Abstract: An apparatus and method are provided herein for solid-state microcellular processing, wherein a gas-saturated solid thermoplastic web is fed into an enclosed processing system comprising a plurality of thermal rollers and nip rollers that work together to constrain the thermoplastic web while expanding it through a series of thermal rollers and nip pressure. By constraining the web and controlling its rate of expansion, the present invention is intended to produce microcellular thermoplastic webs with a uniform thickness and flat surface.