Hi eric, if you had an example model that you could post it would really help to give your question some more specific context.

But in general it sounds like you should avoid using Trim for these kinds of cuts - when you use Trim it just cuts the surface skin of a solid so actually any time you Trim a solid it will no longer be a solid.

Instead, you should be using the Boolean operations for those kinds of cuts - the Booleans operations work on volumes and will maintain solids as solids, and leave the pieces of the cutting object behind to form side walls around the cuts.

Here's an example where I an cutting a box with a rectangle curve:




Here is the result using Trim to cut the box:



And here is the result using Boolean Difference to cut the box:




Here's another example cutting the corner off of the box with a line segment as the cutting object:



Here I did a Trim and kept all the pieces (just right-click or push enter in Trim at the "select pieces to discard" step instead of picking any fragments to get rid of and it will keep all the cut up pieces), and moved the piece a bit:



Here I instead did a Boolean Difference using the line as the cutting object:




So as you can see, Trim is more of a surface modeling operation where it just focuses on cutting up the surfaces of an object.

One other aspect of Trim is that it only cuts an object up and does not do things like automatically incorporate pieces of the cutting object into the result. That's why when you Trim a solid it will no longer be a solid.

The boolean operations on the other hand do all that other stuff - they work off of volumes and will incorporate pieces of both the base object and the cutting object into the final result to keep it as a solid. So if that's the result you want, make sure to use Boolean difference to do the cutting and not Trim.

- Michael