Knitted Fabric Geometry

Introduction The earliest work on the physical and geometrical properties (knitting science) on the knitted structure can be traced back to Tompskin in 1914. A major advance in this field occurred when Doyle and Munden (1959) reported that for an extremely wide range of relaxed plain-knit fabrics, the fabric dimensions (width and length) are completely determined by the knitted loop length

Introduction 2 The following equations were found: cpi =kc/l wpi=kw/l R=cpi/wpi; R=kc/kw (loop shape factor) S=cpi x wpi =ks/l2 Kc, Kw, Ks are constant and termed as the fabric dimensional parameters.

The above relations apply only for plain knit fabrics that are in their relaxed state. During the knitting process, strains are imparted to the fabric as it comes off the knitting machine, and subsequent recovery from these strains occurs until the fabric has attained its relaxed or stable-equilibrium state.

Relaxation Staes Mudnen (1959) defines two baisc relaxed states: the dry relaxed state and wet relaxed state. The yarn in a dry-relaxed fabric is in a condition of stress and when unravelled from the fabric tends to return to its natural stress-free configuration. The stresses in the loop are largely released during wet relaxation. Setting takes place during drying, and as a result, the yarn retains its loop configuration when unravelled from a wet relaxed fabric.

K-value for two relaxed states Dry-relaxed Wet-relaxed Kc 5.0 5.3 Kw 3.8 4.1 Ks 19.0 21.6 Kc/kw 1.3

K-value For a wide range of plain knit wool fabrics, Munden obtained the average numerical values of the fabric dimensional parameters as shown in above Table. He concludes that on relaxation, the knitted loop tends to take up a definite equilibrium configuration, which is independent of the tightness of construction of the fabric and of the physical properties of the yarn.

K-value 2 This loop configuration is the same for all plain knit fabrics in a given of relaxation, and it governs the numerical values of the fabric dimensional parameters. The values of these parameter are constant, and once they are determined, the fabric dimensions are function of the loop length.

Although the dimensions of relaxed knitted fabrics were found to be dependent only on the knitted loop length and are independent of the yarn count, it is expected that other physical properties (such as mechanical properties of the fabric, fabric stiffness, pilling) will depend on the tightness of construction.

Conclusion 1 On relaxation, the knitted loop tends to take up a unique configuration which is independent of the loop length and of the physical properties of the yarn. This unique configuration is determined by the condition of minimum strain energy of the loop and it governs the numerical values of the fabric dimensional parameters, Kc, Kw and Ks

Conclusion 2 The practical importance of this work, is that there is only one variable, namely the loop length, that affects the quality and dimensional properties of a knitted fabric in its relax state. This implies that careful control of the length of yarn being fed into the knitting machine is all that is necessary for controlling the quality of fabric. It is on this principle that HATRA Yarn speed meter and Yarn length meter have been developed.

Conclusion 3 Prior to the work of Doyle and Munden, many yarn and machine variables were recognized as having an influence on knitting quality and the dimensions of a fabric in its relaxed state. However, Munden has shown that a change in any one of these variables affects the knitting quality and fabric dimensions either by altering the knitted loop length or by temporarily distorting the fabric. In the first case, the effect on fabric dimensions is permanent, but in the second case, the effect is only temporary as the fabric tends to recover its stable loop configuration on relaxation.

Conclusion 4 Adjustment of the stitch cam setting on the knitting machine and change in the yarn tension at the knitting point are invariably accompanied by a change in the loop length. Nutting has shown that an increase in kinetic yarn friction causes an increase yarn tension during knitting, so producing a decrease in loop length or tighter fabric. Thus a change in stitch cam setting, yarn tension or yarn friction has a permanent effect on fabric dimension. Adjustment of fabric take down rollers and stretcher boards usually has little influence on the loop length and so does not affect the dimensions of the relaxed fabric. These variables, however, may have a considerable temporary effect on the dimension of the fabric in the form of distortion.