My experience printing TPU  

My experience printing TPU

Now that I've mastered printing PLA and PETG (or at least I can do it repeatably with reasonable quality and not destroy my printer), I decided to try printing TPU. I needed some flexible parts for a tool I was printing, so I bought a 0.5kg roll of NinjaFlex TPU (it was only $8.95 on Amazon - possibly due to a pricing mistake on their part, it's now listed for $24!).  I read that printing TPU was tricky, so I read up on it as much as I could. However, as you'll see below, there still were a few things that I needed to know that I did not learn from my internet searches.  As instructed, I put PVA glue onto my PEI sheet (after washing it with dish soap), I loosened my bondtech gears, I also downloaded the pp_tpu_config Prusa Slicer configuration from here: https://github.com/eoprede/prusa_profiles/tree/master/MK3/Slic3r (this turned out to be a mistake).

Issue #1: The first issue I had was loading the filament. If you've seen TPU, it bends like a cooked noodle. The auto filament loader would not push it into the extruder. It would go in partway, and further extrusion (by pressing "No" when it asked me if the color was right) would typically just make it bunch up inside the extruder housing around the gears.  Not being one to give up easily, I kept trying by unloading (which wasn't easy) and re-loading it, until I actually got it to extrude.

Issue #2: Then I started printing.  It managed to prime ok, but then the filament would come out in chunks creating disconnected lines and poor adhesion to my PVA-coated bed.  Then it stopped extruding completely at which point I stopped the print.

Issue #3: Then, deciding that I will try printing TPU on another day when I had more time (it was after midnight),  I tried unloading the filament.  It started moving out (and I was gently helping it by pulling with my hand) and then got stuck.  I opened the extruder door and saw that it actually wrapped around the bondtech drive gear. I immediately had visions of having to disassemble my extruder yet again. Through careful filament pulling with needle-nose pliers and some snipping here and there, I finally managed to pull it out.  I then loaded some PETG, was happy to see that it was extruding ok and my extruder was not clogged, and started an overnight print.

The next morning (it was Sunday), I decided that I was no quitter and would try TPU again.  I again started googling around and found a few new key pieces of advice: 1. Do not using the auto filament loading function with TPU. It tries to pull it in way too fast and makes it buckle and bunch,  and 2. Use very slow print speed (strangely the pp_tpu profile was trying to print pretty fast).  So armed with this new knowledge, I rushed into the breach again.

I turned-off filament auto-load and used the Settings->Move Axis->Extruder to slowly push the TPU into the extruder. This time it loaded ok and started extruding from the hotend!

I went back to all standard settings in Slicer, but changed all my print speeds to be at most 30mm/s. I used the "Generic FLEX" filament setting. I also increased infill to 60% (not sure that was necessary or not). One video said to set infill to 80%, but Slicer kept crashing when trying to slice with that setting, so I changed it to 60% and it sliced fine.

I re-washed my PEI sheet, dried it on the heatbed, and re-applied glue stick.

I then restarted the print job and.... It went great!  TPU extruded fine, stuck well to the bed, and had great layer adhesion.  Most importantly I was able to get it OFF the bed after the print without damaging the PEI coating (there was one piece that I could not peel off no matter what, but running the PEI sheet under hot water dissolved the glue under the piece and allow it to come off).

Finally, when I was unloading the TPU, I didn't use the "Unload Filament" function.  I just slowly moved the extruder manually (Settings->Move Axis->Extruder) while gently pulling on the filament with my hand, until the filament popped out.

I've since printed TPU one more time and it still works!  So to summarize my key learning:

1. Do not use Auto Filament Load or Unload with TPU. (It would actually be nice if Prusa added an option to slow down the auto load/unload speed)

2. Apply a robust layer of PVA to the PEI sheet (if you're using PEI). I also recommend washing the sheet with dish soap first.

3. Loosen your bondech idler until is seems too loose (the top of my screw head was probably around 1-2 mm above the extruder housing)

4. Use a slow print speed (30mm/s max).

5. Do not use retraction (this is already part of the Generic FLEX settings in Slicer, but in case you're using a different filament).

I hope my experience benefits the next poor slob who decides they want to print TPU.