Print has travel paths which aren't in slicer, combing ignored on ~50% of layers, thick stringing

[Keishii 0]

Hello,

 

I've been going nuts trying to change settings, restoring defaults, googling, etc.

All in an attempt to fix very thick stringing issues on a set of handles I'm printing for a box.

 

It wasn't until I did a test piece, stopping it after ~20 layers, that I noticed something. The strings on the print follow a path which is non-existent in my cura preview. It appears that ~50% of the layers follow the correct combing travel path, and the other half ignore it, moving straight from the end point of one layer, to the start of the next.

 

I finally feel less crazy after my journey down a rabbit-hole trying to fix an issue which I believed was caused by my tinkering with combing. But now I want to see what others think about this. I assume I could get rid of the stringing by changing my combing distance before retraction from 0 to 10 or something. But at the same time, I see the printer taking a path that does not exist in the slicer preview, and it makes me wonder if I've come across a bug or something.

 

Attached are: photos of the print, the slicer preview, and the print with a red line highlighting the occasional correct path being used. I've also attached the project file as well as the gcode, which I've taken from the SD card after the print, to make sure it's the exact file used for the print.

 

Cura 5.7.2

 

bottomless.gcode handles project.3mf

Edited August 5 by Keishii

[Slashee_the_Cow 410]

It does string across that path for a couple of layers:

But your photo looks like more than a couple of layers.

 

It looks like it's aiming directly towards the Z seam - but why it would try and take the direct route instead rather than comb I'm not sure. My only guess so far would be to try disabling Mesh Fixes > Enable Fluid Motion and see if that makes a difference. I don't know if you're printer has a motion planner; if it doesn't, this setting shouldn't be on, if it does, it could be trying a little too hard when it's plotting.

[Keishii 0]

Thank you for the reply.

 

I ran a test with fluid motion disabled, resulting in no change (first attachment).

 

I then closed the project without saving, and reopened it to reset the changes from the last test. I set Max Comb Distance With No Retract from 0mm to 10mm, and the stringing vanished as expected (second attachment). Is doing this the same as setting combing to Off? or would disabling combing modify any other variables?

I'll use this to solve my issue for the printed parts I need, which is nice.

 

The travel deviations are still interesting though, and I'm down to run more tests if anyone has more ideas.

 

Regarding increasing the number of retracts during combing, is there any negative impact this may have on my direct-drive printer? I've seen people mention printer wear and tear, along with filament grinding when a Bowden-style printer has excessive retracts, but I don't know if there is any harm with my direct-drive printer.

 

Edited August 6 by Keishii

[Slashee_the_Cow 410]

5 hours ago, Keishii said:

I then closed the project without saving, and reopened it to reset the changes from the last test. I set Max Comb Distance With No Retract from 0mm to 10mm, and the stringing vanished as expected (second attachment). Is doing this the same as setting combing to Off? or would disabling combing modify any other variables?

Disabling combing would affect a lot more things. Combing is, where possible (and depending on your settings) planning travels to prevent going through walls and possibly skin, since even if you retract it's still entirely possible that there'll be a bit left in the nozzle that could ooze out and you don't want that appearing on an outer surface. The correct path you highlighted in your first post is a perfect example of combing. Disable combing and it'll just take a direct path everywhere and not give a stuff if there's anything in the way:

 

5 hours ago, Keishii said:

Regarding increasing the number of retracts during combing, is there any negative impact this may have on my direct-drive printer? I've seen people mention printer wear and tear, along with filament grinding when a Bowden-style printer has excessive retracts, but I don't know if there is any harm with my direct-drive printer.

Technically yes, practically... only if you're likely to have a lot of retractions. Bowden extruders have to retract a lot further (often 4-5x as far) which means wear and tear on much more of the filament each retraction. Bowden extruders also have to apply more pressure to the filament to get it to move (even just in normal printing) because they have to push it a lot further than the ~1cm or so of internal pipe between my direct drive extruder and the hot end.

[GregValiant 1,342]

Part geometry plays a part in Combing and Travel.  "C" shapes are always tough and this project has 2 of them.

 

Just talking here (I may have had too much coffee)....

Looking at just the first layer of the original posted project, there are 13 "Islands".  Each island really needs a retraction, then movement to the next island, and then a prime.  The chances of stringing are high.

 

But the cross holes are small and the Tree supports aren't doing much except holding up the outer edges of those little cross holes.  Changing to "Normal" supports with "Touching Build Plate" simplifies things as the initial layer now has 5 islands instead of 13.

 

That isn't something that can be done for every project, but for this one it could work.  

 

Another tough spot on this project occurs at around layer 31.  The "support islands" are finished, but the little cross holes cause 4 more islands to develop and we're back to 7 islands.

 

Because the tops of the models aren't planar, at layer 40, more islands develop and we are up to a veritable archipelago of 10 islands.  Each of those islands requires a "travel to" and a "travel from".

 

It's always a balancing act trying to get models to print as we think they should.  Any slicer is going to impose some constraints on what the user can do.  Ya just gotta keep staring at the preview, and making changes, and re-slicing, until you reach a (hopefully) reasonable compromise between what you "think" should happen, and what can actually be made to happen.  

 

Simplifying the project to a single part reduces the number of islands, and depending on the "Z-Seam" location (0,0), the "Layer Start X" (110), the "Layer Start Y" (110), the "Combing Mode" (All), and "Avoid printed parts" (False), you can get rid of a lot of travel.  

At this point (just before the outside islands develop) there have been no travels outside the part.  It's going to take longer because of loading and unloading parts, but it would likely make for a nicer print.

 

It's always about the compromises that you make in the setup, and the settings.

[Keishii 0]

Thank you Greg. I appreciate your skilled analysis and will reference your tips for future prints!