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 manufacturing of composite structures. These have advantages and disadvantages. 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 entrapment of air during the laying-up process. In addition, this process suffers from the evaporation of styrene into the atmosphere, which is an environmental 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 vacuum, 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 preform 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 pressure can be applied when both mold halves are made of stiff metals. The amount of voids that may be present in the final product depends 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 injection 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 reactivity, such as cyanate for SRIM, the infusion takes place within a matter of seconds; whereas for slower reaction systems such as epoxies 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 contain 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 different orientations can be built into the preforms. Composites made from the preforms may have reinforcements along the thickness direction 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 production 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 lightweight 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 applications derived from the injection molding of regular plastic components. Due to its low cost, relatively fast production rate and its ability to provide closed mold conditions that help to address the problem of styrene (in open mold process), LCM has found acceptance for the manufacturing 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 binders 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 molding (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 preform, 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
252 LIQUID COMPOSITE MOLDING Monomer A Ratio Mixer control 84 SuizJawXlod Monomer B Mold FIGURE 7.3 SRIM process. 100 psi(680 kPa).Figure 7.4 shows an RTM mold for automotive parts. Vacuum-assisted resin transfer molding (VARTM):This is similar to RTM except that rather than using pressure,vacuum is used. Because of this,the pressure differential is small.The advantage here is that a rigid mold is used only on one side of the part where on the other side a flexible bag can be used.This can result in significant cost savings.The disadvantage is that due to the low FIGURE 7.4 An RTM mold for a curved piece
100 psi (680 kPa). Figure 7.4 shows an RTM mold for automotive parts. • Vacuum-assisted resin transfer molding (VARTM): This is similar to RTM except that rather than using pressure, vacuum is used. Because of this, the pressure differential is small. The advantage here is that a rigid mold is used only on one side of the part where on the other side a flexible bag can be used. This can result in significant cost savings. The disadvantage is that due to the low 252 LIQUID COMPOSITE MOLDING FIGURE 7.3 SRIM process. FIGURE 7.4 An RTM mold for a curved piece
Introduction 253 vacuum pump resin supply mould cavity vacuum gauge pressure gauge peristaltic pump FIGURE 7.5(a)Vacuum-assisted molding arrangement. pressure,more voids may appear in the part.Figure 7.5 shows schematics of the VARTM. Seaman composite resin infusion molding process (SCRIMP): This process is similar to VARTM in which only vacuum is used to provide the pressure gradient.In the case of RTM,both mold surfaces are hard,meaning both mold surfaces are made of some sort of metal.In the case of SCRIMP,only one mold surface is hard,the other mold surface is a flexible membrane that is used to contain the vacuum.Figure 7.6 shows a schematic of the process. In SCRIMP,the liquid resin flows in between the flexible membrane and the fiber preform.This type of flow is rapid since Cover strip to prevent the vacuum gallery Clamp Resin in from becoming blocked Seal strip Vacuum out Lower tool face FIGURE 7.5(b)Cross section of a VARTM set up
pressure, more voids may appear in the part. Figure 7.5 shows schematics of the VARTM. • Seaman composite resin infusion molding process (SCRIMP): This process is similar to VARTM in which only vacuum is used to provide the pressure gradient. In the case of RTM, both mold surfaces are hard, meaning both mold surfaces are made of some sort of metal. In the case of SCRIMP, only one mold surface is hard, the other mold surface is a flexible membrane that is used to contain the vacuum. Figure 7.6 shows a schematic of the process. In SCRIMP, the liquid resin flows in between the flexible membrane and the fiber preform. This type of flow is rapid since Introduction 253 FIGURE 7.5(b) Cross section of a VARTM set up. FIGURE 7.5(a) Vacuum-assisted molding arrangement
254 LIQUID COMPOSITE MOLDING membrane fluid flow fluid providing direction channel fluid channel release to vacuum film 的 mold FIGURE 7.6 Schematic of the SCRIMP process. the resin does not have to flow through the dense fiber preform along the planar dimensions of the part.To wet the fibers,resin only needs to flow through the thickness of the part.The distance dx in Darcy's law is therefore small and one does not need to have high pressure difference(dp)to get the resin to flow through the fiber beds.The advantage of this process is that it allows the ability to manufacture very large components on the order of several tens of meters (such as boat hulls or large turbine blades) One disadvantage of this method is that good surface finish is only provided on one side (the mold side).The other disadvantage is that the percent void content may be high.For critical applications such as aerospace,the amount of void content needs to be very small. Resin film infusion molding (RFIM):In this process,instead of injecting resin into the mold,thin films of resin are placed at the bottom of the fiber beds or between different layers of the dry preform.Upon heating and application of pressure,the resin film melts and the liquid resin permeates into the dry fiber preform. Figure 7.7 shows a schematic of the process. caul plate preform vacuum bag resin layer vacuum 图 Release film tool sealant dam tape FIGURE 7.7 Schematic of the RFIM process
the resin does not have to flow through the dense fiber preform along the planar dimensions of the part. To wet the fibers, resin only needs to flow through the thickness of the part. The distance dx in Darcy’s law is therefore small and one does not need to have high pressure difference (dp) to get the resin to flow through the fiber beds. The advantage of this process is that it allows the ability to manufacture very large components on the order of several tens of meters (such as boat hulls or large turbine blades). One disadvantage of this method is that good surface finish is only provided on one side (the mold side). The other disadvantage is that the percent void content may be high. For critical applications such as aerospace, the amount of void content needs to be very small. • Resin film infusion molding (RFIM): In this process, instead of injecting resin into the mold, thin films of resin are placed at the bottom of the fiber beds or between different layers of the dry preform. Upon heating and application of pressure, the resin film melts and the liquid resin permeates into the dry fiber preform. Figure 7.7 shows a schematic of the process. FIGURE 7.6 Schematic of the SCRIMP process. 254 LIQUID COMPOSITE MOLDING FIGURE 7.7 Schematic of the RFIM process
Materials 255 2.MATERIALS 2.1.Fibers Fibers used for LCM are usually glass or carbon.Discussion on fibers was presented in Chapter 3.In addition to the fiber forms presented in Chapter 3,there are other forms of fibers that are specifically applicable to the LCM process. 2.1.1.Flow Enhancement Fabrics The macroscopic permeability(permeability referring to the fabric as a whole rather than the individual filaments)can be increased by creating effective flow channels between fiber bundles.This can be achieved by fiber clustering,which may still allow high volume fraction to be at- tained but with a less uniform fiber distribution.Commercially available flow enhancement fabrics are said to offer a number of advantages over the aligned fabrics,in particular,reduced injection times,which may make possible the production of relatively large parts at high volume fractions.The main disadvantage of these materials is the potential re- duction in mechanical properties caused by less uniformity in the fiber distribution. For VARTM or SCRIMP,a layer of impermeable plastic with flow channels can be placed on top of the fiber preform to facilitate fast pene- tration of the liquid resin.Flow of the resin will only need to go through the thickness of the preform (Figures 7.5 and 7.6). 2.1.2.Surface Veils Surface veil is a random reinforcement with low superficial density and is produced from a fine (low tex)glass fiber.This material is used in LCM to provide a high quality surface finish by eliminating fiber strike-through and creating a resin rich surface layer,or alternatively where chemical resistance is required (where a C-glass tissue may be used).The use of a surface veil may eliminate the need for a gel coat.A number of materials are commercially available based on either chopped or continuous filaments held together with either a polyester or PVA binder and having superficial densities in the range of 30-100 g/cm2. 2.1.3.Binder Binder is applied to the fibers during the preform manufacturing stage
2. MATERIALS 2.1. Fibers Fibers used for LCM are usually glass or carbon. Discussion on fibers was presented in Chapter 3. In addition to the fiber forms presented in Chapter 3, there are other forms of fibers that are specifically applicable to the LCM process. 2.1.1. Flow Enhancement Fabrics The macroscopic permeability (permeability referring to the fabric as a whole rather than the individual filaments) can be increased by creating effective flow channels between fiber bundles. This can be achieved by fiber clustering, which may still allow high volume fraction to be attained but with a less uniform fiber distribution. Commercially available flow enhancement fabrics are said to offer a number of advantages over the aligned fabrics, in particular, reduced injection times, which may make possible the production of relatively large parts at high volume fractions. The main disadvantage of these materials is the potential reduction in mechanical properties caused by less uniformity in the fiber distribution. For VARTM or SCRIMP, a layer of impermeable plastic with flow channels can be placed on top of the fiber preform to facilitate fast penetration of the liquid resin. Flow of the resin will only need to go through the thickness of the preform (Figures 7.5 and 7.6). 2.1.2. Surface Veils Surface veil is a random reinforcement with low superficial density and is produced from a fine (low tex) glass fiber. This material is used in LCM to provide a high quality surface finish by eliminating fiber strike-through and creating a resin rich surface layer, or alternatively where chemical resistance is required (where a C-glass tissue may be used). The use of a surface veil may eliminate the need for a gel coat. A number of materials are commercially available based on either chopped or continuous filaments held together with either a polyester or PVA binder and having superficial densities in the range of 30–100 g/cm2. 2.1.3. Binder Binder is applied to the fibers during the preform manufacturing stage Materials 255
256 LIQUID COMPOSITE MOLDING to provide cohesion to the fiber architecture during subsequent handling and processing operations.Binding can be achieved mechanically by needling or stitching with a light yarn or roving,but it is more usual to use a chemical adhesive binder.This may be either a thermoplastic or thermoset in the form of a powder,an emulsion or a solution.For systems using polyester of vinyl ester,binders may be categorized by their solu- bility in styrene.A dissolution time of less than 60 seconds corresponds to high solubility,between 60 and 200 seconds indicates medium solu- bility,and over 200 seconds represents low solubility.Relatively low sol- ubility binders result in improved flow characteristics at the expense of prolonged fiber wet-out times.One potential consequence of binder dis- solution is a change in resin viscosity.It has been suggested [2]that the viscosity of a vinyl ester resin may be doubled by the addition of 5%by mass of thermoplastic polyester binder. 2.2.Preforms Preform is an assembly of fibers having the configuration of the part. Once the preform is wetted by the liquid resin and after the resin is cured, a composite part is obtained.Figure 7.8 shows an example of a preform. There are several good reasons to preform the reinforcement before loading it into the mold. Preforms speed up the process and free the mold from everything except loading,injection,in-mold cure,and demolding. FIGURE 7.8 Knitted glass fiber preform for a stiffened T joint (courtesy of Preform Technologies Ltd.)
to provide cohesion to the fiber architecture during subsequent handling and processing operations. Binding can be achieved mechanically by needling or stitching with a light yarn or roving, but it is more usual to use a chemical adhesive binder. This may be either a thermoplastic or thermoset in the form of a powder, an emulsion or a solution. For systems using polyester of vinyl ester, binders may be categorized by their solubility in styrene. A dissolution time of less than 60 seconds corresponds to high solubility, between 60 and 200 seconds indicates medium solubility, and over 200 seconds represents low solubility. Relatively low solubility binders result in improved flow characteristics at the expense of prolonged fiber wet-out times. One potential consequence of binder dissolution is a change in resin viscosity. It has been suggested [2] that the viscosity of a vinyl ester resin may be doubled by the addition of 5% by mass of thermoplastic polyester binder. 2.2. Preforms Preform is an assembly of fibers having the configuration of the part. Once the preform is wetted by the liquid resin and after the resin is cured, a composite part is obtained. Figure 7.8 shows an example of a preform. There are several good reasons to preform the reinforcement before loading it into the mold. • Preforms speed up the process and free the mold from everything except loading, injection, in-mold cure, and demolding. 256 LIQUID COMPOSITE MOLDING FIGURE 7.8 Knitted glass fiber preform for a stiffened T joint (courtesy of Preform Technologies Ltd.)