How are castings made?
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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 ).
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