Abstract: A method for molding plastic materials which release gas during molding by operating a mechanical rotary crank type press having opposed metal die parts respectively carried on a slide driven by a crank mechanism and on a floating plate supported on a die cushion and movable against a bolster on the press. The method involves heating a blank of a plastic material which releases gas during molding to a molding temperature and placing it between the die parts. Then, the crank mechanism of the press is operated to drive the slide in a direction toward a bottom dead center position to a first position which is within a range of positions in the movement of the slide where only the reacting force of the die cushion is received by the blank and holding the slide at this first position for a time up to several seconds for permitting release of the gas from the plastic material of the blank.