High Pressure Die Casting enables economical quantity production of castings at a rapid rate. These castings can comprise of various holes, screw threads, recesses, and are characterized by a high dimensional accuracy and good surface finish. They enable good economy of metal and require little or no surface finishing. The principal on which pressure die casting is based, consists of forcing molten metal into a mold or the die, under considerable pressure. The machines which are used for this purpose operate on one or two systems:
Hot Chamber Machines (for metals with a low melting point)
Cold Chamber Machines (for metals with a high melting point)
In the hot chamber machines 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 the metal bath. This machinery can be pneumatically operated, or more usually develop the pressure by the action of a ram. The casting metal for a cold chamber machine, is kept liquid in a holding furnace, from which it is transferred to the pressure chamber, by means of a scoop or a special automatic device and is forced into the die by the means of a ram. Some simple machines are hand operated, but fully automatic machines are more commonly employed for high rates of production.
In Pressure Die Casting, precision made dies of intricate multipart design and therefore very expensive are used, and exposed to severe working conditions, by high pressures and high successive variations in temperature. For the production of zinc and zinc alloy castings the dies may be made of unalloyed steel, however for magnesium, aluminum, copper and the alloys of these metals, the dies are generally made of hot-work steel, which has much greater durability.
Another development in high pressure die casting is vacuum die casting. It produces castings which have an even better finish than ordinary high pressure castings. There are two systems, the die is either enclosed in a hood which evacuates the air, or the holding furnace is installed under the Pressure Die Casting Machines, so that on the evacuation of the air from the die, the metal is sucked into the die and is compacted in it, and a process for making iron castings based on this principle has been developed.
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