Injection molding and blow molding systems that include screws having low L:D ratios are provided. The systems are capable of producing microcellular polymeric materials. In some cases, the systems may be formed by retrofitting conventional polymer processing systems. Retrofitting may involve changing (e.g., machining or replacing) existing components of the conventional system, as well as, adding new components to the system. For example, retrofitting generally involves replacing the conventional polymer processing screw with a new screw designed to satisfy conditions needed for processing microcellular materials. In other cases, the systems may be newly manufactured. The retrofitted or newly manufactured systems of the invention are considerably less expensive than newly manufactured microcellular processing systems having higher L:D ratios.