DIE CASTING PROCESS
die casting process |
There
are four major steps in the die casting process. First,
the mold is sprayed with lubricant and
closed. The lubricant both helps control the temperature of the die and it also
assists in the removal of the casting. Molten metal is then shot into the die
under high pressure; between 10-175 MPa (1,500-25,000 psi). Once the die is
filled the pressure is maintained until the casting has solidified.
Finally, the die is opened and the shot (shots are
different from castings because there can be multiple cavities in a die,
yielding multiple castings per shot) is ejected by the ejector pins. Finally,
the scrap, which includes the gate, runners, spruces and flash, must be
separated from the casting(s).
This is often done using a special trim die in a
power press or hydraulic press. An older method is separating by hand or by
sawing, which case grinding may be necessary to smooth the scrap marks. A less
labor-intensive method is to tumble shots if gates are thin and easily broken;
separation of gates from finished parts must follow. This scrap is recycled by
remelting it.
The high-pressure injection leads
to a quick fill of the die, which is
required so the entire cavity fills before any part of the casting solidifies.
In this way, discontinuities are avoided even if the shape requires
difficult-to-fill thin sections.
This creates the problem of air entrapment,
because when the mold is filled quickly there is little time for the air to
escape. This problem is minimized by including vents along the parting lines,
however, even in a highly refined process there will still be some porosity in
the center of the casting. Most die casters perform other secondary operations
to produce features not readily ca stable, such as tapping a hole, polishing,
plating, buffing, or painting
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