How to Fix PETG Over-Extrusion

PETG needs less flow than PLA — noticeably less. Profiles cloned from PLA quietly over-extrude PETG by several percent, and PETG punishes it more than PLA would: the sticky melt piles onto the nozzle, wipes onto walls as blobs, and gives tops a wet, ploughed look.

The 0.95 rule

Look across the industry’s stock profiles and a pattern jumps out: Bambu ships PETG at a flow ratio of 0.95, and Creality, QIDI, Anycubic and FLSun all ship their generic PETG baselines at 0.95 too — against 0.98 for PLA. PETG dies swell out of the nozzle differently and the industry converged on roughly 3 % less flow. So the single most common cause of PETG over-extrusion is simply: the profile was made from a PLA profile and kept PLA’s flow.

Fixes in order

1. Set flow to the PETG baseline, then calibrate. Start at 0.95 and run the slicer’s flow calibration for your actual spool. Generic PETG drifts batch to batch; the calibration pins it.

Filament settings → Filament → Flow ratio; Calibration tab → Flow rate

2. Cool the melt slightly. Overheated PETG flows more freely and bulges more at corners. Stock is 250 °C; if you run near the top of the 230–270 °C window for speed, expect fatter corners and either accept them or come down 10 °C.

3. If it’s blobs rather than fatness, look at moisture. Wet PETG doesn’t over-extrude in the strict sense, but bursting steam bubbles fling extra material onto the nozzle, and the nozzle wipes it back onto the part as blobs and zits — which everyone first reads as over-extrusion. Rough + blobby = dry the spool (8 h at 65 °C) before touching flow.

PETG-specific reading tips

Sanity-check with the single-wall test

Print a single-wall cube in PETG and measure: if the wall matches the sliced line width within a couple of hundredths, flow is right and your “over-extrusion” is one of the impostors above. Fat by several percent: calibrate flow. The general method, plus how flow interacts with pressure advance on corners, is in the main over-extrusion guide; the full PETG picture in the PETG settings guide.

Frequently asked questions

Why does PETG over-extrude when PLA prints fine?

Usually because the PETG profile was cloned from a PLA profile and kept PLA’s flow. The industry converged on roughly 3 % less flow for PETG: Bambu, Creality, QIDI, Anycubic and FLSun all ship generic PETG at a 0.95 flow ratio, against 0.98 for PLA. PETG punishes the excess harder — the sticky melt piles onto the nozzle, wipes onto walls as blobs and gives tops a wet, ploughed look.

What flow ratio should I use for PETG?

Start at 0.95 — the baseline Bambu, Creality, QIDI, Anycubic and FLSun all ship for generic PETG — then run your slicer’s flow calibration for the actual spool, since generic PETG drifts batch to batch. If corners still bulge, cool the melt: stock is 250 °C, and running near the top of the 230–270 °C window means fatter corners unless you come down about 10 °C.

Are PETG blobs over-extrusion or wet filament?

If the surface is rough and blobby rather than uniformly fat, suspect moisture first. Wet PETG doesn’t over-extrude strictly, but bursting steam bubbles fling extra material onto the nozzle, which wipes it back onto the part as blobs and zits — easily misread as over-extrusion. Dry the spool 8 h at 65 °C before touching flow; genuine flow errors show as walls measurably fat by several percent.

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