Materials 257 Preforms improve quality and reduce part-to-part variations.For fast cycle times,the ideal is to make the preform so stiff that it becomes self-locating in the production mold.In other cases, when the mechanical properties are of paramount importance,one often wants to minimize the preform binder since the mechanical properties can be adversely affected by the binder. In general,a good preform is required to be inexpensive to make and stiff enough to be stacked and handled before injection.The fibers must stay in the direction in which they have been placed both during handling and injection.To achieve all these goals it is common to apply some form of preforming agents(binders).Both thermoplastic and thermosetting powders are commonly used for this purpose. The compaction behavior of the preform differs a lot depending on the performing method and the type of reinforcement that has been used.An important observation is that the fiber volume fraction at zero compac- tion pressure can differ significantly between different fabrics.A lower nominal fiber volume fraction can result in movement of the reinforce- ment during filling and incomplete impregnation.Another typical fea- ture of the compaction behavior is that all fabrics behave like nonlinear (stiffening)springs and that the possible increase in fiber volume frac- tion from the value at rest is limited. In practice,most molds are flexible and will deform when the fiber volume fraction becomes too high(which corresponds to a high compac- tion pressure).The forces associated with the compaction can be so high that the mold surface becomes deformed or even that the entire mold breaks (if the clamping force is powerful enough).High compaction pressure is a particularly difficult problem when the thickness of the lam- inate in a part varies in different positions (stepping of the thickness).In this case,even a small error in the placement of the fabric with a corre- sponding increase in local fiber volume fraction can lead to a dramatic in- crease in compaction pressure.Both thermosetting and thermoplastic powders are commonly used to stiffen the preform.Ideally,the preform- ing agent should not decrease the permeability,the wettability,or the me- chanical properties of the finished part,but it should still stiffen the perform so that it can be handled.In practice,however,this compromise is difficult to achieve.For example,the mechanical properties can be sig- nificantly reduced by the preforming operation,but they can also be close to the value without preform binder with a judicious choice of pre- forming agent.The raw material suppliers can usually recommend suit- able preforming agents for a given matrix system or provide pretreated reinforcement with binder
• Preforms improve quality and reduce part-to-part variations. For fast cycle times, the ideal is to make the preform so stiff that it becomes self-locating in the production mold. In other cases, when the mechanical properties are of paramount importance, one often wants to minimize the preform binder since the mechanical properties can be adversely affected by the binder. In general, a good preform is required to be inexpensive to make and stiff enough to be stacked and handled before injection. The fibers must stay in the direction in which they have been placed both during handling and injection. To achieve all these goals it is common to apply some form of preforming agents (binders). Both thermoplastic and thermosetting powders are commonly used for this purpose. The compaction behavior of the preform differs a lot depending on the performing method and the type of reinforcement that has been used. An important observation is that the fiber volume fraction at zero compaction pressure can differ significantly between different fabrics. A lower nominal fiber volume fraction can result in movement of the reinforcement during filling and incomplete impregnation. Another typical feature of the compaction behavior is that all fabrics behave like nonlinear (stiffening) springs and that the possible increase in fiber volume fraction from the value at rest is limited. In practice, most molds are flexible and will deform when the fiber volume fraction becomes too high (which corresponds to a high compaction pressure). The forces associated with the compaction can be so high that the mold surface becomes deformed or even that the entire mold breaks (if the clamping force is powerful enough). High compaction pressure is a particularly difficult problem when the thickness of the laminate in a part varies in different positions (stepping of the thickness). In this case, even a small error in the placement of the fabric with a corresponding increase in local fiber volume fraction can lead to a dramatic increase in compaction pressure. Both thermosetting and thermoplastic powders are commonly used to stiffen the preform. Ideally, the preforming agent should not decrease the permeability, the wettability, or the mechanical properties of the finished part, but it should still stiffen the perform so that it can be handled. In practice, however, this compromise is difficult to achieve. For example, the mechanical properties can be significantly reduced by the preforming operation, but they can also be close to the value without preform binder with a judicious choice of preforming agent. The raw material suppliers can usually recommend suitable preforming agents for a given matrix system or provide pretreated reinforcement with binder. Materials 257
258 LIQUID COMPOSITE MOLDING It should be noted that the permeability of the resin in the fiber preform depends to a great extent on the fiber volume fraction.Since the compac- tion pressure has great influence on the fiber volume fraction,this pres- sure therefore has important influence on the flow of the resin through the preforms. 2.2.1.Preforming Methods The preforming methods can roughly be classified into five basic types:(1)cut and paste,(2)spray-up of chopped fiber on preformed models,(3)thermoforming,(4)weft knitting,and(5)braiding. 2.2.1.1.Cut and Paste In this technique,sheets of fabrics are cut to simple shapes and these pieces are fit together with an adhesive or by stitching to make up the configuration of the part.It should be remembered that an adhesive can adversely affect the mechanical properties. 2.2.1.2.Spray--p In this technique,continuous rovings are chopped using a chopping device.These rovings are deposited on a perforated former having the shape of the perform.Binder solution is sprayed on the chopped fibers to provide adhesion.After spraying,hot air is allowed to circulate for about 1 minute so that the thermoplastic binder melts.After melting,the air stream is switched to cold and the preforming powder solidifies.Figure 7.9 shows an example of a preform made by spray-up. 2.2.1.3.Thermoforming In thermoforming,the fiber bed along with binders is compression molded at the proper temperature.The formability of woven fabrics is limited and only moderately double-curved shapes have been formed commercially.Wrinkles and folds formed by draping of woven fabrics can to some extent be predicted with computer simulations.There is a possibility that the fiber orientation may change during forming. 2.2.1.4.Weft Knitting There are two kinds of knitting operations:weft and warp.Both of these produce interlooped structures.These methods differ in that weft
It should be noted that the permeability of the resin in the fiber preform depends to a great extent on the fiber volume fraction. Since the compaction pressure has great influence on the fiber volume fraction, this pressure therefore has important influence on the flow of the resin through the preforms. 2.2.1. Preforming Methods The preforming methods can roughly be classified into five basic types: (1) cut and paste, (2) spray-up of chopped fiber on preformed models, (3) thermoforming, (4) weft knitting, and (5) braiding. 2.2.1.1. Cut and Paste In this technique, sheets of fabrics are cut to simple shapes and these pieces are fit together with an adhesive or by stitching to make up the configuration of the part. It should be remembered that an adhesive can adversely affect the mechanical properties. 2.2.1.2. Spray-up In this technique, continuous rovings are chopped using a chopping device. These rovings are deposited on a perforated former having the shape of the perform. Binder solution is sprayed on the chopped fibers to provide adhesion. After spraying, hot air is allowed to circulate for about 1 minute so that the thermoplastic binder melts. After melting, the air stream is switched to cold and the preforming powder solidifies. Figure 7.9 shows an example of a preform made by spray-up. 2.2.1.3. Thermoforming In thermoforming, the fiber bed along with binders is compression molded at the proper temperature. The formability of woven fabrics is limited and only moderately double-curved shapes have been formed commercially. Wrinkles and folds formed by draping of woven fabrics can to some extent be predicted with computer simulations. There is a possibility that the fiber orientation may change during forming. 2.2.1.4. Weft Knitting There are two kinds of knitting operations: weft and warp. Both of these produce interlooped structures. These methods differ in that weft 258 LIQUID COMPOSITE MOLDING
Materials 259 Chopping device Roving Binder (emulsion) Perforated former 小八 Air flow FIGURE 7.9 Preform by spray-up. knits are formed in the weft or horizontal direction,whereas warp knits are formed in the warp or vertical direction.The weft-warp knits have the advantages of conformability and can be automated easily.The disad- vantages are that high fiber volume fraction is difficult to achieve and the knit is anisotropic. 2.2.1.5.Braiding Braiding is a good technique for forming tubular structures.Braiding is available in diameters up to about 300 mm and with different types of fibers.Figure 7.10 shows an example of a braided perform. 2.3.Matrix Common requirements for resin systems for LCM processes are: Sufficiently low viscosity (about 500 cP)and long gel time to permit complete impregnation,mold filling and fiber wetting Appropriate curing characteristics to provide acceptable cycle times Adequate mechanical properties and physical characteristics to meet the performance requirements RTM processes rely heavily on polyesters,vinyl esters and(for aero- space applications)epoxies,while SRM is almost exclusively based on polyurethanes
knits are formed in the weft or horizontal direction, whereas warp knits are formed in the warp or vertical direction. The weft-warp knits have the advantages of conformability and can be automated easily. The disadvantages are that high fiber volume fraction is difficult to achieve and the knit is anisotropic. 2.2.1.5. Braiding Braiding is a good technique for forming tubular structures. Braiding is available in diameters up to about 300 mm and with different types of fibers. Figure 7.10 shows an example of a braided perform. 2.3. Matrix Common requirements for resin systems for LCM processes are: • Sufficiently low viscosity (about 500 cP) and long gel time to permit complete impregnation, mold filling and fiber wetting • Appropriate curing characteristics to provide acceptable cycle times • Adequate mechanical properties and physical characteristics to meet the performance requirements RTM processes rely heavily on polyesters, vinyl esters and (for aerospace applications) epoxies, while SRM is almost exclusively based on polyurethanes. Materials 259 FIGURE 7.9 Preform by spray-up
260 LIQUID COMPOSITE MOLDING 2.3.1.Polyester and Vinyl Ester In common with the hand-laminating industry,RTM has been domi- nated (in tonnage)by the use of polyester resins.Unsaturated polyesters are produced via a condensation reaction of organic acids(maleic and phthalic anhydride)with ethylene or propylene glycol to produce esters (Chapter 2).The styrene content of an unsaturated polyester is important in that it controls the resin viscosity and thereby the impregnation pro- cess.Increasing styrene content will decrease the viscosity but will in- crease the heat of reaction and peak exotherms at the expense of the final mechanical properties.Excessive styrene content may be detrimental to product quality since any residual monomer following curing or post-curing may continue to be lost in service with dimensional changes in the finished part. 2.3.1.1.Shrinkage Control Additives (Low Profile Additives or LPA) Polyesters and vinyl esters exhibit a significant amount of shrinkage FIGURE 7.10 Preform made by braiding
2.3.1. Polyester and Vinyl Ester In common with the hand-laminating industry, RTM has been dominated (in tonnage) by the use of polyester resins. Unsaturated polyesters are produced via a condensation reaction of organic acids (maleic and phthalic anhydride) with ethylene or propylene glycol to produce esters (Chapter 2). The styrene content of an unsaturated polyester is important in that it controls the resin viscosity and thereby the impregnation process. Increasing styrene content will decrease the viscosity but will increase the heat of reaction and peak exotherms at the expense of the final mechanical properties. Excessive styrene content may be detrimental to product quality since any residual monomer following curing or post-curing may continue to be lost in service with dimensional changes in the finished part. 2.3.1.1. Shrinkage Control Additives (Low Profile Additives or LPA) Polyesters and vinyl esters exhibit a significant amount of shrinkage 260 LIQUID COMPOSITE MOLDING FIGURE 7.10 Preform made by braiding