Making flat back silicone rtv molds

First off, why do we cast things? We cast anything that we need to make multiple copies of or if we need an item to be lightweight and sturdy. For example, these pins we made for our Rose of Versailles costumes could have been made from sculpey but sculpey is fairly heavy and if it falls, it will shatter. Also, we had to make 2 copies of the pin on the top row.

Obviously, 2 copies is not a big deal but we used this same technique to make our Easterling costumes and in those, we made over 700 copies of the spikes used on that armor. No one in their right mind wants to make 700 pieces by hand. It’s way easier just to cast all the pieces.

So let’s get started…

The first step in casting something is to creat an original and make a mold of it. We like to use sculpey so that we can bake it to harden it. For bigger pieces like chest plates, we use oil-based clay because it’s cheaper than sculpey. Oil-based clays are not bake-able however, so the original gets destroyed after the mold is poured.

Here you can see the original. You then take oil-based clay and build a wall around your original. This will keep your RTV from running all over your table and making a big mess of things.

We use primarily silicone RTV molds for our casting and we get it from Tap Plastics. It’s a mix and pour product which means you read the instructions and it tells you how much of the white goop and how much of the blue goop to mix together. You mix them together and stir very very well.

Now you pour the RTV onto your original. We recommend doing a high pour. What that means is that you pour from way above the table and try to pour the RTV in a thin stream. This forces any air bubbles in the mixture to stretch out and pop. Air bubbles in your mold are really annoying and could possibly make the mold unusable if there are too many or they are too big.

You’ll want to have enough rtv to cover your original with a decent thickness. If you don’t have enough, no worries. Just mix up another batch and pour it on. This stuff cures slow enough that you can just add on. Tap gently on the table the mold is resting on to force any air bubble inside to float to the top. Most of them should have been take care of in the pouring process though.

Now you go away for long time while the mold cures. Curing time will vary from product to product so double check the directions on the product you’re using. Generally, we just leave it overnight.

When the mold is done curing, it will have solidified into a rubbery sheet. (If your mold did not cure all the way and you waited the right amount of time, it means you didn’t mix your product properly and you’ll have to start over. Ugh.)

Pull away the clay wall, and peel the mold off your original and tada, all done! Now you have a mold you can use to make resin casts with.