COMPONENT FORM AND MANUFACTURE 123 There are two established approaches to automating the lay-up process: automated tape layers (ATL)and automated tow placement (ATP)machines. ATL machines normally consist of a gantry with a dispensing head that is free to move over the surface of the tool.Generally,unidirectional pre-preg tape is placed onto the surface (Fig.5.5)according to a programmed routine. As the tape is placed on the surface,the backing layer is stripped away,and the surface of the tool may be heated to aid tack of the pre-preg.Tape width is typically around 300 mm,and the lay-down rate is of the order of 50 m min. Advanced ATLs are capable of laying tape onto a highly contoured surface. However,these machines are very costly and can be justified only where long runs of expensive components,such as tail or wing skins,are to be made. ATLs are also being developed for use with thermoplastic pre-pregs.In this application,a gas flame or laser is used to heat the tape as it is laid down and a consolidation roller is then used to form the composite layer. The limitations to the capability of ATL machines to manufacture more complex shapes has led to the development of automatic tow placement (ATP) systems.These machines lay down multiple pre-preg tows and are able to stop, cut,and restart individual fiber tows.A multi-axis manipulator arrays a group of pre-preg tows into a continuous band and compacts them against the surface of the lay-up tool.This allows more complex shapes to be fabricated,including lay- up onto relatively severe and complex curves and the steering of tows into curved trajectories.Heat and pressure are used to ensure proper adhesion and consolidation of the material. ATPs offer the potential for greater structural optimization by locating fiber where it is most effective.Some systems are combined with a spindle,(Fig.5.6) to allow lay-up of closed shapes such as ducts,combining the advantages of both filament winding and automated tape lay-up while alleviating some of the problems associated with each.However,these are,so far,even more expensive to purchase and operate and have been limited to use on military aircraft Fig.5.5 Schematic diagram of an automatic tape-laying process (left)and a typical product(right)
COMPONENT FORM AND MANUFACTURE 123 There are two established approaches to automating the lay-up process: automated tape layers (ATL) and automated tow placement (ATP) machines. ATL machines normally consist of a gantry with a dispensing head that is free to move over the surface of the tool. Generally, unidirectional pre-preg tape is placed onto the surface (Fig. 5.5) according to a programmed routine. As the tape is placed on the surface, the backing layer is stripped away, and the surface of the tool may be heated to aid tack of the pre-preg. Tape width is typically around 300 mm, and the lay-down rate is of the order of 50 m min- 1. Advanced ATLs are capable of laying tape onto a highly contoured surface. However, these machines are very costly and can be justified only where long runs of expensive components, such as tail or wing skins, are to be made. ATLs are also being developed for use with thermoplastic pre-pregs. In this application, a gas flame or laser is used to heat the tape as it is laid down and a consolidation roller is then used to form the composite layer. The limitations to the capability of ATL machines to manufacture more complex shapes has led to the development of automatic tow placement (ATP) systems. These machines lay down multiple pre-preg tows and are able to stop, cut, and restart individual fiber tows. A multi-axis manipulator arrays a group of pre-preg tows into a continuous band and compacts them against the surface of the lay-up tool. This allows more complex shapes to be fabricated, including layup onto relatively severe and complex curves and the steering of tows into curved trajectories. Heat and pressure are used to ensure proper adhesion and consolidation of the material. ATPs offer the potential for greater structural optimization by locating fiber where it is most effective. Some systems are combined with a spindle, (Fig. 5.6) to allow lay-up of closed shapes such as ducts, combining the advantages of both filament winding and automated tape lay-up while alleviating some of the problems associated with each. However, these are, so far, even more expensive to purchase and operate and have been limited to use on military aircraft Fig. 5.5 Schematic diagram of an automatic tape-laying process (left) and a typical product (right)
124 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Fig.5.6 An automatic tape placement system in use at Bell Helicopter from Automated Dynamics Corporation literature. programs and in cases where the complexity of shape means that the part cannot be practicably fabricated in any other way. 5.3.7 Bagging After all plies have been laid-up and inspected,the lay-up is prepared for curing. An autoclave or vacuum bag will be applied over the surface of the lay-up and sealed to the mold,so that a consolidating pressure can be applied during cure by evacuating the space under the bag,and/or by increasing the outside pressure.As illustrated in Figure.5.7,the bagging process uses a number of different materials.These include: Release film-a smooth non-stick film often made from fluro-polymers,placed over the lay-up,which may be perforated to allow passage of gases or resin Breather fabric-transmits gases even under pressure and is used to allow gases to flow from all over the part to the vacuum fitting .Bleeder fabric-used to soak up excess resin,especially in high-bleed pre-pregs Vacuum bag film,normally nylon .Mastic tape-also called tacky tape and often made from butyl rubber;used to seal the edge of the bag to the mold In addition,for surfaces to be bonded,a peel ply (non-bonding woven cloth, such as nylon)is placed on the surface of the lay-up.During the cure this is incorporated into the surface resin and is subsequently peeled off to create a clean,roughened surface that is ready for adhesive bonding
124 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Fig. 5.6 An automatic tape placement system in use at Bell Helicopter from Automated Dynamics Corporation literature. programs and in cases where the complexity of shape means that the part cannot be practicably fabricated in any other way. 5.3.7 Bagging After all plies have been laid-up and inspected, the lay-up is prepared for curing. An autoclave or vacuum bag will be applied over the surface of the lay-up and sealed to the mold, so that a consolidating pressure can be applied during cure by evacuating the space under the bag, and/or by increasing the outside pressure. As illustrated in Figure. 5.7, the bagging process uses a number of different materials. These include: • Release film--a smooth non-stick film often made from fluro-polymers, placed over the lay-up, which may be perforated to allow passage of gases or resin • Breather fabric--transmits gases even under pressure and is used to allow gases to flow from all over the part to the vacuum fitting • Bleeder fabric--used to soak up excess resin, especially in high-bleed pre-pregs • Vacuum bag film, normally nylon • Mastic tape--also called tacky tape and often made from butyl rubber; used to seal the edge of the bag to the mold In addition, for surfaces to be bonded, a peel ply (non-bonding woven cloth, such as nylon) is placed on the surface of the lay-up. During the cure this is incorporated into the surface resin and is subsequently peeled off to create a clean, roughened surface that is ready for adhesive bonding
COMPONENT FORM AND MANUFACTURE 125 Edge Bleeder Vacuum Sealant Vacuum Bag Breather Cloth Release Film Prepreg Plies Tool Fig.5.7 Schematic diagram of a vacuum bag lay-up,indicating the various layers used Taken from Ref.2. The bagging must allow an even consolidation pressure to be applied to the part,while at the same time allowing any gases trapped in the lay-up or generated during curing to be removed from the system.The gases include volatiles from solvents left in the resin during the pre-pregging process,water,and air. The cost of the non-reusable materials described above is considerable;many companies use permanent,shaped vacuum bags made from high-temperature elastomers.Where thermocouples are not embedded in the mold,these may be inserted into the edge of the lay-up through the edge sealant. Vacuum bags are also applied temporarily during the lay-up process to tack the pre-preg firmly onto the mold,to consolidate previous pre-preg layers,and to allow the removal of air and volatiles.This process is often called debulking and may be required at the introduction of each ply in some complex-shaped parts, especially those with sharp corners. Where it is critical that both surfaces of a part be smooth and of controlled dimensions,matched(usually metal)tooling can be used,as described previously. In these cases,most of the bagging materials are not required,and even the vacuum bag need not be used if the matched molds include integral seals.Careful control of tool contour,pre-preg resin content and placement,cure pressure,and resin bleed are necessary for successful matched-die molding with pre-pregs. Alternatively,if a smooth outer surface is required,but control of tolerance is not required to a high level,a caul plate may be used.This is a stiff,free-floating plate or mold of the outer surface which is placed on the lay-up,just above the release film 5.3.8 Curing The majority of aerospace composite parts with thermosetting matrices are cured at elevated temperatures to ensure that the service temperature of
COMPONENT FORM AND MANUFACTURE Vacuum Bag Breather Cloth Release Film Prepreg Plies Tool 125 Fig. 5.7 Schematic diagram of a vacuum bag lay-up, indicating the various layers used Taken from Ref. 2. The bagging must allow an even consolidation pressure to be applied to the part, while at the same time allowing any gases trapped in the lay-up or generated during curing to be removed from the system. The gases include volatiles from solvents left in the resin during the pre-pregging process, water, and air. The cost of the non-reusable materials described above is considerable; many companies use permanent, shaped vacuum bags made from high-temperature elastomers. Where thermocouples are not embedded in the mold, these may be inserted into the edge of the lay-up through the edge sealant. Vacuum bags are also applied temporarily during the lay-up process to tack the pre-preg firmly onto the mold, to consolidate previous pre-preg layers, and to allow the removal of air and volatiles. This process is often called debulking and may be required at the introduction of each ply in some complex-shaped parts, especially those with sharp corners. Where it is critical that both surfaces of a part be smooth and of controlled dimensions, matched (usually metal) tooling can be used, as described previously. In these cases, most of the bagging materials are not required, and even the vacuum bag need not be used if the matched molds include integral seals. Careful control of tool contour, pre-preg resin content and placement, cure pressure, and resin bleed are necessary for successful matched-die molding with pre-pregs. Alternatively, if a smooth outer surface is required, but control of tolerance is not required to a high level, a caul plate may be used. This is a stiff, free-floating plate or mold of the outer surface which is placed on the lay-up, just above the release film 5.3.8 Curing The majority of aerospace composite parts with thermosetting matrices are cured at elevated temperatures to ensure that the service temperature of
126 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES the composite is sufficiently high.As a typical example,a carbon/epoxy composite cured at 180C for 2 hours might have a glass transition temperature (T)of 200C when dry,but only 160C when saturated with moisture.This would allow the composite to be used at a maximum service temperature of around 135C. As mentioned earlier,composites may be cured in an oven under a vacuum bag,but the best results come from the use of pressure above one atmosphere (compaction pressure),usually generated in an autoclave.The autoclave is basically a very large,internally heated pressure vessel,with internal connections for vacuum hoses and sensors such as thermocouples (Fig.5.8).The autoclave is usually computer controlled,and often pressurized with nitrogen or carbon dioxide to reduce the risk of an internal fire.A standard machine for epoxy composites will be capable of temperatures over 200C and pressures over 700 KPa.Autoclaves for processing thermoplastic composites or high- temperature thermoset composites may be capable of 400C and 1200 KPa or more.The part is normally heated by convection of heat from the fan-forced air circulation,although electrically heated molds are sometimes used.Although more costly,there are several advantages in heating the mold,including more rapid and uniform heating and the ability to use high temperatures as the walls of the autoclave remain cool. Normally the lay-up will be under vacuum from the time it leaves the lay-up room and while it is loaded into the autoclave,to keep the lay-up in position and help remove air and volatiles.The vacuum and sensor connections will be checked before the autoclave door is closed and the cycle commences. Pressurization and heating will begin immediately,and the target pressure will be reached in less than 30 minutes whereas,in thick parts,the target temperature may not be reached for several hours.After more than 100 KPa(gauge)pressure has been reached in the autoclave,the space under the vacuum bag is vented Pressure intet Autptlave wall base plate Vacuum Fig.5.8 Layout of an autoclave and,right,a small typical autoclave
126 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES the composite is sufficiently high. As a typical example, a carbon/epoxy composite cured at 180°C for 2 hours might have a glass transition temperature (Tg) of 200°C when dry, but only 160°C when saturated with moisture. This would allow the composite to be used at a maximum service temperature of around 135°C. As mentioned earlier, composites may be cured in an oven under a vacuum bag, but the best results come from the use of pressure above one atmosphere (compaction pressure), usually generated in an autoclave. The autoclave is basically a very large, internally heated pressure vessel, with internal connections for vacuum hoses and sensors such as thermocouples (Fig. 5.8). The autoclave is usually computer controlled, and often pressurized with nitrogen or carbon dioxide to reduce the risk of an internal fire. A standard machine for epoxy composites will be capable of temperatures over 200°C and pressures over 700 KPa. Autoclaves for processing thermoplastic composites or hightemperature thermoset composites may be capable of 400°C and 1200 KPa or more. The part is normally heated by convection of heat from the fan-forced air circulation, although electrically heated molds are sometimes used. Although more costly, there are several advantages in heating the mold, including more rapid and uniform heating and the ability to use high temperatures as the walls of the autoclave remain cool. Normally the lay-up will be under vacuum from the time it leaves the lay-up room and while it is loaded into the autoclave, to keep the lay-up in position and help remove air and volatiles. The vacuum and sensor connections will be checked before the autoclave door is closed and the cycle commences. Pressurization and heating will begin immediately, and the target pressure will be reached in less than 30 minutes whereas, in thick parts, the target temperature may not be reached for several hours. After more than 100 KPa (gauge) pressure has been reached in the autoclave, the space under the vacuum bag is vented /~ utociave wall basMCipdate II ~. / ,.'"°'GI tt "? Vacuum ~ Fig. 5.8 Layout of an autoclave and, right, a small typical autoclave
COMPONENT FORM AND MANUFACTURE 127 (connected to the atmosphere)to discourage the growth of existing bubbles and the generation of new bubbles,from entrapped gases and volatiles,in the resin as it is heated.Heat-up and cool-down rates are controlled to ensure even curing throughout the part and to reduce the possibility of residual stresses causing structural deficiencies or distortions. The viscosity of the resin falls with increasing temperature until the resin begins to chemically cross-link(gel).It is important that full pressure is applied before gelation occurs to allow removal of entrapped gases and removal of excess resin. Under some circumstances,a dwell is incorporated (isothermal hold),as shown in Figure 5.9,to prolong the time for consolidation and volatile removal. The hold also pre-reacts the resin and reduces the danger of large damaging exothermic reactions that can occur in thick laminates,for example,over 50 plies thick.A hold will also allow the temperature to become more uniform;this is very important in components with large variations in thickness. The need for complex heating/pressure cycles is important for earlier,less viscous epoxy resins and high-temperature resins because this is necessary to accommodate the requirements of the chemical reactions and to ensure that the resin viscosity is optimum when pressure is increased.Most modern non-bleed epoxy pre-pregs,however,can be processed with a simple "straight-up"cure cycle,provided that the component is not too thick or complex. If an autoclave is not available,compaction pressure may also be applied by an inflated rubber bladder or by materials with a high coefficient of thermal expansion(CTE)such as silicone rubber,used in conjunction with matched mold tooling.The expansion force generated by these arrangements requires that the 200 8 175 150- 6 125 100 Temperature 3 Pressure(10*kPa) 75 -----Pressure 2 25 0 50 100 150 200 250 Time (min) Fig.5.9 Typical autoclave cycle incorporating a dwell to allow temperature equilibration in thick lay-ups
COMPONENT FORM AND MANUFACTURE 127 (connected to the atmosphere) to discourage the growth of existing bubbles and the generation of new bubbles, from entrapped gases and volatiles, in the resin as it is heated. Heat-up and cool-down rates are controlled to ensure even curing throughout the part and to reduce the possibility of residual stresses causing structural deficiencies or distortions. The viscosity of the resin falls with increasing temperature until the resin begins to chemically cross-link (gel). It is important that full pressure is applied before gelation occurs to allow removal of entrapped gases and removal of excess resin. Under some circumstances, a dwell is incorporated (isothermal hold), as shown in Figure 5.9, to prolong the time for consolidation and volatile removal. The hold also pre-reacts the resin and reduces the danger of large damaging exothermic reactions that can occur in thick laminates, for example, over 50 plies thick. A hold will also allow the temperature to become more uniform; this is very important in components with large variations in thickness. The need for complex heating/pressure cycles is important for earlier, less viscous epoxy resins and high-temperature resins because this is necessary to accommodate the requirements of the chemical reactions and to ensure that the resin viscosity is optimum when pressure is increased. Most modern non-bleed epoxy pre-pregs, however, can be processed with a simple "straight-up" cure cycle, provided that the component is not too thick or complex. If an autoclave is not available, compaction pressure may also be applied by an inflated rubber bladder or by materials with a high coefficient of thermal expansion (CTE) such as silicone rubber, used in conjunction with matched mold tooling. The expansion force generated by these arrangements requires that the 200 175 - 150- ~ 125- 100- 75 50, 0 .:-7 [ I ! 50 100 150 200 250 Tmle (rain) Fig. 5.9 Typical autoclave cycle incorporating a dwell to allow temperature equilibration in thick lay-ups