There are several ways to approach the making of a flexible mold; variations depend on the type of pattern (object) you are casting and the type of flexible mold making material you choose to use. As you will see, careful and thorough preparation of the pattern is everything!
Let's start with a fairly simple object: A section of flexible wire enforced hose:
I tried to cut the hose even and clip the wire evenly. I then cut a couple of rough circles out of plywood and fitted them to the ends. This makes it easier to make a mold of the hose section. It will register as a solid object, but if I want it to be hollow I can certainly cast it that way later. For now, it is important that the hose be sturdy and that the mold making material not get inside. I braced the object on a scrap piece of board with some clay and filled the joints between the plywood and the hose with sulphur-free oil based clay. The entire object and the wood is sealed with a few coats of thinned shellac. Most flexible compounds require that you seal porous surfaces, including (and especially) wood.
After the object is held steady in place with clay or hot glue and the shellac is dry, I find the center line with a machine square. Where the machine square touches the circular object, I make a mark - this is the center. This is a useful way to find the center of round objects for flexible mold making.
Now I want to make the shim wall for the first half of the flexible mold. I want to use the parting line that I created with the square and I will use a sulphur-free oil based clay the make the shim wall. This is the material specified by the type of rubber I will use for the mold. Most rubber compounds will react badly to water based clay and to any oil-based clay (plastelina) that contains sulphur. Be sure to read all the instructions on the product before beginning so that you have the correct materials to make your mold. Since the object is about 4 -5 inches tall and would take a lot of extra clay to build up a shim wall to the center line, I take a short-cut by cutting out a silhouette in another scrap piece of plywood to fit around the object. I secure it so that it sits just below the center line I made with the square.
I pound out a stick of sulphur-free clay and make a shim wall that sits right at the center line. I carefully work it evenly and smoothly up to the surface of the hose; I don't want any gaps between the object and the clay. Because there is a little bit of a gap between the plywood and the hose, I had to add a little here and there as some of the clay sunk into the gap. I do this all the way around and then trim the clay shim walls to about 3/4" wide.
Presto! The shim wall is complete. If you look closely, I added some keys (thumb-sized depressions in the shim wall that will cast as little bumps in the flexible mold material) and then built a dam to contain the uncured rubber. I am now ready the for next post: Mixing and Applying a Flexible Compound!!
