Tech Briefs

To prevent formation of voids, volatiles are removed before applying consolidation pressure.

A double-vacuum-bag process has been devised as a superior alternative to a single-vacuum-bag process used heretofore in making laminated fiber-reinforced resin-matrix composite-material structural components. This process is applicable to broad classes of high-performance matrix resins — including polyimides and phenolics — that emit volatile compounds (solvents and volatile by-products of resin-curing chemical reactions) during processing. The superiority of the double-vacuum-bag process lies in enhanced management of the volatile compounds. Proper management of volatiles is necessary for making composite material components of high quality: if not removed and otherwise properly managed, volatiles can accumulate in interior pockets as resins cure, thereby forming undesired voids in the finished products.

Figure 1. These Temperature and Pressure Profiles are typical of the curing cycle of a composite laminate containing a reactive resin matrix.
The curing cycle for manufacturing a composite laminate containing a reactive resin matrix usually consists of a two-step ramp-and-hold temperature profile and an associated single-step pressure profile as shown in Figure 1. The lower-temperature ramp-and-hold step is known in the art as the B stage. During the B stage, prepregs are heated and volatiles are generated. Because pressure is not applied at this stage, volatiles are free to escape. Pressure is applied during the higher-temperature ramp-and-hold step to consolidate the laminate and impart desired physical properties to the resin matrix. The residual volatile content and fluidity of the resin at the beginning of application of consolidation pressure are determined by the temperature and time parameters of the B stage. Once the consolidation pressure is applied, residual volatiles are locked in. In order to produce a void-free, high-quality laminate, it is necessary to design the curing cycle to obtain the required residual fluidity and the required temperature at the time of application of the consolidation pressure.

Single-vacuum-bag processing in an oven is one of the most cost-effective techniques for making fiber-reinforced resin matrix composites in cases in which resins undergoing curing do not emit volatiles. However, this technique is often ineffective in cases in which volatiles are emitted. In order to produce a void-free composite laminate, it is imperative to remove the volatiles before commencing forced consolidation. A single-vacuum-bag assembly inherently hinders the removal of volatiles because the vacuum-induced compaction interferes with the vacuum-induced outgassing. The present double- vacuum-bag process eliminates this interference while still providing for vacuum-induced compaction.

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