Abstract: Closed-cell foam particles are pressurized to produce particles useful in a process using secondary foaming to achieve a lower density foam. Closed-cell foam particles are initially compressed in a first stage compression tank and then transferred to a main gas pressurization tank. The main gas pressurization tank comprises multi-level shelves, each with a set of compartments. Charges of foam particles pass sequentially through the shelves while becoming fully pressurized. The multi-level shelves with means to move charges of foam particles from shelf to shelf are responsible for the closed cell foam particles becoming pressurized in an efficient manner.