30 3D Fibre Reinforced Polymer Composites controlled horn gears on the flat bed arrangement as shown in Figure 2.19(Kimbara et al.,1995;Schneider et al.,1998;Laourine et al.,2000). 2多 0 Figure 2.18 Examples of possible 3D braided preforms(Ko,1989b) Figure 2.19 Computer controlled horn gears for the transfer of the yarn carrier across a flat bed braider
30 controlled horn gears on the flat bed arrangement as shown in Figure 2.19 (Kimbara et al., 1995; Schneider et al., 1998; Laourine et al., 2000). 30 Fibre Reinforced Polymer Composites Figure 2.18 Examples of possible 3D braided preforms (KO, 1989b) Figure 2.19 Computer controlled horn gears for the transfer of the yarn carrier across a flat bed braider
Manufacture of 3D Fibre Preforms 31 2.3.4 Multilayer Interlock Braiding A different class of three-dimensional braiding does not rely upon the 2-step and 4-step processes previously described,and is considered to be closer to the traditional process of 2D braiding in its operation.This proprietary braiding process,called "multilayer interlock braiding",was developed at Albany International Research Corporation (Brookstein,1991;Brookstein et al.,1993)and the machinery is analogous to a number of standard circular braiders being joined together to form a cylindrical braiding frame. This frame has a number of parallel braiding tracks around the circumference of the cylinder but the mechanism allows the transfer of yarn carriers between adjacent tracks thus forming a multilayer braided fabric with yarns interlocking adjacent layers (see Figure 2.20).The multilayer interlock braid differs from both the 4-step and 2-step braids in that the interlocking yarns are primarily in the plane of the structure and thus do not significantly reduce the in-plane properties of the preform.The 4-step and 2-step processes produce a greater degree of interlinking as the braiding yarns travel through the thickness of the preform,but therefore contribute less to the in-plane performance of the preform. ●Axials Figure 2.20 Schematic of the multilayer interlock braiding process A disadvantage of the multilayer interlock equipment is that due to the conventional sinusoidal movcment of the yarn carriers to form the preform,the equipment is not able to have the density of yarn carriers that is possible with the 2-step and 4-step machines. The consequence of this is that multilayer interlock braiders will be larger than 2-step and 4-step machines for a comparable number of carriers and are considered to be less versatile in the range of preform architectures produced (Kostar and Chou,1999). However the use of the traditional horn gear mechanisms offers improved braiding speed over the 2-step and 4-step processes. There are a number of disadvantages with all the 3D braiding processes described here(Kostar and Chou,1999).Firstly,compared to other textile processes,braiding can only make preforms of small scale relative to the size of the machinery.Also,the
Manufacture of 30 Fibre Preforms 31 2.3.4 Multilayer Interlock Braiding A different class of three-dimensional braiding does not rely upon the 2-step and 4-step processes previously described, and is considered to be closer to the traditional process of 2D braiding in its operation. This proprietary braiding process, called “multilayer interlock braiding”, was developed at Albany International Research Corporation (Brookstein, 1991; Brookstein et al., 1993) and the machinery is analogous to a number of standard circular braiders being joined together to form a cylindrical braiding frame. This frame has a number of parallel braiding tracks around the circumference of the cylinder but the mechanism allows the transfer of yarn carriers between adjacent tracks thus forming a multilayer braided fabric with yarns interlocking adjacent layers (see Figure 2.20). The multilayer interlock braid differs from both the 4-step and 2-step braids in that the interlocking yarns are primarily in the plane of the structure and thus do not significantly reduce the in-plane properties of the preform. The 4-step and 2-step processes produce a greater degree of interlinking as the braiding yarns travel through the thickness of the preform, but therefore contribute less to the in-plane performance of the preform. Axials Figure 2.20 Schematic of the multilayer interlock braiding process A disadvantage of the multilayer interlock equipment is that due to the conventional sinusoidal movement of the yarn carriers to form the preform, the equipment is not able to have the density of yarn carriers that is possible with the 2-step and 4-step machines. The consequence of this is that multilayer interlock braiders will be larger than 2-step and 4-step machines for a comparable number of carriers and are considered to be less versatile in the range of preform architectures produced (Kostar and Chou, 1999). However the use of the traditional horn gear mechanisms offers improved braiding speed over the 2-step and 4-step processes. There are a number of disadvantages with all the 3D braiding processes described here (Kostar and Chou, 1999). Firstly, compared to other textile processes, braiding can only make preforms of small scale relative to the size of the machinery. Also, the
32 3D Fibre Reinforced Polymer Composites length of preform that can be braided before re-supply of the yarn is necessary is limited by the need for the yarn to be on the moving carriers,which ideally must be small and light for rapid braid production.Thus the production of long lengths of the preform can be slow due to the need to re-stock the yarn carriers.One of the greatest current disadvantages however is the fact that the 3D braiding process is still very much at the machinery development stage.Therefore there are limitations to the type of preform that can be made commercially and there are very few companies that have the necessary experience and equipment to manufacture these preforms. 2.4 KNITTING Knitting may not at first appear to be a manufacturing technique that would be suitable for use in the production of composite components and it is arguably the least used and understood of the four classes of textile processes described here.However,the knitted carbon and glass fabric that can be produced on standard industrial knitting machines has particular properties that potentially make it ideally suited for certain composite components. 2.4.1 Warp and Weft Knitting Two traditional knitting processes,weft knitting and warp knitting,are available to manufacture preforms for composite structures.Both of these techniques can be performed upon standard,industrial knitting machines with high performance yarns such as glass and carbon.One critical issue that must be considered is that the more advanced knitting machines have electronic control systems close to the knitting region where broken fibres can be generated.The use of carbon yarns with these machines should be avoided as loose carbon fibres can generate electrical shorts.In warp knitting there are multiple yarns being fed into the machine in the direction of fabric production, and each yarn forms a line of knit loops in the fabric direction.For weft knitting there is only a single feed of yarn coming into the machine at 90 to the direction of fabric production and this yarn forms a row of knit loops across the width of the fabric (see Figure 2.21). (a) (b) Figure 2.21 Illustration of typical a)weft and b)warp knitted fabric architectures
32 30 Fibre Ueinforced Polymer Composites length of preform that can be braided before re-supply of the yarn is necessary is limited by the need for the yarn to be on the moving carriers, which ideally must be small and light for rapid braid production. Thus the production of long lengths of the preform can be slow due to the need to re-stock the yarn carriers. One of the greatest current disadvantages however is the fact that the 3D braiding process is still very much at the machinery development stage. Therefore there are limitations to the type of preform that can be made commercially and there are very few companies that have the necessary experience and equipment to manufacture these preforms. 2.4 KNITTING Knitting may not at first appear to be a manufacturing technique that would be suitable for use in the production of composite components and it is arguably the least used and understood of the four classes of textile processes described here. However, the knitted carbon and glass fabric that can be produced on standard industrial knitting machines has particular properties that potentially make it ideally suited for certain composite components. 2.4.1 Warp and Weft Knitting Two traditional knitting processes, weft knitting and warp knitting, are available to manufacture preforms for composite structures. Both of these techniques can be performed upon standard, industrial knitting machines with high performance yams such as glass and carbon. One critical issue that must be considered is that the more advanced knitting machines have electronic control systems close to the knitting region where broken fibres can be generated. The use of carbon yarns with these machines should be avoided as loose carbon fibres can generate electrical shorts. In warp knitting there are multiple yams being fed into the machine in the direction of fabric production, and each yarn forms a line of knit loops in the fabric direction. For weft knitting there is only a single feed of yarn coming into the machine at 90" to the direction of fabric production and this yarn forms a row of knit loops across the width of the fabric (see Figure 2.21). Figure 2.21 Illustration of typical a) weft and b) warp knitted fabric architectures
Manufacture of 3D Fibre Preforms 33 The formation of the knitted fabric is accomplished through a row of closely spaced needles(needle bed)which pull loops of yarn through previously formed knit loops (Figure 2.22).The needle bed can be in a circular or flat configuration and an increase in the number of needle beds available in the machine for knitting increases the potential complexity of the fabric knit architecture.For weft knitted fabrics the motion of the yarn carrier as it travels across the width of the needle bed (or around the circumference for circular machines)draws the yarn into the needles for knitting(Figure 2.23).In much the same way as weaving,warp knitting machines have an individual supply of yarn feeding each knitting needle. Figure 2.22 Illustration of knitting process Yarn carrier Needle Bed Figure 2.23 Flat bed knitting machine showing the yarn carrier and needle beds
Manufacture of 30 Fibre Preforms 33 The formation of the knitted fabric is accomplished through a row of closely spaced needles (needle bed) which pull loops of yarn through previously formed knit loops (Figure 2.22). The needle bed can be in a circular or flat configuration and an increase in the number of needle beds available in the machine for knitting increases the potential complexity of the fabric knit architecture. For weft knitted fabrics the motion of the yam carrier as it travels across the width of the needle bed (or around the circumference for circular machines) draws the yarn into the needles for knitting (Figure 2.23). In much the same way as weaving, warp knitting machines have an individual supply of yarn feeding each knitting needle. Figure 2.22 Illustration of knitting process Figure 2.23 Flat bed knitting machine showing the yarn carrier and needle beds
34 3D Fibre Reinforced Polymer Composites Standard warp and weft knitted fabric are regarded by many as 2D fabric,however, machines with two or more needle beds are capable of producing multilayer fabrics with yarns that traverse between the layers.Figure 2.24 shows a schematic of such a fabric and the range of knit architectures that can be produced with current industrial machines is quite extensive.These flat fabrics can also be formed with variable widths,splits to allow multiple,parallel fabrics to be formed,and holes with sealed edges. Figure 2.24 Schematic of a multilayer knitted fabric It is clear from the illustrations of knit architectures that the primary difference between knitted fabric and fabric made by the other textile processes described here is in the high degree of yarn curvature that results from the knitting process.This architecture results in a fabric that will provide less structural strength to a composite(compared to woven and braided fabrics)but is highly conformable and thus ideally suited to manufacture relatively non-structural components of complex shape.This conformability means that layers of knitted fabric can be stretched to cover the complete tool surface without the need to cut and overlap sections.This reduces the amount of material wastage and helps to decrease the costs of manufacturing complex shape components (Bannister and Nicolaidis,1998).Examples of such components are shown in Figure 2.25. Changing the knit architecture can vary the properties of knitted fabric itself quite significantly.In this fashion,characteristics such as fabric extensibility,areal weight, thickness,surface texture,etc,can all be controlled quite closely.This allows knitted fabric to be tailor-made to suit the particular component being produced.Both warp and weft knitting also have the ability to produce fabric with relatively straight,oriented sections of the knitting loop (see Figure 2.26)that can be designed to improve the in- plane mechanical performance of the fabric.Warp knitting in particular has been used to produce fabric with additional straight yarns laid into and bound together by the knit structure,but this will be described more fully in a later section
34 30 Fibre Reinforced Polymer Composites Standard warp and weft knitted fabric are regarded by many as 2D fabric, however, machines with two or more needle beds are capable of producing multilayer fabrics with yams that traverse between the layers. Figure 2.24 shows a schematic of such a fabric and the range of knit architectures that can be produced with current industrial machines is quite extensive. These flat fabrics can also be formed with variable widths, splits to allow multiple, parallel fabrics to be formed, and holes with sealed edges. Figure 2.24 Schematic of a multilayer knitted fabric It is clear from the illustrations of knit architectures that the primary difference between knitted fabric and fabric made by the other textile processes described here is in the high degree of yarn curvature that results from the knitting process. This architecture results in a fabric that will provide less structural strength to a composite (compared to woven and braided fabrics) but is highly conformable and thus ideally suited to manufacture relatively non-structural components of complex shape. This conformability means that layers of knitted fabric can be stretched to cover the complete tool surface without the need to cut and overlap sections. This reduces the amount of material wastage and helps to decrease the costs of manufacturing complex shape components (Bannister and Nicolaidis, 1998). Examples of such components are shown in Figure 2.25. Changing the knit architecture can vary the properties of knitted fabric itself quite significantly. In this fashion, characteristics such as fabric extensibility, areal weight, thickness, surface texture, etc, can all be controlled quite closely. This allows knitted fabric to be tailor-made to suit the particular component being produced. Both warp and weft knitting also have the ability to produce fabric with relatively straight, oriented sections of the knitting loop (see Figure 2.26) that can be designed to improve the inplane mechanical performance of the fabric. Warp knitting in particular has been used to produce fabric with additional straight yarns laid into and bound together by the knit structure, but this will be described more fully in a later section