The use of atmospheric pressure to hold closed molds together during injection was the early process of vacuum assisted resin injection (VARI). Adding vacuum has enabled resin-transfer molding (RTM) to challenge compression molding and autoclaving systems capable of making the best high-performance composites. Vacuum is used in two ways. First, it mixes resin and hardener under a 91-Pa vacuum just prior to injection. Using an impeller, it agitates the mixture to drive air to the surface where the vacuum removes it. Degassing, which takes about 2 h per tank, also removes volatiles and low-molecular-weight by-products.
Some companies place their RTM tools in a vacuum chamber rather than using a vacuum tool. The chamber creates a vacuum that does not vary, even as resin fills the tool. The hard vacuum pulls any air and water vapor off the preform and sucks resin into the mold.
This combination of degassed resin and vacuum keeps voids under 3% and often better. This ensures consistently high structural integrity because voids concentrate stresses that initiate fractures and cause premature failure. It takes only a 2% increase in voids to drain inter-laminar shear strength 20% and flexural modulus 10%. The process matches equivalent compression molding and autoclaving fiber volumes and voids.
Compared with compression molding and autoclaving, VARI does not need to apply pressure over the entire skin surface to vanquish voids, simplifying cocuring. For example, the process can fabricate cores and reinforced skins in a single step rather than bonding them after fabrication.
More importantly, the process makes composites in fewer, more controllable steps. These resin injection and molding processes — RTM, VARI, vacuum resin-transfer molding (VRTM), and vacuum-assisted resin-transfer molding (VARTM) — are much simpler processes to use.
Each process step is also independent and controllable. In VRTM, air evacuation depends on only the vacuum, and resin preparation depends on only the resin mixer. Preform production varies with automated fabric weaving and preform placement, whereas core manufacture depends on molding or a machining process. The cure depends on a programmable heat source.
VRTM composites also show excellent resistance to water, solvents, and chemicals. This is largely a function of resin type and surface finish. Rough surfaces pitted with micropores trap water and chemicals and act as tiny reaction chambers that set in motion their own destruction. VRTM yields parts with less than 20-^in. rms (root mean square) porosity. VRTM can achieve this fine finish repeatedly on all surfaces, depending on the finish of the tool. For high-quality finishes, compression and autoclaving processes depend on uniform resin flow under pressure, which they cannot always maintain.
The main attraction of VRTM, despite its competitive properties, remains cost, where it offers real advantages over compression molding and autoclaving.
Another production process is low-cost VARTM infusion technology.
VARTM is becoming a manufacturing method of choice because of its ability to produce fairly large structures out of the autoclave with the high quality usually associated with higher-priced processes.
