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?
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.