Molds by themselves can only have the external shape of the pattern. That is why cores are placed inside the mold to form internal cavities. Cores are produced in a core box, which is a permanent mold, developed in tandem with the pattern. It helps in flowing the molten metal to all sides of the cores.

Cores are supported either on core prints or by metal supports called chaplets. Generally the foundry molds are made of sand grains bonded together to form the desired shape of the casting. Sand is used because it is cheap, resists deformation when heated, offers a great variety of casting sizes and complexities. It also offers the added advantage of reuse of a large portion of the sand in future molds. However pattern making, melting, cleaning, and finishing operations are essentially the same whether or not sand molds are used in the casting process.