Permanent mold casting is a non-expandable casting technique where the mold is not reformed after each casting is made. In this method, instead of using sand as the mold material, a metal is used as a mold. Typically, cast iron or Meehanite (a dense cast iron) is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate.
Molds are pre-heated up to 200 °C (392 °F) before the metal is poured into the cavity. The cavity design for these molds does not follow the same rules for shrinkage as in sand casting molds, due to the fact that the metal molds heat up and expand during the pour, so the cavity does not need to be expanded as much as in sand casting. However, a proper thermal balance is essential. External water cooling or appropriate radiation techniques may be used to achieve this.
This technique includes at least four different methods: permanent, die, centrifugal, and continuous. Permanent mold castings are mostly used in mass production jobs in the automotive industry. For example, gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons can all be cast using this method. Typical weights for these types of castings range from 50 grams (0.10 lb.) up to 70 kilograms (150 lbs.).
In permanent mold casting, solidification occurs much more rapidly than in sand casting. The main advantage is that a permanent mold can be used repeatedly for multiple metal castings. The mold, also called a die, is commonly made of steel or iron, but other metals or ceramics can be used. Non-ferrous metals are typically used in this process, such as aluminum alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite molds.
A slight variation of the permanent mold casting process involves hydrostatic pressure created by the risers facilitating the casting of metal in the mold, rather than applied external pressure. This process is referred to as gravity die casting. Other variations include slush, low-pressure, and vacuum casting.
The castings produced by a permanent mold method are generally better than sand castings and are stronger than sand or die castings, and less porous. In this method, the castings have finer dendritic arm spacing (DAS) and grain structure. This finer grain structure offers better strength properties than sand castings of similar alloys. Permanent mold castings have fewer inclusion defects than sand castings. The casting designer can use thinner sections and lighter weight for designs. The castings produced by this method have a higher degree of reliability, finer surface finish, and higher production rate.
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