I. Hot compression moulding
In the hot compression moulding process, the material is heated in an oven to a temperature above the melt temperature (340-430℃, or 645-805℉), rapidly (1-10 s) delivered to the forming mould, compression moulded, and solidified and cooled under pressure (700 - 7000 kPa, or 100 - 1000 psi). As shown in Figure 42, in production, the press-forming die is usually a convex-concave combination of steel or aluminium construction. However, materials such as rubber, wood, phenolic, etc. can be used in the prototyping process. The complete mould can be kept at room temperature throughout the forming-solidifying cycle. However, the use of hot moulds (120-200℃, or 250-390℉) allows control of the cooling rate (avoiding part warpage and controlling the morphology of semi-crystalline thermoplastic prepregs, such as PEEK and polyphenylene sulphide) and extends the moulding window and promotes better lay-up slip.
Figure 42: Hot press equipment
The main disadvantage of this method is that the press applies pressure in only one direction, making it difficult to manufacture parts with complex shapes (e.g., beads, closed corners) or parts with near-vertical supports. As the temperature of the complete set of moulds does not need to be cycled with each part, rapid prototyping times between 10 minutes and 2 hours are achievable with press moulding.
II. Thermocouple (Probe)
A thermocouple (TC) is a thermoelectric device used to accurately measure temperature. It can be connected to a simple temperature reading device or to a thermal link, oven, or other type of controller to regulate heat.A TC consists of a single wire, or two wires of different metals, with one end of the wires connected together. The heated connector generates an electric current that is converted to a temperature reading by a TC monitor. Select a wire type (J or K) and connector type that is compatible with local temperature monitoring equipment (thermal connectors, ovens, autoclaves, etc.) TC wires can be connected to different types of insulated equipment; check the manufacturer's product data sheet to ensure that the insulation is capable of withstanding the highest curing temperatures. Teflon insulated wire is typically suitable for curing at 390℉ (198.89℃) and below; polyimide (Kapton) insulated wire should be used for higher temperatures.
III. Thermocouple Layout
Placement of thermocouples throughout the repair process is critical to obtaining proper cure temperatures. In general, thermocouples used for temperature control should be placed as close as possible to the repair material without embedding them in the repair material or creating indentations during the repair process. They should also be placed appropriately hot or cold to ensure that the material is adequately cured, but not exposed to excessive temperatures that could degrade the structural properties of the material. Thermocouples should be placed as close as possible to the area to be monitored. The following steps should be taken when using thermocouples.
-At least no less than three thermocouples to monitor a heating cycle.
-Place the thermocouples near the centre of the patch if bonding pre-cured patches.
-Control thermocouples can be located in the centre of a low temperature (200°F (93.33°C) or lower) co-cured patch as long as they are placed on top of a thin metal sheet to prevent the thermocouples from indenting into the patch. This allows for more precise control of the patch temperature.
-Thermocouples installed around the repair patch should be placed approximately 0.5' from the edge of the glue line.
-Position spill tape below and above the thermocouple tips to protect them from resin spills and to protect the control unit from power shorts.
-Do not place thermocouples under vacuum ports as the pressure may damage the leads and cause false readings to occur.
-Do not place thermocouple wires next to or across heat transfer blanket power wires to prevent magnetic flux lines from causing false temperature readings.
-Do not place any control thermocouples in a two-inch overlap repair of the heat blanket to prevent the controller from trying to compensate for lower temperatures.
-Always maintain slack in the thermocouple wires under the vacuum bag to prevent the thermocouple from being pulled away from the monitoring area during vacuum application.
IV. Thermal monitoring of the repair area
In order to achieve maximum structural bonded composite repairs, it is essential that these materials are cured within the recommended temperature range. Failure to cure at the correct temperature may produce weak patches or bonded surfaces and may result in failure of the repair in service. Thermal measurements should be taken prior to installation of the repair to ensure that proper and uniform temperatures are achieved. The thermal survey determines the heating and insulation requirements, as well as the location of the TC in the repair area. Thermal surveys are particularly useful for determining heating methods (hot air modules, heat lamps, heat blanket methods and monitoring requirements where radiators are present in the repair area). All types of heating methods should be tested to prevent under-, over- or uneven heating of the restoration area.
V. Temperature variations in the repair area
Temperature variations in the restoration area can have a variety of causes. Chief among these are the type of material, the thickness of the material, and the underlying structure of the restoration area. For these reasons, it is important to understand the structural composition of the area to be repaired. Substructures present in the restoration area direct heat away from the restoration area, resulting in a cold spot directly above the structure. Thin surfaces are quickly heated and can easily overheat. Thicker surfaces absorb heat more slowly and take longer to reach soaking temperature. Thermal measurements can identify these problem areas and allow the technician to develop the heat and insulation settings needed to evenly heat the repair area.
VI. Thermal Measurements
During the thermal measurement process, try to identify possible hot and cold areas in the repair area. Temporarily apply a patch of the same material and thickness, several thermocouples, heating blankets, and a vacuum bag to the repair area. Heat the area and record the thermocouple temperatures after the temperature has stabilised. If the thermocouple temperature varies more than 10 degrees from the average temperature, insulation should be added. Areas with long bands and ribs indicate lower temperatures than the middle of the patch because they act as heat sinks. Add insulation to these areas to increase the temperature. As shown in Figure 43.
Figure 43: Thermal Measurement Legend
VII. Solutions to heat dissipation problems
Additional insulation can be placed in the repair area. This insulation can also extend beyond the repair area to minimise heat being carried away. Ventilation valve material and fibreglass cloth work well, either on top of or inside the vacuum bag or on the accessible backside of the structure. Put more insulation in cold areas and less in hot areas. If you have access to the backside of the repair area, you can place additional heat blankets there to heat the repair area more evenly.
VIII. Types of Overlay Coating
The dry fabric is impregnated with resin during the wet lay-up process. The resin system is mixed before the repair is carried out. Lay the repair on a piece of fabric and impregnate the fabric with resin. After the fabric has been impregnated, the repair layers are cut, stacked in the correct lay-up direction and bagged with a hoover. Wet lay-up repairs are often used with glass fibre for non-structural applications. Carbon fibre and kevlar® dry fabrics can also be used with wet-laid resin systems. Many resin systems are cured with room temperature wet lay-ups, which are easy to finish and the material can be stored at room temperature for long periods of time. The disadvantage of room-temperature wet-layering is that it does not restore the strength and durability of the original structures and components that were cured at 250℉ (121℃) or 350℉ (176.67℃) during the manufacturing process. Some wet layup resins have improved properties using high temperature curing. Generally, the properties of wet layup materials are lower than those of prepregs.
Epoxy resins may need to be refrigerated before use. This prevents the epoxy from deteriorating. The labels on the containers indicate the correct storage temperature for each part. Typical storage temperatures for most epoxy resins range from 40℉ (4.4℃) to 80℉ (26.67℃). Some resin systems require storage below 40℉ (4.4℃).
IX. Prepreg
Prepregs are fabrics or tapes impregnated with resin during the production process. The resin system has been mixed and is in stage B cure. The prepreg is stored in a freezer below 0℉ (-17.78℃) to prevent further curing of the resin. The material is usually placed on a roll and the backing material is placed on one side of the material so that the prepregs do not stick together. This prepreg is sticky and tends to adhere to other layers during stacking. You must remove the prepreg from the freezer and allow the material to thaw, which can take up to 8 hours for a full roll. Store the prepreg in a sealed moisture-proof bag. Do not open these bags until the material is completely thawed to prevent the material from becoming contaminated with moisture.
After the material has been thawed and stacked and removed from the backing material, cut it into repair layers, stack them in the correct lay-up direction and vacuum. Don't forget to remove the backing material when stacking. Cure the prepreg at a higher cure cycle; the most common temperatures are 250℉ (121℃) and 350℉ (176.67℃). Hot press tanks, curing ovens, and hot binders can be used to cure prepregs.
If the part is made from several layers of prepreg, curing is necessary because a large amount of air will be trapped between each layer of prepreg. Remove this trapped air by covering the prepreg with a perforated release film and a breathable layer, and apply a vacuum bag. Vacuum for 10 to 15 minutes at room temperature. Typically, the first layer of consolidated plywood is applied to the tooling surface, and the process is repeated every 3 or 5 layers, depending on prepreg thickness and component shape.
Store the prepreg, film adhesive, and foam adhesive in a freezer at temperatures below 0℉ (-17.78℃). If these materials need to be shipped, place them in special containers filled with dry ice. The freezer must not be an automatic defrost type; the automatic defrost cycle periodically heats the inside of the freezer, which can reduce storage life and deplete the allowable factory time of the composite material. Freezers must be capable of maintaining a temperature of 0℉ (-17.78℃) or below; most home freezers meet this standard. Large freezers can be used for high-capacity cold storage. If the use is small, a box freezer may be sufficient. Freezers are used for storing laminating and pasting adhesives and should be kept near 40℉ (4.4℃). As shown in Figure 44.
Uncured prepregs have time limits for storage and use. The maximum time prepregs are allowed to be stored at low temperatures is called the shelf life and is typically 6 months to 1 year, as shown in Figure 45. The material can be tested and the storage life can be extended by the material manufacturer.
Figure 44: Storage of prepreg materials in a small freezing chamber
Figure 45: Storage life of prepreg materials
The maximum time allowed before the material cures at room temperature is called the mechanical life. The recommended time to complete paving and compaction at room temperature is called the operational life. Operational life is shorter than mechanical life. Mechanical life is measured between the time the material is removed from the freezer and the time the material is returned to the freezer. The operator must record the time in and out of the freezer. Material that exceeds its mechanical life needs to be discarded.
Many maintenance facilities cut materials into smaller kits and store them in moisture-proof bags that thaw more quickly after removal from the freezer. This also diminishes the time it takes for large rolls of material to come out of the freezer.
All frozen prepregs need to be stored in moisture-proof bags to avoid moisture contamination. All prepregs should be protected from dust, oil, vapour, smoke and other contaminants. A clean room is desirable for repair lay-ups, but if one is not available, prepregs should be kept in bags or covered with plastic. Cover the unprotected edge of the prepreg with a dividing membrane before starting the lay-up and clean the repaired area immediately before laying the repair layer.
Prepregs are temperature sensitive. Excessively high temperatures will cause the material to start curing and excessively low temperatures will make it difficult to handle. For aircraft repairs in very cold or very hot climates, the repair area should be protected by a tent around the area. Prepare the prepreg repair layer in a temperature controlled environment and bring it to the repair area immediately prior to use.
X. Co-curing
Co-curing is a process in which two parts are cured at the same time. The interface between the two parts may or may not have a bonding layer. Co-curing usually results in a poorer surface quality of the panel, which can be prevented by using a co-cured secondary overlay material in the standard cure cycle or a subsequent filler homogenisation operation. Co-cured surfaces may also have poorer mechanical properties, requiring the use of lower design values.
A typical co-curing application is the simultaneous curing of stiffeners and skins. Often an adhesive film is placed at the interface between the stiffener and the skin to increase fatigue and peel resistance. The main advantages of the co-curing process are a good fit between the bonding components and the assurance of surface cleanliness.
XI. secondary bonding
Secondary bonding utilises pre-cured composite components to bond two pre-cured composite components together with a layer of adhesive. Honeycomb sandwich components are typically bonded using a secondary bonding process to ensure optimum structural performance. Co-cured laminates on honeycomb cores may have deformed layers that have penetrated into the core cells. As a result, compressive stiffness and strength may be reduced by as much as 10 per cent and 20 per cent, respectively.
Pre-cured laminates that have undergone secondary bonding usually have a thin layer of nylon or fibreglass adhesive on the bonding surface. Although the peel-off layer sometimes hinders non-destructive inspection of pre-cured laminates, it has been found to be the most effective method of ensuring a clean surface prior to bonding. Once the peel layer is removed, a pristine interface is obtained. Light abrasive sanding removes excess resin impressions from the peeled layer fabric, which, if broken, can create cracks in the bondline.
Composites can be used for structural repair, restoration, or reinforcement of aluminium, steel and titanium components. Combining composite reinforcements has the ability to slow or stop the expansion of fatigue cracks, replace structural areas lost due to corrosion abrasion, and structurally strengthen small and negative edge areas. This technique is commonly used in conjunction with metal bonding and composite bonded repairs on conventional aircraft. Boron prepreg tapes with epoxy resins are most commonly used for this application.
XII. Co-bonding
In co-bonding, one of the parts is pre-cured and the matching part is cured at the same time as the adhesive. Film adhesives are often used to improve peel strength.
To be continued
Source "Composites Frontier" Public Website