Advancing Automotive Light-weighting: Material–Process–Microstructure–Performance (MP2) Relationships of Supercritical Foam Injection of PP–Graphene Nanocomposite Foams and Over-molding

Vehicle light-weighting constitutes a critical component in the automotive sector’s drive to improve fuel economy and reduce greenhouse gas emissions. Among the various options for lightweight materials, thermoplastic foams are distinguished by their durability, low weight, and environmental sustainability. This study explores the manufacturing of novel graphene-filled polypropylene (PP) foam, employing supercritical nitrogen as an eco-friendly substitute instead of conventional chemical foaming agents, and investigated the role of over-molding a solid skin over a foamed core on the flexural strength of the molded component. Our approach is broken down into four distinct investigations—Study I investigated the effect of different graphene content by weight percentage (wt.%), namely 0.1%, 0.5%, and 1%, on flexural properties and foam morphology obtained for 15 wt.% reduction of the PP thermoplastic, thereby helping identify an optimum graphene loading wt.%. Study II broadened the wt.% reduction horizon for PP to 5 wt.%, 10 wt.%, and 15 wt.%, systematically analyzing the impact of the optimal graphene loading and comparing their cell morphology and flexural properties. This improvement in microstructure and mechanical properties was confirmed in the case of graphene addition to 10 wt.% and 15 wt.% reduction, where cell size was reduced by ~100% for 10 wt.% reduction samples and cell density improved from 4.37 × 10⁵ cell/cm³ to 5.42 × 10⁶ cell/cm³ for the same when compared to baseline PP foams. Study III serves as a demonstrator for a novel hybrid over-molding process designed to improve flexural properties. Over-molding with solid PP was performed over a foamed PP core, generating a composite foam with improved flexural strength and a class-A surface finish and noticeably improved flexural strength from 23.4 MPa to 27.3 MPa, achieving an overall 10 wt.% reduction. This is significant since it translated to a 16% improvement in flexural strength over baseline PP foams and a flexural modulus equivalent to solid PP. Study IV investigated the impact of this light-weighting to assess the potential energy savings over a typical passenger vehicle’s life cycle. The study demonstrates a viable route to achieve sustainable vehicle light-weighting and highlights the role supercritical fluid-assisted foamed thermoplastic nanocomposites may occupy in the vanguard of sustainable material development.