Under-extrusion is the silent print killer. Over-extrusion announces itself with blobs and zits. Under-extrusion produces prints that look complete but fail mechanically — low layer adhesion, weak infill, perimeter gaps that snap under load. Your print finishes, looks okay from a distance, and breaks in half with finger pressure. The root cause is the printer pushing less plastic than the slicer expected, and there are five distinct causes with different fixes.
The Five Causes of Under-Extrusion
Under-extrusion is always one of these (in order of likelihood):
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Partial nozzle clog: The most common cause. A speck of debris, carbonized filament, or dust restricts flow. The printer extrudes 80% of commanded volume — enough to print, not enough to be strong. Unlike a full clog (zero extrusion), partial clogs produce weak, rough-surfaced prints that complete.
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Extruder grip failure: The extruder gear slips on the filament. Common on stock plastic extruders (Creality) where the tension arm cracks, and the idler bearing no longer presses filament against the drive gear. The gear spins, grinds a divot in the filament, and extrusion stops.
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Incorrect e-steps: The firmware’s steps-per-millimeter calibration is wrong. If the firmware thinks 100 steps = 1mm of filament but actually 100 steps = 1.2mm, you’re over-extruding. The opposite (under) means the extruder motor works fine mechanically but the software commands too little filament.
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Temperature too low: Below the filament’s minimum extrusion temperature, the plastic doesn’t melt fast enough. The extruder pushes cold filament into the hotend, pressure builds, and the extruder gear strips the filament. Different from a clog — raising temperature 10°C fixes this instantly.
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Heat creep: The cold side of the hotend gets warm enough to soften filament before it reaches the melt zone. Soft filament expands in the heat break, creates friction, and the extruder can’t push through. Often misdiagnosed as a clog because the symptoms (sudden underextrusion mid-print) are identical.
Step-by-Step Under-Extrusion Diagnosis
Step 1: Cold Pull to Clear Partial Clog
Before adjusting anything else, clear the nozzle:
- Heat nozzle to 170°C (PLA) or 200°C (PETG)
- Push filament through by hand until it flows
- Let nozzle cool to 90°C (PLA) or 120°C (PETG) — the filament is still soft but not liquid
- Pull the filament firmly upward. It should come out with the shape of the nozzle interior molded on the tip
- Inspect the pulled tip: black specks = carbonized filament. Gray specks = dust/debris. Clean tip = nozzle was clear, problem is elsewhere
If the cold pull tip is clean, move to the extruder.
Step 2: Inspect Extruder Grip
Mark the filament with a sharpie 100mm above the extruder entrance. Command the printer to extrude 100mm (set hotend to printing temperature first — don’t extrude cold). Measure the remaining mark distance. If less than 100mm extruded, the extruder is slipping.
Common extruder issues:
– Cracked tension arm (Creality stock plastic extruder): Visible hairline crack on the underside of the arm. Replace with aluminum extruder ($10-15).
– Worn brass drive gear: The gear teeth are rounded instead of sharp. Filament rides over the teeth instead of being gripped. Replace gear.
– Insufficient tension: The spring isn’t applying enough force. Some extruders have an adjustment screw; others need a shim under the spring.
– Filament grinding: A divot has been ground into the filament directly under the gear. The gear now spins in the divot. Cut off the damaged section and reload.
Step 3: Calibrate E-Steps
If the extruder mechanically grips but pushes the wrong amount:
- Heat nozzle to printing temperature
- Mark filament 120mm from extruder entrance
- Command: extrude 100mm at 2mm/s (slow speed eliminates slip as a variable)
- Measure remaining distance from mark to extruder: subtract from 120
– Extruded exactly 100mm → e-steps correct
– Extruded 90mm → under-extruding by 10%
– Extruded 110mm → over-extruding by 10% (different problem)
Calculate new e-steps: New E-steps = Current E-steps × (100 / Actual Extruded)
For example, if Current E-steps is 93 and you actually extruded 87mm: 93 × (100/87) = 106.9. Round to 107. Save with M92 E107 then M500.
Step 4: Temperature Validation
If extrusion is intermittent — prints fine for 20 minutes, then under-extrudes — it’s likely temperature:
- Print a temperature tower spanning 180-230°C (PLA) or 220-260°C (PETG)
- Evaluate layer adhesion by trying to break the tower at each temperature section
- The lowest temperature that produces strong adhesion is your minimum — print 5°C above it
Step 5: Diagnose Heat Creep
Heat creep is confirmed when: the print starts perfectly, under-extrudes 20-40 minutes in, and recovers after cooling down. The timeline is the key — partial clogs are immediate, heat creep takes time as heat soaks up the heat break.
Fix: Check the hotend cooling fan (the one blowing on the heatsink, not the part-cooling fan). It should run at 100% whenever the hotend is above 50°C. If it’s slowing down, replace it. Our all-metal hotend upgrade guide covers heat break selection that prevents heat creep.
Under-Extrusion Diagnosis Table
| Symptom | Cause | Diagnostic Test | Fix |
|---|---|---|---|
| Rough surface, weak layers, print completes | Partial nozzle clog | Cold pull, inspect tip | Cold pull cleaning or nozzle replacement |
| Clicking sound from extruder, filament not moving | Extruder grip failure | Mark filament test; inspect tension arm | Replace cracked arm, tighten tension, replace gear |
| Consistent thin walls across entire print | Incorrect e-steps | Command 100mm extrusion, measure actual | Recalibrate e-steps: New = Current × (100/Actual) |
| Prints fine for 10 min, then underextrusion | Temperature too low | Temperature tower test | Raise nozzle temp 5-10°C |
| Prints fine for 20-40 min, then underextrusion | Heat creep | Check hotend cooling fan speed | Replace fan, reseat heat break, apply thermal paste |
Common Mistakes & How to Avoid Them
Mistake 1: Replacing the Nozzle Without Finding the Cause
A clogged nozzle is a symptom, not always the root cause. If dust in your filament caused the clog, the new nozzle clogs in 10 hours. Fix: After replacing a nozzle, check your filament path. Add a filament dust filter (a small piece of sponge clamped around the filament before the extruder) — costs nothing, catches the particles that killed the last nozzle.
Mistake 2: Calibrating E-Steps Through the Nozzle
Extruding through a hot nozzle adds back-pressure that varies with temperature and nozzle diameter. A 0.4mm nozzle at 200°C offers different resistance than at 220°C. Fix: Disconnect the Bowden tube or remove the nozzle, then calibrate e-steps extruding into open air. Zero back-pressure gives you a pure measurement of extruder mechanics.
Mistake 3: Increasing Flow Rate to Compensate
The slicer’s “Flow” setting at 110% is a bad band-aid for under-extrusion. It masks the mechanical problem and produces inconsistent results — different areas of the print receive different compensation. Fix: Fix the root cause (clog, extruder, e-steps, temperature). Flow rate should stay between 95-105%. If you need 110%+, your printer is broken, not miscalibrated.
Mistake 4: Confusing Heat Creep with a Standard Clog
Heat creep produces the same symptom (sudden underextrusion) but has a completely different cause. Standard clog fix (cold pull, higher temp) does nothing for heat creep. Fix: The timeline is the diagnostic. If underextrusion starts at minute 30 of a print, feel the hotend heatsink — if it’s hot to the touch, it’s heat creep. A properly functioning heatsink stays cool enough to touch.
⚠️ Safety Notice: Always operate 3D printers in well-ventilated areas. PLA emits lactide fumes, PETG releases VOCs at printing temperatures, and ABS/ASA produce styrene gas — all of which require ventilation. Ensure your printer’s electrical components are certified (UL/CE) and never leave a printer unattended during long prints.
Video Resource
For FPV pilots printing TPU mounts and GoPro protectors, under-extrusion on flexible filament is especially common and usually caused by extruder grip. The aluminum dual-gear extruder upgrade solves the flex-filament slip problem — the dual drive gears grip TPU from both sides where single-gear extruders let it deflect.