TPU Extrusion Problems: Diagnose Gaps Before Increasing Flow

Thin walls, interrupted lines, and missing material can all look like under-extrusion. But they do not necessarily have the same cause.

A TPU print that loses material only during fast infill needs a different investigation from one that leaves a gap after every travel move.

The most useful starting question is not “How much should I increase flow?” It is: When does extrusion become inconsistent?

Read the Pattern Before Changing the Profile

Compare the damaged area with the slicer preview. Look for changes in printing speed, extrusion width, travel moves, and feature type.

Pattern	First investigation
Gaps appear throughout slow and fast sections	Material delivery, nozzle condition, and profile
Slow walls look complete but faster infill becomes thin	Extrusion demand and volumetric limit
Gaps appear immediately after travel	Retraction and restart behavior
Small gaps remain consistently after feeding is stable	Flow calibration
Material builds up across top surfaces	Possible excessive flow

These are diagnostic clues, not confirmed causes. Save the original profile before testing.

1. Separate Flow Ratio from Volumetric Speed

These settings solve different problems.

Flow ratio, also called extrusion multiplier in some slicers, adjusts the amount of material commanded for an extrusion move.

Maximum volumetric speed limits how much material the slicer asks the system to process per second. Its appropriate value depends on the material and hotend. Wider or taller extrusion paths can require more material even when movement speed stays unchanged. Prusa’s maximum volumetric speed guide

If the system cannot deliver material consistently at the requested rate, increasing the flow ratio asks it to deliver even more.

Check the preview’s volumetric-flow display if available. This can reveal why a wider infill line fails while a narrower outer wall succeeds at a similar movement speed.

2. When Gaps Appear Only in Faster Sections

Use a small model that reproduces the failure. Keep temperature, material preparation, and flow ratio unchanged.

Reduce the demand in the affected section using the appropriate feature speed or a conservative material-specific volumetric limit. Reslice and confirm that the preview actually shows a reduction.

Prusa identifies high-speed under-extrusion and extruder clicking as possible signs of an excessive volumetric limit. These symptoms do not prove that the hotend alone is responsible; the material and feeding system also matter. Choosing a volumetric limit

If the lower-demand version improves, you have evidence that the original operating point was unsuitable. You have not yet established a universal maximum for every model or temperature.

Keep the successful setting as a working baseline and verify it on the intended part.

3. When Gaps Follow Travel Moves

A line that starts thin and then recovers deserves a different test from one that remains thin throughout.

Check whether the defect follows retraction events in the slicer preview. Also check whether a custom profile introduced unfamiliar restart or retraction values.

BCN3D’s TPU guidance links repeated retraction with feeding difficulties and under-extrusion in its setup. Its numerical settings are hardware-specific, so they should not be transferred directly to every printer. BCN3D TPU printing guidance

Start from the validated profile for your printer and material. Test one small retraction adjustment at a time.

If reduced retraction improves restart consistency but increases stringing, record both outcomes. A cleaner travel path is not an improvement if the next printed line becomes incomplete.

Avoid adding large amounts of restart compensation simply to hide the symptom. First establish whether material delivery remains stable.

4. When Extrusion Is Inconsistent Everywhere

If slow, continuous paths also contain gaps, stop treating the problem as a high-speed calibration exercise.

Check the basics:

• Does the spool unwind without obstruction?

• Is the filament following the intended guide path?

• Is the nozzle diameter correctly selected?

• Is the material profile appropriate for the exact TPU?

• Does the unloaded filament show a folded or heavily damaged section?

Follow the printer manufacturer’s procedure for inspection and cleaning. Do not force repeated extrusion attempts against a suspected blockage.

TPU is also moisture-sensitive. BCN3D notes that absorbed moisture can produce a foamy, uneven surface and other printing defects. Prepare the material according to its own supplier’s instructions before comparing calibration samples. TPU material preparation

A profile adjustment is easier to evaluate once the mechanical and material conditions are repeatable.

5. Calibrate Flow Only After Delivery Is Stable

Flow calibration is useful for persistent, relatively small discrepancies—not as a substitute for fixing intermittent feeding.

Prusa describes extrusion multiplier calibration as a way to address minor under-extrusion and excessive material buildup. It also notes that slicer and firmware flow adjustments can combine. Prusa’s extrusion multiplier calibration guide

Before changing the value, check whether a printer-side flow override is already active. Otherwise, you may unknowingly compensate twice.

Use the calibration method recommended for your slicer and printer. Compare adjacent extrusion paths and top surfaces rather than judging only one corner or seam.

Make small adjustments, retain the previous sample, and confirm that improving the test surface does not worsen dimensions or other features.

Use Three Tests to Answer Three Questions

Instead of printing the same complex model repeatedly, choose focused tests:

Test	Question it helps answer
A simple part with long continuous paths	Can the system deliver material steadily?
A small model with repeated travel moves	Does extrusion recover consistently after travel?
A suitable flow-calibration sample	Is the commanded material amount slightly too low or high?

Keep a record of the exact TPU, nozzle, temperature, profile, and changed setting.

Once the individual tests are acceptable, print the actual part. A successful calibration sample is evidence of improvement, not a guarantee that every geometry will behave identically.

When choosing filament for another project, explore the INKCLOUD filament collection and review the individual material guidance before selecting a profile.

Frequently Asked Questions

Should I increase temperature when TPU under-extrudes?

Only as a controlled test within the material and printer limits. Temperature cannot correct every mechanical obstruction or unsuitable retraction setting.

Does clicking mean the nozzle is clogged?

Not necessarily. It is a reason to inspect the system, not a complete diagnosis.

Can I use the same volumetric limit for every TPU?

Do not assume so. TPU grade, hardware, temperature, and feeding conditions can change the usable operating range.

What should I adjust first?

Identify when the defect occurs. Stabilize material delivery, investigate demand or restart behavior, and then fine-tune flow if a consistent discrepancy remains.