I created this part and when I imported it into Cura for slicing and 3D printing, the two cylinders that were supposed to be hollow were filled.

I exported the .stl with default settings except I set Angle to 2°.



I threw the .stl into the first online stl fixer I could find (https://www.formware.co/onlinestlrepair) and it reported 3 Non-manifold edges and 6041 Inverted faces.

The repaired .stl sliced alright in Cura.

Hi Andre, you've got an area in your model where more than 2 surfaces are coming together at a single edge.

This area here:


That's the non-manifold structure.

When the slicer tries to generate a section through that area, it's going to get a result like this:



It wants for each section to make a well formed closed region with a well defined inside and outside. The touching point here is ambiguous whether it's an inside point or outside point.

So you generally need to avoid constructing models with this type of structure, you need a little bit of clearance there with pieces either a little further apart from each other or pushing through each other a little bit instead of just barely grazing each other at a single point.

Hope that makes sense.

- Michael

Hm, interesting. The model was previously working, it only stopped working when I cut out a piece here to make that neck shorter.



I'm guessing that's just because it changed the order of vertices that the slicer iterates over or something like that.

Hi Andre, yes nonmanifold geometry can behave unpredictably.

Some mechanisms can do things like say determine orientation by intersecting a ray from a known outside point to a vertex and if it doesn't happen to cross the non manifold region it will seem to be ok.

Also another thing is that can be problematic for 3D printing in general are "thin" areas where 2 different points on the model surface are closer together than the building tolerance.

So you kind of want to avoid really pointy needle thin shaping like these:



- Michael