Abstract: Methods and apparatuses for converting expanded polystyrene into a rigid material are disclosed herein. The methods of modifying EPS herein generally include placing EPS workload into a pressurized chamber and applying heat and pressure through a gas. The application of heat and pressure is held for a determined amount of time, the pressure of the internal chamber is then lowered below atmospheric pressure and a hot or cool step is administered. The resulting CPS product can be modified in a multiple number of ways depending on the desired end product.