3D printing troubleshooting
Under-extrusion (gappy layers)
Also known as: thin lines, gaps between lines or missing patches.
The symptom
Lines come out thinner than normal, there are gaps between perimeters or in the infill, and some patches simply have no plastic. The part is weak and splits along layers.
Causes and fixes, ranked by likelihood
- 1
Partially clogged nozzle: charred residue lets through less plastic than the slicer asks for.
Fix: Do a cold pull or clear the nozzle with the cleaning needle at working temperature. If it keeps returning, replace the nozzle — it's a consumable.
Source: Simplify3D — Print Quality Guide: Under-Extrusion ↗ - 2
You're asking for more flow than your hotend can melt: high speed + thick layers + marginal temperature.
Fix: Raise the temp 5-10 °C or slow down. Your hotend's real flow (mm³/s) rules: at 0.2 mm layers with a 0.4 nozzle, 60 mm/s is already ~5 mm³/s. Per-model flow specs are in our comparator.
Source: Prusa Knowledge Base — Under-extrusion ↗ - 3
Wet or inconsistent-diameter filament (it crackles while printing, bubbles in the line).
Fix: Dry the filament (50-60 °C for a few hours, material-dependent) and measure the real diameter with calipers at several points; set it in the slicer.
Source: Simplify3D — Print Quality Guide: Under-Extrusion ↗ - 4
The extruder slips: loose spring tension, dirty drive gear or a sloppy PTFE tube.
Fix: Brush the drive gear clean, adjust the spring tension and make sure the PTFE tube seats fully against the nozzle.
Source: Simplify3D — Print Quality Guide: Under-Extrusion ↗ - 5
Extrusion multiplier / flow set low in the slicer.
Fix: With everything else ruled out, calibrate flow: single-wall cube, measure with calipers and tune the multiplier until the wall matches the theoretical width.
Source: Simplify3D — Print Quality Guide: Under-Extrusion ↗ - 6
The filament arrives braked: a tangled spool (it crossed over itself when let go), the strand rubbing the spool edge, or a dirty PTFE tube adding friction.
Fix: Check the spool spins freely and the filament isn't crossed, then trace the whole filament path hunting for drag. A bearing spool holder and a dust sponge before the extruder prevent relapses.
The material factor
TPU deserves its own chapter: it's soft enough to buckle inside the extruder rather than push — print it at 20-30 mm/s with the most constrained filament path you can manage. PETG and nylon under-extrude mostly from moisture (crackling and bubbles in the line); PLA, from heat creep on long prints. ABS is among the least troublesome here.
hotend tools that help here
Related
FAQ
- How do I tell a partial clog from badly calibrated flow?
- A clog is erratic: it comes and goes, worsens over time, and the nozzle sometimes 'spits'. Low flow is constant and uniform across the part. If extruding in mid-air gives a curly or thin line, suspect the clog.
- Does under-extrusion weaken the part much?
- Massively: gaps between lines are pre-made cracks. A functional part with visible under-extrusion can't be trusted — fix the cause and reprint.
- Why does it only show in the infill while walls come out fine?
- Because infill prints considerably faster than perimeters: it's the first place where the requested flow exceeds what your hotend can melt. Slow the infill down or raise the temp 5-10 °C; if slowing down makes it vanish, there's your diagnosis.
- The extruder chewed a notch into the filament and it won't feed — now what?
- Pull the filament, cut above the notch and reload. But the notch is a symptom, not the cause: the gear slipped because something downstream (a clog, low temp, the nozzle sealed against the bed) blocked the way. Fix that or it will chew again.
Sources & references
This page is cross-checked against public manufacturer documentation and reference technical guides:
- Manufacturer Prusa Knowledge Base — Under-extrusion ↗
- Technical guide Simplify3D — Print Quality Guide: Under-Extrusion ↗
Still stuck? Back to the quick diagnosis or check which materials your printer can handle.