Softened film drapes over the pattern with 300 to 600 mm Hg vacuum acting through the pattern vents to draw it tightly around pattern. Flask is placed on the film-coated pattern. Flask walls are also a vacuum chamber with outlet shown. Flask is filled with fine, dry unbonded sand. Slight vibration compacts sand to maximum bulk density.
Sprue cup is formed and the mold surface leveled.
The back of the mold is covered with unheated plastic film. Vacuum is applied to flask. Atmospheric pressure then hardens the sand. When the vacuum is released on the pattern carrier plate, the mold strips easily. Cope and drag assembly form a plastic-lined cavity.