by DENNIS ALLEN

As a scratch builder, I often find that I’ll need more than one of an individual part. Instead of trying to make a lot of the same piece, each exactly the same, I make a mold of the first one and cast the rest. It's much easier, and a lot less stressful.

Once you've made the original part, 'the master', flatten out some regular (non-hardening) modeling clay on a spare piece of plastic, wood, or cardboard, something that can be moved and leveled. The clay should be at least as thick as half the master. Once the clay is flattened out, press the master into the clay to the halfway point. Press the clay up around the master so that no mold material can get under the piece. If there are any protrusions or overhanging parts, the clay should be used to fill in the space under the overhanging part, so the mold material won't get under it and cause difficulties later. Once the master is set in the clay, press indentations in the clay all around the master. There dents will form alignment bumps in the mold. Next take a small, smooth, round tool and draw a path in the clay for your pouring track location. Now build a form around the master, leaving enough room for the pouring track and vents. The form needs to be tall enough to allow the mold material to cover the master sufficiently. Press the form into the clay, and level it. Now you can mix the mold material. There are several materials on the market that are suitable for making molds. I use RTV-3110 by Dow Corning with the S-type catalyst. This type takes seven hours to cure, allowing most of the bubbles to come to the surface, and not interfere with the quality of the mold. Before the mold material is poured in, some type of mold-release should be put on the master and clay. I use FREEKOTE 1711 by Dexter Products. Pour the mold material in, and let it cure. Once the mold half has cured, remove the clay and push the cured mold half containing the master to the other side of the form, and pour that half the same way. Once both halves are cured, remove the mold from the form. Be careful separating the mold halves, the rubber can tear if you pull too hard. When you have the pieces apart, remove the master, and inspect the mold. If the mold is right, cut the pouring track in both halves following the marks you made in the clay. The pouring track is the path the liquid casting material travels to the main mold. Next, cut the vents in the mold to allow a path for the trapped air to escape. These vents should start at the highest point(s) of the main part of the mold. Now you have a completed mold. Find a way to secure the two halves together. I use either a piece of tape or a clamp from a clamp on light.

Once you've got the mold together, just mix the casting material and pour it in. In about 10 or 15 minutes you can remove the excess plastid from the casting right after removing it from the mold. If you need a hole through the part, you can cast it in by incorporating a piece of brass rod in the mold i.e.. The shaft hole in a propeller.