During the cooling of the part, it will reduce the volume. As the metalic or plastic armatures have very low thermal restrain coefficient compared with rubber, they will act as fixed walls while the rubber try to reduce his volume, so this will create a lot of stress (traction stress) in the rubber. In order to cancel this stressed the parts are mechanical swaged (reduced in diameter), as a postmolding operation or by the customer when they mount in place in case of bushes with plastic armatures. In order to increase the durability, more swage is best, but it has a limit due to the adhesive layer or phosphate/adhesive that will break (as they are more rigid than rubber) and create adhesion issues. In order to predict with some accuracy the performance of the part is needed to include this two steps in the simulation (cooling and swage), that's why is imperative start with hot mold dimensions of the part, and not final model.