Under-extrusion is the printer laying down less filament than the layer needs, so the walls go thin, the top fails to close, and the infill looks sparse. To fix it you work a short list: clear any blockage, raise the flow, and make sure the filament melts. Prusa’s under-extrusion guide and Bambu’s point the finger at a clog, a low flow setting, or a nozzle that runs too cold.
The fix can be a single slicer change or a quick hardware clean. Work the list in order and reprint the same one-wall test after each step.
Wet PETG and nylon under-extrude more than PLA, since the moisture thickens the melt and chokes the flow.
What to fix, in order
| Likely cause | Fix | Severity |
|---|---|---|
| Partial clog or narrowed path | Run a cold pull to clear the blockage, then push fresh filament through. | hi |
| Flow or extrusion multiplier 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 loose | 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 |
Quick fixes to try first
Print a one-wall cube and pinch a wall to feel the thickness, then work top to bottom and reprint after each step.
Raise the flow
Bump the extrusion multiplier up a few percent and reprint.
Raise the nozzle temp
Add 5 to 10 C so the filament melts clean and flows full.
Run a cold pull
If the flow change does nothing, 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.
Deeper tuning to try next
When flow and temp do not fill the walls, the extruder and the path are the next place to look.
Tension and clean the gear
Set the idler to a light, even grip and brush the extruder gear 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.
How the material changes things
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 and keep the path short for flexibles.
Mistakes that swing it the other way
A few choices fix the gaps but cause a new fault.
Avoiddo not
- Raising the flow so far the top layers bulge.
- Pushing the nozzle temp past the range to mask a clog.
- Leaving a slipping extruder gear and blaming the flow.
- Skipping the dry step for PETG or nylon.
Key takeaways
- Under-extrusion leaves thin walls and gaps from a clog, low flow, or a cold nozzle.
- Raise flow and temp, then clear any clog.
- Tension and clean the extruder gear if it slips.
- Dry PETG and nylon. Wet filament thickens the melt.
For the wider write-up, the under-extrusion guide goes deeper, 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