Under-Extrusion

2 Most common cause

Check for a partial nozzle clog

A partial clog is the single most common cause of under-extrusion. The nozzle isn't fully blocked, but material flow is restricted.

Heat the nozzle to 220 °C, load PLA and extrude about 50 mm via the touchscreen. A long, smooth strand that drops straight down means the nozzle channel is clear and the drive gear works; a short, rough or curling strand means a narrowed nozzle or a faulty drive gear (Bambu). If it is the nozzle, clear it with a cold pull: heat to printing temperature (PLA about 220 °C), push filament until it flows, let it cool to 70–100 °C (PLA), then press Retract on the screen and help only lightly by hand; do not pull hard against the extruder gears. Repeat until the tip comes out clean. Full cold pull guide

3 Calibration

Run Flow Rate Calibration

If there's no clog, the flow ratio is likely off. This is especially common with third-party filaments. Even a difference of 0.05 causes visible gaps.

1. In Bambu Studio: go to Calibration → Flow Rate.
2. Pass 1 (Coarse): Prints 9 blocks from 120% down to 80% of the preset's flow ratio, in 5% steps. Select the block with the smoothest top surface (no gaps, no raised ridges). Click "Calibrate" to print the fine pass (or skip it).
3. Pass 2 (Fine): Prints 10 blocks from 91% to 100% of the selected value in 1% steps. Select the smoothest one, name the filament preset and click Finish: Bambu Studio saves the new flow ratio to that preset (use the original preset name to overwrite it).

You can also adjust the flow ratio directly: click the filament box in Bambu Studio → Basic tab → Flow ratio. PETG typically lands at 0.93–0.96. If you're seeing gaps, try increasing by 0.02 at a time.

4 Temperature & Speed

Raise nozzle temperature or lower Max Volumetric Speed

If the nozzle isn't hot enough or the print is too fast, the hotend can't melt filament fast enough → under-extrusion. Both have the same visual result.

Raise temperature: In 5 °C increments from your filament profile's value. Stay inside Bambu's nozzle range for the material:

Ranges: Bambu Lab filament material table, listed with a ±10 °C tolerance. Plain PA is not in that table; its range is from Bambu Studio's Generic PA profile.

Lower Max Volumetric Speed: Click the filament box → scroll to the bottom → Max volumetric speed. Reduce by 20–30% as a test. This is especially relevant when using Generic profiles (Generic PLA defaults to 12 mm³/s); if the filament can't keep up, gaps appear. Also run Calibration → Flow Dynamics (K-value). Missing material specifically at corners is a flow dynamics issue, not a flow ratio issue.

5 Advanced

Less common causes

Wet filament causing 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.

Extruder gear wear

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-specific under-extrusion

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 at the value of Bambu's filament profile for your printer (0.8 mm for Bambu TPU 95A HF on the X1, P1, A1, P2S and X2D printers, 2 mm on most H2 variants), raised only in 0.2 mm steps if it oozes (Bambu: generally no more than 2 mm), 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.

Nozzle worn out

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, H2/P2S/X2D $17.99), on the X1/P1 $34.99 for the Complete Hotend that swaps in minutes; the bare $14.99 Hotend comes without heater, thermistor and fan, which you move over from the old one first (prices checked October 18, 2026).

Generic vs. Bambu filament profiles

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.