CASTING PLATINUM & PLATINUM ALLOY INGOTS FOR FABRICATION

Platinum and Platinum Alloys can be static cast in ingots for rolling sheet or wire products. Several materials are suitable for making molds for casting platinum ingots, high purity graphite, pure copper or 3% chromium copper alloy. The pure copper is rather soft to machine and some manufacturers prefer the 3% chromium copper that has a better hardness for cutting. High purity graphite and copper are excellent conductors capable of rapidly pulling the heat out of the molten platinum. A preference for copper molds has been noted as it reduces the risk of carbon contamination of the platinum.

Ingot size should be as thick as your rolling equipment can handle, a 1 inch thick ingot is ideal. Pouring thicker ingots helps avoid hitting the sides of the mold during the pour preventing large cold shuts in the side of the ingot. A ½ inch thick ingot would be the minimum thickness for static pouring.

Molds should be 2 piece L shaped molds with a base plate of the same material for casting ingots to be rolled in sheets. For wire ingots a 2 piece rod shape mold should be used. Each section of the ingot mold should be 2 to 2 ½ times the thickness of the ingot to provide enough mass to act as a heat sink, pulling the heat out of the molten platinum. If the molds are too thin, distortion, burning or melting of the mold can occur.

Molds should be preheated to 250 degrees F and placed on a sturdy metal table. A high intensity spot light should be directed at the top of the mold to make it visible for pouring. Some workers put a little talc on the top of the mold to make it more visible to pour into. Looking through # 10 lenses when melting platinum will make it hard to see the mold opening. No mold release agent should be used on the mold for platinum. After pouring and removing the ingot allow the mold to cool, use a scotch bright pad to gently remove the white ash like coating left on the mold.

For larger scale melting a high frequency induction melting furnace should be used. Zircon crucibles packed in isul-oxide sand inside a silica crucible liner are normally used with the larger high frequency furnaces. A length of fibre frax rope is packed around the top of insul-oxide sand to prevent it falling into the mold. The crucible life is very limited, usually 1 or 2 melts. Melting areas for platinum should be kept clean to prevent contamination from other metals. More superheat is required in the platinum when pouring ingots due to the slower rate of pouring. Tilt pouring is usually used on larger melt sizes. Small size melts can be made by torch melting in a suitable crucible for platinum. Proper eye protection must be worn when melting platinum, #10 lenses are recommended. A quick steady pouring motion should be used, slowing down at the end of the pour to avoid deep shrinks in the top of the ingot. The platinum ingot should be removed from the mold quickly and quenched in water.

The platinum or platinum alloy can then be rolled in the usual way. Larger reductions can be used on pure platinum and 50% reduction on platinum alloys. Anneal at 1800 to 1850 degrees F for 20 to 30 minutes and water quench. Do not over-anneal the platinum or platinum alloy. The pour marks on the outside of the platinum ingot will roll out during fabrication. Keep work areas and rolling mills clean to prevent contamination.

No guarantees are made for the aforementioned methods that are provided as information to United customers.