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double layer stacking - dual extrusion artifact
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While dual printing, Cura will print 2 layers each tool change vs 1 layer per tool.
Slic3r will switch back to the other tool at the layer change
Here are some pictures of the artifact in question which only occurs while 2 extruders are in use

Here is the same model sliced in Slic3r and printed on the same machine

My hypothesis is that perhaps a layer time that is too short isn't allowing the material below the new layer to cool enough before another layer is applied. My reasoning for this is Slic3r's way of doing it allows more time for the layer to cool before another layer is applied by switching to the other tool/part before applying the next layer.
Another theory I have heard from @kent is that the backlash isn't being compensated for properly when z move occurs without a tool change. He may be able to explain that better.

Perhaps applying this change would mean more frequent tool changes and/or an increase in print time, and may shorten the life of the actuators on quiver.

Event Timeline

logan created this task.Mon, Jan 7, 12:31 PM
logan added a comment.Mon, Jan 7, 2:28 PM

I also noticed Cura prints the second layer faster before changing tool heads, is there a way to control this behavior?

Steven added a comment.Tue, Jan 8, 7:43 AM

Wouldn't you see the same issue with a single tool head then?

logan added a comment.Tue, Jan 8, 8:17 AM

@Steven perhaps that was stated incorrectly,

Another theory I have heard from @kent is that the backlash isn't being compensated for properly when z move occurs with a tool change.

If it is backlash related I can theorize why we'd be seeing it only while 2 colors are printing; because the machine must compensate for up and down Z moves during the tool change whereas when printing with one color the Z axis is moving consistently up. Minus the moves for auto bed leveling of course. Anyway, @kent may have more on that.

The second layer being applied faster than the first I think is a large contributing factor. In a single extruder scenario the individual layers behave more similarly to each other; walls one speed, infill another, etc. In Curas current layer stacking for dual extrusion, one layer is printed noticeably faster than the other so the outer walls are being printed at a different speed every other layer.

alexei triaged this task as High priority.Wed, Jan 9, 2:07 PM
alexei changed the edit policy from "All Users" to "Cura LulzBot Edition (Project)".
logan added a comment.Wed, Jan 9, 2:34 PM

Here is a video showing the model above being printed, sliced with v3.6.2

logan added a comment.Wed, Jan 9, 4:00 PM

karrad moved this task from Backlog to Next Release (3.6) on the Cura LulzBot Edition board.

For some reason, we are seeing an increase in speed after moving up a layer, and using the same tool head. Right at about 26 seconds in the video above, the tool head goes to the next layer and speeds up. Looking at the gcode, we cannot see what/why it is inserting the increased feed rate after that positive Z move

adam added a subscriber: adam.Thu, Jan 10, 12:14 PM

I'm seeing the same behavior in quiver 12 with the second layer that's printed on E2 printing much faster than the layer before.

anolen added a subscriber: anolen.Thu, Jan 10, 3:06 PM

Same here, confirmed issue on my Quivers as well.
Firmware = .68
Cura = 3.6.3

karrad added a subscriber: Yahuba.Fri, Jan 11, 1:08 PM

@Yahuba Can you test this on a TAZ 6 v3 dual extruder?

@Yahuba Here you go, sorry about missing those earlier

Im not sure I was able to replicate this. I did not notice any difference in print speed between layers or extruders. I loaded the two stl files and merged them into the cube on CuraLE 3.6.3 using a Taz6 with Dual v3 extruder.

I don't see any of the artifact in the above picture. When it does speed up I feel it is quite noticeable. So perhaps this is only happening on quiver? Seems odd