PETG under-extrusion is what you see when the printer lays down less PETG than the layer needs, so the walls go thin, the top fails to close, and the infill looks sparse. PETG under-extrudes for the same reasons as any filament, but wet PETG is the lead suspect, since the moisture thickens the melt. Bambu’s PETG guide and under-extrusion page point to moisture, a partial clog, and a nozzle that runs too cold.
The fix runs from a quick dry to a slicer change. Work the list in order and reprint the same one-wall test after each step.
PETG needs a hotter nozzle than PLA, near 230 to 240 C, so a temp set for PLA will starve a PETG print.
What causes it
| Likely cause | Fix | Severity |
|---|---|---|
| Wet PETG | Dry the spool at 60 to 65 C, then reprint. Wet PETG thickens and flows less. | hi |
| Partial clog | Run a cold pull to clear the blockage, then push fresh PETG through. | hi |
| Nozzle too cold | Raise the nozzle toward 230 to 240 C so the PETG melts fully. | hi |
| Flow too low | Raise the flow a few percent and reprint. | md |
| Extruder gear slip | Tension the idler and clean the gear teeth so they grip the PETG. | lo |
Try these first
Print a one-wall PETG cube and pinch a wall, then work top to bottom and reprint after each step.
Dry the PETG
Run the spool at 60 to 65 C for 4 to 8 hours, then reprint.
Raise the nozzle temp
Set the nozzle toward 230 to 240 C so the PETG melts clean.
Run a cold pull
If the flow change does nothing, clear a partial clog with a cold pull and retest.
Raise the flow
Bump the extrusion multiplier up a few percent and reprint.
Deeper fixes to try next
When drying and temp do not fill the walls, the extruder and the path are next.
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 PETG, command a 100 mm feed, and trim the e-steps to match.
How PETG differs
PETG under-extrudes more than PLA when it is wet, because it soaks up moisture fast and the water thickens the melt so less gets through. It also needs a hotter nozzle, near 230 to 240 C, so a temperature tuned for PLA will starve it. PLA under-extrudes less when dry, and nylon is the only common filament that drinks water faster than PETG.
Mistakes that keep PETG thin
A few choices fix the look and leave the cause.
Avoiddo not
- Raising the flow before you dry the PETG.
- Running PETG at a PLA nozzle temperature, which starves it.
- Leaving a slipping extruder gear and blaming the flow.
- Changing flow, temp, and tension in one pass.
Key takeaways
- PETG under-extrudes from wet filament, a clog, or a cold nozzle.
- Dry the PETG first, then raise the nozzle toward 230 to 240 C.
- Clear any clog and tension the extruder gear.
- PETG needs a hotter nozzle than PLA.
For the general version, the under-extrusion guide covers any filament, and the filament storage guide covers keeping PETG dry.
Related guides
Related
More in this area
Cross-reference
Sources & methodology
3 citations · reviewed 2026-07-10- 01Bambu Lab PETG Usage Guide (wiki): PETG drying and flowaccessed 2026-06-29Tier 1
- 02Bambu Lab Wiki: Under-extrusion (flow volume, resistance)accessed 2026-07-09Tier 1
- 03All3DP: Under-extrusion causes and fixesaccessed 2026-07-09Tier 2