Author Topic: Under-extrusion around the edges of circular holes  (Read 8172 times)

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Offline InfravioletTopic starter

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Under-extrusion around the edges of circular holes
« on: May 28, 2023, 02:51:57 pm »
I seem to have a problem where whenever I print a part with a vertically aligned small (2mm to 4mm range of diameters) circular hole through it the printed perimeters of the circle seem to barely touch the main infill (and at the upper facing surfaces barely touch the top covering layers). I found a guide online "Gaps Between Infill and Outline" (https://www.simplify3d.com/resources/print-quality-troubleshooting/gaps-between-infill-and-outline/) and and have tried slowing print speed and increasing extrusion multiplying but still get these. It particularly affects holes for screws to go in to or for thin axles to rotate in. I've also tried all manner of different infills, even fully solid, and tried Cura's setting for having extra perimeters on alternate layers, neither helps. Hole ofset settings don't afect this, they make the bore of the hole wider but then the area where the under-extrusion between the hole's perimeter and the infill happens simply gets shifted to a slightly wider diameter accordingly. I have increased perimter-infill overlap settings to the point it starts to affect the geometry of the outer wall, but while the outer wall is connected to the infill just fine for any setting, the inner walls around holes still keep this under-extruded "gap" area between them and the infill. I adjusted the overlap settings right up to the point where Cura thinks it is too large and turns the box orange, and then larger still, an still get the "gaps". Trying to change printing order to do infill before walls didn't help either.

I have "connect infill lines" and "fill gaps between walls" both turned on.

I'm using Cura as my slicer and the printer is a CR20-Pro.

I'll post some photos later, but the one in that guide is reasonably representative of my situation, a little more severe perhaps in that theirs are full gaps wheras mine is more like serious under-extrusion. And it only happens with circular (or close to circular) holes, and fairly small holes at that, No such issues have occured for holes sized at a cm diameter or more.

Thanks
« Last Edit: May 28, 2023, 02:57:41 pm by Infraviolet »
 

Offline Whales

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Re: Under-extrusion around the edges of circular holes
« Reply #1 on: May 28, 2023, 03:34:40 pm »
Yes photos please :)

It sounds like your small circular shapes are shrinking after being deposited.  This is normal but sometimes too much occurs and it's annoying.  Try measuring the internal radius of the hole with calipers and see how much it has shrunk vs the original model.

In my experience: some filaments are just garbage and shrink a lot.  Others can be improved by drying them.  Some are mostly perfect all of the time.

Offline InfravioletTopic starter

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Re: Under-extrusion around the edges of circular holes
« Reply #2 on: May 28, 2023, 04:01:52 pm »
Can't post photos yet (give me a few hours before I'm in the right place), but in so far as I can measure the internal diameters of small holes (tricky to get caliper tips in to small ones) Thy are printed with 0.3mm clearances for 3mm bars to fit in to, and yet always need some "filing" out with a drill bit for a bar to be fitted in. The holes are always a bit undersized. Surely there is a way to fix this in settings though rather than have to get different filament.
 

Offline Whales

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Re: Under-extrusion around the edges of circular holes
« Reply #3 on: May 28, 2023, 04:14:37 pm »
Undersized holes can be mitigated by:
 - making them bigger in the original model (very effective, but of course annoying)
 - setting XY expansion to a negative number like -0.1mm in Prusaslicer (not sure if this has an equiv in Cura, plus it distorts other parts of the model too)
 - telling your slicer to lay infill BEFORE walls sometimes helps
 - filament drying or replacement (as per my prev post)

The first two options will of course not fix your problem with walls not touching infill for small radius holes.

If you are 0.3mm oversizing your hole but still having them undersize then you are seeing a lot of shrinkage.  What plastic are you using, if I may ask?

Fun fact: different colours of the same brand have drastically different properties.

Offline InfravioletTopic starter

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Re: Under-extrusion around the edges of circular holes
« Reply #4 on: May 28, 2023, 11:13:31 pm »
Photos, here are some of the very worst examples, all of holes in the 2.5mm to 3.5mm range:

Undersizing of the holes themselves isn't really a problem, I've got in to the habits of leaving tolerances for this. The issue is ofcourse the walls being poorly connected to the infill. And infill before walls doesn't sem to have helped. If I could force the slicer to do something like 4 extra walls lines, not just 1 extra, on alternate layers I might have a hope.

I am using "eryone" branded PLA (grey), it has tended to work quite well and is what I've been running on ever since shortly after I got the printer, i went through a while of testing filaments from several brands and this one behaved best. When I say 0.3mm oversizing I mean 0.3 oversizing the diameter, so only 0.15 oversizing the radius, the shrinkage, or perhaps other effects means the actual hole comes out at just under 3mm, not substantially under, almost loose enough to force a 3mm metal rod in but not quite. And this undersizing mostly seems to only apply for small holes, large holes through parts seem to maintain expected dimensions better.

I found the under-extrusion in the areas between inner perimeter and infill particularly problematic recently, I had been tolerating it but I've had a case where a part broke in use when a screw (with nut) was tightened in to a hole because the hole wall and infill weren't well connected. I hoped I might finally find a fix now.
« Last Edit: May 28, 2023, 11:26:09 pm by Infraviolet »
 

Offline Whales

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Re: Under-extrusion around the edges of circular holes
« Reply #5 on: May 29, 2023, 04:14:07 am »
Thanks for the photos.  Eep yes that's bad.

FWIW I have had luck with a roll of dark grey PLA, but the light grey roll I bought (3Dfilies I believe) is absolute trash and does things almost identical to this one.  It also changes colour depending on print speed, so some sections of the model are lighter than others.  Other colours from this brand have been everything from perfect (PLA transparent purple) to utter trash (PETG black that kept clogging, threw in the bin with anger).

I asked about drying the filment earlier: are you able to try that?

The only other thing I can think of is dropping the extruder temperature.  Go all the way to PLA's min of 180degC and see if it reduces this problem.  It might cause some others, but at least you would see if this parameter helps at all.

Offline InfravioletTopic starter

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Re: Under-extrusion around the edges of circular holes
« Reply #6 on: May 29, 2023, 03:24:16 pm »
I don't have any flament drying equipment, but the filament comes in well sealed bags with dessicant packets in them. And the quality across the rest of the prints is very good, I only get troubles in the interface between the inner walls and the infill.

P.S. just to be clear, this was done at high infill densities, those gears (being small) are best printed as fully solid 100% infill (zig zag) with 3 perimeter walls, but I'd like to make inner walls alternate between 3 and 6 perimeters on alternate layers*. The other photos are of a larger part done with a 50% cubic infill. I use high density infills as most of the parts I do are load bearing to an extent.

*I'm trying to pursue that option within Cura's forums, but maybe someone here knwos a magic trick for it already, if not hopefully here can suggest other fixes to be done in slicer settings.
 

Offline Whales

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Re: Under-extrusion around the edges of circular holes
« Reply #7 on: May 29, 2023, 05:11:03 pm »
The desiccant packets in those bags will only be enough to keep already dry filament survive shipping and sale.  Their moisture capacity won't be enough to dry a roll.

I've read that some people have luck using their printer's hotbed as a filament dryer, combined with some sort of covering (foil?).  I'd hazard this with a "unattended fire risk" but I guess that describes all filament dryers.

Best of luck.

Offline thm_w

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Re: Under-extrusion around the edges of circular holes
« Reply #8 on: May 29, 2023, 09:33:42 pm »
Cura version?
Its interesting that the outermost layer of that inner hole seems to look like the extrusion width is thinner than elsewhere. Go to the Preview in Cura, and set color to Line Width. Make sure that its not varying the line width in there for some reason.

Have you printed a hollow calibration cube recently and ensured the outer dimension is right and the wall thickness of the cube is right? (you can use the cura plugin "calibration shapes").


If you want to print solid you can also change "Wall line count" to 999, sometimes this will give you a stronger print over just changing infill to 100% (probably does the same thing as setting infill to 100% and type to concentric though).
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Offline InfravioletTopic starter

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Re: Under-extrusion around the edges of circular holes
« Reply #9 on: May 31, 2023, 07:00:44 pm »
Cura 4.11.0

I'm playing around with using a "cutting mesh" object to give different infill/wall properties in certain regions of the model, namely areas around holes. I'm going to try a series of prints where these regions have varied properties but without affecting the geometry of the outer edges of a model. I've also got a test part with insanely over-boosted infill overlap percentage on solid infill(and no top/bottom layers) to see if that can make infill stick more to hole ringing walls, but not quite so insanely boosted as to risk straying over to affecting air-facing (red in Cura) walls.

Ultra-high wall line counts don't actually work so well for solid printing as zig-zag infill at 100%, simply because certain shapes of feature cause gaps when you've got such a concentric perimeter following pattern, whereas zig-zag fills all of these.
 

Offline Coordonnée_chromatique

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Re: Under-extrusion around the edges of circular holes
« Reply #10 on: May 31, 2023, 07:23:45 pm »
Hello,
The third row around the hole seems to had been splitted in half by a local retraction, there is probably a gradient of temperature that split the circular parts from the rest of the part, don't you think ?
« Last Edit: May 31, 2023, 07:26:44 pm by Coordonnée_chromatique »
 

Offline thm_w

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Re: Under-extrusion around the edges of circular holes
« Reply #11 on: May 31, 2023, 09:44:48 pm »
Try Cura 5, it has variable line width so may affect this problem.

Ultra-high wall line counts don't actually work so well for solid printing as zig-zag infill at 100%, simply because certain shapes of feature cause gaps when you've got such a concentric perimeter following pattern, whereas zig-zag fills all of these.

Can't say I've had that issue with my parts.
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Offline InfravioletTopic starter

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Re: Under-extrusion around the edges of circular holes
« Reply #12 on: June 01, 2023, 05:12:40 pm »
Looks like extreme overlaps might have worked, though not sure yet if they propagated through to cause dimensional inaccuracies on the outer external perimeter walls. The other technique seems to have given some success to, a cutting mesh with altered settings seems to let me affect only area near holes and not the outer external wall areas. Skin overlap I could only dare increase in the cutting mesh example, not the settings-all-over scenario as it did cause lumps in outermost walls.

Also unrealised to me, Extra Skin Wall Count in Top/Bottom was defaulting to 1. I set it to zero to ensure the other setting changes would have the proper effects.
 


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