Casting Technique

© Platinum Guild International USA 2000 All Rights Reserved 1

V6N3

Pouring, Pulling, Spinning,and Pushing:

Various Methods of the Lost WaxCasting of Platinum Both Old

and New

Roger GreeneEisinger Enterprises, Inc.

Various methods of the Lost Wax Casting(LWC) process are employed in thecasting of platinum with a varying degreeof success. We shall concern ourselveswith: “pouring” the Gravity Casting Method;“pulling” the Vacuum Assist CastingMethod, “spinning” the Centrifugal CastingMethod; and “pushing” the PressureCasting Method.

First, it is important to understand thegoals of a “good” casting.

1. No casting defects: deformations,inclusions, misfills.

2. Sufficient mechanical strength through-out the casting.

3. The creation of a good surface for theease of finishing, plating or brazing.

4. No porosity on the inside or outside ofthe casting.

5. Homogeneous mix of precious metaland alloy.

We also make the assumption that thepreparations prior to casting are done withquality casting in mind. Care must betaken with the creation of the originalmodel, the forming of the rubber mold, thewax pattern, the investing process, andthe burnout cycle. In addition, the use ofclean and appropriate alloy for the partscast is essential.

“Pouring” or Gravity Casting

This method, employed for thousands of


V6N3

years, makes use of the basic principle ofgravity, all objects fall towards the earth.Molten metal is either poured from a ladleor bottom poured from a special cruciblewith a stopper and a hole. This methodmakes use of the principle that objectswith heavier specific gravity displacematerials of lower specific gravity, henceheavier liquid (molten metal) displaceslighter gas.

However, because of the inherent surfacetension between the metal and the refrac-tory and the viscosity of the metal, onlylarger objects can be cast this way, as inindustrial settings. Other disadvantagesinclude the need for a large sprue reser-voir and the contamination that exposureto oxygen can bring to the cooling metal.

“Pulling” or Vacuum AssistedCasting

This method relies on the use of a vacuumpump drawn on the refractory mold while

molten metal is poured into it. (See Figure2).

The permeability of the mold allows thecreation of a negative pressure in therefractory. The maximum pressure - 1 bar(30" of mercury) or -1.02 kg/cm2 can only

Figure 1: Gravity Casting


V6N3

be reached at sea level and relies heavilyon the ability for the suction to passthrough the micro grains of the refractorymaterial. Gypsum based refractories aresuitable for this method, and in fact thecasting of gold and silver are done suc-cessfully this way. The main advantage tothe vacuum method is the removal of backpressure pockets of gas formations by thevacuum.

However, for the casting of platinum weare required to use phosphoric acidbonded refractories because of their abilityto withstand greater temperatures thangypsum bonded products and for thestrength of the refractory mold they pro-vide. These investments are very poor atallowing a vacuum to be drawn throughthem and therefore impractical for use inthe casting of platinum. Also, a large“feeder head” or button is required topreserve metal temperature and thereforeextra metal is being wasted as it is alsowith centrifugal and gravity methods.

“Spinning” or Centrifugal Casting

Centrifugal casting uses the action ofspinning to generate a force for the molten

Figure 2: Vacuum-Absorbing Casting


V6N3

mass of metal entering the refractorymold. This formula applies:

Centrifugal force (mass of metal) x (radiusof rotation) x (rpm2)There are three methods of using centrifu-gal force: horizontal rotation, verticalrotation, and slanted rotation, each with itsown characteristics. (See Figure 3)

Increasing the radius increases the cen-trifugal force, however space becomes aconsideration. To compensate for limits ofradius, rotation speed is increased. Springrotation or motor methods can be used.With spring action, the initial force issufficient to move the metal into the mold,but without sustained force, inefficienciesoccur. In motor driven methods, often themotor does not give sufficient initial boostto cause a proper flow of the metal. Of themotor methods, a transmission using aclutch allows for the motor to increase toproper speed before rotation occurs.1

However, as speed increases, so do

casting deficiencies. These include metalspillage upon entering the refractory, theaddition of investment particles in thecasted metals brought in by the metalmoving into the mold, and most detrimen-

Figure 3: Centrigugal Casting


V6N3

tally, the intense effect of the turbulence ofthe metal flow and the concomitant gasformations. The various gas formations,having no where else to go, are thenforced back into the solidifying metal.

Studies show that this creation of gasformations in centrifugal casting methodsmay be the most serious detriment to thiskind of casting.2 Minimizing metal thrust todecrease gas formations causes miss fillsand no compromise to the centrifugalspeed can eliminate this problem.

An additional problem is caused by thevarious specific gravity and varying solidifi-cation rates of themixed alloys used in casting. The centrifu-gal force varies according to the specificgravity, the heavier component of the alloygoing in the direction of the higher centrifu-gal force, while the lighter staying in thecenter. This causes inconsistencies in thehomogeneous mix of precious metal andalloy. This becomes a larger concern with

alloys of greater differentiation in specificgravity.

Also the inherent flow characteristic of themetal into the refractory mold causesadditional agitation to the metal, Thedominant force is the linear movement ofthe metal towards the outside of its spin-ning radius. In addition, you have theinertia force which moves the metal awayfrom the rotation direction, as well as thegravity force moving the metal down-wards. These forces can be contradictoryto the passage the metal is required totake to fill the various components. Fur-ther turbulence and gas formations areresultant.

A side issue involves the crucible shapesused in centrifugal casting (See Figure4)

Typically an upright or vertical cruciblestyle is used with an induction coil of thehigh frequency type casting machine,while a horizontal crucible style is more


V6N3

typically used with a manually meltingmachine (spring type).

The disadvantage to the vertical crucible isthat the centrifugal force causes the metalto spiral up the inner walls of the crucible,impacting the roof of the spout it is meant

to pass through and therefore coolingoccurs. To compensate for the cooling ofthe metal, a larger sprue volume is typi-cally used, thereby wasting additionalmetal. Additional problems such as metalspillage and crucible impingement havebeen mentioned earlier.

“Pushing” or Pressure Casting

The methods of pressure casting are theleast known of the various methods ofLWC. In the dental trade, in particular,compression caps have been used fittedover the “ring” or mold flask. (See Figure5)

Steam pressure is used inside this closedsystem to cast small quantities of metal.3

Pressure techniques are also used in thecasting of steel, aluminum, and ceram-ics.4

Simply put, in pressure casting a pressur-ized force (be it steam, compressed air or

Figure 4: Tyoes of Casting Crucibles in CentrafugalCasting


V6N3

inert gas) pushes against the molten metalin order to fill the mold refractory. In someways it can be argued that centrifugalforce is in fact a pressure system in theseterms. But I have placed centrifugal actioninto its own category as the pressure iscreated through centrifugal force only andnot by an external source. This is preciselywhy centrifugal force is so difficult tocontrol and thereby to minimize the delete-rious effects. For the same matter, in fact,vacuum is technically a pressure system,but is, in particular, a negative pressureand therefore belongs in its own category.

Pressure is applied to the sprue reservoirand continues as the metal is cooling andhardening. Even with the surface appar-ently hardened, the inner mass of themetal is still molten. As the metal solidifiesit is contracting and therefore with pres-sure continuously applied the density ofthe hardening metal increases. As themetal is solidifying at different rates,according to the mass of the design and

its internal alloy components, the pressureapplied to the metal organizes the solidifi-cation process and insures an evencrystalline pattern.5

It is critical at this point to review the

Figure 5: Pressure Casting by Tilt-pouring


V6N3

nature of pure platinum and platinumalloys as metal for casting.

A. The melting point is very high com-pared to gold and silver(1700°C orhigher).

B. There is a large temperature differentialbetween the molten metal and moldrefractory.(1000-1200°C)

C. Surface tension on the molten metal issignificant.

D. The addition of impurities, particularlycarbon, are easily absorbed into themetal and cause deformations.

It has been largely accepted in the westernhemisphere that centrifugal force is theonly way to successfully deal with thedifficult nature of platinum as a castingmetal.6 A variety of solutions have beentaken to ameliorate the problems ofcasting centrifugal. The addition ofvacuum into the centrifugal method hasbeen considered advantageous for theridding of back pocket air pressure but

appears only to accomplish the protectionof the metal from oxide formations duringits melt. Various configurations have beenused to maximize the initial thrust of thecentrifugal force including redesigning thetraditional swing arm with one or even twobreaks in the arm. These solutions appearto minimize the problems of metal spillageand crucible particle embedding.

The drawbacks of the centrifugal method,however, specifically, the gas formationscaused by the excessive turbulence in thefilling of the mold, as well as alloy separa-tion giving rise to the mixing of gas andmetal have been well known in the castingof gold and silver. 7 In fact the centrifugalcasting of gold and silver is done less andless.8

The use of vacuum casting, while provingsuccessful in the casting of gold andsilver, is impractical for casting of platinumbecause of the density of the refractoryused, and additionally, it would appear that


V6N3

the force of the vacuum is too limited forthe characteristics of platinum.

Given the difficulties in casting platinumcentrifugal or with vacuum, it is no wonderthat the impression has been created (weare speaking of the western jewelrymanufacturing market) that working withplatinum is so difficult and that the accept-ing of high failure rates is unavoidable.However, if we consider the advantages ofworking with a pressure system for thecasting of platinum, the problems inherentin centrifugal and vacuum methods canbe avoided.

The introduction of pressure in the castingof platinum began in Japan in 1966. Itsintegration with more modern methods ofinduction melting began in 1985. Thisprocess was introduced in North Americaat the MJSA Expo NY in 1998.

The means by which the metal enters themold refractory during the pressurecasting process is smoother than withcentrifugal. First, the metal enters therefractory mold by the force of gravity, and

Figure 6: Efficient substitution by fluid whirlpool


V6N3

then pressure is applied to the solidifyingsprue as well as throughout the moldchamber. The metal is pushed into thevarious intricacies of the mold whilepressure is maintained. The force isadequate to accomplish the tilling of themold without the excessive conflictcaused by centrifugal filling.

Additionally, the various factors in terms ofdelivery of the force for filling the mold ismuch more easily controlled in pressurecasting. Variations on the amount of force,when the force is applied in relationship tothe timing of the metal cooling, and theduration of the force are all much morecontrollable with the method of pressure.

One difficulty that still needs to be ad-dressed is the problem of gas formationsand back pressure pockets inside therefractory mold. As mentioned before, theapplication of a vacuum on the mold wouldbe ideal, but because of the nature of therefractory used in platinum casting and the

difficulty of balancing the forces of positiveand negative pressure in this very brief actof casting, other methods must be pur-sued.

The use of mold rotation on its own axis(the centrifugal force mentioned earlier) isan interesting addition to the casting cycle.The refractory mold spins at a low rpm(75-100 rpm) while the molten metal ispoured into a funnel shaped opening (a

Figure 7: Our unique spruing method makes the bestuse of your precious metal: less for the button, morepieces sprued per flask. Our funnel feed form creates awhirlpool fill, allowing gases to escape.


V6N3

different method of sprueing is required). Avortex of negative pressure is createdforming a center column that may be usedfor the exiting of remaining air and gaseswithin the mold that would normally embedinto the metal.

This principle can be seen with the use ofa bottle of liquid which has a narrowopening. In turning upside down the bottle,the exiting of the liquid is slow because ofthe narrow opening When the bottle isrotated, a negative pressure is created bythe centrifugal force sending the liquid tothe sides of the bottle. This creates acenter air column allowing easier accessof air into the bottle and thereby creating abetter flow for the liquid. (See Figure 6)

This sprueing method is unique in that themetal enters the top of the refractory mold,spiraling down the center funnel, filling athin donut shape at the bottom and thenentering through the sprues to the jewelryforms. There is no need of a heavy sprue

reservoir as the pressure force actsdirectly upon the metal. A minimumamount of metal is wasted and a largervolume of finished forms can be cast byallowing pieces to be setup both aboveand below the donut reservoir. (See Figure9)

This combination of a more gentle en-trance of the molten metal into the refrac-tory, while at the same time creating ameans for entrapped gasses to escape, isa successful formula for the casting ofplatinum. Cross sections of pressure castplatinum forms show remarkably denseformations without the porous problemstypically associated with other means ofcasting platinum (See Figure 10).While the use of pressure in the casting ofplatinum is relatively recent, at least in thiscountry, the potential for success with thismethod is much greater than with theother methods we have discussed.

This paper: “Pouring, Pulling, Spinning,


V6N3

and Pushing: Various Methods of the LostWax Casting of Platinum Both Old andNew”was presented by Roger Greene duringPlatinum Day IV, sponsored by the Plati-num Guild International on March 27, 1999with the direct collaboration of MinoruYoshida.

Mr. Roger Greene is the president ofEisinger Enterprises, a jewelry supplycompany located in Newark, NJ, USA.Eisinger Enterprises has been activelyinvestigating new casting technologies,particularly for platinum.

The Yoshida Cast Industry of Japan hasbeen pioneers in casting by the pressurecasting method since early 1966. A pres-sure casting machine using inductionheating was introduced in Japan in 1985.The Yoshida company began to export itstechnology into China, India and Thailandduring the early 90s. Pressure castingwas introduced to North America in 1998.

Figure 8: Microphotography of cross-section ofpressure-cast platinum.(Photo courtesy of Tiffany & Co.)


V6N3

Footnotes

1. A newer method of centrifugal rotationexists which uses the spinning of arefractory on its own axis. But thismethod is employed using the pres-sure cast method and will be dis-cussed later.

2. “High turbulence increases the prob-ability of gas entrapment and favorsthe formation of gas porosity.” IgorShersher Platinum ManufacturingProcess, Vol. II, PGI.

3. “Another one is a kind of static pres-sure casting, where the pressuregenerated by the steam produced byan application of a potato is used toimprove mold filling.”Dr. Valeno Faccenda, GOLD TECH-NOLOGY Issue #23, April 1998, pg. 21

4. There is a great deal of informationavailable on pressure casting of steel,

aluminum, and ceramics. For anexcellent article on ceramic pressurecasting consult: “Ceramic Technology”Chalmers School of Chemical Engi-neering Dec 23,1998, seehttp: //www. chalmers. se/researchprofile/ceramic. Html

5. “The results show that pressureaccelerates the solidification process,reassigned to the crystal structure andincreases the formation of steelcastings under pressure” E.D.Taranov. CASTING PROCESSESNo.11997,pg. 22-28

6. “A Primer for Platinum Casting” ElaineCorwin, American Jewelry MagazineAugust 1998 pg. 28-35

7. “Investment casting: Centrifugal orstatic vacuum assist?” Dr. ValenoFaccenda, GOLD TECHNOLOGYIssue#23, April 1998, pg. 21


V6N3

8. Although a definitive source for thisinformation couldnot be documented, I am deducing thisbased on thediminishing sales of centrifugal equip-ment over vacuum that we have seen.Also much discussion of this issuecan be found on the Internet,particularly at: “orchid~ganoksin. com”

Notes: (figure 10 on following page)1. Hardnesses and Ductilities in table are

for the annealed state except thosemarked (C), which are for as-caststate.

2. Under Countries of Main interest (lastcolumn):EUR refers to most European coun-tries but Germany (GER) has addi-tional alloy interests.EU2 refers to most European coun-tries who would stamp higher gradealloys as standard 950 Fine.

3. Pt/PGMs fineness are in parts perthousand.


V6N3

Casting Technique

Documents

Transcript of Casting Technique