When your 3D print comes out with bulging top layers, thick rough walls, or fine details that have filled in with extra plastic, the printer is pushing too much filament. The dial-back is quick and lives mostly in the slicer: lower the flow, confirm the filament diameter, and the surface cleans up.
Bambu’s print-quality catalogue ties the bulge to three things: an extrusion multiplier set too high, a wrong filament diameter in the slicer, or a nozzle worn wider than its rating. Check the flow first, since it is the most common and the easiest to fix.
Any filament over-extrudes the same way when the flow is high, but a wet spool makes it worse. Boiling moisture forces extra plastic out at the nozzle, so the bulge grows even with the flow set right.
What to reduce, in order
| Likely cause | Fix | Severity |
|---|---|---|
| Flow or extrusion multiplier too high | Lower the flow a few percent and reprint. | hi |
| Filament diameter set wrong | Set the filament diameter to 1.75 mm in the slicer. | hi |
| Nozzle temp too high | Lower the nozzle about 5 C so the filament runs less runny. | md |
| Worn nozzle wider than spec | Swap in a new nozzle of the right size. | md |
| E-steps set too high | Recalibrate the extruder steps against a measured 100 mm. | lo |
Quick fixes to start with
Print a one-wall cube or a flat top, then work top to bottom and reprint after each step.
Lower the flow
Drop the extrusion multiplier by a few percent and reprint.
Check the diameter
Confirm the slicer is set to 1.75 mm for the filament.
Lower the nozzle temp
Cut the nozzle about 5 C if the walls still look fat.
Dry the filament
If the surface is rough and pops, dry the spool, since wet filament adds volume.
When the basics are not enough
When flow and temp look right, the hardware and the calibration are next.
Inspect the nozzle
A nozzle worn wide lays too much plastic, so swap in a new one of the right size.
Recalibrate e-steps
Mark 100 mm of filament, command a 100 mm feed, and trim the e-steps to match.
Measure the filament
Some spools run thick, so measure with calipers and set the real diameter if needed.
How your material changes things
Over-extrusion hits PLA, PETG, and the rest the same when the flow is high. A wet spool makes it worse, since boiling moisture pushes extra plastic out at the nozzle. PETG shows the bulge as rough, droopy top layers more clearly than PLA, so it is a good material to tune flow on.
Mistakes that swing it the other way
A few choices fix the bulge but thin the print.
Avoiddo not
- Dropping the flow so far the top goes thin and the walls gap.
- Setting the diameter by guess instead of a real measurement.
- Ignoring a worn nozzle and blaming the flow.
- Changing flow, temp, and diameter at once.
Key takeaways
- Over-extrusion lays too much filament, from flow, diameter, or a worn nozzle.
- Lower the flow a few percent and confirm 1.75 mm first.
- Swap a worn nozzle and recalibrate e-steps if it returns.
- Change one setting and reprint the same test.
For the full guide, the over-extrusion page covers the same fault in depth. For the opposite fault, the under-extrusion guide covers thin walls and gaps, and the filament storage guide covers keeping spools dry.
Related guides
Related
More in this area
Cross-reference
Sources & methodology
2 citations · reviewed 2026-07-10- 01Bambu Lab Wiki: Common print quality problems (over/under-extrusion)accessed 2026-07-09Tier 1
- 02All3DP: Over-extrusion causes and fixesaccessed 2026-07-09Tier 2