You can see Josh's post about Moulding and Casting the Mail Man's heads HERE. I will be using the same process.
A general rule for mould making is -
When casting hard objects, a soft mould is needed. When casting soft objects, a hard mould is needed.
As my head sculpt is hard and I will be casting copies in solid plastic, I will be moulding the heads in soft silicone.
Firstly I decided where the 'pour spout' will be located. The pour spout is the cone shaped hole in the mould where the resin will poured in later. I start by making a small cone of baked sculpey and gluing it the the sculpture using a glue gun. The hot glue holds onto the sculpt securely but can be removed easily later without damaging the head. In the picture above you can see the pour spout located under the head. The second function of the pour spout is to suspend the sculpture up off the surface.
Locating where to put the spout is very important. When it is cut off later it might leave a visible mark. Therefore its best to put it somewhere where there's no detail and it won't be seen. I've chosen to put it on the top of the head where it will be covered by hair.
Another important consideration is air bubbles. If the spout is incorrectly placed then air can get trapped in the mould causing voids in the castings. If the pour spout is positioned in the 'highest point' then air will be able to flow out of the mould without getting trapped.
I built a box around the sculpture made from foam board. The sides are glued together with hot glue to create a water tight seal. I left a gap of about 1cm around the head so that the silicone mould will have sturdy sides.
The silicone I'm using is white in colour and cures in 24 hours with 5% catalyst. When mixing the two parts together small air bubbles are introduced. These annoying bubbles can cause problems later and without a vacuum chamber they are difficult to get rid of. My solution is to paint a layer of silicone onto the sculpture and let that set. This helps to capture all the details and reduces bubbles on the surface. I then top up the mould box with silicone. I pour it in a thin stream into a corner of the box (not directly onto the sculpt).
Using this method means any bubbles that are in the mould won't touch the surface of the sculpture and therefore shouldn't cause any problems.
Once them silicone has set I removed it from the foam board box. Using a scalpel I now have to remove the master sculpt from the silicone. I cut along the back edge in a zig zag pattern and then bend the mould open to remove the sculpt. Cutting only the back edge reduces the amount of visible seam lines on the castings. The zig zag pattern will help the mould line up when it closes back up.
To cast the head I used Polyurethane (Fast Cast) Resin. This is a two part resin that mixes together in equal parts and cures in 30 mins. No mould release is needed. The resin is most fluid and viscous in the first minuet of mixing so I had to work fast and get the resin mixed and poured into the mould as quickly as I could. (There was no time to take a picture). I gently shake, rotate and tap the mould to coax any bubbles out. After about 3 minuets the resin starts to thicken up.
Above is my first casting next to the master sculpture. I cut the pour spout off the top of the head and lightly sanded away the seam line. The fast cast copy of the head is much lighter than the original. Weight is always an issue for stop motion puppets in the fight against gravity. Another positive is that fast cast is much stronger than the sculpey original meaning it is much less likely to break during filming.
Above you can see that the silicone eye socket and eyes fit snugly into place, just as they did in the original head. I repeated this method to mould and cast the brow and mouth masks. I've cast 10 copies of the mouth and 3 brows that I will use to make all the different facial expressions required for the film.
The masks fit into place neatly but I will be adding a magnet system to hold them in place.
In by next post I will be doing more mould making, only this time they will be hard Polyurethane moulds for casting flexible silicone body parts.