it penetrates andfills the mould completely. This same pressure mayalso be used to effect transfer of the molten metal tothe mould Erom the crucible when this is part of themachine. Centrifugal force, pressure or vacuum, or acombination of them, are used to perform these twofunctions. The machine may also have built into it ameans of melting the metal or a crucible or hearth in

which the metal can be melted by external torchheating. Ancillary functions provided on the moresophisticated machines are melt temperature indica-tion and regulation, atmosphere control and castingpressure regulation.

Centrifugal Casting Machines`Centrifugal' machines for casting jewellery are not

truly centrifugal, the mould does not rotate on its ownaxis but is fitted at one extremity of a balanced arm,the rotation of which develops the necessary pressureto force the metal into the mould (Figure 2).

Such casting machines were initially spring-drivenbut modern machines are almost invariably power-driven at speeds up to around 300 r/min and the mostsatisfactory types have rotation speed or torqueregulators. Power-driven rotation provides constantspeed and ensures that the centripetal force continuesto be applied until the metal has solidified. Also, thepossibility of run-back is eliminated and, in completecontrast to spring-driven machines, a slow take-offspeed can be followed by an increase in speed toreduce the possibility of turbulence and allow maxi-mum consolidation of the metal in the mould. Spring-driven rotation would in any case be impractical forlarge capacity machines which may be able to cast upto 7 kg of 18 carat gold alloy.

Torch, electrical resistance and high frequency in-duction melting are all used on centrifugal castingmachines. There are a few large capacity machines inwhich the metal is melted in a conventional cruciblein a separate furnace, the crucible with its moltencharge being transferred to a carrying cradle on themachine immediately before the release of the castingarm. However, direct melting on the machine is byfar the most common practice.

•f 1:, __

MELTING COMMENCEMENT OF ROTATION MOULD FILLED

Fig. 3 Schematic representation of casting in a torch melting centrifugal machine such as that of Figure 2. The axisof rotation is to the right of the illustration

Torch Melting Machines

Torch melting is carried out in shallow hearth-typecrucibles which are hooded at the front end wherethere is a central hole through which the metal istransferred to the investment mould (Figure 3). Thecrucibles for torch melting are normally moulded inhigh-alumina fireclays and, if protected frommechanical damage, have good lives. Considerablethought has gone into their design in recent years toensure efficient melting, smooth transfer of the meltto the mould and avoidance of tangential losses dur-ing the acceleration of the casting machine arm.While a shallow configuration of the hearth of thecrucible provider for rapid melting, it may also pro-mote oxidation of the charge or gas absorption as aresult of the large exposed surface of molten metal.However, skilled melters using an efficient torch havelittle difficulty in producing gas- and oxide-free meltsof any normai jewellery gold alloy. Town gas, naturalgas, propane or acetylene are used as torch fuels withcompressed air or oxygen as supporters. Preferencesare dictated by availability but combinations whichcould lead to overheating of the melt are to beavoided.

Torch melting suffers from the great technicaldisadvantage that temperature control and protectionof the melt against oxidation are not possible. Thepreparation of the metal in optimum condition forcasting is dependant on the skill of the operator andthe use of refined melting aids such as vacuum orinert atmosphere is precluded.

It is unlikely that any major development can be ex-pected in torch melting centrifugal casting. No doubt,improvement of the details of torch and machinedesign will continue but it must be concluded thatthis process is being superseded. Although it remainsa very adequate production method, particularly forsmall-scale work, it would appear inevitable that forjewellery of the highest quality it must give way totechniques which permit better control of the condi-tion of the melt.

Resistance Melting Machines

The use of resistance melting has been confinedprincipally to only two types of casting machines.

One machine utilizes a horizontal, cylindrical, wire-wound resistance furnace. A cylindrical graphitecrucible is used with a charging hole at one end and acasting hole at the other. The furnace is rigidlymounted on a straight casting arm and the mould isclamped against the casting end of the furnace by astepped sliding plate. A thermocouple fitted in agroove in the outer crucible wall, indicates and con-trols the furnace temperature. The enclosed design ofthe graphite crucible ensures clean gas-free melts ofalloys melting at up to around 1000°C. The machine,however, has limited capacity with a maximum ofabout 620 g of 18 carat gold alloy. Moreover, meltingis slow, it is difficult to observe the melt conditionand the arm is spring-driven.

A resistance melting machine of greater current in-terest is produced in a range of sizes with maximum18 carat gold capacities of 200 to 2000 g. Originallydesigned for dental applications, the system is uniquein that carbon resistance heating is employed togetherwith metal transfer by tilting in conjunction with thecentrifugal configuration. The furnace is of tubularform with the axis vertical for melting and is rotatedon trunnions to the horizontal position for casting. Itis mounted on the edge of a power-driven horizontalturntable which carries a counterweight or a secondfurnace on the opposite edge. Initially, the mould isplaced mouth downwards over the top of the meltingcrucible and clamped in position. When the metal ismelted and the turntable rotated, a trip releases thefurnace and it tilts to the horizontal position with themould outwards (Figure 4). Tilting is controlled byan oil dash-pot, so that smoother transfer of the metalto the mould is obtained than with other systems.

The metal may be melted in a graphite or ceramiccrucible fitted inside a ceramic liner to the tubularresistance element. There is no practical limit to alloymelting temperature as far as jewellery alloys are con-

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TILTING

MOLTEN MOULD FILLEDMETAL

Fig. 4 Schematic representa-tion of casting in a carbonresistance melting een-trifugal machine. Metalpouring is achieved by pro-gressive tilting of the mould-furnace assembly to the MOULD

horizontal position at the SECURE

commencement of rotation: IN POSIT

The rotation axis is to theright of the illustration. Afterdocuments from Arno Lind-ner Fabrik, Munich, WestGermany

CARBONRESISTAIHEATER

CRUCIBL

READY FOR CASTING

cerned. Melting conditions are inevitably reducingand very satisfactory for normal coloured gold alloys.With white gold alloys, however, particularly thehigh melting range soft alloys, there is a danger ofpick-up of deleterious impurities such as sulphurfrom the graphite or silicon from the ceramic, leadingto serious hot shortness and apparent embrittlement.Furnace temperature may be controlled by a ther-mocouple positioned between the resistance elementand the furnace liner but with this configuration theremay be a considerable differente between the in-dicated and the actual metal temperatures.

In some respects, the system described aboveembodies the most satisfactory application of the cen-trifugal principle to jewellery casting. Melting condi-tions are excellent, some control of the melttemperature is possible, smooth, controlled metaltransfer is obtained and the tost of the equipment ismodest. Disadvantages are the limited maximumcapacity, the impossibility of loading bulky scrap,slow melting, the fact that observation of the melt im-mediately prior to casting is not practical and limitedheating element life. While resistance melting lendsitself more readily to smooth centrifugal metaltransfer than induction melting (as will be seenbelow), it can be predicted that this advantage will notbe relevant in future if, as is likely, static castingprocesses become pre-eminent.

High Frequency Induction Melting Machines

At the present time, induction melting centrifugalcasting machines are probably the most widely pro-duced of all types for jewellery casting. They aremarketed by some ten or more manufacturers inEurope and the U.S.A.

Basically, all these machines are very similar withintegral water-cooled valve or solid state generators of

3 to 18 kVA nominal output through short retractablecoils. Melting is in vertical crucibles generallymounted on a rigid counter-balanced power-drivencentrifugal casting arm rotating in the horizontalplane. Melting capacities range from 150 g to 5 kg of18 carat gold alloy.

It is considered that the weak point of most suchmachines is the mechanism of metal transfer from thecrucible to the mould. This depends on the fact thatthe crucible walls slope outwards a few degrees fromthe vertical and a pouring hole is provided at the topof the side facing outwards from the centre of rotationand aligned with the centre of the mouth of thehorizontal mould. With ceramic crucibles the half ofthe crucible on the pouring hole side may be coveredwith an integrally moulded hood, but with the morecommonly used graphite crucibles the pouring hole is

MOULDCRUCIBLE_—SUPPORT

CRUCIBLE-

Fig. 5 Schematic representation of metal transfer inan induction melting centrifugal casting machine.Once the required melt temperature has beenreached, the induction coil is retracted and rotationis triggered. Upon attainment of sufficient een-tripetal force, the metal is projected through theopening in the crucible wall and into the mouldcavity. The rotation axis is to the right of the il-lustration

a semi-circular depression in the top rim, which iscovered with a loose flat lid. When the centrifugalarm is set in motion, the centripetal force causes themolten metal to climb up the inclined crucible wallopposite to the centre of rotation until it reaches thepouring hole through which it is rapidly flung intothe mould (Figure 5). It is evident that only a smallcomponent of the centripetal force is available tocause the metal to climb the crucible wall but that assoon as the metal reaches the pouring hole the fullforce is suddenly applied to it and it is very rapidlyand turbulently thrown into the mould. If the rota-tion speed and thus the centripetal force is high, thenexcessive entrainment of air in the metal streammay result. This effect can cause cunsiderable andsometimes very puzzling porosity in castings. If air isentrapped in the metal and solidification is rapid, as itnormally is, there may not be time for the air to beforced out of the mould cavity before an initial solidmetal skin forms on the mould walls. With the slowersolidification of metal within the interior of thecasting, the centrifugal action causes air bubbles to betrapped below the surface of castings at pointstowards the axis of rotation. It is therefore essentialthat some form of speed control which permits theprogressive application of centrifugal force be fittedonto machines of the above design.

When temperature control is fitted to inductionmelting machines it may be by use of radiationpyrometers or immersion thermocouples. Pyrometer

CRUCIBLE

HIGH TEMPERATURE

PERFORATED / SEAL

FLASK \/

'F'CASTING CHAMBER

Fig. 6 Principle of vacuum assisted statie investmentcasting with a perforated flask. As the metal ispoured from the crucible, vacuum from the reser-voir is applied to the casting chamber and istransmitted to the mould cavity via the perforationsin the flask and the pores in the investment

readings can be affected by oxide films at the surfaceof the melt, by fumes or by inefficient stirring in radiofrequency melting. Thermocouples can be affected bystray currents and sheath life is often too short. Thus,both control methods have limitations but opticalpyrometry is much simpler for use with a cruciblewhich has to move and to which it is thereforedifficult to arrange electrical connections.

Melting of coloured gold alloys is normally inmachined graphite or electrically conducting Biliconcarbide crucibles. With the very high frequency cur-rents employed, coupling to the metal charge is nottoo efficient and more effective melting is obtained bythe use of the crucible as a susceptor. The use ofgraphite crucibles also provides some measure of pro-tective atmosphere over the melt, though for alloys ofhigh base metal content this is insufficient to preventoxidation and most machines have facilities for in-troducing an inert or reducing atmosphere above thecrucible. Atmospheres used have included naturalgas, nitrogen-hydrogen mixtures and argon. With thelatter gas, difficulties can arise if it contains moisture.Reduction of water vapour by the hot graphite cruci-bie can occur, resulting in hydrogen absorption intothe melt and subsequent gas porosity in the castings.

Statie Casting MachinesSimple air- or steam-pressure casting machines

have been in use in parallel with centrifugal machinesin dental casting for far longer than jewellery castinghas been a commercial proposition. These machineswere only suitable for casting small melts made in thetop of the mould itself and early commercial jewellerycasting was primarily with centrifugal machines. Formanufacture of jewellery on a small scale, simplevacuum assisted machines began to appear some yearsago in the United States and limited use was made oflarger equipment of the same general type for heavysection silverware casting. The principle of all thesemachines was that the mould was placed on a heat-resisting gasket on a flat table over a hole connected toa vacuum pump and vacuum was applied to the baseof the mould while the molten metal was poured. Thelow pressure in the mould cavity caused atmosphericpressure to force the metal into the mould, fillingbeing assisted by reduction of the cushion effect ofthe air in the mould cavity.

These techniques though effective for small mouldsor heavy section casting are not suitable forproduction-scale jewellery casting and it was not until1970 that commercial equipment for statie invest-ment casting became available to manufacturers. Theprocess, correctly described as a `vacuum assistedcasting' and not as `vacuum casting', is simple. A per-forated moulding flask is used which is fitted with aheavy flange on the upper or molten metal entry end.

Moulds are invested in the usual way with a singlesprue entry to the cavity. The machine (Figure 6)consists of a cylindrical casting chamber, largeenough to accommodate the largest mould used, andwhich has an open top with a flange corresponding tothe flange on the flask. The casting chamber is fittedinside a vacuum chamber of large volume and isisolated from it by a large diameter quick-openingvalve. The vacuum chamber is provided with asimple gauge and is evacuated by a rotary pump. Themetal is melted independently in any suitable cruciblefurnace and when casting is to be carried out, thevacuum chamber is pumped out with the valve closedand the hot investment mould is seated on the flangeof the casting chamber with a heat-resisting gasket in-terposed between the Hanges. The molten metal issimply poured into the mould by hand, the vacuumvalve being opened momentarily before the metalstream strikes the mould surface.

The technique is giving excellent service for boththe high volume production of small jewellerycastings and for large size castings much too heavy forproduction on centrifugal machines. Its disadvantagesare the expense of the specialty designed flasks andthe difficulty of seating them accurately and removingthem from the casting machine at temperatures up toaround 700°C. Several developments have, however,been introduced by equipment manufacturers whichaim at simpler and more economical operation.

Although vacuum assisted casting techniques workextremely well in some operators' hands, there is stiltcontroversy about their effectiveness for thin sectioncasting. This is because correct sprueing is probablymore critical than in centrifugal casting and the timeof opening the vacuum valve in relation to pouringthe metal is of prime importance. For this reason,automatie machines where the mould cavity isevacuated in advance with the mould in a sealedchamber have been developed. These are probablythe most sophisticated casting machines currentlyavailable.

The machines (Figure 7) feature a fully enclosedcasting chamber. The pouring hole in the chamberlid is sealed to the bottom of the melting furnace,which is fitted with a bottom pouring crucible closedfrom above by a graphite stopper carrying a ther-mocouple. Melting in the larger machines is bymedium frequency induction. Simple transfer of themolten metal from the crucible to the mould ismonitored by the attainment of either the desiredtemperature of the metal or the partial pressure in thecasting chamber. In automatie machines of recentdesign, the induction coil, crucible and metal releasemechanism are completely enclosed in a separatemetal vacuum chamber, thus enabling full vacuum orinert atmosphere melting and casting. Machines are

rIVCUMHI II. NAM

Fig. 7 Schematic representation of the bottom pour-ing type of vacuum assisted statie investment castingmachine with induction melting. The castingchamber is evacuated in advance. Upon attainmentof casting conditions the graphite stopper rod islifted and the molten metal fills the mould cavityunder the combined effects of gravity and at-mospherie pressure

also being developed in which the induction coil andthe mould are enclosed in a single vacuum chamber.Pouring is generally by tilting of the crucible via ex-ternal controls and in the simplest case, castingpressure is established by evacuating the chamberduring melting and then exposing to atmosphericpressure at the moment of pouring the metal into themould. A refinement of this process is to provide avacuum take-off from the mould support table so thatvacuum can be maintained on the base of the mouldwhen atmospheric pressure is admitted to the maincasting chamber.

Most lower carat gold alloys contain zint andshould not be melted at low pressures. For such alloysfull vacuum chamber machines are used with backfilling with an inert gas to a partial pressure sufficientto eliminate volatilization of alloying elements. In thiscase the possibility of applying full vacuum to thebase of the mould at the moment of casting is a decidedadvantage.

Hybrid MachinesA development of centrifugal induction melting

casting machines has been to enclose the melting coiland casting arm in a vacuum chamber. This move in-troduced many design problems which inevitably

OPPER

JCTION COIL

PHITE CRUCIBLE

:STMENT MOULD

UUM CHAMBER

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Fig. 8 A wax pattern and a finished 18 carat gold and dia-mond ring cast from the same patternPhotograph by courtesy of Julie Crossland

resulted in machines much more expensive than theirair melting counterparts with possibly little improve-ment in the quality of the castings.

Another approach has been the application of avacuum to the bottom of investment moulds whilecasting was carried out by normal centrifugalmethods. The reasoning is that the vacuum draws`harmful' gases out of the mould before the metalenters and that atmospheric pressure on the metalhelps mould filling. It is unlikely, however, that any`harmful' gases would be present in properly fired in-vestment moulds. Vacuum is not applied to the baseof the mould until rotation of the casting arm com-mences, thus, no significant purging of the mouldoccurs before the entry of the metal and in the best ofcases only very partial vacuum is achieved beforemetal solidification. Clearly, a factor in the invest-ment casting process is that air or gas must be forcedout of the unvented mould cavity through the lowporosity investment material by the pressure of themolten metal. There is ample practical evidence thatassistance by either centripetal force or atmosphericpressure and gravity suffeces to overcome this prob-lem. The beneficial results, if any, of compoundedrotation and inefficient vacuum assistance do notjustify the complication and expense of such hybridmachines.

Discussion

While it is true that centrifugal casting has been thebackbone of the development of jewellery casting toits present level of efficiency (Figure 8), it is sug-gested that persistente in the use of the technique isnow more based on habit than on necessity. However,it must be admitted that with the current level ofknowledge of the parameters of casting, the power-driven centrifugal machine with torch melting is stillthe most economical, versatile and reliable method ofcasting the whole range of jewellery alloys.

The combination of efficient induction meltingtechniques with centrifugal methods of metal transferhas led to a decrease in the efficiency of the latter andto a loss of some of the advantages of the former. Fur-ther, the turbulent transfer of large quantities ofexpensive metal in the molten state by high speedspinning and climbing of a near-vertical crucible wallappears inefficient, physically and economicallyhazardous and mechanically expensive. With melts ofthe average size cast today there is ample time totransfer metal smoothly into moulds by simple tiltingof the crucible by the well tried Durvifle method orperhaps most effectively, but with greater technicaldifficulty, by bottom pouring. Altogether, it seems adecided advantage to work with gravity, instead ofagainst it, in order to transfer the metal, fill the mouldand obtain a sharp impression.

Casting machines of the future should have flex-ibility allowing large or relatively small melts to bemade under vacuum or controlled atmosphere withefficient temperature control. Pouring should be bymeans allowing smooth entry of the metal stream intothe mould and the necessary pressure on the metalshould be as uniform as possible throughout themould and automatically or semi-automatically ap-plied at the correct moment. However, before suchideal machines can be fully developed, further workneeds to be done on the many interdependentparameters which affect the efficiency of the castingmachine itself, which after all is only one factor in amany facetted overall process.

AcknowledgementsThe writer is indebted to the Worshipful Company of

Goldsmiths for permission to publish this article and to his col-league Chris Walton for many stimulating discussions.

Interested readers are invited to contact theauthor for more information. Mr. Gainsbury isDirector of Research, Technical Department, TheWorshipful Company of Goldsmiths, Goldsmiths'Hall, Foster Lane, London EC2V 6BN, England.