Previously (Part One) we introduced composite materials, reviewed the vacuum bagging method for composite manufacture, and discussed the role vacuum systems play. Here we will take a closer look at these systems, and some of the unique requirements of vacuum equipment for composite manufacture.
The Vacuum Pump
The most common vacuum pump used in industrial scale composite curing units is the rotary vane pump. A rotary vane pump is considered a wet, positive displacement pump. Wet, because the air being pumped is exposed to oil used as a lubricant and to help provide the seal. Positive displacement refers to the fact that the pump works by mechanically trapping a volume of gas and moving it through the pump, creating a low pressure on the inlet side.
This type of pump is highly reliable and reasonably energy efficient. It features an eccentrically mounted rotor that contains two vanes which slide in diametrically opposed slots (Fig. 1). The vanes are either spring loaded, or rely on centrifugal force to push outward against the stator wall. As the rotor turns, the tips of the vanes trap a volume of incoming air and move it around the circumference of the pump chamber toward the exhaust outlet. This creates a vacuum at the pump inlet due to the void created. During this process the air is compressed and discharged out of the pump. As the vane continues to rotate, the opposing vane seals the inlet and the process is repeated. A small amount of oil is discharged with the air as a mist. A filter system is employed to remove the oil from the discharged air.
In vacuum curing of composite parts, it is common for dozens of parts to be simultaneously placed in the cure oven, vacuum bagged, then cured under heat. In other cases, one or two very large parts are processed. Either way, the parts being cured are expensive, since many hours of manual layup are required to create each part. Therefore it is important the oven and vacuum system be extremely reliable. A failure in the middle of the process can cost many thousands of dollars if the parts have to be scrapped. In the case of large space rocket components, the value can be several million dollars per load. For this reason, a duplex pump arrangement (Fig. 2) is commonly employed to insure the reliability of the vacuum system.
The duplex pump arrangement consists of two pumps piped in parallel. Each pump is designed to handle the full flow rate and ultimate vacuum required by the system. During normal operation both pumps are in use, and evenly share the load. In the event of failure of one of the pumps, the remaining pump still has the full capacity required to provide vacuum for the system. Control logic is employed such that upon failure of one of the pumps, its inlet valve is automatically closed, isolating it from the system, allowing the other pump to carry the load. This design has two advantages. First, each pump is normally only required to run at half of the usual system flow rate. This reduces the wear on each pump and extends the service intervals and pump life. Second, this redundancy greatly increases the reliability of the overall system since the likelihood of both pumps failing simultaneously is very low. For example, if each pump has a .005 (.5%) likelihood of failure during a given run, the likelihood of both pumps failing during the same run would be .005 x .005, or .000025 (.025%).
The duplex pump arrangement is not to be confused with a two-stage rotary vane pump, in which two pumps are piped in series and the inlet of one is connected to the outlet of the other. The two stage pump arrangement is not used to improve system reliability, but rather to enhance the pump performance by increasing the ultimate vacuum achievable.
In a duplex pump arrangement, the inlets to both pumps are piped to a common receiver (aka accumulator). The receiver is then piped to the main header that is in turn connected to the bagged parts. The receiver is a tank that stores a reservoir of vacuum, allowing the pump to cycle less frequently and serves as a buffer between the pump and the bagged parts. A 75 m3/h (44 ft3/min) pump, for example, might incorporate a 0.454 m3 (16 ft3) receiver tank. Receivers are used for both duplex and simplex pump arrangements.
Pump Performance and Draw-Down Time
Both the volumetric capacity of the pump (i.e., the pump “speed”) and the ultimate vacuum of the pump must be considered when selecting a pump for composite curing applications. The speed of the rotary vane pump is not consistent over the entire operating range of vacuum. The pump speed is highest when the pressure at the pump inlet is highest (i.e. the vacuum is the lowest). This often occurs when the system is first evacuating a bagged part and the maximum vacuum has not been reached. The flow capacity decreases dramatically after most of the air is removed from the bagged part and the vacuum on the part increases. When ultimate vacuum is reached, the pump flow rate approaches zero. The pump manufacturer provides curves that show the relationship between the pumping speed and the vacuum level (pressure) at the pump inlet (Fig. 3).
To properly select a pump, it is necessary to determine the time required to draw the air out of the part bags (often referred to as the “draw down” or “pull down” time) and achieve the desired vacuum. If this is ignored, the pump may be selected incorrectly such that it will take an unacceptable length of time to draw a vacuum on the bagged parts. As is seen on the pump curve, the relationship between the pump pressure and the pumping speed is highly nonlinear. This is due to the fact that as the pump draws the air out of the bagged parts, the remaining air gets thinner and expands to fill the remaining volume, making the pump less effective at removing the remaining air. There is a relationship (Table 1) between the pump speed at atmospheric pressure (SCFM) and the actual speed at reduced pressure (ACFM). For example, the pumping speed at one half of atmospheric pressure (roughly 15 in. Hg), is ½ the speed at atmospheric pressure (0 in. Hg). This is shown by the fact that the ratio of the volume of SCFM (unloaded pump at standard conditions) to ACFM (pump operating under vacuum) is 2 at 15 in. Hg.
The above factors make it difficult to calculate the pump draw down time because the pump speed (flow rate) continuously drops as higher vacuum is achieved during pull down. There is, however, a way to estimate the draw down time with reasonable accuracy (Fig. 4). This estimation requires a “pull down” factor be applied. Variable A, tank size, represents the desired volume under bag to be evacuated. For example, if an ultimate vacuum of 28 in. Hg is desired, the pull down factor is 2.75. If the volume under bag is 100 ft3 and the pump speed is 10 ft3/min, the pull down time is 100 x 2.75 / 10 = 27 minutes. Another way to think of this is that it takes 2.75 times longer for the pump to displace a given volume of air in order to attain 28 in. Hg than it would take the pump to displace that same volume at atmospheric pressure.
The most common arrangement for vacuum bagging and curing involves placing many parts in a cure oven and connecting the vacuum system to the parts through penetrations in the oven wall (Fig. 5). The vacuum pump is then turned on and the vacuum is drawn on the part bags. A header pipe connects the vacuum source lines to the pump.
Each of the individual source lines includes a separate solenoid valve, to allow them to be turned on or off individually (Fig. 6). The solenoids are wired to a master control system so they can be actuated remotely. Each static line is connected to the bagged part, usually on the end of the part opposite from the source line(s). The static lines are capped off and don’t allow air to pass to the parts. Their purpose is to monitor the vacuum. A transducer located on each static line verifies vacuum is penetrating the part from the source line and reaching the transducer. Furthermore, the transducer measures the vacuum level and transmits this information to a data acquisition system. The data acquisition system collects and stores this data for each part. Since many of the parts being cured are critical, high value components for aerospace and automotive applications, it is important this information be stored and traceable to each individual part. The master control system also stores recipes that control the vacuum level, oven cure time and oven temperature for each part.
Individual Vacuum Control
The design described above results in all the bagged parts being at the same vacuum level, since they are all connected to a common manifold. However, it is sometimes desirable to have an arrangement where different parts in the same cure cycle are at different vacuum levels. This requires an alternative design where each source line incorporates a second solenoid valve located between the first solenoid valve and the part. This “bleed” valve allows a slight amount of air into the system at a controlled rate, reducing the vacuum on that part.
When the bleed solenoid valve opens and closes, a closed loop controller monitors the vacuum transducer. If the vacuum is above set point, the bleed valve opens, reducing the vacuum on the part until it reaches set point. If the vacuum on the part is too low, the bleed valve closes, which increases the vacuum level on the part until set point is achieved. Individual vacuum control requires separate vacuum reservoirs be located at each source line.
Since the bagged parts put under vacuum consist of a woven fibrous material impregnated with resin, loose fibers or resin vapor can possibly pass into the vacuum system, causing contamination. It is common to incorporate a filter system to prevent this. The preferred design is to locate a small filter in each source line, to catch contaminants before they reach the vacuum header. An alternative design is to rely on a single main filter located near the pump inlet.
Composite manufacturing is a growth-orientated technology with aerospace and transportation related industries leading the way. The increased use of composites, particularly epoxy-based glass, aramid, and carbon fiber materials in recent years has paralleled the growth in commercial aviation and is positioned to meet the demands of “all composite” transportation vehicles in the future. Vacuum treatment of these materials will continue to play a significant role in the success of this technology.
1. Edwards Vacuum (www.edwardsvacuum.com)
2. Dekker Vacuum Technologies (www.deckkervacuum.com)
3. Kurt J. Lesker Company (www.lesker.com)
4. Combined Fluid Products (www.cfpco.com)
5. Wisconsin Oven Corporation (www.wisoven.com)