I designed a basic structure with FreeCAD. For the base, I started with a 40x40 mm square:

Screenshot of FreeCAD showing a 40x40 mm square

And I extruded it with 6.5 mm:

Screenshot of FreeCAD's Pad parameters dialog box with a 'Length' of 6.5 mm

Then, I added a circle sketch with a radius of 19 mm:

Screenshot of a FreeCAD sketch showing a circle with a radius of 19 mm

And applied 5.5 mm pocket:

Screenshot of FreeCAD's Pocket parameters dialog box with a 'Length' of 5.5 mm

The end result looks good:

Screenshot of a square base with a circular recess 3D model in FreeCAD

However, any slicer I know will produce a strange shape around the middle of the sides both inside and outside:

Screenshot of a sliced model with unknown artifacts at the tangent edges of the circle and square sides

There is a 1 mm thickness there, so I don't understand why are those shapes presented.

I'm looking for:

  1. The reason for those failed shapes
  2. The solution to eliminate them (preferably without resizing either the square or the circle)

I have provided the STL file for analysis.

  • $\begingroup$ Can you include the Stl? As a link or, if it's small enough, as a code block in the question ? $\endgroup$
    – Mołot
    Commented Dec 3, 2023 at 15:13
  • $\begingroup$ Also, include your slicer settings as these may be part of the problem. $\endgroup$
    – agarza
    Commented Dec 3, 2023 at 15:32
  • $\begingroup$ I tried more than 10 combinations of slicer settings, but those strange artifacts always stayed there. So, @agarza: if you have any idea on which settings worth trying, let me know please. $\endgroup$
    – Daniel
    Commented Dec 3, 2023 at 15:35
  • 2
    $\begingroup$ It could be a problem with the number of walls, the thickness of the walls, etc. Hence the reason for asking for slicer settings. We need somewhere to start. $\endgroup$
    – agarza
    Commented Dec 3, 2023 at 15:37

1 Answer 1


It's not FreeCad, it is the shape you want to create. The wall has 1.0 mm thickness. I assume that you are using 0.4 mm nozzle. With default extrusion width of 0.45, you can get either 0.9 or 1.35 mm thickness, 0.9 obviously being closer to desired.

1.0 mm wall width

The other issue is, there's no space for four perimeters. So, at the point where internal perimeters end, slicer need to move external perimeters closer together, to make them fuse properly. Thus, even at 0.5 mm extrusion width you will see this collapse. To prevent this effect, you need to either use smaller nozzle that would allow for internal and external perimeters to be continuous, or change your geometry to make it compatible with FDM process.

Note that, if you want consistent prints, you shouldn't set extrusion width (or wall thickness, or however it's called in your slicer) to the value lower than the diameter of your nozzle. Even making it equal is risky. For this situation, I'd suggest 0.2 mm extrusion width on 0.15 or 0.18 nozzle. Or redesign.

Wall collapse

  • $\begingroup$ In case I change the wall thichkness in Slicer software to 0.2mm it eliminates the problem. Is that a wrong way to solve this? $\endgroup$
    – Daniel
    Commented Dec 4, 2023 at 6:08
  • $\begingroup$ @Daniel wall thickness needs to be slightly larger than nozzle diameter if you want a consistent print. The same if you are lucky and nozzle is extraordinarily precise. Thus, it is a good solution, but only if you will change nozzle to 0.2 or smaller. $\endgroup$
    – Mołot
    Commented Dec 4, 2023 at 6:12
  • $\begingroup$ @Mołot I don´t see problems using Simplify 3D, I can have a little air gap of 0.2mm using nozzle 0.4mm. $\endgroup$ Commented Dec 4, 2023 at 23:20
  • $\begingroup$ @FernandoBaltazar if your prints looks good and fuse properly with extrusion width smaller than nozzle diameter, I really want to see them. Most people can't manage that. $\endgroup$
    – Mołot
    Commented Dec 4, 2023 at 23:29

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