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pressure created by the vacuum bag alone, and is cost effective for composite fabrication. In general, the SVB process is best suited for molding epoxy matrix based composites because of the absence of reaction by-products or other volatiles. However, the superior fluidity (low viscosity) of epoxy matrix may cause an excessive resin flash, which results in dry fibers and extensive void networks in the laminate [1]. Furthermore, SVB processing is often ineffective in composite molding when a reactive resin matrix with volatile by-products or solvent containing prepreg is present. The SVB assembly inherently hinders and/or retards the volatiles depletion mechanisms during fabrication because a compaction force (i.e., atmospheric pressure) is created and exerted onto the laminate during the volatile depleting B-stage period. In the past two decades, a double vacuum bag procedure was devised at the Naval Air Warfare Center, Warminster, PA to replace SVB for aircraft composite repair applications [2-8]. The double vacuum assembly consists of a rigid enclosure over a flexible vacuum bag. Prepreg plies are exposed to a vacuum but do not experience a consolidation force. When bonding the wet lay-up repair patch of epoxy matrix, the double vacuum procedure eliminates problems of excessive flash during the B-stage period [4-8]. Very limited studies and conflicting results were noted in the applications of high temperature bismaleimide based wet lay-up repair patches. The double vacuum procedure resulted in a lower interlaminar shear strength when compared to the SVB process, presumably due to the poor wetting out of the fibers and high void content [1]. 2. DVB Assembly and Processing Concept A schematic drawing to illustrate the concept of a traditional SVB in composite manufacturing is shown in Figure 2. Fiber reinforced reactive resin matrix prepregs (composite) are laid up between the caul and tool steel plates. The composite is then enclosed by a vacuum bag, which is sealed around the edges onto the tool plate. A vacuum port is built on the tool plate inside the bag. This assembly is typically installed in a forced air circulation oven and subjected to a cure cycle for composite molding. Prior to the application of vacuum, the bag rests at an equilibrium balanced by the same atmospheric pressure (i.e., 14.7 Psi) from either side of the bag as shown in Figure 2(a). Under this circumstance, the composite is not subjected to any external compaction forces and remains bulky and loose. During the B-stage, the resin softens and becomes molten at elevated temperatures. Reaction by- products may be generated or volatile solvent is released and chemo-viscosity builds-up. In order to fully deplete the volatiles at lower temperatures, vacuum is pulled on the composite. However, because of the pressure differential, the vacuum causes the bag to collapse tightly onto the caul plate and compact the composite. At the same time, there is a thickness reduction due to softening of the resinPDF Image | Evaluation of Double-Vacuum-Bag Process For Composite Fabrication
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