I. Hand tools
Both prepreg and dry fibres can be cut with hand tools such as scissors, pizza cutters and knives. Materials made from Kevlar are more difficult to cut than glass or carbon fibres and tools wear out more quickly. A rubber scraper and a brush are used to impregnate the dry fibre with the resin wet layup. Markers, rulers and a circular template were used to create the restoration layout. This is shown in Figure 31.
Figure 31: Hand laminating tools for lay-ups
II. Air-driven tools
Air-driven power tools such as drill motors, routers and grinders are used for composites. Electric motors are not recommended as carbon fibre is a conductive material and can cause short circuits. If power tools are used, they need to be of the fully enclosed type. As shown in Figure 32.
Figure 32: Pneumatic tools for composite repairs
III. Tire Film Plate
An aluminium sheathing plate is usually used to support the part during the curing process. A mould release agent, or parting film, is applied to the formwork so that the part does not adhere to the formwork. Thin coated panels are also used at the top of the repair when a thermal binder is used. The coated sheet provides a more uniformly heated area, which finishes with a smoother composite laminate.
IV. Supporting Tools and Moulds
Some repairs require tools to support the part/ or to maintain the surface profile during curing. A variety of materials can be used to make these tools. The type of material depends on the type of repair, the curing temperature and whether it is a temporary or permanent tool. Oven and hot press tank curing require supporting tools due to the high curing temperatures. If support tools are not used, the part will warp. There are many types of tooling materials available. Some are moulded to a specific part profile and others are used as rigid supports to hold the profile during curing. Plaster is an inexpensive and convenient material for contouring. It can be filled with fibreglass, hemp or other materials. Plaster is not very durable, but can be used as a temporary tool. A layer of glass fibre reinforced epoxy resin is usually applied to the side surface of the tool to improve the quality of the finish.
Moulding resins are used to impregnate glass fibre, carbon fibre or other reinforcing materials to make permanent tools. Complex parts are made from metal or high-temperature mould plates and machined on 5-axis CNC equipment to make basic tools that can be used to manufacture aircraft parts. This is shown in figure 33/34.
Figure 33: 5-axis CNC tool and mould making equipment
Figure 34: Air intake unit mould
V. Vacuum bagging materials
Repair of composite aircraft components is usually carried out using the vacuum bag pressurisation technique. A plastic bag is sealed around the repair area. Air is then pumped out of the bag so that the repair layers are pulled together and no air is left in between. The atmospheric pressure creates a strong and secure bond during the repair process.
Several process materials are available for vacuum bagging parts for use. These materials do not form part of the repair and are discarded at the end of the repair process.
VI. Release Agents
A release agent, also known as a film tool release agent, allows the cured part to be easily removed from the mould or tyre plate.
VII. Separation Plate
The separator layer creates a path for air and volatiles to escape from the repair. Excess resin is collected in a separator. The separator material can consist of a layer of glass fibre, non-woven polyester, or can also be a perforated Teflon (Teflon)® coated material. The Structural Repair Manual (SRM) describes what type and how many layers of separation are required. Generally, the thicker the laminate, the more layers are required.
VIII. Surface Laminates
Surface laminates are typically used to create a clean surface for bonding purposes. A thin layer of glass fibre is cured with the repaired part. Just before the part is attached to another structure, the skin layer is removed. The skin layer is easily removed and leaves a clean surface for bonding. The skin layers are manufactured from polyester, nylon, fluorinated ethylene propylene (FEP), or coated fibreglass. If they overheat, they can be difficult to remove. Some coated skins can leave undesirable contamination on the surface. The preferred stripping material is polyester, which can be heat cured to eliminate shrinkage.
IX. Adhesive Tape
Vacuum bag sealing tape, also known as duct tape, is used to seal vacuum bags to parts or tools. Always check the temperature rating of the adhesive tape before applying it to ensure that you are using the proper temperature rating adhesive tape.
X. Porous Release Film
Porous release film is used to allow air and volatility at the repair, which prevents the release layer from sticking to the part or repair. Different sizes of holes and hole spacing are available depending on the required discharge volume.
XI. Solid Separation Membrane
The use of solid separating film prevents the prepreg or wet layer from sticking to the work surface or the cladding plate. If solid separation film is used, it also prevents the resin from leaking and damaging the heat blanket or coating plate.
XII. Air-permeable materials
The breathable material described is used to provide a path for air to exit the vacuum bag. The breathable material must be in contact with the separator. Typically, polyester fibre is used in weights of 4 oz. or 10 oz. 4 oz. is used for applications below 50 pounds per square inch (psi) and 10 oz. is used for 50 - 100 psi.
XIII. Vacuum Bags
Vacuum bag materials provide a tough layer between the repair and the air.
Vacuum bag materials are available in a variety of temperature ratings, so be sure that the material used for the repair can handle the curing temperatures. Most vacuum bag materials are single-use, but those made of flexible silicone rubber are reusable. Two small cuts are made in the bagging material to allow for the installation of the vacuum probe valve. Vacuum bags are not very flexible, and if you want to fit complex shapes, you need to make layers in the bag. Sometimes, envelope type bags are used, but the disadvantage of this method is that the vacuum pressure may crush the parts. Reusable bags made from silicone rubber are more flexible. Some have built-in heating blankets that simplify the bagging task. This is shown in figures 35/36/37.
Figure 35: Packaging materials
Figure 36: Bags for complex parts
Figure 37: Self-sealing vacuum bags for heating elements
XIV. Vacuum equipment
Vacuum pumps are used to draw air and volatiles from the vacuum bag to make atmospheric pressure fixed laminates. Specialised vacuum pumps are used in repair shops. For aircraft repairs, mobile vacuum pumps can be used. Most hot bonding adhesives have a built-in vacuum pump. Special air hoses are used as vacuum lines because ordinary air hoses can be flattened during vacuum. Vacuum lines used in ovens or autoclaves need to be able to withstand the high temperatures in the heating unit. Vacuum pressure regulators are sometimes used to reduce vacuum pressure during bagging.
XV. Vacuum Compaction Table
A vacuum compaction table is a handy tool for breaking down multi-layer composite laminates. The compaction table is essentially a reusable vacuum bag consisting of a metal table top with a hinged lid. The lid includes a sturdy frame, a flexible membrane, and a vacuum seal. The repair layer is placed on the table surface and sealed under the lid with a vacuum to remove trapped air. Some of the compacted tables were subjected to heat, but most were not.
XVI. Oven
Composites can be cured in an oven using a variety of pressure application methods. Typically, a vacuum jacket is used to remove volatiles and trapped air and cured using atmospheric pressure, as shown in Figure 38. Another method of pressure application for oven curing is the use of shrink wrap or shrink tape. The oven cures the material system using hot air circulating at high speeds. Typical oven cure temperatures are 250°F (121°C) and 350°F (176.67°C). The oven has a temperature sensor that feeds temperature data back to the oven controller. The oven temperature may differ from the actual part temperature depending on the location of the oven sensor and the position of the part in the oven.
Figure 37: Double-door curing oven
The thermal mass of the part inside the oven is usually greater than the thermal mass of the surrounding oven, and the temperature of the part will lag the oven temperature by a significant amount during the warming process. To handle these differences, at least two thermocouple (chemistry) must be placed on the part and connected to a temperature sensing device (separate chart recorder, thermal binder, etc.) located outside the oven. Some oven controllers can be controlled by thermocouple (chemistry) placed on the repair part.
XVII. Hot Press Tanks
Hot Press Tank systems allow complex chemical reactions to occur within a pressure vessel to treat a wide variety of materials based on specified time, temperature and pressure distributions. As shown in Figure 39, variations in materials and processes increase the operating conditions of high-temperature hot press tanks from 120°C (250°F) and 275 kPa (40 psi) to over 760°C (1400°F) and 69,000 kPa (10,000 psi). High-temperature hot press tanks operating at lower temperatures and pressures may be pressurised with air, but if higher temperatures and pressures are required for the curing cycle, a mixture of air and nitrogen at a ratio of 50/50 or 100% nitrogen should be used to reduce the incidence of high-temperature hot press tank fires.
Figure 39: High Temperature Hot Press Tank
The main components of an autoclave system are: a vessel to hold the pressure, a source to heat the gas stream and circulate it uniformly within the vessel, a subsystem to apply a vacuum to the parts covered by the vacuum bag, a subsystem to control the operating parameters, and a subsystem to load the moulds into the autoclave. Modern autoclaves are controlled by computers that allow the operator to program and monitor all types of curing cycles. The most accurate way to control the curing cycle is to control the hot press tank controller with thermocouples placed on the actual part.
Most parts processed in the hot press tank are covered with a vacuum bag that is used primarily to compact the laminate and provide a path for removal of volatiles. The vacuum bag allows the part to exist with a differential pressure outside the hot press without direct exposure to the hot press environment. Vacuum bags are also used to apply varying degrees of vacuum to parts.
XVIII. Thermal Binders and Heat Lamps
Typical on-board heating methods include resistance heat blankets, infrared heat lamps, and hot air devices. All heating devices must be controlled in some way so that the correct amount of heat can be applied. This is particularly important for repairs using prepregs and adhesives, where controlled heating and cooling rates are often specified.
XIX. Thermal connectors
A thermal connector is a portable device that automatically controls the heating based on temperature feedback from the repair area. The thermal binder also has a vacuum pump to supply and monitor the vacuum equipment in the vacuum bag. The thermal binder controls the curing cycle with thermocouples placed near the repair. Some repairs require up to 10 thermocouples. Modern thermal bonders can run many different types of curing programmes and the curing cycle data can be printed out or uploaded to a computer. This is shown in Figure 40.
Figure 40: Hot-linking equipment
XX. Heat blankets
A heat blanket is a flexible heater. It is made of two layers of silicone rubber with a metal resistance heater between the two layers. Heat blankets are a common method of providing heat for aircraft maintenance. Heat blankets can be manually controlled; however, they are usually used with a thermal binder. Heat is transferred from the blanket by conduction. Therefore, the heat blanket must conform and be in 100% contact with the part, which is usually accomplished by vacuum bag pressure. As shown in Figure 41.
Figure 41: Hot Blanket
XXI. Heat lamps
Infrared heat lamps can be used to cure composites at high temperatures if vacuum bags are not used. However, they are usually ineffective when curing temperatures exceed 150 degrees Fahrenheit or when the area exceeds 2 square feet. It is also difficult to control the heat with a lamp, and the lamps tend to produce high surface temperatures very quickly. Heat lamps can be used to apply curing heat to large or irregular surfaces if controlled by a thermostat. Thermal binders can be used to control the heat lamp.
XII. Hot Air Systems
Hot air systems can be used to cure composite repairs and are primarily limited to small repairs and dry repair areas. After vacuum bagging is complete, a heat generator provides hot air directly into an insulated enclosure set around the repair area. The hot air surrounds the repair with a uniform temperature rise.
To be continued
Source "Composites Frontier" Public Website