Under-extrusion: 10 causes and fixes for 3D prints
Under-extrusion is when the nozzle lays down less plastic than the slicer planned, leaving gaps between lines, thin walls, pinholes in top surfaces and layers that split apart. It's usually a physical problem, such as a partial clog, a slipping extruder or printing faster than the hotend can melt plastic, so check the hardware and speed before you touch the flow rate.
- Reprint at half speed. If the gaps disappear, you were hitting the hotend's flow limit.
- Check the spool turns freely and isn't tangled, and listen for clicking from the extruder.
- Extrude 50 mm in midair at print temperature. The strand should fall straight down at an even thickness. Curling or spurting points to a partial clog.
- Dry the filament if the nozzle pops or hisses.
- Only then calibrate e-steps and flow ratio.
What under-extrusion looks like
- Gaps between lines on top surfaces, or pinholes you can see infill through.
- Thin or gappy walls, with perimeters that don't touch each other.
- Lines that are full in one place and thin or missing in the next.
- Weak parts that snap or split cleanly along a layer when you flex them.
- Clicking, knocking or grinding from the extruder motor.
If the top surface is rough and ridged instead of gappy, you're on the wrong page: that's over-extrusion.
Diagnose under-extrusion by when it happens
The single most useful clue is when the gaps appear. Match your print to a row before reaching for a fix.
| When it happens | Most likely cause | Jump to |
|---|---|---|
| Only on fast sections, infill or long straight lines | Speed beyond the hotend's volumetric flow limit | Cause 1 |
| Random, intermittent gaps, sometimes with clicking | Partial clog or extruder slip | Causes 2 and 3 |
| Starts suddenly and gets worse until extrusion stops | Tangled spool or heat creep | Causes 4 and 6 |
| Gets worse partway through long prints, often with PLA in an enclosure | Heat creep | Cause 6 |
| Began after hotend maintenance on a Bowden printer | Gap between the PTFE tube and nozzle | Cause 5 |
| Nozzle pops or hisses; bubbly, fuzzy lines | Wet filament | Cause 7 |
| First layer only: thin, see-through, scraped lines | Nozzle too close to the bed | Cause 9 |
| Every print, every speed, every filament by the same amount | E-steps, flow ratio or filament diameter setting | Cause 10 |
Cause 1: Printing faster than the hotend can melt
Every hotend has a maximum volumetric flow, measured in cubic millimetres of plastic per second. Past that point the plastic can't heat through fast enough, pressure builds and output drops, even with a perfectly clear nozzle.
Work it out with one multiplication: flow (mm³/s) = speed × line width × layer height. A 0.45 mm line at 0.2 mm layers and 150 mm/s needs 13.5 mm³/s. That's fine for a high-flow hotend and too much for a classic V6-style one.
| Hotend (0.4 mm nozzle) | Approximate max flow |
|---|---|
| E3D V6, E3D Revo and similar standard hotends | About 11 mm³/s |
| Dragonfly BMO, Dragon SF | About 13–15 mm³/s |
| Mosquito | About 20 mm³/s |
| Dragon HF, Rapido HF | About 24 mm³/s |
| Rapido UHF, Mosquito Magnum | About 30 mm³/s |
| Bambu Lab standard 0.4 mm hotend | 24 mm³/s with PLA Basic (manufacturer figure) |
| Bambu Lab High Flow 0.4 mm hotend | 40 mm³/s with PLA Basic (manufacturer figure) |
The third-party figures are Ellis' approximate values for a brass nozzle. Hardened steel usually flows less, larger nozzles flow more, and the real limit also depends on the material and temperature. Treat the table as a starting point, then test your own machine.
- Calculate the flow you're asking for on your fastest feature, usually infill or outer walls.
- Compare it with your hotend's limit and stay comfortably under it.
- Set the limit in the slicer so it slows down automatically: Max volumetric speed in the filament settings of PrusaSlicer, OrcaSlicer and Bambu Studio. Cura has no direct equivalent, so cap the speeds by hand.
- Raise temperature 5–10 °C for fast prints if you need a little more flow, staying within the filament's range.
Your walls look perfect at 50 mm/s but turn gappy at 200 mm/s, with 0.45 mm lines and 0.2 mm layers on a V6-style hotend. What's the most likely cause?
At 200 mm/s the hotend needs 200 × 0.45 × 0.2 = 18 mm³/s, well past the roughly 11 mm³/s a V6-style hotend can melt. Wrong e-steps or diameter would show at every speed, including 50 mm/s.
Cause 2: Partial nozzle clog
A partial clog lets some plastic through, so prints run but come out thin and inconsistent. Debris, burnt plastic, or a small piece of a different material left from a filament change are the usual culprits.
- Extruding in midair, the strand curls hard to one side or comes out thinner than the nozzle.
- Output pulses: a fat bead, then a thin one.
- The extruder clicks during fast moves but not slow ones.
Clear it with a cold pull (also called an atomic pull), and replace the nozzle if that doesn't help. Nozzles are consumables, and brass wears quickly with abrasive filaments like carbon-fibre-filled or glow-in-the-dark blends. Use hardened steel or another wear-resistant nozzle for those. The full routine is in clogged nozzle and hotend.
Cause 3: Extruder slipping, worn or under-tensioned
The drive gear grips the filament with its teeth. When the grip fails, the motor turns but the filament doesn't move as far.
- Tension too loose: the gear slips, especially on fast moves. Tighten the idler a little at a time.
- Tension too tight: it flattens the filament, which then jams. Flexible filaments like TPU are especially sensitive.
- Dirty gear teeth: packed plastic dust stops them biting. Clean them with a stiff brush.
- Cracked lever arm: plastic extruder arms on some budget printers crack near the pivot, which feels exactly like low tension.
Look at a piece of filament that has passed through the gear. Even, shallow tooth marks are healthy. Ground-down, chewed or flat spots mean the gear has been slipping.
Cause 4: Tangled spool or feed path drag
If a loop of filament gets trapped under a later wrap, the extruder suddenly has to drag the whole spool. Prints thin out, then stop extruding entirely. A stiff spool holder, a tight filament guide or a kinked Bowden tube cause a milder version of the same thing.
- Spin the spool by hand. It should turn with almost no effort.
- Pull filament out of the extruder with the idler released and feel for snags along the path.
- Always clip or tuck the loose end when you take a spool off, so it can't cross under a wrap.
Cause 5: Gap between the PTFE tube and nozzle (Bowden hotends)
On many Bowden hotends, including the stock hotend on Ender 3-style printers, the PTFE tube must press firmly against the back of the nozzle. If it's cut at an angle or has backed out of its coupling, molten plastic fills the gap, forms a plug and chokes flow. Under-extrusion often starts right after a nozzle change.
- Heat up and remove the tube following your printer's manual.
- Trim the end perfectly square with a tube cutter or a sharp blade.
- Push it fully home against the nozzle and lock the coupling, using a retaining clip if your printer has one.
Cause 6: Heat creep
Heat creep is when heat travels up past the heat break and softens the filament too early, so it swells and jams. It shows up partway through a print, gets worse over time, and is most common with PLA in an enclosed printer or when the heatsink fan is weak.
- Check the hotend heatsink fan spins whenever the hotend is hot, and clean dust from its fins.
- Open the door or remove the lid when printing PLA in an enclosure.
- Avoid very long or frequent retractions, which pull soft plastic back into the cold zone.
The full diagnosis is in the heat creep guide.
Cause 7: Wet filament
Moisture boils as the filament melts. You'll hear popping or hissing and see bubbly, fuzzy lines with gaps, often with extra stringing. Dry the spool: PLA at 40–50 °C for 4–6 hours, PETG at 60–70 °C for 4–6 hours. The filament drying guide covers every material.
Cause 8: Nozzle temperature too low
Cooler plastic is thicker and harder to push, which lowers the flow you can reach. Stay inside the filament's range: PLA 200–220 °C, PETG 225–245 °C, ABS 235–255 °C. If you're at the bottom of the range and printing fast, try 5 °C hotter. If you're already at the top, the fix is slower speeds, not more heat.
Cause 9: Nozzle too close on the first layer
If the nozzle is squashed against the bed, plastic has nowhere to go. The first layer looks thin and see-through, the extruder may click, and the problem vanishes from layer two. Raise the Z-offset slightly; see bed leveling and Z-offset.
Cause 10: E-steps, flow ratio or filament diameter
If everything above checks out and every filament is short by the same amount, the firmware or slicer is asking for too little plastic. Check the filament diameter setting first (1.75 vs 2.85 mm), then calibrate e-steps on Marlin (M92, saved with M500) or rotation_distance on Klipper, then run a flow ratio test per filament. On Bambu Lab and recent Prusa printers, skip e-steps and tune flow ratio only.
The step-by-step tests are in the over-extrusion fix order, because the calibration is identical in both directions, and in the broader 3D printer calibration guide.
Free PrintPal toolE-Steps CalculatorTurn a 100 mm extrusion test into corrected e-stepsOpen toolDon't paper over a hardware problem with flow. If you raise the flow ratio to hide a partial clog or a speed limit, slow sections will over-extrude and the gaps come back at the next speed bump.
Frequently asked questions
What causes under-extrusion on a 3D printer?
The most common causes are printing faster than the hotend can melt plastic, a partial nozzle clog, an extruder that slips on the filament, and a spool that drags or tangles. Heat creep, wet filament, low temperature, a PTFE gap in Bowden hotends and wrong e-steps or flow settings cause the rest.
Why does my printer under-extrude only at high speed?
You are asking for more plastic per second than the hotend can melt. Multiply speed by line width and layer height to get the volumetric flow, compare it with your hotend's limit, and set that limit as the max volumetric speed in your slicer's filament settings.
Why is my extruder clicking?
Clicking means the extruder motor is skipping because it cannot push filament through. Usual causes are a partial clog, a nozzle too close to the bed on the first layer, printing beyond the hotend's flow limit, a temperature that is too low, or heat creep.
Can wet filament cause under-extrusion?
Yes. Moisture boils in the hot end, so the nozzle pops, hisses and spits uneven, bubbly lines with gaps. Dry the spool before changing any flow settings.
Should I just raise the flow rate to fix under-extrusion?
Only if every filament under-extrudes by the same amount at all speeds. Raising flow to hide a clog, a slipping extruder or a speed limit makes slow sections over-extrude and leaves the real problem in place.
Related articles
Sources & further reading
- Prusa Knowledge Base — Under-extrusion
- Prusa Knowledge Base — Cold pull
- Ellis' Print Tuning Guide — Determining maximum volumetric flow rate
- Klipper Docs — Rotation distance
- Bambu Lab Wiki — Flow rate calibration
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