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Saturday, May 24, 2008

Casting

Casting is a process for producing a shaped object by pouring a fluid substance into a mold and allowing the substance to solidify. Although most materials that have a fluid state can be cast, and such substances as glass, clay, and metal have been cast since ancient times, modern industrial casting processes involve primarily metals and plastics.
The mold is a matrix made from an impressionable material such as sand. A pattern of the object to be molded is pressed into the sand, and the impression left is filled by the casting material, which hardens into the shape of the pattern. Of the many types of molds employed in casting, most use sand as the mold material. Other type can be made using metal, cement, or ceramic.
Sand Casting
The most common molding material used in sand casting is green sand, which is a mixture of sand, usually ordinary silica sand, clay, water, and other binding materials. The molding sand is placed in a two-part form and is packed around the pattern. When the sand is compacted sufficiently, the two parts are separated and the pattern is removed.
Opening in the mold, called gates and risers, are added to allow for the escape od air and gases as the meal is poured into the mold. Large castings may be made with a central core of sand that forms an internal cavity in the finished casting. For metal alloys that react negatively with moisture, dry sand molds are used.

Investment Casting
Investment casting is an adaptation of the cire perdue, or Lost Wax Process. It is used industrially to produce highly detailed precision parts. The pattern is melted out by heating the mold or vaporized as the molten metal is poured in. the pattern may be made of wax coated with fine silica, or of foamed plastic.

Shell Molding
In this casting process, the pattern is made of metal heated to a high temperature (177o-371oC or 350o-700oF). The molding material is a mixture of sand and heat setting resin. When packed around the heated pattern, the resin sets, binding the sand into a thin shell that reproduces the pattern exactly.

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