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.)