I've been struggling with this problem:
Image 1: I have an object that is composed of stacked layers that I'd like to split (so that I can print it in different colors, without an AMS). The STL file is only 8MB and 167,00 triangles (well under the import limits of 25MB and 300,000 triangles). Splitting it was expected to be straight forward (with holes and union groups), but before I could get that far ...
Image 2: The imported STL had missing mesh pieces (mostly in the vertical walls, rather than the xy plane, if that helps).
Image 3: I've imported the STL into Orca Slicer and it looks perfectly fine, and slices without errors (no non-manifold edges, etc.). I've imported it into MeshMixer (and, under analysis, there are no errors). I used MeshMixer to decimate it (reducing the mesh by 50%), and still it imports into Tinkercad with missing mesh pieces. 3DViewer (dot net), used in image 1, was used to export it as an OBJ ... still no luck.
I will gladly take any recommendations for next steps. The STL is from MakerWorld, entitled "Mom mandala - mothers day gift", if you want to go straight to the source.
