[Solved] Badly manufactured PTFE tube in hotend causing TPU print jams
I've been fighting with TPU prints on my brand new (received beginning of Feb, testing protocol 28.01.2019 serial number CZPX0419X004XK06___) i3 mk3 kit and having no luck. Regardless of settings, software, and adjustments, the filament would spool happily into a jam between the extruder bondtech gears and the ptfe tube. PLA worked fine and I got some beautiful prints out, but for a direct drive printer to be even worse at TPU printing than my Creality Ender 3 Pro was confounding to say the least.
After the 30th failure I disassembled the extruder, took the hotend out, and removed the PTFE tube from the hotend. It looked fine, but was full of what looked like diagonal gear marks along the whole length. Taking various filaments and pushing them through they all stuck quite badly and needed a fair amount of force to be pushed through, less than ideal. Testing the same filaments on another piece of PTFE tube I had, they all had no resistance. So it looks like I had the bad luck of receiving a badly extruded PTFE tube that got squished during manufacture, thus causing my TPU filament to jam regardless of the speed. Maybe Prusa and/or E38 needs to do a filament movement check of their hotends before sending them out.
I fashioned a replacement tube out of my spare PTFE with a bit of a V at the gear end and angles at the hotend side, and now I'm printing CCTree TPU with the standard "Semiflex or Flexfill 98A" setting in Slic3r PE with no jams and a perfect print at pretty high speed.
Long story short: if your TPU prints jam constantly and you've tried every temperature, speed, volumetric setting, and gear alignment, then please check your PTFE tube in the hotend for problems. Hopefully not too many people got one from this faulty batch.