5 Component Form and Manufacture 5.1 Introduction Because fiber reinforcement is essentially a one-dimensional strengthening process,a major function of the component-forming process is to orientate the fibers in the matrix in the appropriate directions and proportions to obtain the desired two-dimensional or 3-dimensional mechanical properties.The forming process must also produce the shape of the component and develop the required properties of the matrix and the fiber/matrix bond.The forming process must not damage the fibers and must ensure that they are reasonably evenly distributed in a matrix,free from significant voiding or from large areas devoid of fibers. The simplest method that satisfies these requirements is to infiltrate an appropriately aligned fiber bed with a liquid,which is then converted by chemical reaction (in the case of thermosets)or simply by cooling (in the case of thermoplastics)to form a continuous solid matrix with the desired properties. Techniques based on liquid resin are known as liquid molding,with several subcategories according to various modifications of the process. Altematively,sheets of aligned fibers may be pre-coated with matrix precursor and the continuous matrix formed by flowing the coatings together(and curing,if a thermoset matrix)under heat and pressure.In this widely used form,the material is known as pre-preg(pre-impregnated). There are several methods that can be used to arrange the fibers when forming the composite structure.The main method for the manufacture of aircraft components is laminating woven cloth,or aligned fiber sheets,with the fibers orientated in appropriate directions in each layer. There are also several methods based on continuous fiber tow or yarn;these include: (1)filament winding onto a rotating mandrel;(2)braiding onto a rotating mandrel (the process of braiding is covered in detail in Chapter 14);(3)tow placement;and (4)pultrusion. The main differences between the use of thermosets and thermoplastic matrices are the need for extended times to cure (cross-link)the thermosets and the relatively high viscosities of the thermoplastics melts and the consequential requirement for high processing temperatures and pressures.Table 5.1 lists generic aircraft components made using these manufacturing procedures. 113
5 Component Form and Manufacture 5.1 Introduction Because fiber reinforcement is essentially a one-dimensional strengthening process, a major function of the component-forming process is to orientate the fibers in the matrix in the appropriate directions and proportions to obtain the desired two-dimensional or 3-dimensional mechanical properties. The forming process must also produce the shape of the component and develop the required properties of the matrix and the fiber/matrix bond. The forming process must not damage the fibers and must ensure that they are reasonably evenly distributed in a matrix, free from significant voiding or from large areas devoid of fibers. The simplest method that satisfies these requirements is to infiltrate an appropriately aligned fiber bed with a liquid, which is then converted by chemical reaction (in the case of thermosets) or simply by cooling (in the case of thermoplastics) to form a continuous solid matrix with the desired properties. Techniques based on liquid resin are known as liquid molding, with several subcategories according to various modifications of the process. Alternatively, sheets of aligned fibers may be pre-coated with matrix precursor and the continuous matrix formed by flowing the coatings together (and curing, if a thermoset matrix) under heat and pressure. In this widely used form, the material is known as pre-preg (pre-impregnated). There are several methods that can be used to arrange the fibers when forming the composite structure. The main method for the manufacture of aircraft components is laminating woven cloth, or aligned fiber sheets, with the fibers orientated in appropriate directions in each layer. There are also several methods based on continuous fiber tow or yarn; these include: (1) filament winding onto a rotating mandrel; (2) braiding onto a rotating mandrel (the process of braiding is covered in detail in Chapter 14); (3) tow placement; and (4) pultrusion. The main differences between the use of thermosets and thermoplastic matrices are the need for extended times to cure (cross-link) the thermosets and the relatively high viscosities of the thermoplastics melts and the consequential requirement for high processing temperatures and pressures. Table 5.1 lists generic aircraft components made using these manufacturing procedures. 113
114 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 5.1 Typical Aircraft Fiber Composite Forms Made by the Different Techniques,as Listed Type of Structure Typical Application Laminates Sheets,thick monolithic Wing skins Sheets,integrally stiffened Tail skins Sandwich panels Control surfaces,floor sections Shells Fuselage sections Beams Spars/ribs Complex forms Aerofoils Filament Wound Closed shells Pressure vessels Open shells Radomes Rocket motors Tubes Drive shafts Secondary formed tubes Helicopter blades Braided Tubes Drive shafts Complex tubes Curved pipes Truss joints Ducts Closed shells Pressure vessels Secondary formed Fuselage frames Aircraft propellers Helicopter blades Tow Placed See laminates See laminates Complex wraps Grips Shafts Ducts Pultrusion Beams Floor beams Stringers Spars Ribs Longerons Considerable structural and cost efficiency can be obtained by using the composite in the most highly stressed regions,for example,in the upper and lower surfaces of components subject to bending or buckling.This is achieved by using a sandwich construction,as also listed in Table 5.1,with the composite laminate forming the outer skins,which are bonded to a metallic or polymeric
114 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 5.1 Typical Aircraft Fiber Composite Forms Made by the Different Techniques, as Listed Type of Structure Typical Application Laminates Sheets, thick monolithic Sheets, integrally stiffened Sandwich panels Shells Beams Complex forms Filament Wound Closed shells Open shells Tubes Secondary formed tubes Braided Tubes Complex tubes Closed shells Secondary formed Tow Placed See laminates Complex wraps Pultrusion Beams Wing skins Tail skins Control surfaces, floor sections Fuselage sections Spars/ribs Aerofoils Pressure vessels Radomes Rocket motors Drive shafts Helicopter blades Drive shafts Curved pipes Truss joints Ducts Pressure vessels Fuselage frames Aircraft propellers Helicopter blades See laminates Grips Shafts Ducts Floor beams Stringers Spars Ribs Longerons Considerable structural and cost efficiency can be obtained by using the composite in the most highly stressed regions, for example, in the upper and lower surfaces of components subject to bending or buckling. This is achieved by using a sandwich construction, as also listed in Table 5.1, with the composite laminate forming the outer skins, which are bonded to a metallic or polymeric
COMPONENT FORM AND MANUFACTURE 115 composite honeycomb or polymeric foam core.The metallic honeycomb is generally an aluminum alloy such as 5052,often with a coating or anodized layer to resist corrosion.The composite honeycomb would generally be glass- reinforced epoxy or phenolic;however,the most usual honeycomb material is Nomex,which is the trade name for a composite based on random meta-aramid fibers in a phenolic matrix.The foam core used for aerospace applications is generally made of PVC,but this material is not generally used in applications exposed to high temperatures.Polyetherimide (PED)and polymethacrylimide (PMI)polyimide foams are alternative cores for higher-temperature applications. This chapter deals primarily with pre-preg laminating procedures in some detail because this is the prime method for manufacturing aircraft composite components.Methods based on liquid resin are then considered,followed by details of the various processes,resin transfer and infusion,and filament winding and pultrusion.Finally,the particular processes for manufacturing with thermo- plastic resins are covered. 5.2 Outline of General Laminating Procedures Most reinforced-plastic components based on long fibers are manufactured by some form of laminating procedure.In this process,sheets of reinforcement, pre-coated with resin(pre-preg)or with resin freshly applied,are forced against the surface of a mold under the required conditions of pressure,temperature,and time.Chapter 3 provides details of some of the cloth materials available,and details of the pre-pregging process are provided later in this chapter. 5.2.1 Open Die Molding Open die molding involves the use of only one mold surface,over which the layers of fiber are placed or "laid-up."If dry cloth is used,the resin may be applied by brushing or spraying.With care and suitable materials,this method (which is still widely used outside the aircraft industry)can produce good-quality parts.However,handling wet resins can be messy and can raise occupational health and safety (OH&S)concerns.In addition,a particular concern with the use of wet lay-up in aircraft-part production is the lack of repeatability of the process, especially the control of resin content and therefore the weight,thickness,and mechanical properties.Some smaller companies,notably in the German Glider Industry,have adopted wet pre-preg dispensing machines,which saturate reinforcement fabric on demand with a controlled amount of liquid resin, normally epoxy,and hardener.This solution is cheap and flexible,and it does not require cold storage. Various methods are engaged to apply pressure to consolidate the lay-up.In contact molding,which is generally used only for fairly low-stress applications of
COMPONENT FORM AND MANUFACTURE 115 composite honeycomb or polymeric foam core. The metallic honeycomb is generally an aluminum alloy such as 5052, often with a coating or anodized layer to resist corrosion. The composite honeycomb would generally be glassreinforced epoxy or phenolic; however, the most usual honeycomb material is Nomex, which is the trade name for a composite based on random meta-aramid fibers in a phenolic matrix. The foam core used for aerospace applications is generally made of PVC, but this material is not generally used in applications exposed to high temperatures. Polyetherimide (PEI) and polymethacrylimide (PMI) polyimide foams are alternative cores for higher-temperature applications. This chapter deals primarily with pre-preg laminating procedures in some detail because this is the prime method for manufacturing aircraft composite components. Methods based on liquid resin are then considered, followed by details of the various processes, resin transfer and infusion, and filament winding and pultrusion. Finally, the particular processes for manufacturing with thermoplastic resins are covered. 5,2 Outline of General Laminating Procedures Most reinforced-plastic components based on long fibers are manufactured by some form of laminating procedure. 1 In this process, sheets of reinforcement, pre-coated with resin (pre-preg) or with resin freshly applied, are forced against the surface of a mold under the required conditions of pressure, temperature, and time. Chapter 3 provides details of some of the cloth materials available, and details of the pre-pregging process are provided later in this chapter. 5.2.1 Open Die Molding Open die molding involves the use of only one mold surface, over which the layers of fiber are placed or "laid-up." If dry cloth is used, the resin may be applied by brushing or spraying. With care and suitable materials, this method (which is still widely used outside the aircraft industry) can produce good-quality parts. However, handling wet resins can be messy and can raise occupational health and safety (OH&S) concerns. In addition, a particular concern with the use of wet lay-up in aircraft-part production is the lack of repeatability of the process, especially the control of resin content and therefore the weight, thickness, and mechanical properties. Some smaller companies, notably in the German Glider Industry, have adopted wet pre-preg dispensing machines, which saturate reinforcement fabric on demand with a controlled amount of liquid resin, normally epoxy, and hardener. This solution is cheap and flexible, and it does not require cold storage. Various methods are engaged to apply pressure to consolidate the lay-up. In contact molding, which is generally used only for fairly low-stress applications of
116 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES glass/polyester composites,the pressure is developed by hand-rolling over a sheet of plastic film placed over the surface of the lay-up. The bag procedure involves the use of a flexible plastic membrane that is formed over the surface of the lay-up to form a vacuum-tight bag.In vacuum bagging,the bag is evacuated and atmospheric pressure used to consolidate the lay-up against the surface of the mold.The vacuum initially removes most of the air and volatile materials.Vacuum bagging is an inexpensive and versatile procedure;however,it can provide only limited consolidation pressure and may produce voided laminates due to the enlargement of the bubbles(formed by any residual gases or volatile material)trapped in the resin in regions where the bag is unable to apply pressure,for example,because of local bridging.To minimize this problem,autoclave procedures,described later,are used to manufacture most of the high-quality laminates used in the aircraft industry. Alternatively,pressure may be applied to the surface of an open mold by means of a flexible plunger mounted in a press,by gas-bags,or by thermal expansion of an entrapped rubber or metallic insert. Temperature,generally required to cure the resin,can be applied to the open mold in various ways,including exteral methods such as hot-air blowers and ovens or interally by electric elements or steam or oil pipes buried in the mold. Temperatures up to 180C may be required in aerospace-grade epoxy resin systems. 5.2.2 Compression Molding Compression or matched-die molding involves the use of matching male and female dies that close to form a cavity of the shape of the component (Fig.5.1). The dies,generally made of tool steel,can be internally heated,if required,by electric elements or steam,or hot oil pipes.The fiber layers are placed over the lower mold section,and the two halves of the mold are brought together in a press.Lands built into the mold usually control the thickness of the part. Advantages of matched-die molding include excellent dimensional control;high- quality surface finish,produced on both surfaces;high production rates;and good consolidation and high fiber content. However,the cost of the matching dies(with hardened faces)is very high,and the size of the available hydraulic presses used to apply the closing pressure limits the size of parts that can be produced. Wet laminating procedures may be used,in which case the dry fiber is laid in the mold and the resin added.High-quality fiber composite components are generally based on the use of pre-pregs or by the use of a solid,but uncured,resin film that is laid on the mold surface,followed by dry fiber layers or a fiber preform. Alternatively,a liquid resin can be injected into the sealed and evacuated mold cavity,as discussed later
116 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES glass/polyester composites, the pressure is developed by hand-rolling over a sheet of plastic film placed over the surface of the lay-up. The bag procedure involves the use of a flexible plastic membrane that is formed over the surface of the lay-up to form a vacuum-tight bag. In vacuum bagging, the bag is evacuated and atmospheric pressure used to consolidate the lay-up against the surface of the mold. The vacuum initially removes most of the air and volatile materials. Vacuum bagging is an inexpensive and versatile procedure; however, it can provide only limited consolidation pressure and may produce voided laminates due to the enlargement of the bubbles (formed by any residual gases or volatile material) trapped in the resin in regions where the bag is unable to apply pressure, for example, because of local bridging. To minimize this problem, autoclave procedures, described later, are used to manufacture most of the high-quality laminates used in the aircraft industry. Alternatively, pressure may be applied to the surface of an open mold by means of a flexible plunger mounted in a press, by gas-bags, or by thermal expansion of an entrapped rubber or metallic insert. Temperature, generally required to cure the resin, can be applied to the open mold in various ways, including external methods such as hot-air blowers and ovens or internally by electric elements or steam or oil pipes buried in the mold. Temperatures up to 180 °C may be required in aerospace-grade epoxy resin systems. 5.2.2 Compression Molding Compression or matched-die molding involves the use of matching male and female dies that close to form a cavity of the shape of the component (Fig. 5.1). The dies, generally made of tool steel, can be internally heated, if required, by electric elements or steam, or hot oil pipes. The fiber layers are placed over the lower mold section, and the two halves of the mold are brought together in a press. Lands built into the mold usually control the thickness of the part. Advantages of matched-die molding include excellent dimensional control; highquality surface finish, produced on both surfaces; high production rates; and good consolidation and high fiber content. However, the cost of the matching dies (with hardened faces) is very high, and the size of the available hydraulic presses used to apply the closing pressure limits the size of parts that can be produced. Wet laminating procedures may be used, in which case the dry fiber is laid in the mold and the resin added. High-quality fiber composite components are generally based on the use of pre-pregs or by the use of a solid, but uncured, resin film that is laid on the mold surface, followed by dry fiber layers or a fiber preform. Alternatively, a liquid resin can be injected into the sealed and evacuated mold cavity, as discussed later
COMPONENT FORM AND MANUFACTURE 117 Fig.5.1 Matched-die mold and resulting top-hat stiffened component. 5.2.3 Wrapping Wrapping is an alternative procedure to filament winding,described later,for producing tubular components.A pre-preg sheet,either wrap sheet or cloth,is wrapped onto a removable metal mandrel and cured under pressure.Special machines are available to perform the wrapping operations.The pressure during an elevated temperature cure may be applied by the use of shrink film(applied by a tape-winding machine),vacuum bag,or autoclave.Alternatively,a silicon- rubber bladder may be placed over the mandrel before the wrapping of the laminate.Pressure is applied to the laminate through-inflation of the bladder that forces the laminate against an outer mold surface.This technique is often used to make fishing rods,golf clubs,and tennis rackets. 5.3 Laminating Procedures For Aircraft-Grade Composite Components Major aircraft manufacturers and their subcontractors,especially in the United States,use B-staged epoxy pre-preg as their preferred material form.In this material,the reinforcement is pre-impregnated by a supplier with a resin already
COMPONENT FORM AND MANUFACTURE 117 Fig. 5.1 Matched-die mold and resulting top-hat stiffened component. 5.2.3 Wrapping Wrapping is an alternative procedure to filament winding, described later, for producing tubular components. A pre-preg sheet, either wrap sheet or cloth, is wrapped onto a removable metal mandrel and cured under pressure. Special machines are available to perform the wrapping operations. The pressure during an elevated temperature cure may be applied by the use of shrink film (applied by a tape-winding machine), vacuum bag, or autoclave. Alternatively, a siliconrubber bladder may be placed over the mandrel before the wrapping of the laminate. Pressure is applied to the laminate through-inflation of the bladder that forces the laminate against an outer mold surface. This technique is often used to make fishing rods, golf clubs, and tennis rackets. 5.3 Laminating Procedures For Aircraft-Grade Composite Components Major aircraft manufacturers and their subcontractors, especially in the United States, use B-staged epoxy pre-preg as their preferred material form. In this material, the reinforcement is pre-impregnated by a supplier with a resin already