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4 vacuum bagging setup, the inside and outside pressures of the bag are the same i.e. one atmospheric pressure. Once the air evacuation process from inside of the bag starts, the inner pressure starts to drop while the outer pressure of the bag remains at 1 atm [14]. This pressure difference forces the bag to tightly hold everything inside together, applying uniform pressure, as shown in Figure 2(b). For composites to be cured at a temperature higher than ambient conditions, the SVB setup is then placed inside an air circulating oven. The required cure cycle is then followed to increase the temperature with an appropriate ramp rate and dwells [15, 16]. Modified SVB (MVSB) is similar to SVB, except that it incorporates caul- sheets or pressure strips as pressure intensifiers to facilitate the intensification of pressure distribution on composites, especially for composites with complex shapes. MSVB is suitable for composites with convex, concave, or semi-spherical corners. The intensifiers, however, block the air-passages for entrapped air to escape which causes porosity [1]. Breather Cloth Film Tool/Mould (a) Caul Sheet Perforated Film Vacuum Bag Vacuum Pump Through Bag Connector Perforated Vacuum Bag Vacuum Pump Through Bag Connector Laminates Figure 2: An example of (a) vacuum-bagging-only schematics and (b) laminates subjected to atmospheric pressure using a vacuum-bagging-only technique. The concept of the double-vacuum-bagging technique was first introduced in 1990 by NASA Langley Research Centre. DVB technique is a superior alternative to an SVB and offers exceptional void management [17]. There are two types of DVB setups. For the first DVB type, vacuum bagging setup (similar to SVB) is placed inside a rigid chamber, which acts as a second vacuum environment, as shown in Figure 3 [18]. The rigid chamber is vacuumed during a low-temperature hold stage to create a second vacuum environment, as shown in Figure 3(a), and then purged to the atmosphere during a high-temperature hold, as shown in Figure 3(b). Initially, a partial vacuum is drawn in the vacuum bag during low temperature while the bag is fully vacuumed during the high-temperature hold. This technique is, however, not favorable because of its limitation on composite size flexibility. VBO process was introduced to eliminate the need for autoclave shells. Introducing rigid chambers would again limit the sizes of composites, Laminates Peel Ply Breather Cloth Tool/Mould (b) Peel PlyPDF Image | Comparison of Single-Vacuum-Bag and Double-Vacuum-Bag Methods
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