TPU friction in TPFE tubes when pulling from extruder end
I am having a problem printing with TPU. The printer keeps telling me that the filament needs changing. I have found that the filament is not feeding through the PTFE tubes. From the extruder end, I have to pull to the point of stretching the filament to get it to come out. However, it is easily pushed into the PTFE tube from the spool end of the tubes. What could be causing this?
Different TPU's and TPE's have different characteristics, all of them grip the inside of the PTFE tubes and stretch more or less.
In many cases just mounting the filament on a holder above the printer and feeding direct to the nextruder bypassing the tubes is enough - of course the idler pressure still needs to be reduced as usual.
However, with the softer filaments some elastic tension is necessary for retraction to work without jamming; direct feed doesn't provide the tension unless you use a rewinding spool holder or a weighted tensioner, one trick that sometimes works instead is to thread through the tube but disconnect the tube at the nextruder end and ensure that there is no unwinding tension at the spool - use an active unwinder or for smaller jobs just pull a metre or so off the spool by hand. I have one filament that prints OK through a fully connected tube as long as there is no unwind tension.
You will need to experiment; unfortunately, due to the pigment, different colours of nominally identical filaments often behave differently.
Cheerio,