Under-extrusion is when your 3D printer puts down less plastic than the slicer asked for — gaps in walls, missing lines, weak parts. Here's how to diagnose the cause (clog, wet filament, low E-steps, or low temp) and fix each.
Under-extrusion is what's happening when your 3D printer puts down less plastic than the slicer asked for. It shows up as gaps between adjacent lines, holes or pits in walls, top surfaces that don't fully close out, and parts that snap easily under light pressure. Under-extrusion has four common causes, and the right fix depends on which one you're hitting. This article walks through how to identify the cause from the visual evidence, then the calibration and hardware fixes for each.
For the broader failure-mode catalog, the master diagnostic guide covers everything else. For symptoms that overlap with under-extrusion (a partial nozzle clog, wet filament, retraction grinding), the nozzle clog and filament drying deep dives are the right places to start.
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Quick summary
The four causes of under-extrusion, in rough order of frequency:
- Partial nozzle clog. The bore is constricted by built-up residue. Symptom: extrusion is starved consistently, regardless of print speed or settings. Cold pull or replace.
- Wet filament. Steam pockets disrupt extrusion volume. Symptom: extrusion is inconsistent — fine in places, starved in others — and you may hear faint hissing during extrusion. Dry the filament.
- E-steps mis-calibrated or flow rate too low. The extruder is pushing less filament than it thinks. Symptom: extrusion is steady but uniformly thin. Calibrate with a 100 mm test and digital calipers.
- Print temperature too low for the speed. The nozzle physically can't melt plastic fast enough at the requested print speed. Symptom: extrusion thins out at high speed, looks fine at lower speed.
The rest of this article unpacks each plus the diagnostic process to figure out which one you're actually hitting.
What does under-extrusion look like?
Visual symptoms, ordered from "subtle" to "obvious":
- Slightly thinner walls than the slicer's preview. Walls that should be 0.4 mm wide print at 0.35 mm. Often invisible unless you measure with calipers.
- Visible gaps between adjacent extrusion lines. Top and bottom surfaces look "lined" rather than smooth. You can see the bed through a single-layer test print.
- Holes or pits in walls. Small voids where plastic should be solid. The wall is structurally weak — it cracks under thumb pressure.
- Top surfaces that don't fully close out. The top layer leaves visible gaps even with multiple top layers configured.
- Stringy weak walls that look like the nozzle was ghosting plastic rather than depositing it.
- No extrusion at all on certain layers. The print has visible gaps where the extruder was running but nothing came out — usually a clog or extruder skip.
Distinguishing under-extrusion from other failures:
- Under-extrusion vs over-extrusion. Over-extrusion piles plastic up where it shouldn't be — bulging walls, blobs at corners, a rough top surface. Under-extrusion does the opposite — too little plastic, gaps, weak structure. Both can be caused by mis-calibrated E-steps or flow rate but in opposite directions.
- Under-extrusion vs stringing. Stringing is plastic between features (web-like strands across travel moves). Under-extrusion is missing plastic within features (gaps in walls). The two can co-occur (wet filament causes both) but the underlying mechanism is different. The stringing article covers stringing specifically.
- Under-extrusion vs layer adhesion failure. Layer adhesion failure is when each layer prints fully but the layers don't bond to each other (the print splits along layer lines under stress). Under-extrusion is missing material within a layer. They can coincide if the under-extrusion is severe enough that there isn't enough plastic to fuse with the layer below.
- Under-extrusion vs first-layer Z too high. A first layer that's too high (round un-squished noodles) can look like under-extrusion on the bottom. The fix is Z-offset, not extrusion. The first-layer guide walks through the seven first-layer patterns.
Diagnostic test: the 100 mm extrusion check
Before diving into fixes, run this 30-second test to figure out whether you have an extrusion-volume problem or a flow problem:
- Heat the nozzle to print temperature.
- Mark the filament 120 mm above where it enters the extruder.
- Use the printer's UI to extrude exactly 100 mm of filament at slow speed (5 mm/s).
- Measure the remaining distance from the mark to the extruder entrance with digital caliper 6 inch metric. The difference between 120 mm and that measurement is how much filament actually advanced.
- If the measured distance is 95–100 mm: extrusion volume is correct. Your problem isn't E-steps or flow — it's a partial clog, wet filament, or temperature/speed mismatch. Skip to the relevant section below.
- If the measured distance is under 95 mm: the extruder is under-feeding. Either E-steps need calibration or the extruder is grinding. Continue to the calibration section.
- If the measured distance is over 100 mm: the extruder is over-feeding (causes over-extrusion, opposite problem). Same calibration but in reverse.
This test cuts the diagnostic time in half. It tells you whether to chase the hotend (clog, temp) or the extruder (E-steps, gear wear) before you waste an hour on the wrong fix.
How to fix under-extrusion
These are listed in order of how often each is the actual cause. Most under-extrusion problems clear up at step 1 or 2.
1. Check for a partial nozzle clog
Run the manual filament push test: heat the nozzle to print temperature, then push filament through by hand. If it pushes with noticeable resistance and the extruded line is thinner or wobblier than expected, you have a partial clog. The plastic is making it through the bore but not freely.
The fix is a cold pull (atomic pull) — heat, feed cleaning filament, cool to ~90°C, pull the filament back out. The plug pulls residue out of the bore. The full nozzle clog article covers this in detail with the exact temperatures and procedures for different filament types.
If two cold pulls don't restore clean flow, the nozzle is worn or the residue is hardened beyond cleaning. Replace the nozzle. A new 0.4mm hardened steel nozzle is $5–15 and lasts months to years depending on materials.
2. Verify filament dryness
Wet filament produces inconsistent extrusion — sometimes a fine line, sometimes a starved one, sometimes a small blob, sometimes nothing — because the steam expansion inside the hotend disrupts the steady flow of melted plastic. The pattern you see depends on how wet the filament is.
Test: feed filament into the hot nozzle slowly by hand. Wet filament hisses faintly and produces visible steam wisps; dry filament extrudes silently with a clean line.
If the filament is wet: dry it. PLA at 45–50°C for 4–6 hours. PETG at 65–70°C for 4–6 hours. Nylon at 80°C for 8–12 hours. The full drying guide has the material-by-material temperature/time table and storage recommendations.
If you've already verified dryness via a steam test, skip ahead.
3. Calibrate E-steps
E-steps is the firmware setting that translates extruder motor rotations into millimeters of filament fed. If E-steps is set wrong, the extruder under-feeds (or over-feeds) by a consistent percentage. The 100 mm test from earlier identifies this — if the test shows 95 mm of filament advanced when you asked for 100 mm, E-steps needs a 5% bump.
Klipper-based printers (Ender 3 V3 KE, Bambu Lab — though Bambu doesn't expose this in the UI, Creality K1 family, custom Klipper builds):
Klipper uses rotation_distance instead of E-steps directly, but the calibration is conceptually identical. The community has detailed guides for calibrating rotation_distance for specific extruders. Search "Klipper extruder rotation_distance calibration" for current procedures.
Marlin-based printers (older Ender 3, original Prusa MINI, many budget printers):
The standard procedure:
- Run the 100 mm test, record actual distance advanced (call this X mm)
- Get current E-steps: send
M503via printer console; look forM92 Exxxline - Calculate new value:
new_e_steps = current_e_steps × (100 / X) - Apply:
M92 E<new_value> - Save:
M500 - Re-test to confirm
After calibration, run a flow rate calibration print (OrcaSlicer and PrusaSlicer both have one) to fine-tune the slicer-side flow setting per filament. Different filaments have slightly different actual diameter and density; a 5–10% per-filament flow adjustment is normal even after E-steps is correctly calibrated.
4. Check the extruder gear and idler
If E-steps was calibrated correctly previously and the printer is suddenly under-extruding, the extruder gear may be worn or the idler may be loose. The gear's teeth eventually wear smooth from continuous filament feeding; once smooth, the gear slips on the filament rather than gripping it.
Visual inspection: open the extruder and look at the drive gear. Worn gears have rounded or polished tooth edges; healthy gears have crisp tooth profiles. A photo comparison against a fresh gear (search "3D printer extruder drive gear close up" for reference images) makes the difference clear.
The idler is the spring-loaded wheel on the opposite side of the gear that presses the filament against the drive teeth. Idlers loosen over time, reducing the grip force. Tighten the idler tension screw — the tension should be firm but not so tight that the filament gets visibly indented.
For Bowden printers with weaker stock extruders, the BMG dual gear extruder upgrade is a popular upgrade — dual-gear designs grip filament with twice the contact area and hold up significantly better under demanding conditions. Direct-drive printers usually have stronger stock extruders that don't need this upgrade.
5. Increase nozzle temperature 5–10°C
If the prior steps don't fix it, the issue may be that the nozzle is running too cold for the print speed. Plastic flows much more easily as it gets hotter; at higher print speeds, the nozzle needs more heat to keep up with the volume of plastic being extruded.
Quick temperature test: print a temperature tower (search "temperature tower stl" — they're free) at the filament's recommended temperature range. Look for the temperature where extrusion looks cleanest without stringing.
For typical materials at typical print speeds:
| Material | Slow print (50 mm/s) | Fast print (150+ mm/s) |
|---|---|---|
| PLA | 200–210°C | 215–225°C |
| PETG | 230–240°C | 240–250°C |
| ABS | 235–245°C | 245–260°C |
| TPU | 215–225°C | Don't print TPU fast |
If your print speed is high and your temperature is at the lower end of the material's range, raising 5–10°C often clears under-extrusion that wasn't responding to other fixes.
6. Check the maximum volumetric speed
A more advanced cousin of the temperature issue. Every hotend has a maximum flow rate measured in mm³/s — the maximum volume of plastic it can melt and push through the nozzle per second. If the slicer asks for a higher flow rate than the hotend can deliver (usually because print speed × layer height × line width is too high), the hotend physically can't keep up and the result is under-extrusion.
The simple test: drop print speed by 25%. If under-extrusion clears, the previous speed exceeded your hotend's max flow.
OrcaSlicer has a max-volumetric-speed calibration print that finds the threshold for your specific hotend + filament combination. The output is a number you set in your filament profile that caps print speed at your hotend's actual capability rather than some theoretical higher number.
Hotend max flow varies widely:
| Hotend type | Approximate max flow |
|---|---|
| Stock Ender 3 (V5/V6 clone) | ~10 mm³/s on PLA |
| Bambu A1 / A1 Mini | ~16 mm³/s on PLA |
| Bambu P1S / X1 | ~24 mm³/s on PLA |
| Prusa MK4 / MK4S high-flow | ~30 mm³/s on PLA |
| Creality K1 / K1 Max | ~24 mm³/s on PLA |
| Volcano-style hotend | 30+ mm³/s |
PETG and ABS push less than PLA at the same temperature; PA and PC less still. Your "max volumetric speed" setting should be material-specific.
7. Inspect the filament path
Sometimes the extruder is fine and the filament can't get to it freely. Three things to check:
The PTFE tube (Bowden printers). A kinked or worn PTFE tube creates drag that the extruder fights against. Inspect the full length of the tube; replace if you see visible kinks, wear, or melted spots near the hotend.
Spool drag. A heavy or sticky spool that doesn't rotate freely on its holder makes the extruder pull harder than it should. Lubricate or replace stuck spool holders. For tall heavy spools, a filament spool holder bearings kit reduces drag substantially.
Filament path bends. On printers with multiple filament path bends (especially AMS-style multi-spool systems), tight bends can compress softer filaments enough that they don't feed cleanly. Soft TPU especially struggles with this — see the P1S guide for AMS-specific advice.
What you may need
A short list of products that genuinely help with under-extrusion diagnosis and fixes. We earn a small commission if you buy through these links at no additional cost to you.
- digital caliper 6 inch metric — required for the 100 mm test and flow rate calibration. Single most useful tool for under-extrusion diagnosis.
- nozzle cleaning needles 0.4mm — second-line tool for partial clog clearing.
- nozzle cleaning filament natural color — for cold pulls.
- 0.4mm hardened steel nozzle — replacement for worn or fully clogged nozzles.
- Sunlu S4 filament dryer — wet filament accounts for a large share of "under-extrusion" cases.
- BMG dual gear extruder upgrade — for older Bowden printers with weak stock extruders.
- filament spool holder bearings — reduces drag for heavy or sticky spools.
- hex key set metric — extruder idler tension and most other 3D printer maintenance.
Diagnostic checklist
When prints look starved:
- Run the 100 mm extrusion test with calipers. Tells you in 30 seconds whether to chase the hotend or the extruder.
- Manual hot push. Heat the nozzle, push filament by hand. If it pushes with noticeable resistance, you have a partial clog.
- Cold pull if a partial clog is suspected.
- Verify filament dryness with a hand-feed listening test. Dry if needed.
- Calibrate E-steps if the 100 mm test showed under-feeding.
- Calibrate flow rate per filament in your slicer (OrcaSlicer / PrusaSlicer have built-in tests).
- Raise nozzle temperature 5–10°C if under-extrusion appears at high print speed.
- Reduce print speed by 25% as a diagnostic. If under-extrusion clears, your speed exceeded the hotend's max flow.
- Inspect the extruder gear for wear; tighten the idler.
- Snap a photo and run it through the WhyItFailed AI diagnosis tool if the cause isn't obvious from the above.
FAQ
What's the difference between under-extrusion and stringing?
Stringing is plastic between features — fine strands stretched across travel moves between separated parts of the print. Under-extrusion is missing plastic within features — gaps in walls, holes in surfaces, weak parts. They can coexist (wet filament causes both) but the underlying mechanism is different. Stringing is about retraction and ooze management; under-extrusion is about volume.
How do I calibrate E-steps?
Mark filament 120 mm above the extruder, command the printer to extrude 100 mm at slow speed (5 mm/s), then measure how much actually advanced. If you got less than 100 mm, your E-steps is too low; multiply your current E-steps value by (100 / actual_distance) to get the new value. Save with M500. Re-test to confirm. The exact procedure varies by firmware (Marlin uses M92, Klipper uses rotation_distance) — search "[your firmware] e-steps calibration" for printer-specific steps.
Should I calibrate flow rate per filament?
Yes, if print quality matters. Different filament brands and colors have slightly different actual diameter and density, which means the same E-steps setting produces different actual extrusion volume. A flow rate (or extrusion multiplier) of 95–105% is normal — outside this range usually means E-steps itself is mis-calibrated, not just filament variation. OrcaSlicer and PrusaSlicer both have one-click flow rate calibration prints.
Why does my print look fine but break easily under load?
Almost certainly under-extrusion or weak layer adhesion. Visible quality and structural strength are different things — a print can look perfect (smooth surface, accurate dimensions) but be 10% under-extruded, which means walls have hairline gaps that compromise strength. Run the 100 mm test and verify E-steps. If E-steps is correct, the issue is layer adhesion: try a 5°C temperature increase, reduce part cooling fan speed, or increase wall count.
Can wet filament cause under-extrusion?
Yes. Steam pockets inside the hotend disrupt the steady flow of melted plastic, which produces inconsistent extrusion volume — sometimes correct, sometimes starved, sometimes a small blob. The pattern is usually intermittent rather than uniform under-extrusion. Dry the filament; that resolves it.
Why does under-extrusion get worse at high print speed?
Two reasons. First, your hotend has a maximum flow rate (mm³/s of plastic it can melt). Faster prints ask for more flow; if you exceed the hotend's max flow, the nozzle physically can't keep up and extrusion thins out. Second, faster prints often need higher temperatures to maintain melt; running at the same temperature you used for slower prints means cooler plastic that resists flow. Either reduce speed or raise temperature 5–10°C.
What's a normal extruder current setting?
Stepper current settings are firmware-managed and printer-specific. Most consumer printers ship with correct current settings — you shouldn't need to change them unless you've replaced the stepper motor with a different model. If you suspect the stepper is under-powered (consistent missed steps despite all other fixes), consult your printer's documentation rather than guessing.
Why does my extruder click during printing?
The extruder gear is slipping on the filament because the filament can't advance at the rate the motor is trying to push it. Three causes: a partial clog (test with manual hot push), retraction set too aggressively (gear chews through filament), or print speed too high for the hotend's flow rate. Address the cause; the clicking is a symptom, not the problem itself. Persistent clicking grinds the filament, which can leave debris in the extruder path and create downstream problems.
Should I increase wall count to compensate for under-extrusion?
That's masking the cause, not fixing it. Increasing wall count from 2 to 3 makes the part stronger but uses more time and material; it doesn't fix the underlying volume mismatch. Diagnose and fix the actual cause (clog, wet filament, E-steps, temperature) and your default wall count will produce strong parts.
Can a worn extruder gear be fixed without replacement?
Sometimes. If the gear teeth are still recognizable but slipping, tightening the idler tension is the first move — sometimes that's enough. If the teeth are visibly rounded or polished smooth, no idler adjustment will restore grip; replacement is the answer. A new dual-gear extruder is $20–60 depending on printer, takes 15 minutes to install.
If your under-extrusion doesn't match a single cause from this guide and persists after calibration, snap a photo and run it through the WhyItFailed AI diagnosis tool. The free first diagnosis examines the specific failure pattern and tailors fixes to your printer + filament + surface combination. Under-extrusion often has multiple contributing causes (slightly worn nozzle + slightly mis-calibrated flow + slightly cool temperature) that visual diagnosis catches more reliably than any single-axis test.