How to Fix PETG Under-Extrusion
PETG melts slower than PLA, and modern fast printers are tuned to PLA's pace. Run a PLA-speed profile with PETG in the nozzle and the extruder asks for more melt than the hotend can make — the print comes out with thin walls, gaps and sparse infill that crunches when squeezed.
The melt-rate budget
Every hotend has a ceiling on how many cubic millimetres of plastic it can melt per second, and every filament has its own limit on that ceiling. Bambu’s stock profile caps PETG at 16 mm³/s (the high-flow variant at 23 mm³/s) versus 21 mm³/s for regular PLA. Vendor baselines for slower hotends go lower still — Prusa’s generic PETG allows 8 mm³/s. When a slicer respects this limit it quietly slows fast moves down; when a profile overrides it, the hotend falls behind and under-extrudes.
Fixes in order
1. Check the volumetric limit is set for PETG, not PLA. If your filament profile was cloned from a PLA profile, the max volumetric speed is probably too high. Set it back to the stock value for your machine class; on Bambu machines that is 16 mm³/s for standard PETG.
Filament settings → Setting overrides → Max volumetric speed2. Dry the spool. Wet PETG foams: steam bubbles take up volume that should be plastic, so every line comes out thin even though the extruder is feeding correctly. If walls are both rough and thin, moisture is the likelier cause — 8 h at 65 °C, then retest.
3. Give it heat. More temperature buys melt rate. The stock nozzle value is 250 °C within an allowed 230–270 °C; if you print PETG fast, run the top half of the window. (Yes, hotter PETG strings more — fast printing is the one case where you trade that away deliberately.)
4. Calibrate flow for generic spools. Stock flow ratio is 0.95; generic PETG can sit several percent off. Run the flow calibration rather than guessing.
Rule out the mechanical suspects
- Partial clog: under-extrusion that worsens over a print, or comes and goes, smells like a half-blocked nozzle — cold pull time; see the clogging guide.
- Tangled or dragging spool: PETG is stiff enough to fight through most feed friction, but a spool wound over itself stalls any extruder.
- Worn nozzle: if this machine has printed carbon-filled filament (PETG-CF counts), the bore may be eroded and extrusion permanently inconsistent. Swap to a fresh hardened nozzle.
How to confirm the diagnosis
Slice a test cube twice: once at your normal profile, once with every speed halved. If the slow print comes out solid and heavy while the fast one is gappy, the hotend cannot keep up — volumetric limit or temperature. If both prints are equally starved, the problem is upstream: moisture, clog, feed path or flow ratio. That one comparison print separates the two whole families of causes. The material-agnostic version of this diagnosis lives in the under-extrusion guide; PETG’s full value sheet is in the PETG settings guide.
Frequently asked questions
Why does PETG under-extrude on fast printers when PLA prints fine?
PETG melts slower than PLA, and modern fast printers are tuned to PLA's pace. Bambu's stock profile caps PETG at 16 mm³/s versus 21 mm³/s for regular PLA — run a PLA-speed profile with PETG in the nozzle and the extruder asks for more melt than the hotend can make, producing thin walls, gaps and sparse infill that crunches when squeezed.
What max volumetric speed and temperature should PETG use?
On Bambu machines, set max volumetric speed to the stock 16 mm³/s for standard PETG (23 mm³/s for the high-flow variant; Prusa's generic PETG allows just 8 mm³/s). The stock nozzle temperature is 250 °C within an allowed 230–270 °C window — if you print PETG fast, run the top half of that window, because more temperature buys melt rate.
How do I tell whether PETG under-extrusion is the hotend or something else?
Slice a test cube twice: once at your normal profile, once with every speed halved. If the slow print comes out solid and heavy while the fast one is gappy, the hotend cannot keep up — fix the volumetric limit or temperature. If both prints are equally starved, the problem is upstream: moisture (dry 8 h at 65 °C), a partial clog, the feed path or the flow ratio.
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- Common print quality problems and solutions — Bambu Lab Wiki
- Filament Drying Recommendations — Bambu Lab Wiki
- OrcaSlicer vendor profiles — SoftFever/OrcaSlicer (GitHub)