Thursday, January 10, 2013

Construction of Knit Fabric

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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