Your print has gaps between perimeters, the top layers look like a screen door, and you can crush the infill with your fingers. That’s under-extrusion — the printer is pushing less plastic than the slicer asked for. The cause is never “just turn up the flow rate.” Here’s how to find and fix the real problem.
Step 1: Check Extruder Tension
The extruder gear grips the filament and pushes it through the hotend. If the tension arm is too loose, the gear slips on the filament instead of advancing it. You’ll hear a clicking sound as the gear teeth skip over the filament surface, and you’ll see plastic dust accumulating around the extruder — that’s ground-up filament from the slipping gear.
How to check: Mark the filament 100mm above the extruder entrance with a Sharpie. Heat the hotend to printing temperature. Use the LCD menu or OctoPrint/Klipper to extrude 100mm at a slow speed (2mm/s). Measure how much filament actually moved past your mark. If it’s less than 100mm, you’re under-extruding.
Adjustment for spring-loaded extruders (Ender 3 style): Tighten the tension screw until the extruder gear leaves visible teeth marks on the filament but doesn’t deform it. You want bite, not crush. A crushed filament has flat spots that increase friction in the Bowden tube, making the problem worse.
Adjustment for dual-gear extruders (BMG, Orbiter): The tension screw on these is more sensitive. A quarter-turn too tight, and the gears deform flexible filaments so badly they jam. Start loose, print a test cube, and tighten only if you hear skipping.
Common pitfall: The stock plastic extruder arm on Creality printers cracks on the underside, where the crack is invisible without disassembly. The crack reduces tension just enough to cause skipping, and the symptoms look exactly like a clogged nozzle. If you’re chasing under-extrusion on an Ender 3 with the stock plastic extruder, replace it with a metal one. The metal upgrade costs $12 and eliminates the most common mechanical failure point on these printers.
Step 2: Check for Partial Nozzle Clogs
A fully clogged nozzle produces zero extrusion. A partial clog restricts flow and causes under-extrusion that comes and goes — the filament pushes through the obstruction sometimes, then catches and underextrudes for a few layers.
Cold pull technique: Heat the hotend to printing temperature. Push filament through manually until it flows. Cool the hotend to 90°C for PLA (or 120°C for PETG). At this temperature, the filament is soft but not molten. Pull the filament out firmly and steadily. The tip should come out with the shape of the nozzle’s internal bore. If it comes out with black specks or a rough surface, you’ve pulled out carbonized plastic — the partial clog.
Needle method: Heat the nozzle to printing temperature. Use an acupuncture needle (0.4mm for a 0.4mm nozzle) and insert it from the nozzle tip upward. Do not wiggle aggressively — brass nozzles are soft and you’ll ovalize the orifice, creating a permanent extrusion problem. A few gentle in-and-out passes dislodge the debris.
If cold pulls and needles don’t fix it, replace the nozzle. Nozzles are consumables. A brass nozzle costs $0.50 and lasts 200-400 print hours before the orifice wears and loses dimensional accuracy.
Step 3: Verify Temperature
Filament extruded below its optimal temperature range doesn’t melt completely. The unmelted core creates back-pressure that the extruder has to fight, causing slippage. The extruder motor clicks, and extrusion volume drops.
Run a temperature tower for every new filament brand and type. The manufacturer’s “190-220°C” range is a suggestion, not a guarantee. I’ve had “220°C” PLA that printed best at 235°C, and “210°C” PLA that oozed and stringed above 195°C. The only way to know is to test.
Temperature tower procedure: Slice a temperature tower model with temperature changes at each block (Cura has a built-in post-processing script for this). Print it. Break the tower at each block junction and check layer adhesion. The coldest block that breaks cleanly without delamination is your minimum printing temperature. Add 5°C for a safety margin. That’s your actual printing temperature for that filament.
Step 4: Check Volumetric Flow Limits
Every hotend has a maximum volumetric flow rate — the amount of plastic it can melt per second. Exceed this rate and the filament doesn’t fully melt, creating the same under-extrusion symptoms as a clog or temperature issue.
Calculate your actual flow rate: Volumetric flow (mm³/s) = layer height × line width × print speed. A 0.2mm layer height, 0.4mm line width, at 100mm/s = 0.2 × 0.4 × 100 = 8mm³/s.
A stock Ender 3 hotend maxes out around 10-12mm³/s for PLA depending on temperature. A Volcano hotend handles 20-25mm³/s. A Mosquito Magnum pushes 35mm³/s. If your calculated flow rate exceeds your hotend’s capability, you either reduce speed, reduce layer height, or upgrade the hotend.
The symptom of exceeding the flow limit is specific: perfect extrusion on slow sections, under-extrusion on long straight moves where the printer actually reaches full speed. If your calibration cube prints fine but large flat surfaces show gaps, you’re hitting the volumetric ceiling.
| Symptom | Likely Cause | Diagnostic Test | Fix |
|---|---|---|---|
| Clicking sound from extruder | Loose tension, partial clog, low temp | Extrude 100mm and measure | Adjust tension, cold pull, raise temp |
| Gaps in top layers only | Low flow rate, incorrect line width | Increase top layer flow 5% for test | Calibrate E-steps, adjust slicer flow |
| Under-extrusion only on long moves | Volumetric flow limit exceeded | Lower speed 20% and reprint | Reduce speed, upgrade hotend |
| Alternating under/over-extruded layers | Z-axis binding, inconsistent filament diameter | Measure filament diameter at 10 points | Fix Z-axis, use higher-quality filament |
| Under-extrusion after filament change | Different temperature requirement | Temperature tower for new filament | Adjust to actual optimal temperature |
Common Under-Extrusion Mistakes
Mistake 1: Cranking Flow Rate to Compensate
Print looks under-extruded, so you bump flow rate to 115% in the slicer. The print looks better on the outside, but the actual problem — a worn extruder gear with flat teeth — gets worse because the extra back-pressure accelerates the wear. Two weeks later you’re at 130% flow and the extruder is grinding filament into dust.
Fix: Flow rate compensation masks the problem. Diagnose the root cause — extruder tension, clog, temperature, or flow limit — and fix it. Flow rate should stay within 95-105% for any properly functioning printer.
Mistake 2: Ignoring Filament Diameter Variation
Cheap filament varies in diameter from 1.68mm to 1.78mm along a single spool. The slicer assumes 1.75mm constant, so a 1.68mm section under-extrudes by 9% and a 1.78mm section over-extrudes by 5%. The print alternates between starved layers and blobby layers, and no amount of calibration fixes it because the calibration was done on a different section of the spool.
Fix: Measure filament diameter in 10 spots along 2 meters of filament. If the standard deviation exceeds 0.03mm, the filament is low quality. Switch brands. Hatchbox, eSun PLA+, and Prusament have diameter tolerances under ±0.02mm.
Mistake 3: Tightening the Extruder Tension Too Much
A tight extruder gear chews through filament, especially on retraction-heavy prints. Each retraction pulls the filament back over the gear teeth, and an over-tightened gear shaves a tiny amount of plastic off. After 1000 retractions, the gear is packed with filament dust and loses grip completely.
Fix: Extruder tension should leave visible teeth marks without deforming the filament. If you can feel flat spots on the filament with your fingernail after it passes through the extruder, back off the tension.
⚠️ Regulatory Notice: 3D printer operation should comply with the latest 2026 safety and electrical regulations in your country or region. Hotend temperatures up to 300°C present burn and fire hazards. Always operate printers in well-ventilated areas, especially when printing materials that emit VOCs. Verify compliance with local electrical safety standards and fire codes for unattended printing. Regulations vary between the US (NFPA), EU (CE marking), UK (UKCA), China (CCC), and other authorities.
For calibrating your extruder steps per millimeter, see our E-step calibration guide. If you’re dealing with nozzle clogs specifically, check our nozzle clog clearing guide.
A reliable extruder is the foundation of consistent prints. The BMG dual-gear extruder clone, available at uavmodel, provides 3:1 gear reduction that eliminates skipping on any filament — and it mounts directly to Ender 3, CR-10, and most i3-style printers with no bracket mods.