Plastic injection moulding machine with automatic shut-off valve

A plastic extruder comprising a rotating helical screw within a cylindrical shell (10). An automatic shut-off valve (24) is mounted at the forward end of the helical screw. The valve includes a valve body (34) and a poppet (36) which is axially movable and is rotatable relative to the valve body. A rearwardly extending axial bore (48) is provided at the forward end of the screw and the valve body is secured to the screw within said bore. The poppet has an axial stem (60) which extends through the valve body into the axial bore. A compression spring (52) is mounted on the rearward end of the stem to urge the poppet against the valve body and maintain the valve in a normally closed position. Flow of plastic through the valve upon rotation of the screw causes the valve to open. However, the valve automatically closes upon cessation of screw rotation to prevent back-flow of plastic during the injection step.