casting

casting or founding, shaping of metal by melting and pouring into a mold. Most castings, especially large ones, are made in sand molds. Sand, mixed with a binder to hold it together, is pressed around a wooden pattern that leaves a cavity in the sand. Molten metal is poured into the cavity and allowed to solidify. Permanent metal molds are used to make many small, simple parts; shell molding gives greater accuracy for a large volume of semiprecision parts. A two-step process, investment casting, produces small, complex shapes. Wax or plastic replicas of the parts are molded in accurate metal molds. These replicas are covered with sand in a box to make the final mold. When the whole mold is heated, the replica melts, leaving behind a cavity into which metal is poured. Large numbers of small, precise parts of metals that have a low melting point, such as zinc, are made by die-casting ; in an automatic process, molten metal is forced under pressure into metal molds. Cast iron and cast steel are more brittle than forged iron and forged steel (see forging ).

casting Forming objects by pouring molten metal into moulds and allowing it to cool and solidify. Specialized processes, such as plastic moulding, composite moulding, cire perdue casting and die casting give greater dimensional accuracy, smoother surfaces and finer detail.