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3 potential to produce autoclave quality high-performance composite structures, mostly with the use of partially impregnated prepregs [7]. VBO is an economical and cost-effective manufacturing process. It solves major limitations associated with autoclave processes such as high operational and setup cost, longer curing cycles, and size dependency on autoclave chambers. It only uses atmospheric pressure differential to consolidate the prepregs to achieve the required shape of molds [8, 9]. VBO prepreg processing allows the composite parts to be cured in conventional ovens. A void is the most commonly known manufacturing-induced defect associated with VBO. The acceptable void content in polymer composites may vary depending on desired applications. However, many researchers often use aircraft standards for benchmarking purpose. The primary structures of the aircraft industry have less than 1 % void content while the secondary structures allow up to 5 % void content [10]. Manufacturing of such quality composite parts is a challenging activity. It depends on several factors including consolidation pressure, curing cycle, raw material, prepregs, breather density, and release film perforation. Usually, a vacuum is pulled in VBO at room temperature prior to the curing of composites at elevated temperature [11]. This technique is efficient to prevent any leakages in the bag at an early stage of the process. Conventionally, a single bag is used to form a vacuum chamber in vacuum-bagging method, hereby known as single-vacuum-bagging (SVB). A typical SVB setup consists of composite plies stacked on a mould/tool of the desired shape, covered with a release fabric, perforated (or unperforated) film, and a breather cloth layer, and then sealed in a vacuum bag using sealant tape [12]. Vacuum pump with fittings (i.e. value, pressure gauge, pressure regulator, hoses) is connected to the vacuum bagging setup using a through-bag-connect, as shown in Figure 2(a). Release fabric is a smooth woven fabric which is placed on top of the laminate plies to create a separation film between the breather cloth and the laminate. It allows the excess epoxy to penetrate though laminate and still can be peeled off, as it does not bond with resin. An example of a commonly used release fabric is a finely woven polyester fabric. A perforated plastic film is often placed on top of a release fabric which aims in holding the resin within laminate when a high consolidation pressure is applied. Clamping pressure, resin’s viscosity and its out- time define the selection of a suitable perforated film which varies in pore sizes and its patterns [13]. On top of a perforated film, a breather cloth is placed to allow air evacuation from the inner environment to through-bag-connector by providing a separation between the bag and the laminate. [1, 13]. Edge breather or porous dams are also used on the edges of a composite; they prevent resin loss while allowing air evacuation [12]. Before applying sealant bag on top of the setup, the through-bag connector is placed inside which helps evacuating entrapped air to the atmosphere through a vacuum pump. The setup is then sealed with a plastic bag using sealant tape to provide an airtight environment. It is recommended to use a transparent bag for easy inspection. After setting a vacuum bag on top of the setup and sealing to avoid any leakages, the vacuum port is then connected to the vacuum pump. Any leakages in the bag or near through-bag- connector can be sealed using sealant tape. Before starting the vacuum pump of aPDF Image | Comparison of Single-Vacuum-Bag and Double-Vacuum-Bag Methods
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