Knit fabrics are constructed by interloping one or more sets of yarns
Common examples of apparel utilizing weft knitted fabric are socks. Knitting is a more versatile manufacturing process, as entire garments can be manufactured on a single knitting machine, and it is much faster than weaving. However, due to the looping, more yarn is required to manufacture a knitted garment than a comparable woven garment. Thus any cost savings gained in manufacturing speed are offset by the higher materials cost.
Knits are comfortable fabrics, as they adapt to body movement. The loop structure contributes to elasticity beyond what is capable of the yarns or fibers alone. A knit fabric is prone to snagging, and has a higher potential shrinkage than a woven fabric. The loop structure also provides many cells to trap air, and thus provides good insulation in still air. Knits are not typically very wind- or water-repellent.
Knit fabrics are composed of intermeshing loops of yarns. There are two major types of knits: weft knits and warp knits, as illustrated in Fig. 4.7. In weft knits, each weft yarn lies more or less at right angles to the direction in which the fabric is produced, and the intermeshing yarn traverses the fabric crosswise. In warp knits, each warp yarn is more or less in line with the direction in which the fabric is produced, and the intermeshing yarn traverses the fabric lengthwise. Similar to the way that woven fabrics have warps and wefts, knit fabrics have courses and wales, which lie in the crosswise and lengthwise direction, respectively. However, unlike woven fabrics, courses and wales are not composed of different sets of yarns; rather are formed by a single yarn.
Weft blend knitted fabrics are produced predominantly on circular knitting machines. The simplest of the two major weft knitting machines is a jersey machine. Generally, the terms circular knit and plain knit refer to jersey goods. The loops are formed by knitting needles and the jersey machine has one set of needles. Typical fabrics are hosiery, T-shirts, and sweaters.
‘Loop’ is the basic unit of knit fabric. As illustrated in Fig, 4.7a, in weft knits, a loop, called a needle loop, consists of a head and two legs, and the section of yarn connecting two adjacent needle loops is called the sinker. In warp knits, the needle loop is divided into overlap and underlap, as illustrated in Fig. 4.7b. Each loop in a printed fabric is a stitch. Alternative to fabric count for woven fabrics, cut (or gauge) and stitch density are used to represent the closeness of the intermeshing loops. Cut or gauge indicates the number of knitting needles per unit length along the crosswise or lengthwise direction. The greater the number, the closer together the loops are to each other. Stitch density is the number of stitches per unit area, obtained by multiplying the number of courses per inch (25 mm) by the number of wales per inch (25 mm). Like woven fabrics, a knit fabric also has a technical face and a technical back and can differ in appearance on each side. The technical face is the side where the loops are pulled toward the viewer. Knit fabric also has an effect side, which is intended to be used outermost on a garment or other textile product. In some cases, the technical face and the effect side are the same; but in others, they are opposite.
The two common warp-knit fabrics are tricot and raschel (Fig. 10.9). Tricot, solely composed of knit stitches, represents the largest quantity of warp knit. It is characterized by fine, vertical wales on the surface and crosswise ribs on the back. Tricot fabrics may be plain, loop-raised or corded, ribbed, cropped velour or patterned designs. It is commonly used for lingerie owing to its good drapability. It is used for underwear, night-wear, dresses, blouses and outerwear.4 Tricot fabric is used in household products such as sheets and pillowcases. It is also be used for upholstery fabrics for car interiors.Most warp cotton stripe jersey knit fabrics tend to curl, including the most important type known as Jersey stitch (in the USA) or Locknit stitch (in the UK). If they receive appropriate heat treatment, synthetic warp knit fabrics do not curl. In dyeing, finishing, cutting and sewing garments, it helps to know the face and back of the fabric and its curling propensity. When a greige nylon Jersey stitch fabric is put on a table technically upright (having the loop side up), the top and bottom edges of the fabric will curl upwards or towards the loop side or technical face. However, the side edges will curl under the fabric towards the float or technical backside of the fabric.
MWKs have evolved through structural modifications of warp-knitted fabrics and are predominantly fabrics with inlay yarns in the warp (90°), wale (0°) and bias (± θ°) directions. Warp, weft and bias yarns are held together by a chain or tricot stitch through the thickness of the fabric (Du and Ko, 1996). Layers of 0° need to be placed somewhere other than the top or bottom layer to ensure structural integrity. The amount of fibre and the orientation of the inlay yarns can be controlled, which is advantageous for preform engineering. As a result, the insert yarns are made from a much higher linear density yarn than the stitch yarns, since they form the load-bearing component of the fleece fabric structure (Du and Ko, 1996). Figure 8.4 shows the configuration of the chain and tricot MWK structures.Yarns in a simple weft-knitted structure, as shown in Figure 11.13a, lack the long continuous paths found in woven fabrics and there would be stress concentrations where yarns cross one another. This limits their mechanical performance, but as shown in Chapter 3, they do have applications as composites. In the free state, the knit fabric shows a low resistance to extension and shear, with accompanying area change, until the yarns jam together. This means that they are easily draped into complex shapes.