Many thanks Torsten. Nailed the fault in the program generating that PolyHedron! Why did the faulty PolyHedron render OK with F6 while by itself. The subsequent STL file was happily sliced by Slic3r Mystery, Greg ---------------------------------------------------------------- Turn on "Thrown Together" view. Flip winding order (reverse points) of the face shown in pinkish color. ciao, Torsten.

On 11/7/2022 1:58 AM, Greg Leonard wrote: > Why did the faulty PolyHedron render OK with F6 while by itself. The problem is detected only when OpenSCAD uses the full-powered CGAL subsystem to do things like unions.  When you only have one object, CGAL doesn't get involved. > The subsequent STL file was happily sliced by Slic3r Consumers vary in how tolerant they are of problems in the STL.