More and more application uses are being found for composite materials and their use is expanding beyond just aerospace applications. As such, you may be wondering how they are manufactured. Composites are manufactured using multiple layers of material, each with different properties, combined into a single structure. By joining them in this manner, the resultant “composite” material is unusually strong and light. It has greater strength, flexure and more favorable mechanical properties than any of the individual materials used to make the composite. Metal matrix and ceramic matrix composites enjoy widespread use in the wind power, automotive and aerospace industries, among others (Fig. 1), as they offer unique advantages over steel and aluminum. Composite manufacturing is a growing industry and has strong continued prospects for expansion.
The most popular composites, valued for their very high strength to weight ratio, are made of a woven base material such as fiberglass, aramid (Kevlar®) or carbon fiber impregnated with a resin that is hardened into a plastic using heat and/or pressure. The woven material is known as a “prepreg” since it has not yet been impregnated with resin. The resin can be epoxy, polyester, polyurethane, or other plastic in liquid form. The base fiber, such as carbon, has high tensile strength but is not stiff enough on its own to be used as a structural element. The resin can be molded and formed into various shapes but is not strong enough on its own to serve as a structural member. When the base fiber and the plastic are combined, the resultant composite material has both the strength of carbon fiber and the retained shape of the cured epoxy or other hard plastic. This feature, the ability to incorporate the most favorable mechanical properties of each of the constituent materials, is what makes composites such an attractive choice for a wide variety of lightweight structures.
Composite materials are manufactured in different ways, but one of the most common is the vacuum bagging method in which the uncured material is formed in a mold, placed under vacuum, then cured in an oven while vacuum is maintained. This involves starting with a fabric weave of carbon fiber (mat), similar in appearance to a thick woven cloth (Fig. 2), placing it in a mold (also referred to as a tool), and forcing epoxy resin into the weave. Successive sheets of the fabric are then layered onto the first with the epoxy imbedded into and between each layer. This manual process is referred to as hand layup where the successive layers are placed into the mold and the epoxy resin is forced into the mat using a small roller (Fig. 3). Each mat layer can be oriented with the weave rotated 45° or 90° to the one below it to maximize the strength of the final product. The result is a cross-plied layup (as opposed to a unidirectional layup where all the fibers are oriented in the same direction). After the layup is complete, the mold gives the finished part the desired shape, and the part is ready for bagging. The epoxy forms a matrix that allows the part to hold its shape with the carbon fibers residing in the matrix providing the required strength.
In order to create a useful finished part, the composite material must be held tightly in the mold while the resin is curing. Vacuum bagging is a technique that uses atmospheric pressure to squeeze the resin-impregnated layers (the laminate) together, forcing them to conform to the shape of the mold. After layup is complete, the laminate layers are sealed in an airtight vacuum bag. The bag may consist of a plastic sheet that covers the layers (Fig. 4) or it may utilize top and bottom sheets that enclose the laminate and the mold. In either case, after the bag is sealed and before vacuum is applied, pressure on the inside of the bag is the same as the outside and is equal to atmospheric pressure, which is approximately 1,000 mbar (29.9 in. Hg, or 14.7 psi). A vacuum hose is connected to the bag and a vacuum pump draws the air from it. The air pressure inside the bag is reduced while air pressure outside of the bag remains at atmospheric pressure. As a vacuum is drawn on the inside of the bag, atmospheric pressure on the outside of the bag presses on the bag (Fig. 5) and squeezes the laminated layers of resin-saturated mat together against the mold, serving as a clamp that puts very uniform pressure over the entire surface of the part.
The pressure differential between the inside and outside of the bag governs the pressure exerted on the laminate. The maximum pressure that could be exerted on the laminate by atmospheric pressure if the vacuum pump removed all the air from the bag and achieved a perfect vacuum, would be 1,000 mbar (29.5 in. Hg). As a practical matter, the vacuum system is operated at 400 to 840 mbar (11.8 to 24.8 in. Hg), which generates sufficient pressure, 4,200 – 8,400 kg/m3 (6 to 12 psi) on the laminate to hold the layers together and force it to conform to the mold. This high available pressure permits very complex molds to be used without voids remaining between the layers of laminate or between the laminate and the mold. Atmospheric pressure is not only pushing down on top of the mold but is also pressing on all sides and the bottom of the mold with the same force. Therefore the mold does not need to be rigid enough to withstand the entire force created by the 8,400 kg/m3 (12 psi) pressure on its surface, since the sides and bottom are supported by atmospheric pressure. This allows the mold to be relatively lightweight and inexpensive.
Vacuum Pumps for Composite Manufacturing
The central component of the composite cure vacuum system is the vacuum pump. The pump must be capable of drawing the necessary vacuum and it must also be able to provide sufficient displacement. The level of vacuum necessary depends on the shape and complexity of the part being molded. For example, a part with many contours and complex features will require a high vacuum to squeeze the part via atmospheric pressure on the bag. This forces the laminate into the concave mold cavities without creating voids. A flat panel shaped laminate does not have these cavities and therefore needs a lower vacuum level vacuum to be drawn on the bag.
The flow rate (displacement) of the pump is also important for two reasons. First, the initial drawdown must evacuate all the air beneath the bag within an acceptable time period. This air is a result of voids and cavities between the bag and the laminate prior to the vacuum being drawn, which are a normal result of the manual layup process. The greater the pump displacement, the faster the pump will remove this air resulting in a shorter drawdown period. The second reason pump displacement is important is due to leakage that occurs at full vacuum. After drawdown, the pump no longer needs to evacuate the air under the bag but must be able to displace the air being leaked into the bagged volume through the perimeter mastic or tape as well as through any fittings or pipe connections. It is important to recognize that during initial drawdown, the pump inlet is at or near atmospheric pressure so the pump is allowed to operate at its maximum flow rate. While under full vacuum after drawdown the pump is operating near maximum vacuum and minimum flow rate. Therefore both the maximum pump flow rate (referred to as the pumping speed) must be considered as well as the flow rate at full vacuum. Pump manufacturers provide pump curves illustrating the flow rate at both conditions. If one of these ratings is ignored during pump selection, the system will either take too long to draw down (while under minimal vacuum) or fail to maintain the required vacuum level after drawdown (at maximum vacuum) due to system leakage.
Vacuum Components for Composite Cure Vacuum Systems
After the laminate materials are laid up in the mold and placed under vacuum, they are cured in an industrial oven (Fig. 6). It is common practice in aerospace and other applications for many parts (50, 100, or more) to be cured at the same time. In these cases, the layup process is often done inside the oven.
The molds are placed on worktables in the oven while they are at ambient temperature. The workers perform the layup operation inside the oven, connect the vacuum system to the layup (part under bag) via flexible connections, and then the vacuum system is turned on. The vacuum valves, transducer, pump, and other components are located outside the oven, and there are penetrations through the oven wall for the vacuum lines.
One common arrangement is for one or more vacuum source (aka active) connections to provide the vacuum to the part, and for a static (aka passive) line to be connected to the other end of the part (Fig. 7). The source line delivers the vacuum to the part. A transducer on the static line senses the vacuum level. This information is sent to a master control system for recording and control. It is common for a composite cure system to process 50 or more parts having 50 or more sets of active and static vacuum lines. The vacuum pump selection is critical since the flow rate necessary for initial drawdown and leakage can both be considerable. Rotary vane pumps are very common in industrial applications such as these due to their reliability and reasonable cost.
Next Time: We will focus on the vacuum systems and the pumps used in the processing of composite materials.
- Composites Manufacturing magazine (www.compositemanufacturingmagazine.com)
- 3MB Co., LTD (www.3mb.asia)
- University of Liverpool (www.liverpool.ac.uk)
- West System (www.westsystem.com)
- Nextcraft.com (www.nextcraft.com)
- Wisconsin Oven Corporation (www.wisoven.com)