Liquid molding comprises a number of composite fabrication processes that are capable of fabricating extremely complex and dimensionally accurate parts. Resin transfer molding (RTM), the most widely used of the liquid molding processes, uses a two-part, matched, closed mold that is made of metal or composite material. Dry reinforcement (typically a preform) is placed into the mold, and the mold is closed. Resin and catalyst are mixed in dispensing equipment, then pumped into the mold under low to moderate pressure through injection ports, following predesigned paths through the preform. Both mold and resin can be heated, as necessary, for particular applications. RTM is a process that is well suited to fabricating three-dimensional structures requiring tight dimensional tolerances on several surfaces. Excellent surface finishes are possible, mirroring the surface finish of the tool. The major limitation of the RTM process is the relatively high initial investment in the matched die tooling. Sufficient part quantities, usually in the 100 to 5000 range, are necessary to justify the high nonrecurring cost of the tooling. The main advantages of using the RTM process are: - Excellent tolerance control. - Cycle times can be very short. - Molded-in inserts, fittings, ribs, bosses, and reinforcements are possible. - Volatile emissions are controlled by a close mold process. - Lower labor intensity and skill levels. - Considerable design flexibility: reinforcements, lay-up sequence, core materials, and mixed materials. - Mechanical properties are somewhat comparable to autoclave parts (void content <1%). - Smooth finish on both surfaces. Check out this video, which shows an automated HP-RTM (High-Pressure Resin Transfer Molding) machine in action!