Extruder for a three-dimensional printer

An extruder for a 3-dimensional (3D) printer, the extruder comprising:

• a motor having an upright drive shaft;
• and
• a print head adjacent the motor and having a gearbox, a hopper body, and a shroud sequentially coupled in a top-down arrangement with the hopper body positioned adjacent the motor, the print head comprising:
• a nozzle disposed at a bottom portion of the shroud;
• a barrel in fluid communication with the nozzle and the hopper body;
• and
• a screw conveyor moveably supported within the barrel and the hopper body such that the screw conveyor is disposed in rotatable engagement with the upright drive shaft of the motor via gearing arrangement of the gearbox, and wherein the screw conveyor is axially retractable in relation to the nozzle, upon rotation by the upright drive shaft in a first direction, for selectively defining a positive displacement pump that controls, by stopping or reversing, a flow of feedstock output by the nozzle.