Mold filling in SRIM and RTM: controlling a critical processing parameter

Structural reaction injection molding (SRIM) exploits the advantages of RIM processing (fast cycles, low processing temperatures, low clamping pressures, design flexibility) in making complex fiber-reinforced parts. In SRIM, a preformed continuous or woven fiber mat is placed into the mold prior to filling. The reacting resin stream is then injected into the closed mold, allowing the resin and mat matrix system to be demolded as a composite. Mold filling in SRIM involves pumping low-viscosity liquid reactants at relatively high velocities (5-20 cm./sec.) into a mold containing a glass-fiber reinforcement mat. Like RIM, RTM is a reactive polymer processing method in which liquid reactants cure in a mold containing a glass fiber reinforcement to produce a composite part. The focus of this study is to characterize the mechanism associated with mold filling in SRIM and RTM processes. By combining results obtained from flow-visualization studies and from monitoring the pressure drop of a fluid as it flows through a mold containing pre-located glass fiber reinforcements, a model based on Darcy's law of the flow of fluids through porous media can be either substantiated or modified to include previously unknown mold filling phenomena. Also, high-speed feeding methods (i.e., RTM) can be compared using a variety of process parameters, including different flow rates.