UNDER-EXTRUSION (GAPS, THIN WALLS, MISSING LINES)

Walls have visible gaps between adjacent lines, layers look thin or pitted, and infill has missing sections - the printer isn't delivering enough plastic to complete each line.

Under-extrusion is the gap between how much plastic the slicer commanded and how much actually arrived at the bed. The slicer calculates volume per second based on layer height, line width and speed. The extruder motor is supposed to deliver exactly that volume - but anywhere along the chain from spool to nozzle, restrictions can prevent it. The most common restriction is a partial nozzle clog - carbonised filament that has built up inside the nozzle and reduces the cross-sectional area through which plastic can flow. The extruder pushes harder, the motor starts to slip on the filament (you'll hear clicking), and the volume coming out drops below what was commanded. A full clog stops extrusion entirely; a partial clog produces under-extrusion that looks like everything else on this list. Melt rate is the second restriction. A standard nozzle on a standard hotend can melt about 8 mm3 per second of PLA at 210C. If the slicer commands more than that - through high speed, high layer height or wide lines - the extruder is pushing colder, more viscous plastic that resists flow. The output drops. The fix is either slowing down or upgrading to a high-flow hotend.

Based on 244 confirmed fixes from r/FixMyPrint, these are the causes ranked by how often they actually worked: 1. Partial nozzle clog - cold pull at 90C (PLA) or 100C (PETG/ABS) cleared the most cases. Even users who 'just printed yesterday with no problem' often had a developing partial clog. 2. E-steps wrong - calibrating extruder steps-per-mm fixed many cases where flow was off by 5-15% from spec. 3. Print speed too high for the hotend - dropping from 80+ mm/s to 50 mm/s on a stock Ender-class printer fixed melt-rate-limited under-extrusion. 4. Nozzle temperature too low - raising 5-10C let the plastic flow faster. 5. Extruder tension wrong - both too loose (gear slipping) and too tight (gear deforming filament) cause under-extrusion. Adjusting the extruder spring tensioner solved many cases on bowden Ender 3s. Data sourced from r/FixMyPrint - one of the largest 3D printing troubleshooting communities on Reddit. This represents real user-reported issues and community-confirmed fixes, not theoretical advice.

If you hear the extruder clicking or grinding rhythmically - the motor is trying to push and something is stopping it. Almost always a partial clog. Cold pull first. If extrusion is consistent but everything is uniformly thin - e-steps are off. Calibrate. If under-extrusion only appears at high speeds or layer heights - hotend melt rate limit. Slow down or upgrade. If you find pellets of filament dust near the extruder - the gear is grinding the filament. Loosen extruder tension slightly or the cause is elsewhere (clog, low temp). If under-extrusion started suddenly after a filament swap - moisture in the new spool, or different filament diameter than slicer setting. If only the first 5-10 minutes are under-extruded then it self-corrects - this is heat creep / soft cold zone, not regular under-extrusion. See the clogging guide.

Cause 1: Cold pull Heat to print temp. Push filament through manually. Drop to 90C (PLA) or 100C (PETG/ABS). At temp, pull filament out firmly in one fast motion. The tip should come out shaped like the nozzle interior with debris attached. Repeat 3-5 times until clean. Do this before tweaking any settings. Cause 2: Calibrate E-steps Mark filament 100 mm from extruder inlet. Command 100 mm extrusion. Measure actual movement. New e-steps = current e-steps x (100 / actual_mm). Update firmware (Marlin: M92 E<value> then M500). Klipper: edit `rotation_distance` in printer.cfg. Re-check yearly or after extruder hardware changes. Cause 3: Slow down Drop print speed to 40-50 mm/s. If under-extrusion clears, you were melt-rate limited. Increase gradually until you find your hotend's limit. Stock Ender 3 hotend: ~60 mm/s on PLA at 210C. Volcano/Revo HF: 100+ mm/s. Bambu hotend: 200+ mm/s. Cause 4: Raise nozzle temperature Increase 5-10C. PLA can run reliably to 220C. PETG to 250C. Hotter plastic flows faster, helping at high speeds. Don't push past 230C on PLA - degradation accelerates and stringing increases. Cause 5: Adjust extruder tension Bowden (Ender 3 family): the spring lever should hold filament firmly without grinding. Look at the filament after a few prints - tooth marks should be uniform indentations, not chewed gouges. Loosen the tension screw if you see chewing; tighten if filament is slipping freely. Bambu/Prusa direct drive: tension is fixed and rarely wrong - if it is, replace the extruder gear.

Bowden printers (Ender 3 family): Stock hotends limit print speed to 50-60 mm/s before melt rate becomes the bottleneck. The plastic extruder lever wears out and is the #1 hardware cause of intermittent under-extrusion - upgrade to a metal extruder ($10-15) before debugging settings further. Direct drive printers (Bambu, Prusa MK4S): High-flow hotends remove melt rate as a common cause. Under-extrusion on these printers is almost always partial clog, wrong filament profile, or wet filament. Enclosed printers (Bambu P1S, K1 Max): Heat creep risk is higher in enclosed chambers. If under-extrusion gets worse over the course of a long print, it's heat creep - reduce retraction distance and check the hotend cooling fan.

PLA / PLA+: Most forgiving. Cold pulls clear PLA debris easily. Stock e-steps usually within 5% of correct. PETG: Sticky and hard to cold-pull. Use 100C cold pull temp. PETG also requires lower print speeds than PLA (40-50 mm/s on stock hotends) because of higher viscosity at print temperature. ABS / ASA: Easy to cold pull. Print speeds similar to PLA. Heat creep more likely because of higher print temperature and longer print times. TPU: Frequently under-extrudes because soft filament gets squeezed by the extruder gear instead of pushed. Reduce extruder tension and slow to 20-30 mm/s. Direct drive only - Bowden TPU underextrudes constantly. Carbon fibre / glow / abrasive blends: Wear brass nozzles fast. After 10-20 hours of abrasive printing, the nozzle bore is worn enough to underextrude. Switch to a hardened steel nozzle.

Cold-pull every 200 hours of printing as preventative maintenance, even if extrusion looks fine. It catches developing partial clogs before they cause failures. Re-calibrate e-steps yearly and after every extruder change. Drift over time is real and matters. If you print a lot of abrasive filament, switch to hardened steel nozzles permanently. Brass nozzles wear in dozens of hours with carbon fibre.

Use FixMyPrint to get nozzle temperature, print speed, flow rate and retraction values tuned for your specific printer and filament - based on what actually worked for similar issues in the community. Go to /settings-generator to generate yours.