Under-Extrusion

Water in the filament creates steam bubbles in the hotend → inconsistent material flow that looks like under-extrusion. Confirmed by: crackling sounds during printing, steam or bubbles at the nozzle tip, rough/bubbly surface texture. Dry before using: PLA at 50 °C for 8 h, PETG at 60–65 °C for 8 h, Nylon at 75–85 °C for 8–12 h. If the problem returns quickly, your storage conditions are the root cause. Seal filament with fresh desiccant.

After extensive printing (especially with abrasive CF/GF filaments), the extruder drive gear can wear smooth. Signs: clicking persists even with no clog, filament shows grinding marks rather than the normal serrated grip pattern. The P1S has a Hardened Steel Extruder Upgrade available from Bambu that addresses this. For other models, the extruder assembly can be replaced.

TPU under-extrudes on Bambu printers because the flexible material compresses under extruder pressure instead of advancing. Key settings: Max Volumetric Speed at or below the 3.2 mm³/s of Bambu Studio's Generic TPU profile (roughly 35–40 mm/s at 0.2 mm layers with a 0.4 mm nozzle), retraction 0–0.8 mm (not the standard 1.0+), Pressure Advance disabled or set to 0.00–0.01. Print from the external spool or the rear TPU outlet of an AMS HT (which then only works as a dry box); the long AMS feed path makes things worse. Only Bambu's "TPU for AMS" is made to feed through the AMS.

A worn nozzle opening (especially stainless steel used with abrasive materials) can be larger than nominal, which actually causes over-extrusion. But a worn, chipped tip can also create inconsistent flow that appears as under-extrusion. If no other fix has worked and the printer has many hours on it with CF/GF filaments, replace the hotend assembly. On Bambu printers this takes 3–5 minutes and costs about $10 to $18 in the Bambu US store (A1/A2 series $9.99 to $11.99, P1 series $14.99, H2/P2S/X2D $17.99; checked October 7, 2026).

The biggest hidden difference between Generic and Bambu filament profiles isn't temperature; it's Max Volumetric Speed. Bambu PLA: 21 mm³/s. Generic PLA: 12 mm³/s. The slicer caps the sliced speeds at that value, but Sport and Ludicrous are printer speed levels (124 % and 166 % of speed and acceleration) applied on top at print time, so they can push a Generic profile past its MVS and cause under-extrusion (Bambu). But if MVS is set too high for your filament's actual capability, you'll get under-extrusion. Create a custom profile and find your filament's real MVS limit by gradually increasing until extrusion becomes inconsistent.