Step 1: The pattern (with vent holes) is placed on a hollow carrier plate.
Step 2: A heater softens the .003″ to .008″ plastic film. Plastic has good elasticity and a high deformation ratio.
Step 3: 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.
Step 4: The flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with the outlet shown at right.
Step 5: The flask is filled with dry, unbonded sand. A slight vibration compacts sand to maximum bulk density.
Step 6: A sprue cup is formed and the mold surface leveled. The back of the mold is covered with unheated plastic film.
Step 7: 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.
Step 8: The cope and drag assembly form a plastic-lined cavity. During pouring, molds are kept under vacuum.
Step 9: 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.