How to Fix TPU Under-Extrusion

Rigid filament is pushed through a printer; TPU is coaxed. Because flexible filament stretches, every source of friction between spool and nozzle turns into elastic under-extrusion — the extruder feeds the right length, but a stretched, thinned strand arrives at the melt zone.

The stretch problem

Pull gently on rigid PLA and nothing happens; pull on TPU and it lengthens like a rubber band. Now add spool-holder friction, a long PTFE path, sharp feed angles — the extruder gears see filament under tension, effectively thinner than 1.75 mm, and the print under-extrudes in slow motion. This is why TPU under-extrusion is nearly always a path problem before it is a settings problem.

Clear the path first

Feed from a free-spinning spool holder, straight down into the extruder. No AMS: standard 95A TPU is too soft for multi-material feed systems — on Bambu machines only the special 68D “TPU for AMS” grade may run through the AMS, and everything else goes on the external holder (details in the TPU + AMS guide). Check the spool spins with a fingertip touch; a sticky holder is enough to starve TPU.

Unload and inspect for buckling. If TPU ever jammed in this extruder, a kinked section may still be wedged in the feed channel, rubbing every strand that follows. Cut generously past any deformed filament before reloading.

Then respect the speed ceiling

Keep the volumetric limit at stock. Bambu caps TPU at 12 mm³/s; most vendor baselines for bedslingers cap flexibles far lower — Creality’s generic TPU allows just 2 mm³/s and Prusa’s 3.2 mm³/s. These numbers look painfully slow next to PLA’s 21 mm³/s — that is the price of flexibility. Raising the cap doesn’t make TPU print faster; it makes it print thinner.

Filament settings → Setting overrides → Max volumetric speed

Temperature: stock first. 230 °C is the Bambu default inside a 200–250 °C window. Too-cold TPU extrudes stiffly and skips; if you lowered the temperature to fight stringing and under-extrusion appeared, meet in the middle.

Moisture: the impersonator

Wet TPU forms bubbles in the melt, and bubbled extrusion is thin extrusion — it looks exactly like mechanical under-extrusion but no path-clearing fixes it. If lines are thin and the surface is rough, or you hear popping, dry the spool 8 h at 70 °C (never hotter) before blaming the drivetrain.

Retraction sabotage

Aggressive retraction settings copied from a rigid-filament profile cause cyclic under-extrusion with TPU: each long, fast retraction stretches the strand, and the deretraction never quite recovers, so the line after every travel move starts starved. Bambu’s TPU profile retracts just 0.8 mm at 10 mm/s — if your profile says anything dramatically larger, that alone can be the whole answer. The broader flexible-filament playbook is in the TPU printing guide; the cross-material diagnosis method in the under-extrusion guide.

Frequently asked questions

Why does TPU under-extrude even though the extruder is feeding?

Because TPU stretches. Spool-holder friction, a long PTFE path or sharp feed angles put the filament under tension, so the extruder gears see a strand that is effectively thinner than 1.75 mm and the print under-extrudes in slow motion. TPU under-extrusion is nearly always a path problem before it is a settings problem: feed from a free-spinning spool holder straight down into the extruder, and keep standard 95A TPU out of the AMS.

What volumetric speed limit should TPU use?

Keep the limit at stock: Bambu caps TPU at 12 mm³/s, while most bedslinger vendor baselines cap flexibles far lower — Creality’s generic TPU allows just 2 mm³/s and Prusa’s 3.2 mm³/s, versus PLA’s 21 mm³/s. Raising the cap doesn’t make TPU print faster; it makes it print thinner. Also check retraction: Bambu’s TPU profile retracts just 0.8 mm at 10 mm/s.

When is TPU under-extrusion actually moisture?

When thin lines come together with a rough surface or audible popping. Wet TPU forms bubbles in the melt, and bubbled extrusion is thin extrusion — it looks exactly like mechanical under-extrusion, but no amount of path-clearing fixes it. Dry the spool for 8 hours at 70 °C (never hotter) before blaming the drivetrain.

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