3.4 VACUUM DIE CASTING MANUFACTURING EQUIPMENT
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Figure 3.3 Example operating curve for a vacuum pump.
ification shrinkage porosity. However, the small gates typically used in conventional die casting freeze quickly, creating a barrier,
which inhibits pressurization within the die cavity. Vacuum die casting offers no additional benefits with regards to solidification
shrinkage porosity, in comparison to conventional die casting.
3.4 ELEMENTS OF VACUUM DIE CASTING
MANUFACTURING EQUIPMENT
The vacuum die casting process utilizes a conventional die casting machine coupled with a vacuum system. This system is composed
of a vacuum pump, a vacuum shut-off valve, a vacuum control system, and an unvented die.
Vacuum pumps must be sized in accordance with the volume of the gas, which must be evacuated from the die cavity during
fill. To aid in sizing, all vacuum pumps have a characteristic op- erating curve that compares air flow with vacuum level. An ex-
ample operating curve is presented in Figure 3.3. Vacuum pumps should be capable of pulling a vacuum of at least 725 mm Hg for
use with vacuum die casting.
Rotary vane vacuum pumps are commonly used for vacuum die casting. Figure 3.4 is an illustration of such a pump. Within the
36
VACUUM DIE CASTING
Oil Mist Eliminator
Exhaust Gas
Ballast Inlet
Inlet Screen
Anti- Suckback
Valve
Vane
Rotor Oil
Return Line
Oil Return
Valve
Exhaust Valve
Spin-On Oil Filter
Main Oil Feed Line
Figure 3.4 Illustration of a rotary vane vacuum pump. Courtesy of Busch,
Inc.
pump cylinder is an eccentric rotor that pulls the vacuum. As the rotor turns, gases are trapped and compressed between several
vanes and the walls of the pump cylinder. The compressed gases are discharged into the exhaust box. The gases then pass through
an oil eliminator that extracts oil vapors from the exhaust gases prior to discharging them to the environment. Numerous vacuum
pumps are commercially available, as shown in Figure 3.5. Often the systems are portable, making for easy placement within the
die casting facility.
In vacuum die casting, a shut-off valve is needed to prevent liquid metal from entering the vacuum pump. A runner is used to
