Flex filament underprints and fails constantly

Hi,

I've searched and tested and can't really pin point the problem.

Printer: Zortrax M300 Dual. 0.4mm nozzle.

Filaments: Zortrax Z-Flex, Fiberlogy Flex 40D, F3dFilaments TPU

Issues: Zortrax Z-Flex and Semi-flex print allright, there are small underprints but the outcome never fails and is acceptable. However any other flex filament starts off fine, then little underprints appear and in the end when prints are bigger than several hours worth of printing time filament jams, printer starts to "print" in the air. Every other type of material prints well.

I've run through various settings, broad range of temperatures, print speeds, everything. I've changed the nozzle, disassembled and reassembled the feed mechanism. I think Z-flex is a bit more rigid than other TPU based filaments but other than that I can't say what's different about them.

Here are some photos of a recent failed print:

Attachment 18690Attachment 18689Attachment 18692Attachment 18693

Tips and guides appreciated.

bigger nozzle .5mm or larger, slow speed 8-10mms, No retraction, gravity feed with bearings spool support so filament will pull really easy. Adjust extruder tension while printing to get the best pull without locking up the filament drive (assuming tension is adjustable on the particular printer) TPU is not easy to use and is never going to work like PLA no matter what you do. The lower the hardness and more stretchy it is the more issues you will have..
Good luck!