Troubles with TPU in dual extrusion

[paoletto 3]

Hi, so i'm sort of troubled with a dual extrusion print PLA/TPU.

I had the cura project set up and somewhat successfully printed on UM3 back in 2022.12.
Then i had to leave, and i picked it up now.

However, somehow now printing the tpu topping is pretty terrible, underextruding A LOT..
The only difference i can think of is that ambient temperature is now some 10 degrees higher, and the tpu roll isn't sealed anymore.
I read mr5 suggesting that tpu is hygroscopic? If that's the issue, how do i dry it? In the owen at 50 degrees?
If not, what can i try? slowing down more?

I cleaned the printcore, for the records (somehow it was half clogged not even sure how lol)

 

[paoletto 3]

I think the problem might be lying in the fact that TPU filament compresses a lot, and therefore oozes a lot during the print. Typically the first couple of layers are fine, that is, right after the second extruder, at some point during the print (15th layer or so) is activated, primed, and start with its first layer. After that, once is kept warm, tpu keeps oozing out because of the pressure built inside.

I have done this disabling retraction all the way (also in successful prints months ago). I now tried to re-enable retraction, but the problem is that the filament gets also retracted multiple times during the layer and that fucks everything up, even worse than with no retraction.

So i wonder: is there a way to enable retraction only *between* first and second extruder stints?

 

[paoletto 3]

Seems like i overlooked the settings under "Dual Extrusion" that become available once "enable retraction" is enabled. I managed to use them to improve the quality. Still not great but a lot better than before.
I think extra prime amount might also help, except that i'm not 100% sure what that does

[Dadkitess 19]

I've had multiple issues recently using PLA/TPU and PETG/TPU : TPU in the second AA printcore will trigger the flow sensor. Not sure if it's relevant, I've not seen particular under-extrusion before error.

 

Oh, btw, it's using the new CURA 5.3 interlocking function, maybe it's important to mention.

[paoletto 3]

@Dadkitess thanks for your feedback. I use an Ultimaker3 which afaik does not have a flow sensor. And still cura 4.13 (as long as it does what i need, i'd rather not change. Maybe at some point i'll try 5.x on another machine not to wreck settings and such).
If i remember correctly i ended up disabling retraction for second extruder (by setting it to 0mm though), increasing flow for the same, and enabling/increasing retraction on second extruder on extruder change. After i did this, i managed to print pretty decently PLA+TPU, and also the oozing and the tpu mess left around decreased substantially. Not entirely sure how i managed to print it just fine some 6 months ago without doing this, but alas..

[paoletto 3]

IIRC UM3 doesn't have a flow sensor, does it?

Btw i'm facing this issue again.. Projects that i managed to tune to print dual PLA/TPU just fine, pretty much in fire and forget mode now started to underextrude the TPU and stop extruding at all after few layers.
I cleaned all the printcores repeatedly, also the cool-ends cooling fan that usually gets clogged with thin plastic, but no luck.. The spool is still the same, so i don't think it's a material fault.
Could be the heat (it got hotter these days) but i cranked the AC up.. so really not sure what could be going on..
Right now i'm stuck.. Can try one more go with printcore cleaning, but honestly doubt it's going to do magic-.

[paoletto 3]

After more failures i have noticed something interesting, related to the setting nozzle switch retraction distance.
It seems that, when combined with flow > 100%, this distance is scaled up. In particular, if i manually increase flow during the print, this retraction goes so far to pull the filament out of the printcore (!).

It seems something firmware-related, since the retraction changes at runtime depending on what i change in the tune menu.
Can someone confirm?

[Dadkitess 19]

Can't confirm, but I do relate that lowering the print speed will definitely help printing TPU.

I was struggling with my UMS5 lastly, I've always been struggling with TPU printing actually, flow sensor would trigger the "no more material left", while it would resume perfectly fine.

 

I tried lowering the speed to 75% in the machine control and I did it fine for 3 consecutive prints that would have failed systematically at 100%, after less that an hour printing.

 

Note that I was already using the TPU preset speed, and I even set it lower at 20mm/s instead of 25mm/s IIRC. It was still to high, apparently... So yeah, you can print TPU on UM machine, but at damn slow speed, insanely slow I'd say, 15 mm/s is really too slow, even for a bowden machine.

[paoletto 3]

Thanks for your feedback. I have it already at 8mm/s and when it works, it works pretty good (aka the first 2-3 layers, right now).
For some reason i almost found a setup "fire and forget" that does not seem to work anymore.
But perhaps i should go back to that one, because my subsequent tuning might be the issue here.
I feel i might have exaggerated with nozzle switch retraction distance in the last failed attempts..

[paoletto 3]

So i tried to reduce the nozzle switch retraction distance, add extra prime amount, increase prime tower minimum volume, but it lasted just few more layers.

I also noticed that it starts failing when the "resting temperature" cranks up from 100 degrees (that is standby temperature setting) to 160, presumably due to smaller layers and therefore less time between one material and another.

In any case, right after aborting the print, i went to Material 2 -> Move, and there i had to feed a lot of filament (maybe 2cm or even more, by the feel of it.. i kept a finger just below the feeder to sense how much it was pulling) before something started to come out of the TPU extruder. So the TPU extruder starts failing because it gradually loses pressure..

This tells me the filament is retracted more than its pushed during those initial 3-4 layers.
 

Edited June 26, 2023 by paoletto

[paoletto 3]

Last episode of the series: i found the project file from March and gave that one a go after cleaning everything once again.
With this one the TPU started failing later, approximately where it failed last time i used this project (i still don't know why). And the extruder wasn't extruding anything anymore.
At this point i tried the improbable: decreasing flow to 99%.
Well, it restarted printing TPU.
The print was sent out with flow at 106% on TPU extruder. Idk if this was the issue. I wonder how to achieve higher flow if higher flow is causing lower flow..
 

[paoletto 3]

when calculating the amount of extra prime amount, would it be correct to count 6mm³ = 1mm of filament ?