How to Fix Under-Extrusion in 3D Printing

Under-extrusion means your printer is depositing less plastic than it should. Signs:

• Gaps between perimeter lines
• Holes in top surfaces
• Weak, crumbly layers that separate easily
• Lines that look thin or incomplete
• Grinding/clicking sound from the extruder

---

Cause 1: Partial Clog (Most Common)

A partial clog lets some material through but restricts flow. This is the most common cause of under-extrusion.

Fix: Cold pull

1. Heat nozzle to print temp, then reduce to 90°C (PLA) or 120°C (PETG/ABS)
2. While cooling, push filament manually
3. When cooled, firmly pull the filament out — it brings gunk with it
4. Repeat 2–3 times until the pull comes out clean and cone-shaped

Fix: Atomic pull (more aggressive)

1. Heat to 250°C, push fresh filament through
2. Let cool to 90°C (PLA)
3. Pull sharply — you'll extract debris from the nozzle

---

Cause 2: Temperature Too Low

If the nozzle is too cold, filament doesn't melt fast enough to keep up with print speed.

Fix: Increase nozzle temperature by 5–10°C. If that cures it, either your temp was too low or your thermistor is slightly off.

---

Cause 3: Print Speed Too Fast

At high speeds, the hotend can't melt filament fast enough. This is "thermal bottlenecking."

Fix: Reduce print speed by 20–30%. If under-extrusion goes away at lower speeds, either slow down permanently or upgrade to a faster-melting hotend (Volcano, Rapido, etc.).

---

Cause 4: Extruder Not Gripping Filament

The extruder wheel (hobbed gear) needs to bite into the filament firmly. If it's slipping:

• Filament grinds into dust
• You hear clicking
• Extrusion stops or stutters

Fix:

• Increase extruder tension — there's usually an adjustment screw/lever on the extruder arm
• Clean the hobbed gear — use a stiff brush to remove ground plastic debris
• Replace worn gear — a worn hobbed gear loses grip over time

---

Cause 5: Flow Rate Too Low

If your slicer's flow rate (extrusion multiplier) is set below 100%, you're intentionally under-extruding.

Check: Cura → Material → Flow | PrusaSlicer → Filament Settings → Extrusion Multiplier

Fix: Reset to 100% and recalibrate if needed.

---

Cause 6: Bowden Tube Issues

On Bowden printers, a loose or damaged PTFE tube causes gaps between the tube and nozzle, creating a space where molten plastic accumulates and causes inconsistent extrusion.

Fix:

• Make sure the PTFE tube is fully inserted into the hotend fitting
• Replace PTFE tube if it's kinked or the end is worn/melted
• Check for gaps with a cold pull (you'll see a bump where the gap is)

---

Cause 7: Wet Filament

Moisture causes steam bubbles inside the nozzle, interrupting flow.

Signs: Hissing/popping sounds, rough surfaces, foamy-looking filament coming out.

Fix: Dry filament at correct temperature for 4–6 hours.

---

Diagnostic Flowchart

1. Hear clicking? → Extruder slipping (causes 4 or partial clog)
2. Happens at high speeds only? → Thermal bottleneck (cause 3)
3. Consistent gaps across all speeds? → Clog or temperature (causes 1 or 2)
4. Hissing/popping? → Wet filament (cause 7)
5. Suddenly started after a print? → Partial clog from retraction

---

Calibrating Flow Rate

After fixing the mechanical issue, calibrate flow rate:

1. Print a single-wall cube (0% infill, 1 perimeter)
2. Measure wall thickness with calipers
3. Wall should equal your nozzle diameter (0.4mm for 0.4mm nozzle)
4. If it's 0.36mm, increase flow by ~10%
5. If it's 0.44mm, decrease flow by ~10%