Quality Control of Knitted Garments Ref. Knitted Clothing Technology, by Terry Brackenbury, Chapter 10 (1) Fabric Quality The word quality in knitted fabric sometimes is used to describe the loop density of particular fabric. This dimension, loop density, is the most important one in defining knitted fabric properties, and is directly related to fabric appearance, weight per unit area, thickness, drape. The loop length is the single unit cell formed in knitted fabric and is directly related to loop density In general, as the loop size increases, the loop density decrease. The fabric relationship is S=K/l 2 where S is loop density, l is loop length and K is constant for particular construction. (2) Course Length For some structure with complex geometry (like fancy jacquard with different loop length), the average loop length value is largely irrelevant. The "quality" is expressed indirectly, either as a course length or loop density. (2.1) Measurement of course length A course length is determined by unroving yarn from a known number of loops of fabric and measuring its length using a crimp tester (BS 5441:1977). For circular machine, which is normally composed with different number of feeders, measurement must be done on each individual feeder. For V-bed machine, care must be taken for the position of -cam system, as the machine changes the direction on each alternative course. 1
(3) Weight per unit area and cover factor The "weight" of a knitted fabric is depended on two factors, namely the loop size and the yarn size. The effect of loop size is simple to express: if the size of the yarn remains constant, then increase loop size produces a decrease of weight per unit area. The effect is an inverse ratio. Example Using the equation of S=K/Pl 2, and let K=20 (a) if l = 0.4cm, then S=20/0.4 2, S = 125 loops/cm 2 Length of yarn in 1 cm 2 of fabric = 125 X 0.4 = 50cm (b) if l = 0.8cm, then S=20/0.8 2, S=31.25 loops/cm 2 Length of yarn in 1 cm 2 of fabric = 31.25 X 0.8 = 25 cm Conclusion :Double the loop size means half the weight per square unit. Cover Factor In a knitted fabric, for fabric of similar construction, as the loop length increase, the size of yarn increase. Cover is a simple ratio of the area of a knitted fabric covered by yarn to the area of covered by the gaps in between loops. It can be shown that for a given knitted structure, if the cover ratio is maintained through a range of fabrics with different loop lengths, then those fabrics are related in characteristics of tightness/looseness and other physical properties. The formula of cover factor that take into account of yarn diameter, loop length and loop density is Cover factor = tex/1 Relationship of (a) yam count against loop length; 2
(b) weight per square meter against loop length see the diagrams The graph of the relationship between loop length and weight per square meter for a given construction while maintaining a cover factor, is a straight line. i.e double the loop length, double the diameter of the yarn, double the weight/m 2. Measurement of fabric (Relaxation) It is difficult to measure the dimensions and quantities of cotton fabric because of the extensible nature of the material and the possibility that it is under stress in the time of measurement. Knitted fabric may change dimensions with time, handling and with subsequent wet treatments including steaming, and such change can occur after garment has been produced and sold to public. The concept of relaxation stated for knitted fabric is used. Quality control must ensure that before knitted garments are cut, the fabric is in a relaxed or near relaxed condition (ie. There will be little shrinkage of the fabric/garment when it is in the consumer hands) Relaxation procedure (BS 1955:1981(86)). Most test procedures involve agitation in aqueous solution followed by measurement under water,, and spinning and tumble drying. They attempt to reproduce the conditions under which the garment will be laundered during usage. Knitted Faults It can be grouped into horizontal and vertical components. Different course lengths variation is one of the horizontal fault. Most faults in horizontal barring are connected in some repect of yarn used. Horizontal faults * Uneven counts between yam; * Uneven counts within yarns; * Different dyeing shades between yarn; * Different dyeing within one yarn; * Different spinning/batch source of yarn; 3
* Different heat treatment given to man-made yarns. Vertical faults are resulted from the knitting process but can occur with finishing. A common vertical fault is a needle line. It is because a faulty needle may produce a wale slightly larger or smaller or distorted loop. Really faulty needles result in vertical lines of occasional tuck stitches. Stains The knitting industry, unlike weaving, uses oil to lubricate machinery where the fabric is being produced. (i.e on needles themselves). Such oil mixed with dust and metallic powder can, cause stain on the fabric being knitted. Minimise the stains 1. Control of the oil itself, by applying it in minimum quantities and delivering it continuously, so that it present in the fabric but does not show up in any build up of soiled material. 2. Using oils that are readily mixed with water, so called scourable or clean oils. The majority of knitted piece goods are wet finished before cutting and therefore, little oil staining will remain in finished fabric. On stitch-shaped garment,, tight control is needed as they are not wet finished, stains are dealt with during examination by solvent based spotting guns. The problem is negligible in fully fashioned where oil is not used on the needles and where wet finishing is usual. Summary Fabric portions at the start of the garment making process may exhibit the following undesirable observable qualities : 1. The wrong construction, with loop length, width and weight per square meter variations from the specification; 2. Variable loop length/course construction, showing horizontal barring; 4
3. Horizontal barring from a number of yam characteristics; 4. Vertical faults that are machine determined 5. Stains due to oiling or lint contaminating the fabric 5