Most of the molding and casting materials I used for this came from Reynolds Advanced Materials.
They are a great resource with a ton of great information on how to use their products. I spent a ton of time on there website going through all of the different materials and learning as much as I could. I was also fortunate enough to have one of their brick and mortar stores right here in town where I live. Their staff was also really helpful in helping me with any remaining questions I had.
Due to the size that these molds will need to be, I'll be using the hard shell mold technique. This technique consists of an inner silicone liner covered with a hard exterior shell to help it maintain the proper shape. There are a bunch of resources online for different techniques, if you interested in casting and mold making you should definitely check them out. I decided to go with Smooth-On Rebound 25 rubber for the molds with a Plasti-Paste shell. Both of these are available at Reynolds in smaller trial sizes as well as a 1 gallon size.
Since this mold will be split into two pieces, the first step was to create the division line for the first half of the mold. It actually worked out really nicely that a piece of foam core perfectly fits into the center groove of the body that exists as part of the design. After adding some registration keys, you can use anything really, I used acorn nuts, and a clay dam around the perimeter, its time for rubber. The clay damn will stop the rubber from just running right off the side of the foam core.
The silicone comes in two parts that you mix equal volumes of to activate. It was applied in layers to build up the proper thickness. A thin initial coat was applied over the whole surface to make sure there weren't any air bubbles in the mold. After this coat started to set, I applied a thicker second coat. After this coat started to set I added in registration keys for the hard shell mold. This will ensure that the rubber always fits properly into the hard shell and that there isn't any distortion of the shape. For the third and final coat of silicone, I added a few drops of Thi-Vex II silicone thickener to thicken up the rubber allowing me to apply a thicker coat. The thickener stops the rubber from flowing down and settling in the low spots.
After the rubber had fully cured which takes about 6 hours, The next step was to apply the hard shell. The first thing to do was to move the clay damn out about 1 inch to make room for a hard shell rim. The Plasti-Paste works similarly to the rubber. It comes in two parts that need to be mixed together. The Plasti-Paste is applied in a single step. There is no need to do layers like the silicone. After a few hours this is fully cured.
Time to do the other half of the mold. After removing the foam core, the second half is pretty much the same as the first so I'm not going to bother repeating the steps. Once the two half were made, The last thing I did was to drill holes around the edge. There are for bolts to close up the mold and make it easier to cast pieces.
Materials used for this stage
oil based clay
Smooth-on Rebound 25 silicone rubber
Thi-Vex II silicone thickener
Mold release agent