Not Solid, just a "Joined Surface

 From:  Michael Gibson
5786.2 In reply to 5786.1 
Hi Bill, when this happens it means that there was some problem in setting up regions. Once you have no naked edges, then what is supposed to happen is there are 2 region structures formed, one for the "infinite region" of all volume on the outside of the object and then another region for the interior solid volume area.

An object will only read as solid if it has at least 2 regions (more than 2 means having internal void cavities as well), and no naked or non-manifold edges.

So if there was some error in generating the regions you could end up with no naked edges but not registering as a "solid". This is pretty rare though.

It may be that you have some area of the model that has either a degenerate trimming boundary or a degnerate tiny surface sliver either of which can mess up face traversal when it comes to volume type calculations.

The easiest thing to try is to select your models and use Edit > Separate to break it into individual surfaces and then follow that with a Join, that will try to force region calculation again.

If you do not have any naked edges it could be possible that your mode will actually be ok anyway, but it's somewhat likely that if regions can't be calculated that there is some problematic area of geometry like some kind of self intersection or degenerate boundaries somewhere.

- Michael