Rhino problem splitting OBJ file Part1

Ok I have a file imported into Rhino from ZBrush (obj)

I need to create a parting plane for molding. If I create a solid or surface sphere I can split the object into two halves using a plane or a plane that is created from 4 curves that resembles a sheet blowing in the wind without any problems.

Once I move the plane back into position to cut through my model I perform the exact same operation Rhino takes about 20-25 minutes to do it's "thinking" and then it's done. I then try to move one half or the other and the model is still one solid piece.

I've tried split as the object were a mesh and a solid. Rhino won't select it if I try to do a solid so I have to use the mesh boolean split.

Any ideas? It's a 1.2 million poly model which I think is the issue.

Rhino's mesh tools are not extremely reliable, especially anything that needs a mesh-mesh intersection. Hoping for success in trying to split a 1.2 million poly mesh is very optimistic...

As you surmised, the only tools you can use on your object are the mesh ones, normal split/trim/boolean ops for surfaces do not work on meshes.

I do not have an answer for you on this, aside from finding another tool to work with meshes... --ch

Rhino's strength is Nurbs. I would split the mesh before it comes into Rhino.