Pressure die casting is the injection of melted metal or plastic under high pressure into a mould cavity.

Before injecting the melted metal into the cavity, the mould is ?closed? i.e. the two halves of the mould, called dies are brought together, after which dies are held together while the melted metal is forced into the cavity they form. The metal is allowed to solidify in the shape of mould cavity and then the dies are pulled apart so that solidified object may be ejected and the cycle repeated.

To manufacture die castings free of pores and shrink holes it is normal practice to fill the mould at high pressure and to let the metal solidify while under high pressure so as to effect compression of die casted metal. The apparatus which closes the dies and holds them together needs to have capacity to withstand this high pressure and it is required to work for long hours.

To simplify the clamping procedure for holding the dies together the apparatus uses a die casting machine which has one die fixed on the machine base and second die half is removable into and out of apparatus. Thus movement and control apparatus might be given for one half of die. This has an additional advantage that the hot melted metal can be fed into the mould cavity or chamber between the dies through a sprue in the fixed die.

Aluminium and copper alloys that attack and erode machine components with which they are in regular contact are generally produced in what is called cold chamber machine, whereas tin, lead and zinc die castings are processed in a chamber called hot chamber machine. The pressure die casting machine is equally well used in hot and cold chamber pressure die casting.

A die casting machine uses a fixed and moveable die. Four tie bars or guideways are rigidly connected to two plates upstanding from the machine lease; one die is placed on one plate while the other die is slidably mounted on the tie bars. Pressure oil is given to an operating cylinder containing an operating piston which is linked to a displacement yoke linked in turn with guide spars these are externally threaded to receive nuts between which the yoke is clamped.

The dies are closed and melted metal is entered into cavity of dies from the sprue in the fixed die and required pressure is given. After the metal solidifies the pressure on the dies is released and two halves of dies are separated and finished product is taken out and process of pressure die casting is repeated.

The same process of pressure die casting is used in plastic moulded die casting products the only difference being that instead of melted aluminium, melted plastic is poured into the die from a sprue fixed in lower die and required pressure is given. After the plastic cools down and solidifies the pressure is released, and die opened and finished product is taken out and process repeated.