Some casting techniques are simple and can be easily mastered by hobbyists and craftsmen who are willing to operate a furnace and work with liquid metal. Other techniques are more demanding and the province of a larger operation.
Metal casting is the pouring of liquid metal into a mold, which has a hollow cavity in a desired shape. The liquid metal is then allowed to cool and harden. The now-solid casting is then ejected from or broken out of the mold. This is done to create objects that would be uneconomical to create by sculpting, boring, carving or other means.
This type of metal casting involves making a mold in a sand mixture and then pouring liquid metal into the sand cavity. This is a simple six-part process: (1) Use a pre-existing pattern to create a sand mold, or craft one by hand. (2) Add your gating system (to control the liquid metal). (3) Remove the pattern (if used). (4) Pour in the metal. (5) Cool. (6) Remove the casting. This is the best form of casting for a small operation that will be making castings in small batches. A variant of this is shell molding. This is similar to sand casting, except with the added touch of using resin to hold the sand together. This requires a much longer lead time and requires the heating of the mold between castings, but it has the advantage of allowing castings to be turned out more rapidly.
Using plaster to make a mold has the advantage that a plaster mold is easy to make. If the mold is damaged in ejecting a metal casting, turning out a new one is a simple affair (often, a plaster mold is, in itself, a plaster casting). This method is used for metal alloys based on aluminium, zinc and copper (i.e., not things like iron) for the manufacture of lock components, fittings, gears, ornaments and valves.
This involves forcing molten metal into cavities under high pressure. First, the mold is lubricated and closed. Then liquid metal is shot into the die/mold under high pressure. The pressure must be sustained during the casting process. Finally, the die is opened and the shots (not casting; shots differ from castings since there may be multiple cavities in a die, yielding multiple castings per shot) are removed. This is an industrial process for producing a large number of small- or medium-sized pieces that must have a high quality of detail and consistent features. It is commonly encountered when a toy or part is "die-cast."
Permanent Mold Casting
This is what most people think of when they imagine casting. Two halves of a mold are joined, and liquid metal is poured into the mold through a hole in the top. The metal is allowed to cool, and the casting is struck by separating the two halves of the mold. Old-fashioned bullet casting is done this way.