Spin-casting

Spin casting, also known as centrifugal rubber mold casting (CRMC), is a method of utilizing centrifugal force to produce castings from a rubber mold. Typically, a disc-shaped mold is spun along its central axis at a set speed. The casting material, usually molten metal or liquid thermoset plastic, is then poured in through an opening at the top-center of the mold. The filled mold then continues to spin as the metal solidifies or the thermoset plastic sets.

The two defining characteristics of spin casting are semi-permanent (non-expendable) rubber molds and the use of centrifugal force. These make the process relatively unique compared to machined die-based and expendable mold casting methods. These qualities also encourage operators to use casting materials specially formulated for low melting points and viscosities. Most spin casting is done with pewter and zinc alloys or thermoset plastics.

The spin casting process typically uses vulcanized silicone or organic rubber as the mold-making substrate. Vulcanization is an integral step that occurs halfway through the mold-making process. Prior to vulcanization, the mold rubber is a soft and malleable solid-like fluid, in many ways very similar to Silly Putty. Because of the clay-like nature at this stage, the mold is easily cut or shaped to accommodate irregular models. Vulcanization serves two purposes: establishing the negative space inside the mold as well as hardening the rubber so it will remain strong and rigid during casting.

After vulcanization, before it is usable, the mold must undergo gating and venting. This involves carving channels to ensure proper air and material flow during the casting process. Gating and venting is typically done by hand using a sharp knife or scalpel and varies in time depending upon the complexity of the mold. The final product is a cured rubber mold which can withstand anywhere from hundreds to over a thousand casting cycles before it needs replacement.

Generally, the casting materials used for competing processes like metal die casting and injection molding are similar, but not suitable for spin casting. For example, a typical zinc die-casting alloy such as Zamak 3 can be used but will solidify too rapidly from a molten state when cast with centrifugal force. This typically results in incomplete filling of the mold as well as a rough, porous finish, called orange peel. Zamak 2, of a slightly different composition, was originally developed as a gravity-cast alloy with greater finished strength but was found to work well with spin-casting. Its extra copper content encourages the eutectic behaviour and gives a lower freezing point. It has become known as 'Kirksite' and has given rise to a range of dedicated spin-casting alloys, some with additional components such as magnesium, to control the surface finish.

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