9321 Casting Types

Sand castings are made with a sand mold that is used only once. This method is used for larger castings without intricate details and that are produced in small quantities. The mold material is sometimes referred to as green sand or dry sand. Aluminum sand castings as large as 7000 lb (3000 kg) have been produced.

Permanent mold castings are made in reusable molds; sometimes the flow is assisted by a small vacuum but otherwise is gravity induced. Permanent mold castings are more expensive than sand castings but can be held to tighter tolerances and finer details, including wall thicknesses as small as 0.09 in. (2 mm). Semipermanent molds made of sand or other material are used when the geometry of the casting makes it impossible to remove the mold in one piece from the solidified part.

Die castings are made by injecting the molten metal under pressure into a reusable steel die at high velocity. Solidification is rapid, so high production rates are possible. Die castings are usually smaller and may have thinner wall thicknesses and tighter tolerances than either sand or permanent mold castings.

Investment castings are made by surrounding (investing) an expendable pattern (usually wax or plastic) with a refractory slurry that sets at room temperature. The pattern is then removed by heating and the resulting cavity is filled with molten metal.

Not all casting alloys are appropriate for all production methods, but some may be produced by multiple methods. Fewer alloys are suitable for die casting than the other methods.

New methods, such as squeeze casting and thixocasting, are showing promise in producing high-strength, ductile castings but have not been proven in aluminum yet. Thixocasting has more recently been called semisolid forming and may be thought of as a cross between casting and forging. Semisolid forming stock has a special globular crystal structure that behaves as a solid until sufficient shearing forces are applied during forming, upon which the material flows like a viscous liquid.

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