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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.

Sources for this page: Prusa Knowledge Base · Manufacturer Simplify3D · Technical guide

Causes and fixes, ranked by likelihood

  1. 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. 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. 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. 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. 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 ↗

hotend tools that help here

Printer comparator → Compare your printer's flow (mm³/s) and max temps against others — asking for more than it delivers makes under-extrusion inevitable.

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.

Sources & references

This page is cross-checked against public manufacturer documentation and reference technical guides:

Still stuck? Back to the quick diagnosis or check which materials your printer can handle.