《纺织复合材料》课程参考文献（Principles of the Manufacturing of Composite Materials）CHAPTER 7 Liquid Composite Molding

Copyrighted Materials Copyright 2009 DEStech Publications Retrieved from www.knovel.con CHAPTER7 Liquid Composite Molding 1.INTRODUCTION The last three chapters have presented a few processes for the manu- facturing of composite structures. These have advantages and disadvan- tages. The hand-lay-up on open molds provides flexibility and versatility in terms of different configurations; However it does not provide good quality of the part due to the lack of control of compaction and the entrap- ment of air during the laying-up process. In addition, this process suffers from the evaporation of styrene into the atmosphere, which is an environ- mental concern. For the autoclave molding process, even though it also uses laying-up either by hand or by tape laying machine, the environmental concern is not critical because prepreg tapes are used and the evaporation of volatiles is not serious. The quality of the parts is very good due to the fact that the impregnation of the fibers is done off-line. The use of vac- uum, pressure and temperature control also gives parts of good quality. However the autoclave molding process has disadvantages as follows: . Since prepregs have to be used, the cost is high compared to cases where dry fibers are used. . The prepregs have a shelf life, which imposes time constraint on their usage. This also can produce waste if the prepregs are not used during their shelf lives. . Since laying up is required, the component cannot have fiber orientations other than in-plane. Having fibers along the thickness 247

CHAPTER 7 1. INTRODUCTION The last three chapters have presented a few processes for the manu￾facturing of composite structures. These have advantages and disadvan￾tages. The hand-lay-up on open molds provides flexibility and versatility in terms of different configurations; However it does not provide good quality of the part due to the lack of control of compaction and the entrap￾ment of air during the laying-up process. In addition, this process suffers from the evaporation of styrene into the atmosphere, which is an environ￾mental concern. For the autoclave molding process, even though it also uses laying-up either by hand or by tape laying machine, the environmental concern is not critical because prepreg tapes are used and the evaporation of volatiles is not serious. The quality of the parts is very good due to the fact that the impregnation of the fibers is done off-line. The use of vac￾uum, pressure and temperature control also gives parts of good quality. However the autoclave molding process has disadvantages as follows: • Since prepregs have to be used, the cost is high compared to cases where dry fibers are used. • The prepregs have a shelf life, which imposes time constraint on their usage. This also can produce waste if the prepregs are not used during their shelf lives. • Since laying up is required, the component cannot have fiber orientations other than in-plane. Having fibers along the thickness 247

248 LIQUID COMPOSITE MOLDING direction of the part can improve properties such as interlaminar strength and toughness. The process requires an autoclave,which can be a substantial investment.The autoclave needs to be heated to a certain temperature and sometimes this can be costly as in the case where a large autoclave is heated to cure a small composite part. For parts with very large dimensions,such as those of a boat or a wind turbine blade,the use of an autoclave is economically impractical. The filament winding and pultrusion processes are geared towards parts of special shapes such as those having surfaces of revolutions,or those having constant cross section along their length. Liquid composite molding(LCM)is a process that may respond to the concerns mentioned above.The main steps of the process are shown schematically in Figure 7.1 and discussed below. 1.Preforming:During this step,dry fibers are packaged into a pre- form having the configuration of the part.The starting materials can be tows,random mats,or woven fabrics.The finished preform is usually woven,compression molded,braided or knitted together. Small amounts of adhesive or small-diameter stitches are usually used to hold the preform in shape. 2.Tool:After the preform is made,it is placed inside a tool(mold)for further processing.Usually the mold has two halves.Both of these can be made out of stiff metals (such as the case of SRIM,RTM. VARTM or RFIM)or one-half of the mold can be made out of stiff metal and the other half made out of a flexible membrane (such as the case of SCRIMP or its variations).The surface of the final part Injection Ratio Control Tool De-mold Cure Preform Resin Catalyst FIGURE 7.I Schematic of the LCM process

direction of the part can improve properties such as interlaminar strength and toughness. • The process requires an autoclave, which can be a substantial investment. The autoclave needs to be heated to a certain temperature and sometimes this can be costly as in the case where a large autoclave is heated to cure a small composite part. • For parts with very large dimensions, such as those of a boat or a wind turbine blade, the use of an autoclave is economically impractical. The filament winding and pultrusion processes are geared towards parts of special shapes such as those having surfaces of revolutions, or those having constant cross section along their length. Liquid composite molding (LCM) is a process that may respond to the concerns mentioned above. The main steps of the process are shown schematically in Figure 7.1 and discussed below. 1. Preforming: During this step, dry fibers are packaged into a pre￾form having the configuration of the part. The starting materials can be tows, random mats, or woven fabrics. The finished preform is usually woven, compression molded, braided or knitted together. Small amounts of adhesive or small-diameter stitches are usually used to hold the preform in shape. 2. Tool: After the preform is made, it is placed inside a tool (mold) for further processing. Usually the mold has two halves. Both of these can be made out of stiff metals (such as the case of SRIM, RTM, VARTM or RFIM) or one-half of the mold can be made out of stiff metal and the other half made out of a flexible membrane (such as the case of SCRIMP or its variations). The surface of the final part 248 LIQUID COMPOSITE MOLDING FIGURE 7.1 Schematic of the LCM process

Introduction 249 depends on the quality of the surface of the mold.Also,high pres- sure can be applied when both mold halves are made of stiff metals. The amount of voids that may be present in the final product de- pends on the ability of the resin to penetrate into small interstices between the fibers,and this may require high pressure.The type of tool used therefore depends on the required quality of the final part. 3.Resin infusion:After the preform is placed inside the mold and the two halves of the mold are closed,resin is infused into the mold. The objective of the infusion is to wet the fibers and to fill up any cavity within the preform.The infusion can be in the form of injec- tion where high pressure [several hundred psi(tens of MPa)]for the case of SRIM,or moderately high pressure(around 100 psi or 6.89 MPa),for the case of RTM,is used.It can also be simply suction created by vacuum(such as the case of VARTM or SCRIMP).The duration of time for the infusion of resin depends on the size of the part and on the reactivity of the resin system.For resin with fast re- activity,such as cyanate for SRIM,the infusion takes place within a matter of seconds;whereas for slower reaction systems such as ep- oxies for RTM,the infusion time can be on the order of minutes or hours. 4.Curing:After the resin has been infused completely into the cavity of the fiber preform,curing takes place.Normally the resin already contains curing agents and catalysts for curing.It is important that the resin does not gel during the infusion process.If the resin gels before the preform is infused,short shots are obtained.Curing can be accelerated by heating. 5.Demolding:The part is demolded and removed from the mold. The advantages of LCM are as follows: 1.The preforms are made using dry fibers and they do not have to con- tain the partially cured resin as in the case of prepregs (preforms may contain binders,which are small amounts of resin used to hold the shape of the preforms together).Because of this,fibers with dif- ferent orientations can be built into the preforms.Composites made from the preforms may have reinforcements along the thickness di- rection in addition to those in-plane.Different techniques such as weaving,braiding,stitching,and knitting can be used to make the preforms. 2.The dry preforms do not have the constraint of shelf life. 3.The process is done in a closed mold.For manufacturing involving

depends on the quality of the surface of the mold. Also, high pres￾sure can be applied when both mold halves are made of stiff metals. The amount of voids that may be present in the final product de￾pends on the ability of the resin to penetrate into small interstices between the fibers, and this may require high pressure. The type of tool used therefore depends on the required quality of the final part. 3. Resin infusion: After the preform is placed inside the mold and the two halves of the mold are closed, resin is infused into the mold. The objective of the infusion is to wet the fibers and to fill up any cavity within the preform. The infusion can be in the form of injec￾tion where high pressure [several hundred psi (tens of MPa)] for the case of SRIM, or moderately high pressure (around 100 psi or 6.89 MPa), for the case of RTM, is used. It can also be simply suction created by vacuum (such as the case of VARTM or SCRIMP). The duration of time for the infusion of resin depends on the size of the part and on the reactivity of the resin system. For resin with fast re￾activity, such as cyanate for SRIM, the infusion takes place within a matter of seconds; whereas for slower reaction systems such as ep￾oxies for RTM, the infusion time can be on the order of minutes or hours. 4. Curing: After the resin has been infused completely into the cavity of the fiber preform, curing takes place. Normally the resin already contains curing agents and catalysts for curing. It is important that the resin does not gel during the infusion process. If the resin gels before the preform is infused, short shots are obtained. Curing can be accelerated by heating. 5. Demolding: The part is demolded and removed from the mold. The advantages of LCM are as follows: 1. The preforms are made using dry fibers and they do not have to con￾tain the partially cured resin as in the case of prepregs (preforms may contain binders, which are small amounts of resin used to hold the shape of the preforms together). Because of this, fibers with dif￾ferent orientations can be built into the preforms. Composites made from the preforms may have reinforcements along the thickness di￾rection in addition to those in-plane. Different techniques such as weaving, braiding, stitching, and knitting can be used to make the preforms. 2. The dry preforms do not have the constraint of shelf life. 3. The process is done in a closed mold. For manufacturing involving Introduction 249

250 LIQUID COMPOSITE MOLDING : Compression moulding Hand laminating RTM Production Volume Cost effective range for liquid moulding FIGURE 7.2 Cost versus production volume of different manufacturing processes. polyester and vinyl ester resins,the issue of styrene evaporation into the atmosphere is not a great concern. 4.The cost-effective range for LCM is in the middle range in the pro- duction volume.Figure 7.2 shows the relation between production volume and the unit cost index for a few processes.LCM can be more cost-effective compared with the autoclave molding process when the production volume is on the order of 20,000-60,000 units per year [1]. 5.The molds required for LCM are generally considered to be light- weight and low cost compared with conventional compression molding and metal forming,resulting in a lower investment to enter production. Initially liquid composite molding was developed for low-cost appli- cations derived from the injection molding of regular plastic compo- nents.Due to its low cost,relatively fast production rate and its ability to provide closed mold conditions that help to address the problem of sty- rene (in open mold process),LCM has found acceptance for the manu- facturing of composites for automotive applications

polyester and vinyl ester resins, the issue of styrene evaporation into the atmosphere is not a great concern. 4. The cost-effective range for LCM is in the middle range in the pro￾duction volume. Figure 7.2 shows the relation between production volume and the unit cost index for a few processes. LCM can be more cost-effective compared with the autoclave molding process when the production volume is on the order of 20,000–60,000 units per year [1]. 5. The molds required for LCM are generally considered to be light￾weight and low cost compared with conventional compression molding and metal forming, resulting in a lower investment to enter production. Initially liquid composite molding was developed for low-cost appli￾cations derived from the injection molding of regular plastic compo￾nents. Due to its low cost, relatively fast production rate and its ability to provide closed mold conditions that help to address the problem of sty￾rene (in open mold process), LCM has found acceptance for the manu￾facturing of composites for automotive applications. 250 LIQUID COMPOSITE MOLDING FIGURE 7.2 Cost versus production volume of different manufacturing processes

Introduction 251 The disadvantages of LCM are as follows: 1.Preforms need to be held together by binders.The presence of bind- ers may interfere with the flow of resin to wet the fibers.Binders also need to be dissolved in the resin to avoid the bundling of fibers, which may affect the resulting mechanical properties. 2.Preforms need to fit well into the tool.For the resin transfer mold- ing (RTM)process,if the preforms do not fit well into the tool such that there is looseness at the peripheries of the preform,liquid resin can run quickly along these easy paths resulting in resin rich areas in the final part. 3.The permeability of the preform depends on many factors,such as the volume fraction of fibers,the compression pressure on the pre- form,the type of fiber form used,and the stacking sequence of the fibers.The variability of the permeability of the fiber preforms makes it difficult to predict the speed of flow of the liquid resin in them.This can result in lack of wetting,voids,and low mechanical properties such as interlaminar shear strength. 4.The quality of the part can be affected by the presence of voids,dry spots or resin rich areas. Depending on the fiber volume fraction and the end-use applications, there are many variants of the LCM process as follows: Injection molding (IM):This is a pure plastic injection process where there are no fibers involved,which has been used to make injection molded parts for a long time.The resin is mainly engineering thermoplastics such as polypropylene,polystyrene, and polymethylmethacrylate (PMMA).Sometimes short fibers (such as short glass or carbon fibers)can be incorporated into the thermoplastics to make reinforced plastic components.In this case,the fibers are mixed with the resin and injected together, rather than in the form of fiber preform. Structural reaction injection molding (SRIM):This is similar to IM above except that in this case a fiber preform is placed inside the mold cavity before injection.Figure 7.3 shows a schematic for this process.Due to the high rate of reaction,the pressure in the mold is usually high and the duration of the reaction is on the order of seconds. Resin transfer molding(RTM):This is similar to the SRIM process except that the duration of the injection step lasts on the order of minutes and the pressure inside the mold is in less than

The disadvantages of LCM are as follows: 1. Preforms need to be held together by binders. The presence of bind￾ers may interfere with the flow of resin to wet the fibers. Binders also need to be dissolved in the resin to avoid the bundling of fibers, which may affect the resulting mechanical properties. 2. Preforms need to fit well into the tool. For the resin transfer mold￾ing (RTM) process, if the preforms do not fit well into the tool such that there is looseness at the peripheries of the preform, liquid resin can run quickly along these easy paths resulting in resin rich areas in the final part. 3. The permeability of the preform depends on many factors, such as the volume fraction of fibers, the compression pressure on the pre￾form, the type of fiber form used, and the stacking sequence of the fibers. The variability of the permeability of the fiber preforms makes it difficult to predict the speed of flow of the liquid resin in them. This can result in lack of wetting, voids, and low mechanical properties such as interlaminar shear strength. 4. The quality of the part can be affected by the presence of voids, dry spots or resin rich areas. Depending on the fiber volume fraction and the end-use applications, there are many variants of the LCM process as follows: • Injection molding (IM): This is a pure plastic injection process where there are no fibers involved, which has been used to make injection molded parts for a long time. The resin is mainly engineering thermoplastics such as polypropylene, polystyrene, and polymethylmethacrylate (PMMA). Sometimes short fibers (such as short glass or carbon fibers) can be incorporated into the thermoplastics to make reinforced plastic components. In this case, the fibers are mixed with the resin and injected together, rather than in the form of fiber preform. • Structural reaction injection molding (SRIM): This is similar to IM above except that in this case a fiber preform is placed inside the mold cavity before injection. Figure 7.3 shows a schematic for this process. Due to the high rate of reaction, the pressure in the mold is usually high and the duration of the reaction is on the order of seconds. • Resin transfer molding (RTM): This is similar to the SRIM process except that the duration of the injection step lasts on the order of minutes and the pressure inside the mold is in less than Introduction 251