Knitting - technological compulsions on quality of cotton yarn

Professor and Head, Dept. of TextileTechnology

Shri Vaishnav Institute ofTechnology and Science, Indore

1.                  INTRODUCTION

Excellent comfort properties of weft knits have made their entryinto formal wears for men and women. But with the technological advancement inmanufacturing of cloths and the awareness of consumers to quality, theexpectations in knit goods too have gone high. However, knit goods are knownfor their high structural sensitiveness to deformation during manufacturing process or at their end use. The research work of the past focused on macro level aspects ofquality control while the market demand today is on micro level. The qualitycriteria in the future would be much different than what is being countedtoday.

The improvement of knit structure at micro level calls forbetter understanding of mechanics of loop formation, fluidity of knitstructures and their influence on quality of knit fabrics. The quality ofhosiery yarn has to be considered with due weightage to these aspects. If theyare not addressed, probably satisfying the customer at global level may become difficult.

This article aims at initiating a thought provoking process on above lines for assessing fabric quality as well as that of yarn for manufacturingsuperior quality fabrics apart from highlighting role of certain yarn properties on knit fabrics. The technological compulsions, not of the knitting machinetechnology but of the future demand of quality level in the fabric by thecustomer and also progress in technology in assessing fabric quality leading toreconsideration of machine, material and process parameters at micro level on forsuperior in yarn quality are explained.

2.                  Quality of Knit Fabrics

At present knit fabric quality is decided by few physicalparameters namely GSM, loop dimensions (loop length, wales and courses per unitlength), fabric width, dimensional stability and defects in the fabrics.However, aesthetic value of the fabric or the appeal to the customer regardingthe fabric, which ultimately accounts for satisfying the customer, is notmeasured by these parameters. At the same time the comfort properties of the fabric like smoothness and fluidity of loops that influences shear and lowstress mechanical properties are also not sufficiently covered by above listedparameters. Further, more than just the stability of the fabric/garment, forwhich knits are known to be poor, the localized variation in dimension wouldhamper the appearance and useful life of it. Though objective assessment ofthese parameters is not possible commercially today, with the introduction ofimage analysis technique for fabric quality inspection such an assessment maybecome reality in the future. The customer too may consider subjectiveassessment of the same by observing the garment under different light sourceswhich enhances the localized variation and get a feel of quality level.

2.1              Parameters of Fabric Quality

The quality of fabrics at micro level could be, loop to loopvariation in their dimension (rather than an averaged value) including loopshape (not as a shape factor but as geometrical shape of the loop) andlocalized variation in loop density (rather than GSM). The important loopdimensions are loop length, loop width (wale spacing) and loop height (coursespacing). The uniformity in dimension of loops provides attractive appearanceto fabric, as it eliminates blurring effect of irregular dimensions. The fabricwould be more elegant, lustrous, smoother, softer and stable. This is likebetter cover in woven fabric. In woven fabrics those produced on shuttleless machinesare better in appeal to eyes than those from auto-looms or plain power looms.In woven fabrics the balance of crimp between warp and weft yarns also plays avital role on its aesthetic property.

The measurement of dimension of each loop for a large number of loops that statistically represents the whole lot of the fabric is a very hard task by using existing tools. However, for research purpose it can be executed with satisfactory accuracy. As the measurement of yarn imperfection for a unit length of 10mm is made possible and with statistical quality control it is capable of representing a whole lot of yarn or the production of a large spinning mill. The development of a suitable instrument to measure individual loops for their dimensions could be practically possible. The present day image analysis technique may be the appropriate technology for this purpose.

The uniformity in geometrical shape of the loop is another parameter which affects the elegancy of the fabric and its fluidity. In most of the structures the loop is distorted during relaxation, chemical processing or during usage resulting in dullness, rough or ridged effect in the fabric. A standard loop shape is shown in fig.1 for single jersey structure [1]. The geometrical shape of a standard loop should have same curvature for crown and sinker loop [2] (normally sinker loops are larger than crown). Both the arms of loop should be in the same plane [2]. The bending of crown and sinker loop should be to an equal depth and without twisting or turning. The shape factor, ratio of width to height of the loop should be about 1.3 [3]. The contact places of yarn in loop interlacement should be at the junction of loop arm and the crown/sinker loop i.e. at points A, B, C and D in fig.1. [5]. The variation in this loop shape and the dimension should be minimum. Such structures can be more resilient because the mobility of loops or redistribution of yarn in loops during any deformation would be easier. This would improve the dimensional stability of the fabric.

The geometrical shape of the loop, its variation, twisted or deformed loops, etc. can be assessed by the same image analysis technique. If any other method is suitable that can be explored. Typical examples of uniform and twisted/deformed loops [4] are shown in fig.2 (a) and (b) respectively.

The localized variation in loop dimensions, i.e. group of loops in few wales and courses covering a small area in the fabric having dimension different from their neighboring group of loops, is again a common problem but goes as accepted till it leads to an unpleasant appearance. However, this variation certainly affects luster and elegancy of the fabric. This aspect is appreciated when two fabrics with and without such variations are placed side-by-side. Obviously higher the uniformity better is the appearance and texture of the cloth or say appeal to the customer.

This parameter of the quality, as well, can be assessed by image analysis technique. The images obtained from the cloth have to be analyzed for all these three parameters and scanning could be a single operation.

1.1               Machine Parameters

Machine parameters and technology of the machine influence the fabric quality as well as the demand on yarn quality. The role of machine parameters such as gauge, needle type, cam type, yarn feeding system, number of feeders, take down system, cloth rolling or spreading, monitoring and control systems, etc. are well established by extensive research work. However, selection of machine or its parameters for knitting a particular yarn for manufacturing given GSM is crucial. The ideal count range for a given gauge has to be followed [5]. The coarser gauge machine can knit with much ease compared to finer one for a given yarn. However, the present tendency is to knit more on finer gauges and with very short loops. That means the curvature of the yarn in loop would be sharper and the space available between two needles to form a loop or for slipping of loops at the needle hook would be less. Then, the stress and strain on yarn would be much higher and knit structure could approach a jammed condition. This reduces the fluidity of the loops and their relaxation at dry or wet or both conditions and their final dimension may not be uniform. Such conditions call for stringent quality in yarn; else fabric is prone to develop all the three types of quality variations.

1.2               Process Parameters

The process parameters such as cam setting, speed, yarn tension, sinker setting (in single jersey), delay time (in double jersey), stitch length, take down rate, condition of machine, etc. play vital role in deciding the quality of the fabric. Extensive research on these aspects has given sufficient guidelines to manufacturers. However, the manufacturer still has to grapple with his expertise to achieve accuracy in GSM and quality of fabric. Variation in GSM, spirality and many other defects are the problems encountered regularly. The knit fabric from similar yarn knit on similar machine (make and condition) with similar process parameters produce fabrics of different quality and some times beyond acceptable limits [6,7]. This speaks about influence of variation in yarn and process parameters other than those considered today in the industry. They can create difference in strain at different loops as also variation in their relaxation. This leads to loop to loop variation in dimension, geometrical shape as well as the localized variation in loop density.

2.                   YARN QUALITY

The practice in the industry in assessment of hosiery yarn quality is on the lines with the established norms for weaving or for general understanding of yarn grade rather than anything specific to knitting, except waxing. The purchase of yarn is based on the general parameters like count, U%, imperfections, strength and elongation and TPM. Most of the knitters in SMEs test only the count for setting the GSM of the fabric.

If so, do the knit structure and knitting process have no specific requirements compared to the weaving! As mentioned under section 2 the knit fabrics and their process requirements are definitely much different from weaving. It has not been appreciated so far, may be due to lack of proper tools to assess them. The day has come to think on these lines and make the knit fabrics superior in quality.

The special quality testing for hosiery yarns with their technical reasons are discussed in the foregoing part of this paper.

3.1   Twist in Yarn

Twist in hosiery yarn should be less, a fact known to all technologists. Still in few cases one finds yarn of higher twist being preferred on the ground that it performs well in knitting in terms of lesser yarn breakages. That is true but the benefit is at the cost of fabric quality.

Unlike woven fabrics knit structures are formed by bending the yarn into a loop and then interlacing them to create a fabric. The curvature of loop would be smooth and well defined if the bulkiness of the yarn is higher. The bulkiness eliminates sharp bending and improves resiliency of the structure, these fabrics are expected to stretch easily and recover during use. The very purpose of using low twist yarn is to achieve this smooth curvature to loops and high resiliency to fabric.

Then, what should be the gauge length for testing twist? Longer gauge lengths, as practiced in industry, would provide information about averaged twist and CV% would be low. Where as for the type of fabric quality discussed here the gauge length should be less. This would provide information about the likely variation in loop shape and its dimension as a result of short term variation in yarn twist.

However, the exact gauge length that is practically feasible needs to be investigated. The twist in yarn also has a role to play in the geometrical shape of the loop. When a loop is bent in third dimension, as shown in fig.3 [8], for interlacement of loops the arms of the loop are twisted in opposite directions [8], as shown in fig. 4. As a result the effective twist in each loop arm may change to the extent of 400 to 600 tpm (10 to 15 tpi) e.g. in a 20s cotton yarn (29.5 tex) of 3.6 TM (34 tpc.tex of TF) twisted in Z direction, or of 633 tpm (16tpi), would have a reduction in twist to the extent of 400 to 600 tpm (10 to 15 tpi) in left arm and an addition of the same amount in right arm of the loop. Such a great change in twist or strain in yarn at loop arms in association with strains experienced in the formation of loops would lead to deformation of loop shape. This change in strain at loop arms would vary from loop to loop due to change in yarn characters, including variation in yarn friction of the type shown in fig. 5. The basic yarn, therefore, should have minimum torsional rigidity to achieve good geometrical loop shape.

3.1   Yarn Irregularity

For obtaining smooth curvature to loop and its uniformity the yarn should be uniform in thickness and imperfections should be minimum.

Establishing correlation between yarn imperfections, short term twist variation and variation in loop dimensions or shape could be an interesting work. The thin place in yarn receives more twist resulting in compact structure i.e. high torsional rigidity or sharp bends in loop while thick place receives less twist and forms a large curvature at loop. The coefficient of friction at thin places might be higher due to increased twist, which might be further aggravated by probable low wax pick-up. This variation in bending, twisting and surface friction can vary tension in yarn during loop formation. This would result in shift of loop forming point in knitting zone [11] leading to variation in loop dimension. The shape of loop, obviously, has changed. Though yarn uniformity and imperfections can not be improved beyond a limit the twist flow in these zones might have the influence of various spinning parameters. A study of correlation between dimensional change in loop and yarn irregularity can show severity of this phenomenon.

3.2   Coefficient of Friction

Waxing to cotton hosiery yarns is common. A great improvement in quality of wax and there by the reduction in coefficient of yarn friction is observed today. The friction value in waxed yarn has come down from 0.24 to 0.14 today. The methodology of its application at machine too has changed from friction driven wax discs to motor driven. This change ensures more uniform application of wax. However, an interesting observation by the author is worth sharing here.

When 18 cotton hosiery yarns were collected from industry, picked from their regular export lot, who manufacture 5% level Uster standard yarns and tested for coefficient of friction some surprising results were observed. The data is given in table 1, while the typical plots of continuous recording of friction are given in fig.5. From the data given in the table 1 one can note that in several instances the CV% of mean value is less but the overall CV% is very high. This indicates that within sample variation is high or waxing may not be uniform. The low overall CV% in few cases clearly demonstrates that higher uniformity in wax application can be achieved. In the fig.4a and 4b a typical case is shown where the average value of coefficient of friction in two yarns are same but the CV% of friction within a yarn is very high in yarn b compared to yarn a (in few cases the average value of friction itself had changed significantly). The implications of such high variation in friction can definitely change knitting tension and loop dimensions. The sensitiveness of knitting process and loop dimension to yarn friction is well established [11, 12]. Then for achieving fabric quality in terms of micro level dimensions, as discussed in this paper, would be difficult to achieve unless yarn is tested for within variation of friction. The modification in the existing instrument can provide such information. An interesting observation is that the unwaxed warp yarn has high friction value but very low variation within as well as between samples.

3.1   Flexural Rigidity

Flexural rigidity is the resistance of the yarn to bending. Formation of loop involves torsional, flexural and tensile deformations [13]. The study by Prabhakar Bhat [13] has shown that flexural rigidity influences knitting tension and loop dimension.

The flexural rigidity is the result of fibre properties and yarn structure. Even if all fibre properties and certain yarn properties are same the change in spinning condition can form yarn of different flexural rigidity. The correlation between irregularities in yarn and flexural rigidity for similar yarn has to be established to investigate their influence on micro level variation in loop dimension or loop shape or the localized variation in loop density. Technically, there should be good agreement between these parameters. There are few methods for testing this property but needs further standardization and sophistication for commercial application.

3.2   Torsional Rigidity

The torsional rigidity of spun yarns is difficult to test and an instrument is developed at IIT Delhi for testing the same for such yarns [13]. The study by Prabhakar Bhat [13] can help in understanding the importance of this property. Design of the instrument and testing of cotton yarn for torsional properties is explained by Banerjee and Prabhakar [14].

Table 1 Variation in yarn to metal friction coefficient

K = carded yarn, C = combed yarn W = warp yarn

However, the point to be noted here is the importance of this property and the factors that can influence this rigidity in hosiery yarns. Torsional properties of spun yarns depend on torsional, tensile and bending properties of staple fibers [15], twist in yarn, thickness of yarn, compactness and strain energy stored in yarn, etc. All these parameters can vary from yarn to yarn, though their general properties are more or less same, due to changes in spinning condition and yarn conditioning after spinning. The values in table 2 and 3 show the relation between few crucial yarn properties and knitting tension (cam force) and loop dimension [13].

Table 2 Correlation between yarn properties and cam force

Note: Due to narrow distribution of yarn friction coeff. in samples the correlation is low

Table 3 Correlation between yarn properties and loop dimension

LL Loop length CS Course spacing

WS Wale spacing NS Not significant

The torsional rigidity in a yarn can vary due to any structural change or variation in strain energy during spinning. The loop dimensions can, therefore, vary when yarns of different torsional properties are mixed or if the yarn has continuous variation in its torsional properties. This property is yet to be standardized commercially, however technology is available.

3         CONCLUSION

The quality of the knit fabrics in the future would be defined for variation in loop parameters at micro level such as loop to loop variation in dimension, geometrical shape of loop and localized variation in loop density. The tools required to assess them could be developed. However, the yarn quality should meet such demands and under such circumstances the yarn parameters considered so far may not be adequate.

Then the crucial yarn properties to be considered would be twist in yarn for shortest possible gauge length, yarn irregularity, within variation of friction in yarn, flexural rigidity and torsional rigidity.

An experimental study has shown that loop dimensions are strongly influenced by torsional rigidity followed by flexural rigidity. Apart from fibre and yarn parameters the process condition has a significant influence on these two properties and hence testing the yarns for properties discussed so far might be a serious requirement in the future. Any study on above lines might be worth considering for the benefit of knitting industry.

Acknowledgement

The author expresses his sincere thanks the principal and management of Shri Vaishnav Institute of Technology and Science, Indore for permitting him to publish this paper.

References

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13. Prabhakar Bhat, A Study on the Role of Yarn Properties in Double Jersey Loops, Ph.D. Thesis, Dept. of Textile Technology, IIT Delhi, 2003.

14. Banerjee, P.K. and Prabhakar Bhat, Torsional Properties of Cotton Yarns, IJFTR, March 2005.

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