Hi Petr,
thank you for the answer!

The IGES file from iges2step, tested in Okino Polytrans, seems to be ok.
If I turn off JoinOnImport obviously the surfaces are all separated and I need to 'clean' all the surfaces not involved in rendering.
The strange thing is that the 'glued' parts have no relationships or contact between them.
It is possible to correctly select all the different parts in Moi but the resulting mesh is a join of the parts indicated in the jpg shown.

Gianrico