I have made a model which is going to be a 3D printed core for a sand cast impeller. The geometry of the impeller is fixed, and so are the surrounding surfaces. I need to hollow it out so that it is lighter when handling during manufacturing. At the moment I am using the shell feature and I can only get a wall thickness of 2mm, any more and it throws an error "Rebuild Errors - One of the faces may offset into an adjacent face, a small face may need to be eliminated, or one of the faces may have a radius of curvature which is smaller than the shell thickness" etc etc. Does anyone have any ideas on how to get a wall thickness of 10mm without changing any of the geometry that is already in place?

I have attached the part file and an image of a section view.

Many thanks.