Screw design and method for metal injection molding

A plastic injection molding machine configured for processing metal, metallic alloys, and metal matrix composites in a plastic injection molding machine, comprising:

• a temperature-controlled barrel having a metering zone, transition zone and feed zone;
• a feed throat configured for dispensing material for molding into the feed zone of the barrel;
• a nozzle extending from the metering zone of the barrel;
• a screw having:
• a shank;
• a screw shaft extending from the shank, the screw shaft having a rear portion proximate to the shank, a middle portion proximate the rear portion, and a front portion proximate the middle portion, said front portion extending to and terminating in an end of the screw shaft, said screw shaft having a substantially uniform diameter the length thereof;
• flights extending from the rear portion of the screw shaft for advancing metal, metallic alloys, and metal matrix composites through the barrel of the plastics injection molding machine into the middle portion of the screw shaft and the transition zone of the barrel, said flights being absent from the front portion and the middle portion of the screw shaft, said front portion and said middle portion of the screw shaft being unobstructed;
• and
• a motor driving the shank of the screw;
• said screw shaft configured and arranged to be rotated and reciprocated within the barrel of the plastic injection molding machine, said middle portion of said screw shaft being proximate to the transition zone of said plastic injection molding wherein said metal, metallic alloys and metal matrix composites are heated to form a semi-solid slurry with fluctuating viscosity whereby the absence of flights on said middle portion of said screw shaft allow said screw shaft to be rotated and reciprocated within the barrel of said plastic injection molding machine without binding with said semi-solid slurry when said fluctuating viscosity is high.