Double-vacuum-bag technology for volatile management in composite fabrication

A nonautoclave vacuum bag process using atmospheric pressure alone that eliminates the need for external pressure supplied normally by an autoclave or a press is an attractive method for composite fabrication. This type of process does not require large capital expenditures for tooling and processing equipment. The traditional single‐vacuum‐bag (SVB) process is best suited for molding epoxy matrix‐based composites because of their superior flow and the absence of reaction byproducts or other volatiles. This is not the case for other classes of materials such as polyimides and phenolics. Polyimides and phenolics are cured by condensation reactions which generate water as a reaction byproduct. In addition, these materials are commonly synthesized as oligomers using solvents to facilitate processability. Volatiles (solvents and reaction byproducts) management therefore becomes a critical issue. SVB molding, without additional pressure, normally fails to yield void‐free quality composites for these classes of resin systems. A double‐vacuum‐bag (DVB) process for volatile management in composite fabrication using common molding equipment was designed and built at the NASA Langley Research Center. This experimental DVB process affords superior volatiles management compared with the traditional SVB process. Void‐free composites are consistently fabricated as measured by C‐scan and optical photomicroscopy for high‐performance polyimide and phenolic resins. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers