Under-Extrusion in 3D Printing: Causes, Diagnosis and Fixes

What Is Under-Extrusion?

An FDM printer builds a part by laying down thin lines of melted plastic. For every move, your slicer calculates how much filament needs to be pushed into the hot end so that the line comes out at the planned width and height. Under-extrusion is simply what happens when the amount that actually comes out is smaller than that plan.

The printer doesn't know this is happening. It keeps moving as if every line were full width, so the shortfall becomes visible as gaps, thin walls or weak bonds between layers. That is why under-extrusion is best thought of as a symptom rather than a single problem: anything that restricts, slows or miscounts the flow of plastic can produce it.

What Does Under-Extrusion Look Like?

The signs range from subtle to obvious. The most common ones are:

• Gaps between extrusion lines. On top surfaces and in solid infill, you can see daylight or the infill pattern between lines that should be touching.
• Thin or incomplete walls. Perimeters look narrow, slightly translucent, or have small holes where the inner and outer walls don't fuse.
• Weak layers. Thin lines have less contact area with the layer below, so parts can snap or peel along layer lines with little force.
• Inconsistent extrusion. Some lines look normal while others look stringy or thin, sometimes changing within a single layer.
• Missing sections of lines. Short segments simply aren't there, often right after a travel move or retraction.
• Symptoms that get worse during a print. The bottom of the part looks fine, but the problem appears or grows as printing goes on.

When and where the symptom appears is often more useful than how it looks. A problem that affects every layer points in a different direction from one that only appears on fast infill, which points elsewhere again from one that starts an hour into the print.

How Is Under-Extrusion Different From Other Print Defects?

Several problems look similar at a glance. A few quick ways to tell them apart:

• Over-extrusion is the opposite — too much plastic. You see blobs, bulging walls, rough top surfaces and parts that are slightly oversized.
• Rough or bumpy surfaces can come from either over- or under-extrusion, wet filament or temperature. If your main complaint is texture rather than gaps, our guide to why 3D prints come out rough covers that broader picture.
• A bad first layer with gaps is very often a Z-offset issue — the nozzle is too far from the bed — rather than true under-extrusion. If only the first layer is affected, check bed leveling and Z-offset first.
• Layer shifts move whole layers sideways. That's a motion problem (belts, stepper motors), not a flow problem.
• Pillowing (bumpy, sunken top surfaces) is often caused by too few top layers or insufficient cooling, though under-extrusion can make it worse.

Symptom → Likely Cause → What to Check → Fix

Use this table to decide where to look first. These are diagnostic clues, not proof: one symptom can have several causes, and more than one cause can be present at the same time.

What Causes Under-Extrusion?

Every cause below reduces flow in one of three ways: it restricts the path the plastic travels, it stops the filament being pushed reliably, or it makes the slicer miscalculate how much to push. Knowing which group a cause belongs to makes it easier to test.

Is a partial nozzle clog restricting flow?

A full clog stops extrusion entirely and is easy to spot. A partial clog is sneakier: a bit of burnt plastic, dust or a particle from the filament narrows the opening, so the extruder has to push harder to get the same amount through. Often it can't, and the result is thin lines across the whole print or intermittently. A good test is to heat the nozzle and extrude a short length by hand from the printer menu — a healthy nozzle produces a smooth, straight strand that falls down; a partially clogged one tends to curl sharply to one side, come out thin, or sputter.

Is the printing temperature too low?

Colder plastic is thicker and harder to push. If the nozzle is at the bottom of — or below — the filament's recommended range, the hot end may not melt filament fast enough, and the extruder meets more resistance. This often appears together with weak layer bonding. Filament makers print a recommended range on the spool or in their data sheet; start there, and see our 3D printing temperature guide for how to find a good value within that range.

Are you printing too fast?

Speed itself doesn't cause under-extrusion; the amount of plastic the speed demands does. At a higher speed, the hot end has less time to melt each millimeter of filament. Past a certain point it can't keep up. This is the most common reason gaps appear only on fast infill or long straight walls, and it's explained in detail in the volumetric flow section below.

Is the extruder tension wrong?

Most extruders squeeze the filament between a drive gear and an idler, adjusted by a spring or screw. Too little tension and the gear slips over the filament instead of pushing it. Too much and the gear can deform soft filament, digging a groove until it loses grip, or flatten it enough to add friction further down the path. Your printer or extruder manufacturer usually documents a recommended setting.

Is the extruder gear slipping or grinding?

When something downstream resists — a clog, cold nozzle or excessive speed — the gear can't advance the filament, so it spins against it. You'll often hear a rhythmic clicking and find fine plastic dust or a chewed section on the filament. Slipping is usually a result of resistance elsewhere, so treat it as a clue rather than the root cause. Teeth clogged with plastic dust can also lose grip on their own; cleaning them with a small brush can help.

Is there resistance in the filament path?

Between the spool and the nozzle, filament can rub against guides, sharp bends, a filament sensor or the entry of the extruder. Each bit of drag uses up some of the extruder's pushing force. You can feel this by pulling filament through the path by hand with the extruder tension released: it should slide smoothly with gentle effort.

Is the spool feeding freely?

A spool that binds on its holder, a crossed-over winding or a tangle can suddenly stop filament feeding, creating random bands of under-extrusion or a complete stop. Spool drag can also rise as the spool empties, because the extruder has to pull a smaller-diameter coil that's harder to rotate on some holders. Watching the spool for a few minutes during a print tells you a lot.

Is the Bowden tube causing problems?

On printers with a Bowden setup (the extruder is mounted on the frame and pushes filament through a PTFE tube), a tube that has slipped out of its fitting can leave a gap at the hot end where molten plastic collects and partially blocks flow. Worn fittings, tight bends and a damaged tube end can also add friction. This doesn't apply to direct-drive printers, where the extruder sits on the toolhead.

Is the flow rate or extrusion multiplier set too low?

The slicer's flow (also called extrusion multiplier) scales how much filament is pushed for every move. A value below what your setup needs produces consistent under-extrusion on every layer. Some printer profiles and filament profiles stack two flow settings, so check both. Flow calibration should come after the mechanical and thermal checks: raising flow to cover a clog or speed limit only masks the real problem.

Is the filament diameter setting correct?

Slicers calculate extrusion volume from the filament diameter you enter. If a printer that uses 1.75 mm filament is set to 2.85 mm, or vice versa, extrusion will be dramatically wrong. It's a rare mistake on a stock profile but common after importing settings or switching printers. Filament that is consistently thinner than its labeled diameter will also under-extrude slightly, which is why diameter tolerance matters.

Is the filament wet or degraded?

Many filaments absorb moisture from the air. When wet filament hits the hot end, the water turns to steam, which can cause popping, bubbles and voids that look like inconsistent extrusion. Old or heat-damaged filament can also become brittle and snap in the path. If extrusion became inconsistent after a spool sat out for weeks, drying is a reasonable step; our filament dryer guide explains the options. Follow the filament maker's drying temperature and time — they differ by material.

Filament quality can matter here too. Spools with loose diameter tolerance or inconsistent material can cause flow to drift along the roll. When you've ruled out the printer and settings, printing the same file with a spool from a manufacturer that publishes its tolerances and recommended settings — for example, a spool from Polymaker's filament store (affiliate link) — can act as a control: if the problem disappears, the original spool was a factor; if it doesn't, keep looking at the printer.

Are worn or damaged components involved?

Nozzles wear over time, especially with abrasive filaments such as carbon-fiber or glow-in-the-dark blends, which can change how plastic flows. Drive gears wear smooth, PTFE liners degrade, and thermistors or heater cartridges can fail or come loose, giving incorrect temperatures. These are usually the last things to suspect, after simpler checks.

Could heat creep be the cause?

Heat creep happens when heat travels up from the hot end into the area where filament should still be solid. The filament softens too early, swells and jams, causing extrusion to fade or stop — typically after the printer has been running for a while. A failed or clogged hotend cooling fan, a very warm enclosure, or very slow printing with low-temperature materials like PLA can contribute. If extrusion fades mid-print and recovers after cooling down, heat creep is worth investigating.

What Is Volumetric Flow and Why Does It Matter?

Think of your hot end like a kettle with a fixed-size heating element. It can melt only so much plastic per second. If you ask for more than that, you don't get faster printing — you get plastic that isn't fully melted, more resistance at the nozzle, and less material coming out.

That amount is called volumetric flow, usually measured in cubic millimeters per second (mm³/s). The flow a line needs can be estimated with a simple formula:

Multiplying millimeters by millimeters by millimeters-per-second gives cubic millimeters per second — the volume of plastic the hot end must deliver every second.

Illustrative examples

The numbers below are worked arithmetic examples, not measurements from any specific printer:

• 0.2 mm layer × 0.45 mm line × 50 mm/s ≈ 4.5 mm³/s
• 0.2 mm layer × 0.45 mm line × 150 mm/s ≈ 13.5 mm³/s
• 0.3 mm layer × 0.5 mm line × 150 mm/s ≈ 22.5 mm³/s

The same printer that handles the first case easily may not be able to deliver the third. Hot end capacity depends on its design, the nozzle size, the temperature and the filament, so check your printer or hot end manufacturer's specification where one is published, and treat it as an upper bound rather than a target.

This explains a very common pattern: walls printed slowly look perfect, while infill — which slicers often print faster and sometimes with thicker lines — shows gaps. The slicer's speed setting may also be capped by acceleration on short moves, so the problem often appears only on long, straight sections where the printer actually reaches full speed. Many slicers let you set a maximum volumetric speed so they automatically slow down when a move would exceed it. If you print fast regularly, our guide to PLA for high-speed printing covers which materials are designed for higher flow.

Diagnostic Decision Tree

Work down this sequence in order. Each step is ordered so that the easiest and least invasive checks come before ones that require heating, disassembly or replacement parts.

1. 1

   Is it only the first layer?

   Yes → check Z-offset and bed leveling before anything else. No → continue.

2. 2

   Are the slicer's filament diameter and flow values correct?

   No → fix them and reprint. Yes → continue.

3. 3

   Is the spool turning freely and the filament untangled?

   No → free the spool and path. Yes → continue.

4. 4

   Does slowing the print down make the problem go away?

   Yes → likely a volumetric-flow limit; reduce speed, layer height or set a max volumetric speed. No → continue.

5. 5

   Is the temperature within the filament maker's range?

   No or at the very bottom → raise it in small steps and retest. Yes → continue.

6. 6

   Is the extruder clicking, or is there ground plastic at the gear?

   Yes → something is resisting flow; check for a partial clog and correct tension. No → continue.

7. 7

   Does a manual extrusion produce a smooth, straight strand?

   No → partial clog likely; clean or replace the nozzle. Yes → continue.

8. 8

   Does the problem get worse the longer the print runs?

   Yes → check the hotend cooling fan for heat creep and watch spool drag. No → continue.

9. 9

   Has the spool been stored openly, or does it pop or hiss?

   Yes → dry it and retest. No → continue.

10. 10

    Do settings and hardware check out?

    Yes → fine-tune flow calibration, and try a known-good spool as a control.

In What Order Should You Troubleshoot Under-Extrusion?

Change one thing at a time and reprint a small test piece after each change. Changing several settings at once makes it impossible to know which one helped.

1. Quick checks (no tools, printer cold or idle)

• Confirm the slicer's filament diameter matches your filament.
• Check the spool turns freely and the filament isn't crossed or tangled.
• Look at the path from the spool to the extruder for sharp bends or snags.
• Check whether the problem appears everywhere or only on certain features.

2. Settings checks

• Compare nozzle temperature to the filament maker's recommended range.
• Reduce print speed, especially infill speed, and reprint the same test.
• Calculate the volumetric flow of your fastest feature using the formula above.
• Check flow / extrusion multiplier in both printer and filament profiles.

3. Filament checks

• Dry the spool if it's been exposed, following the manufacturer's guidance.
• Measure the diameter at a few points along the filament with calipers.
• Look for brittleness, kinks or chewed sections.
• Try a different, known-good spool as a comparison.

4. Hardware checks

• Heat the nozzle and extrude manually to look for a partial clog.
• Check extruder tension against the manufacturer's recommendation.
• Clean filament dust from the drive gear teeth.
• Check that the hotend cooling fan spins whenever the hot end is warm.
• On Bowden printers, check the tube is fully seated at both ends.
• Only then consider replacing the nozzle or other worn parts.

How Can You Prevent Under-Extrusion?

• Store filament sealed with desiccant when it's not in use.
• Keep the spool holder smooth and the filament path free of tight bends.
• Use the filament maker's published temperature range as your starting point.
• Set a maximum volumetric speed in your slicer that suits your hot end.
• Check that the hotend cooling fan runs freely and keep it clear of dust.
• Clean the extruder drive gear periodically.
• Inspect nozzles more often if you print abrasive materials.
• Recalibrate flow when you change filament brand or type.
• Keep a note of settings that work for each spool.

Final Thoughts

Under-extrusion is rarely mysterious once you look at when it happens. Problems on every layer usually point to settings or a restriction; problems on fast features point to flow limits; problems that grow during a print point to heat creep, a developing clog or spool drag. Work through the cheap, quick checks first, change one thing at a time, and save flow calibration for last.