Make sure your filament roller doesn’t have too much resistance as well. Since TPU stretches linearly way more than any other filament if it does have a lot of resistance then the extruder won’t be able to pull more filament until it overcomes that resistance and the stretch and it ends up physically thinning out the filament. Just a thought, hope it helps!

It's been a while since I last printed TPU.
I think I've always had to set a slightly higher flow rate for TPU, probably because it always compresses a bit when pushed by the extruder.
My TPU roll printed somewhat colder than yours at about 220C.
I usually start with a very low speed (15-30mm/s), retraction completely off, cooling fan very low (25%).
Tune the flow rate and speed then add a tiny amount of retraction (<1mm).

YMMV

For me, completely disabling retraction helped out a ton. It went from a stringy mess to mostly clean prints. Other than that, slow speeds and finding the right temperature for my printer/filament got my prints pretty nice.

zero retraction, dry the filament out (If It's not a short-notice print, I'll dry it out in a food dehydrator the night before, timing it so it comes out just as I'm starting the print.). Slow it down. Slow it down some more.

If the under extrusion is constant, try checking the filament's diameter on the roll, if that's fine, try upping the flow just a bit. TPU is fairly squishy and stretchy at the same time, so it may need tweaking. Also, consider running a temperature calibration tower.

New Lemmy Post: Any tip for printing TPU? (https://lemmy.world/post/11869100)
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