Hi Andrei, MoI's shell function is not sophisticated enough to deal with situations where the thickening would cause one vertex on the base surface to branch out into 2 different vertices on the offset. It's unfortunately a big limitation in the geometry library's solid offsetting function.

Probably your best bet would be to import the object into a different CAD program like ViaCAD which has an engine that handles shelling better than MoI.

Otherwise, the manual way to do it is to create offsets on individual separated surfaces and build extensions between the offsets as needed. That will involve quite a bit of work so probably using a different CAD program with a more robust thickener would be better.

- Michael