“If you don’t hit that temperature,
there is a good chance you will get insufficient fill, as the metals cool too quickly,” Arvay said. The molds used for casting precious
metals generally are either cast iron or graphite, with some proprietary blends used by different manufacturers. The molds must be heated prior to pouring and coated with the appropriate mold release agent. According to Arvay, iron molds are less expensive than graphite molds and last longer.
Preparing the Metal The thought of alloying a precious
metal seems heretical. Why would you want to change the composition of a metal that is worth the greatest amount at its most pure? But refining the alloys to their purest form is itself a trick. “If anything, we de-alloy,” Arvay
said. “We remove any of the base met- als through refining.” Traditional alloying also occurs,
just as it does in commercial casting. Introducing small amounts of certain elements can either make precious metal alloys more castable, or elements can be used to produce a piece of bul- lion with a certain color. Rose gold, for example, is gold alloyed with copper. White gold has large amounts of pal- ladium. De-oxidized silver is actually an alloyed silver that contains boron,
Gold and other precious metals are refined into small grains to achieve the highest levels of purity possible before they are melted and poured into casting molds.
silicon and other metals that are less attractive to oxygen molecules. With the cost of gold in particular
so high, casters of precious metals must be careful to maximize the alloys used in their processes. Metal loss, a common concern in most (if not all) commercial metalcasting facilities, is unacceptable in the world of precious metal bar production. And pre-measur- ing prior to pouring is critical. “It’s all about material preparation
The Golden Rules What can the designer and purchaser of engineered metal castings take away from the lessons
of the precious metals production and sales process? Following are a few golden rules to follow, taken from conversations with precious metals casters.
Rule #1: Monitor the Metal—Not all metals are pre- cious, but no metal is free. Be aware of the cost of the metal you purchase by following scrap prices, and audit your metalcaster to determine they are achieving appropriate yield rates.
Rule #2: Allow the Right Alloy—Be flexible when specifying an alloy for your caster to pour. Metalcast- ers are intimately familiar with the way different al- loys react to the sometimes volatile process of melt- ing and solidification. Be open to suggestions from your metalcaster about alloys that are more resilient against high oxygen entrainment, for example.
Rule #3: Perfect the Parameters—Make sure your metalcaster knows the basics, and consistently audit the supplier for proper handling of alloys and mold material. Regardless of how geometrically complex your component, without proper procedures in place to ensure the quality of each step of the casting process, you won’t receive quality metal castings.
March/april 2010
Rule #4: Know Your Finishing Processes—While the metalcasting process has a reputation of produc- ing rough-hewn products, suppliers can employ a range of finishing processes to achieve the effects designers and purchasers need.
Rule #5: Don’t Specify Unnecessary Finishing—Ex- pensive finishing processes are not always neces- sary. Some metalcasters make products that require finishing; others can produce a finely finished product in the casting process. Make sure to weigh the costs of finishing steps when considering the piece price of a metal casting.
Rule #6: Seek the Right Supplier—While metalcast- ing is one of the oldest manufacturing processes in the world, not all casting suppliers use old world technology. When sourcing a cast component, re- member that a range of facility types are out there, from the hand pouring job shops to the fully auto- mated production shops.
METAL Metal casting Design anD purchasing 25
beforehand,” said the Valcambi spokes- person. “We measure out metal shot to the exact weight you need before melt- ing and pouring it. You have to meet exact standards in weights, measure and fineness.” Refining scrapped precious met-
als also is an exacting process. Silver in particular must undergo extensive refining (typically performed by an outside firm) to overcome its propen- sity to attract oxygen during melting.
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