《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