Foam injection molding of polypropylene/stainless steel fiber composites for efficient EMI shielding

Lightweight polypropylene/stainless-steel fiber (PP-SSF) composites with 15-35% density reduction were fabricated using foam injection molding and supercritical carbon dioxide (CO₂). The electrical percolation threshold, through-plane electrical conductivity, and electromagnetic interference (EMI) shielding effectiveness (SE) of the PP-SSF composite foams were characterized and compared against the solid samples. The effects of the plasticizing gas and the void fraction on fiber breakage and orientation were also investigated. Microstructure characterization showed that the presence of dissolved CO₂ decreased fiber breakage by about 30%, and together with foaming action, contributed to less preferential orientation of fibers. Consequently, the percolation threshold decreased up to four folds from 0.85 to 0.21 vol.% as the void fraction increased from 0 to 35%. The specific EMI SE was also significantly enhanced. A maximum specific EMI SE of 75 dB.g^-1cm³ was achieved in PP-1.1 vol.% SSF composite foams, which was highly superior to 38 dB.g^-1cm³ of the solid PP-1.0 vol.% SSF composites. The results reveal that light and efficient products with a lower fiber content can be developed by foam for EMI shielding applications.