Of all the casting processes, i.e. casting is by far the best suited to the high production of quality parts. There are some restrictions as to the metals which can be supplied as die castings and as to the complexity of the shapes produced, but other than that the process has much to offer.
Several types of die casting machines are used, some of them specifically for certain metals. All of them involve the forcing of carefully measured quantities of molten metal under pressure into closed dies. When small parts are involved, multiple cavity dies are used to multiply production rates.
Most machines now in use are capable of handling a minimum of 100 shots per hour and some can be rated at even higher speeds. The high rates cannot be used on extremely large pieces, however, because of the longer times required to fill the dies, permit the metal to solidify and eject the finished parts from the machines.
The mechanical properties of die castings tend to be somewhat lower than the properties of identical metals in other cast or wrought forms. This situation can be charged chiefly to a slight porosity which results from the rapid freezing of the metal in the steel dies. Despite this, many die castings are used for applications where reasonably high mechanical strength is required. Usually the metal structure on the outside surfaces of die castings is excellent due to the formation of fine grained structures through rapid cooling.
Generally speaking, the complexity of design of die castings is restricted because of the problem of removal of parts from solid dies. However, ingenious die design involving the use of complicated sliding die mechanisms can provide greater flexibility of design. Such die construction is expensive; thus, the savings expected through the reduction of further processing of the part would have to be great to justify the added expense.
Most difficulty is encountered with undercuts and cross holes. For many years die castings were considered for small parts only, but in recent years much work has been done on larger pieces such as the internal mechanisms of automobile doors and automotive engine blocks. If special machines are available, parts weighing as much as 75 lb in aluminum and 200 lb in zinc are practicable.
Some die casters specialize in extremely small pieces and produce them at attractively low cost. As with most processes in the precision group, die casting is capable of producing parts to extremely close tolerances.
Again, within limits, nearly any degree of accuracy can be furnished if costs are no object. Ordinarily, the best tolerances are to be had with zinc die castings, and these are in the order of ±0.001 in. per in. of casting length. Somewhat more liberal tolerances must be allowed for other metals. Minimum draft allowances of from 0.005 in. for zinc and 0.020 in. for copper-base alloys are required for removal of castings from the dies. More liberal allowances are desirable, however.
Large masses of metal are not good in die castings, and usually section thickness should be less than 1 in. In greater thicknesses, internal shrinkage can become a problem. On small parts, section thicknesses as low as 0.025 in. are possible. Minimum walls must be greater on larger pieces and with other metals. The surface finish of die castings is such that machining is seldom necessary.With highly polished dies, a finish of 60 rms is possible, although ordinarily 100 to 125 rms is to be expected.
Most die castings are made of zinc, aluminum or magnesium, but some are produced in copper-base alloys, lead and tin. The relatively high melting point of copper-base alloys results in considerable die wear and, therefore, makes the process more expensive for these metals. Zinc has the advantage of taking plated coating readily without any preliminary work.