Metal casting molds are classified in two types: open and closed. With open molds, molten metal cools down inside a cavity with the opening exposed to the environment. Closed molds, the preferred type for manufacturing, have a small delivery system through which molten metal reaches the enclosed cavity and hardens inside. Metal casting processes depend on the material used to make the molds. These processes are expendable mold casting and permanent mold casting.
Expendable mold casting employs nonreusable materials to make the mold for a single casting. This allows for casting complex shapes but the mold is destroyed to remove the finished piece. The preferred mold materials for expendable mold casting are sand, plaster and rubber.
Permanent mold casting reuses the same mold repeatedly. Opening and closing different sections of the mold allows the removal of the finished piece, limiting the complexity of the shape produced. Gravity, vacuum or gas pressure are used to fill the mold. Permanent molds are made of metal or refractory ceramic. Graphite is also used when casting tin, zinc, lead alloys and iron.
Patterns used to shape the molds are made with wood, plastic, aluminum, cast iron or steel. The material used to make the pattern depends on the chosen casting process, the mold type, the shape complexity and the amount of castings to be made.