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Casting methods

Casting methods

Iron castings can be made in many ways, but sand-casting is the most common. First, a pattern of the required shape (slightly enlarged to allow for shrinkage) is made in wood, metal, or plastic. It is then placed in a two-piece molding box and firmly packed in sand that is held together by a bonding agent. After the sand has hardened, the molding box is split open to allow the pattern to be removed and used again, and then the box is reassembled and molten metal poured into the cavity to create the casting.

A greensand casting is made in a sand mold bonded with clay, the name referring not to the colour of the sand but to the fact that the mold is uncured. Dry-sand molds are similar, except that the sand is baked before receiving any metal. Alternatively, hardening can be effected by mixing sodium silicate into the sand to create chemical bonds that make baking unnecessary. For heavy castings, molds made of coarse loam sand backed up with brick and faced with highly refractory material are used.

Sand-casting produces rough surfaces, and a much better finish can be achieved by shell molding. This process involves bringing a mixture of sand and a thermosetting resin into contact with a heated metal pattern to form an envelope or shell of hardened sand. Two half-shells are then assembled to make a mold. Wax patterns also can be used to make one-piece shell molds, the wax being removed by melting before the resin is cured in an oven.

For some high-precision applications, iron is cast into permanent molds made of either cast iron or graphite. It is important, however, to ensure that the molds are warmed before use and that their internal surfaces are given a coating to release the casting after solidification.

Most castings are static in that they rely on gravity to cause the liquid metal to fill the mold. Centrifugal casting, however, uses a rotating mold to produce hollow cylindrical castings, such as cast-iron drainpipes.

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