Under-extrusion: less material comes out than it should

Likely causes

In order: the most frequent cause comes first.

1. 01A partially blocked nozzle: some material passes but not enough
2. 02A nozzle worn by abrasive filament, with an oval, enlarged orifice
3. 03Temperature too low for the flow rate the speed demands
4. 04Badly set extruder gear tension: it slips on the filament
5. 05Filament tangled on the spool or rubbing on the holder, slowing the feed
6. 06A miscalibrated extrusion multiplier or flow rate in the slicer

How to fix it

Apply one fix at a time and reprint the same test part. Changing three settings at once tells you nothing about which one worked.

1. 01

   Check the filament feeds without resistance

   Easy

   Before dismantling anything, untangle the spool and check it turns freely on its holder. A crossed strand trapped under an earlier wrap slows the feed just enough to cause intermittent under-extrusion, and it is the easiest cause to rule out.

2. 02

   Raise the temperature or lower the speed

   Easy

   A hotend can only melt so much material per second. If you print fast with the temperature at the low end, the material does not melt in time. Raise it by 10 °C or drop the speed by 20 % and see whether the gaps disappear. It is a two-minute test that separates a thermal problem from a mechanical one.

3. 03

   Clean the hotend with a cold pull

   Medium

   Heat the hotend, load filament, let it cool to the range your maker specifies and pull firmly: out comes a plug shaped like the nozzle's interior, carrying the carbonised residue with it. Prusa recommends it as a preparatory step before printing fibre composites, and it is also the standard repair for a partial clog.

4. 04

   Replace the nozzle if you have run abrasive material

   Medium

   A brass nozzle wears out within hours on carbon- or glass-filled filaments, and the wear is invisible: it shows up precisely as progressive under-extrusion. If you have printed PLA-CF or PETG-CF with a brass nozzle, swap it for hardened steel and recalibrate flow.

How to prevent it

• —Fit a hardened steel nozzle from the first gram of any fibre-filled filament
• —Calibrate flow whenever you change filament brand
• —Check the spool turns freely and the filament rubs nowhere along its path
• —Do not raise print speed without also raising temperature

Sources

1. [1]Under-extrusion | Prusa Knowledge BasePrusa Research · Manufacturer · accessed July 27, 2026
2. [2]Composite materials (filled with carbon, kevlar or glass) | Prusa Knowledge BasePrusa Research · Manufacturer · accessed July 27, 2026