Hi to all.
I have noted that, whne i do an extrusion from a plane mesh, it result in a surface, and not in a Body. This causes, when I cut the object by boolean operation, in a shell rather then a solid body.
The concept of solid objects is unknown to Blender. Therefore true CSG (constructive solid geometry) is impossible. If you do a boolean operation, a new surface is created, mostly with lots of faces.