Casting Rubber

Lesson: Rubber Casting

For the casting process, rubber is very similar to the resins/plastics. We are using 1:1 ratio rubbers. The biggest differences are in cure time and type of rubber. Like types of rubbers will bond together- silicone rubber will bond to a silicone rubber mold. Polyurethane rubber will bond to a polyurethane rubber mold.

We have a silicone rubber and 2 polyurethane rubbers for casting.

Oomoo is a silicone rubber. We used it for mold making as well. Naturally it is an opaque lavender color when mixed and cured.

Vytaflex is a polyurethane rubber, pale peach color when mixed and cured- it has some translucency but is not clear. This is much more flexible rubber than our other polyurethane rubber.

Polytek 74 series is the other polyurethane rubber, also one we used for mold making. It is a clear light amber color when mixed and cured.

All rubbers can be colored with any of the pigments we used for the plastic. They can’t be colored using the colores pigments, or the ceramic paints- only the pure pigments.

Prepare your piece or mold.

If pouring rubber into a piece, make sure all finishing is done. This should be the last step.

If pouring into a mold, spray your mold with mold release. Make sure the release works with the type of rubber you are using! (read the label)

For delicate/thin castings:

The rubber is thicker than the resin and plastic. You may want to use a disposable syringe or eyedropper to inject the mold. This pushes the material in with greater force then simply pouring.

Thickening rubber for brush-on casting:

You can mix in some of the PolyFiber material to thicken the rubber, allowing it to just stay where you want it to when pouring, or to be able to brush on the material to create a hollow form. The PolyFiber does tint the rubber- making it a lighter shade and more opaque.

To Use:

Mix your rubber as directed. Add in PolyFiber a little at a time (half spoonful) and mix thoroughly. Add enough until the rubber resembles icing- it holds soft peaks.

Imbedding:

The rubber cures slower, but it is more viscous than the plastics. If the embedding material is light, pour a layer of rubber, lay the material in, and then complete the pour.

If the material is heavy and will sink through the rubber, you will need to do a multi-stage pour.

Connecting:

Rubber bonds to itself better that the plastic. In addition to the other connection methods, you can also connect castings by pouring a little extra material to connect them. Set your castings up on parchment or other non-stick surface at the correct distance. Block off the aarea/shape of the connection with plasticine, sculpey, or coated cardboard (i.e. anything that the rubber will not stick to). Pour, and let cure.