Gaps in walls, perimeters that don’t close, top surfaces that look like a mesh. The printer is moving correctly but not enough plastic is coming out.
Narrow It Down First
Under-extrusion has four distinct causes that require different fixes. Applying the wrong fix wastes time.
Test 1 — Extrude 100mm by hand. Heat the nozzle, mark the filament 110mm from the extruder, command 100mm extrusion. If less than 95mm moves through, your e-steps are off or the extruder is slipping. If exactly 100mm moves, the extruder is fine — the problem is downstream.
Test 2 — Cold pull. Heat to print temp, then cool slowly to 90°C (PLA) or 120°C (PETG/ABS) and pull firmly. If the pulled filament tip is rough, discolored, or has debris embedded, your nozzle is partially blocked.
Test 3 — Listen. A clicking extruder during printing is the motor skipping — the resistance downstream is too high for the motor to push through. Clicking = blockage or temperature issue, not an e-steps problem.
Fixes
Partial nozzle clog (most common cause)
Do a cold pull first. If under-extrusion started gradually over sessions, not suddenly, it’s usually partial clogging. Heat to 10°C above your normal print temp for 5 minutes to let any carbonized debris soften, then print slowly at reduced speed. Replace the nozzle if cold pulls keep coming out dirty — a worn or debris-filled brass nozzle isn’t worth cleaning multiple times.
E-steps miscalibration
Command 100mm, measure actual movement, scale: new_steps = current_steps × (100 ÷ actual_mm). Do this cold (no heat) to measure only motor movement, not melt flow. A single calibration is usually enough; e-steps don’t drift unless you swap extruder hardware.
Temperature too low for speed
If gaps appear only at high speed or on long perimeters, the hotend can’t melt fast enough. Either raise temperature 5°C or reduce speed by 20%. Volumetric flow rate matters: a 0.4mm nozzle at 0.2mm layer height at 100mm/s is pushing ~3.2mm³/s — most hotends are fine with this, but budget clone hotends cap around 2–3mm³/s.
Filament diameter inconsistency
Cheap filament varies ±0.1–0.2mm in diameter, which translates directly to ±10–20% flow variation. Measure at 5 points along 30cm of filament with calipers. If range exceeds 0.05mm, set your slicer diameter to the average measured value, not the nominal 1.75mm.
Worn extruder gear
The drive gear teeth get rounded off over time, especially with abrasive filaments. A worn gear grips less and slips under load. Look at the filament after extrusion — if you see shallow grooves instead of sharp teeth marks, the gear is worn. Replacement gears are under $5 and fix this permanently.
What Doesn’t Fix It
Increasing flow rate in the slicer is a band-aid — it adds more commands but the root restriction is still there. You’ll hit the same wall sooner. Find the actual cause.