Last updated: October 22, 2026

How to fix under-extrusion on 3D prints

Your print has gaps between the lines, thin see-through walls, or it crumbles when you squeeze it. That's under-extrusion: the printer is putting down less plastic than the slicer asked for. The good news is that it's nearly always one of a handful of causes, and most are quick to check. Work down this list in order and stop when your lines look solid again.

How to recognize under-extrusion

Gaps between lines, or between the walls and the infill
Thin, wispy walls and a top surface you can see through
Missing or very thin layers, sometimes in a band part-way up the print
Prints that feel light and break or crumble easily
A clicking or knocking sound from the extruder as it slips on the filament

Bambu Lab splits it into two kinds, and it's a useful clue. Global under-extrusion shows on the whole print, which points to speed, temperature, flow or a clog. Local under-extrusion shows only in some places. If it's only at the corners, it's usually a calibration setting, not a hardware problem (more on that in step 8).

Before you change anything, ask yourself what changed recently: a new spool, a new nozzle, a faster print profile, a move to a warmer room? Prusa's under-extrusion guide makes the same point. The cause is often the last thing you touched.

Step 1: Check the spool and the filament path

The most common cause is also the easiest to miss: the filament can't get to the nozzle freely. Bambu Lab lists these first:

The spool can't turn freely, or the filament is tangled or crossed over itself. Turn the spool by hand; it should spin easily.
The tube is kinked, worn or has something stuck in it. Follow it from the spool to the print head.
The filament has snapped or flattened inside the path, often from a spool that sat out for a long time.

Step 2: Make sure the first layer isn't squished

If the gaps start right on the first layer, your nozzle may be too close to the bed. Prusa warns against "squishing" the first layer to fix adhesion: when the nozzle is too close, it physically blocks the plastic from coming out. Set the height properly using how to level a 3D printer bed, and redo it after you move the printer, change the nozzle or swap build plates.

Step 3: Use the right profile, and don't outrun the hotend

A hotend can only melt plastic so fast. Ask it to print faster than that and it can't keep up, so lines come out thin. Both Prusa and Bambu Lab recommend starting from the stock profile for your exact printer, nozzle size and filament type.

Third-party filament on a fast printer: Bambu Lab suggests choosing the Generic profile for that material, which prints more slowly, instead of a Bambu filament profile.
"Sport" or "Ludicrous" speed modes: these push the hotend hard. Try normal speed first.
Check the nozzle size and filament diameter in the slicer match what's actually on the printer (almost always 0.4 mm and 1.75 mm for beginners).

Step 4: Try a slightly hotter nozzle

If the whole print is under-extruded, printing a little hotter helps the plastic melt and flow faster. Bambu Lab suggests raising the nozzle temperature slightly or reducing speed, and about 10 °C more for its fastest mode. Prusa suggests adjusting by 5 to 15 °C for filament brands its slicer doesn't list. Stay inside the temperature range printed on the spool, and raise it 5 °C at a time: going too hot causes stringing. To find the best temperature for a spool in one print, use a temperature tower.

Using a hardened steel or stainless nozzle? Prusa notes these usually need 5 to 10 °C more than brass.

Step 5: Clear a partial clog

A nozzle doesn't have to be fully blocked to cause trouble. A partial clog gives thin lines, filament that curls sideways out of the tip when you load it, and clicking. Bambu Lab notes it's common after long prints or after fiber-filled filaments. A hot push or a cold pull clears most of them in a few minutes. Our step-by-step guide covers both: how to fix a clogged 3D printer nozzle.

If you recently took the nozzle out, check it went back in correctly. Prusa says an incorrectly installed nozzle can cause both clogs and leaks, so follow your printer's own nozzle guide.

The nozzle runs at 200 °C or more. Use heat-resistant gloves or pliers, keep your face away from the nozzle when pushing filament, and follow your printer maker's guide when it differs from ours.

Step 6: Dry your filament

Damp filament boils in the nozzle. You'll hear popping or hissing and see bubbly, rough, uneven lines that can look a lot like under-extrusion. Prusa says moisture especially affects PETG, ABS and soluble filaments, and recommends storing spools in their bag with silica gel, ideally in a dry box. See how to dry 3D printer filament for temperatures and methods.

Changing material? Prusa's tip: load the new filament at the higher of the two temperatures. For example, if you printed PETG before PLA, load the PLA at PETG temperature so it pushes the old plastic out.

Step 7: Check the extruder gears and the hotend fan

Extruder gears: plastic dust packed into the teeth means they slip instead of grip. Clean them following your printer's guide. Prusa also notes the extruder tension can cause under-extrusion if it's too loose or too tight, so leave it at the factory setting unless your maker says otherwise.
Hotend fan: this fan keeps the top of the hotend cool so filament only melts at the nozzle. If it's blocked by a stray strand or not spinning, plastic softens too high up and jams. This is called heat creep. Prusa says to check the fan is clear and, in an enclosure, that it isn't too hot inside. PLA in a closed, warm enclosure is a classic cause, so many makers suggest opening the door or lid for PLA.

Step 8: Calibrate flow, last

Only when the mechanical causes are ruled out should you touch flow settings. Modern slicers have a built-in test that prints a set of samples so you can pick the best one:

Small gaps everywhere: run the flow rate (or "extrusion multiplier") calibration. Prusa and Bambu Lab both have step-by-step guides (see sources). Bambu Lab recommends keeping the default flow ratio if you use its own filament.
Gaps only at corners: Bambu Lab says this usually means the pressure advance value is wrong. Run its flow dynamics calibration rather than raising flow.

Raising flow by guesswork can make things worse: blobs, rough top surfaces and parts that don't fit.

Quick reference: symptom and likely cause

What you see	Most likely cause	Go to
Gaps on the first layer only	Nozzle too close to the bed	Step 2
Thin lines everywhere, worse at high speed	Printing faster than the hotend can melt	Steps 3 and 4
Clicking extruder, curly filament when loading	Partial clog	Step 5
Popping or hissing, rough bubbly lines	Wet filament	Step 6
Starts fine, then fades or stops part-way	Tangled spool, partial clog or heat creep	Steps 1, 5 and 7
Gaps only at corners	Pressure advance not calibrated	Step 8
Small gaps between lines after everything else checks out	Flow slightly low	Step 8

Sources

Checked October 22, 2026: Prusa Knowledge Base, Under-extrusion, Under-extrusion (Nextruder printers), Extrusion stopped mid-print (heat creep) and Extrusion multiplier calibration; Bambu Lab Wiki, Under-extrusion and Flow rate calibration.

Your next step

Lines solid again? If your prints now look hairy, read how to fix stringing. If they won't stay down on the bed, try why your first layer isn't sticking. Layers stepping sideways part-way up? Read how to fix layer shifting.

Frequently asked questions

What does under-extrusion look like?

Gaps between lines, thin or patchy walls, a top surface you can see through, weak infill, missing layers, and prints that snap or crumble easily. It can be everywhere on the print (global) or only in some places, like the corners or one band of layers (local).

What is the most common cause of under-extrusion for beginners?

Something slowing the filament down before it ever reaches the nozzle: a spool that catches or is tangled, a kinked tube, or a partly clogged nozzle. Printing too fast or too cold for the filament is the next most common, especially on fast printers with third-party filament.

Should I just increase the flow rate?

Not first. Raising flow hides the real problem, and if the cause is a clog or a catching spool it comes back. Bambu Lab recommends keeping the default flow ratio with its own filament. Fix the mechanical and temperature causes first, then use your slicer's flow rate calibration if small gaps remain.

Why does under-extrusion only show up halfway through a print?

Common reasons are the spool tangling or catching part-way, filament with a thicker or damaged section, a partial clog building up, or heat creep, where filament softens too high up the hotend. Heat creep is common with PLA in a hot enclosure. Prusa's guidance is to make sure the hotend fan works and the enclosure isn't too hot.

Can wet filament cause under-extrusion?

Yes. Damp filament turns to steam in the nozzle, which gives bubbly, rough, inconsistent lines. Prusa notes moisture especially affects PETG, ABS and soluble filaments. Drying the spool usually brings it back.