Permanent mold casting is a process of fabricating a cast part using a mold that can be reused multiple times, as opposed to processes such as lost wax and lost foam sand casting. The molds used in permanent mold casting are generally made from cast iron or machined from high temperature steel to withstand the heat of the molten material poured into it.
Permanent Molds
A permanent mold has at least two parts that are clamped together during the casting process. The mold can be made from cast iron, steel, or other metals. Ejector pins can be incorporated into the mold to assist with removing the part from the mold after the cast material has solidified. Cores can be inserted into the mold to create interior shapes. These cores are generally made from bonded sand, plaster, or graphite.
Permanent Mold Casting Process
The interior of the mold is generally coated with a wash to prevent the molten material from sticking to the mold as the material cools. The mold is pre-heated before casting so it does not crack from the thermal shock of the molten material. Gravity is generally used to feed the molten material into the mold, but low pressure can also be used to ensure material flow into complex designs. Because the mold expands with the heat of the molten material, and shrinks as the material cools, shrinkage does not have to be designed into the mold, as must be done for other casting methods.
The same part design considerations for other casting methods also apply to permanent mold casting, including draft angle, wall thicknesses, and dimensional tolerances.
Materials Used in Permanent Mold Casting
The permanent mold casting process can be used for any metal with a low melting point, including aluminum, brass, copper, and magnesium. Materials with higher melting points can damage the mold, reducing their life. The materials are heated just past their melting point to prevent thermal damage to the mold during casting.
Benefits of Permanent Mold Casting
Permanent mold casting is generally used for larger production quantities to keep the cost per part down. It is easier and less expensive to cast complex parts using permanent mold casting. Gears, pistons, and pipe fittings are examples of parts created using the permanent mold casting process. The metal mold has better heat transfer characteristics than the sand used in lost wax methods, so the resulting part cools faster and has a smaller grain structure in permanent mold casting.
