A mechanical shut-off valve member for use with gas-assisted injection
molding systems. A piston member is slidingly positioned in a body member
or housing and spring-biased to a closed position. The valve member is
opened by the injection of plastic material through the valve member and
into a mold cavity. The spring biasing force closes the valve member and
shuts off the flow of plastic once the mold cavity is filled with the
requisite amount of plastic material. The shut-off valve member prevents
any backflow of plastic material into the nozzle or injection molding
machine during further steps in the injection molding process, including
injection of gas or fluid into the plastic in the mold cavity.