plaster casting, as a sculpture process, is of three kinds. One employs a waste mold, another a piece mold (both plaster of paris), and the third a gelatin mold; all reproduce the original clay or wax model executed by the sculptor. The waste mold is chipped away (wasted) to free the hardened cast, which was poured in as liquid plaster. The gelatin mold, being pliable, may with care be sprung from the cast and removed intact and used for replicas. The piece mold also may be used again, being so divided as to be readily drawn away from the undercutting of the cast without damage to either. Plaster casts are used not only for the creation of new sculptures, but also for the numerous replicas of famous marble or stone statues. The ancient Egyptians used models of plaster taken directly from the human body. The Romans cast in plaster many thousands of copies of Greek statues. In another sense of the term, plaster casting refers to the surgical technique of encasing in a plaster-of-Paris cast any part of the body in which bones are broken so that the bones may set smoothly without interference by motion, jarring, or physical shock.

lost-wax casting: see cire perdue.

die-casting, process by which molten metal is forced by a plunger or compressed air into a metallic die and the pressure maintained until the metal has solidified. Die castings are accurate, are sharply outlined, have a good surface finish, and can be made in complicated designs. Zinc, aluminum, and magnesium alloys are the principal metals used. The high cost of the die usually limits the process to large-scale, high-speed production. Typical products are carburetor bodies and zippers. Type-casting machines are specialized die-casting machines.

casting, plaster: see plaster 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).

Traditional method of producing molds for metal sculpture and other castings. It requires a positive, a core made of refractory material and an outer layer of wax. The positive can be produced either by direct modeling in wax over a prepared core (direct lost-wax casting), or by casting in a piece mold or flexible mold taken from a master cast. The wax positive is invested with a mold made of refractory materials and heated to melt the wax, leaving a narrow cavity between the core and the investment. Molten metal is poured into this cavity. When the metal has solidified, the investment and core are broken away. Seealso investment casting.

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Precision casting for forming metal shapes with minutely precise details. Casting bronze or precious metals typically involves several steps, including forming a mold around the sculptured form; detaching the mold (in two or more sections); coating its inside with wax; forming a second mold, of heat-resisting clay, around the wax shell, and filling the interior with a clay core; baking the assembly (hardening the clay and melting the wax, which escapes through openings in the outer mold); pouring molten bronze into the space vacated by the wax; and breaking the mold to expose the cast form. In modern foundries, plastics, or occasionally frozen mercury, are used instead of wax. Seealso lost-wax casting, die casting.

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Forming metal objects by injecting molten metal under pressure into dies or molds. An early and important use of the technique was in the Linotype machine (1884), but the mass-production automobile assembly line gave die casting its real impetus. Great precision is possible, and products range from tiny parts for sewing machines and automobiles to aluminum engine-block castings.

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