in the various force curves of fig8 a and 8 b so that during inward displacement from full closed position 40 to full open position 44 intermediate positions 42 exhibit greater closing forces thus increasing the snap type closure effect upon release of the valve actuator furthermore by constructing each of the valve elements as discussed above namely forming the valve body from a polypropylene copolymer having a minimum average wall thickness of 0 0625 inches and forming the valve seal from a thermoplastic rubber having an average thickness of about 0 032 inches the valve structure may be subjected to the vigorous sterilization processes necessary for using the valve in food applications including irradiating the structure at up to 5 0 mrad and subjecting the structure to high temperature chemical and steam sterilization processes without causing the valve structure to become brittle or otherwise jeopardizing the integrity of the valve 39 s structure or operation the non linear spring characteristic provides several significant advantages it can provide a substantial closing force at the full closed position and hence an effective seal with a low holding force at the full open position the user can keep the valve open while the liquid is flowing with only moderate effort the highest actuating forces are encountered only.