128 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES tooling be stiff to resist the bending forces applied to the tool.(Bending forces are low in autoclave tooling because the pressure acts on all sides.) Autoclave molds must be vacuum tight and free of distortions under temperature.Ideally,they should have a low thermal mass to avoid slow heating and cooling and should have a low CTE,similar to that of the laminate. 5.3.9 Cocuring of Complex Components Complex integrally stiffened components,such as those shown in Figure 5.10, can be manufactured using internal pressurization3 (Fig.5.11).There are essentially two methods of applying internal pressure:thermal expansion of a rubber or metallic mandrel,and expansion under autoclave pressure of a rubber bladder.In both cases,the approach is to wrap pre-preg around the internal mandrel,which is inserted into an outer mold containing the outer skins.As shown in Figure 5.10,it may be desirable to arrange for one,usually the top outer skin,to be removable to allow equipment to be installed or for inspection purposes.This can be achieved by introducing a release film between the outer (removable)skin lay-up and the substructure.The outer skin,though cured at the same time as the substructure,can be separated,the release film removed,and the skin mechanically fastened or bonded in a secondary operation. 5.3.10 Processing Problems The main processing problems encountered in autoclave molding include overheating (caused by excessive exothermic reactions),porosity,resin-rich Fig.5.10 Cocured control surface components with integral lower skin and ribs and removable top skin.Courtesy of CRC-ACS
128 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES tooling be stiff to resist the bending forces applied to the tool. (Bending forces are low in autoclave tooling because the pressure acts on all sides.) Autoclave molds must be vacuum tight and free of distortions under temperature. Ideally, they should have a low thermal mass to avoid slow heating and cooling and should have a low CTE, similar to that of the laminate. 5.3.9 Cocuring of Complex Components Complex integrally stiffened components, such as those shown in Figure 5.10, can be manufactured using internal pressurization 3 (Fig. 5.11). There are essentially two methods of applying internal pressure: thermal expansion of a rubber or metallic mandrel, and expansion under autoclave pressure of a rubber bladder. In both cases, the approach is to wrap pre-preg around the internal mandrel, which is inserted into an outer mold containing the outer skins. As shown in Figure 5.10, it may be desirable to arrange for one, usually the top outer skin, to be removable to allow equipment to be installed or for inspection purposes. This can be achieved by introducing a release film between the outer (removable) skin lay-up and the substructure. The outer skin, though cured at the same time as the substructure, can be separated, the release film removed, and the skin mechanically fastened or bonded in a secondary operation. 5.3.10 Processing Problems The main processing problems encountered in autoclave molding include overheating (caused by excessive exothermic reactions), porosity, resin-rich Fig. 5.10 Cocured control surface components with integral lower skin and ribs and removable top skin. Courtesy of CRC-ACS
COMPONENT FORM AND MANUFACTURE 129 Cocured Vacuum Spar Pump Lightweight Steel Tooling Cavities Open Skins -to Autoclave Pressure Autoclave Protected Pressure Silicone Rubber Pressure Bags Fig.5.11 Schematic diagram of the tooling used to make a cocured carbon/epoxy wing structure,using rubber bladder expansion for internal pressurization. Adapted from Ref.3. areas,resin-dry areas,poor surface finish,insufficient consolidation,uneven cure, and distortion. Many of the problems can be resolved by correct timing of application of temperature and pressure and use of pre-preg materials with a wide processing window with (ideally)low exothermic cures. The formation of voids is generally caused by the entrapment of volatiles, water,and air that have remained after debulking.At the high processing temperatures in the autoclave,more solvents are liberated,and the volume of the solvents and other entrapped gases increases.To avoid the formation of severe porosity,it is necessary that the hydrostatic pressure in the resin before gelation exceeds the partial pressure of the gases,allowing them to be expelled.Once the resin gels no further,void removal or consolidation is possible.Water is often considered to be the main cause of void formation so that the applied pressure needs to exceed the partial pressure of the water.2 While a low temperature hold is often used to increase the time at low resin viscosity for the reasons stated above,excessive pressure or over-efficient resin-bleed when the resin viscosity is low may lead to dry spots.Resin-rich areas result when areas in the lay-up have lower resistance to resin-flow and insufficient pressure is applied before gelation
COMPONENT FORM AND MANUFACTURE 129 Cocured Pump Vacuum I Spar Lightweight -~ Steel \ Tooling --~ T t B~] •B Cavities Open I~Jl ]]" I .-- HH-- --Hll ~ to Autoclove ~ BB ~ / //~,BH--~ ' / "-'HI-- Pressure --II-- / " " Silicone Rubber Pressure Bogs Skins Fig. 5.11 Schematic diagram of the tooling used to make a cocured carbon/epoxy wing structure, using rubber bladder expansion for internal pressurization. Adapted from Ref. 3. areas, resin-dry areas, poor surface finish, insufficient consolidation, uneven cure, and distortion. Many of the problems can be resolved by correct timing of application of temperature and pressure and use of pre-preg materials with a wide processing window with (ideally) low exothermic cures. The formation of voids is generally caused by the entrapment of volatiles, water, and air that have remained after debulking. At the high processing temperatures in the autoclave, more solvents are liberated, and the volume of the solvents and other entrapped gases increases. To avoid the formation of severe porosity, it is necessary that the hydrostatic pressure in the resin before gelation exceeds the partial pressure of the gases, allowing them to be expelled. Once the resin gels no further, void removal or consolidation is possible. Water is often considered to be the main cause of void formation so that the applied pressure needs to exceed the partial pressure of the water. 2 While a low temperature hold is often used to increase the time at low resin viscosity for the reasons stated above, excessive pressure or over-efficient resin-bleed when the resin viscosity is low may lead to dry spots. Resin-rich areas result when areas in the lay-up have lower resistance to resin-flow and insufficient pressure is applied before gelation
130 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES To reduce surface porosity,a surfacing resin film or fine glass/epoxy scrim ply may be placed on the mold surface before the pre-preg is placed. The use of honeycomb core in the composite component can result in several problems,of which the most common is core-crushing.The reason for this is illustrated in Figure 5.12,which shows how a lateral force can arise in an autoclave,causing inward collapse of the core.2 Methods for avoiding this problem include the use of reduced pressure in the autoclave (reduced from 700 to 300 KPa with a concomitant reduction in laminate quality, however)and use of friction grips to prevent the inner pre-preg skin sliding inwards.The gripped skin region must be surplus to the component and must be removed after processing. Distortion can be a serious problem,and can arise from uneven cure, unbalanced fiber lay-ups,or the expansion differential between the composite Edgeband Chamfer a)】 Honeycomb Core Pressure Skin/Core- Compression nmmmmmmn Frictional Resistance b) Tool Reaction Fig.5.12 Schematic diagram,showing a)a typical honeycomb arrangement incorporating a chamfer and b)the origin of lateral crushing forces.Adapted from Ref.2
130 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES To reduce surface porosity, a surfacing resin film or fine glass/epoxy scrim ply may be placed on the mold surface before the pre-preg is placed. The use of honeycomb core in the composite component can result in several problems, of which the most common is core-crushing. The reason for this is illustrated in Figure 5.12, which shows how a lateral force can arise in an autoclave, causing inward collapse of the core. 2 Methods for avoiding this problem include the use of reduced pressure in the autoclave (reduced from 700 to 300 KPa with a concomitant reduction in laminate quality, however) and use of friction grips to prevent the inner pre-preg skin sliding inwards. The gripped skin region must be surplus to the component and must be removed after processing. Distortion can be a serious problem, and can arise from uneven cure, unbalanced fiber lay-ups, or the expansion differential between the composite - Edgeband Chomfer.~ .f" "\ a) Honeycomb Core~ "~" "J" b) t t l t e Tool Reaction Fig. 5.12 Schematic diagram, showing a) a typical honeycomb arrangement incorporating a chamfer and b) the origin of lateral crushing forces. Adapted from Ref. 2
COMPONENT FORM AND MANUFACTURE 131 part and the tooling.It will be found that long parts such as spars may appear to have"grown"with respect to the tooling,especially if this is made of a high-CTE material such as steel or aluminum.This phenomenon occurs because the resin is solidified at the curing temperature,and,compared with the tool,the composite shrinks little during cooling.This also can make it difficult to remove some complex components from their mold without damage. Parts such as"C"sections made on male mandrels may grip the mandrel due to a condition known as spring-in,in which composite angles close up slightly (about 1)during cool-down because of CTE differences between the resin and fiber.Allowance has to be made in the tool design to compensate for this. 5.3.11 Debagging,Finishing,and Painting The part is normally cooled down to below 60C before it is removed from the autoclave.The bagging layers are stripped off,and the part is carefully separated from the mold.If the release coating is imperfect or the mold does not have sufficient draught angle for deep parts,this may present processing difficulties. The part should be smooth on the tool side,but unless matched molds are used, there will be some texture or roughness on the bag side of the part;however,this is minimized if a stiff caul plate is used.Due to slight variations in pre-preg fiber areal weight and resin contents,and in resin-bleed during curing,it is difficult to specify the thickness of a pre-preg part to less than about+5%.This becomes a serious concern in thicker parts such as wing skins,where the choice may be between having a smooth outside surface with the correct aerodynamic contour (outer mold line tooling),and controlling the inner surface dimensions(inner mold line tooling)to allow easy assembly to the substructure. Any surface blemishes may need to be filled with special putty.For epoxy composites,a typical paint scheme is an epoxy primer coat followed by a polyurethane topcoat.Any residue from the release coating applied to the mold may cause problems with poor adhesion of the paint.For this reason,many parts may be abraded lightly on the surface before painting. Painting may either be carried out by traditional hand-operated methods or with robots.Robotic painting is normally controlled by computer-aided-design- generated off-line process trajectories.Computer modelling and test simulations can verify the programs before production commitment.Paint application robotics can vary the paint thickness applied that would be specified to suit the service environment.Contemporary systems for automatic paint spraying can be applied to a series of small parts through to a working envelope of up to 3 million cubic feet using gantry-mounted robots 5.3.12 Trimming and Drilling Increasingly,trimming and drilling processes"are also being carried out automatically by robots
COMPONENT FORM AND MANUFACTURE 131 part and the tooling. It will be found that long parts such as spars may appear to have "grown" with respect to the tooling, especially if this is made of a high-CTE material such as steel or aluminum. This phenomenon occurs because the resin is solidified at the curing temperature, and, compared with the tool, the composite shrinks little during cooling. This also can make it difficult to remove some complex components from their mold without damage. Parts such as "C" sections made on male mandrels may grip the mandrel due to a condition known as spring-in, in which composite angles close up slightly (about 1°) during cool-down because of CTE differences between the resin and fiber. Allowance has to be made in the tool design to compensate for this. 5.3.11 Debagging, Finishing, and Painting The part is normally cooled down to below 60°C before it is removed from the autoclave. The bagging layers are stripped off, and the part is carefully separated from the mold. If the release coating is imperfect or the mold does not have sufficient draught angle for deep parts, this may present processing difficulties. The part should be smooth on the tool side, but unless matched molds are used, there will be some texture or roughness on the bag side of the part; however, this is minimized if a stiff canl plate is used. Due to slight variations in pre-preg fiber areal weight and resin contents, and in resin-bleed during curing, it is difficult to specify the thickness of a pre-preg part to less than about _+ 5%. This becomes a serious concern in thicker parts such as wing skins, where the choice may be between having a smooth outside surface with the correct aerodynamic contour (outer mold line tooling), and controlling the inner surface dimensions (inner mold line tooling) to allow easy assembly to the substructure. Any surface blemishes may need to be filled with special putty. For epoxy composites, a typical paint scheme is an epoxy primer coat followed by a polyurethane topcoat. Any residue from the release coating applied to the mold may cause problems with poor adhesion of the paint. For this reason, many parts may be abraded lightly on the surface before painting. Painting may either be carried out by traditional hand-operated methods or with robots. Robotic painting is normally controlled by computer-aided-designgenerated off-line process trajectories. Computer modelling and test simulations can verify the programs before production commitment. Paint application robotics can vary the paint thickness applied that would be specified to suit the service environment. Contemporary systems for automatic paint spraying can be applied to a series of small parts through to a working envelope of up to 3 million cubic feet using gantry-mounted robots. 5.3.12 Trimming and Drilling Increasingly, trimming and drilling processes 4 are also being carried out automatically by robots. 5