Pressure die casting makes possible the production of intricate castings at a rapid rate. The castings may comprise of various recesses, holes and so on, and are characterized by high dimensional accuracy, good surface finish and excellent economy of metal, and usually require very little or no final machining.
The principals of high pressure casting consist of forcing molten metal into a mould or a die under very high pressure. The machines used for this purpose operate on various systems; hot chambers for zinc metals and other metals with lower melting points and cold chamber for aluminium metals with a high melting point. With the hot type, the metal is kept liquid in a crucible inside the machine, and the pressure chamber that delivers the metal into the die is located in a crucible. This type of machine can have a pneumatic operated or hydraulic operated injection system. The casting metal for the cold chamber machine is kept in a holding furnace from which it is transferred to the pressure chamber by means of manual ladling or special automatic device, and is forced into the die by means of a ram. The simplest machines are hand operated, but fully automatic machines are more usually employed, which makes possible the high rates of production.
There are fully automatic machines that can produce small zinc alloy castings at a rate of more than fifteen hundred cycles per hour. In pressure die casting, precision made dies, sometimes of very intricate multipart design are employed, which are exposed to very severe working conditions characterized by very high pressures and a large number of successive variations in temperature.
For the production of zinc and zinc-alloy castings the dies may be made of unalloyed steel; however for magnesium, copper, aluminium and the alloys of these metals the dies are usually made of hot-work tool steel which has greater durability.
A complete cycle of operations comprises closing the die, forcing the molten metal into it, withdrawing the cores, opening the die, ejecting the casting, and if required shearing off the sprue, deburring the casting and cleaning the die. The number of cycles per hour that a casting maching can achieve will depend on the size and shape of the castings and the metal used. With zinc alloys it is possible within a given period of time to produce about seven times the number of castings that can be made with brass.
Another type of casting is vacuum die casting, and this method produces castings which have an even better finish than ordinary pressure castings. There are two systems; either the die is enclosed within a hood which evacuates the air, or the holding furnace is so installed under the casting machine that on evacuation of the air from the die, the metal is sucked into the die and is compacted into it.