Thick TPU Parts at 100% in-fill, warping, is this just a 'material tension' issue.

[klippeNZ]

I have been printing for about 18 months, so far only PLA and TPU. Part of what I do for a living is mount GoPro cameras on various vechicles for a small TV crew I work for.

One of those tasks involved putting cameras on cars that are designed to crash into one-another (we call them, here in NZ, "StockCars"). Given that nature of that motorsport, I cannot get away with standard GoPro mounts. So I have designed some clamps and arms that keep my cameras in place. They are printed in 95a TPU.

Given the nature of the sport, and how I have to mount the clamps using through-bolts, I cannot use anything other than a 100% infill. The parts simply get crushed at anything under around 80% infill. And if they take an impact, then that last crush resistance is super important.

One issue I am having, is what seems to be a problem more with the behaviour of the material itself, than a build-plate adhesion issue. Once the material reaches a certain height (normally around 15-20 mm) I can often see the part starting to warp. I have tried the following;

1. Print speeds, slower, faster
2. Build plate(s) PEI, textured, glass
3. Build plate temp 0, 15, 30, 60 (none of it seemed to matter - on any of the plate materials I tried) (also differing hot end temps from 200 to 250c - same results across all temps)
4. Retraction settings (I have no idea how this one was supposed to affect a warping issue, but it was suggested, so I tried it).
5. Tape/glue/anything-that-is-sticky on the build plate (no difference)
6. Different brands of TPU (local NZ supplier's TPU, CrealityTPU, ESun TPU etc etc)
7. Enclosure (with and without all the various combinations above)

Am I right in thinking that this might just be an inner-tension issue with heating/cooling across the thick cross-section of the material itself? I mean, short of allowing it to cool completely between every single layer, I'm not sure there's anything that can be done. I can print thinner TPU parts no issue at all.

And just getting back to the bed adhesion thing, I have managed to pick the entire weight of the printer up off the bench whilst trying to lift these parts off the plate - they are stuck - which makes me think I am fighting a "material tension" issue.

Am I an idiot, or is this likely more a material properties problem than an 'adhesion' problem?

Thanks!

[klippeNZ]

I apologise for the lack of line breaks. The edit function is not working for me either. (Line breaks now working, yay!)

[airscapes]

Thanks for the detailed explanation! I do not normally make TPU parts that large and have never had an issue with them warping but I also don't normally use 100% infill.
You did not mention nozzle size or layer height.
I would suggest if you are not already doing it is to get a .8mm or 1mm nozzle and print your parts with .5mm layers.
I would suggest that you filet your corners where the part touches the build plate, the larger the radius you can use the better.
You must, if you are not, verify your Z offset and first layer is the proper height. If you do not have them model some 16mm x16mm squares with 2 wall which will be the thickness of your set extrusion width and a height with divides evenly such as 3mm tall for .5mm layers. Once printed the model should be 3mm tall +.1. Make adjustments in the slicer/firmware or however it is done on your particular machine.
If you can get some NinjaTeck Cheetah (probably not as made in US) that stuff is far better than others.. and more expensive as well.
TPU also need to be dried.. you did not mention any issue with water pops so I assume you have that under control.
I typically print TPU at the mid to top end of the manufacture range and use their top bed temp.
You did not mention if you are using a bed probe and height map? Does the warping happen in the same locations on the bed every time? Could be height variations of the bed surface. Hope something i mentioned helps
Good luck!

[klippeNZ]

Thank you very much for getting back to me.

Nozzle is a .4mm (standard one that comes with the Ender 3 S1 Pro). 'Sprite Pro' extruder, .2mm layer height.

I use a Sunlu drier and the material has been sitting at 55 degrees celcius for days. It's likely as dry as it is going to get. Relative humidity in the box gets as low as 18%.

I have a bed probe and a mesh is taken before each print. The first layer goes down very well indeed and I very rarely have bed adhesion issues on any print I am doing. I semi-regularly tram/level the bed and check the z-offset. All seems to be as it should.

I like the idea of a bigger nozzle and layer height, however there's detail in the print that I need to "index" an arm that gets clamped in the front of the mount (see attached image) so a semi-decent resolution is needed there.

Must mention that I have tried printing these using a 40% infill, with a 6 wall perimeter. Although the warp was less than with 100% infill, it was still present (sadly the clamps really need the 100% infill as they are really clamped tight with the bolts etc and they do collapse when I really tweak up the torque on the bolts). I went with six walled-perimeter as it means that the long back curve on the clamps essentially get's printed solid (as the 6 walls on ether side essentially take up the approx 5mm thickness of the clamp at that point).

I might try a bigger nozzle just to see what I get away with, although I do doubt the ability of the Sprite extruder to push more than it already does, perhaps a .6mm nozzle might help?

So, the image;
Point A: was my first issue. That's a critical point because the warp ruins the index radial pattern that I use to help 'lock' the arms in place.
Point B: I simply rotated my build plate (NOT the part on the build plate, but the plate only) and it started suddenly warping there instead (this, curiously, stayed the same when I used tape too - having this change with JUST a build plate rotation makes no real sense to me either)
Point C: That's where the current issue is, without having changed any settings etc. Both TPU spools I am using at the moment have the same warping in that area (spools are from different manufacturers).

I am beginning to think that this is just an inner-tension issue with the TPU itself, but given that the point of warp has "shifted" I am wondering about different positioning on the build plate with regards to heating consistency.

Thank you very much again for taking the time to reply, I really appreciate it.

[airscapes]

May want to try making the part more blocky with out the nice rounded ends. This way you have nice adhesion to the build plate.. Looks real nice as it is but not all that great for 3d printing.. I try and stay away form overhangs and tapered ramp.. Good luck in finding a solution.. Maybe Nylon 910 alloy rather than TPU??

[klippeNZ]

Cheers. I have had someone else (who never has warping issues with TPU) print it for me and he's had very similar results to me (same warp in the same places), so I am beginning to think that it's purely an "amount of material, coupled with inner tension, due to heat/cooling" issue - but I may be wrong.

The warping is less with infill at less than 100% (of course, there's less material to warp, so it makes sense).

Have previously tried square edges etc, same warping issues in the same point (sorry, forgot to mention that above).

Thanks heaps for getting back to me.

[airscapes]

you got me now.. can I try and print it for you? Just really curious to see how it would come out. if yes, pm me and I will send you my email address to send the stl.