《纺织复合材料》课程参考文献（Principles of the Manufacturing of Composite Materials）CHAPTER 8 Long Fiber Thermoplastic Matrix Composites

Preliminary Material Combinations (PMCs) 299 3.3.Commingling/Bundling and Microbraiding As long as the thermoplastic matrix can be spun into fiber form,it is possible to combine the thermoplastic matrix and reinforcing fibers by commingling(Figure 8.7).Wetting of the fiber in this case is deferred un- til the composite is consolidated.This approach leads to flexible pre- forms since the independence of the structural elements is maintained until the preform is consolidated.Because the matrix filament has not wetted out the reinforcing fiber until consolidation,more effort is re- quired to complete consolidation compared to preconsolidated prepreg. Another way of bundling the matrix filament together with the rein- forcing fibers is to microbraid the matrix filaments with the reinforcing (Figure 8.8). 3.4.Fabric/Film Sandwich Film stacking with dry fiber or commingled slit film and dry fiber are examples of combining processes where wetting is deferred until consol- Nylon Wrapping Nylon Wrapping Nylon outside Wrapping 6K 6K Commingled Commingled Bundle Bundle reinforcing thermoplastic fibre fibre (b) FIGURE 8.7 Commingling matrix filaments with reinforcing fibers

3.3. Commingling/Bundling and Microbraiding As long as the thermoplastic matrix can be spun into fiber form, it is possible to combine the thermoplastic matrix and reinforcing fibers by commingling (Figure 8.7). Wetting of the fiber in this case is deferred un￾til the composite is consolidated. This approach leads to flexible pre￾forms since the independence of the structural elements is maintained until the preform is consolidated. Because the matrix filament has not wetted out the reinforcing fiber until consolidation, more effort is re￾quired to complete consolidation compared to preconsolidated prepreg. Another way of bundling the matrix filament together with the rein￾forcing fibers is to microbraid the matrix filaments with the reinforcing (Figure 8.8). 3.4. Fabric/Film Sandwich Film stacking with dry fiber or commingled slit film and dry fiber are examples of combining processes where wetting is deferred until consol￾FIGURE 8.7 Commingling matrix filaments with reinforcing fibers. Preliminary Material Combinations (PMCs) 299

300 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES Axial fiber Multiple axial fibers FIGURE 8.8 Microbraiding of matrix filaments with reinforcing fibers.The left shows one single reinforcement fiber tow at the center surrounded by matrix filaments.The right shows four reinforcement tows surrounded by matrix filaments(adapted from Ref- erence [4],with permission from Canadian Association for Composite Structures and Materials). idation(Figure 8.9).These options are less expensive than commingled fibers,but the degree of physical mixing is not as good as with commingled fibers. 3.5.Surface Polymerization Surface polymerization of the polymer on the fiber consists of elec- tro-polymerization on graphite fibers or vapor deposition polymeriza- Carbon fiber weave PEKK Film 0° 90 FIGURE 8.9 Fabric/ilm sandwich

idation (Figure 8.9). These options are less expensive than commingled fibers, but the degree of physical mixing is not as good as with commingled fibers. 3.5. Surface Polymerization Surface polymerization of the polymer on the fiber consists of elec￾tro-polymerization on graphite fibers or vapor deposition polymeriza- 300 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.8 Microbraiding of matrix filaments with reinforcing fibers. The left shows one single reinforcement fiber tow at the center surrounded by matrix filaments. The right shows four reinforcement tows surrounded by matrix filaments (adapted from Ref￾erence [4], with permission from Canadian Association for Composite Structures and Materials). FIGURE 8.9 Fabric/ilm sandwich

Fabrication of the Final Product 301 tion of paraxylene on graphite fibers,analogous to chemical vapor deposition.These approaches can produce individually coated fibers, hence the prepreg can have virtually the same flexibility as commingled fibers as well as complete fiber wetting. 4.FABRICATION OF THE FINAL PRODUCT The transformation of the preliminary material combinations into the final thermoplastic composite product is mostly done using compression molding,and to a certain extent,fiber placement process. 4.1.Compression Molding All of the PMCs(tape,tows with clinging powder,commingled tows, microbraided tows,tows with surface polymerization,or fabric/film sandwich)can be stacked up and compression molded to make the final composite product.The form of the PMC depends on the complexity of the configuration of the final product. Thermoplastic tapes do not have sufficient drape and tack at room tem- perature to permit lay-up of complex shapes.Normally,flat pieces are made if tapes are to be used.Since cold thermoplastic tapes lack tack, they must be spot welded to the ply immediately below the ply being laid down.Either heat gun or ultrasonic gun is normally used for this task. Tows with commingled matrix filaments,microbraided tows,tows with surface polymerization,or fabric/film sandwiches are more flexible than tapes and they can be arranged to make shapes other than flat plates. However,since the fibers in these configurations are not well con- strained,alignment and tension need to be applied to the tows to assure proper fiber orientation control.For example,Figure 8.10 shows the ar- FIGURE 8.10 Springs are used to align and tension commingled fiber tows

tion of paraxylene on graphite fibers, analogous to chemical vapor deposition. These approaches can produce individually coated fibers, hence the prepreg can have virtually the same flexibility as commingled fibers as well as complete fiber wetting. 4. FABRICATION OF THE FINAL PRODUCT The transformation of the preliminary material combinations into the final thermoplastic composite product is mostly done using compression molding, and to a certain extent, fiber placement process. 4.1. Compression Molding All of the PMCs (tape, tows with clinging powder, commingled tows, microbraided tows, tows with surface polymerization, or fabric/film sandwich) can be stacked up and compression molded to make the final composite product. The form of the PMC depends on the complexity of the configuration of the final product. Thermoplastic tapes do not have sufficient drape and tack at room tem￾perature to permit lay-up of complex shapes. Normally, flat pieces are made if tapes are to be used. Since cold thermoplastic tapes lack tack, they must be spot welded to the ply immediately below the ply being laid down. Either heat gun or ultrasonic gun is normally used for this task. Tows with commingled matrix filaments, microbraided tows, tows with surface polymerization, or fabric/film sandwiches are more flexible than tapes and they can be arranged to make shapes other than flat plates. However, since the fibers in these configurations are not well con￾strained, alignment and tension need to be applied to the tows to assure proper fiber orientation control. For example, Figure 8.10 shows the ar￾FIGURE 8.10 Springs are used to align and tension commingled fiber tows. Fabrication of the Final Product 301

302 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES Melting Consolidation Solidification Time (t) FIGURE 8.1I Heating and pressurizing cycles. rangement where tension springs are used to tighten the commingled tows before being placed inside a compression molding press to make a flat plate. Commingled tows,microbraided tows or tows with clinging powder can also be woven into fabrics,or braided into preforms before molding. Subsequent to the preliminary assembly,the assembly of the dry fibers is placed in a compression molding machine for final molding.The molding parameters consist of application of heat and pressure for a cer- tain amount oftime.The general schedule for this application is shown in Figure 8.11,where the temperature needs to be increased before the pres- sure is applied.This allows the resin to be liquefied and to flow through the interstices to wet the fibers before the liquid resin is squeezed out for its minimization.The pressure is also maintained after the temperature is reduced from the maximum temperature.This prevents the fiber network from springing back before the resin solidifies. There are two distinct approaches to making shapes from thermoplas- tic composite materials:preimpregnate the fibers with the resin and then mold into shape,or arrange the reinforcement and resin together in the mold and then consolidate and impregnate simultaneously. Impregnation after shaping.In film stacking technology,layers of reinforcement fiber are interleaved with layers of film and the whole is consolidated into simple shapes.The principal advantage of this approach,especially for drapable products such as co-woven or hybridized fiber is the potential for forming a

rangement where tension springs are used to tighten the commingled tows before being placed inside a compression molding press to make a flat plate. Commingled tows, microbraided tows or tows with clinging powder can also be woven into fabrics, or braided into preforms before molding. Subsequent to the preliminary assembly, the assembly of the dry fibers is placed in a compression molding machine for final molding. The molding parameters consist of application of heat and pressure for a cer￾tain amount of time. The general schedule for this application is shown in Figure 8.11, where the temperature needs to be increased before the pres￾sure is applied. This allows the resin to be liquefied and to flow through the interstices to wet the fibers before the liquid resin is squeezed out for its minimization. The pressure is also maintained after the temperature is reduced from the maximum temperature. This prevents the fiber network from springing back before the resin solidifies. There are two distinct approaches to making shapes from thermoplas￾tic composite materials: preimpregnate the fibers with the resin and then mold into shape, or arrange the reinforcement and resin together in the mold and then consolidate and impregnate simultaneously. • Impregnation after shaping. In film stacking technology, layers of reinforcement fiber are interleaved with layers of film and the whole is consolidated into simple shapes. The principal advantage of this approach, especially for drapable products such as co-woven or hybridized fiber is the potential for forming a 302 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.11 Heating and pressurizing cycles

Fabrication of the Final Product 303 complex shape by hand and subsequently consolidating.The most obvious disadvantage of this approach is that it requires a relatively protracted molding cycle in order to carry out the impregnation stage and that impregnation stage contributes to product quality in two ways:by defining the microstructure of the molding,and by establishing the interface between the resin and the fiber on which the service properties of the composite depend. Preimpregnated product forms.Each product form has its own particular advantages:impregnated woven fabrics are most appropriate for thin skinned structures;woven single tow tapes, including tied uniaxial materials,offer modest drapability and can be made as very broad products.Uniaxial tapes are the most versatile product especially suitable for designing anisotropic composites. Table 8.3 shows a comparison of strengths of samples made from the two types of products.It appears that the products relying on a post im- pregnation technology do not generate the full potential strength of the fiber reinforcement.Cogswell [3]attributed this difference to fiber attri- tion during the molding impregnation stage where the forces necessary to squeeze the resin into the spaces between the fibers also force the fi- bers together.By contrast,in preimpregnated products each fiber is lubri- cated by a protective coating of viscous polymer. 4.1.1.Heat Transfer-Melting As discussed in Chapter 1,in order to make a composite,the resin has to wet the fibers.To do this,the resin has to have low viscosity (in liquid TABLE 8.3 Properties as a Function of Impregnation Route [3]. Axial Flexural Short Beam Impact Energy Strength Shear Strength 2 mm Sheet (MPa) (MPa) (U) Preimpregnated products Cross plied uniaxial 907 76 23 Woven single tow 929 68 23 Woven fabric 1052 80 29 Impregnation after shaping Co-woven fibers 782 60 13 Film stacked 680 67 9 Powder coated fabric 545 54

complex shape by hand and subsequently consolidating. The most obvious disadvantage of this approach is that it requires a relatively protracted molding cycle in order to carry out the impregnation stage and that impregnation stage contributes to product quality in two ways: by defining the microstructure of the molding, and by establishing the interface between the resin and the fiber on which the service properties of the composite depend. • Preimpregnated product forms. Each product form has its own particular advantages: impregnated woven fabrics are most appropriate for thin skinned structures; woven single tow tapes, including tied uniaxial materials, offer modest drapability and can be made as very broad products. Uniaxial tapes are the most versatile product especially suitable for designing anisotropic composites. Table 8.3 shows a comparison of strengths of samples made from the two types of products. It appears that the products relying on a post im￾pregnation technology do not generate the full potential strength of the fiber reinforcement. Cogswell [3] attributed this difference to fiber attri￾tion during the molding impregnation stage where the forces necessary to squeeze the resin into the spaces between the fibers also force the fi￾bers together. By contrast, in preimpregnated products each fiber is lubri￾cated by a protective coating of viscous polymer. 4.1.1. Heat Transfer-Melting As discussed in Chapter 1, in order to make a composite, the resin has to wet the fibers. To do this, the resin has to have low viscosity (in liquid TABLE 8.3 Properties as a Function of Impregnation Route [3]. Axial Flexural Strength (MPa) Short Beam Shear Strength (MPa) Impact Energy 2 mm Sheet (J) Preimpregnated products Cross plied uniaxial 907 76 23 Woven single tow 929 68 23 Woven fabric 1052 80 29 Impregnation after shaping Co-woven fibers 782 60 13 Film stacked 680 67 9 Powder coated fabric 545 54 Fabrication of the Final Product 303