a unitary vehicular frame formed of composite plastic material with a medial expanded core and fiber reinforced periphery . the core material is of a lower density and may be formed homogeneously or with various non - homogeneous configurations to provide additional strength . the peripheral material is of a higher density to provide rigidity and strength both by reason of its physical nature , configuration , and spatial position . the core material comprises an in place arbor to aid the formation of the frame . the frame provides the strength , rigidity and durability of a metallic auto frame with less weight and greater insulative value , corrosion resistance and crash protection .