Under-Extrusion

Under-extrusion means the printer is delivering less plastic than the slicer expects. Treat it like a supply chain problem: first confirm the symptom, then remove drag or slipping in the feed path, then rule out a partial clog/heat creep in the hotend, and only then adjust slicer demand (speed/temperature/flow) and validate with a small repeatable test.

TL;DR

If you see thin lines, gaps, or missing top skin, first check for filament drag or extruder slipping (spool tangles, tight guides, wrong idler tension), then rule out a partial clog/heat creep, and validate with a single-wall cube before touching flow %.

Under-Extrusion Decision MapTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.MoistureRetractionTemperatureTravel
Start at the symptom, then check feed (spool to extruder), melt (hotend/nozzle), and finally slicer flow limits.

What Under-Extrusion Looks Like (and Why the Pattern Matters)

Under-extrusion shows up as gaps between lines, perimeters that don’t touch, infill that doesn’t weld to walls, weak or see-through walls, and top surfaces with holes. Constant under-extrusion across the whole print often points to slicer demand (too fast/too cool/flow limited) or a consistently restricted nozzle. Intermittent missing segments, clicking, or “good then bad” behavior is more typical of feed problems (snags, friction, slipping) or heat creep/partial jams that come and go.

Fast Checks Before You Change Settings

  • Confirm the right profile: correct filament diameter (1.75 vs 2.85) and material type.
  • Confirm nozzle diameter in the slicer matches the installed nozzle.
  • Watch the spool while printing: it should unwind smoothly with no jerks, tangles, or filament crossing under a loop.
  • Check the filament path for friction: sharp bends, tight guide holes, runout sensor drag, or debris in a filament guide.
  • Inspect the extruder drive area: plastic dust/“shavings” usually means slipping or too much idler pressure.
  • Set idler tension to grip without crushing: too loose slips; too tight grinds and increases drag.
  • Verify the hotend heatsink fan is actually spinning and unobstructed (a common cause of heat creep and intermittent jams).

Symptom → Likely Cause → First Fix

Gaps everywhere; lines consistently thin

Likely cause: You’re asking for more flow than the hotend can melt at that speed/temperature, or the profile is limiting flow/line width

Fix: Lower print speed 20–30% or lower max volumetric speed; raise nozzle temperature 5–15 C; re-test the same small model.

Extruder clicking; filament chewed; missing segments

Likely cause: Back-pressure is spiking (partial clog, too low temperature, excessive retraction, heat creep) causing the drive to slip

Fix: Raise temperature 5–15 C and reduce retraction distance/speed; if it persists, clean/replace nozzle and check the filament path for drag.

Starts fine, then under-extrudes after minutes/layers

Likely cause: Heat creep softening filament above the heatbreak, or feed resistance that worsens as the spool position changes

Fix: Check heatsink fan airflow and ducting; reduce retraction; for PLA try printing with enclosure open; confirm the spool doesn’t snag as it unwinds.

Top layers have holes but walls look OK

Likely cause: Not enough top thickness or top speed too high for reliable fill; sometimes compounded by mild under-extrusion

Fix: Increase top layers/thickness and slow top surface speed; only tune these after extrusion is stable on a single-wall test.

Only happens with a new roll

Likely cause: Material needs different melt settings, is wet, or has inconsistent diameter/ovalness

Fix: Dry the filament, then raise temperature 5–15 C and re-test; if still inconsistent, measure filament and consider a different roll.

Fails around the same Z height repeatedly

Likely cause: A repeatable snag (tangle, guide contact) or a recurring jam trigger (kinked filament section, partial clog catching)

Fix: Watch the full filament path at the failure height; if feed is clean, inspect/clear the nozzle and heatbreak path for partial blockage.

Fix Order (Least Risk, Most Likely First)

  1. Remove feed resistance: untangle spool, reduce sharp bends, clean/relieve runout sensor or guides.
  2. Restore extruder grip: clean the drive gear, remove filament dust, adjust idler tension so it grips without grinding.
  3. Reduce melt demand: slow the print (or lower max volumetric speed) and then add a small temperature bump if needed.
  4. Clear restrictions: cold pull where appropriate, replace a suspect nozzle, and verify the PTFE path (if your hotend uses PTFE) is cut cleanly and seated fully.
  5. Only after hardware/feed are stable: tune extrusion multiplier/flow using a repeatable calibration print.