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molded by the DVB process. A photomicrograph of the resultant laminate in Figure 5(b) shows a superior void-free laminate than that fabricated by the SVB process. When the prepreg was laid-up at RT, the composite was filled with air, especially between the prepreg plies. In the SVB process, as vacuum was pulled at RT, and while the temperature was ramped to the B-stage condition, atmospheric pressure created by the vacuum compacted the composite because of the softening of matrix resin. The compacted fibrous architecture retarded the volatile depletion rates and a significant amount of “air-filled pockets” surrounded by the molten resin were formed. These pockets were concentrated between prepreg plies. Similarly additional “volatile-filled pockets” were formed inside the fiber bundles that were matrix resin impregnated. The formation of these two types of pockets in the SVB process led to the voided network pattern observed in Figure 5(a). On the other hand, in the DVB process, there was no atmospheric pressure created by the Inner Bag vacuum, and the composite (even though under vacuum) was not compacted during the B-stage allowing passageways for the air and volatiles to escape. “Vacuum pockets” surrounded by the molten resin matrix were formed in this case both inside the fiber bundles and between the prepreg plies. These “vacuum pockets” were then easily filled by the flow of low viscosity resin at elevated temperatures when atmospheric pressure was applied, resulting to a void-free consolidation as seen in Figure 5(b). The enhanced capability of volatile management offered by the DVB process is due to the following two attributes: 1) vacuum suction in the Inner Bag lowers the vapor pressure (and the boiling point) of the volatiles, resulting in an acceleration of the depletion rates at lower temperatures (which helps to maintain processability of the resin matrix after B-stage); and 2) the absence of compaction force during volatiles depletion period (i.e., the B-stage) relieves physical constraints of the volatiles passageways resulting in a further enhancement of volatiles depletion rates. 4.2 CYTEC 7781 E-glass/Cycom‚ 6070 phenolic composite system An identical temperature profile was used for this composite molded by either the SVB or DVB process: a single ramp from RT to 138°C (280°F) at ~ 3°C/min (5°F/min) with a 1 hr hold, followed by a cool down to RT. The laminate lay-up scheme is 6” x 6” - [0]8 . Atmospheric pressure created by the vacuum was applied throughout the cure cycle for the SVB process. In the DVB process, a full vacuum (30” Hg) is applied to the Outer Bag, while a slightly lower vacuum level (i.e., 28” Hg) is set in the Inner Bag during the temperature ramp from RT. When the temperature reached 105°C (220°F), the Outer Bag was purged to atmosphere and the Inner Bag vacuum was increased to 30” Hg. The atmospheric pressure created by the Inner Bag vacuum helped to consolidate the composite during the remaining ramp-and-hold period.PDF Image | Evaluation of Double-Vacuum-Bag Process For Composite Fabrication
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