The fuel efficiency of vehicles can be increased by the lightweighting of autobody parts. To achieve this goal, this study focused on developing “syntactic foams”. Hollow glass spheres (HGS) were incorporated into unsaturated polyester resin (UPR), via a Sheet Molding Compound (SMC) line, in which the resulting product is a fiber reinforced syntactic foam. By replacing the heaviest filler, calcium carbonate (CaCO3), of typical SMC resin paste with HGS, the density of SMC composites can be decreased from the industry standard of 1.9 g/cm3 to 1.5, 1.2, and 1.0 g/cm3, called mid-, low-, and ultra-low density, respectively. The HGS characteristics investigated were density, diameter, loading level and surface functionalization. In all formulations investigated, the resin paste was kept at 40 vol%. The syntactic foam composites were characterized in terms of tensile, flexural, and impact properties and were compared to standard density ones. The viscosity and microstructure of every composite formulation were also analyzed. Statistical analysis of a design of experiments (DOE) of a limited set of runs was utilized, and it was concluded that there was no significant difference between the samples. Therefore, manufacturers should produce the most cost-effective formulations, in this case: S and L28. The water uptake of samples with three different HGS loadings was also studied. The effect of HGS, GF, and CaCO3 surface areas in the final composite were investigated and mechanisms for water absorption were discussed. It was shown that the low-density samples exhibited a 2.33 and an 8.53 percentage point increase in water uptake when compared to standard and mid-density, respectively.M.S
