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7 For each sample, prepregs of dimensions 220 mm x 220 mm were stacked on a flat aluminum mould. Laminates using SVB technique were covered with first release fabric, followed by a perforated film and on top, a breather fabric was placed. A plastic bag 20 % larger than the laminate dimensions was cut to accommodate the stacking without disturbing its orientation. Composites were then sealed within the vacuum bag using sealant tape on the edges. A second bag was used in the DVB to create a second vacuum environment which was separated from the inner-bag by a steel perforated tool. A manufacturer’s recommended cure cycle (MRCC) was used for SVB and DVB techniques, as shown in Figure 5(a) and (b). The MRCC for SVB technique consists of 4 h dwell at 70°C (B-stage) and 1 h dwell at 120°C (C-stage). Two ramp rates are used: 1°C/min up to 70°C, and 2°C/min from 70°C to 120°C. A full vacuum is pulled at room temperature by applying 1.01 bar pressure until the end of C-stage. The temperature profile in the MRCC for DVB is similar to SVB. However, it consists of two pressure profiles due to the two vacuum bags used. The pressure inside the inner-bag is 0.95 bar (partial vacuum) during B-stage and 1.01 bar pressure (full vacuum) during C- stage. A full vacuum is drawn inside the outer-bag during B-stage (to create a ballooning effect in the inner-bag) while it is purged to the atmosphere during C- stage. B-stage B-stage C-stage C-stage (a) (b) Figure 5: Manufacturer’s recommended cure cycle used in (a) SVB and (b) DVB techniques. 4 Quantification of Void Content Content of surface porosity and through-thickness void is quantified using MATLAB code for image processing. First, high-resolution images of mould- laminate and laminate-bag sides were acquired for all samples produced, using a Nikon D7000 DSLR camera with an AF-S XD NIKKOR 18-105 mm VR lense. Specimens were then cut into 15 mm x 200 mm dimensions using a diamond blade cutter. Specimens’ cross-sectional areas were then polished using 1000-grit and 1500-grit sandpapers, respectively. High-resolution images were taken to quantify through-thickness void content. To avoid discrepancies and human error,PDF Image | Comparison of Single-Vacuum-Bag and Double-Vacuum-Bag Methods
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