Under-extrusion

Less plastic is coming out than the slicer asked for. The result is gappy walls, weak parts and a top surface you can see through. The causes split cleanly into "something is blocking it" and "the numbers are wrong" — check the blockage first, because tuning numbers around a partial clog wastes hours.

Looks like: Thin, patchy or missing lines; walls with gaps between them; weak brittle parts.

Fixes, in the order worth trying

1 · Try this first

Check for a partial clog

A partially blocked nozzle under-extrudes intermittently, which is why the problem seems to come and go. Test it before touching any setting: heat the nozzle and push filament through by hand.

Heat to printing temperature, release the extruder tension, and push filament through by hand. It should come out in a smooth straight line with light pressure. If it curls hard to one side, dribbles, or needs real force, do a cold pull (heat, cool to ~90 °C for PLA, pull the filament out sharply) and inspect the tip for burnt residue.

2 · Try this first

Raise the nozzle temperature

Plastic that is too cool is too viscous to push at print speed. This shows up first on fast prints and thick layers — the same profile that works at 40 mm/s under-extrudes at 150 mm/s because the hotend cannot melt fast enough.

Printing Temperature: Cura → Material → +5 °C at a time, up to the high end of the spool label
Nozzle temperature: OrcaSlicer → Filament → Filament → +5 °C per test

3 · Common cause

Set the correct filament diameter and calibrate flow

The slicer computes volume from a diameter you told it. If the profile says 2.85 mm and your filament is 1.75 mm, it will extrude a fraction of what is needed. After that, flow rate trims the remaining error.

Diameter: Cura → Material → 1.75 mm for almost all desktop printers
Flow: Cura → Material → 100 %, then ±2–5 % after a calibration cube
Flow ratio: OrcaSlicer → Filament → Filament → 0.98–1.02 typical after calibration

4 · Common cause

Check extruder tension and the drive gear

If the gear cannot grip, it grinds a flat into the filament and stops feeding. Look for plastic dust in the extruder and a shiny flat patch on the filament just above the gear.

Open the extruder, blow out any ground plastic packed into the gear teeth, and set the idler tension so the filament is gripped firmly but not crushed out of round. Over-tightening deforms the filament and makes feeding worse.

5 · Less common

Slow down or raise line width

There is a hard limit to how many cubic millimetres per second a hotend can melt — roughly 8–12 mm³/s for a stock V6-style hotend with PLA. Past that no setting helps; you are asking for more plastic than the heater can melt.

Print Speed: Cura → Speed → −20 % as a test
Max volumetric speed: OrcaSlicer → Filament → Filament → 8–12 mm³/s stock hotend · 20+ mm³/s high-flow

Change one thing at a time. If you adjust temperature, retraction and speed together and the problem goes away, you have learned nothing and you will be back here next month. Re-print the same test object after each single change.

Frequently asked questions

How do I tell under-extrusion from a clog?

Consistency. A clog is usually intermittent or gets progressively worse within one print, and hand-extruding feels wrong. Genuine under-extrusion from settings is uniform — every layer is equally thin, and the nozzle extrudes cleanly by hand.

Can wet filament cause under-extrusion?

Yes, indirectly. Steam bubbles in the melt zone interrupt the flow and leave voids, which looks like intermittent under-extrusion and often comes with popping sounds. If the filament also strings badly, moisture is the likely common cause.

Is a bigger nozzle a fix?

It raises the ceiling rather than fixing a fault. A 0.6 mm nozzle can push far more plastic per second than a 0.4 mm, so it helps if you are genuinely at the melt limit on fast or large prints. It will not help at all if the real cause is a clog, moisture or extruder slip.