Under-Extrusion
Gaps in walls or infill, thin top surfaces, parts weaker than expected. Follow the steps in order.
Is the extruder clicking?
A regular clicking or grinding sound from the extruder during printing is the clearest signal of under-extrusion. The drive gear is skipping because it can't push filament fast enough or against too much resistance.
If you hear clicking: the most likely causes are a partial nozzle clog or a temperature that's too low for the current speed. Start there.
Also check: pull the filament out after removing from the extruder. If you see flat, chewed-up sections (gear grinding marks), the gear was slipping, usually caused by a clog further down.
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 280 °C and try extruding 50 mm manually via the touchscreen. The material should flow in a consistent, straight strand. If it comes out thin, curling, or sputtering, you have a partial clog. Clear it with a cold pull: heat to 250 °C, push filament until it flows, cool to 70–90 °C (PLA), then pull sharply. Repeat until clean. Full cold pull guide →
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.
- In Bambu Studio: go to Calibration → Flow Rate.
- Pass 1 (Coarse): Prints 7 blocks with flow values from ~1.12 down to ~0.88. Select the block with the smoothest top surface (no gaps, no raised ridges). Click "Next".
- Pass 2 (Fine): Prints 10 blocks in a narrow range around your Pass 1 result. Select the best one. Bambu Studio saves the result to the filament profile automatically.
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.
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. Reference targets:
| PLA | 210–220 °C |
| PETG | 235–255 °C |
| ABS / ASA | 250–265 °C |
| PA / Nylon | 250–270 °C |
| PC | 270–290 °C |
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.
Less common causes
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 45 °C for 6 h, PETG at 65 °C for 8 h, Nylon at 80 °C for 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 max 5–8 mm³/s, retraction 0–0.8 mm (not the standard 1.0+), Pressure Advance disabled or set to 0.00–0.01. Print from external spool, as the long AMS path makes things worse. Never try to run TPU 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 around €11–16.
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. If you use a Bambu print speed profile ("Sport" or "Ludicrous") with a Generic filament profile set to 12 mm³/s, the slicer will throttle speeds correctly. 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.
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