A shutoff valve mechanism for a plastic injection molding system and process. The shutoff valve mechanism is positioned between the nozzle of the injection molding machine and the hot runner manifold. Force applied by the machine nozzle moves an inner body member in the shutoff valve mechanism and opens a valve allowing molten plastic material to flow through the valve and into the hot runner, sprue bushings, and mold cavities. A biasing mechanism, such as a plurality of spring washer members, returns the inner body member to its rest position, thereby closing the valve mechanism when the machine nozzle is retracted.