The pattern (with vent holes) is placed on a hollow carrier plate.
A heater softens the .003" to .008" plastic film. The plastic has good elasticity and a high deformation ratio.
The softened film drapes over the pattern with 200 to 400 mm Hg vacuum acting through the pattern vents to draw it tightly around the pattern.
The flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with the outlet shown at right.
The flask is filled with dry, unbonded sand. A slight vibration compacts sand to maximum bulk density.
A sprue cup is formed and the mold surface leveled. The back of the mold is covered with unheated plastic film.
Vacuum is applied to the flask. Atmospheric pressure then hardens the sand. The vacuum is released, pressurized air is introduced into the carrier and the mold is stripped
The cope and drag assembly form a plastic-lined cavity. During pouring, molds are kept under vacuum.
After cooling, the vacuum is released and free-flowing sand drops away leaving a clean casting, with no sand lumps. The sand is cooled for re-use.
