It is well established that knitted fabrics of all constructions and fiber blends are inherently more prone to shrinkage as compared to wovens. Because of the inability of a knitter to form a knitted fabric with no shrinkage, it is important for the dyer and finisher to make an effort to remove as much shrinkage from the product as possible. However, the ease with which cotton knitted fabric is distorted during processing makes it especially difficult to deliver fabrics with no shrinkage. Some detail regarding aspects of knitted fabric construction and wet processing and how they are related in terms of shrinkage.
The factors that influence the level of dimensional stability can be summarized as follows:
processing tensions after knitting,
relaxation techniques in finishing, and
mechanical and chemical finishes.
Each of these areas can be broken down into fundamental aspects.
The amount of shrinkage for any given knit fabric is primarily dependent upon the product specifications and the knitting parameters used to meet those specifications. The predominant fabric specifications that determine the shrinkage of a knitted fabric are the weight, stitch counts, and width at which the fabric is sold.
The knitter uses those specifications to establish another set of specifications for knitting. Whether or not these knitting specifications are achievable is determined by the knitting machinery available to the knitter. The gauge of machinery (number of needles per circumference inch of the cylinder) determines the range of yarn counts that can be used. For a given knitting machine gauge, a restricted range of yarn counts can be knit.
Finer yarns are more expensive compared to coarser counts. In order to maintain target weights in knitting finer yarns, the stitch length must be changed. The stitch length is the amount of yarn in one stitch repeat of the pattern. Heavier yarns when knit at the same stitch length as fine yarns will result in a heavier fabric. In essence, for any given yarn count, knitting a smaller loop results in a heavier, more stable fabric. Knitting a shorter loop also results in less length shrinkage, but higher width shrinkage for any specified finished width. The width of a fabric is related to the number of needles in the cylinder and the stitch length. Each needle equals one wale. The longer or looser the stitch in each wale, the wider the fabric, and the lighter the weight. This is important because should there be a need to knit a fabric wider by increasing the stitch length, this will affect the weight and shrinkage.
In summary, the choice of yarn count, machine gauge, the number of needles in the cylinder, and the stitch length has a profound effect on shrinkage performance of a fabric.
1 PROCESSING TENSIONS
The knitting set-up is the single most important aspect in shrinkage. Yet, the tensions in wet processing and apparel manufacturing can render a knit unacceptable with respect to weight, width, and shrinkage. In the purest form, linear tension in processing increases (stretches) the fabric length and decreases the width.
Where does processing tension occur? Almost every time the fabric is moved over rolls and from one point in processing to another.
A. Inspection: During inspection, rolls should move freely to prevent excessive tension, and efforts should be exercised to prevent sudden starts and stops. Care should be taken to properly unwind and roll-up the fabric. If the fabric is to be re-rolled, make sure excessive tension does not cause the width to neck-in or be reduced. The flat tube should be kept open to prevent wrinkles and folds so as not to cause permanent creasing of the goods.
B. Lot Pulldown (Batching): This is another dry area where the fabric can be stretched in the length. It is important to keep speeds at start-up slow and to increase gradually. Do not allow the strand to rope tightly if plaited into a storage bin or truck.
If the fabric is plaited, take care to prevent the plaited pile from tumbling or falling over. This will create extreme linear tensions when the fabrics are subsequently moved again. If the fabrics are batched into a wet-scray, all these precautions are even more important.