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Above: A pencil is used to create alignment holes in the clay.
Above: Rubber is created by using the same amounts of each part, A and B. The two parts need to be mixed until they become a solid purple color ( in this case because Polytek 71-20 is being used ).
Above: Rubber is dribbled around and into the crevices of each part.

The left side of the photo shows the clay that has been smoothed with alignment holes having been placed as well. Now when rubber is poured over this, the surface of the rubber will conform to each part and come away with a nice, even surface.

Once all the clay has been added to the "floor" and around all the parts and smoothed, then a wall must be fashioned to hold the rubber mixture while it cures. The best and probably cheapest thing to use is Foam Core™, which is available in just about any drugstore or arts and crafts outlet. You'll want to make the walls
about 1/2" above the highest point so that the parts are fully covered, yet it won't be so high that you'll be wasting a good deal of rubber, which isn't cheap! The more rubber used, the less flexible the mold will be. This may result in tears and/or premature aging.
This is a bit of a guessing game here to be able to judge. There should be at least 1/2" to 3/4" between the edge of the parts and the clay. You'll need room to create alignment holes. You will cut the clay all the way around in straight lines (square, rectangle, triangle, etc., whatever the design of the mold dictates) and then
add the wall. If your mold is circular, then it will be difficult to use Foam Core, but some molds work better in the round depending upon the part as we found out for one particular part.

When creating the wall, you'll want to have the wall as one long strip, sliced on the outside so that you can bend it around a corner. You won't fully cut it off until you get to the last bend and join. Use Scotch™ tape to tape the two ends together and then tape the inside corner where these two ends meet as well to eliminate
a seam line on the corner of the rubber.

Once you have the clay smoothed, the walls built and taped, you are ready to create the alignment holes. These were done by using the eraser end of a pencil and pushing it gently into the clay, while twisting slightly. They were placed at intervals so that the second part of the mold would "marry" itself into these holes and create the male side of the alignment holes. These holes are there so that the process of aligning the two sides of the mold when casting is accomplished much easier.

This particular rubber was created by using the same amounts of both parts. Once you have equal amounts of both parts, mixing is necessary to ensure that both parts meld fully. One part was pink and one part was blue, so when they are thoroughly mixed, they turn a nice shade of purple. As you can see in the photos on the next page, we used a throwaway brush to spread the rubber around, after dribbling the rubber onto the top of the mold around each of the parts, as opposed to just pouring in the rubber and letting it settle. This method avoids trapping air bubbles, which would then become part of the mold. You'll also want to take care to gently pull on the brush tip before using it to spread the rubber so that any loose brush strands will come out in your hand and not become embedded in the rubber.

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Bare-Metal would like to thank Fred DeRuvo at Modeler's Resource Magazine for allowing us to share this material with our visitors. Images and text Copyright © 2002 Adroit Publications.