How to slice rounded corners in thin walls with Cura

Question

I'm trying to print a small box with 1 mm thick walls. The box has rounded corners.

Cura slices the rounded corners with straight internal lines instead of rounded lines:

The reason for these straight lines is that Cura is using infill to fill the walls, because it determines that there's not enough space in a 1 mm wall for three 0.4 mm passes.

This causes the walls to have small imperfections in the final print. I was able to solve this by increasing the model wall thickness to 1.2 mm (to allow for 3 0.4 mm lines), but even then it wasn't doing it as a single continuous line, but instead breaking it into short lines at the corner:

Is there a way to avoid this issue without increasing the wall thickness, and have 3 continuous lines?

Answer 1

Fixing the model

My solution to this problem is, to increase the thickness in such corners by 0.05 mm by pushing the inner wall's radius towards its center. This allows some extra space as a round corner is not actually saved round but as a pattern of straight wall facets, and those can get squished together just enough to make the slicer believe the wall is too thin for a proper wall. As you see, the new corner rounding starts a little bit before the outer one - in this exaggerated model I use 1 millimeter extra - violet are the normal corners 10 mm apart, black is the new inner corner, the light blue area between them is the extra bulk that will massively reduce the problem.

origin of the error

As an example, let's put the number of facets to 1 and 2 when we form them from the rounded corner. The more facets there are, the closer the distance between the walls will get to the full wall thickness, but it will always stay a fraction below.

How does the model fix work?

Well, we shifted the inner corner away, and with 2 facets we get a fidelity to the intended wall thickness of 96.21 % instead of 92.39 % for the non-modified wall - and with even more facets the fidelity gets better more quickly. However, there is a point where your gaps will start to diverge in the corner, so just using a tiny alteration is advisable - in our model case, 9 facets (a 10°) is the point where the inner wall will start to become more distant to the outer wall. That's why I only need such a tiny bit of extra space if I export the model with the most fidelity in the .stl generation my CAD has available.

making it work in the slicer

Another thing I do when I can't alter the model is, knowing that I usually slice width a line width of 0.45 mm. In case such a corner appears, I can then alter my line width in the slicer down to 0.44[5] mm (or more) and get pretty much the same result at the cost of possibly needing to account for errors in other areas.

Answer 2

Contrary to the other answer, this is not (exclusively) a model / geometry issue.

Frame challenge: There's no good / easy solution to this when using Cura, at least that I know of. Better slicers can easily handle this situation though.

I replicated all the tests in SuperSlicer, which is a PrusaSlicer branch. All tests were done with models with relatively high resolution:

As well as a model which I created to replicate a horrible quality .stl:

I checked the continuity of the added internal line with Simplify3D's g-code preview.

For a 0.4 mm extrusion width and a 1 mm wall thickness, PrusaSlicer simply adds a 0.2 mm line in the middle. This line is completely uninterrupted:

The same happens with the very rough .stl:

The same thing happens when using a 1.2 mm wall thickness:

Conclusion: You should not be adding esoteric "fixes" to your models to prop up terrible slicer performance.