In the two permanent-mold processes described previously, the molten metal flows
into the mold cavity by gravity. In pressure casting (also called pressure pouring or
lou/-pressure casting), the molten metal is forced upward by gas pressure into a graphite or metal mold. The pressure is maintained until the metal has solidified
completely in the mold. The molten metal also may be forced upward by a vacuum,
which also removes dissolved gases and produces a casting with lower porosity.
Pressure casting generally is used for high-quality castings, such as steel railroad-car
wheels, although these wheels also may be cast in sand molds or semipermanent
molds made of graphite and sand.