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The Die Casting Process
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The Die Casting Process uses two permanent mold halves of a die, mounted in a press and brought together. Molten metal is injected into the die through a runner and gate with pressures in the area of 2000-5000 psi. Air escapes into overflow wells, and out vents, as metal fills the molds. The mold is chilled, and the injected metal freezes. the mold is separated, and knockout pins eject the part (see diagram below). Cycle times are typically given in seconds. The parts are cut off the runners and sprues.
The Metals used in die casting are low melting point, non-ferrous metals. Most die casting is done using aluminum and zinc, with smaller, but growing, industries in magnesium, copper, lead and tin.
The Dies are typically made of carbon steel or special alloys and must withstand high pressures and extreme temperature fluctuations. Dies may incorporate multiple cavities to lower part cost, which is proportional to machine time and materials. Dies often require water lines to enhance cooling and decrease cycle time..
1) Molten Metal is ready for injecting and the die halves are pressed together
2) Molten Metal is injected into the die
3) The die halves are separated
4) Knockout pins eject the part
Advantages of Die Casting: Although the initial tooling cost is high, die cast parts are produced in short cycles and thus have a low unit cost. Die castings parts are created under high pressure, allowing for thinner wall sections, better surface finish, more intricate geometry and tighter tolerances. These advantage provides the benefit of a cast to net shape or nearly net shape, thus cutting down on the cost of the secondary machining and finishing operations. Also, inserts are feasible.
Case Study: Although die casting is most often used for large production runs, in many instances the savings in part price is enough to justify the tooling costs even at relatively low yearly quantities. The connector part shown at right was designed by a major computer manufacturer. Since the yearly requirement was only 750 pieces, the computer manufacturer had used a lost wax investment casting process. The tolerance requirements for the part, however, included one parallelism and three flatness specifications. With all the secondary machining required to meet those specs, part price was $65.
Using modern die casting technology, Twin City Die Castings was able to meet the flatness and parallelism requirements directly out of the cast machine. Secondary machining was limited to tapping two holes. All this reduced part price to $3 a copy, and the customer regained the $20,000 cost of the die cast tooling before the 350th part had been produced.
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Last modified February 24, 2008