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Strange purging behavior with a prime tower

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Posted · Strange purging behavior with a prime tower

I've got a dual extruder/single nozzle printer (a CR-X Pro, if that makes a difference) and today I tried printing my first dual color test print, the usual traffic cone. I picked the two STLs that would require the fewest tool changes, which would be the ones that only need one color per layer.

I started printing and kept an eye on it because I wanted to make sure the tool change went well and then I noticed some weird behavior. Up to the first tool change the prime tower was just being made with extruder 1 only. Then after the first tool switch I would have expected it to do the same with extruder 2, but instead what it did was it printed what it needed to print with extruder 2, parked the print head near the edge of the bed, did a tool change to reselect the other color, do a layer of the prime tower, park the print head again, do a tool change, print some more on the prime tower and finally go back to the model to print the same color that it had used there exclusively on the previous layer. And it did that for every single layer until it got back to the first layer that needed the color from extruder 1 again.

Now obviously this isn't costing me more filament overall, but it is wasting a lot of time and it also means that there is more risk of colors bleeding into each other. Am I missing some obvious setting with which I can tell Cura to raise the prime tower with whichever filament is currently loaded if the next layer is going to be using the same filament?

 

I'm using Cura 4.4.1 and these settings:

  • Enable Prime Tower: on

  • Prime Tower Size: 20mm

  • Prime Tower Minimum Volume: 6mm³ (default setting but seems a bit low as my colors kept bleeding into one another)

  • Wipe Inactive Nozzle on Prime Tower: off (didn't seem to make sense for a single-nozzle printer)

  • Prime Tower Brim: on

  • Enable Ooze Shield: off

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Posted · Strange purging behavior with a prime tower

Finally had some time to play with it some more and the behavior is even weirder than I thought. I tried the traffic cone again, so never more than one color per layer. It would start with red PLA at the bottom, prime tower first, then the actual layer. Layer 2, same story. Right up to the point where the first white layer begins. That's when it would first deposit red filament on the prime tower, then swap tools, then print the next layer. It's not purging the nozzle at the critical time when it should...

 

And even just that wouldn't be an issue, that would just mean a single pink layer between a red and a white one, would probably not even be noticeable. But it keeps doing this: it will print that "white" (pink) layer, swap to red, deposit a layer on the purge tower, swap back to white and print the next layer...in muddy pink.

 

I have no idea what's going on here but this seems to very much defeat the entire purpose of the prime tower.

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Posted · Strange purging behavior with a prime tower

No one?

 

Is Cura just incapable of doing dual extrusion right?

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Posted · Strange purging behavior with a prime tower

WOW !! I could have written this same post word for word. I've just been googling the very same question, I'm even printing the traffic cone :D

Did you get anywhere with the issue? It does seem stupid behaviour to purge the inactive extruder, causing risk of colour bleed when it's totally unnecessary 

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Posted · Strange purging behavior with a prime tower

Let's say that I don't really agree with the developers' views on how to "save filament." They're doing all of this to allow for more than two extruders but it just means more wasted filament the way it is now. They will consider adding a feature that optionally uses the currently active extruder to build the outer perimeter of the prime tower though, which helps a little I guess.

 

https://github.com/Ultimaker/Cura/issues/5282#issuecomment-594646291

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