In my slicing software (Slic3r) some of the vertices/walls of my model seem to have disappeared, so that the inside of the model - which should be solid - is visible, while the surface appears as a thin shell.
Why does this happen? Is it still safe to export the model for printing?
My understanding is that this occurs when the object is not a true solid. Since an STL holds the triangulation of each face and spline, the slicing engine is not "smart" enough to determine if there is a gap in the model and therefore if it should be filled in and how. When the slicer encounters a gap, it will either treat the endpoint as the end of the feature or navigate to the next point on the layer, resulting in either gaps in the print or extra inclusions that don't make sense.
I've noticed that my models will fail as a true solid when I use complex solid tools such as Union, Subtract, and Trim. A lot of times there will be a rounding error in how these tools interact with the solid model that will result in a small (sometimes not very small) gap in the outer shell of the solid model. When exported to an STL, the gap is retained.
@kareem mentioned it in their answer, but Microsoft does provide online tool(s) for 3D Printing including a solid repair tool. Use Microsoft 3D Tools to upload your STL and try to automatically detect and fix issues with your file(s).
rounding error That explains a lot. I often have to add or subtract a tiny fudge factor (eg 0.001 mm) when I'm using OpenSCAD to make it unambiguous when two faces or lines would otherwise coincide. Now I know why I need to do that and why it works.
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Some STLs aren't exported properly from the CAD software.
Use a repair tool or service like https://modelrepair.azurewebsites.net .
The repaired STL shouldn't have those issues anymore.
In the cad software, it could be that your normals are reversed or inconsistent. Recently I used Cinema4D to export some models, and the faces were acting super weird in the slicer.
What I mean by 'reversed normals' is that your polygon faces could be oriented inward. Some slicing softwares don't care about that, and treat both sides of each face equally. In those that observe reverse normals, faces may be completely invisible, or appear with a lack of shading or strange shading.
There should be tools in your cad software to align the normals, or flip them on each face at least. Alternatively, some tools like MeshLab and Meshixer I believe have tools to fix this. Also of course make sure your model is watertight and that you don't have too many errors like duplicate vertices, faces, etc.
Had similar problems with blender <-> Slic3r, modified a stl in blender then slic3r started acting up, showing strange faces where it shouldn't. The solution was (as suggested by @tbm0115) to solidify the exported object. Just add a Solidify modifier to the object(no need to apply), and when exporting to stl make sure to check the "Apply modifiers".
