Amts-swp-0016-a-vacuum Bagging

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Vacuum Bagging AMTS-SWP-0016-A-2008 AMTS STANDARD WORKSHOP PRACTICE _________________________________________ Vacuum Bagging Reference Number: AMTS_SWP_16_2008 Date: July 2008 Version: A 1 Vacuum Bagging AMTS-SWP-0016-A-2008 Contents 1 Technical terms .................................................................................... 3 2 Scope ..................................................................................................... 3 3 Primary References .............................................................................. 4 4 Advantages of using vacuum bagging ............................................... 4 5 Typical applications.............................................................................. 5 5.1 Considerations .............................................................................................5 6 Tools and materials required............................................................... 6 6.1 Hardware......................................................................................................6 6.1.1 Vacuum pump ................................................................................6 6.1.2 Fittings and connectors..................................................................7 6.1.3 Vacuum gauge ...............................................................................7 6.1.4 Vacuum relief .................................................................................8 6.2 Consumables ...............................................................................................8 6.2.1 Release agent ................................................................................8 6.2.2 Bag sealant tape ............................................................................9 6.2.3 Peel ply...........................................................................................9 6.2.4 Release film ...................................................................................9 6.2.5 Bleeder material ...........................................................................10 6.2.6 Bag ...............................................................................................10 7 Bagging method ................................................................................. 11 8 Advanced vacuum processes ........................................................... 12 8.1 Vaccum bagging with heating....................................................................12 8.2 RIFT (Resin infusion under flexible tooling) process ................................13 8.3 Autoclave moulding (with prepregs) ..........................................................13 8.4 Repairs .......................................................................................................13 8.5 Multiple bag and multiple port applications ...............................................14 8.6 Vacuum moulding presses ........................................................................14 2 Vacuum Bagging AMTS-SWP-0016-A-2008 1 Technical terms Atmosphere: At sea level, the uniform pressure by air surrounding the earth equals 101.3 kPa (10130 kg/m2). Vacuum bagging adds one atmosphere of pressure to a system. Autoclaves: A pressurizing device used to elevate pressure and/or temperature within a sealed container part of the machine itself. Bag: Made from simple polymer film or fabric reinforced with a rubber coating. It may enclose the entire part, being open at only one end, or alternatively only single-sided to seal the outer face of laminate within the mould. Bridging: Instead of following the contour of a curved mould, a material pulls taut and away from the mould, “bridging” the gap. Curing: A chemical reaction whereby the polymer chains interlink to form a three dimensional network of covalent bonds. Laminate: Material made by joining multiple material layers by some means. This can form a composite material part with widespread uses. Prepreg: Reinforcement fabrics preimpregnated with a unique, partly cured resin Resin: Synthetic resins are viscous materials having thermosetting properties or are capable of hardening. 2 Scope The use of composite materials offers immense strength-to-weight advantages in any application. On its own, the reinforcement (be it fibreglass, aramide, carbon etc.) is not particularly strong in the textile state. Thermosetting resins on the other hand, such as polyester and epoxy, are quite brittle if cured without any reinforcement. Vacuum bagging does not have to be an extremely complex process. A manufacturer who would like to improve the strength-to-weight ratio of a part will generally profit from a simple vacuum bag construction. As the process is refined or expanded, the project may become more and more complex or if larger parts are made, the technique may become quite sophisticated. These elaborate techniques unfortunately tend to scare fabricators from using this helpful and effective approach to improving almost any hand laminates. Summarized, features of vacuum bagging methods include: low- to medium capital expenses; less labour-intensive than with hand laminates; greatly increased mechanical characteristics of finished parts. 3 Vacuum Bagging AMTS-SWP-0016-A-2008 3 Primary References J.S.U. Jonker & J.P. Schümann, Training Manual – Composites, Jonker Sailplanes CC, 2007. J.F.A. Kessels, Modelling the Resin Infusion under Flexible Tooling process, 2006. R&G GmbH, Komplett Faserverbundwerkstoffe Handbuch, 8th Ed. U.S. Department of Transportation, Aircraft Inspection, Repair & Alterations, 1998. 4 Advantages of using vacuum bagging Vacuum bagging is a technique employed to create mechanical pressure on a laminate during its curing cycle. This serves several purposes: • Removes trapped air between laminate layers. • Compacts the fibre layers for more efficient force-transmission amongst fibre bundles • Prevents shifting of fibre orientation throughout the curing process. • Reduces humidity. • Optimizes the fibre-to-resin ratio in the composite part (discussed in further detail later on). • To compress layers with foam cores into complex and curved shapes, for example an aircraft wing profile. Hand laminated moulds can be significantly strengthened by vacuum bagging. This is a result of the increased fibre-to-resin content, as well as the elimination of trapped air, which might have weakened certain points in the finished part. Simple or small shapes may be formed in moulds by applying weights. When moulding complex or large parts, vacuum bagging has several advantages over using weights. • • • • • Pressure is more evenly applied during the formation process, in stead of small highpressure points. Higher pressures can be achieved than when using weights. A uniform part thickness is obtained in the finished product. Less air is trapped within the laminate, which otherwise might have resulted in rejection. The repeatability of the process is enhanced when many conforming parts are to be made. 4 Vacuum Bagging AMTS-SWP-0016-A-2008 5 Typical applications With the numerous advantages from vacuum bagging in mind, these are a few typical applications where vacuum bagging will be beneficial: • • • • • • • General wet lay-up parts to improve quality Complex shapes Structures with core materials Repairs RIFT (Resin infusing under flexible tooling) process Autoclave with prepregs Vacuum moulding machines 5.1 Considerations The following aspects should be considered when deciding whether or not to use vacuum bagging in the manufacturing process: 1. Curing time: The time necessary for the cure cycle to complete is dependant mainly on the resin used and the curing temperature. If heat is applied to accelerate the curing process, the use of vacuum bags is normally required to prevent bubbles from forming when trapped air expands. 2. Production rate: When a high production rate is needed for complex shape laminate parts, vacuum bagging is a good possibility. For small complex parts, double-moulds and clamping is faster than vacuum bagging. 3. End quality of part: Vacuum bagging will provide a more uniform spread of resin, as well as increase the fibre-to-resin content throughout a part. This makes vacuum bagging a better process to improve the aesthetic as well as mechanical quality of parts, when compared to normal hand lay-ups. 4. Complex shapes In complex moulds the fabric may not want to follow sharp contours. Vacuum bagging the part will resolve this issue. 5. Removal of trapped air: When hand lay up techniques are used the chances are good some air pockets or bubbles may become caught within the laminate. This air is then trapped in the structure after hardening and will weaken the part in these areas. When strength and resilience are critical factors, it is important to eliminate these air pockets to guarantee requirements are met. Vacuum bagging the part will also resolve this issue. 5 Vacuum Bagging AMTS-SWP-0016-A-2008 6. Extended lifetime of mould: In some cases vacuum bagging methods may extend the lifetime of costly moulds, when compared to other techniques. This should be well though over if a long service life is expected from any mould. 7. Resin-to-fibre content: There are a few considerations to be made regarding the ratio of fibre to resin content in the finished part. Vacuum bagging guarantees a low resin content per fibre weight. A typical percentage can be around 60% resin-to-fibre and values as low as 40% may be achieved with a RIFT-process. 8. Cost: When a few compromises are made with regard to the materials used in the bagging process, vacuum bagging can be very cost effective and economically viable, especially when the high production rate is also taken into account. Should the RIFT-process be used, the initial capital expenditures will increase. A decent cost-analysis should be done in most cases where a choice has to be made between methods, but is beyond the scope of this manual. 6 Tools and materials required Vacuum bagging requires specific hardware and tools as well as consumable materials as listed below: 6.1 Hardware Together with the consumable materials (discussed in 6.2) the following hardware items are essential when building a vacuum bagging system. 6.1.1 Vacuum pump The first step in creating a successful vacuum bagging system is to select a quality vacuum pump. The pumps are typically rated by the horsepower (or watts) of the motor, type of pumping mechanism (rotary vane, diaphragm, piston, etc.,) the volume of air displaced and the maximum % attainable vacuum. The following should be matched with that of the pump for optimum results: • • • Bag size Desired rate to establish vacuum Ultimate vacuum pressure While a smaller pump can often achieve a desired vacuum level on a large system, it will take much longer to attain that level. Moreover, if a leak is present in a system which exceeds the maximum volume displacement of the pump being used, it will be difficult to achieve even a partial vacuum. 6 Vacuum Bagging AMTS-SWP-0016-A-2008 Note: If a pump cannot achieve full vacuum in less than five to eight minutes, the pump is probably too small. Also, by squeezing as much air out of the bag as soon as possible before sealing the bag and applying the vacuum, the work needed by the pump can be greatly reduced. As a rule of thumb, determining the correct pump size for a specific application should be based on the area of the mould being surrounded by the bag. One typical pump that may be used for vacuum bagging purposes is a two stage rotary vane pump (able to handle low viscosity, non-lubrication liquids – such as air) rated at around 400 Watts for, medium duty applications. 6.1.2 Fittings and connectors The next step is connecting the pump to the bag itself. This is done with: • • • Tubing Valves Connectors Tubing: The primary consideration for tubing is its ability not to collapse whilst vacuum is applied. A connector between the tubing and the pump is required as well as a means of fitting the tubing into the bag. Pump fitting: A vacuum port (also referred to as a nozzle) is used to connect the pump to the bag assembly itself. The port can be fitted to the bag or mould, depending on the use. • • • NB: If the port is located above the part itself, several layers of bleeder material (see 5.3.5) should be placed beneath the port to prevent resin being sucked into the port and enhances airflow to the port. If an area for the port has been left on the mould, there should be a bleeder path(s) leading to the part. Take care not to damage the bag when making a hole for the port. It is important to ensure all connections are sealed as tight as possible to maintain the vacuum. 6.1.3 Vacuum gauge The primary reasons for adding a vacuum gauge are to: • • • Monitor the vacuum level Detect leaks Confirm that leaks have been eliminated 7 Vacuum Bagging AMTS-SWP-0016-A-2008 Another scenario is where one pump is used to apply vacuum to more than one bag. It is often helpful to know or vary the vacuum pressure within an individual bag. A gauge and valve should then be attached before each bag. Note: Gauge readings enable manufacturers to keep accurate records of the vacuum pressures achieved during the fabrication of a part. Accurately recorded data on the time elapsed at each of these vacuum pressure level readings enables precise duplication and fine tuning of the fabrication process. Some pressure will be lost in the system due to frictional air resistance, and the gauge’s remote location will then only record the effective pressure reaching the bag. In a system employing multiple bags, these gauges can measure the pressures in each one of the bags independently. This is especially helpful when the bags have to operate at different vacuum levels. 6.1.4 Vacuum relief To allow for pressure relief, a bleeder valve assembly would be installed. At the same time, it can be used as connector for the vacuum gauge, instead of a T-attachment. This permits adjustment to the vacuum, in addition to gauge readings. As an example, it is sometimes beneficial to only create a partial vacuum during the initial stages of the cure cycle, and then gradually increase it. Note: If full vacuum is applied before the resin begins to gel, too much resin may be pulled from the laminate. This could leave a dry, resin-starved surface resulting in an inherent weak spot within the finished laminate. So a partial vacuum during this early phase allows proper compaction of the composite layers Full vacuum is delayed until the thin resin section begins to gel. Excess resin will still be bled off after this point. 6.2 Consumables Each vacuum bagging process involves using a few materials which are discarded after use. They are briefly explained below. A few select materials may be used more than once. The bag, for example, may be reused when making the next part, if thoroughly cleaned of any excess resin and checked for punctures. 6.2.1 Release agent Prior to laying up the part, a release agent should be applied to the mould. This may be one of the following: • • • Liquid coating Wax based coating Solid barrier (e.g. Teflon tape) 8 Vacuum Bagging NB: AMTS-SWP-0016-A-2008 It is important to mask off the mould’s edges well before applying release agent, otherwise the bag sealant tape might not stick to the mould properly. 6.2.2 Bag sealant tape Sealant tape has the consistency of putty and is usually bought as rolls, with release paper on one side. Steps when applying tape: • • • Roll down the required length of tape Press it against the mould (edge) Do not remove the release paper until ready to apply the bag. Tape is usually applied once the part is laid up, particularly with a wet lay-up. 6.2.3 Peel ply Peel plies are a firmly woven fabric (frequently nylon) and impregnated with a type of release agent. This allows it to not only stick to the laminate, but also pull away easily. Peel ply is optional and results in a coarse finish. Note: Peel ply will absorb a small amount of resin, and should be specified in the datasheet. This should be taken in account to ensure the laminate does not come out too dry. 6.2.4 Release film Depending on the specific application, release film may or may not be necessary. We assume it will be used, referring to fig. 7-1. The release film will be some type of thin plastic, usually treated not to bond with the laminate. When using peel-ply this is prevented from happening altogether. Ensure this film is highly stretchable when moulds have complex curving geometries. This film can also be bought with or without perforations, and the spacing of holes on perforated release films can vary. Those with more frequent holes will increase the amount of and rate at which resin is bled out from the laminate. There are a few exceptions where release film is incompatible with certain resin systems. Read the specifications of the film being applied carefully, to establish whether compatible with the particular resin being used. 9 Vacuum Bagging AMTS-SWP-0016-A-2008 6.2.5 Bleeder material For a bleeder material, hessian / burlap can be used. Generally this bleeder cloth should be thick and feel felt-like. At least one layer of this material is placed on top of the release film. This layer serves two purposes: • • It absorbs excess resin seeping through the porous release film It provides clear path for airflow in order to maintain the vacuum Where the bag would otherwise trap air where it wrinkles against the saturated laminate, the cloth will prevent this from happening. NB: It is a good idea to make this layer thick enough not to become fully saturated with the excess resin. This will prevent resin from coming in contact with the bag, making it easily removable and even reusable. Note: Even though the initial fibre-to-resin content is chosen at, say 60%, and the correct amount of resin is mixed and applied, it should be remembered that this value may decrease as a result of resin being soaked up in the bleeder material. A lower percentage will mostly be beneficial, but this should be kept in mind nonetheless. 6.2.6 Bag The final item to be placed is the bag, made from a relatively thick plastic layer or reinforced cloth sealed with rubber. Points to follow when applying the bag: • • • • Apply the bag along one edge at a time, starting from the corners. Gradually remove the release paper form the sealant tape as the bag is applied all along the edges. Ensure no wrinkles are present along the edges to prevent leakage. Attach the vacuum port fitting before closing the bag completely. Note: If possible, moulds should be designed to include flanges if a one-sided bagging method is used. This ensures that the bag can be sealed and removed easily, saving time and trouble. 10 Vacuum Bagging AMTS-SWP-0016-A-2008 7 Bagging method The figure below shows the basic structure of a general vacuum bag assembly. The vacuum port / nozzle, tubing, gauge and pump are not shown. Perforated release film Bag Bleeder material Laminate Peel Ply Sealant tape Release agent Mould Figure 7-1: Schematic representation of vacuum bagging. Preparation: If the one-sided bagging method is used (where the mould acts as the other sealing surface), it is advisable to cut the required layers in advance. This saves time during the actual bagging procedure. If the sealed bag method is used, the fitting of the female mould inside the vacuum bagging assembly should be checked before the actual lay up. Peel ply layer: Peel ply is normally draped over any portion of the mould coming in contact with the resin. If the inside surface has to be painted after moulding, or another layer bonded to it, a peel-ply layer provides a good finishing surface. Peel-ply will also allow excess resin to seep through it and be absorbed in the bleeder material. If a smooth, finished surface is desired, the peel ply layer may be omitted and the release film may be applied directly onto the part. Release film: The release film ensures that the bleeder layer releases easily from the part or the peel ply. The type of release layer will vary, depending upon the desired surface texture and resin content the finished component should have. 11 Vacuum Bagging AMTS-SWP-0016-A-2008 Note: While perforated release film will allow surplus resin to pass through, non-perforated release film retains the resin in the moulded part. In order to guarantee that the material will not bridge or be pulled taught across corners, leave some excess folds of loose release material in any deep or hard to reach portions of the mould. A gap would leave an air pocket below the material. If the part is too complex to have the release draped evenly, however, sections may be cut and applied individually to the contours. Nevertheless, the layers must overlap at all joints to ensure adequate protection. Bleeder layer: An important layer to be added next is the bleeder/breather material, and it provides two vital functions: • • The material absorbs any excess resin from the laminate (when used in conjunction with perforated release film). This layer insures that the vacuum is evenly distributed within the bag. By rolling or folding the breather material into tight areas, the air flow within the bag can be directed, thus ensuring vacuum is achieved on edges far from the pump inlet. The breather layer can also be used to apply pressure in hard to reach areas of a mould, by placing rolled or folded sections deep into sharp angles or troughs. It is important to have excess breather cloth in all tight radii to prevent bridging. NB: The bleeder/breather layer should extend all the way to the point where the vacuum couplings are located. Here a triple layering will guarantee the bag union not to seal itself from the part. 8 Advanced vacuum processes 8.1 Vaccum bagging with heating In addition, a heating blanket may be added to the pile. This allows heating of the part during the vacuum bagging and curing process. Placing the bagged part in an autoclave will provide both heat and increased pressure. This in turn allows pressures up to 300kPa and temperatures close to 200°C in general. It should be noted that monitoring devices such as gauges and thermo couples are needed to monitor pressures and temperature against time during the cure cycle to make sure curing parameters are met. 12 Vacuum Bagging AMTS-SWP-0016-A-2008 8.2 RIFT (Resin infusion under flexible tooling) process This method more or less involves the same steps as used with conventional vacuum bagging. The only difference being resin is fed to the bag only after it is under vacuum. No resin is added beforehand and this in turn means no porous film or absorbent material has to be added. This not only ensures that very little excess resin is used, but also saves the costs of not using the extra materials in the bag and resin wastage. Figure 7.2-1: Schematic representation of the RIFT-process. 8.3 Autoclave moulding (with prepregs) Autoclaves can both elevate the temperature and pressure surrounding the part and/or mould while curing, resulting in parts with high fibre-to-resin content. This process is mostly applied when prepregs are used. Because prepregs are expensive and require stable and exact Companies who insist on maximum fibre-to-resin content use autoclaves to mould parts that have to meet the highest requirements in their field or industry (aircraft, motor sport, high performance boats etc.) Generally, the applied pressure can be increased to two or three atmospheres when using this method. While this can be quite desirable, autoclaves are expensive and not readily available to the general public. 8.4 Repairs Localized repairs can easily be done to a part by means of vacuum bagging. Repair plies of preimpregnated laminate can be used together with an adhesive layer placed between the laminate and part that is to be repaired, for example. The inclusion of a heating blanket further extends the possibilities of utilising vacuum bagging methods for a wide range of repair purposes. The use of vacuum bagging for repairs is discussed in further detail in the SWP on Composite repairs. 13 Vacuum Bagging AMTS-SWP-0016-A-2008 8.5 Multiple bag and multiple port applications As already mentioned, it is possible to have more than one bag working from a single pump at a time. It is also possible (and usually necessary on long moulds like those for glider wings) to have more than one port going into a single bag. A fitting is then inserted between the pump and the tubing connections. Multiple vacuum couplings, gauges, etc. completes the setup. 8.6 Vacuum moulding presses A vacuum moulding press allows manufacturers to run a refined process of vacuum bagging where most steps are automated. This is useful in the automotive industry where composite parts have to be mass produced or in other applications such as the moulding of reinforced rubber boots and laminating chipboards. Advantages of using presses include proper cavity filling when using rubber or silicon and better curing of laminates through the addition of heated press platens, thus providing both heat and pressure by the same means. Presses are mainly used when: • • The fibre-to-resin content should be exceptionally high Lightweight support materials (like honeycombs) have to be joined with laminate overlays, producing a highly rigid core-structure part. 14 Vacuum Bagging AMTS-SWP-0016-A-2008 Appendix 1 Atmosphere explained Vacuum bagging adds one atmosphere of pressure to a system. Although the technique is relatively inexpensive and easy to perform, it is worthwhile to understand the principle of atmospheric pressure and how it is measured, enabling manufacturers to effectively diagnose and remedy faulty systems. The atmosphere that surrounds the earth can be considered a reservoir of low pressure air. This low pressure air has some weight which exerts a uniform force that varies with temperature, humidity, and altitude. At sea level, that uniform pressure equals 101.3 kPa or 10130 kg/m2 in terms of force. As the name implies, vacuum bagging is a technique which creates a vacuum; an airtight barrier between the open atmosphere and the closed system. When a vacuum is created within this bubble by means of a pump, the atmospheric air presses down on the outside skin and laminate within. This is because the pressure inside the bag decreases while the outside pressure remains at 101.3 kPa (at sea level) or approximately 85kPa at 4500ft AMSL. A pressure differential develops between the closed system within the bag and the open atmosphere. It is this pressure differential which provides the uniform mechanical “clamping force” which is desired from this technique. As a result, even complex shapes may be clamped at pressures approaching the 101 kPa figure of perfect vacuum. The best pressure that can be achieved through vacuum bagging in Gauteng is close to 8.5 metric tons per square meter. At sea level this will increase up to 10 tons/m2. 15