The ever increasing demand of knitted apparels has attracted attention in global niche market. In comparison to woven garment, around 50% of the clothing needs are met by the knitted goods. It is well known that weft knitted fabrics tend to undergo certain dimensional change that causes distortion in which there is a tendency of the knitted loops to bend over, causing the wales to be at diagonal instead of perpendicular to the courses (Figure 1).
Figure 1: Angular relationship of course and wales in a knitted structure
In other words, spirality occurs in knitted fabric because of asymmetric loops which turns in the wales and course of a fabric into an angular relationship other than 90 degree. This is a very common problem in single jersey knits and it may exist in grey, washed or finished state and has an obvious influence on both the aesthetic and functional performance of knitwear. However, it does not appear in interlock and rib knits because the wale on the face is counter balanced by a wale on the back.
Course spirality is a very common inherent problem in plain knitted fabrics. Some of the practical problems arising out of the loop spirality in knitted garments are: displacement or shifting of seams, mismatched patterns and sewing difficulties. These problems are often corrected by finishing steps such as setting / treatment with resins, heat and steam, so that wale lines are perpendicular to the course lines. Such setting is often not stable, and after repeated washing cycles, skewing of the wales normally re-occurs.
Causes of generation:
The residual torque in the component yarn caused due to bending and twisting is the most important phenomenon contributing to spirality. The residual torque is shown by its twist liveliness. Hence the greater the twist liveliness, the greater is the spirality. Twist liveliness of yarn is affected by the twist factor or twist multiple. Besides the torque, spirality is also governed by fibre parameters, cross-section, yarn formation system, yarn geometry, knit structure and fabric finishing. Machine parameters do contribute to spirality. For instance, with multi-feeder circular knitting machines, course inclination will be more, thus exhibit spirality.
1. Influence of yarn properties :
Count: Degree of freedom of yarn movement in the fabric structure contributes significantly to the increase in spirality. Dimensional parameters of fully relaxed single jersey fabrics depend on the yarn linear density and tightness of construction. If diameter is reduced, its resistance to deformation is lowered. It indicates that, deformation of loop structure is influenced by yarn count. In other words, the finer the yarn, the more will be the spirality due to more twisting.
Twist: Usually in knitting, low twisted yarns are used. High twisted yarn has a great impact on spirality due to its unrelieved torque. With the increase in twist, the twist liveliness increases, this in turn, causes the angle of spirality to increase. The direction of spirality in the fabrics knitted from short staple ring spun single yarns is determined by the yarn twist direction. Thus, the technical face of single jersey fabric exhibits spirality in the Z direction if a Z twisted yarn is knitted.
a) Yarn Twist Multiplier (TM):
This index is represented by the following formula:
TM = T.P.I. / √N, where T.P.I. indicates twist per inch and N represents yarn number in an indirect system, the cotton system unless otherwise specified. With the increase in twist multiplier, the angle of spirality increases.
(b) Yarn Twist Factor (TF): TF is related with the following formula:
TF = TPCm x √ T, where T signifies yarn number in Tex.
Raising the twist factor of two ply yarn increases the left hand or S-direction spirality, whereas increasing the twist factor of single yarn increases the right hand or Z-direction spirality.
Conditioning: The minimum Spirality level that can be achieved by several ways such as storing yarn at appropriate temperature and relative humidity or by thermal conditioning with low temperature saturated steam in vacuum that results in a speedy relaxation. This process balances the twist so that it does not regain its original state. However, there is no systematic study carried out to understand the effect of yarn conditioning on spirality of single jersey fabrics.
Spinning method: Yarn produced by different spinning technique has a direct bearing on spirality of knitted fabric. Friction spun yarn made of 100% cotton produce fabrics with highest degree of spirality, followed by ring spun yarns. Both rotor spun and air jet yarns produce fabrics with a low degree of spirality.
Blend: In general, 50/50 cotton/polyester blends have a lower tendency to produce spirality in fabrics than the 100% cotton yarns. Spirality can be virtually eliminated by using 50/50 cotton/polyesters blend of air jet and rotor yarns.
2. Influence of fabric properties
Fabric stitch length: This is the length of one loop in knitted fabric. Spirality increases with the length of loop.
Fabric structure: More spirality in single jersey due to non-arrest of loops. By adding moisture to such a structure, the twist will try to revert as it swells, that distorts the shape of the loop. In double jersey, the effect of spirality is nullified. Pique and honey comb also show spirality even if sometimes two beds are used. Spirality can be noticed in certain jacquard structures. In stripe pattern, it increases with the size. No appreciable problem of spirality is there in ribs and interlocks.
Fabric tightness: Slack fabric presents higher spirality angle compared to tightly knitted fabrics. At each level of yarn twist factor, the degree of spirality decreases linearly with fabric tightness factor.
Fabric relaxation: Fabric relaxation (dry and wet) treatment removes the residual knitting tension in the yarn introduced during the knitting process. The relaxation treatment relieves the residual yarn torque as a result of changes in the molecular structure and increasing yarn mobility.
3. Influence of machine parameters
Number of feeders: The number of feeders in a circular knitting machine also influences the angle of spirality. Due to more course inclination, spirality will be more.
Direction of machine rotation: The direction of machine rotation has influence on spirality. For Z twist yarns, the wales go to the right and thus, giving Z skew and S twist yarns makes the wales go to the left, giving S skew to the fabric. With multifeed machines, the fabric is created in helix, which gives rise to course inclination and consequently wale spirality. Direction of spirality depends on the rotational direction of the knitting machine. Earlier research work revealed that, for a clockwise rotating machine, the wale would be inclined towards the left, thus producing the S spirality.
Gauge: In knitting terminology, number of needles per inch is called the gauge. Smaller the gauge, lesser will be the spirality keeping other parameters constant. A proper combination of linear density and gauge is required to reduce spirality e.g. torque can be controlled in 20 gauge and 40s count.
Knitting tension: The effects of various knitting tensions including the whole process of loop formation on fabric spirality had been investigated by the researchers. Experimental investigation could not establish consistent trends with respect to variations in fabric quality with knitting tensions.
The twist factors of ply and single yarn, loop length, and fiber diameter have significant effects on the angle of spirality, while yarn linear density and fabric tightness factor have comparatively lesser effect.
Compacting: Compaction reduces the length of the fabric based on its elongation during processing which, in turn, reduces the width. It helps in controlling the shrinkage of the fabric. There are two types of compactors - open and tubular. In tubular compactor, the squeezing line gets on the sides in this process and is done on natural movement thus controlling spirality. If the wales are straightened manually then it results in spirality.
Resin treatment: Cross-linking the fabric by means of inter fibre bonding also reduces spirality. Resin in the form of aqueous solution is applied and set by passing the fabric through a high temperature stenter. This method is not recommended for cotton fabrics, since it weakens the cotton yarn.
Heat setting: Steam or hot water setting reduces twist liveliness and hence spirality. Mercerization is recommended for cotton yarns, so that fibres are made to relax permanently.
Balancing yarn twist factor: In an earlier investigation on plain knitted wool fabrics, it was been revealed that raising the twist factor of a ply yarn increases the left-hand of S-direction spirality of fabrics. But while increasing the twist factor of a single yarn, there is decrease in left-hand of S-direction spirality with an increase in right-hand or Z-direction spirality. Thus, there is possibility to balance twist factors for both ply and single yarns with a view to achieve zero spirality. Experimental study by Chen, Q H on wool knits indicates that, when the ratio of twist factor of the ply yarn to the twist factor of single yarn is about 0.73, zero spirality may be achieved.