Plastics-processing machine with combined performance of extrusion and injection

A plastics extrusion-injection molding machine having:

• a cylinder including three portions for supply, compression and melting through which plastic material is conveyed in that order,
• a screw having a longitudinal axis rotatably inserted in said cylinder for conveying plastic material through the clearence between said cylinder and said screw,
• a nozzle attached to the downstream end of said cylinder for extruding and injecting the plastic material there through into a mold which is removably attached thereto, and
• a valve mounted in said nozzle for controlling the opening of the same, the improvement which comprises:
• boosting means formed in the supply portion of said cylinder for allowing the plastic material to fully occupy such portion of said clearance as corresponds to said supply portion so as to boost the pressure upon the plastic material within the cylinder,
• back-flow means in the melting portion of said cylinder for allowing the plastic material to flow backwardly therethrough to mitigate abnormally high pressure from being built up in said melting portion, said back-flow means comprising a plurality of grooves that extend through the threads of said screw, and are bounded by surfaces which are parallel to the longitudinal axis of said screw.