Underextrusion Troubleshooting

What is underextrusion? (And how to spot it)

Underextrusion means the printer isn't pushing out enough plastic. The nozzle extrudes less material than the slicer expects, resulting in weak, gappy prints.

Visual signs of underextrusion:

• Perimeter gaps: Outer walls have small holes or gaps. Light passes through the printed shell.
• Sparse infill: Interior fill pattern has big holes instead of a solid grid. Part is hollow-looking inside.
• Weak, fragile layers: Print looks OK from outside but crumbles easily when you squeeze it.
• Text and features fail: Thin walls (0.4mm or smaller) don't print. Text becomes invisible or just a trace.
• First layer looks thin: First layer has visible gaps between lines instead of solid coverage.
• Different materials behave differently: One material underextrudes while another works fine (diagnostic clue)

Quick diagnosis: Print a 20mm cube with 0% infill and 1 perimeter

A single-perimeter cube shows underextrusion instantly. If the perimeter has gaps or looks thin (< 0.4mm), you're underextruding.

Root causes of underextrusion (in priority order)

1. Flow/extrusion multiplier set too low (20% of cases): Slicer is telling printer to extrude only 80-90% of normal. Easy to fix.
2. Nozzle temperature too low (15% of cases): Cold plastic doesn't flow well. Especially common with materials like PETG or ABS.
3. Clogged or partially clogged nozzle (20% of cases): Blockage restricts plastic flow. Most common culprit.
4. Filament diameter wrong in slicer (10% of cases): Slicer thinks you have 1.75mm filament but you have 2.85mm. Under or overextrudes significantly.
5. Extruder slipping (15% of cases): Stepper motor can't push filament through. Usually indicates high resistance (clogged nozzle or jammed filament).
6. Nozzle size mismatch (10% of cases): Slicer configured for 0.6mm nozzle, but you're using 0.4mm. Creates underextrusion.
7. Z-offset too high (5% of cases): Nozzle too far from bed. Plastic forced out at low pressure, extrudes poorly.
8. Filament material mismatch (3% of cases): Some materials (carbon-filled, glass-filled) extrude differently than standard.
9. Extruder skipping consistently (2% of cases): Rare. Indicates extruder drive gear stripped or filament worn smooth.

Fix sequence (follow in order)

Step 1: Check slicer flow/extrusion multiplier (fastest fix)

• Open your slicer (Cura, PrusaSlicer, SuperSlicer)
• Look for "Flow" or "Extrusion Multiplier" setting
• Should be 1.0 (100%)
• If it's 0.9 or lower, increase to 1.0
• Slice and print a test cube

Common reason it's wrong: You may have set it to 0.9 after a previous overextrusion problem and forgot to reset it.

Step 2: Verify filament diameter

• Open your slicer
• Settings ? Filament ? Diameter
• Usually 1.75mm or 2.85mm
• Check your filament package for actual diameter
• If wrong, change in slicer

Common mistake: Wrong diameter = proportionally wrong extrusion. A 1.75mm setting on 2.85mm filament extrudes 2.6× less plastic (wrong by huge margin).

Step 3: Check nozzle size in slicer matches your physical nozzle

• Slicer settings: "Nozzle diameter"
• Should match your printer's actual nozzle (usually 0.4mm, sometimes 0.6mm or 0.8mm)
• If mismatch, change in slicer

Step 4: Raise nozzle temperature by 5-10°C

• Hotter plastic flows more easily
• Example: 210°C → 215°C for PLA, or 245°C → 250°C for PETG
• Print a test cube

Tip: If temperature was the issue, underextrusion stops immediately.

Step 5: Swap the nozzle (if still underextruding)

• Even "clean-looking" nozzles can have partial clogs invisible to the eye
• Cost: $2-7 per nozzle
• Install new nozzle and test
• If underextrusion stops, old nozzle was clogged

Step 6: Clean the nozzle while installed (if you want to try first)

• Heat to 280°C
• While hot, use a thin brass wire to gently push through the nozzle opening
• Cool and test
• (Only works if clog is reachable; deep clogs need nozzle swap)

Step 7: Check Z-offset (nozzle height)

• If nozzle is too high, it can't push filament with enough pressure
• Run a bed level test: nozzle should have slight drag on paper
• If too high, adjust Z-offset down 0.1mm and test

Step 8: Verify filament is not stripped (advanced)

• Remove filament from extruder
• Look at the part that was gripped by the drive gear
• If it's smooth with no teeth marks, the gear slipped (underextrusion)
• If teeth marks look deep/ground, filament was worn. Swap for new filament.

Material-specific underextrusion issues

PLA underextrusion:

• Usual cause: Nozzle partially clogged (PLA clogs easily). Swap nozzle.
• Temperature fix: Raise from 210°C to 215°C
• Prevention: Replace nozzle every 50+ hours of printing (PLA is abrasive)

PETG underextrusion:

• Usual cause: Nozzle temperature. PETG is sensitive to temp (240-260°C range).
• Temperature fix: Raise from 245°C to 250°C (cold PETG doesn't flow)
• Also check: Flow rate using volumetric flow calculator. Keep under 12 mm³/s.

ABS underextrusion:

• Usual cause: Nozzle temperature. ABS needs 230-250°C minimum.
• Temperature fix: Raise to 245°C minimum
• Also check: Bed temperature. ABS needs 100°C+ bed temp. Cold bed makes first layer weak ? rest of print underextrusites as compensation.

TPU/Flexible underextrusion:

• Usual cause: Direct drive extruder required. Bowden extruders can't push flexible filament reliably.
• Fix: Switch to printer with direct drive, or reduce print speed to 30 mm/s and accept slow prints.

When underextrusion is actually overextrusion in disguise

Sometimes what looks like underextrusion is actually overextrusion causing the nozzle to jam. Confusing, but here's how to tell:

• Underextrusion: Walls are thin and gappy, infill is sparse, perimeters have visible holes.
• Overextrusion masquerading as underextrusion: Nozzle clogs from overextrusion, then as it clogs, flow decreases, making prints look underextruded. Stepper may slip trying to push through the clog.

Diagnosis: If swapping nozzle fixes it immediately, it was a clog (overextrusion caused the clog). If raising temperature fixes it, it was underextrusion (cold material).

Tracing a gappy print back to its source

Underextrusion is an endpoint, not a cause. Walk the chain backwards from the symptom and you will almost always find the decision that broke first:

1. You are here: gappy or weak output. The print is already wrong. Resist the impulse to swap hardware immediately. The fault is usually a slicer setting, and the sections above give the full fix sequence for hardware cases once you have ruled that out.
2. Was volumetric flow already at the hotend ceiling? Speed times layer height times line width gives mm3/s. Exceed what the hotend can actually melt and the extruder loses ground on every fast move, producing exactly the matte banding and perimeter gaps that look like a partial clog. The volumetric flow calculator gives you this number before you touch any hardware.
3. Did line width create back-pressure? A line width that creeps above 150% of nozzle diameter forces the bead sideways with nowhere to go, raising resistance until the extruder skips. The extrusion width calculator shows where your setting sits relative to that ceiling. The useful range sits between 100% and 120% of nozzle diameter.
4. Was layer height inside the safe zone? Once layer height exceeds 75% of nozzle diameter, the nozzle cannot shape the bead properly and adhesion between layers collapses. The layer height guide maps the safe envelope for each nozzle size.
5. What did the slicer profile actually lock in? A profile chosen for aesthetics rather than hotend limits can silently fix flow at a speed that consistently exceeds capacity on infill passes. What slicer presets actually change explains which fields matter.
6. Was nozzle diameter entered correctly from the start? Every calculation above inherits this number. A 0.6mm nozzle logged as 0.4mm in the slicer throws off layer height limits, line width targets, and flow ceilings simultaneously. The nozzle diameter decision guide and nozzle size chart are the right place to audit this.

The flow ceiling varies enormously by hardware. On the Ender 3 with a stock hotend, pushing past 8 to 10 mm3/s produces exactly the matte surface and gappy perimeters described on this page, and it looks like a clog when it is actually a speed problem. Running the volumetric flow calculator first saves a nozzle swap that was never needed.