Each one is specifically formulated for increased cut-resistance and memory retention. Used properly, a urethane can easily last ten years or longer. Its purpose is to provide the matching half of the form that you create in your press; it forces the metal into every crevice of the die, precisely conforming to the design. To ensure good results and to prevent damage to the urethane, choose pieces thicker than the design. Designs should no more than 2/3 of the thickness of the urethane.