Die-casting and metal injection moulding

Die-casting is the name given to the production of castings by processes which make use of permanent metal moulds or dies. The term permanent is used since the moulds can be used to produce thousands of castings before they require replacement, unlike those used in sand casting, where each mould is destroyed to remove the casting. 

The basic principle of all die-casting is for the die, which contains a cavity of the required shape, to be filled with molten metal. The die is usually in two parts, but may often have movable pieces or cores, depending on the complexity of shape being produced. 

With the two die halves securely locked in the closed position, molten metal is introduced into the die cavity. When the molten metal has solidified, the die halves are opened and the casting is removed or ejected. The die halves are then closed and the operation is repeated to produce the next die-casting. With fully automatic machines this complete cycle can be done extremely quickly. 

Each fill of metal is known as a shot. It is the way in which filling is done which distinguishes the different die-casting methods. 

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The Injection moulding process originated with metal die casting and was first used for polymers in 1878 . The technique Is recorded as being used for ceramics fabrication In 1937 but development of the ceramics process over the last fifty years has not been systematic and reporting of experimental detail has been sparse. In the last ten years there has been renewed Interest In the process as ceramics are used Increasingly In mass produced devices. 

Permanent mould casting involves the injection of molten metal into a metal mould. The mould is opened after solidification and the easting is taken out for finishing. Chemieally-bonded sand cores are used to a limited extent in gravity and low-pressure die-casting. 

For higher melting point alloys, e.g. aluminium-based alloys injected at temperatures above 500°C, cold-chamber die casting is employed. Here a transfer chamber adjacent to the mould is charged with molten metal, which is then forced into the die. Both vertical and horizontal machines are available. An example of the horizontal cold-chamber process is shown in Fig. 8.16. 

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