Thick Tread Polyurethane on Cast Iron Core - PX. Composed of a polyurethane elastomer, open cast or injection molded, on clean, close tolerance cores. Polyurethane tread is chemically bonded to a cast iron core. High capacity polyurethane protects floors, is non-marking and delivers quieter operation than standard metal wheels. Hardness: 95 Shore A. Wheel Face: Available in flat or crown tread. Finish: Green tread on black core. Temperature Range: -40 degrees up to +180 degrees Fahrenheit.