So occasionally I will have a budget of pretty darn close to $0 to do a build. I am working on a prop for a steam punk web series called Mantecoza ( http://mantecoza.wordpress.com/ )and I needed to make 4 copies of this oversized fuse. In a perfect world I would just run down to my local supply store Ball Consulting
and pick up some platinum Polytek mold making silicone or order some from Smooth-On. But we don’t live in a perfect world, and I don’t have the extra cash to spend on materials right now. So I started Google-ing different ways to make molds, and ran across people using hardware store %100 silicone caulk. Now everybody seems to have there own recipe, some thinning it with naphtha or adding acrylic paint and glycerin, but that would mean more money and time. So I used what I had: clay, %100 silicone caulk, soapy water, and Bondo fiberglass resin jelly. (Ok so I cheated a bit. I also used some mold release. Its the only non-hardware purchase you need to make for this to work, and if you are going to be casting in your new cheap-ass mold you will need it anyways. Pics with the can to follow, also a good video on this is here: http://www.youtube.com/watch?v=R0KrE3G4t_o) After getting the fuse set half way down in the clay, and using the domed head of a bolt to make registration keys I was ready to start applying the caulk. I laid down a bead all the way around the base where the fuse met the clay. After getting my fingers good and wet I patted down the caulk. That seems to work better than trying to spread it out. I then laid 3 beads going along the length of the fuse and patted them down as well. The fist layer is the most important, so I tried not to trap any air between the silicone and the fuse. The internet also taught me that this kind of silicone cures by moisture, so the water isn’t going to harm anything. It also taught me that if you lay down to thick of a layer that the outside will skin over and the caulk will not cure all the way through. So for this mold I did 3 layers on each side, letting each layer cure overnight. Now 6 days to do a mold might seem like a long time, but lets remember that I just bought 2 tubes of Ace brand 50 year %100 silicone caulk for FIVE DOLLARS. Cheap is not going to be fast, but it is going to be cheap. A mold this size took a little over one tube to do.
After getting one side done, I sprayed mold release over the silicone and clay and did a jacket of Bondo fiberglass resin jelly. Its like regular Bondo but it has fiberglass fibers in it already. I don’t really like the stuff, and doing a regular fiberglass and resin jacket would be lighter and stronger, but I’ve had half a can sitting around my shop for over a year and it seemed like a good time to use it. I cut a piece of mattboard and Bondo-ed it to the back as well so when I flip the mold over and remove the clay it will stand on its own. You could use cardboard as well.
As you can see the mold came apart nicely! I was pretty surprised actually. With my luck I figured that not only was it not going to work, but it would ruin my original as well. A quick test cast proved what I had hoped, that you can get a decent looking cast from this method. Now let me take a moment here to say that this IN NO WAY is better than buying professional mold making silicone. But if you have a tight budget and don’t mind having to do some clean up on your cast pieces, this way works.