Permanente mold

At foundry instead of using sand a metal is used as a mold, typically cast iron is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate. The molds are pre-heated before the metal is poured into the cavity and care has to be taken to ensure proper thermal balance by using external water cooling or appropriate radiation techniques.

Permanent mold castings, while not as flexible as sand castings in allowing the use of different patterns, lower the cost of producing a parta, at a production run of a thousand or more parts permanent mold castings produce a lower piece cost part.

(via Efunda)