Ever-increasing demand for faster production rates has pressed the industry to replace hand layup with alternative fabrication processes and has encouraged fabricators to automate those processes wherever possible.
A common alternative is resin transfer molding (RTM), sometimes referred to as liquid molding. RTM is a fairly simple process: It begins with a two-part, matched, closed mold, 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 metered and mixed in dispensing equipment, then pumped into the mold under low to moderate pressure through injection ports, following predesigned paths through the preform. Extremely low-viscosity resin is used in RTM applications for thick parts, to permeate preforms quickly and evenly before cure. Both mold and resin can be heated, as necessary, for particular applications. RTM produces parts that do not need to be autoclaved. However, once cured and demolded, a part destined for a high-temperature application usually undergoes postcure. Most RTM applications use a two-part epoxy formulation. The two parts are mixed just before they are injected. Bismaleimide and polyimide resins are also available in RTM formulations. "Light RTM"is a variant of RTM that is growing in popularity. Low injection pressure, coupled with vacuum, allow the use of less-expensive, lightweight two-part molds.
The benefits of RTM are impressive. Generally, dry preforms for RTM are less expensive than prepreg material and can be stored at room temperature. The process can produce thick, near-net shape parts, eliminating most post-fabrication work. It also yields dimensionally accurate complex parts with good surface detail and delivers a smooth finish on all exposed surfaces. It is possible to place inserts inside the preform before the mold is closed, allowing the RTM process to accommodate core materials and integrate "molded in"fittings and other hardware into the part structure during the molding process. Moreover, void content on RTM'd parts is low, measuring in the 0 to 2 percent range. Finally, RTM significantly cuts cycle times and can be adapted for use as one stage in an automated, repeatable manufacturing process for even greater efficiency, reducing cycle time from what can be several days, typical of hand layup, to just hours - or even minutes.
In contrast to RTM, VARTM (for vacuum-assisted resin transfer molding) uses vacuum pressure only to draw resin into the mold to saturate the reinforcements. Because of the minimal pressures involved, and the fact that it is done at ambient temperature, VARTM can be accomplished with low-cost tooling, making it the method of choice for large, complex parts like boat hulls or large transportation parts like bus bodies. Fiber reinforcements are placed in a one-sided mold, like fiberglass, metal or even wood, and the layup is sealed using vacuum bagging materials to create a vacuum-tight seal. A lightweight flexible mold cover such as silicone can also be used. Resin is introduced into the mold at strategically-placed ports and is drawn by the vacuum pump throughout the layup.
A resin flow simulation tool is helping to optimize liquid molding of aerospace parts.
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