This is to describe the steps I took in making simple rubber shapes which I intend to use as cosplay belt buckle pieces; they won't be working buckles but rather attached to vinyl belts to give that classic superhero look (like Superman's simple yellow oval).
After experimenting with Flex Foam 3, I decided to try a different rubber compound for my belt buckle castings. My friend at Smooth-On, Jason recommended their Dragonskin product (you've probably seen it used on Mythbusters). It's a two-part equal ratio mix. It can only be pigmented not painted so that was also a new thing for me to try. The Silc-Pigs are pricey but a little bit goes a long way so it actually works out to be a relatively inexpensive thing when you break it down.
To begin, I wanted to go with simple shapes. I think I was trying to run before walk when I sculpting with wax and making molds for F4 and Tech Support. I pulled myself back and bought things in the shapes that I wanted: the circle was a 3" metal button from the sewing department of my local craft store; the oval was a cardboard oval container with a lid which is the part I used. I used a cardboard box as my mold container. The box had a layer of masking tape inside and then smothered with petroleum jelly. I used the duoMatrix Neo instead of gypsum for the mold. It's great to work with even for a beginner like me.
The Dragonskin is great because it's a 1:1 ratio and I don't have any precision measuring devices or scales. I basically eyeball stuff that I mix. For this project, I used a small cup that I would guess holds an ounce of liquid; first poured in the A part, dumped it into a paper cup, then poured the B part into the small cup and added that to the paper cup. With no expansion, it's much easier to handle than the foams and you can take the time to mix and pour whereas the foam dries within seconds. Then comes the pigment - I just used a popsicle stick to dip into the Silc-Pig and then just used that stick to continue mixing the Dragonskin. It took maybe a teaspoon at most to color my two ounce mixtures.
I sprayed my mold with Krylon's Crystal Clear before pouring the Dragonskin. The spray dries quickly within 15 minutes. When I did the blue ones, I added one step: to the circle mold, I lightly brushed a layer of Cast Magic Goldfinger casting powder to added a gilded look to it.
I think my yellow castings came out best. When it came time to do the blue then red, I found that I should have let them set a lot longer. My blue and red oval shapes are a bit "chewed" up on the sides because they weren't ready to be demolded. My circles are really good but I noticed that they are lopsided from a side view because using my washing machine as a work surface is not at all level. However, for a costume piece, I actually don't think anyone would notice this.
Since the hero I'm working on next has a plain buckle, I don't have too many ideas yet for how to get more complex in my casting. I think the first one I will try that's more than just basic, will be the Legion of Superheroes logo.