Under-Extrusion in 3D Printing
Under-extrusion occurs when the printer deposits less plastic than the slicer expects, producing gaps, weak walls, and rough surfaces. It is one of the most common FDM problems and has a clear diagnostic order: check flow rate, then temperature, then hardware.
What under-extrusion looks like
On top surfaces: gaps between lines, visible holes, or a swiss-cheese texture. On walls: visible layer lines from the outside, perimeters that are thinner than expected, or walls that snap apart under light pressure. During printing: a clicking or grinding sound from the extruder motor (the hobbed gear is grinding against the filament because it cannot push fast enough).
Severe under-extrusion produces a lacy, scaffold-like structure where solid infill should be. Mild under-extrusion only appears on close inspection as slight roughness or small gaps.
All under-extrusion causes
| Cause | Category | Symptom | Fix |
|---|---|---|---|
| Flow rate too low | Settings | Thin walls, gaps visible in top surface, weak layer adhesion throughout the print | Print a single-wall cube and measure wall thickness. If it reads less than your nozzle diameter, increase flow rate by that percentage in the slicer. |
| Print temperature too low | Temperature | Rough or grainy surface, partial extrusion at corners, filament grinding at extruder | Increase nozzle temperature by 5°C increments. Run a temperature tower to find the optimal range for your filament brand. |
| Print speed too high for temperature | Settings | Under-extrusion at speed peaks but OK at slower areas | Reduce print speed or increase temperature. The extruder is starving — plastic cannot melt fast enough for the speed demanded. |
| Partial clog | Hardware | Under-extrusion that gets worse over time, filament grinding | Cold pull (atomic pull): heat to printing temp, then cool to 90°C (PLA) or 110°C (ABS/PETG) and pull sharply. Repeat 3–5 times. Needle-clear if cold pull fails. |
| Bowden tube gap at nozzle | Hardware | Intermittent under-extrusion on direct drives or Bowden setups, blobs forming | Disassemble hot end. Ensure PTFE tube seats flush against nozzle — no gap. A 0.5 mm gap causes molten plastic to pool, causing intermittent blockage. |
| Extruder not gripping filament | Hardware | Clicking or grinding sound from extruder motor, visible wear marks on filament | Check extruder tension arm — spring may be worn. Clean the hobbed gear with a stiff brush. If worn, replace the extruder assembly. |
| Wet filament | Filament | Bubbling, popping sounds during extrusion, rough surface, inconsistent line width | Dry filament at material-appropriate temperature (PLA: 45°C/4–6 h, PETG/ABS: 65°C/4 h, Nylon/PC: 80°C/6–12 h). See the wet filament guide. |
| PTFE degradation (above 240°C) | Hardware | Under-extrusion at high temps (ABS/ASA/PC), black specks in extrusion | Replace all-PTFE hot end with an all-metal hot end (CR6-SE, Dragon, Revo) for materials above 240°C. |
Flow rate calibration (step by step)
Flow rate calibration is the first thing to check — most slicer defaults are off by 2–8% from real-world filament diameter variation. This is a 10-minute fix that eliminates the most common cause.
How to do a cold pull (clearing partial clogs)
Heat to printing temperature. Load filament and push manually until clean plastic flows. Lower temperature to 90°C for PLA, 110°C for PETG/ABS. Wait until you feel resistance, then pull the filament out sharply in one smooth motion. The tip should come out with a perfect cast of the inside of the nozzle, often with a plug of black or discoloured debris. Repeat until the tip comes out clean white/natural.
If the clog is fully solid (no extrusion at all), use a 0.35 mm nozzle needle to poke through from the nozzle tip while hot, then follow with a cold pull.