The walls are thin enough to see light through. The top surface never closes, so the infill shows as a grid of ridges and holes. Somewhere on the side there is a line that simply stopped for two centimetres and started again. Under-extrusion is the machine putting out less plastic than the slicer asked for, and unlike a warp or a layer shift it is rarely one dramatic event — it is a shortfall that repeats on every layer until the part is measurably weaker than the one you designed. There are five causes, they are not equally likely, and the print in front of you narrows them down before you touch a setting. This guide reads the symptoms, works the fixes in the order that costs least, and ends with the case where the right answer is having the part printed by a service.
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What Under-Extrusion Actually Is
The slicer calculates a volume of plastic for every line it draws, converts it to a length of filament, and tells the extruder motor to advance exactly that much. The motor almost always turns as instructed. What the machine has no way to check is whether that filament actually arrived at the nozzle and left it as a bead of the right cross-section.
Everything between the drive gear and the nozzle tip is an unmonitored assumption. The gear can spin against filament it is not gripping. The path can be restricted so that the pressure needed exceeds what the extruder can push. The molten plastic can be less dense than expected because it is full of steam. The orifice can be partly blocked or worn oversize. In each case the firmware believes the correct volume was delivered, and the part is short.
That is why raising the flow multiplier so often fails as a fix. It asks the same restricted path for more plastic, and a restricted path is exactly what cannot supply it.
Read the Print Before Touching a Setting
The pattern of the shortfall tells you which half of the machine to inspect:
| What you see |
What it usually means |
| Gaps between every line, consistently, from the bottom up |
Under-calibrated extruder steps, or flow set too low |
| Extrusion that starts fine and fades over the print |
Partial clog building, or a hotend at its thermal limit |
| Intermittent — good, gone, good again, at random heights |
Slipping extruder, spool drag, or wet filament |
| Only on fast sections, solid infill or thick layers |
Volumetric flow ceiling of the hotend |
| Sputtering, faint crackling, tiny surface pits |
Moisture in the filament |
| Gradually worse over weeks on abrasive filament |
Worn nozzle, oval orifice |
Take the failed part to a window and look along the wall. If every layer is equally short, it is a calibration or flow problem, which is free to fix. If it deteriorates with height or time, it is a hardware problem, which is not.
Fix It in This Order
Cheapest and most likely first. Most cases resolve before step five:
- Watch the extruder for two minutes at the start of a print. A gear that stutters backwards or clicks is slipping — that is a grip or a pressure problem, not a flow number.
- Push filament through by hand with the nozzle hot and the extruder disengaged. It should come out in a steady, straight strand with mild thumb pressure. Heavy resistance or a strand that curls hard to one side is a partial clog.
- Do a cold pull, then another, until the tip comes out clean.
- Check the filament for moisture — dry a spool that has been open for weeks before blaming the machine.
- Inspect the nozzle and the tube for wear, scorching and gaps.
- Calibrate extruder steps, then flow, in that order and last.
The Nozzle: Partial Clogs and Wear
A total blockage is easy — nothing comes out and you know immediately. A partial clog is the one that produces weeks of quietly bad prints, because plastic still flows, just less of it and inconsistently.
The usual cause is a fragment of carbonised material or dust sitting in the melt zone, which restricts the channel without sealing it. A cold pull removes it: heat the nozzle, feed filament through, drop the temperature to around 90°C for PLA, and pull sharply so the plug comes out attached to the tip. A cleaning kit with acupuncture needles and cleaning filament is the right tool here; a needle alone clears the final orifice but does not reach the debris sitting above it.
Wear is the other half. Brass is soft, and printing carbon-fibre, glass-filled or glow-in-the-dark material erodes the orifice from round to oval within a spool or two. The line width then no longer matches what the slicer assumed, and the shortfall creeps up so gradually that people tune around it for months. A nozzle costs about a dollar — replacing it is faster than diagnosing it, so keep a set of spare 0.4mm nozzles in the drawer, and if you print abrasives at all, fit a hardened steel nozzle and stop replacing brass every fortnight. Our guide to nozzle sizes and materials covers which diameter suits which job.
The Feed Path: Extruder, Tube and Spool
If the nozzle is clear and the strand still will not come through under hand pressure, the restriction is behind it.
The drive gear. Look at the filament where the gear grips it. Clean teeth marks mean it is driving properly; a polished flat or a pile of shaved plastic dust in the housing means it is slipping. Tighten the idler tension a little, clean the gear teeth with a brush, and check the spring is not collapsed. A single-sided drive gear is the weak point on many budget machines under high flow, and a dual-drive extruder that grips from both sides is the standard fix.
The tube. On a bowden or a PTFE-lined hotend, the tube must sit hard against the back of the nozzle. If it has been removed and refitted without cutting the end square, a gap opens where molten plastic collects, cools and forms a plug that resists every retraction after it. A scorched, narrowed or brown-tipped tube is finished — replace it with fresh PTFE tubing, cut it square, and seat it fully.
The spool. A spool that binds on its holder, or filament that has crossed under itself, adds a load the extruder was never sized for. It shows up as intermittent under-extrusion with no other pattern. Lift the spool and turn it by hand — if it needs a real pull, that is the print's problem too.
Moisture, and the Spool You Assumed Was Fine
Wet filament under-extrudes without any mechanical fault at all. Absorbed water turns to steam in the melt zone, disrupting the flow and leaving voids, pitting and a faint crackle you can hear from a metre away. PETG, nylon, TPU and polycarbonate get there in days in an ordinary room; PLA takes longer but gets there too.
Dry the spool at the manufacturer's temperature for four to six hours and reprint the identical file before changing anything else. If that fixes it, storage is the real problem — a heated dryer that also feeds the printer keeps it fixed, and our round-up of filament dry boxes compares the practical options. Moisture is also the cause behind most stringing, so if your prints are hairy as well as gappy, one change fixes both.
While the spool is off, measure it. Filament sold as 1.75mm should be within about 0.03mm along its length; a spool running consistently at 1.68mm delivers roughly eight percent less plastic per millimetre fed, which looks exactly like a machine fault. A pair of digital calipers and five measurements along a metre settles it in two minutes, and choosing filament on tolerance rather than price avoids the repeat.
Flow, Temperature and the Speed Ceiling
Only now do the settings matter.
Extruder calibration comes before flow. Mark 120mm of filament above the extruder, command a 100mm extrusion, and measure what is left. If it pulled 92mm, the steps-per-mm or rotation distance is wrong, and every print you have ever made was short by that fraction. Fix that number rather than compensating for it downstream.
Temperature buys flow. Plastic pushed too cold is viscous, and the pressure needed exceeds what the extruder can deliver, so it slips. If under-extrusion appears only at speed, print a temperature tower and take the highest temperature that does not string or sag — that is often the whole fix.
And there is a hard ceiling. Line width times layer height times speed is a volumetric demand in mm³/s, and a stock hotend supplies roughly 8-15mm³/s on PLA, less on PETG. Beyond it the filament simply has not melted by the time it must leave, and no slicer number changes that. Slow down, thin the layers, or change the hotend. Where the machine has been rebuilt to its limit, a newer hot end designed for high flow is the honest answer — both the ELEGOO store's FDM machines and hotend spares and Anycubic's FDM range and hotend parts list by model, which avoids matching a thread pitch or a heat-break length by eye.
When To Stop Tuning and Send the File Out
Under-extrusion is worth fixing on a machine you will keep using. It is not worth fixing on a deadline. Each diagnostic cycle is a test print plus a change, a partial clog can take an evening to chase, and a machine at its flow ceiling will meet that ceiling again on the next large part — a structural limit, not a fault.
If the part is large, in an engineering material, or due to someone, the shorter path is a shop whose machines are calibrated and monitored as a matter of routine. Browse providers by location and process, send the same STL to two or three of them, and compare the quote against another scrapped spool and a lost weekend.
Hero photograph by Jakub Żerdzicki on Unsplash.
