Arbitrary missing extrusion at layer start
This is a weird one that has me scratching my head. I've recently started having issues with missing extrusion at the start of layers - but not every layer, and not every de-retraction. I'm hoping perhaps someone here has figured out what can cause this and what I should check/adjust to fix it.
The fact it doesn't happen everywhere leads me to believe that while it's a seam issue, it's not LA or calibration related. I've already checked for the usual culprits like a loose E drive gear, cold pull comes out clean, and there are no signs of underextrusion anywhere else.
What gives? I've seen a few reports of this on the slicer issue reports, but I'm also not convinced it's that per-se as I don't see this sort of thing on the Mini.
https://github.com/prusa3d/PrusaSlicer/issues/3295
RE: Arbitrary missing extrusion at layer start
Have you looked at travel? Is the gapped seam coming from the same distance / place as the prior seam?
You might also want to look at lubrication of the Bondtech set - is it dry? Or debris in some of the Bondtech teeth causing less grab in spots? This would explain some but not all seams: and randomness.
A third possibility is filament tension from the spool binding as it rotates.
Honestly, I doubt you'd see any under-extrusion with that small a volume missing over a distance.
RE: Arbitrary missing extrusion at layer start
Ah, good call - I did have travel turned on for visualizing during initial inspection and didn't see it immediately - the blue line was hidden because the travel is inside the same volume as the internal perimeters. I had to use the step-by-step in the 2.3 alpha gcode previewer to see it.
I looks like that may be the cause - travel ooze. I suspect the reason it may be more visible now is a combination of things:
- This is a marble filament so naturally a little stringier than usual
- Recent P'licer changes to improve seam placement for "aligned" seams.
- My printing a little hotter than usual because of said marble filament.
So I should be able to resolve this by increasing the retraction a little, printing a little cooler, and perhaps switching back to "nearest" for seams depending on what I'm printing. Definitely with this print, there are lots of places where seam zits are not going to be visible.
I'm open to other suggestions, of course, but I suspect the above would resolve it.
RE: Arbitrary missing extrusion at layer start
Not unless you know of someone making a nozzle with a valve on it. Once upon a time back when I was doing fun stuff, we used piezo transducers to control rapid pressure changes, if only one could be inserted into the hot end melt zone and used to do all the retractions more locally than pulling up on a stick of plastic.
RE: Arbitrary missing extrusion at layer start
Yes... I can come up with a few ideas that work in theory but the problem with any sort of valve arrangement is likely to be added dead volume and gumming up from molten plastic getting into said mechanism...