Theshim has a hollow interior 21 that contains a fluid, preferably a liquid that can migrate within the shim to the point of least resistance, i.e. the portion thereof that lies adjacent any portion of the mold having wear or a gap that can cause theformation of flash.As is evident in FIG. 2, the shim 20 is formed of two circular sheets 22 and 23 of elastomer material that are fused or joined toge