Under-Extrusion: Causes and How to Fix It

You start a print and immediately notice something is off. The first layer has gaps between lines instead of being solid. The walls look like they have tiny trenches running through them. Infill lines are thin and barely touching each other. Your print is under-extruding, and if you let it continue, the result will be a weak, ugly part that probably falls apart.

Under-extrusion means your printer is pushing out less filament than the slicer expects. The root cause can be mechanical, thermal, or software-related. This guide covers every common reason and exactly how to fix each one.

How to Identify Under-Extrusion

Under-extrusion shows several telltale signs:

• Gaps between perimeter lines — you can see light through the walls
• Weak or missing infill — infill lines are thin, broken, or completely absent
• Rough, pitted top surfaces — not enough material to fill the top layers
• Poor layer adhesion — layers separate easily because they are not fully bonded
• Clicking or grinding from the extruder — the extruder gear is slipping on the filament

If you see any combination of these, under-extrusion is your problem.

Cause 1: Partially Clogged Nozzle

The single most common cause of under-extrusion is a partial clog. A full clog stops extrusion entirely — you would notice that immediately. A partial clog restricts flow just enough to cause thin, inconsistent extrusion that gradually worsens.

How to diagnose: Heat the nozzle to printing temperature and manually push filament through. It should come out in a smooth, consistent stream that curls slightly. If it comes out thin, inconsistent, or curves sharply to one side, you have a partial clog.

How to fix:

1. Cold pull method: Heat the nozzle to 250°C, push nylon or cleaning filament through, then cool to 90°C and pull firmly. The filament will bring debris out with it. Repeat until the pulled filament tip is clean. The eSUN Cleaning Filament is designed specifically for this.
2. Acupuncture needle: With the nozzle at temperature, insert a 0.3mm or 0.35mm needle (for a 0.4mm nozzle) from below and gently work it in and out.
3. Replace the nozzle: Nozzles are consumable. If cleaning does not work, swap it. Keep a pack of Micro Swiss MK8 nozzles on hand — they are compatible with most printers.

Cause 2: Incorrect Nozzle Temperature

If the nozzle temperature is too low for your filament, the plastic will not melt fast enough to keep up with the extrusion rate. This is effectively a flow restriction.

How to fix:

• Increase nozzle temperature in 5°C increments until extrusion is smooth.
• Print a temperature tower to find the sweet spot for your specific filament. Search 3DSearch for "temperature tower" to find printable models.
• Different brands of the same material can have different optimal temperatures. Always check the manufacturer's recommendation on the spool.
• As Simplify3D's troubleshooting guide explains, even 5°C too low can cause significant under-extrusion with some materials.

Cause 3: Incorrect E-Steps Calibration

E-steps (extruder steps per millimeter) tell the firmware exactly how many motor steps are needed to push 1mm of filament. If this value is wrong, every print will under-extrude or over-extrude by a consistent percentage.

How to diagnose: Mark your filament 120mm above where it enters the extruder. Command the printer to extrude 100mm. Measure how far the filament actually moved.

How to fix:

1. Calculate new e-steps: New E-steps = (Current E-steps × 100) / Actual distance extruded
2. Set via G-code: M92 E<new value> then M500 to save
3. Re-test to confirm accuracy

The TeachingTech calibration guide has an excellent interactive calculator for this process.

Cause 4: Filament Grinding (Extruder Chewing Through Filament)

If the extruder gear grinds a flat spot into the filament, it loses grip and can no longer push filament forward. You will often hear a clicking or ticking sound from the extruder when this happens.

Common reasons for grinding:

• Nozzle is partially clogged (causing back-pressure)
• Print speed too fast for the melt rate
• Extruder tension too high (crushing filament) or too low (slipping)
• Retraction distance too long (repeatedly grinding the same spot)

How to fix:

• Address the root cause first (clog, temperature, speed).
• Check extruder gear tension. You want firm grip without crushing the filament flat. The filament should have light tooth marks, not a deep gouge.
• Reduce retraction distance. For direct-drive extruders, 0.5-2mm is typical. For Bowden systems, 3-6mm. Going beyond these ranges risks grinding.
• Upgrade to a dual-gear extruder like the Bondtech BMG Clone, which grips filament from both sides and rarely grinds.

Cause 5: Bowden Tube Issues

On Bowden-style printers (where the extruder motor is separate from the hotend), the PTFE tube connecting them can cause under-extrusion if:

• The tube is not seated properly. A gap between the tube and the nozzle creates a pocket where melted filament pools and carbonizes, eventually clogging.
• The tube has worn or deformed. PTFE degrades above 240°C. If you print PETG or ABS regularly, the tube end inside the hotend can deform.
• The tube has too much friction. Low-quality PTFE tubes have higher internal friction that resists filament movement.

How to fix:

• Cut the tube end cleanly with a Bowden tube cutter and re-seat it firmly against the nozzle.
• Upgrade to a Capricorn XS tube, which has tighter tolerances and lower friction than standard tubes.
• On printers like the Ender 3, consider the "Luke Hatfield fix" — a small modification that ensures zero gap between the tube and nozzle, as described on Creality's community forum.

Cause 6: Filament Diameter Inconsistency

Slicer software assumes a consistent filament diameter (typically 1.75mm). If your filament varies — some cheap filaments swing between 1.65mm and 1.85mm — the extrusion amount will be wrong wherever the diameter deviates.

How to diagnose: Measure your filament diameter with calipers at several points along the spool. Quality filament stays within ±0.02mm. If you see ±0.05mm or more, the filament is the problem.

How to fix:

• Switch to a reputable filament brand. Hatchbox PLA and Polymaker maintain tight diameter tolerances.
• In your slicer, set the filament diameter to the average of your measurements rather than the nominal 1.75mm.

Cause 7: Insufficient Flow Rate / Extrusion Multiplier

Even with correct e-steps, you may need to fine-tune the flow rate (also called extrusion multiplier) in your slicer. Different filaments, nozzle sizes, and speeds can all require slightly different flow rates.

How to calibrate:

1. Print a single-wall cube (1 perimeter, 0 infill, 0 top layers).
2. Measure the wall thickness with calipers.
3. It should equal your nozzle diameter (e.g., 0.4mm for a 0.4mm nozzle). If it measures 0.36mm, your flow is about 90% and needs increasing.
4. Adjust flow rate: New flow = (Expected width / Measured width) × Current flow

As All3DP's flow calibration guide details, most filaments print best between 95-105% flow rate.

Cause 8: Wet Filament

Filament that has absorbed moisture from the air will produce steam during extrusion. This causes popping sounds, inconsistent extrusion, and rough surfaces that look similar to under-extrusion.

How to identify: Listen while printing. If you hear popping, crackling, or hissing, your filament is wet. You may also see tiny bubbles or steam wisps at the nozzle.

How to fix:

• Dry your filament in a filament dryer like the SUNLU FilaDryer S2 or a food dehydrator at the appropriate temperature for your material (45°C for PLA, 65°C for PETG, 80°C for nylon).
• Store filament in sealed bags with desiccant when not in use.
• PETG, nylon, TPU, and PVA are especially hygroscopic and should be dried before every use if stored in open air.

Cause 9: Print Speed Exceeding Volumetric Flow Limit

Every hotend has a maximum volumetric flow rate — the maximum amount of plastic it can melt per second. If your combination of speed, layer height, and line width exceeds this limit, the extruder will starve.

Typical limits:

• Standard V6-style hotend: ~12 mm³/s
• Bambu Lab / Creality K1 hotend: ~24-32 mm³/s
• Volcano-style hotend: ~20-25 mm³/s

How to calculate your flow: Speed × Layer Height × Line Width = Volumetric Flow

Example: 100 mm/s × 0.2mm × 0.4mm = 8 mm³/s (fine for any hotend)

But: 200 mm/s × 0.3mm × 0.5mm = 30 mm³/s (exceeds most standard hotends)

How to fix:

• Reduce speed, layer height, or line width to stay within your hotend's limit.
• Upgrade to a high-flow hotend if you want to print faster.
• As explained in E3D's volumetric flow guide, setting a volumetric flow limit in your firmware prevents the printer from ever exceeding the hotend's capacity.

Systematic Approach to Diagnosis

If you are not sure which cause applies, follow this order:

1. Extrude manually at temperature. If flow is inconsistent, it is a clog or temperature issue.
2. Check e-steps. If manual extrusion is fine but prints under-extrude, e-steps or flow rate is off.
3. Listen for grinding. Clicking means the extruder is fighting something — usually a clog or too much speed.
4. Listen for popping. Popping means wet filament.
5. Measure filament diameter. Rule out inconsistent filament.
6. Calculate volumetric flow. Ensure you are within hotend limits.

Prevention

• Clean or replace nozzles every 500 print hours or when switching materials.
• Calibrate e-steps once and re-check when changing extruder hardware.
• Store filament in dry conditions with desiccant.
• Use quality filament from brands with tight diameter tolerances.
• Set a volumetric flow limit in firmware to prevent exceeding hotend capacity.

Final Thoughts

Under-extrusion is the printer telling you that something is restricting flow. The restriction could be physical (clog, worn tube), thermal (too cold, wet filament), calibration (wrong e-steps, wrong flow), or physics (exceeding the hotend's melt capacity). Work through the causes systematically, fix the root issue, and your extrusion will be clean and consistent. Do not just crank up the flow rate to compensate — find and fix the actual cause.