Knit Fabric; Different types and classification of knitted fabric

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Single Jersey Knits Fabric  

Knitting Machines with one needle bed and one set of needles are called jersey machines or single-knit machines. With one set of needles used for knitting and one needle bed, all needles face the same direction; all stitches are pulled to the same side of the knitted fabric. As a result, single jersey fabrics have a smooth face with a vertical grain on the right side of the fabric and a width wise grain on the back side. The knittig loops formed by the jersey machine are formed in one direction only, which gives a different appearance to each side of the fabric. The basic knit fabric produced by this knitting machine is known alternately as a plain, single knit, or jersey. The terms are interchangeable.Jersey stretches slightly more in the crosswise than the lengthwise direction. If one stitch breaks, the fabric may ladder, or run. Jersey fabrics tend to curl at the edges and are less stable than are some other types of knits. This is the result of the pressures exerted during knitting. In addition jersey knits may twist or skew after laundering, as the twisting tensions imposed during the knitting process are relaxed. 

Special finishing techniques are used to overcome these tendencies and maintain fabric stability; the principal ones use starches, gum mixtures, polyvinyl acetate emulsions, and resins. 

A great many items of hosiery, sweaters, and other wearing apparel are made from plain jersey knits. Consumer Brief 16.1 highlights one of the common uses of jersey knit fabrics: Tshirts. Plain knit fabrics can also be made into designs of two or more colors by use of a patterning mechanism that controls the selection and feeding of yarns and types of stitches to create jacquard knits. 

Double Jersey Knit Fabrics 

The term double knit is generally applied by consumers to fabrics that are, technically, double jersey fabrics. Double jersey fabrics are also made on two-bed knitting machines, but the arrangement of the needles is different from that for knitting rib fabrics. The layers of loops alternate from one side to the other, locking the two layers together. Double knit fabrics have the same appearance on both sides of the fabric, that is, exhibiting the appearance of the face or outer side of a single knit on both sides. Twice as much yarn is incorporated into double knit fabrics as into comparable single knits 

Interlock Knit fabric

Interlock knits are produced on a special machine that has alternating long and short needles on both beds. Long and short needles are placed opposite each other. Long needles knit the first feeder yarn; short needles knit the second feeder yarn. The fabric created is an interlocking of two 1 X 1 rib structures. The resulting fabric, like double knit fabrics, is thicker than single knit fabric, and more stable in the width wise direction. Interlock fabrics have been traditionally used for underwear. They are produced more slowly than are other rib knits and are generally made in plain colors or simple patterns because the addition of pattern slows down the manufacture even further 

High Pile Fabrics 

High-pile fabrics, such as imitation furs and plushes, are usually knitted by a jersey machine. While the knitting is taking place, a sliver of staple fiber is fed into the machine. These fibers are caught in the tight knit and are held firmly in place. Although any staple fiber can be used for the pile, the greatest quantity of these fabrics are made with acrylic and modacrylic fibers in the pile. By using staple fibers of varying lengths, adding color through fiber dyeing or printing on the surface of the pile, and by shearing or brushing the pile, an enormous variety of effects can be achieved. The use of knitted pile fabric ranges from excellent imitations of furs, such as leopard, tiger, mink, or mouton, to colorful pile outerwear, coat linings, or pile carpet fabrics.

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Loop Formation in knit fabric structure

Loop Formation 

The spring beard needle is formed from one piece of thin wire. One end of the needle is drawn into thinner dimensions and is curved to form a hook. The flexible outer side of the hook can be pressed against the stem of the needle to close the hook for sliding a formed loop off and beginning a new loop. In 1847 Matthew Townshend invented a different type of hook known as the latch needle, which has come to be the most widely used type of needle. Its operation s similar to that of the spring beard needle, except that instead of having to mechanically press the flexible wire of the needle closed so that the forming yarn loop will not slide off, a latch closes to hold the yarn in place. 

1. The old loop is held on the stem of the needle. The latch is open (a). 

2. The hook grasps the yarn to begin forming a new loop (b). 

3. The needle falls, the old loop rises, closing the latch of the needle (c). 

4. The old loop is cast off (d and e). 

5. The needle tises, and the new loop slides down to the stem of the needle, pushing the latch open again, and the needle is ready to repeat the cycle (f).

Loop formation in knit fabric

Yet a third type of needle, the compound needle, is used almost exclusively for warp knitting. The compound needle has two components, a tongue and a hook Its motion is as follows: 

1. The old loop encircles the hook; the tongue is in such a position as to leave the hook open. 

2. Both tongue and hook rise; a new yarn is fed to the hook. 

3. Both tongue and hook descend, but the tongue descends more slowly, thereby closing the hook. 

4. As the needle descends, the held loop slides off, forming a new loop. 

5. The needle returns to its initial position, the hook ascending more rapidly, thereby opening the hook again. 

For weft knitting with either needle type, a cam system provides the action for lifting the needles as the yarn is fed in. A small projection called a butt is located at the bottom of the needle. The butt is held in a groove formed by a system of cams or shaped pieces. The movement of the butt in the grooves between the cams causes the needle to rise and fall. 

The engaging by the needle of a new piece of yarn is called feeding. Devices called feeders are located to introduce the yarn to the needles. The number of feeders can vary, but obviously the more feeders a machine has, the higher will be the speed of fabric forming on the machine, since each needle produces a loop each time it is activated and if many needles are activated more frequently, many courses can be formed at the same time. 

Another important element of some knitting machines is the sinker. The already formed fabric may need to be controlled as the subsequent knitting action takes place. A thin steel device called the sinker may be used to hold the fabric as the needle rises, support the fabric as the needle descends, and push the fabric away from the needle after the new loop has been formed. Sinkers are generally mounted between the needles. Some machines, however, do not use sinkers but instead use the tensions placed on the completed fabrics for control.

Construction of Knit Fabric

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The construction of knitted fabrics is evaluated by the number of stitches or loops. When the interlocking loops run lengthwise, each row is called a wale. A wale corresponds to the direction of the warp in woven fabrics. When the loops run across the fabric, each row is called a course. A course corresponds to the filling, or weft. Thus, a knitted fabric having 40 loops or stitches in 1 inch of width, and 50 loops in 1 inch of length, is said to have 40 Wales and 50 courses.

Construction of Knit Fabric

The major difference between knitted and woven structures lies in the way the yarns are interconnected geometrically. In weaving, two sets of parallel yarns are interconnected by interlacing them at right angles. Different woven structures are produced by varying this basic principle In knitting, the yarns are initially formed into loops, and then these loops are interconnected in order to produce a textile structure. The term interlooping is used to describe this technique of forming fabrics. Based on this principle, a textile fabric is produced by using only one set of yarns. Thereby, a horizontal set of yarns (weft) could be interlooped to produce a weft knitted fabric, and a vertical set of yarns (warp) could be used to produce a warp knitted fabric. As a result of this interlooping of yarns, the surface of a weft or a warp knitted fabric is more open when compared to the surface of a woven fabric. Due to this interlooping of yarns a knitted fabric could be stretched more than a woven fabric, even when a small force alone is applied. 

Once this force is eased the fabric slowly returns to its original dimensions. In fact, weft and warp knitted fabrics have higher elongation values than woven fabrics due to their structure, and their elastic behaviour generally exceed the elastic properties of the yarns used to knit the fabric. 

Yarns have poor bending and tensional properties compared to their longitudinal elastic properties, and so once a knitted fabric is stretched and then released, it would slowly go back to its original state. The absolute elongation and the elastic behavior of the fabric are both determined by the knitted structure and the mechanical properties of the yarns used to knit the fabric. Due to the structure and good elastic behaviour of knitted fabrics, garments made of knitted fabrics (knitted garments) are comfortable to wear. The air trapped in the loops of a knitted garment insulates the human body against cold. At the same time the relatively loose and open structure helps the perspiration process of the human body, especially when the knitted fabric is made of yarns spun from natural fibers. Due to the interlooping of yarns, the knitted fabrics also have better crease recovering properties compared to fabrics woven from similar yarns. 

The term binding can be used to describe the connection of one or more yarns in a textile fabric. The structure of a knitted fabric can be evaluated by studying how the yarns in weft and warp knitted fabrics are bound or interconnected, and this can be illustrated using stitch (loop) diagrams (charts). The actual interlooping of yarns in order to produce knitted structures depends on the knitting principle that was adopted to produce the structure, i.e. weft or warp knitting, and on the patterning elements. Knitting is the process of making cloth with a single yarn or set of yarns moving in only one direction. Instead of two sets of yarns crossing each other as in weaving, the I single knitting yarn is looped through itself to make a chain of stitches. These chains or rows are connected side by side to produce the knit cloth” (American Fabrics and Fashions Magazine 1980, 370). The interlocking of these loops in knitting can be done by either vertical or horizontal movement. When the yarns are introduced in a crosswise direction, at right angles to the direction of growth of the fabric, and run or interlock across the fabric, the knit is known as a weft knit. (Some sources may refer to these knits as filling knits, but the term weft knit is used in the knitting industry.) 

When the yarns run lengthwise or up and down, the knit is known as a warp knit. In knitting terminology, the rows of stitches that run in columns along the lengthwise direction of the fabric are known as wales. This corresponds to the warp direction of woven fabrics. Crosswise rows of stitches or loops are called courses. The direction of the courses corresponds to the filling of woven goods. 

Both warp and weft knits are made by machine. Knitting machines may be either flat or circular. The flat-type knitting machine has needles arranged in one or two straight lines and held on a flat needle-bed. The cloth is made by forming stitches on these needles. The resulting fabric is flat. Machines with flatbeds are used to make both warp and weft knits. 

The circular knitting machine has needles arranged in a circle on a rotating cylinder. The resulting fabric is formed into a tube. Circular knitting machines produce weft knits almost exclusively. For nearly two hundred years after its invention in 1589, Lee’s machine was used without further improvement. Using a spring beard needle, Lee’s machine produced flat knitted fabrics by mechanically passing one loop of yarn through another.

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Care of Knit Fabric

Care must be taken for Knit Fabrics 
Although there is a great variety in the quality of knitted goods sold, and the performance of any individual knit may differ markedly from that of other knits, some general guidelines for the care of knitted goods can be observed. The problems that consumers seem to encounter most often in the performance of knitted fabrics are in the areas of dimensional stability, snagging, and pilling.

Dimensional Stability
One reason for the popularity of knits for wearing apparel is their comfort. The looped construction of knit fabrics permits the fabric to give with the body as it moves. But the stretchiness of knits also results in lessened dimensional stability. Consumers have complained about shrinkage, stretching, and distortion of knits, although interlock and double knit fabrics are usually more stable and display little or no shrinkage. Similarly, fabrics with weft or warp inserted yarns are more stable. Shrinkage control treatments, heat setting of synthetics, and special resin finishes can provide good dimensional stability for knits. Unfortunately, not all manufacturers provide such treatment for their products. Consumers should check labels for percentage of shrinkage or for other special treatments to judge potential dimensional stability. (About 3 percent shrinkage is one garment size.) If products fail to live up to specified performance standards, items should be returned to the retailer or the manufacturer.

Knits are considered to be easy-care fabrics, and many care labels recommend machine washing. Some labels will also specify that the fabric can be dried in an automatic dryer. In general, however, knits will shrink more in the dryer than if air dried. Knits maintain their shape best if they are dried flat. The weight of a wet knit, hung on a line, may cause the fabric to stretch out of shape. The dimensions of knits usually will be retained best by professional dry cleaning. Hand knits, sweaters of wool or animal hair fiber, and other knits with an open construction may require special hand laundering and blocking (stretching back into shape). Such items should be laid on a sheet of wrapping paper before washing, and the outlines traced. After washing, the garment should be stretched out on the paper to dry. While still damp, the garment should be gently stretched to fit the outline of the original dimensions.

Aside from stretching or shrinking, an additional problem with knitted items is skewing or twisting as the fabric is relaxed during laundering. Side seams of garments may pull to the front or back and hems may hang unevenly. In general, knits made of synthetics will have better resistance to stretching out of shape than will cotton, acetates, and rayons. Blending of synthetics with cottons, acetates, and rayons will improve the resiliency and dimensional stability of knitted fabrics made from these fibers. Price is a good guide-especially for children’s knits.

Mechanical Damage
The loop structure of knitted fabrics makes them especially susceptible to snagging. If a loop catches on another object, it may be pulled up from the fabric surface and a long snag, or pull, of yarn may be formed. If the yarn that has been snagged is not broken, it can be pulled to the back of the fabric. It may be possible to gently stretch the fabric and work the pulled yarn back into place. This is difficult to do with tightly knitted fabric structures. If the yarn has been broken, the snag may produce a hole in the fabric. A few hand stitches with needle and matching thread should be made to secure the yarns so that the hole does not become enlarged during wearing or laundering. Synthetic double knits or knits made from loosely twisted yarns may be subject to pilling. Weaker fibers, such as cotton, rayon, acetate, and wool, generally break off the fabric, but the stronger synthetic fibers cling to the fabric, making an unsightly area on the fabric surface. The use of textured yarns for knitting synthetics decreases the likelihood of pilling. Knits may be damaged by sharp objects puncturing the fabric. If yarns are cut, a hole will result, and further pressure and strain on the fabric may enlarge the open area, as loops are dropped in the interlocking structure.

Effects of Knit Structure on Knitted Fabric Performance

Knit fabrics are most comfortable, durable and make most flexible effect on human body. I am going to describe the effect of knitted structure on knit fabric performance in details. 

Durability Factors 

Strength of knitted fabrics is considered to be less important for durability than it is in woven goods. Knitted fabrics are easily stretched to accommodate changes of shape as a result of stresses imposed in wear and care. When knits are made from resilient fibers and yarns, the ability to stretch and recover from stretching will be enhanced. In comparison to knitted fabrics, woven fabrics are generally firmer and have less elongation. In knits the loops in the structure can be deformed horizontally or vertically, increasing the stretch in both directions. But, as noted earlier, variations in knitting techniques can increase or decrease extensibility of knitted fabrics. Double knit fabrics and warp knit fabrics are usually less extensible than single knit fabrics.A major problem in the durability of knits is the runs that can develop in weft knits when one of the loops is broken. If stronger fibers and yarns are used in these knits, they will be less likely to run. 

Appearance 

Shear ability, discussed in chapter 15, is an important factor in the appearance of textiles. In general, because of the ways in which yarns are combined, woven fabrics have higher shearability than do knitted fabrics. Knitted fabrics have good flexibility and are easily extended. Warp knits do not shear as easily as weft knits. 

These qualities must be taken into account by designers, although they may not be aware of the technical terminology used to describe the fabric properties. For example, a warp knit tricot fabric with low shearability would not be used to make a bias-cut garment in which the intention is to take advantage of high shearability. Instead, the designer would be likely to use a soft, gathered construction where the high flexibility of the fabric would work to good advantage, but where most of the draping would fall in the vertical direction of the fabric. In general, knits wrinkle less than do other fabrics. This is because loosely constructed fabrics generally allow more fiber redistribution and motion. However, knitted fabrics, because of their greater extensibility, are more likely to lose their shape in laundering. The stresses applied in knitting distort the shapes of the loops rather than just stretching the yarns as in weaving. On relaxation, the loops broaden, shrinking the fabric length and increasing the width. As with woven fabrics, the tighter the structure, the higher the shrinkage until the structure becomes so tight that further shrinkage is not possible. Such fabrics may, however, buckle. 

Knitted fabrics tend to have lower cover than do woven fabrics, with weft knits having substantially more porous structures than warp knits, unless the fabric is deliberately made to have an open, lacy construction as in some Raschel knits. Use of thinner or thicker yarns can increase the cover of knitted fabrics. 

Comfort Factors

Knits usually entrap more air than woven fabrics, although the tightness of the knit is a factor as well. Pile or napped knit constructions are especially good for cold weather because the yarns or fibers perpendicular to the surface provide numerous spaces for dead air. This effect is maximized if such fabrics are worn with the napped or pile surface next to the body, or if they are covered with another layer. The flexibility of knits contributes to a feeling of fabric softness. Fibers and yarns used can enhance or detract from the smoothness of knitted fabrics. 

Elongation and Recovery 

The majority of bathing suits for women, and many for men, are tight fitting, with stretch required for getting the garments on and off. Since knits have much higher elongation than woven or nonwoven fabrics, they are the preferred fabric construction for bathing suits that fit tightly to the body. Fibers too play an important role in achieving tightness of fit. Those with lower modulus and high resilience will enhance the stretch and recovery properties of knits. Not only elastomeric fibers but also nylon have low modulus and good recovery. Nylon and spandex are often chosen for swimwear because of these properties. Polyester is not usually used because it has a higher modulus and therefore does not stretch as easily. Knitted fabrics of nylon blended with elastomeric fibers, such as spandex or rubber, will provide the highest amount of stretch. The elastic fibers also have high recovery from stretch. 

Colorfastness 

Bathing suits are exposed to sunshine, chlorine and other chemicals in swimming pools, and salt water in oceans. Fibers and yarns are both important in maintaining the colorfastness of swimming suits that are often made of bright-colored fabrics. Light-fastness of dyed spandex is fair to good, while that of nylon dyed with acid dyes is good. Spandex normally makes up only a small percentage of the fiber content of swimwear fabrics and is often used in core-spun yarns wrapped with nylon. This helps to protect ir from sunlight. Colorfastness of nylon and spandex exposed to pool and salt water is fairly good but these fabrics will show some loss over time. 

Absorbency 

Since bathing suits are worn while swimming in water, the absorbency of the fabric is a consideration. If a material absorbs and retains a significant amount of water, the weight of the suit will increase, affecting its comfort and function. Synthetic fibers have low water regain and are more appropriate for bathing units than the natural fibers that were used many years ago. 

Environmental Resistance 

Light, chlorine, and salt water can also degrade textile fibers. When this occurs, bathing suit fabrics may lose some of their recovery from stretch. Nylon and polyester are more resistant to ultraviolet light and chlorine than spandex. That is another reason why spandex is usually used in small percentages blended with nylon. The use of rubber as an elastomeric fiber in swimwear has decreased because of its susceptibility to degradation by light and other environmental conditions. Because resistance of nylon to degradation by light is higher for fibers that have not been delustered, brighter nylons are usually used in swimwear.

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