This study investigates the effect of calcium carbonate (CaCO3) fillers of varying particle sizes and hollow-glass microspheres (HGM) on process-property relationships in sheet molding compound (SMC) composites. A stepwise experimental approach assessed viscosity evolution, filler dispersion, curing enthalpy, and mechanical performance. Thickening kinetics depended on filler type: HGM slowed thickening, while fine CaCO3 accelerated viscosity build-up via polymer bridging. Two thickening regimes were identified: CaCO3-rich systems formed denser polymer-filler networks, whereas HGM-CaCO3 blends produced sparser agglomerates. Curing enthalpy correlated with storage modulus, peaking in HGM systems. Finer CaCO3 influenced fiber bundle distribution and tensile strength; flexural strength remained stable. HGM enhanced impact strength through energy absorption from microsphere rupture, while hardness increased with dense carbonate packing.
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