Minimum feature width?

I’m modeling what will eventually be a small injection molded component. It has two vertical walls that start straight for a centimeter or so and then curve about 90 degrees. The walls are 2.5mm tall and 0.5mm thick, and since my Mini has a 0.5mm nozzle it’s right at the limit of resolution. I knew that going in but wanted to see what was possible.

Amazingly, the curved portion comes out just fine - but the straight portion does not! On one of the walls the straight portion is utterly absent. On the other wall, the nozzle deposits a single drop (no horizontal movement) in two locations where the wall should be, thus building two extremely narrow pillars instead of the wall.

As an experiment, I thickened both walls to 0.75mm. Bingo - the walls appeared just like they were supposed to, other than being 50% too thick.

This has to be Cura’s slicing algorithm, since the Mini will only do what the GCode tells it to do. Hence the reason for putting this in the “Software” forum. I would presume Cura is enforcing a minimum feature width based on the nozzle diameter, except that the curved portion is also 0.5mm and it prints just fine.

Is this basically a “try it and see if it works” sort of thing? Or is there some tribal folklore that says “you can print narrower walls if they’re curved than if they’re straight?”

One thing that may help with this would be the Print Thin Walls option in Cura, I have had instances where I modeled something that was supposed to be perfectly even but when I checked the wall thickness some areas were just barely thinner than others, it is possible that the curved walls were actually just over the .5mm threshold which is why they registered. If that was the case then enabling the print thin walls option should allow the software to overcome those slightly thinner areas. It also would be helpful if you posted the .STL file so we could have a look at it. For what it is worth if you are ever questionable on if a model will print or not you can check the layer view in Cura which will show you a rough outline of how the print would come out with your current settings.

I’ve just checked again, and I can’t find this option in the Basic, Advanced, nor Expert settings in Cura. I’m using version 21.04, so perhaps this option didn’t appear until later versions. I realize 21.04 is an older version but 1) the forum remarks about the Lulzbot-specific later versions aren’t very encouraging, and 2) while the download prompts on the official Cura page for the latest version (3.6.0) do list the Lulzbot Mini, once you install 3.6.0 no Lulzbot devices are listed at all. Theoretically I could install the Mini as a Custom, but this entails all sorts of specifications for the Mini that I do not have at my disposal. I also haven’t been able to find a Lulzbot Mini config file from any third sources for 3.6.0. So I’ve stuck with 21.04 for now, at least until the Lulzbot-specific later versions start to sound more reliable on the forums here.

For what it is worth if you are ever questionable on if a model will print or not you can check the layer view in Cura which will show you a rough outline of how the print would come out with your current settings.

I believe this may also be a feature that appeared after 21.04. I see it in 3.6.0, but can’t find it in 21.04. For what it’s worth, the walls do appear in 21.04’s on-screen rendering of the objects that will be printed so it definitely “knows” the walls are there in their entirety.

Thanks for responding, I do appreciate it!

Sorry for the confusion on that! You are correct in your assumption that the print thin walls setting was enabled in the later versions of Cura, for what it is worth it sounds like when you were looking at the newer version of Cura you may have been looking at the Ultimaker version. The most current stable version of Cura Lulzbot Edition is 3.2.32 however I believe that Ultimakers version is in the 3.6 range, this would also explain why the printer profiles for Lulzbot weren’t populating in the newer version of Cura that you looked at. If/when you want to download the new version of Cura you will want to download it from here to get our version:

For the Layer view that I mentioned that should be available in your version, it is available in Cura 21.08 which is from the same branch of Cura and had the same basic layout. In legacy Cura this is in the upper right of the screen in a box like the load model and control boxes, it should say normal but if you open it down at the bottom will be an option for Layers which will show you a layer render of the way the print will come out.

Bingo, you’re correct and I found it. Sadly, it’s less definitive than I’d hoped… the Layers view shows the walls perfectly. As I slide through the layers, the walls “build” themselves exactly the way you’d expect.

From this, I’d presume that Cura does in fact “see” the walls and would render them into the slices as expected. If this were true, the printhead would trace them out. But that’s the key: The printhead does NOT trace them out, other than the two specific “dots” (which become little columns) mentioned earlier. Since the Mini will only do what the GCode tells it to, I presume Cura is not including the walls in its GCode output. Unless, of course, the firmware in the Mini can choose to ignore CGode for some reason. Can you confirm it does (not) do that? It’s not how I would design the system but I didn’t design it.

What I expect is that the Mini does exactly what the GCode says - no more, no less. Which means since the printhead does not trace out these walls, the walls are missing from the GCode. Since the GCode comes from Cura, but Cura clearly shows the walls in Normal AND Layers view, it is Cura that is deciding to delete the walls from the GCode.

Which then raises the question: Why? Under what circumstances does Cura know (for certain, we can see the walls on screen) that a feature is present yet deliberately delete it from the GCode?