Hi Graham, that's correct MoI will not open STL files, only export them.

That's because STL files contain a different type of object than what MoI normally works with - MoI works with smooth surfaces which is a technology called "NURBS", while STL files do not contain smooth surfaces, instead they contain a bunch of faceted triangles.

MoI can convert from a smooth NURBS surface into a triangulated mesh, but it is not very easy to go from a triangulated mesh back into a smooth NURBS surface.

It's kind of like how you can make apples into applesauce, but it is not easy to take applesauce and make it back into apples again...

- Michael