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Processes 2020, 8, 1147 2 of 16 increase extensively in the next decade. However, composite manufacturing processes are relatively time-consuming and costly. It poses challenges to meet the rising composite demands which certainly requires innovative and robust production techniques. Composites for industrial/mass production applications are typically produced by autoclave process under elevated temperature and pressure [13]. In this method, fully saturated prepreg plies with resin are placed on a tool/mold, the setup is enveloped inside a plastic bag. This setup is vacuumed using a vacuum pump and it is then placed inside a pressurized autoclave vessel. The application of elevated pressure and temperature in autoclave processing safeguards nominal void content and first-rate shape conformation [14,15]. However, major drawbacks of the autoclave manufacturing process include the high costs associated with equipment procurement, maintenance and operation, costly nitrogen gas required for pressurizing autoclave vessel, and lengthy cure cycles [16–18]. The capacity of autoclaves limits the size and design of parts, and the production rate is primarily affected by autoclave availability [19,20]. The industries are seeking more economical and robust out-of-autoclave (OOA) composite manufacturing techniques as it will be very challenging to meet the growing demand. OOA prepreg processing plays a crucial role to substitute the metallic structures with a lighter and stronger composite structure to meet the needed production rates. Shifting from autoclave to OOA process will allow a more economical, less time-consuming manufacturing process with increased part size flexibility. The autoclave process uses a high pressure to minimize manufacturing-induced defects, while OOA methods only involve a pressure differential of 1 atm maximum, which makes it vulnerable to certain manufacturing-induced defects. Thus, a sound scientific understanding of low-pressure processing methods is required for successful implementation. Several researchers have addressed the issue of low-pressure application in OOA by proposing alternative methods to generate sufficient consolidation pressure. Some of the noteworthy techniques are the use of high-power permanent magnets [21,22] and hot press molds [23]. Among these techniques, magnet assisted composite manufacturing (MACM) is an impressive option with less costs associated with acquisition, tooling and maintenance when compared to hot press molding. However, the major drawback of MACM is that the compression pressure degrades exponentially with the increase in layup thickness which makes this process vulnerable to thicker laminates [22]. Conventionally, a two-step ramp and dwell temperature and pressure profiles, usually referred to as manufacturer’s recommended cure cycle (MRCC), are recommended by the prepreg suppliers. The low-temperature dwell of a typical cure cycle, also known as B-stage or TB, permits the depletion of volatiles and impregnation of resin into dry tows, whereas, the high-temperature dwell, also referred to as C-stage or TC, allows the resin cross-linking and consolidation into a solid final product [24]. The mechanical properties for commercially available prepregs could be further improved by taking induced residual stress/strain into consideration during curing [25–27]. Takagaki et al. [26] conducted an in situ residual stress/strain investigation using MRCC and proposed two new cure cycles which ultimately reduced residual strain by 12–18% while tensile strength improved up to 26%. Extended manufacturer’s recommended cure cycle (EMRCC) consists of an extra temperature dwell TB2 after the first TB. The drawback of EMRCC is it requires a relatively longer curing time compared to that of MRCC. Direct cure cycle (DC), on the other hand, raised TB closer to TC. Samples produced using DC are reported to suffer from reduced tensile strength, despite an 18% improved residual strain when compared to those of MRCC. The modified cycles showed no significant changes in the void content. To date, the information on the effects of various bagging techniques for composites with complex shapes produced using these cure cycles are not readily available. Among bagging techniques, most conventionally used is a typical one bag setup which creates one vacuum environment, hereby referred to as single vacuum-bag-only (SVB) in this study. This technique is readily used for composites with flat or streamline structures [27]. However, SVB is vulnerable to thickness deviation in composite structures with blunt corners [28]. A modified single vacuum-bag-only (MVSB), also uses one bag to create a vacuum environment but it incorporates the use of metallic or polymeric intensifiers, caul-sheets or pressure strips in the corners of complex structures [29–31].PDF Image | Effects of Processing Parameters for Vacuum-Bagging
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