Under-extrusion is what happens when the printer lays down less filament than the layer needs. The walls go thin, the top layers show gaps, and the infill looks sparse or broken. Per Prusa’s under-extrusion guide, the cause is usually a partial clog, a flow set too low, a nozzle that runs too cold, or an extruder gear that slips.
The fix runs from a quick slicer change to a hardware clean. Work the causes in order and reprint the same test after each step.
Your material changes the odds. PETG and nylon under-extrude more when they are wet, because the moisture thickens the melt. PLA stays more forgiving.
The causes, ranked
| Likely cause | Fix | Severity |
|---|---|---|
| Partial clog or narrowed path | Run a cold pull to clear the blockage, then push fresh filament through. | hi |
| Extrusion multiplier or flow too low | Raise the flow a few percent and reprint. | hi |
| Nozzle temp too low | Raise the nozzle 5 to 10 C so the filament melts fully. | hi |
| Extruder gear slipping or under-tensioned | Tension the idler and clean the gear teeth so they grip. | md |
| Wet filament | Dry the spool, then reprint. Wet filament thickens the melt. | lo |
Fast fixes to try first
Print a one-wall cube or a hollow test, then work top to bottom and reprint after each step.
Raise the flow
Bump the extrusion multiplier or flow up a few percent and reprint.
Raise the nozzle temp
Add 5 to 10 C to the nozzle and reprint so the filament melts clean.
Run a cold pull
If flow does not help, clear a partial clog with a cold pull and retest.
Dry the filament
If the print is rough and pops, dry the spool and reprint.
If the quick fixes stall
When flow and temp do not fill the walls, the extruder hardware and the calibration are the next place to look.
Tension and clean the gear
Set the idler to a light, even grip and brush the extruder gear teeth clean.
Check the Bowden path
On a Bowden machine, look for a loose coupling or a tube gap that chokes flow.
Recalibrate e-steps
Mark 100 mm of filament, command a 100 mm feed, and trim the e-steps to match.
What your material changes
PLA under-extrudes the least when it is dry, because it melts clean at a low temperature. PETG and nylon pull in moisture fast, and a wet spool thickens the melt so less gets through. TPU under-extrudes when it is pushed too fast, so slow down for flexibles and keep the path short.
Habits that backfire
A few common choices swing the print the other way or hide the cause.
Avoiddo not
- Raising the flow so far the top layers bulge and over-extrude.
- Pushing the nozzle temp far past the range to mask a clog.
- Ignoring a slipping extruder gear and blaming the flow.
- Skipping the dry step for PETG or nylon.
Key takeaways
- Under-extrusion lays too little filament, from a clog, low flow, or a cold nozzle.
- Raise flow and temp, then clear any partial clog.
- Tension and clean the extruder gear if it slips.
- Dry PETG and nylon first. Wet filament thickens the melt.
For the sound that often comes with it, the extruder clicking guide covers the stepper skipping under load, and the filament storage guide covers keeping spools dry.
Related guides
Related
More in this area
Cross-reference
Sources & methodology
3 citations · reviewed 2026-07-10- 01Prusa Knowledge Base: Under-extrusion (calibration, slicer, nozzle, gears)accessed 2026-07-09Tier 1
- 02Bambu Lab Wiki: Under-extrusion (flow volume, resistance, pressure advance)accessed 2026-07-09Tier 1
- 03All3DP: Under-extrusion causes and fixesaccessed 2026-07-09Tier 2