Nozzle Injection of Physical Blowing Agents in the Injection Molding of Microcellular Foams

Abstract Results obtained from the implementation of a physical foaming system for injection molding are presented. The implemented design involves the injection of gas in the upstream side of a special nozzle composed by a static mixing element (SMX type) and a shut-off valve. In order to avoid the formation of large pockets of gas in the injection point, a porous metal plug is used to create a multiple point injection area with 20 lm size pores. Gas injection simultaneously with polymer melt injection gave a consistent foamed material. However, given the total pressure drop in the system of static mixers, runners and mold, there is an inherent operational pressure limitation. The polymer injection pressure needs to be kept at a pressure below the gas injection pressure, therefore limiting the speed at which the polymer can be injected in the mold.