henderpa

Huh, okay... I'll space some holes apart by different amounts and see what that does.

Update—the only single setting that has worked so far to eliminate the gaps is Thanks burtoogle! I have just tested with these settings and so far they're the only thing that has gotten rid of the gaps around the holes. I just did two prints; the first one was after I'd tried a bunch of stuff and it still made swiss cheese of interior round holes. For the second print, I _only_ added comb distance without retract as 10mm and switched combing mode to not in skin. Printing the same model with only those two settings changed, and I'm not getting any problems with my round holes. They are perfect. So... anyone willing to venture a theory as to why those two small tweaks in particular solved it when everything else I tried didn't? (I'm admittedly still cautious in my optimism, and will certainly want to reprint some old troubled models with these two things changed to see if that fixed it on them as well...)

Yup, the walls of the holes are blue; the rest is green.

Thanks, I had been printing with a range of higher temps and was told the temps were abnormally high and that that was a likely cause (which didn't make sense to me, but process of elimination... I can't remember if it was this thread or another, as this topic is plastered around the Ultimaker fora). So I tried lowering it as low as I could still print, and the issue is still happening. I have also been running with a 50% small feature speed but that hasn't fixed it. At any rate, my main question is still: why is extrusion noticeably (almost unusably) worse specifically with curved holes, and only in Cura? I'm not having any other extrusion-based problems whatsoever. Not even on other (straight edged) hole-walls.

Thanks! I have. I did that when I switched firmware and checked it when I put on the new hotend.

Hoping the comb setting burtoogle suggests above produces a change! Trying that next...

I've tried adjusting some of the settings suggested above, and I can confirm that it's not that my speeds/temps are too high. I tried a print at 190 degrees (the lowest temp advised for this particular PLA, confirmed with an external thermal sensor) and 30mm/s print speed, line width of 0.4mm and layer height of 0.2mm, no combing, no acceleration, no jerk control, etc. Still came out with crummy round inner walls and perfectly good straight inner walls. I know burtoogle is skeptical that it's anything but a setting issue, but then I really don't understand why it also happens at default Cura settings, and why it's only curved lines and not straight ones (or why it's only inner curves and not outer curves). Also: tried a brand new hot end with a brand new nozzle, so it's not from clogging...

Perhaps "small feature speed" is the crucial factor, here. Because I've tried upping the extrusion, even just around holes or on walls, but that makes it _overextrude_ any straight lines... because the underextrusion is only happening (for me) on curves. Anyhow, thanks for the suggestion, I'll try pulling "small feature speed" way down.

@Arak0n, this is the best example of this that I’ve seen so far, and shows why it’s so frustrating—that peg is going to be very weak indeed, because the slicer turned it into pastry. It also demonstrates to me that it’s not an issue of cooling time between parts of a layer; my theory at one point had been that it’s due to Cura telling the printer to print interior of those hole-walls first, which then cooled (and contracted) while the printer did the rest of the layer before returning and doing the others, which no longer contacted the now-contracted lines. But since on your model there’s only the peg part for those higher layers, there’s nowhere for the nozzle to go to let the lines cool and yet it’s -still- happening. This furthermore tells us that it’s nothing to do with infill. Because if too little infill were pulling the walls away from each other on the holes, the peg wouldn’t be a problem. I am increasingly convinced this is a bug within Cura; that there’s some calculation it applies when determining line width or extruder flow for rounded edges that causes it to underextrude only curved lines, and only at certain scales. (Since you posted your printers: I was seeing this on an Anycubic Mega S, both with stock and Marlin firmware. It happens even if I print with default settings [0.2mm Normal] from Cura.)

That’s... disconcerting. Haha.

Update: no luck with hole expansion—it appears to do the same thing as horizontal expansion, but applied only to holes. That is, it makes the hole itself larger or smaller, but does nothing to fix the gaps where the walls meet the rest of each layer.

Hi Tomahawk_101, I haven’t! I hadn’t realized there was a hole-specific expansion setting! I’ll try that out today and report back, thank you!

Hi Greg, thanks. It’s a good thought, but I have actually tried a range of line widths with each nozzle, up to 0.1mm in either direction (printing 0.4mm to 0.6mm on a 0.5mm nozzle, I mean). I’ve adjusted flow as well, but as I point out turning up flow even just for inner walls causes will then cause overextrusion on straight lines (excess filament buildup, stringing, messy prints everywhere else).

Hi burtoogle, thanks for your thoughts. I currently have a 0.5mm nozzle on it, and when equipped that way my lines range from 0.4 to 0.6mm in width, and heights from 0.1-0.4mm depending on what I want to achieve with the model. I sometimes print with a nozzle as small as 0.2mm and the issue remains (albeit with smaller gaps). Haven’t seen any other underextrusion issues apart from around the circles, and as I mention above the same model and machine via a different slicer doesn’t end up having the gaps (with same speed and temp, which I hadn’t pointed it out but when I was testing Prusaslicer, for example, it was set as close to my Cura settings as I could get). Also, it doesn’t matter how slow I go or where I set the temp—if the rest of the model is bang on, the circular holes are still an issue. (Oh, re: temps, the filament I was using on that one was a PLA that likes it a bit hotter. The temperature was arrived at after several bridging and retraction tests.) Anyhow, I’ll see if I can lower the temp to as low as it will go and still allow adhesion, and I’ll drop the volumetric output accordingly, to see if those are the issue. I suspect it’s some combo of the sequence in which Cura is programmed to prioritize the holes vs the rest of the structure; the way it handles travels (have tried a range of travel speeds between 90 and 150mm/s); and the amount to which the holes cool before their adjacent walls are laid down. I -do- notice that in a given layer, it will print each hole in its entirely (all walls) first before connecting them, so I gather they’re shrinking before the walls get built around them.

Thanks, @burtoogle. Here's a sample file, but it's not exclusive to this one by any means. GapHoleSample.3mf