Coaxial injector screw providing improved small shot metering

An injection molding machine comprising:

• an injector barrel having a bore extending along an axis and terminating in a nozzle to abut a mold held by the injection molding machine;
• a screw positioned within the injector barrel, the screw having outer threads to advance a molten plastic through the bore toward the nozzle with rotation of the screw about an axis, the screw further having a central bore along the axis;
• a first actuator for rotating the screw about the axis;
• a piston slidably received within the central bore of the screw movable along the axis with respect to the screw;
• a second actuator providing relative movement between the piston and the screw,
• wherein the second actuator is a screw translation actuator moving the screw in translation along the axis;
• and
• a controller communicating with the first actuator and the second actuator to execute a stored program of the controller that (i) rotates the screw relative to the injector barrel to fill a front of the barrel with molten plastic and (ii) translates the screw along the axis of the bore and relative to the injector barrel toward the nozzle of the injector barrel while concurrently retracting the piston relative to the injector barrel and with respect to the screw to eject molten plastic from the nozzle.