This invention relates to plastic molding presses and other types of pressure plates (16, 10), and more particularly to novel means by which to ensure against application of high closure pressure to the mold sections of a press when a solid impurity is on one of the confronting mold surfaces.;This invention involves the passage of a gas under a predetermined superatmospheric pressure from an inlet opening (30) in the plane of the confronting surface of one of a pair of pressure plates (16) to an aligned outlet opening (40) in the plane of the confronting surface of the other of the pair of pressure plates (10), and the detection of the air pressure in the outlet opening (40), a drop in air pressure indicating that a solid obstacle or other impurity between the confronting surfaces has prevented closure of the pressure plates (16, 10).
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