Permanent-Mold Process

In permanent-mold casting processes, the mold (die) is made of a metal, and the shape is limited to geometries that can be realised from two or more mold sections. Internal cavities and holes can be formed with retractable cores, as in casting aluminum-alloy pistons, but at the expense of higher cost and greater complexity of machinery. Even greater complexity is achievable when expendable cores are inserted. In all permanent mold casting, porosity can be a problem when casting alloys of dendritic solidification pattern.

The metal mold can be fed by gravity (permanent-mold or gravity die casting) or under pressure (low-and high-pressure die casting). The latter ensures filling of thinner mold sections, especially when allows of lower-melting points are cast. Highly complex shapes, such as machine housings and transmission cases are cast to close tolerances. Casting of higher-melting temperature alloys is limited by rapid wear of the die. Direct transfer of the melt (hot-chamber die casting) is also limited to casting of lower-melting point alloys (aluminum as the maximum), this allows reducing or eliminating sprue and feeders. Porosity and voids are reduced or eliminated if high pressure is applied just prior to final solidification, or if the alloy is formed at a temperature between the liquidus and solidus temperatures. Such mushy-state forming (variously know as squeeze casting, melt forging, or semisolid metal working) represents a transition between casting and hot forging processes.