Fiber weaving structure processing techniques
Traditional 2D weaving involves the interlacing of warp and weft yarns on a loom or multi-arm weaving machine to create plain, twill, and satin weave structures. The weaving process consists of five basic actions: shedding, picking, beating, taking-up, and letting-off. There are various shedding techniques available, such as shuttle, rapier, and air-jet. The single-layer 2D weaving process can also be applied to weave certain 3D woven structures, including 3D orthogonal and angle-interlock structures, 3D hollow spacer structures with fabric interlayers and honeycomb structures, 3D shell structures, and 3D node structures. Figure 1 illustrates the 2D weaving principles for manufacturing both conventional 2D and 3D angle-interlock woven structures.
Figure 1: 2D weaving principles for 2D and 3D woven structures
While traditional 2D weaving techniques can be used to produce various solid 3D woven structures, the thickness dimension is limited. For this reason, specialized 3D weaving machines have been developed to manufacture 3D woven fabrics. One of the earliest machines developed abroad is a special loom used to produce orthogonal structures with X, Y, and Z yarns, as shown in Figure 2.
Figure 2: Specialized 3D weaving machine for manufacturing 3D orthogonal structures
In the weaving process, the Z-direction fibers remain stationary while the X fibers are first inserted and beaten into the appropriate position, followed by the insertion and beating of the Y fibers into their respective positions. This operation is repeated to generate a compact structure until the desired height is reached, resulting in a 3D rectangular cross-section structure. Subsequently, a dual-opening operation 3D weaving machine was developed abroad. This opening system allows the warp yarns to interlace with the weft yarns both horizontally and vertically. This special 3D weaving technique can also directly produce woven molded materials, providing ultimate structural integrity even when the fabric is cut or damaged.
The manufacturing of three-axis weaving structures is achieved through the integration of traditional 2D weaving and automated weaving techniques. A typical three-axis weaving machine, designed by Dow and manufactured by Barber-Colman, is shown in Figure 2.28. This equipment utilizes a rotating wheel with spindles to lay down the warp yarns and employs a rapier edge to create a shed for the insertion of weft yarns.
Figure 3: Rotating wheel with spindles for manufacturing three-axis weaving structures
Processing techniques for fiber knitting structures
The principles of warp knitting and weft knitting are illustrated in Figure 4. In warp knitting structures, each needle on the needle bed continuously feeds and forms loops with the same warp yarn within a knitting cycle. Specifically, needles A, B, C, and D are sequentially fed with the same warp yarn, resulting in a section of looped fabric (E, F, G, H). In weft knitting structures, within the same knitting cycle, yarn feeding and loop formation occur on each needle in the needle bar. All needles in the needle bar (A, B, C, and D) are individually overlapped by separate weft yarn guides (E, F, G, and H).
Figure 4: Knitting principles of fiber structures: (top) warp knitting; (bottom) weft knitting
Circular weft knitting is characterized by the production of tubular fabric structures. However, flat weft knitting offers greater flexibility in constructing various types of tubular structures, including single tubes, bifurcated tubes, and multi-branch tubes, due to its ability for individual needle selection, loop transfer, multi-system knitting, and the use of sinkers and pressers. Figure 5 illustrates the knitting of a single tube using selected needles on a computerized flat knitting machine.
Figure 5: Knitting of a single tube on a computerized flat knitting machine
Tubular knitting is achieved by alternately knitting a yarn on two needle beds and transferring the yarn from one bed to the other only at the edges to form a tube. By combining tubular knitting with internal knitting techniques, various variations of single-tube knitting structures can be achieved.
Intarsia knitting technology enables knitting machines to use multiple different fibers to knit different parts of the fabric. Fibers can be used individually or in combination. With this technique, a single tube can be formed by initially knitting a certain length with one fiber and then introducing another fiber to simultaneously form two tubes, resulting in a bifurcated tube. Similarly, by using more fibers, multi-branch tube structures can be formed.
The versatility of computerized flat knitting machines enables the possibility of weaving 3D structures with more complex shapes, such as domes, spheres, and boxes, as shown in Figure 6. A 2D repeating shaping segment can form a knitted dome structure (Figure 6(b)). This 2D segment is achieved by repeatedly increasing and decreasing the number of needles in action. Each shaping segment represents an operation of gradually widening and then narrowing the fabric. The type of shaping segment affects the angle and height-to-base ratio of the dome, while the number of shaping segments affects the shape of the dome. By replacing the elliptical segments of the dome with triangular segments, a box-like structure can be formed.
Figure 6: (a) Circular dome, (b) Knitted dome structure, (c) Knitted sphere, (d) Knitted box
As shown in Figure 6(d), for dome structures, the lines representing the decrease or increase in the number of operating needles are linear rather than curved. The type of shaping segment affects the angle of the resulting cuboid. The ratio between the number of shaping and non-shaping needles determines the aspect ratio of the obtained box. The ability to change the number of knitting needles provides the greatest potential for computerized flat knitting machines to create various 3D shapes.
Interval structures are produced using two sets of needles on circular, flat weft knitting, or warp knitting machines. Circular weft knitting machines equipped with a cylinder and a disc can produce interval fabrics, where the individual outer layers are connected by fibers. Interval fabrics on circular weft knitting machines are created by knitting two different fabrics separately using the latch and cylinder needles, and then connecting the two layers with tucks on the latch and cylinder needles (Figure 7).
Figure 7: Production of interval fabrics on a circular weft knitting machine: (a) Double-bed circular knitting machine; (b) Knitting interval fabric on a circular machine
The distance between the two separate fabric layers can be adjusted by changing the height of the latch needles relative to the machine cylinder. The preset thickness of the interval fabric in this manner can range from 1.5 to 5.5 millimeters. Similarly to producing interval fabrics on circular machines, interval fabrics with yarn interval layers are produced on flat knitting machines by forming two independent fabric layers on the front and back needle beds and then connecting them with tucks on both needle beds (Figure 8).
Figure 8: Production of interval fabrics on a computerized flat knitting machine: (a) Computerized flat knitting machine; (b) Knitting interval fabric on a flat machine
The distance between the two needle beds determines the thickness of the interval fabric. Unlike circular weft knitting machines, the distance between the two needle beds in flat weft knitting machines is usually fixed at around 4 millimeters. The difference between weft-knitted interval fabrics and other types of interval fabrics is that their three basic structural elements (i.e., top layer, bottom layer, and interval layer) are knitted together in the same knitting cycle. Weft-knitted interval fabrics are produced on double-needle bar Raschel machines, as shown in Figure 9(a). When guide bars 1 and 2 overlap the front needle bar and guide bars 5 and 6 overlap the back needle bar (knitting the top and bottom layers, respectively), guide bars 3 and 4 sequentially overlap the interval yarn around both needle bars. Figure 9(b) illustrates the process of producing interval fabrics on a double-needle bar Raschel machine RD 6.
Figure 9: Production of interval fabrics on a double-needle bar Raschel machine: (top) Schematic illustration of the principle; (bottom) Equipment diagram