Extruder: the mechanism that pushes filament into the hotend

How an extruder works

The word extrusion means pushing material through an opening. In a 3D printer this is done by a small stepper motor with a toothed gear that grips the filament and pushes it forward. The plastic then goes on into the hotend, where it melts and comes out of the nozzle.

The motor can turn by an exact angle, so the printer can meter plastic in fractions of a millimetre. The same mechanism pulls the filament back during a move across empty space (retraction), so that the nozzle does not drip.

The two main layouts

With direct drive the extruder sits right above the hotend. The filament path is short, so it responds more precisely and also copes with soft materials. With a Bowden setup the extruder is fixed to the frame and a long PTFE tube guides the filament to the head. The head is lighter, but the flexibility of the tube makes metering less precise.

Today most new printers use direct drive, because it is easier to set up and more reliable.

What can be wrong

• The gear slips. The filament is not fed, and the print comes out thin (under-extrusion).
• Over-pressure. With wrong settings more plastic is printed than needed, and the surface is lumpy (over-extrusion).
• Damp filament. It crackles and hisses as it is fed, and the plastic comes out rough (filament moisture).

Metering plastic and the price

The extruder does not set the price by itself, that comes from the grams and the print time. More colours, however, mean more filament changes, and more plastic ends up on the wipe tower, which is why multicolour printing costs more. If one colour is enough for you, you save.

If the extruder delivers five percent less plastic than the slicer expects, the part is too sparse: walls have gaps and are less strong. If it delivers more, dimensions are a little bigger and the surface is rough. Flow is therefore set separately for each material, and the values are slightly different for PLA and PETG (flow rate).

The condition of the filament matters too. Damp plastic forms bubbles in the nozzle as it is fed, which shows up as crackling and a rough surface. For the customer, the outcome is what you see on the part: even walls, accurate dimensions and no holes.