Although both silicone and urethane rubber molds are considered durable enough for repeat castings, damage can occur over time. Sometimes a rubber mold will wear away in some sections, causing the walls to become thin, or you may accidentally tear the rubber when handling the mold. Repairing a rubber mold by patching the tear can be tricky, depending on the degree of damage. This patched mold can be used again, but the fix is not permanent and the mold will eventually need to be replaced.
Assess the type and size of the tear in your rubber mold. If a piece has broken off the mold, or if the mold has split in half, the repair will be much more difficult and, in some cases, impossible.
Assemble the materials for the type of damage your mold has. For a small tear, use some of the original rubber mixture, and for a large tear, use a rubber adhesive. You can also use gasket caulk or aquarium sealant in place of these materials, although they will not create as strong a bond.
Clean the torn rubber mold with acetone. Put the mold back together as best you can.
Mix together a small amount of RTV rubber, either silicone or urethane depending on what you used to make the original mold, or prepare your rubber adhesive.
Paint a small amount of rubber onto the damaged portions of the mold. If there are broken pieces, paint rubber into the "seams" created by the breaks and fit the pieces back together, then paint more rubber onto the outside edges. If you are using rubber adhesive, do the same.
Let the rubber or rubber adhesive dry completely. If you have used adhesive, test the repaired mold to make sure it is sturdy enough for use.
Fit the mold back into its original molding box or mother mold, if you have used the rubber mixture to make your repairs.
Pour a small amount of silicone over the mold, covering the surface you repaired. Let the new rubber cure completely, then remove it from the molding box. Test the mold for sturdiness.
You can also try fitting the mold onto a flat sheet of modeling clay, covering the damage. Make sure that the modeling clay is perfectly flat, however.
If you are having trouble making the rubber stick to itself, first clean the surface with a rubber cement thinner to make it tacky.