《纺织复合材料》课程参考文献（Principles of the Manufacturing of Composite Materials）CHAPTER 4 Techniques for Composites Manufacturing

Copyrighted Materials Copyright 2009 DEStech Publications Retrieved from www.knovel.com PART 2 Techniques for Composites Manufacturing Theoretically, manufacturing of composites can be broken down into the following items: . Aligning of fibers . Single filaments . Tows . Fabrics (mats, weaves, braids, knits) . Bed consisting of many layers of fabrics .Filling the interstices between filaments with liquid matrix . Wetting the fibers . Curing the resin Filling interstices between the filaments with liquid resin can be done at the levels of single filaments, tows, fabrics, or a bed consisting of many layers of fabrics (refer to the list below). The arrangement of the fibers in different configurations was shown schematically in Figure 3.1. It is of no practical use to fill and wet single filaments with liquid resin so it is rarely done. a. If resin filling happens at the level of many tows, preimpregnated tapes (for thermosets) or preimpregnated tow preg (for thermoplas- tics)is made. b. If resin filling happens at the level of fabrics, preimpregnated fab- rics are made. In order to make the final composite part, these preimpregnated tapes or fabrics need to be laid up on top of each other to form the thickness of the laminate. 137

PART 2 Techniques for Composites Manufacturing Theoretically, manufacturing of composites can be broken down into the following items: • Aligning of fibers • Single filaments • Tows • Fabrics (mats, weaves, braids, knits) • Bed consisting of many layers of fabrics • Filling the interstices between filaments with liquid matrix • Wetting the fibers • Curing the resin Filling interstices between the filaments with liquid resin can be done at the levels of single filaments, tows, fabrics, or a bed consisting of many layers of fabrics (refer to the list below). The arrangement of the fibers in different configurations was shown schematically in Figure 3.1. It is of no practical use to fill and wet single filaments with liquid resin so it is rarely done. a. If resin filling happens at the level of many tows, preimpregnated tapes (for thermosets) or preimpregnated tow preg (for thermoplas￾tics) is made. b. If resin filling happens at the level of fabrics, preimpregnated fab￾rics are made. In order to make the final composite part, these preimpregnated tapes or fabrics need to be laid up on top of each other to form the thickness of the laminate. 137

138 TECHNIQUES FOR COMPOSITES MANUFACTURING c.If resin filling occurs at the bed of many layers of fabrics,then one can make the final composite part at the same manufacturing ses- sion with the liquid filling. No matter which fiber level the liquid resin is introduced,it is essential that it wets the filaments to ensure good bonding between matrix and fi- bers.In the manufacturing process involving activities(a)and(b),the liquid resin has an opportunity to wet the fiber two times.First,when the preimpregnated tapes or fabrics are made,and second,when these layers are placed on top of each other and processed to make the final part.For processes involving activity(c),the liquid has only one opportunity to wet the fibers.Therefore it is necessary that sufficient time is allowed for the resin to properly wet the fibers. The curing of resin is normally done after the resin has already filled all interstices and wet all filaments.However,due to the exothermic reac- tion,in order to avoid a peak in temperature rise,sometimes curing is done in stages(i.e.a thin layer may be cured first before additional layers are placed to add more thickness). Above are the theoretical activities.For making a composite part,a manufacturer can combine or alternate these steps,depending on the re- quirements for quality and cost,as follows: For hand-lay-up in open mold for fiber glass/polyester,dry tows or dry fabrics are laid on a mold,liquid resin is then poured and spread onto the fiber beds.A few layers are wetted and left to cure in open air.After these layers are cured,more layers are added. For autoclave curing,pre-impregnated layers made from tows and fabrics are first manufactured by one group of people.The second group of people obtain these prepregs and lay them up to fit the the thickness and configuration of the part.These are placed inside an autoclave for curing. For filament winding,dry tows of fibers are run through a bath of resin to be wetted.These are then deposited onto the surface of a rotating mandrel.Many of these layers are deposited until the desired thickness is obtained.These layers are then left to cure either in room temperature or under some form of heat lamp. For pultrusion,the dry tows are run through a bath of resin to be wetted.These are then fed into a heated die.The fibers and resin are subjected to compaction and heating.When the assembly of fibers and resin exit the die,they are compacted and cured

c. If resin filling occurs at the bed of many layers of fabrics, then one can make the final composite part at the same manufacturing ses￾sion with the liquid filling. No matter which fiber level the liquid resin is introduced, it is essential that it wets the filaments to ensure good bonding between matrix and fi￾bers. In the manufacturing process involving activities (a) and (b), the liquid resin has an opportunity to wet the fiber two times. First, when the preimpregnated tapes or fabrics are made, and second, when these layers are placed on top of each other and processed to make the final part. For processes involving activity (c), the liquid has only one opportunity to wet the fibers. Therefore it is necessary that sufficient time is allowed for the resin to properly wet the fibers. The curing of resin is normally done after the resin has already filled all interstices and wet all filaments. However, due to the exothermic reac￾tion, in order to avoid a peak in temperature rise, sometimes curing is done in stages (i.e. a thin layer may be cured first before additional layers are placed to add more thickness). Above are the theoretical activities. For making a composite part, a manufacturer can combine or alternate these steps, depending on the re￾quirements for quality and cost, as follows: • For hand-lay-up in open mold for fiber glass/polyester, dry tows or dry fabrics are laid on a mold, liquid resin is then poured and spread onto the fiber beds. A few layers are wetted and left to cure in open air. After these layers are cured, more layers are added. • For autoclave curing, pre-impregnated layers made from tows and fabrics are first manufactured by one group of people. The second group of people obtain these prepregs and lay them up to fit the the thickness and configuration of the part. These are placed inside an autoclave for curing. • For filament winding, dry tows of fibers are run through a bath of resin to be wetted. These are then deposited onto the surface of a rotating mandrel. Many of these layers are deposited until the desired thickness is obtained. These layers are then left to cure either in room temperature or under some form of heat lamp. • For pultrusion, the dry tows are run through a bath of resin to be wetted. These are then fed into a heated die. The fibers and resin are subjected to compaction and heating. When the assembly of fibers and resin exit the die, they are compacted and cured. 138 TECHNIQUES FOR COMPOSITES MANUFACTURING

Techniques for Composites Manufacturing 139 For liquid composite molding,layers of dry fibers are stacked on top of each other in a mold.Liquid resin is then injected into the mold to wet the fibers.Heat is applied to cure. For thermoplastic composite processing,two different approaches are taken.In the first approach,the fiber tows are first impregnated with the liquid thermoplastic resin.It is then allowed to cool so that tow pregs of themoplastic resin and fiber are obtained.These tow pregs are then placed on top of each other to make up the thickness.The stack of tow pregs is compression molded to make the part.In the second approach,the thermoplastic resin is brought to the vicinity of the fibers.This can be done by drawing the thermoplastic resin into small filaments,calendering the liquid thermoplastic resin into thin sheets,or grinding the thermoplastic resin into powders.These forms of thermoplastic resin are placed close to the dry fibers. The whole assembly of dry fiber and thermoplastic resin is placed inside a mold.Compression molding is used to consolidate the final part. The rule of thumb is that for good quality,the whole process should be broken down into small steps and each step done at a different time.For low cost manufacturing,many steps may be combined so the process can be done at one time or a lesser number of times.The selection of the pro- cess depends on weighing these two parameters:quality and cost.The following chapters present current techniques for manufacturing.By un- derstanding the principles used for manufacturing,new techniques can also be developed

• For liquid composite molding, layers of dry fibers are stacked on top of each other in a mold. Liquid resin is then injected into the mold to wet the fibers. Heat is applied to cure. • For thermoplastic composite processing, two different approaches are taken. In the first approach, the fiber tows are first impregnated with the liquid thermoplastic resin. It is then allowed to cool so that tow pregs of themoplastic resin and fiber are obtained. These tow pregs are then placed on top of each other to make up the thickness. The stack of tow pregs is compression molded to make the part. In the second approach, the thermoplastic resin is brought to the vicinity of the fibers. This can be done by drawing the thermoplastic resin into small filaments, calendering the liquid thermoplastic resin into thin sheets, or grinding the thermoplastic resin into powders. These forms of thermoplastic resin are placed close to the dry fibers. The whole assembly of dry fiber and thermoplastic resin is placed inside a mold. Compression molding is used to consolidate the final part. The rule of thumb is that for good quality, the whole process should be broken down into small steps and each step done at a different time. For low cost manufacturing, many steps may be combined so the process can be done at one time or a lesser number of times. The selection of the pro￾cess depends on weighing these two parameters: quality and cost. The following chapters present current techniques for manufacturing. By un￾derstanding the principles used for manufacturing, new techniques can also be developed. Techniques for Composites Manufacturing 139

CHAPTER 4 Hand Laminating (or Wet Lay-up)and the Autoclave Processing of Composites 1.HAND LAMINATING(OR WET LAY-UP) The hand laminating or wet lay-up processing technique is presented in the same chapter with autoclave processing because these two pro- cesses have one thing in common:the depositing of the different layers is done by hand.(Initially the lay-up for autoclave has been done by hand; recently many large companies have adopted the automated fiber place- ment machine for laying laminates for autoclave curing.)Apart from that,the materials used,the components to be made and the quality of the parts are very different.The hand laminating or wet lay-up process is used to make low cost components such as boats and corrosion-resistant components in the chemical process industry,while the autoclave pro- cess is used to make components for the aircraft industry.Hand laminat- ing or wet lay-up works with fiber glass and polyester(or vinyl ester) while the autoclave process mostly works with epoxy resins. Hand laminating is a primitive but effective method that is still widely used for prototyping and small batch production.The most common ma- terials are E glass fiber and polyester resin,although higher performance materials can also be used.The single sided mold is invariably operated at room temperature using an ambient curing resin.The reinforcement may be in the form of chopped strand mat or an aligned fabric such as wo- ven rovings. The usual feature of hand laminating is a single sided female mold, which is often itself made of glass fiber reinforced plastics(GRP),by taking a reversal from a male pattern.The GRP shell is often stiffened with local reinforcement,a wooden frame or light steel work to make it 141

CHAPTER 4 1. HAND LAMINATING (OR WET LAY-UP) The hand laminating or wet lay-up processing technique is presented in the same chapter with autoclave processing because these two pro￾cesses have one thing in common: the depositing of the different layers is done by hand. (Initially the lay-up for autoclave has been done by hand; recently many large companies have adopted the automated fiber place￾ment machine for laying laminates for autoclave curing.) Apart from that, the materials used, the components to be made and the quality of the parts are very different. The hand laminating or wet lay-up process is used to make low cost components such as boats and corrosion-resistant components in the chemical process industry, while the autoclave pro￾cess is used to make components for the aircraft industry. Hand laminat￾ing or wet lay-up works with fiber glass and polyester (or vinyl ester) while the autoclave process mostly works with epoxy resins. Hand laminating is a primitive but effective method that is still widely used for prototyping and small batch production. The most common ma￾terials are E glass fiber and polyester resin, although higher performance materials can also be used. The single sided mold is invariably operated at room temperature using an ambient curing resin. The reinforcement may be in the form of chopped strand mat or an aligned fabric such as wo￾ven rovings. The usual feature of hand laminating is a single sided female mold, which is often itself made of glass fiber reinforced plastics (GRP), by taking a reversal from a male pattern. The GRP shell is often stiffened with local reinforcement, a wooden frame or light steel work to make it 141

142 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES sufficiently stiff to withstand handling loads.The mold surface needs to be smooth enough to give an acceptable surface finish and release prop- erties and this is provided by a tooling gel coat that is subsequently coated with a release agent.The latter prevents the matrix resin from bonding to the mold surface and facilitates the de-molding operation.It is common practice to use a surface tissue immediately after the gel coat to mask any reinforcement print-through on the outer surface. Once the gel coat has hardened sufficiently,the reinforcement is laid in,one layer at a time.Catalyzed resin is then worked into the reinforce- ment using a brush or roller.This process is repeated for each layer of re- inforcement until the required thickness is built up.For thick laminates, pauses need to be taken after a certain number of layers have been depos- ited to allow the exothermic heat to dissipate before additional layers are deposited.Local reinforcements can be used to provide stiffness in spe- cific areas and lightweight formers such as foams or hollow sections can be laminated in for the same purpose.Figure 4.1 shows a schematic of the hand laminating process. The major limitation of hand laminating is that the mold has only one smooth surface.The absence of direct control over part thickness,fiber content,void fraction and surface quality on the other surface means that the moldings are used in very low stress applications and in areas where dimension accuracy is noncritical.Although capital costs are low,pro- duction is labor intensive and quality control is relatively difficult.The quality of the final part is highly dependent on the skill of the operator. Resin(catalyzed) Hand roller Mold Fiberglass reinforcement Gel goat Release agent FIGURE 4.I Schematic of the hand laminating process

sufficiently stiff to withstand handling loads. The mold surface needs to be smooth enough to give an acceptable surface finish and release prop￾erties and this is provided by a tooling gel coat that is subsequently coated with a release agent. The latter prevents the matrix resin from bonding to the mold surface and facilitates the de-molding operation. It is common practice to use a surface tissue immediately after the gel coat to mask any reinforcement print-through on the outer surface. Once the gel coat has hardened sufficiently, the reinforcement is laid in, one layer at a time. Catalyzed resin is then worked into the reinforce￾ment using a brush or roller. This process is repeated for each layer of re￾inforcement until the required thickness is built up. For thick laminates, pauses need to be taken after a certain number of layers have been depos￾ited to allow the exothermic heat to dissipate before additional layers are deposited. Local reinforcements can be used to provide stiffness in spe￾cific areas and lightweight formers such as foams or hollow sections can be laminated in for the same purpose. Figure 4.1 shows a schematic of the hand laminating process. The major limitation of hand laminating is that the mold has only one smooth surface. The absence of direct control over part thickness, fiber content, void fraction and surface quality on the other surface means that the moldings are used in very low stress applications and in areas where dimension accuracy is noncritical. Although capital costs are low, pro￾duction is labor intensive and quality control is relatively difficult. The quality of the final part is highly dependent on the skill of the operator. 142 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES FIGURE 4.1 Schematic of the hand laminating process

Autoclave Processing 143 The process remains an important one for low volume manufacture,al- though increasingly stringent emission regulations are forcing several manufacturers to explore the use of closed mold alternatives.Hand lami- nating using open molds has traditionally been used for making struc- tures out of fiberglass and polyester,but there are environmental concerns about evaporation of styrene into the atmosphere.There are new techniques of liquid composite molding that may produce similar parts with the closed mold,thus avoiding the environmental issue. 2.AUTOCLAVE PROCESSING 2.1.Introduction Autoclave processing is commonly used for manufacturing composite components for the aerospace industry.The process produces composite components of high quality,but it requires a considerable amount of time.The main steps of the autoclave processing of composites are: ·Prepregs ·Tool preparation Laying up prepregs on the tool to make the part ·Curing of the part Removal of the part from the tool ·Inspection 。Finishing steps In the Chapter 1,Figure 1.7(a)shows the different stages for the manu- facturing of composites.For the manufacturing using autoclave,the stages involved are a,b and c or a,b and d.As one moves from stage a to b,the dry fibers and the liquid resin are combined to make a semi-fin- ished form.This semi-finished form consists of both fibers that are uni- formly spread out over a rectangular cross section area and partially cured matrix resin that holds the fibers in place.This semi-finished form is called prepreg(pre-impregnated).As one moves from stage bto stage c or from b to d,many layers of the prepregs are stacked on each other to form a sheet of sufficient thickness for practical use.Note that one does not move from stage c to stage d.In the case where the process involves stages a,b and c,flat plate laminates (as in stage c)are made.Flat plate laminates are usually made for the purpose of studying and characteriz- ing the properties of the laminate.For real practical applications,nor- mally laminates may have curvatures and more complex configuration such as those shown in stage d.For this latter case,the process goes di-

The process remains an important one for low volume manufacture, al￾though increasingly stringent emission regulations are forcing several manufacturers to explore the use of closed mold alternatives. Hand lami￾nating using open molds has traditionally been used for making struc￾tures out of fiberglass and polyester, but there are environmental concerns about evaporation of styrene into the atmosphere. There are new techniques of liquid composite molding that may produce similar parts with the closed mold, thus avoiding the environmental issue. 2. AUTOCLAVE PROCESSING 2.1. Introduction Autoclave processing is commonly used for manufacturing composite components for the aerospace industry. The process produces composite components of high quality, but it requires a considerable amount of time. The main steps of the autoclave processing of composites are: • Prepregs • Tool preparation • Laying up prepregs on the tool to make the part • Curing of the part • Removal of the part from the tool • Inspection • Finishing steps In the Chapter 1, Figure 1.7(a) shows the different stages for the manu￾facturing of composites. For the manufacturing using autoclave, the stages involved are a, b and c or a, b and d. As one moves from stage a to b, the dry fibers and the liquid resin are combined to make a semi-fin￾ished form. This semi-finished form consists of both fibers that are uni￾formly spread out over a rectangular cross section area and partially cured matrix resin that holds the fibers in place. This semi-finished form is called prepreg (pre-impregnated). As one moves from stage b to stage c or from b to d, many layers of the prepregs are stacked on each other to form a sheet of sufficient thickness for practical use. Note that one does not move from stage c to stage d. In the case where the process involves stages a, b and c, flat plate laminates (as in stage c) are made. Flat plate laminates are usually made for the purpose of studying and characteriz￾ing the properties of the laminate. For real practical applications, nor￾mally laminates may have curvatures and more complex configuration such as those shown in stage d. For this latter case, the process goes di￾Autoclave Processing 143

144 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES (a)Prepreg (d)Vacuum Bag (b)Tool (c)Lay up ww8w8888 (f)Final product (e)Curing in autoclave FIGURE 4.2 Main steps in the autoclave manufacturing process. rectly from stage b to stage d.The properties of flat laminates can usually be applied to curved laminates with appropriate consideration for the change of the coordinate system.Figure 4.2 shows the main steps for au- toclave manufacturing.These are described below: 2.2.Prepreg 2.2.1.Prepreg Manufacturing and Handling Prepregging involves the incorporation of the partially cured resin with the fibers.Figure 4.3 shows a schematic of the prepregging ma- chine.In the prepregging process,dry fibers are fed from creels through stations of combs where the fibers are spread out.The fibers then enter into a bath of wet resin where they are wetted.Subsequently the fi- ber/resin combination is heated to change the liquid resin into a partially cured state.The partially cured resin is viscous enough to help keep the fibers in the configuration of flat sheets.This fiber/viscous resin combi- nation is called prepreg.Normally sheets of backing paper are placed on both sides of the prepreg for handing purposes.Then the prepregs are rolled up for storing and shipping

rectly from stage b to stage d. The properties of flat laminates can usually be applied to curved laminates with appropriate consideration for the change of the coordinate system. Figure 4.2 shows the main steps for au￾toclave manufacturing. These are described below: 2.2. Prepreg 2.2.1. Prepreg Manufacturing and Handling Prepregging involves the incorporation of the partially cured resin with the fibers. Figure 4.3 shows a schematic of the prepregging ma￾chine. In the prepregging process,dry fibers are fed from creels through stations of combs where the fibers are spread out. The fibers then enter into a bath of wet resin where they are wetted. Subsequently the fi￾ber/resin combination is heated to change the liquid resin into a partially cured state. The partially cured resin is viscous enough to help keep the fibers in the configuration of flat sheets. This fiber/viscous resin combi￾nation is called prepreg. Normally sheets of backing paper are placed on both sides of the prepreg for handing purposes. Then the prepregs are rolled up for storing and shipping. 144 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES FIGURE 4.2 Main steps in the autoclave manufacturing process

Autoclave Processing 145 Tensioner 三目 Feeding Comb Resin Heater Take up creels spreader bath roll FIGURE 4.3 Schematic of a prepregging machine. Resins are usually thermosets such as epoxies,although recently prepregs made of vinyl ester resins have also been available.The par- tially cured resin has about 30%of the crosslinks already formed.With the incorporation of fibers(such as carbon,glass or Kevlar at about 60% by volume),prepregs are flexible sheets of fibers about 150 mm thick. This is similar to a sheet of wallpaper except that it is sticky on both sides. Figure 4.4 shows a roll and a sheet of the carbon/epoxy prepreg. FIGURE 4.4 A roll and a sheet of carbon/epoxy prepreg

Resins are usually thermosets such as epoxies, although recently prepregs made of vinyl ester resins have also been available. The par￾tially cured resin has about 30% of the crosslinks already formed. With the incorporation of fibers (such as carbon, glass or Kevlar at about 60% by volume), prepregs are flexible sheets of fibers about 150 mm thick. This is similar to a sheet of wallpaper except that it is sticky on both sides. Figure 4.4 shows a roll and a sheet of the carbon/epoxy prepreg. Autoclave Processing 145 FIGURE 4.4 A roll and a sheet of carbon/epoxy prepreg. FIGURE 4.3 Schematic of a prepregging machine

146 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES The thermoset resins(such as epoxy)inside the prepregs contain all of the curing agents necessary for the complete cure of the resin.Only a por- tion of the crosslinking,however.has taken place,due to the addition of certain amounts of inhibitors into the resin,and/or by allowing the reac- tion to take place at low temperature and within a limited amount of time The time from the date of manufacturing of the prepregs to the time when the actual part is done may be several months.If the prepregs are left at room temperature on the shelf,the reaction in the resin may continue and the resin becomes hard (more crosslinking has already taken place). Once the resin is hard,it is no longer sticky and one cannot bond the prepregs together to make the composite component.Therefore it is es- sential to slow down the reaction of the resin until the prepregs are laid up to make the composite component.Slowing down is usually done by storing the prepregs at low temperature.Normally during shipping the prepregs are contained inside refrigerated bags.Once received,the prepregs need to be stored inside a freezer at about-5C.Usually the supplier of the prepregs provides specifications as to the storage temper- ature and the maximum amount of time of storage(shelf life)at that tem- perature.This means that even at that low temperature,the resin in the prepregs may keep on with the process of crosslinking and become mostly cured at the end of the period. Prepregs are available in the form of tapes(unidirectional fibers)or fabrics (woven).Widths of prepreg rolls vary from 25.4 mm(1 inch)to 305 mm (12 inches). 2.2.2.Prepreg Cutting When one is ready to make the composite component,the prepregs are taken out of the freezer and left at room conditions for a few hours.This allows the prepregs to rise to room temperature,reducing viscosity of the resin.Since normal room conditions contain a certain amount of humid- ity,equilibrating between the prepregs and room conditions means that the prepreg may acquire a similar amount of moisture.This has implica- tions for the formation of voids,which will be discussed later in this chapter. Once the prepregs have become pliable (viscosity of the resin is low enough for the prepregs to be shaped to conform to the contour of the mold),they are cut into the desirable configuration.Cutting the fabrics of the prepregs is similar to cutting cloth fabrics to make a suit,except that here carbon fibers are involved,rather than textile fibers.This means that the contour of the cut depends on the configuration of the part to be made For example,to make the nose cone of an airplane,the geometric config-

The thermoset resins (such as epoxy) inside the prepregs contain all of the curing agents necessary for the complete cure of the resin. Only a por￾tion of the crosslinking, however. has taken place, due to the addition of certain amounts of inhibitors into the resin, and/or by allowing the reac￾tion to take place at low temperature and within a limited amount of time. The time from the date of manufacturing of the prepregs to the time when the actual part is done may be several months. If the prepregs are left at room temperature on the shelf, the reaction in the resin may continue and the resin becomes hard (more crosslinking has already taken place). Once the resin is hard, it is no longer sticky and one cannot bond the prepregs together to make the composite component. Therefore it is es￾sential to slow down the reaction of the resin until the prepregs are laid up to make the composite component. Slowing down is usually done by storing the prepregs at low temperature. Normally during shipping the prepregs are contained inside refrigerated bags. Once received, the prepregs need to be stored inside a freezer at about −5°C. Usually the supplier of the prepregs provides specifications as to the storage temper￾ature and the maximum amount of time of storage (shelf life) at that tem￾perature. This means that even at that low temperature, the resin in the prepregs may keep on with the process of crosslinking and become mostly cured at the end of the period. Prepregs are available in the form of tapes (unidirectional fibers) or fabrics (woven). Widths of prepreg rolls vary from 25.4 mm (1 inch) to 305 mm (12 inches). 2.2.2. Prepreg Cutting When one is ready to make the composite component, the prepregs are taken out of the freezer and left at room conditions for a few hours. This allows the prepregs to rise to room temperature, reducing viscosity of the resin. Since normal room conditions contain a certain amount of humid￾ity, equilibrating between the prepregs and room conditions means that the prepreg may acquire a similar amount of moisture. This has implica￾tions for the formation of voids, which will be discussed later in this chapter. Once the prepregs have become pliable (viscosity of the resin is low enough for the prepregs to be shaped to conform to the contour of the mold), they are cut into the desirable configuration. Cutting the fabrics of the prepregs is similar to cutting cloth fabrics to make a suit, except that here carbon fibers are involved, rather than textile fibers. This means that the contour of the cut depends on the configuration of the part to be made. For example, to make the nose cone of an airplane, the geometric config- 146 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES

Autoclave Processing 147 uration of the surface of the cone when it is opened up onto a flat surface needs to be worked out,in order to learn how the flat piece(s)of fabric will cover that surface.On surfaces with double curvatures,where sev- eral flat pieces of fabric are required,care should be taken to assure conti- nuity of fiber orientation between adjacent pieces. Cutting used to be done by hand.Recently,however,this work has been mechanized and computerized,not only to speed up the process but also to reduce waste,which ordinarily increases very significantly the cost of manufacturing a composite part.Waste occurs due to the require- ment of cutting prepregs at an angle with the axis of the prepreg roll.By computerization,one may nest the different parts to minimize waste. With the use of the prepregs,the availability issue mentioned in chap- ter 1 is not a concern.For the autoclave process this issue is already taken care of during manufacturing 2.3.Tool Preparation Manufacturing using autoclave is a molding process.As such,molds (also called tools)are required.The mold provides the shape and surface finish for the part.As such the size of the mold depends on the size of the part.Large parts require large molds and these can be very expensive. Advanced composites must be cured at about 180C and at pressures of about 600 kPa;molds would be required to sustain these conditions for periods of several hours.In addition,there are many other considerations when designing and building tools.These include tool cost,life,accu- racy,weight,machinability,strength,thermal expansion,dimensional stability,surface finish,and thermal mass and thermal conductivity. Over a wide range of material systems and processing scenarios used for composites,there are many materials suitable for tooling.In general, the choices fall into three categories: 1.Reinforced polymers,for low to intermediate temperature ranges 2.Metals,for low to high temperatures 3.Ceramics and bulk graphite,for very high temperatures In addition,one may use cast plaster and other inexpensive and rela- tively easy-to-process materials for small scale part runs like those needed for prototype verification. For production tooling for advanced composites,the choice is usually made between metals,including aluminum,steel,nickel alloys (Invar), electroformed nickel,and graphite/epoxy tooling.Elastomeric tooling is

uration of the surface of the cone when it is opened up onto a flat surface needs to be worked out, in order to learn how the flat piece(s) of fabric will cover that surface. On surfaces with double curvatures, where sev￾eral flat pieces of fabric are required, care should be taken to assure conti￾nuity of fiber orientation between adjacent pieces. Cutting used to be done by hand. Recently, however, this work has been mechanized and computerized, not only to speed up the process but also to reduce waste, which ordinarily increases very significantly the cost of manufacturing a composite part. Waste occurs due to the require￾ment of cutting prepregs at an angle with the axis of the prepreg roll. By computerization, one may nest the different parts to minimize waste. With the use of the prepregs, the availability issue mentioned in chap￾ter 1 is not a concern. For the autoclave process this issue is already taken care of during manufacturing. 2.3. Tool Preparation Manufacturing using autoclave is a molding process. As such, molds (also called tools) are required. The mold provides the shape and surface finish for the part. As such the size of the mold depends on the size of the part. Large parts require large molds and these can be very expensive. Advanced composites must be cured at about 180°C and at pressures of about 600 kPa; molds would be required to sustain these conditions for periods of several hours. In addition, there are many other considerations when designing and building tools. These include tool cost, life, accu￾racy, weight, machinability, strength, thermal expansion, dimensional stability, surface finish, and thermal mass and thermal conductivity. Over a wide range of material systems and processing scenarios used for composites, there are many materials suitable for tooling. In general, the choices fall into three categories: 1. Reinforced polymers, for low to intermediate temperature ranges 2. Metals, for low to high temperatures 3. Ceramics and bulk graphite, for very high temperatures In addition, one may use cast plaster and other inexpensive and rela￾tively easy-to-process materials for small scale part runs like those needed for prototype verification. For production tooling for advanced composites, the choice is usually made between metals, including aluminum, steel, nickel alloys (Invar), electroformed nickel, and graphite/epoxy tooling. Elastomeric tooling is Autoclave Processing 147