Casting is a process in which a liquid is poured into a mold in order to produce a product. There are several types of molds that are used in the casting process. Some are temporary and are destroyed during the casting process. Others are permanent and are reused again and again. The type of mold chosen is usually based on the requirements of the final product.
Sand molds are one of the oldest and most basic types of molds. Dry sand is pressed into a box (called a flask). A pattern made of wood or metal is pressed into the sand, creating a mold. Liquid metal or another material can then be poured into the mold, and the excess sand is brushed away when the part has cooled.
Vacuum forming can create many thin, weak molds in a short period of time. A sheet of plastic is suspended over a form, or "buck." The plastic is heated until it is soft, then pressed down on the buck. A vacuum is then turned on beneath the plastic to form it to the shape of the buck. These molds are strong enough to be reused, but thin enough to be cut from the final casting if needed.
Ceramic molds are typically employed with the "lost wax" process of casting. A wax model is created of the desired item. It is then encased in ceramic, leaving a small channel open. When the ceramic is fired the wax melts away, leaving a ceramic mold. The casting material can now be poured in, creating an exact replica of the wax model. When the casting material has hardened, the mold is shattered, freeing the cast.
Permanent molds are often made of steel, iron, silicone or urethane. The casting material is either poured in or injected. Once the cast has hardened, it is removed from the mold. Permanent molds can only be made of hard materials if there are no undercuts in the cast (areas where the cast and the mold lock together). Soft mold materials do not have this issue.
For especially complex castings, multi-piece molds must be used. Often these are two halves that are locked together while the casting is taking place, but molds comprised of many more pieces are used as the casting gets more intricate.
Die casting is the preferred method for creating many small castings. Liquid metal is forced under pressure into a steel mold, or "die." Often many identical castings are built into the same die, allowing rapid manufacture of parts.