TPU 95 HF Stops Printing

Maybe this is a know issue with TPU, and I am just discovering something that is widely known, but I though I would record it for posterity if it can be helpful for someone else.

TLD:DR if your TPU stops extruding and then won’t extrude again, it could be because it is getting elongated and thinned. You have to trim of the elongated part of the filament to get it to feed again.

Twice I had TPU 95 HF just stop printing mid-print and then couldn’t get it extrude after that. I ended up replacing extruder gears and nozzle etc. Thinking that those parts were worn out…Maybe, but also discovered that the TPU was getting elongated (I assume by the tension from the spool to the extruder drive) during the failed prints. then when I fed it back into the extruders, it was too thin to be engaged by the extruder drive gear and would just sit in the printer head melting.

After cutting off about 12 inches of TPU I found the it would print again.

Also I think 12mm^3/s might be pushing the volume just a bit too much for my setup which is why I was getting the elongation in the first place.

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I have the same problem with 95HF. The flowt test is OK and the prints starts with a good first layer. Second layer starts ok but half-way through feeding seems to be erratic (some tearing), then the feeding stops completely. I have tried to gently feed the filament but to no avail, it seems to be stuck. Unloading works ok and I can drag the filament out, however there is more resistance than expected. The last time I did not “help” feeding, but stopped in the sam way. I measured the diameter of the filament, from the spool and up to about 11mm from the cut it was 1.70-1.77 mm (mostly 1.71-1.73), for the last 10mm it varied from 2.15mm to 1.83mm at the very end, this end was also noticebly stiffer. The islightly larger diameter may be the cause of the increased force required for unloading
If I select a non TPU filament from the AMS it works ok, so there is no clogging and the extruder seems to work as normal…

I just bought some HF TPU yesterday and ran tests on it and have been very impressed.

I used the Bambu Lab HF TPU preset.

It was more than twice as fast as my generic TPU and excellent quality.

For those having issues, I would suggest checking for heat creep. TPU is very susceptible to it and it will show just as most have described, seemingly clogging and/or under extruding. I’m able to print the HF TPU at 12mm3 or higher, even with it closed up, for prints under 2 hours. But I use a printer profile and filament profile that is good for that. For the P1P users that upgraded to the P1S, things can get a little sketchy, as well as, people who use higher bed temps. Higher bed temps heat up the enclosure, and speed up the onset of heat creep.

We have to be super careful with the profiles as they can cause problems just as easily as they cure them.

Things to note:

  1. TPU will heat creep if you get into the 40-50C chamber temp range (and may start below that).
  2. Different printer profiles use different bed temps which contribute to heat creep. For example, the P1P profile may have higher bed temps for better adhesion with an open printer.
  3. Different printer profiles use different fan speeds which contribute to heat creep. For example the P1P profile may have lower fan setting because its an open printer.
  4. We should all be printing TPU with open doors and lids (even though I never do :stuck_out_tongue_closed_eyes:)

Things I like to add to the profile when printing TPU:

  • 100% Chamber fan to evacuate the hot air. This sucks for noise, but does wonders for heat creep. But I assume opening it up would equally be as good.
  • 35C bed temps for enclosed printers. Less heat in the chamber, closed or not. I haven’t run into a TPU that doesn’t stick like glue, so adhesion is rarely a problem and this will likely be better by knocking it down a hair.