Vacuum Bagging

After ply collation, the laminate is sealed in a vacuum bag for curing. A typical bagging schematic is shown in Fig. 7.27. To prevent resin from escaping from the edges of the laminate, dams are placed around the periphery of the lay-up. Typically, cork, silicone rubber, or metal dams are used. The dams should be butted up against the edge of the lay-up to prevent resin pools from forming between the laminate and dams. The dams are held in place with either double-sided tape or Teflon pins.

Nylon Vacuum Bag Breather Material

Inner Bag

Bleeder Material

Porous Release Material

Peel Ply (Optional)

Composite Lay-up t Peel Ply (Optional)

Non-porous Release

Double Sided

Vacuum Bag Sealant

Tool

Fig. 7.27. Typical Vacuum Bagging Schematic1

Vacuum Bag Sealant

Tool

Fig. 7.27. Typical Vacuum Bagging Schematic1

A peel ply may be applied directly to the laminate surface if the surface is going to be subsequently bonded or painted. Then, a layer of porous release material, usually a layer of porous glass cloth coated with Teflon, is placed over the lay-up. This layer allows resin and air to pass through the layer without having the bleeder material bond to the laminate surface. The bleeder material can be a synthetic material (e.g., polyester mat) or dry fiberglass cloth, such as 120 or 7781 style glass. The amount of bleeder material depends on the laminate thickness and the desired amount of resin to be removed. For the newer net resin content prepregs, bleeder cloth is not required since it is not necessary to remove any excess resin.

After the bleeder is placed on the lay-up, an inner bag made of Mylar (polyester), Tedlar (PVF), or Teflon (TFE) is placed over the lay-up. The purpose of the inner bag is to let air escape while containing the resin within the bleeder pack. The inner bag is sealed to the edge dams with double-sided tape and then perforated with a few small holes to allow air to escape into the breather system. The breather material is similar to the bleeder material, either a synthetic mat material or a dry glass cloth can be used. If dry glass cloth is used, the last layer next to the vacuum bag should be no coarser than 7781 glass. Heavy glass fabrics, such as style 1000, have been known to cause vacuum bag ruptures during cure. The nylon bagging material can be pushed down into the coarse weave of the fabric and rupture. The purpose of the breather is to allow air and volatiles to evacuate out of the lay-up during cure. It is important to place the breather over the entire lay-up and extend it pass the vacuum ports.

The vacuum bag, which provides the membrane pressure to the laminate during autoclave cure, is normally a 3-5 mil thick layer of nylon-6 or nylon-66. It is sealed to the periphery of the tool with a butyl rubber or chromate rubber sealing compound. Nylon vacuum bags can be used at temperatures up to 375° F. If the cure temperature is higher than 375° F, a polyimide material called Kapton can be used to approximately 650° F, along with a silicone bag sealant. Higher temperatures usually require the use of a metallic bag (e.g., aluminum foil) and a mechanical sealing system. It should be noted that Kapton bagging films are stiffer and harder to work with than nylon. Some manufacturers have invested in reusable silicone rubber vacuum bags to reduce cost, and reduce the chance of a leak or bag rupture during cure. These normally require some type of mechanical seal to the tool. Also, if the part is large, reusable rubber vacuum bags become heavy and difficult to handle, so they may require a handling system to facilitate their installation and removal, as shown for the extremely large bag in Fig. 7.28. There are suppliers who sell both the materials to make silicone rubber vacuum bags, or will provide a complete bag and sealing system ready for use.

Caul plates, or pressure plates, can be used to provide a smoother part surface on the bag side. Caul plates are frequently made of mold release coated aluminum, steel, fiberglass, or glass reinforced silicone rubber. They range in thickness from as thin as 0.060 in. up to about 0.125 in. The design of a caul plate and its location in the lay-up are important considerations in achieving the desired surface finish. The caul plate may be placed above the bleeder pack or within the bleeder pack, but close to the laminate surface to provide a smooth surface. However, it will then require a series of small holes (e.g., 0.060-0.090 in. diameter) to allow resin to pass through the caul plate into the top portion of the bleeder pack. It should be noted that a caul plate containing holes is usually not placed next to the laminate surface, because the holes will mark-off on the laminate surface. In general, the further the caul plate is from the laminate surface, the less effective it is in producing a smooth surface, due to the cushioning effect of more and more bleeder material.

Fig. 7.28. Large Reusable Silicone Rubber Vacuum Bag Source: The Boeing Company

The current trend in the composites industry is toward net or near-net resin systems (32-35% resin by weight), which require little or no bleeding, in contrast to the more traditional 40-42% resin systems. Since the labor and cost of the bleeder material is eliminated, this simplifies the bagging system. However, when using this type of material, it is even more important to properly seal the inner bag system, to prevent resin loss during cure, or resin-starved laminates may result. The edges are a particularly critical area, where excessive gaps or leaks in the dams can result in excessive resin loss and thinner-than-desired edges. In addition to the elimination of the need for a bleeder pack, net resin content prepregs produce laminates with a more uniform thickness and resin content. The problem with the traditional 40-42% resin content prepregs is that as the laminate gets thicker (i.e., larger number of plies) the ability to bleed resin through the thickness decreases. As more and more bleeder is added, the plies closest to the surface become overbled, while those in the middle and on the tool side of the laminate are underbled.

Once the vacuum bag has been successfully leak checked and the thermocouples applied, it is ready for autoclave cure. A slight vacuum should be maintained on the bag while it is waiting for autoclave cure to make sure that nothing shifts, or that wrinkles will not form, when the full vacuum is applied in the autoclave. If the lay-up contains honeycomb core, the maximum vacuum that should be applied during leak checking, or cure, is 8-10 in. of mercury vacuum. Higher vacuums have been known to cause core migration, and even crushing, due to the differential pressure that can develop in the core cells.

7.9 Curing

Autoclave curing is the most widely used method of producing high quality laminates in the aerospace industry. An autoclave works on the principle of differential gas pressure, as illustrated in Fig. 7.29. The vacuum bag is evacuated to remove the air, and the autoclave supplies gas pressure to the part. Autoclaves are extremely versatile pieces of equipment. Since the gas pressure is applied isostatically to the part, almost any shape can be cured in an autoclave. The only limitation is the size of the autoclave, and the large initial capital investment to purchase and install an autoclave. A typical autoclave system, shown in Fig. 7.30, consists of a pressure vessel, a control system, an electrical system, a gas generation system, and a vacuum system. Autoclaves lend considerable versatility to the manufacturing process. They can accommodate a single large composite part, such as a large wing skin, or numerous smaller parts loaded onto racks and cured as a batch. While autoclave processing is not the most significant cost driver in total part cost, it does represent a culmination of all the previously performed manufacturing operations, because final part quality (per ply thickness, degree of crosslinking, and void and porosity content) is often determined during this operation.

Autoclavi Vessel

Autoclavi Vessel

Fig. 7.29. Principle of Autoclave Curing

Working Space: 12 ft Diameter x 40 ft Long Heating: Electrical - 3120 kW

Max Temperature: 650° F Gas Movement: 60 000 ft3/min at 600 Rpm

Max Pressure: 150 psi Part Monitoring: 48 Vacuum Supply Outlets

24 Vacuum/Pressure Monitoring Outlets 108 Thermocouple Jack Outlets

Working Space: 12 ft Diameter x 40 ft Long Heating: Electrical - 3120 kW

Max Temperature: 650° F Gas Movement: 60 000 ft3/min at 600 Rpm

Max Pressure: 150 psi Part Monitoring: 48 Vacuum Supply Outlets

24 Vacuum/Pressure Monitoring Outlets 108 Thermocouple Jack Outlets

Fig. 7.30. Typical Production Autoclave Schematic1

Autoclaves are normally pressurized with inert gas, usually nitrogen or carbon dioxide. Air can be used, but it increases the danger of a fire within the autoclave during the heated cure cycle. The gas is circulated by a large fan at the rear of the vessel and passes down the walls next to a shroud containing the heater banks, usually electrical heaters. The heated gas strikes the front door and then flows back down the center of the vessel to heat the part. There is considerable turbulence in the gas flow near the door,15 which produces higher velocities that stabilize as the gas flows toward the rear. The practical effect of this flow field is that you can often encounter higher heating rates for parts placed close to the door; however, the flow fields are dependent on the actual design of the autoclave and its gas flow characteristics. Another problem that can be encountered is blockage, in which large parts can block the flow of gas to smaller parts located behind them. Manufacturers typically use large racks to insure uniform heat flow and also maximize the number of parts that can be loaded for cure.

Composite parts can also be cured in presses or ovens. The main advantage of a heated platen press is that much higher pressures (e.g., 500-1000 psi) can be used to consolidate the plies and minimize void formation and growth. Presses are often used with polyimides that give off water, alcohols, or high boiling point solvents, such as NMP (N-methylpyrrolidone). On the other hand, presses usually require matched metal tools for each part configuration, and are limited by platen size to the number of parts that can be processed at one time. Ovens, usually heated by convective forced air, can also be used to cure composite structures. However, since pressure is provided by only a vacuum bag (^14.7psia), the void contents of the cured parts are normally much higher (e.g., 5-10%) than those of autoclave cured parts (<1%).

7.9.1 Curing of Epoxy Composites

A typical cure cycle for a 350° F curing thermoset epoxy part is shown schematically in Fig. 7.31. It contains two ramps and two isothermal holds. The first ramp and isothermal hold, usually in the range of 240-280° F, is used to allow the resin to flow (bleed) and volatiles to escape. The imposed viscosity curve on the figure shows that the semi-solid resin matrix melts on heating and experiences a dramatic drop in viscosity. The second ramp and hold is the polymerization portion of the cure cycle. During this portion, the resin viscosity initially drops slightly due to the application of additional heat, and then rises dramatically, as the kinetics of the resin start the crosslinking process. The resin gels into a solid and the crosslinking process continues during the second isothermal hold, usually at 340-370° F for epoxy resin systems. The resin is held at this cure temperature for normally 4-6 h, allowing time for the crosslinking process to be completed. It should be noted that as the industry has moved toward net resin content systems, the use of the first isothermal hold, which allows time for resin bleeding, has been eliminated by many manufacturers, resulting in a straight ramp-up to the cure temperature.

Fig. 7.31. Typical Autoclave Cure Cycle1

High pressures (i.e., 100psi) are commonly used during autoclave processing to provide ply compaction and suppress void formation. Autoclave gas pressure is transferred to the laminate due to the pressure differential between the autoclave environment and the vacuum bag interior. Translation of the autoclave pressure to the resin depends on several factors, including the fiber content, laminate configuration, and the amount of bleeder used. Even though a relatively high autoclave pressure (e.g., 100psi) may be used during the cure cycle, the actual pressure on the resin, the hydrostatic resin pressure, can be significantly less.

7.9.2 Theory of Void Formation

Porosity and voids have been one of the major problems in composite part fabrication. As shown in Fig. 7.32, voids and porosity can occur at either the ply interfaces (interlaminar) or within the individual plies (intralaminar). The terms "voids" and "porosity" are used fairly interchangeably in industry; however, the term "void" usually implies a large pore whereas "porosity" implies a series of small pores. Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles.16 Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure), while the resin is a liquid (Fig. 7.33). The prevailing relationship is:

If PVoid > PHydrostatic ^ then void formation and growth

If PVoid > PHydrostatic ^ then void formation and growth

Was this article helpful?

0 0

Post a comment