What under-extrusion is and how to confirm it
Under-extrusion means less plastic comes out of the nozzle than the slicer planned. Lines come out thin, walls do not bond to each other, top surfaces show gaps and infill looks lacy. Parts are weaker and can feel hollow when squeezed.
Confirm it with a simple test: print a single-wall cube or a 20 mm box with two perimeters and measure the wall thickness with calipers. If a 0.4 mm line measures 0.32 mm, you are about 20 percent short. Also watch the extruder: clicking, a gear that jumps back, or filament dust are clear signs of slipping. If gaps appear only on fast sections, the hotend cannot keep up.
Causes, most likely first
- Partial clog from dust, burnt residue or a previous material.
- Printing too fast or too cold. The hotend cannot melt plastic quickly enough.
- Extruder problems: worn or dirty drive gear, too little tension, or a cracked arm.
- Friction in the filament path: a tangled spool, a tight spool holder or a kinked Bowden tube.
- Wrong slicer settings: filament diameter set to 2.85 instead of 1.75, or flow below 100 percent.
- Wet filament, which bubbles and leaves gaps.
Fixes, cheapest and fastest first
1. Check the slicer basics
Confirm filament diameter is 1.75 mm for standard filament, flow or extrusion multiplier is around 95 to 100 percent, and the material profile matches what is loaded.
2. Free the filament path
Make sure the spool turns easily and the filament is not crossed over itself. Pull filament through the extruder by hand with the hotend hot; it should move smoothly. On Bowden printers, check the tube is fully seated at both ends.
3. Raise temperature or lower speed
Raise the nozzle temperature by 5 to 10 °C, or drop speed by 20 to 30 percent. As a rough guide, a standard hotend handles about 10 to 15 mm³/s of PLA; high-flow hotends handle more. Your slicer can show the volumetric rate of each move.
4. Clean or replace the nozzle
Do a cold pull: heat to printing temperature, push filament through, cool to about 90 °C for PLA and pull firmly. Repeat until the tip comes out clean. A nozzle that keeps clogging should be replaced.
5. Service the extruder
Brush plastic dust out of the drive gear teeth. Adjust tension so the gear leaves visible bite marks without flattening the filament. Replace worn gears and check plastic extruder arms for cracks, a common failure.
6. Calibrate e-steps or rotation distance
Mark filament 120 mm above the extruder, command 100 mm of extrusion at a slow rate, and measure. If only 95 mm went in, correct the steps per millimeter proportionally. Do this only after the mechanical checks above.
Material notes
- PLA: usually a speed or clog issue. 200 to 215 °C covers most fast printing.
- PETG: more viscous; if fast prints thin out, add temperature rather than flow.
- ABS: tolerant of high temperatures; check for burnt residue from long heat soaks.
- TPU: print slowly, often 15 to 30 mm/s, with low extruder tension. Soft filament buckles in loose paths, especially in Bowden setups.
Prevention checklist
- Keep filament dry and free of dust; a foam filter on the filament path helps.
- Do not exceed your hotend's realistic flow rate.
- Clean the drive gear every few spools.
- Do a cold pull when switching between materials with very different temperatures.
- Check that spool holders spin freely.
- Recalibrate flow when you change filament brands.