In metering machines, two polyurethane components are heated and mixed together in a certain ratio. High-quality polyurethane wheels are mixed together at elevated temperatures (lesser quality wheels are mixed at room temperature). This step creates a liquid. If the wheels are to be colored, the pigment is now added and the resulting mixture is poured into aluminum molds via a mix chamber (if the urethane is high quality it is heated again) and allowed to harden into a solid.