Here is my basic "how-to" guide that covers plastic resin molding and casting . Many of my projects, over the last 10 years (B9 Robot, Twilight Zone Mystic Seer and Back to the Future Flux Capacitor) have forced me to learn how to make molds in silicone and to cast plastic resin parts. Hopefully you will pick up a few tips here that will help you avoid some of the problems I encountered. These resin casting tips can be used by the hobbyist to make plastic resin copies of anything from missing toy parts to turn signal lenses for that vintage car.
When molding parts, I use Smooth-On's "Smooth Cast 300" series (300, 305, 321 & 326) with great success. You measure it out by volume so it is pretty easy to use. It's also pretty forgiving if your ratio is off a little.
I have also used a 2 part resin called Por-A Kast "Mark-2".
If you want your resin part to have a specific color you will have to experiment with some coloring. I use So-Strong colorants. There are very strong and very reliable once you figure out how much to use. To get the desired pink-flesh tone I have to mix a drop of orange with a drop of red. That is enough to tint about 4 ounces of resin. I mix up a small batch of the Part A of the resin and keep it in an air tight container.
Silicone Mold Release Spray - If you want to extend the life of your molds you'll want to have to spray on mold release. Lightly spray both halves of your mold and let it dry a few minutes.
Measuring the liquid plastic resin - Measure out an appropriate amount of Part A and Part B of the plastic resin. I use three 2 oz plastic "portion" cups for smaller jobs. One for Part A and one for Part B. The third one is for the final combined resin.
Mixing the liquid plastic resin - Combine the two parts. I pour them back and forth a few times to make sure I get it all mixed. Then pour it into a third clean cup. Now use a new Popsicle stick so that you don't have any unmixed resin on it. This in key to getting a fully mixed solution.