Two blocks of heat resistant metal which are made to meet along parting line are used for die casting. The blocks have cavities to form opposite halves of shape to be cast. Vents are cut to allow air to escape as the metal enters the cavity so that formation of blow holes can be prevented.
Molten or semi-molten metal is poured into the cavity under gravity or under high pressure. When the metal is poured under gravity, it is known as gravity die casting. When it is poured under pressure, known as pressure die casting.
Gravity die casting: The molten metal is poured into the cavity under head. A feeder is used to maintain the required head. The level of metal in feeder is kept above the highest point of casting.
Pressure die casting: Force is applied to fill the mould cavity within fraction of second. Good surface finish and close dimensional tolerances can be obtained. It is advantageous for mass production.
Die casting machines:
There are two classifications of base metals for die casting i.e. metals having high melting point (above 500°C) and low melting point (below 500°C). The metals having low melting point are zinc, lead and tin. These metals have low die maintenance cost and lower cost of production. Zinc is best suited for die casting. The metals having high melting point are Aluminium, Copper and Magnesium based alloys. Due to high temperature there are lots of difficulties in casting these alloys.
Depending upon the location of melting chamber, there are two types of die casting machines.
Hot chamber die casting machine: This type of die casting machine is used for base metals having low melting point i.e. zinc, tin and lead. The melting unit forms the integral part of the machine. Metals having high melting point can not be cast because they can damage the dies. The die is made in two parts i.e. stationary die platen and moveable die platen.
Cold chamber die casting machine: The machine is used for materials having high melting point and require high pouring pressure such as Aluminium, magnesium and brass. The melting chamber is not an integral part of the machine. The metal is melted in an auxiliary furnace. The castings produced are very dense so defects like porosity and blow holes are eliminated.
Advantages of die casting:
- Close dimensional accuracy can be obtained.
- Good surface finish is obtained thus finishing operations are minimum.
- Due to high density of casting, defects are eliminated.
- Less labour cost.
- High production rate.