Under-Extrusion in 3D Printing: Causes Ranked by Likelihood and How to Diagnose It

Recognizing under-extrusion by what you see

Three symptoms come up most often. Top layers that show holes or an uneven texture instead of a solid surface, even though the top layer count is set correctly in the slicer. Thin wall lines, almost see-through in places, that do not match the expected width for the nozzle diameter in use. Infill lines that no longer touch, leaving visible gaps inside the part once it is cut open.

According to the Prusa Knowledge Base, pronounced under-extrusion leaves parts fragile and prone to breaking apart, because the layers no longer have enough material to bond to each other well. Mild under-extrusion mostly shows up visually on outer surfaces before it turns into a mechanical problem.

The most likely causes, in order

Wet or tangled filament tops the list of common causes because it is easy to miss: filament that has absorbed moisture can crackle or foam slightly as it leaves the nozzle, cutting the real material flow without anything looking mechanically blocked. A tangle on the spool, harder to spot, jerks the filament's advance and produces uneven rather than constant under-extrusion.

Next comes a partially clogged nozzle: a deposit of carbonized filament or an impurity shrinks the effective diameter of the channel without fully blocking it, limiting the maximum flow the nozzle can deliver at a given speed. The clogged nozzle page covers how to confirm and fix that.

A nozzle temperature too low for the chosen print speed keeps the filament from melting fast enough to keep up with what the slicer is asking for, especially at higher speeds or with engineering filaments. A slipping extruder, from insufficient gear tension or filament shavings built up in the grooves, grinds against the filament instead of feeding it, an issue the Prusa Knowledge Base recommends checking regularly by cleaning the Bondtech gears and idler tension. Finally, filament diameter out of tolerance, common on cheaper spools, throws off the flow calculation the firmware applies continuously.

A step-by-step diagnosis method

Start with the simplest check: pull out the filament and look at it. Filament that crackles, swells slightly, or gives off a steam smell at the nozzle has likely absorbed moisture; drying it before printing fixes this without touching any slicer setting. If the filament looks dry, unroll a few meters from the spool to check for a knot or a crossed wrap that could jam mid-print.

Next, isolate mechanics from electronics: with the printer off, push the filament by hand between the extruder inlet and the heated nozzle. Unusual resistance there points to a partially clogged nozzle rather than a software setting. If the filament slides freely when cold, test a 5-10 °C bump above your usual profile on a simple test piece: if the under-extrusion disappears, the cause was thermal rather than mechanical.

Change only one variable between tests. Adjusting temperature and speed at the same time, for example, makes it impossible to tell which one actually fixed the problem, which often leads to over-correcting a setting that was not the culprit.

Checking the extruder and filament grip

A slipping extruder leaves a visible mark: small filament shavings or a flattened, smooth patch where the gear bites the filament without feeding it properly. Per the Prusa Knowledge Base, these deposits build up over time in the gear grooves and eventually reduce their grip, which calls for regular cleaning rather than automatic replacement. The idler tension screw also deserves a check: too tight, it crushes the filament instead of feeding it cleanly; too loose, it lets the gear slip with no effect.

A hotend cooling fan that is not spinning properly, or a printer caked in dust, can also trigger early heat creep that softens filament higher up in the heat break than expected, which contributes to extruder slipping even when the root cause is not mechanical.

Slicer settings to check before touching hardware

Before taking anything apart, check the extrusion multiplier (or flow rate) in the slicer: a value set too low deliberately reduces material flow and produces the exact symptoms of under-extrusion, with no physical fault on the printer at all. The Prusa Knowledge Base offers a calibration procedure for this setting to find the right value for your filament and nozzle combination.

A print speed too high for the nozzle diameter and chosen temperature asks the extruder to push more material per second than the nozzle can actually melt, which produces under-extrusion that worsens on the fastest parts of the model. Dropping speed by 10-20% on a test often confirms or rules out this cause before touching temperature at all.

When to replace the nozzle instead of cleaning it

A successful cold pull, described on the clogged nozzle page, fixes most partial clogs without replacing anything. If under-extrusion comes back quickly after a clean cold pull, or persists despite dry filament, an extruder that is not slipping, and a correctly calibrated extrusion multiplier, nozzle wear itself becomes the most likely explanation: brass nozzle openings widen slightly over time, especially with fiber- or metal-filled filaments, which changes the real flow without any clog being involved.

In that case, swapping in a new nozzle of the same diameter, and moving to a hardened-steel nozzle for abrasive filaments, fixes a problem that cleaning alone cannot solve.

Sources and limits

• Prusa Knowledge Base — Under-extrusion
• Prusa Knowledge Base — Under-extrusion (Nextruder printers)
• Prusa Knowledge Base — Extrusion multiplier calibration
• Bambu Lab Wiki — Under-Extrusion

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.