Cast solid urethane elastomers can be formulated to vary some properties. The injection molding grade urethanes can be blended with other plastic resins to lower cost and to modify properties. There is now a trend toward custom formulation for specific end use applications. Microcellular urethane elastomers combine the “toughness” of solid urethanes with the advantages of a cellular structure. This material performs as a self-compensating bumper. It responds gently to light loads but firms up under heavy loads. Microcellular elastomers are now used commercially in a number of automotive applications such as auxiliary springs, under-car bumpers, vibration dampeners and seals.