Vacuum Bagging in Composites
A vacuum bagger is not necessary for composite parts. Traditionally, a carbon fiber body panel would begin as a mold or tool. Next, the epoxy resin would be applied to the panel and built to the desired thickness. Vacuum bagging can also be used to reduce the flow of epoxy out of the laminate. Its advantages are discussed in this article. But, you should know that vacuum bagging does not apply to all composite parts.
The perforated film reduces the flow of epoxy out of the laminate.
Using perforated film to contain the epoxy helps reduce the resin’s flow out of the composites during curing. The perforated film can be a simple plastic or polyethylene with holes about 3/8″ apart. Large projects can use a specially designed release fabric to control the epoxy flow. Small projects can be perforated on their own with holes placed in a grid pattern.
This technique best suits medium-volume production since the resin flows through the laminate and fully impregnates the reinforcements. Because the film is perforated, it is easy to handle multiple parts and requires a structural change in the shop layout. Therefore, CCBM is most appropriate for low and medium-volume production and has minimal cosmetic requirements on the B-side. This method is also very economical as it does not require an extra operator.
Double bagging achieves a 90o (1″) female radius.
The concept of double vacuum bagging was first developed in the 1980s to reduce porosity and increase the mechanical properties of composites. NASA and the Boeing Company have since applied the concepts to the repair of wet-layup composites, prepreg, and infusion processes. The basic principles of double vacuum bagging techniques have also been successfully used in vacuum-assisted resin transfer molding. NASA first applied double bagging to the repair process of composite parts. A decade later, Russell Emanis attempted the process with resin infusion.
Infusion bagging schemes are difficult to employ in large-scale productions. Infusion bagging schemes are also not conducive to the use of caul plates. However, double vacuum bagging achieves a 90o (1″) female radius in composites. Therefore, the technique is advantageous for complex parts and provides the highest quality composite. The method of double vacuum bagging also provides improved overall structural integrity.
Cost of vacuum bagging
Vacuum bagging is a commonly used process in the composites industry. It creates uniform pressure on the surfaces of the parts inside the bag, ensuring that the adhesive cures properly. The process also removes trapped air between layers, preventing the fiber orientation from shifting during cure. Several benefits of vacuum bagging include reducing humidity and improving the fiber-to-resin ratio. But the cost of vacuum bagging is not cheap, and it can be prohibitive for large-scale production.
The cost of vacuum bagging in composites varies depending on the size of the parts, which will determine how much the process will cost. Larger projects require more powerful vacuum equipment. Double wobble vacuum pumps, for example, can deliver more CFM and accommodate larger bags. They also require less maintenance. While specialized equipment is needed for this process, some parts can be made with less expensive equipment. Also, if the project is large, it is vital to choose the right vacuum pump for the job.
Duplex pump arrangement
A duplex pump arrangement uses two pumps piped in series, one inlet to the other. The duplex pump arrangement improves system reliability by enabling the pump to run at full capacity. The duplex pump arrangement can also increase the ultimate vacuum achievable. A duplex pump arrangement has two major advantages: first, it enables the vacuum bagging system to achieve the maximum possible flow rate. The second advantage is that the duplex pump arrangement can achieve higher vacuum levels than a single-stage pump arrangement.
When selecting a vacuum pump, volumetric capacity and ultimate vacuum must be considered. For example, a rotary vane pump’s speed is variable over the vacuum range. At atmospheric pressure, the speed is the highest, and when the vacuum is the lowest, it decreases dramatically. In contrast, when the vacuum is greatest on a bagged part, the pump’s speed is the lowest. As a result, the ultimate vacuum increases the vacuum on the bagged part while reducing the pump’s flow rate.