Metal moulding involves heating metal to a liquid state then pouring the liquefied metal into a hollow cavity and allowing it to resolidify in the desired shape. This process is known as casting. The solidified part, also called a casting, is removed from the mould, after which further processes may be used to complete the finished item. This is a versatile method, allowing the manufacture of complex metal shapes which would otherwise be either too difficult or simply not cost-effective to make by other means.
The first step in the process of metal moulding is design. Good mould design is complex, involving many important elements. Firstly the purpose of the item must be considered. Will the finished item be strong enough? Will it be sufficiently resistant to cold, heat, and other physical effects? Will it be too brittle or not hard enough?
Just as important is the way the metal will behave while being poured into the mould and then in turning from liquid to solid. Making sure the molten metal can be poured correctly is very important. If it moves to quickly or too slowly, or if there is too much turbulence, flaws and defects can occur in the finished product.
The cavity the metal is poured into is formed by a pattern that is a model of the intended metal casting, used to create the cavity where the molten metal will go. Common materials for pattern-making include wax and wood.
The mould is made by packing or covering the pattern in moulding material (for example, ceramic, sand or another metal with a higher melting point). When the mould is prepared, the liquid metal can be added. This is called "filling."
As the metal in the mould cools, it solidifies. It is very important that this process is controlled because it is here that many problems can develop leading to defects in the finished casting. As it cools the metal shrinks, and this shrinkage is a source of potential defects. Other problems can be caused if the metal solidifies too fast or too unevenly.
This general process falls into two main types: expendable and non-expendable casting. These categories can be further broken down according to the material used to make the mould, and the method used for pouring the molten metal into the mould.
The simplest and probably oldest method is to allow gravity to force the metal into the shape of a mould; other methods include high- and low-pressure moulding and vacuum moulding.
Expendable casting uses temporary, single-use moulds that are destroyed in the process of removing the casting. These include some plaster moulds, sand moulds and more. One type of expendable casting is called investment casting. Non-expendable casting involves permanent reusable moulds, often made from metal, as in die-casting, where a permanent metal "die" is filled with the molten metal (usually through gravity, but sometimes using high or low pressure).