Introduction: Cement Skulls!!!

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 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
MakerBot Replicator
Filler Primer
Silicone Spray lubricant
gorilla tape
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:
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:

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

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!

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