During this period of temperature dwell,an external pressure is applied on the prepreg stack that causes the excess resin to flow out into the bleeder papers. The resin flow is critical since it allows the removal of entrapped air and volatiles from the prepreg and thus reduces the void content in the cured laminate.At the end of the temperature dwell,the autoclave temperature is increased to the actual curing temperature for the resin.The cure temperature and pressure are maintained for 2 h or more until a predetermined level of cure has occurred.At the end of the cure cycle,the temperature is slowly reduced while the laminate is still under pressure.The laminate is removed from the vacuum bag and,if needed,postcured at an elevated temperature in an air- circulating oven. The flow of excess resin from the prepregs is extremely important in reducing the void content in the cured laminate.In a bag-molding process for producing thin shell or plate structures,resin flow by face bleeding(normal to the top laminate face)is preferred over edge bleeding.Face bleeding is more effective since the resin-flow path before gelation is shorter in the thickness direction than in the edge directions.Since the resin-flow path is relatively long in the edge directions,it is difficult to remove entrapped air and volatiles from the central areas of the laminate by the edge bleeding process. The resin flow from the prepregs reduces significantly and may even stop after the gel time,which can be increased by reducing the heat-up rate as well as the dwell temperature (Figure 5.11).Dwelling at a temperature lower than the curing temperature is important for two reasons:(1)it allows the layup to achieve a uniform temperature throughout the thickness and(2)it provides 140 135℃Dwell B 125C Dwell 120 C 100 100 80 Simulated autoclave temperature B C 60 cycle Gelation 50 40 20 0 0 20 40 60 80 100 Time(min) FIGURE 5.11 Effect of dwelling on gel time.(Adapted from Purslaw,D.and Childs,R., Composites,,17,757,1986.) 2007 by Taylor Francis Group,LLC
During this period of temperature dwell, an external pressure is applied on the prepreg stack that causes the excess resin to flow out into the bleeder papers. The resin flow is critical since it allows the removal of entrapped air and volatiles from the prepreg and thus reduces the void content in the cured laminate. At the end of the temperature dwell, the autoclave temperature is increased to the actual curing temperature for the resin. The cure temperature and pressure are maintained for 2 h or more until a predetermined level of cure has occurred. At the end of the cure cycle, the temperature is slowly reduced while the laminate is still under pressure. The laminate is removed from the vacuum bag and, if needed, postcured at an elevated temperature in an aircirculating oven. The flow of excess resin from the prepregs is extremely important in reducing the void content in the cured laminate. In a bag-molding process for producing thin shell or plate structures, resin flow by face bleeding (normal to the top laminate face) is preferred over edge bleeding. Face bleeding is more effective since the resin-flow path before gelation is shorter in the thickness direction than in the edge directions. Since the resin-flow path is relatively long in the edge directions, it is difficult to remove entrapped air and volatiles from the central areas of the laminate by the edge bleeding process. The resin flow from the prepregs reduces significantly and may even stop after the gel time, which can be increased by reducing the heat-up rate as well as the dwell temperature (Figure 5.11). Dwelling at a temperature lower than the curing temperature is important for two reasons: (1) it allows the layup to achieve a uniform temperature throughout the thickness and (2) it provides 140 120 100 80 60 40 20 0 0 20 40 Time (min) A A B B C C Gelation Temperature (C) Viscosity (simulated output) 1258C Dwell 1358C Dwell Simulated autoclave temperature cycle 60 50 0 100 80 100 FIGURE 5.11 Effect of dwelling on gel time. (Adapted from Purslaw, D. and Childs, R., Composites, 17, 757, 1986.) 2007 by Taylor & Francis Group, LLC.
time for the resin to achieve a low viscosity.A small batch-to-batch variation in dwell temperature may cause a large variation in gel time,as evidenced in Figure 5.11. The cure temperature and pressure are selected to meet the following requirements: 1.The resin is cured uniformly and attains a specified degree of cure in the shortest possible time. 2.The temperature at any position inside the prepreg does not exceed a prescribed limit during the cure. 3.The cure pressure is sufficiently high to squeeze out all of the excess resin from every ply before the resin gels (increases in viscosity)at any location inside the prepreg. Loos and Springer [9]developed a theoretical model for the complex thermo- mechanical phenomenon that takes place in a vacuum bag-molding process. Based on their model and experimental works,the following observations can be made regarding the various molding parameters. The maximum temperature inside the layup depends on(1)the maximum cure temperature,(2)the heating rate,and (3)the initial layup thickness.The maximum cure temperature is usually prescribed by the prepreg manufacturer for the particular resin-catalyst system used in the prepreg and is determined from the time-temperature-viscosity characteristics of the resin-catalyst sys- tem.At low heating rates,the temperature distribution remains uniform within the layup.At high heating rates and increased layup thickness,the heat gener- ated by the curing reaction is faster than the heat transferred to the mold surfaces and a temperature "overshoot"occurs. Resin flow in the layup depends on the maximum pressure,layup thickness, and heating rate,as well as the pressure application rate.A cure pressure sufficient to squeeze out all excess resin from 16 to 32 layups was found to be inadequate for squeezing out resin from the layers closer to the bottom surface in a 64-ply layup.Similarly,if the heating rate is very high,the resin may start to gel before the excess resin is squeezed out from every ply in the layup. Loos and Springer [9]have pointed out that the cure cycle recommended by prepreg manufactures may not be adequate to squeeze out excess resin from thick layups.Since the compaction and resin flow progress inward from the top,the plies adjacent to the bottom mold surface may remain uncompacted and rich in resin,thereby creating weak interlaminar layers in the laminate. Excess resin must be squeezed out of every ply before the gel point is reached at any location in the prepreg.Therefore,the maximum cure pressure should be applied just before the resin viscosity in the top ply becomes suffi- ciently low for the resin flow to occur.If the cure pressure is applied too early, excess resin loss would occur owing to very low viscosity in the pregel period. 2007 by Taylor&Francis Group.LLC
time for the resin to achieve a low viscosity. A small batch-to-batch variation in dwell temperature may cause a large variation in gel time, as evidenced in Figure 5.11. The cure temperature and pressure are selected to meet the following requirements: 1. The resin is cured uniformly and attains a specified degree of cure in the shortest possible time. 2. The temperature at any position inside the prepreg does not exceed a prescribed limit during the cure. 3. The cure pressure is sufficiently high to squeeze out all of the excess resin from every ply before the resin gels (increases in viscosity) at any location inside the prepreg. Loos and Springer [9] developed a theoretical model for the complex thermomechanical phenomenon that takes place in a vacuum bag-molding process. Based on their model and experimental works, the following observations can be made regarding the various molding parameters. The maximum temperature inside the layup depends on (1) the maximum cure temperature, (2) the heating rate, and (3) the initial layup thickness. The maximum cure temperature is usually prescribed by the prepreg manufacturer for the particular resin–catalyst system used in the prepreg and is determined from the time–temperature–viscosity characteristics of the resin–catalyst system. At low heating rates, the temperature distribution remains uniform within the layup. At high heating rates and increased layup thickness, the heat generated by the curing reaction is faster than the heat transferred to the mold surfaces and a temperature ‘‘overshoot’’ occurs. Resin flow in the layup depends on the maximum pressure, layup thickness, and heating rate, as well as the pressure application rate. A cure pressure sufficient to squeeze out all excess resin from 16 to 32 layups was found to be inadequate for squeezing out resin from the layers closer to the bottom surface in a 64-ply layup. Similarly, if the heating rate is very high, the resin may start to gel before the excess resin is squeezed out from every ply in the layup. Loos and Springer [9] have pointed out that the cure cycle recommended by prepreg manufactures may not be adequate to squeeze out excess resin from thick layups. Since the compaction and resin flow progress inward from the top, the plies adjacent to the bottom mold surface may remain uncompacted and rich in resin, thereby creating weak interlaminar layers in the laminate. Excess resin must be squeezed out of every ply before the gel point is reached at any location in the prepreg. Therefore, the maximum cure pressure should be applied just before the resin viscosity in the top ply becomes sufficiently low for the resin flow to occur. If the cure pressure is applied too early, excess resin loss would occur owing to very low viscosity in the pregel period. 2007 by Taylor & Francis Group, LLC
TABLE 5.2 Cure Time for 90%Degree of Cure in a 32-Ply Carbon Fiber-Epoxy Laminatea Cure Temperature, Heating Rate, Cure Time ℃(F) C/min(℉/min (min) 135(275) 2.8(5) 236 163(325) 2.8(5) 110 177(351) 2.8(5) 89 177(351) 5.6(10) 65 177(351) 11.1(20) 5 aBased on a theoretical model developed by Loos and Springer- Loos.A.C.and Springer,G.S.,J.Compos.Mater..17,135.1983. If on the other hand the cure pressure is applied after the gel time,the resin may not be able to flow into the bleeder cloth because of the high viscosity it quickly attains in the postgel period.Thus the pressure application time is an important molding parameter in a bag-molding process.In general,it decreases with increasing cure pressure as well as increasing heating rate. The uniformity of cure in the laminate requires a uniform temperature distribution in the laminate.The time needed for completing the desired degree of cure is reduced by increasing the cure temperature as well as increasing the heating rate (Table 5.2). Besides voids and improper cure,defects in bag-molded laminates relate to the ply layup and trimming operations.Close control must be maintained over the fiber orientation in each ply,the stacking sequence,and the total number of plies in the stack.Since prepreg tapes are not as wide as the part itself,each layer may contain a number of identical plies laid side by side to cover the entire mold surface.A filament gap in a single layer should not exceed 0.76 mm(0.03 in.),and the distance between any two gaps should not be <38 mm(1.5 in.)[10]. Care must also be taken to avoid filament crossovers.Broken filaments,foreign matter,and debris should not be permitted.To prevent moisture pickup,the prepreg roll on removal from the cold storage should be warmed to room temperature before use. 5.3 COMPRESSION MOLDING Compression molding is used for transforming sheet-molding compounds (SMC)into finished products in matched molds.The principal advantage of compression molding is its ability to produce parts of complex geometry in short periods of time.Nonuniform thickness,ribs,bosses,flanges,holes,and shoulders,for example,can be incorporated during the compression-molding 2007 by Taylor Francis Group,LLC
If on the other hand the cure pressure is applied after the gel time, the resin may not be able to flow into the bleeder cloth because of the high viscosity it quickly attains in the postgel period. Thus the pressure application time is an important molding parameter in a bag-molding process. In general, it decreases with increasing cure pressure as well as increasing heating rate. The uniformity of cure in the laminate requires a uniform temperature distribution in the laminate. The time needed for completing the desired degree of cure is reduced by increasing the cure temperature as well as increasing the heating rate (Table 5.2). Besides voids and improper cure, defects in bag-molded laminates relate to the ply layup and trimming operations. Close control must be maintained over the fiber orientation in each ply, the stacking sequence, and the total number of plies in the stack. Since prepreg tapes are not as wide as the part itself, each layer may contain a number of identical plies laid side by side to cover the entire mold surface. A filament gap in a single layer should not exceed 0.76 mm (0.03 in.), and the distance between any two gaps should not be <38 mm (1.5 in.) [10]. Care must also be taken to avoid filament crossovers. Broken filaments, foreign matter, and debris should not be permitted. To prevent moisture pickup, the prepreg roll on removal from the cold storage should be warmed to room temperature before use. 5.3 COMPRESSION MOLDING Compression molding is used for transforming sheet-molding compounds (SMC) into finished products in matched molds. The principal advantage of compression molding is its ability to produce parts of complex geometry in short periods of time. Nonuniform thickness, ribs, bosses, flanges, holes, and shoulders, for example, can be incorporated during the compression-molding TABLE 5.2 Cure Time for 90% Degree of Cure in a 32-Ply Carbon Fiber–Epoxy Laminatea Cure Temperature, 8C (8F) Heating Rate, 8C=min (8F=min) Cure Time (min) 135 (275) 2.8 (5) 236 163 (325) 2.8 (5) 110 177 (351) 2.8 (5) 89 177 (351) 5.6 (10) 65 177 (351) 11.1 (20) 52 a Based on a theoretical model developed by Loos and Springer — Loos, A.C. and Springer, G.S., J. Compos. Mater., 17, 135, 1983. 2007 by Taylor & Francis Group, LLC
Movable platen Movable mold half Shear edge Fixed Charge mold half Ejector pin Fixed platen FIGURE 5.12 Schematic of a compression-molding process. process.Thus,it allows the possibility of eliminating a number of secondary finishing operations,such as drilling,forming,and welding.The entire molding process,including mold preparation and placement of SMC in the mold,as well as part removal from the mold,can be automated.Thus,the compression- molding process is suitable for the high-volume production of composite parts. It is considered the primary method of manufacturing for many structural automotive components,including road wheels,bumpers,and leaf springs. The compression-molding operation begins with the placement of a precut and weighed amount of SMC,usually a stack of several rectangular plies called the charge,onto the bottom half of a preheated mold cavity (Figure 5.12).The ply dimensions are selected to cover 60%-70%of the mold surface area. The mold is closed quickly after the charge placement,and the top half of the mold is lowered at a constant rate until the pressure on the charge increases to a preset level.With increasing pressure,the SMC material in the mold starts to flow and fill the cavity.Flow of the material is required to expel air entrapped in the mold as well as in the charge.Depending on the part complexity,length of flow,and fiber content (which controls the viscosity of SMC),the molding pressure may vary from 1.4 to 34.5 MPa(200-5000 psi).Usually,high pressures are required for molding parts that contain deep ribs and bosses.The mold temperature is usually in the range of 130C-160C (270F-320F).After a 2007 by Taylor&Francis Group.LLC
process. Thus, it allows the possibility of eliminating a number of secondary finishing operations, such as drilling, forming, and welding. The entire molding process, including mold preparation and placement of SMC in the mold, as well as part removal from the mold, can be automated. Thus, the compressionmolding process is suitable for the high-volume production of composite parts. It is considered the primary method of manufacturing for many structural automotive components, including road wheels, bumpers, and leaf springs. The compression-molding operation begins with the placement of a precut and weighed amount of SMC, usually a stack of several rectangular plies called the charge, onto the bottom half of a preheated mold cavity (Figure 5.12). The ply dimensions are selected to cover 60% –70% of the mold surface area. The mold is closed quickly after the charge placement, and the top half of the mold is lowered at a constant rate until the pressure on the charge increases to a preset level. With increasing pressure, the SMC material in the mold starts to flow and fill the cavity. Flow of the material is required to expel air entrapped in the mold as well as in the charge. Depending on the part complexity, length of flow, and fiber content (which controls the viscosity of SMC), the molding pressure may vary from 1.4 to 34.5 MPa (200–5000 psi). Usually, high pressures are required for molding parts that contain deep ribs and bosses. The mold temperature is usually in the range of 1308C–1608C (2708F–3208F). After a Movable mold half Fixed mold half Fixed platen Shear edge Charge Ejector pin Movable platen FIGURE 5.12 Schematic of a compression-molding process. 2007 by Taylor & Francis Group, LLC
Peak exotherm temperature Centerline Mold temperature Subsurface layers Surface layers Time to reach reak exotherm Time FIGURE 5.13 Temperature distribution at various locations across the thickness of an SMC during the compression-molding operation.(After Mallick,P.K.and Raghupathi, N,Polym.Eng.Sci.,19,774,1979.) reasonable degree of cure is achieved under pressure,the mold is opened and the part is removed,often with the aid of ejector pins. During molding,a complex heat transfer and a viscous flow phenomenon take place in the cavity.A review of the current understanding of the flow and cure characteristics of compression-molded SMC is given in Ref.[11]. Temperature-time curves measured at the outer surface,subsurface,and cen- terline of thick E-glass fiber-SMC moldings(Figure 5.13)show that the charge surface temperature quickly attains the mold temperature and remains rela- tively uniform compared with the centerline temperature.However,owing to the low thermal conductivity of E-glass fiber-SMC,the centerline temperature increases slowly until the curing reaction is initiated at the mid-thickness of the part.Since the SMC material has a relatively low thermal conductivity, the heat generated by the exothermic curing reaction in the interior of the SMC charge is not efficiently conducted to the mold surface and the centerline temperature increases rapidly to a peak value.As the curing reaction nears completion,the centerline temperature decreases gradually to the mold surface temperature.For thin parts,the temperature rise is nearly uniform across the thickness and the maximum temperature in the material seldom exceeds the mold temperature. Since the surface temperature first attains the resin gel temperature, curing begins first at the surface and progresses inward.Curing occurs more rapidly at higher mold temperatures(Figure 5.14);however,the peak exotherm temperature may also increase.Since peak exotherm temperature of 200C or higher may cause burning and chemical degradation in the resin,high molding temperatures in thick parts should be avoided. 2007 by Taylor Francis Group,LLC
reasonable degree of cure is achieved under pressure, the mold is opened and the part is removed, often with the aid of ejector pins. During molding, a complex heat transfer and a viscous flow phenomenon take place in the cavity. A review of the current understanding of the flow and cure characteristics of compression-molded SMC is given in Ref. [11]. Temperature–time curves measured at the outer surface, subsurface, and centerline of thick E-glass fiber-SMC moldings (Figure 5.13) show that the charge surface temperature quickly attains the mold temperature and remains relatively uniform compared with the centerline temperature. However, owing to the low thermal conductivity of E-glass fiber-SMC, the centerline temperature increases slowly until the curing reaction is initiated at the mid-thickness of the part. Since the SMC material has a relatively low thermal conductivity, the heat generated by the exothermic curing reaction in the interior of the SMC charge is not efficiently conducted to the mold surface and the centerline temperature increases rapidly to a peak value. As the curing reaction nears completion, the centerline temperature decreases gradually to the mold surface temperature. For thin parts, the temperature rise is nearly uniform across the thickness and the maximum temperature in the material seldom exceeds the mold temperature. Since the surface temperature first attains the resin gel temperature, curing begins first at the surface and progresses inward. Curing occurs more rapidly at higher mold temperatures (Figure 5.14); however, the peak exotherm temperature may also increase. Since peak exotherm temperature of 2008C or higher may cause burning and chemical degradation in the resin, high molding temperatures in thick parts should be avoided. Temperature Peak exotherm temperature Time Centerline Mold temperature Surface layers Time to reach reak exotherm Subsurface layers FIGURE 5.13 Temperature distribution at various locations across the thickness of an SMC during the compression-molding operation. (After Mallick, P.K. and Raghupathi, N., Polym. Eng. Sci., 19, 774, 1979.) 2007 by Taylor & Francis Group, LLC