Quick diagnosis
Thin, separated or missing lines on a PETG print, worse right after travel moves or on a spool that has been open for a while, even though the spool path is clear and the extruder gear looks fine. If that description fits any material, start with Fix Under-Extrusion first and rule out the spool path, hotend readiness and a partial clog. The causes below are specific to how PETG melts and absorbs moisture, not the mechanical checklist that guide walks through.
A PLA-style profile runs cold on PETG
PLA prints comfortably around 190 to 220°C. PETG needs meaningfully more heat to flow and bond: Prusa’s knowledge base lists 230°C for the first layer and 240°C after that on Prusament PETG, and PrusaSlicer’s newer default for the same filament goes as high as 250°C. MatterHackers and Simplify3D both give the same 230 to 250°C band for PETG generally. A profile copied from PLA, or a generic “PETG” preset left at a cautious low setting, can sit well under that range.
Polymaker’s own troubleshooting documentation names the result directly: printing PETG too cool produces weak prints, poor surface finish, and under-extrusion, not just rougher looks. The nozzle is being asked to push plastic through faster than the melt at that temperature actually allows, so the line thins out or breaks rather than laying down full width. If the generic guide’s mechanical checks come back clean, confirm the actual nozzle temperature against the 230 to 250°C band before touching flow or retraction.
Moisture doesn’t just cause stringing, it starves the flow
PETG absorbs airborne moisture faster than PLA, and most advice about that stops at stringing. Polymaker’s documentation goes further: damp PETG causes popping and inconsistent extrusion during printing, and can leave tiny bubbles or pockmarks in the walls of a print. That inconsistency is under-extrusion with a cause that has nothing to do with the spool path or the extruder gear, trapped water flashing to steam mid-nozzle, breaking up the flow for a moment at a time.
Bambu Lab’s own wiki is just as direct about its PETG HF line: damp filament causes pores, stringing, oozing, holes, rough surfaces and low strength, worsening with humidity and exposure time. Its documented fix is a blast-drying oven at 65°C for 8 hours, then storage in an AMS or a sealed box with desiccant during printing. Bambu’s wiki also publishes a specific wet-filament fallback profile, nozzle at 220°C with volumetric speed capped at 16mm³/s, and warns plainly against raising the speed on that profile because it risks clogging and extrusion failure. That warning is a useful tell: if a spool that strung or popped recently is now under-extruding at a normal speed, the filament is still wet, not newly out of tune.
Retraction built for PLA leaves gaps right after travel
Polymaker’s documentation also flags a cause that is easy to miss because it points the opposite direction from what people expect: too much retraction causes under-extrusion, clogs or inconsistent flow after travel moves, not just on the line itself. PETG’s melt stays tacky across a wider window than PLA’s, so a retraction distance that is fine for PLA can pull back further than PETG’s softer, stretchier melt wants, leaving the nozzle under-primed for the first few millimeters after each travel.
Retraction numbers for PETG also depend heavily on extruder type. A reasonable starting point is 5 to 7mm on a Bowden setup and only 1 to 4mm on direct drive, for the same material, because the tube length between gear and nozzle changes how much compression happens before the filament actually moves. A value borrowed wholesale from a PLA profile, rather than tuned on the specific printer, is a common way to land outside either range. If gaps cluster right after corners or long travel moves rather than appearing evenly across a wall, test a shorter retraction distance before assuming a mechanical cause.
One thing worth flagging honestly
A recurring claim on Prusa’s own forum is that some printers’ stock PETG default sits at 230°C while a specific filament’s recommended range starts closer to 250°C, and that the gap alone is enough to jam the extruder as it grinds against cooled, under-melted material with nowhere to go. That is one forum account, not a confirmed, documented cause, and other users in the same thread printed the same Prusament PETG without issues at 230°C, and even as low as 210°C. Treat a printer’s stock PETG default as a starting point to verify against the specific spool’s printed temperature range, not as a number that is guaranteed to be wrong.
PETG under-extrusion checklist
- Generic mechanical checks from Fix Under-Extrusion already ruled out: spool path, filament diameter, extruder tension, partial clog.
- Nozzle temperature confirmed within PETG’s 230 to 250°C band, not a PLA-level setting.
- Spool checked for moisture: popping or crackling at the nozzle, pockmarked walls, or a spool that has sat open for weeks.
- If a wet-filament fallback profile is in use, print speed left alone rather than raised, since that profile’s low speed is what keeps a damp spool from clogging.
- Retraction distance matched to extruder type (roughly 5 to 7mm Bowden, 1 to 4mm direct drive) rather than copied from a PLA profile.
- Gaps checked for a pattern: evenly spread points to temperature or moisture, clustered right after travel moves points to retraction.
If temperature, moisture and retraction are all confirmed and the problem persists, the mechanical causes in Fix Under-Extrusion are the next step. For the full PETG settings baseline these numbers come from, see PETG Print Settings. PETG’s other well-known flow symptom, stringing from the same wide soft-melt window, is covered in Fix PETG Stringing.