After I downloaded the .stl file from Thingiverse I opened it in Autodesk Inventor 2013.
Using the mesh enabler (found here: http://labs.autodesk.com/utilities/inventor_mesh
I converted the skull to a solid. Then I made a cube, totally surrounding the skull with
one side flush to the base (a new solid). Using the combine feature I cut the skull out of
the cube, leaving a hollow cavity. On the base of the cube I did a simple sketch to make
split lines for the 3 part mold. I also made a split line on the back side. And lastly,
the most complex parting ling was made on the side wall. First I split the side to make a
front and back. Then split the back side in half to make the three parts.
Once the three parts were made I used Create Mold Design to analyze each part of the mold
to determine if there were any undercuts. Since I eyeballed the parting line on the side
wall I knew that if any changes needed to be made all I would need to do was edit the
sketch that made that split. In Mold Design, using Adjust Orientation, I selected the face
where the skull will be removed and looked to see if any undercuts were detected (they
apear as blue faces). Once everything was green I was mostly satisfied.
Many thanks to macouno for posting Lisa the skull to Thingiverse.
found here: http://www.thingiverse.com/thing:36110