Under-extrusion means the printer deposits less plastic than the toolpath expects. It can create gaps between lines, thin walls, weak layers, incomplete infill, or a surface that alternates between normal and starved flow.
Quick diagnosis
Inspect the slicer preview before the machine. Sparse infill, a deliberate seam gap, top-surface patterns, and very thin model walls can look incomplete by design.
Under-extrusion is more likely when you see:
- adjacent perimeter or top lines that do not meet;
- intermittent missing sections across several features;
- walls thinner or weaker than the same known-good profile normally produces;
- feeder clicking or ground filament along with reduced output;
- a free-air purge that is unusually thin or inconsistent.
A sudden X/Y offset is layer shifting, not an extrusion problem.
Fix 1: restore the exact profile
Check printer, nozzle diameter, filament diameter, material, and quality preset. Re-slice if any hardware or material changed. An old 0.4 mm toolpath cannot simply be sent after fitting a 0.6 mm nozzle.
Remove experimental flow, speed, pressure-advance, and temperature overrides. A trusted baseline lets later tests answer one question at a time.
Verify
Use the slicer preview to confirm that walls and top surfaces are actually present. Print a small known-good part rather than retrying a multi-hour job.
Fix 2: remove spool and path resistance
Follow the filament from spool to feeder:
- Make sure no winding traps the loose end.
- Check that the spool holder turns without binding.
- Inspect guides, sensors, multi-material paths, and Bowden tubing for sharp bends or debris.
- Confirm that flexible filament is not buckling into an open gap.
Manually move components only under the printer maker’s safe power and temperature conditions.
Fix 3: inspect the feeder
Unload filament and check the drive gear. Packed plastic reduces grip. Clean it with the recommended brush or procedure, then set idler tension to the documented range.
More tension is not always better: it can crush filament, increase drag, and make TPU or other flexible materials buckle. Less tension can let hard filament slip.
Mark a short section of incoming filament and observe it during a slow purge. Smooth advance with poor nozzle output points downstream; stuttering at the gear points to feed resistance or inadequate grip.
Fix 4: check for a partial clog
A partial obstruction can allow some flow while starving high-demand moves. Purge at the material’s supported temperature using the printer’s loading routine and compare with a known-good result. Curling by itself does not establish a clog: residue on the tip can deflect a strand. Interrupted output together with feed symptoms deserves the clogged-nozzle diagnostic sequence.
Do not compensate for a blockage by permanently raising flow percentage. That can overload the feeder and hides the actual fault.
Fix 5: reduce melt demand
The hotend must melt a volume of plastic per second. A tall layer, wide line, high speed, or larger nozzle all increase volumetric flow. When demand exceeds capacity, extrusion can fall behind even when the mechanics are healthy.
Reduce print speed or the slicer’s maximum volumetric flow using the printer/material profile. If consistency returns, the original toolpath was probably too demanding for that combination.
Fix 6: tune temperature within limits
If the stock profile still shows flow starvation at a reasonable speed, raise nozzle temperature in small increments within the filament and hardware ranges. Keep cooling and speed unchanged during the test.
Look for continuous lines and good bonding without new stringing, sagging, discoloration, or heat-creep symptoms. The PLA temperature guide explains a controlled tower test.
Fix 7: evaluate filament condition
Check actual diameter only with a suitable measuring method and compare it with the product specification; do not change the slicer diameter based on one reading. Inspect for severe ovality, contamination, or brittle sections.
Popping, bubbling, rough flow, and increased stringing can be consistent with moisture. Dry only according to the filament and spool supplier’s procedure. Learn the difference between dry storage and active drying in How to Store 3D Printer Filament.
Fix 8: calibrate flow only after hardware checks
Flow ratio or extrusion multiplier can tune a healthy system, but it should not mask a clog, slipping feeder, wrong nozzle profile, or excessive speed. Use a documented calibration model and measurement method for your slicer and printer.
E-steps or rotation distance are firmware-level motion settings and are not appropriate adjustments on every machine. Change them only when the printer or firmware documentation explicitly directs you to do so.
How to verify the fix
Repeat the same coupon at the same orientation. Inspect continuous perimeters, joined top lines, stable feeder motion, and layer strength. Then increase demand gradually toward your normal profile.
If the printer passes a slow test but fails a fast one, focus on volumetric flow and temperature. If it fails at every speed, return to the path, feeder, and nozzle.
Separate a flow limit from a global feed problem
Use a short sample with slow outer walls and a faster solid region, then inspect where lines become incomplete. If slow regions look continuous and faster regions do not, test a lower demand while keeping temperature and material fixed. If all regions are weak, inspect the path, profile, and material condition instead. Record actual slicer flow estimates where available; a speed number alone omits line width and layer height. A slower successful test does not prove that the hotend is clean, but it gives a useful next comparison.
Source and scope
Prusa’s under-extrusion documentation separates first-layer clearance, profile, nozzle installation, feeder gears, hotend cooling, and material checks. Its service instructions apply to the listed Prusa models; use the equivalent procedure for your own machine.