Mesh Boolean union

I make a Mesh Box from Mesh primitive. I extract one face with Extract mesh part. Without any modifications of extracted surface or the rest of box, I want to union back extracted surface with box with Mesh Boolean union, but it fails. All surfaces are dissapear. I use Rhino 5 SR6 64-bit Commercial with latest update. I use this technique for repairing complex mesh surfaces. In Rhino 4 it works fine.

Why would you be using Union (which is for closed volumes essentially) instead of Join (which should join the mesh face back to the original)?


Thanks for your replay Mitch. I use Union instead of Mesh Boolean union with multiple closed mesh volumes and after 3ds export, some mesh volumes (for wich I used Union for extracted surfaces) are missing, . But in the last days Rhino made some updates and after I did some tests, it seams like this problem is resolved and it works fine with Union.