edit: solved by printing at 20% of regular speed. This seems to give the filament enough time to ooze out of the nozzle, and the print result was excellent.
My Prusa MINI+ works like a charm, except with TPU. We have a 5-hour print task that starts well but fails after 2-3 hours because the TPU filament is no longer being pushed into the nozzle; instead it comes out of the extruder!
What could be causing this? Is the TPU just too soft and bendy? Is the shape of the extruder housing at fault?
It looks as if the TPU gets stuck and is then pushed into the extruder housing when the extruder continues to push. This happens again and again, but it's weird that it works well for hours before failing. The object is basically just a long block, so absolutely straightforward and no retractions.
We have checked that the nozzle is clean and has no obstructions. We have opened the extruder every time it happens, and there's no obvious problem to see (see photo 2 here).
We are considering to print a new lid for the extruder housing, see photo 3 here: (1) is the exit hole, and (2) is the cavity where the TPU ends up so it might help to change the lid (3) to a shape that does not leave a cavity there. Or is the problem that the roller (4) is too narrow or too soft?
For reference, the filament is Tinmorry black TPU from Amazon.
That looks like it's not melting fast enough for the extruder speed. Try higher temp, or slow it down, or both.