Abstract: In the manufacture of O-rings by compression molding, the mold parts are so shaped as to provide the O-ring with a flattened sealing surface along the outer and inner peripheral portions of the ring, with the mold parting line along one margin of each sealing surface. Optionally the ring may be provided with additional annular flattened sealing surfaces at the upper and lower peripheral portions of the ring body.
Abstract: In accordance with the present invention a positive pressure mold is provided wherein the mold cavity is sealed during the initial movement of the dies. The entrapped air is caused to escape at a controlled rate as the dies move together, building up a sufficient air pressure between the dies to distribute the workpiece or "preform" throughout the die cavity.