Hi Gianrico, is it possible for you to send the .igs file to me through e-mail (moi@moi3d.com) ? That would make it a lot easier for me to understand what is going on. I can understand if you are not able to send it, but if you do I will keep any file you send to be confidential.

Otherwise it is pretty hard for me to guess what is happening... But like Petr mentioned it sounds like something is not going completely well with the joining process.

Looking at your screenshot, it is possible that model scale is an issue. MoI generally uses a tolerance of 0.001 units. If your objects are very large (so that 0.001 is too small), or very small (so that 0.001 is a large number in comparison to your object), then things based on tolerance may not behave very well, and joining is one of the operations that depends on tolerance.

One quick note on Edit/Separate - if you want to, you can select a set of surfaces from an object instead of selecting the whole object, and then when you run Edit/Separate, just that piece will break off into its own separate object instead of everything going to individual surfaces.

- Michael