Preform: Legal vs Illegal Geometry


In preparing STL’s for print, I was wondering if the following situations are “legal”, and if not, how “bad” is it when encountered by Preform for slicing of layers…I have two situations I’d like to understand.

  1. Two single-walled sphere’s that are identical in size, but displaced along a horizontal axis by 1/4 of their radius.
  2. Two double-walled spheres with a small “air hole” in the bottom of each, separated exactly as above in #1. Where the hole in each is topologically clean (so that each sphere is ‘manifold’ - no border edges)

The point of the question is to understand two things in each case:

A) Structural integrity aside, is it generally ok to overlap individual parts that are each are clean&solid. Will this mess up the slicer in anyway?

B) In terms of structural integrity, for the final print, can this approach be used during construction of models to ensure things stay together (rather than just creating a single continuous, closed mesh to represent what could have more easily been created with overlapping “solids”.

Sorry - I know this is a bit of an abstract question, and I’m sure it depends a bit (at least for case #2) on the thickness of the sphere wall. But if I could understand it a bit better, I think it could open up some nice modeling short cuts.


I once had some models that were overlapping a bit when I printed them. Everything went fine, I just had to break them apart with a knife. Also, I once overlapped the bases with lots of small parts to make removal easier. It worked great.

A) You can use the slider on the right side of the screen in PreForm to preview what regions will be cured when printed. The single-walled spheres would be cured as solid spheres. The double-walled spheres would be cured as hollow spheres. In both cases, the slicer will be fine, but the parts will be printed intersecting and attached.

B) Multiple overlapping manifold meshes in your stl is fine. The printed model will effectively be the union of all the meshes in the stl.

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