How to Fix Under-Extrusion on Bambu Lab Printers

Under-extrusion means the printer is laying down less plastic than each line needs: gaps between infill lines, walls you can see through, weak layers, sometimes a clicking sound from the extruder. On fast printers like Bambu's, the most common cause is simple physics — the hotend can't melt plastic as fast as the motion system can move.

Why it happens on a fast printer

Every hotend has a melt limit, measured as volumetric flow in mm³/s. Bambu’s stock profiles cap the slicer at each filament’s limit — for example 21 mm³/s for Bambu PLA Basic, 16 mm³/s for PETG and ABS, and just 8 mm³/s for PA-CF. Push past the cap (a modified profile, a filament that melts slower than Bambu’s own) and the extruder physically cannot keep up: every fast section prints starved.

Quick fixes

Rule out a partial clog

Persistent under-extrusion that doesn’t care about speed usually means something is in the way:

• Do a cold pull: heat the nozzle, push cleaning or nylon filament through, let it cool to about 90 °C, then pull it out in one firm tug. Repeat until the tip comes out clean.
• A clicking extruder is the sound of the gears skipping because flow resistance is too high — lower the speed and clean the nozzle.
• Check the filament path: a tangled spool or a kinked PTFE tube adds drag that starves the extruder. Details in the clogging guide.

Material-specific notes

• PLA in an enclosed printer: PLA softens at about 45 °C, so a warm chamber can make it go soft above the melt zone and bind (heat creep). Open the door and take the top off for PLA, and never use active chamber heating with it.
• TPU: soft filament can’t be pushed hard. Keep max volumetric speed at 3–8 mm³/s when struggling, feed it from an external spool holder with zero drag, and never run standard TPU through the AMS. More in the TPU printing guide.
• Carbon-fibre filaments: a 0.2 mm nozzle is a guaranteed clog with CF material, and even in a 0.4 mm nozzle, fibres demand a hardened steel nozzle. PA-CF also has a deliberately low 8 mm³/s flow cap — don’t raise it.
• Wet filament extrudes unevenly (steam interrupts the flow). PETG, TPU, PA and PVA are the usual suspects — see the drying guide.

Know your printer’s flow budget

A quick sanity check you can do in your head: flow = layer height × line width × speed. A 0.2 mm layer at 0.42 mm line width printed at 200 mm/s needs about 17 mm³/s — already above the 16 mm³/s cap of standard PETG, which is exactly why the slicer slows those sections down. If you raise speeds or layer height without thinking about the volumetric cap, you aren’t printing faster; you’re printing starved.

Note that Bambu’s newer machines also offer high-flow hotend variants with much higher limits in the same profiles (PLA Basic jumps from 21 to 40 mm³/s on the high-flow extruder). If you have the high-flow hardware, make sure the profile variant matches it — and if you don’t, make sure you’re not running a high-flow profile on a standard nozzle.