MK3S changeable fails of TPU Flexible print
I recently upgraded my MK3 to a MK3S. Using the MK3 extruder I was able to print TPU models perfectly after dialling in the right settings (225 nozzle and 35 heatbed). I've been trying to print the same file with the MK3 and the results are disappointing and confusing. I've spent about 10 hours adjusting and testing settings and so far I've achieved one good print, one ok-ish print and loads of failed prints displaying a combination of issues.
I last printed with the TPU filament (using a MK3) about 8 weeks ago and it's been stored in an air-tight box fitted with a dehumidifier.
The prints started failing (under extrusion) due to the bond tech gears not feeding the filament into the extruder after a few minutes. So I increased the nozzle heat in increments trying from 215 up to 235. The heat bed was 35.
The prints still failed due to under extrusion but the filament was stringing so I assume heat creep was causing the issue and decreased the nozzle temp trying increments between 220-225 and the heat bed set to 30.
The filament started intermittently under extruding for a period of time, then be ok, then under extrude again. If I stopped the print and started again it might be ok for a while and then under extrude again. This caused the first layer to appear ok on first inspection whilst printing but not adhere properly to the print surface, and the lines of filament would not be connected when lifted from the build plate. So I adjusted the bond tech gears screw.
The screw at its loosest wouldn't grip the TPU filament enough causing it to under extrude. Tightening it so that it was 'half way' (so the screws tip is beyond/outside the nut but still within the recess of the extruder door) seemed to do it - until prints over extruded toward the end pf the 35 minute run (too much filament came out the nozzle deforming the print and slopping the plastic about).
So I guessed the screw tightness was seemingly ok-ish, but too much filament was being expelled at the end of the print which I guessed was due to the nozzle temp being too high. So I reduced the nozzle temp to 220 with the bed still set to 30 and started again.
Then I suffered under extrusion on the first later again. And this is what it's been like.
I've cleaned the extruder by opening the door and blowing out any fine dust. I've used rigid ink floss to clean the extruder 4 or 5 times. I've performed a PLA cold pull twice (no dirt or debris on the removed filament at all) and have even performed live Z and first layer calibrations as I got more and more desperate.
And this is all 4 days after removing a serious PETG blockage from the upgraded MK3S (which never happened on the MK3) and replacing the PTFE/Teflon extruder throat tube. After doing this PLA and PETG printed ok but TPU... not so much!
I'm out of ideas and would really appreciate some help. I've included a few photos below of the one random successful print and a couple failed prints.
RE: MK3S changeable fails of TPU Flexible print
Failed print using same settings as successful print.
RE: MK3S changeable fails of TPU Flexible print
Same failed print closer up.
RE: MK3S changeable fails of TPU Flexible print
And these are pics of the most recent failed print featuring random under extrusion.
RE: MK3S changeable fails of TPU Flexible print
This..
RE: MK3S changeable fails of TPU Flexible print
And I should just mention that the last three pictures are of the same failed print, a g-code file that printed flawlessly using the same TPU with the MK3 before I upgraded to the MK3S.
RE: MK3S changeable fails of TPU Flexible print
The MK3S has now managed to complete a handful of successful TPU prints. I've set the idler door screw so that it's protrouding a hairs breadth beyond the face of the idler door, reduced the speed of the print fan for bridging from 80% to 50%, preheated the enclosure before starting the print, increased the first layer filament width from 0.42 to 0.45, slowed the speeds down by a further 10-15% and turned retraction back ON (which oddly seems to help).
The first layer is tricky as it's one sheet of filament deep, so it has to be suitably durable yet just 0.2mm thin. It's still not perfect, but it's getting there.
RE: MK3S changeable fails of TPU Flexible print
I'm updating just incase anyone comes across this thread when searching due to MK3S and TPU/Flex printing issues. Looks like I've cracked it and my MK3S is now cranking out consistent quality TPU prints. The issues I experienced were overcome by quite a few unexpected tweaks to the print settings. Turns out the relocation of the print fan (since updating my MK3 to a MK3S) and the new cooling shroud design of the MK3S parts mean the standard official Prusa Slicer system presets (that were supplied for and have been left unchanged since the MK3) are no longer compatible with the printer - or mine, at least.
Many adjustments were necessary including increasing perimeters and slowing down their print speed, increasing the travel speeds to 300mm/s, increasing the bridge flow ratio, increasing the filament temp to 225 (220 first layer) and reducing the bed temp to 30, turning the fans on for all layers except the first 2 and setting their min and max speed to 50% and 70% respectively, setting 100% bridge fan speed, turning retraction ON and increasing the length to 2mm, turning Z-Lift OFF, increasing retraction speed to 40 and deretraction to 20.
Print speeds were set to: Perimeters 25, Small Perimeters 20, Ext Perimeters 25, Infill 50, Solid Infill 50, Top Solid Infill 30, Bridges 15 and Gap Fill 30. First Layer speed was set to 60%.
That seemed to do the trick and now the balance between heat, cooling and speed seems to be pretty much there for the TPU I've been using.
