I finally have Imp all clayed up. I’ve checked to make sure I haven’t missed any undercuts and that I’ve followed my mold lines faithfully. You can see I’ve cut in natches – the indented circles around the outside of the clay. Those create keys so the mold pieces don’t slip during use. At this point he’s all ready for me to pour the first side of the rubber mold.
These are the essential tools for rubber mold-making. No matter how well I mix my rubber, what’s left on the rim of the bowl and the spatula never cures. (And what does cure never comes off!) So I stock up on throw-away food containers and rubber spatulas at the local dollar store.
I use a polyurethane rubber that comes in two parts which have to be mixed in a 4:1 ratio. Normally I would do this mixing outside – instead of on my dinner table – since I dislike the smell of the rubber as it cures. Unfortunately for me it’s only 37° outside, which is too cold for the catalyst to work properly. (I had to learn this the hard way a few years ago. You end up with something that looks and feels a bit like school paste, only it never dries.)
Here I’ve just added the catalyst, which is a golden brown color. Once cured, the rubber is translucent amber.
I start by pouring a small amount of rubber and then tipping the mold around so the rubber flows around the original. What I want to do is make sure no air bubbles get trapped in the detail of the original.
In this picture I have moved the rubber around enough that the entire design area has been covered with a thin layer that is relatively bubble-free. Now I can safely pour the remaining rubber into my Lego mold box.
The original is still almost visible there inside the translucent rubber. The challenge at this point is to come back in an hour or so and add the marbles to key the back of the mold. I often forget until it is too late and the rubber has cured enough that the marbles won’t work. I am setting the timer this time, so maybe I won’t have to cut any keys.