Hi Bob, it's problematic because STL files only contain triangle mesh geometry. For example something that looks like a sphere in STL is actually made up of many hundreds or thousands of little flat triangle facets. MoI is not designed to use that type of data for modeling, that's why STL is an export only format and not for import.

You would need to use a polygon mesh modeling program to load an STL file, not MoI.

- Michael