Plastic mold key sheets ideal for molds using the shim technique. Pour in extra mold material to create keys that may be embedded in rubber later. May be used to around the flange on brush-on molds (see example) to aid in registration. Key sheets work with silicone and urethane rubber products. A light spray of release (2500 for silicone, 2300 for urethane) may be necessary to aid in the release of some rubber products.
Package of 10 sheets of Mold Keys

As seen in the Shim Technique For Multi-Piece Rubber Molds DVD.