INJECTION-MOLDING OF UNSATURATED POLYESTER COMPOUNDS

A bulk-molding compound made of unsaturated polyester resin, glass fiber, calcium carbonate fillers, and low profile additives is studied. The viscosity of the compound in the absence of cure reaction is measured by capillary rheometry. The compound exhibits a shear-thinning behavior. Injection molding in a rectangular plaque equipped with pressure transducers shows that the crosslinking reaction can begin during mold filling for low flow rate or high mold temperature. Fiber orientation in the plaque is complex as the reinforcement appears under two aspects, bundles or filaments. Their lengths and orientations are different. A layered structure throughout the thickness is observed at the mold entrance, whereas the orientation becomes progressively unidirectional in the plaque. Two fiber-free layers near the mold walls are observed. A numerical simulation of mold filling assuming inelastic non-Newtonian kinetic dependent behavior is presented. The results agree well with pressure measurements. A simplified decoupled fiber motion calculation is finally proposed. A qualitative explanation of the basic phenomena which induce fiber orientation is presented.