I've been wanting to do some cement sculptures for a while now. I wanted a fairly simple,
yet still cool, design for my first project. After looking through Thingiverse I found
http://www.thingiverse.com/thing:36110 by macouno. It is simplified but has all the major
features that make up a skull. It was also complex enough that I got to learn a little bit
about Autodesk Inventor's Mold Making features to help me figure out the parting lines.
(this is NOT a tutorial for Inventor Mold Design, I only learned a little to get me
through the steps of making my parting lines).

I used:

TechShop San Jose!
Autodesk Inventor
Thingiverse
MakerBot Replicator
Cut3D
ShopBot
MDF
Filler Primer
Lacquer
Silicone Spray lubricant
gorilla tape
Quikrete
Bar clamps
Deadblow hammer
sand paper and files
Durhams waterputty

Step 1: Design the Mold

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

Step 2: 3D Print Test

Before I went to the large scale skull I decided to make a smaller version as a sanity
check, and I also wanted a small mold as well. I 3D printed all three mold parts and
made a small cement skull. This confirmed the Mold Design analysis, and I was ready to
move onto the ShopBot for the bigger mold.

Step 3: ShopBot

Using Cut3D I made the tool paths for my larger size mold. I cut and glued MDF into
2 inch blocks. I had to do each part in two sections, so after cutting six pieces
I had all the parts to my mold cut out.

Step 4: Assemble Mold

I applied glue and clamped all the parts of my mold to get my three parts together.
Using Durhams water putty, sand paper, and files I cleaned up and smoothed out any
imperfections.

Step 5: Seal Mold

After the molds were glued and smoothed out I used Filler Primer to create a base layer.
I did some light sanding after applying a few generous coatings. Then I used a spray
Lacquer to create a smooth, hard, shiny durable surface.

Step 6: Fill Mold

The mold is now complete. Using bar clamps, and a some gorilla tape to seal the parting
lines, I secured my three part mold, ready to accept the Quikrete. I mixed up a batch
poured it into my mold, and attempted to shake out as much of the bubbles as I could.
After a day or two to cure I busted open the mold, and I had a pretty sweet cement skull!