Under Extrusion in 3D Printing: The Complete Troubleshooting Guide
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Time to read 10 min
Table of contents
Identify Under Extrusion and Narrow the Cause 1.1 Diagnose Overall Under Extrusion 1.2 Diagnose Corner-Specific Under Extrusion 1.3 Diagnose Intermittent Under Extrusion During Acceleration 2. Check the Filament Look for moisture Verify diameter and consistency Eliminate spool and feed-path resistance 3. Inspect the Extruder 4. Check for a Partial Nozzle Clog 5. Verify Temperature and Hotend Stability Watch for heat creep 6. Use a Controlled Diagnostic Test Symptom-to-Cause Checklist Final Troubleshooting Order
Under extrusion happens when a 3D printer deposits less material than the slicer expects. The result may be thin or missing lines, small holes in walls, weak infill, visible layer gaps, or a rough surface.
The fastest way to solve it is to troubleshoot in order: settings first, then filament and feed resistance, then the extruder, nozzle, and hotend. This guide walks through that process without replacing parts unnecessarily.
🔎Quick rule: change one variable at a time, print a small test model, and record the result. Randomly changing temperature, flow, speed, and retraction together makes the real cause harder to identify.
Identify Under Extrusion and Narrow the Cause
Common signs of under extrusion include gaps between adjacent lines, small holes or pitting on walls, inconsistent line width, weak or incomplete infill, missing sections after retractions, clicking at the extruder, and poor layer bonding.
Do not confuse under extrusion with a seam, insufficient wall thickness, wet-filament bubbles, or a model-mesh defect. Inspect the toolpath preview before changing settings or hardware: if the slicer already shows a gap, correct the model or slicing settings first.
Next, use the distribution of the defect to decide where to begin:
- Overall under extrusion: sparse lines, incomplete infill, or layer gaps appear across most of the model. Check feed resistance, melt capacity, nozzle restriction, and flow ratio first.
- Localized under extrusion: gaps appear mainly around corners, seams, or specific features. Check pressure advance, seam placement, retraction, and rapid speed changes.
- Intermittent under extrusion: missing lines, small surface holes, or extruder clicking appear unpredictably during the print. Check filament condition, feed stability, volumetric flow, and extruder grip.
The following sections use controlled tests to distinguish these causes before settings are changed or parts are replaced.
1.1 Diagnose Overall Under Extrusion
Use this section when sparse lines, incomplete infill, narrow extrusion, or layer gaps appear across most of the model.
- Inspect the sliced preview. Confirm that the missing material is not already absent from the toolpath.
- Restore a known-good profile. Start with the printer or filament manufacturer's validated settings. For third-party filament that may not support high-speed printing, use an appropriate Generic profile as a conservative baseline.
- Calibrate flow ratio or extrusion multiplier. An incorrect value can make every line too narrow. Do not use a large flow increase to hide a mechanical restriction.
- Test the volumetric-flow limit. Reduce print speed while keeping other settings unchanged. If the defect improves, the requested flow may exceed the melting capacity of the filament and hotend. Lower speed, layer height, or line width as needed.
- Test temperature cautiously. If melting cannot keep up, slightly increase nozzle temperature within the material's recommended range. In high-speed modes, approximately 10°C can be used as a controlled starting test, not a universal setting.
- Look for a sudden change in behavior. If a previously reliable profile begins under-extruding, suspect feed resistance, contamination, extruder wear, or a partial nozzle clog before changing flow.
If the entire print remains under-extruded at conservative speed and a validated temperature, continue with the filament, extruder, nozzle, and hotend checks below.
1.2 Diagnose Corner-Specific Under Extrusion
Use this section when gaps are concentrated at corners, seams, or other locations with rapid speed changes. These defects are more likely to involve pressure advance, seam placement, retraction, or acceleration than a general shortage of flow.
- Inspect the sliced preview. Locate layer start and end points and determine whether the defect matches the seam or appears at every sharp corner.
- Move the seam for a controlled test. Place it on a stable, non-overhanging surface. If the defect moves with the seam, focus on retraction and restart behavior.
- Calibrate flow ratio and pressure advance separately. Establish a reliable average flow ratio first, then run Flow Dynamics, K-value, or Linear Advance calibration for the current filament and printer profile.
- Confirm that the saved value is active. Recalibrate after changing filament, nozzle size, hotend, extruder components, slicer profile, or printing conditions.
- Compare speed transitions. Use the speed preview to find abrupt slow-fast changes around the affected corner. Temporarily reduce speed or acceleration and compare the result.
- Review smoothing-related settings cautiously. Begin with the validated profile default, change one option at a time, and verify it with a small test print rather than copying a universal value.
Slicer preview with supports hidden, revealing the start and end points beneath the part
If the defect remains at every sharp corner after moving the seam, prioritize pressure advance and speed transitions. If gaps extend across the whole model, return to the overall under-extrusion workflow.
1.3 Diagnose Intermittent Under Extrusion During Acceleration
Use this section when missing lines, scattered weak patches, or small surface holes appear unpredictably during acceleration, deceleration, or repeated speed changes.
- Establish volumetric-flow headroom. Test the exact filament, nozzle, and hotend combination, then use a working value below the first visible quality loss.
- Reduce speed before adding heat. If a modest reduction removes the defect, the profile is probably too close to the melting or extrusion limit.
- Adjust temperature within a validated range. A small increase may improve melt flow, but it cannot reliably compensate for demand beyond the hotend's capacity.
- Recalibrate pressure advance. Confirm that the saved value for the current filament profile is active.
- Test extrusion-smoothing options deliberately. Compare the default with one controlled change because behavior varies by slicer and firmware version.
- Lower outer-wall acceleration incrementally. Reduce it in steps and compare both surface quality and print time.
- Make outer-wall speed more consistent when appropriate. Use the speed preview to identify repeated slow-fast-slow transitions, while checking for trade-offs in overhang quality, cooling, and minimum-layer-time behavior.
- Review retraction if gaps follow travel moves. Excessive distance, speed, or frequency can delay pressure recovery and may contribute to heat creep or filament deformation.
Severe extrusion transition defects at a smoothing index of 180
If the defect disappears only after reducing speed or acceleration, keep the more conservative value or investigate hotend and extruder capacity. If it persists at low demand, continue with the filament resistance, extruder grip, nozzle restriction, and temperature-stability checks below.
2. Check the Filament
Look for moisture
Moist filament may hiss, pop, foam, or produce an irregular surface as water turns to steam in the hotend. PETG, TPU, nylon, and some filled materials can absorb enough moisture to affect extrusion quality.
Dry the spool according to the material manufacturer's guidance, store it in a sealed container, and compare it with a known-dry spool. Moisture is not the only cause of under extrusion, but it can imitate or amplify the symptom.
Verify diameter and consistency
Confirm that the slicer is configured for the correct nominal filament diameter. Inspect the filament for severe ovality, damaged sections, tangles, or a swollen tip after unloading.
For unexplained flow inconsistency, measure several points with calipers. Large variation may indicate a poor-quality or damaged spool.
Eliminate spool and feed-path resistance
The extruder should not have to fight the spool holder, a crossed winding, a sharp PTFE-tube bend, or a restrictive filament guide.
With the printer idle and safe to handle:
- Check that the spool rotates freely.
- Look for crossed or trapped filament.
- Inspect the full feed path for tight bends.
- Check PTFE tubes and connectors for wear, deformation, or debris.
- Bypass an automatic material system temporarily, if applicable, to isolate the source of drag.
If direct feeding solves the problem, repair the upstream feed path before replacing the hotend.
3. Inspect the Extruder
Extruder gears transfer motor torque to the filament. Debris, insufficient tension, worn teeth, misalignment, or a cracked idler can reduce grip.
Unload the filament and inspect it. Deep grinding marks suggest the gear is slipping or repeatedly trying to push against excessive resistance. Light, consistent tooth marks are normal on many systems.
Then inspect the drive mechanism:
- Remove accumulated filament dust
- Check that both drive surfaces rotate correctly
- Verify the tension mechanism is assembled and adjusted as intended
- Look for worn, chipped, or contaminated gear teeth
- Confirm that the motor connector and gear fasteners are secure
Do not overtighten an adjustable idler. Excessive pressure can deform softer filament and increase friction.
🛠️ If inspection confirms worn drive components on a compatible machine, consider a replacement extruder gear assembly.
4. Check for a Partial Nozzle Clog
A partial clog can still pass filament, but the flow may be restricted or curl to one side. It may appear only at higher speeds.
Possible causes include degraded plastic, dust, foreign particles, abrasive filler, or residue left after changing materials. Follow the printer manufacturer's safe cleaning procedure. Depending on the nozzle and material, this may involve a purge, a cold pull, cleaning filament, or nozzle replacement.
Never force tools into a hot nozzle without appropriate precautions. Nozzles and heater blocks can cause burns, and careless probing can damage the nozzle or wiring.
Replace the nozzle if the orifice is visibly damaged, badly worn, or cannot be cleared reliably. Abrasive carbon-fiber, glass-fiber, glow, and metal-filled filaments can wear soft brass nozzles faster than standard materials.
You can browse Bambu-compatible nozzle and hotend options if diagnosis confirms that replacement is appropriate.
5. Verify Temperature and Hotend Stability
A nozzle temperature that is too low for the selected material and flow rate can increase melt resistance. Start with the filament manufacturer's recommended range, then use a temperature tower or controlled test to refine it.
Also check whether the reported temperature remains stable during printing. A loose heater, failing thermistor, damaged cable, or cooling airflow directed at the heater block can affect extrusion. Stop using the printer if wiring is damaged or temperature behavior is unsafe.
Watch for heat creep
Heat creep occurs when heat travels too far up the filament path, softening filament before it reaches the intended melt zone. The softened filament can swell, deform, and jam.
Typical clues include:
- Printing starts normally, then flow declines
- Failure occurs during long prints or after many retractions
- The unloaded filament has a swollen or distorted section
- The hotend heatsink fan is slow, obstructed, or not running
Inspect the heatsink fan, airflow path, hotend assembly, and retraction settings. Make sure the fan specified for hotend cooling runs as intended whenever required by the printer.
6. Use a Controlled Diagnostic Test
A repeatable test prevents guesswork:
- Load a known-dry, reliable filament.
- Use a known-good printer and filament profile.
- Print a small single-wall or extrusion-calibration model.
- Reduce speed while keeping other settings unchanged.
- Bypass optional feed-path components if possible.
- Inspect the extruder and nozzle only if the symptom remains.
Record the material, nozzle size, temperature, speed, line width, layer height, and result. This makes it easier to identify whether the limit follows the filament, profile, feed path, extruder, or hotend.
Symptom-to-Cause Checklist
| Symptom | Likely area to inspect first |
|---|---|
| Gaps throughout the entire print | Flow ratio, volumetric flow, nozzle restriction |
| Gaps mainly after travel moves | Retraction, pressure advance, heat creep |
| Gaps concentrated at corners | Pressure advance, seam placement, acceleration, speed transitions |
| Defect moves when the seam is relocated | Seam position, retraction, restart behavior |
| Random gaps during speed changes | Volumetric flow, K-value, acceleration, outer-wall speed consistency |
| Defect disappears after reducing acceleration | Pressure transitions, hotend or extruder response limit |
| Clicking or grinding at the extruder | Feed resistance, low temperature, clog, gear grip |
| Random roughness with popping sounds | Filament moisture |
| Flow declines during a long print | Heat creep, fan performance, spool drag |
| Problem appears only at high speed | Volumetric-flow limit, temperature, partial clog |
| Direct feeding fixes the issue | Spool holder, tube, connector, or material-system resistance |
Can increasing flow fix under extrusion?
It can correct a genuinely low flow-ratio setting, but it should not be used to compensate for a clog, slipping gear, wet filament, or excessive feed resistance. Diagnose sudden changes before increasing flow.
Why does my Ender 3 under-extrude?
The same process applies: verify slicer settings and temperature, check the spool and Bowden path, inspect the drive gear and idler, and test for a partial nozzle clog. On a Bowden setup, also inspect the tube ends and couplers for movement or damage.
Why does PETG under-extrude?
PETG may under-extrude when it is wet, printed too cold or too fast, restricted by a partial clog, or affected by excessive retraction. Start with a dry spool and a conservative manufacturer-recommended profile.
Why does TPU under-extrude?
TPU is sensitive to feed-path gaps, excessive speed, high retraction, spool resistance, and drive-gear pressure. Use a supported filament path and slower, controlled settings appropriate for the material.
When should I replace the extruder gear or nozzle?
Replace a component after inspection shows wear, damage, persistent slipping, an enlarged or deformed nozzle orifice, or a restriction that safe cleaning cannot resolve. Confirm compatibility before installation.
Final Troubleshooting Order
For most cases, use this sequence:
- Preview the sliced model and confirm the gaps are not in the toolpath.
- Restore a known-good profile and calibrate flow ratio and flow dynamics separately.
- For corner-only gaps, inspect seam placement and speed transitions.
- Reduce speed to test the volumetric-flow limit.
- For intermittent gaps, test lower acceleration and more consistent outer-wall speed.
- Test with known-dry filament.
- Remove spool and feed-path resistance.
- Clean and inspect the extruder gears.
- Clear or replace a restricted or worn nozzle.
- Check hotend temperature stability, cooling, and heat creep.
A structured process is faster and safer than replacing multiple parts at once—and it gives you a reliable fix instead of a temporary workaround.
Enderwick Pei
Enderwick Pei (EP) has worked in the 3D printing industry for more than 13 years. He previously served as a curator for TCT Asia and as editor-in-chief of a TCT MAG CN 3D printing industry magazine. Since 2020, He has served as Marketing Director at Raise3D, developing deep expertise in the technologies, applications, markets, and products of professional and industrial 3D printing.
