The extruder is the motor-and-gear assembly of an FDM 3D printer that grips solid filament and pushes it into the hotend at a controlled rate, forming the machine's cold end.
The extruder controls how much filament enters the melt zone and when, making it central to dimensional accuracy, retraction, and consistent flow in fused deposition modeling (FDM) 📝3D Printing. A stepper motor turns a toothed drive gear—or a pair of counter-rotating gears in a dual-drive design—that clamps the filament against a spring-tensioned idler and advances it forward. Because the filament is still solid and unheated at this stage, the extruder is called the cold end, in contrast to the hotend that melts it.
Extruders are configured in two main layouts. In a direct-drive extruder the motor sits directly on the moving print head, immediately above the hotend, giving short, responsive filament control that handles flexible materials well but adds mass to the carriage. In a Bowden extruder the motor is mounted to the printer's frame and pushes filament through a length of PTFE tube to a lighter, faster-moving head, at the cost of precision on retraction and flexible filaments.
Extruders are further described by gear reduction, from a direct 1:1 unit to a geared extruder that multiplies torque for more reliable feeding of stiff or abrasive filament. The assembly's grip and calibration—steps or rotations per millimeter—directly set the volume of material delivered.