Yarn Irregularity
Dr. N.Balasubramanian

index Modern Developments Yarn and Cloth Test Methods

Yarn Irregularity

Importance of Irregularity
Irregularity is the most important quality characteristic of yarn. In quality monitoring it plays a crucial role nowadays. Importance of irregularity arises from the following factors

• Irregularity has a profound influence on appearance of yarn and fabric. More regular the yarn, better will be the appearance and aesthetic value of the product. As a result, better sale value can be achieved. With advent of Uster Evenness tester and Uster standards, importance of irregularity has become even higher. Buyers insist on yarns meeting certain Uster standards. In export market, especially in developed countries, Yarns have to meet Uster 5 or 10% standards. Mills who produce consistently more regular yarn get a premium in the selling price
• Regularity contributes to a smoother feel. In apparel and most of other textiles, smoothness is most desired characteristic. Sale value of fabric is dependent, among other things, on smoothness.
• Regular yarns will have fewer weak places and and, as yarn breaks at weakest place, will have a better strength. Better strength realisation from fibre can be achieved if regularity of yarn is improved. It is for this reason good mills, which produce more regular yarn, are able to produce a yarn of much higher strength from the same cotton
• Because of the lower incidence of weak places, fewer end breaks are encountered with regular yarns in weaving preparatory, weaving and knitting. Efficiency in these processes is improved leading to higher productivity. Importance of reguality will be appreciated from the better performance achieved by rotor spun yarns in weaving processes compared to ring spun yarns. Rotor yarns have a lower strength than ring yarns but have a better regularity. Though mean strength is lower, strength of weak place is higher in rotor yarn than ring yarns and so it performs better.
• Fabric defects and rejections are critically influenced by irregularity of yarns. Periodic and quasi periodic irregularities in yarn result in warp way streaks and weft bars in woven fabrics leading to fabric rejections. Yarn defects like slubs, crackers, long thick places and long thin places downgrade the fabric and cause considerable value loss. Mills which produce more regular yarns therefore get better realisation and contribution and as a result higher profitability

Types of Irregularity

• Weight per unit length
  Variation in weight per unit length is the basic irregularity in yarn. All other irregularities are dependent on it. This is because weight per unit length is prportional to fibre number i.e; number of fibres crossing a section of yarn. Variations in fibre number is the factor influenced by drafting. So any improvement in drafting or spinning will first reflect in improvement in variability of weight per unit length.
• Diameter
  Variability in diameter is important because of its profound influence on appearance of yarn. Variations in diameter are more easily perceived by eye. Latest models of evenness testers have therefore a module for determining diameter variability. Diameter variability is however caused by weight variability. As twist has tendency to run into thin place, variability in weight gets exaggerated in diameter variability.
• Twist
  Twist variation is important because of its influence on performance of yarn and fabric dyeability and defects. Soft ends are a major cause of breaks in weaving preparatory and loom shed. They arise from twist variations. Soft twisted yarns take more dye and so uneven dyeing is caused by high twist variation. Weft bars and bands are also caused by low twisted yarns. Twist variations come from slack spindle tapes, jammed spindles . A certion amount of variation is also present along the chase of cop.
• Strength
  Importance of strength variation is easy to appreciate. Yarn breaks at the weakest element and so yarns with high strength variability will result in high breakages in further processes. Strength variability is partly dependent upon count variability and partly upon spinning conditions and mechanical defects.
• Hairiness
  High variation in hairiness leads to streaky warp way appearance and weft bars in fabric. More light will be reflected from portions of weft where hairiness is more and this leads weft bands. High hairiness disturb warp shed movement in weaving and result in breaks, stitches and floats. Among other factors, worn out rings and travellers, vibrating spindles, excessive ballooning and variation in humidity in spinning room cause variations in hairiness from bobbin to bobbin
• Colour
  Variations in colour of yarn cause batch to batch varation in fabric colour, which leads to rejects. This is particularly critical in cloth marketed to garment units. Variations in colour of yarn and fabric are caused by variations in colour of cottons used in mixing. Larger lot sizes made from a large number of bales help to mitigate this problem. Checking of cotton and mixing for colour will also minimise large variations in colour. HVI testing equipments have therefore a module for checking colour.

Measures of Irregularity

Range
Measurements of weights or diameter are classified in number of rows, each row containing about 4 or 5 items as shown below
x₁₁ x₁₂ x₁₃ x₁₄
x₂₁x₂₂ .........
..............
..............
x_n1 ............ x_n4.
Range, which is the difference between maximum and minimumvalue is determined in each row. Mean range,r_m for all the rows is then calculated. Range% = (r_m/x_m)100. Higher the range% higher the variability.If the irregularity is random, there is a relationship between range and standard deviation, σ .Let mean range be r_m. Then σ = r_m/a. The value of a is dependent upon upon the number of units in a row and its values are shown in Table 1 below.

Table 1

Number in a row	2	3	4	5	10	12
a	1.128	1.693	2.059	2.326	3.078	3.258

• Mean Deviation%
  Let the individual weights of specimen be as given in Table 2.

  Table 2

  Weight of specimen	Deviation	Square of Deviation
  x1	x1 -xm	(x1 -xm)2
  x2	x2 -xm	(x2 -xm)2
  .	..	..
  .	..	..
  .	..	..
  .	..	..
  .	..	..
  xn	xn -xm	(xn -xm)2

  
  Deviation of each specimen weight from mean is determined, ignoring the sign.
  Mean Deviation%, MD% = 1/x_m(∑ (|x_i - x_m|)/n)100. Mean deviation % is measured by linear integrator of Uster Evenness tester. If a recorder chart of weight variations is available, Mean deviation is given by the area enclosed by the chart and avearage value. The area can be determined by planimeter
• Standard Deviation
  For estimating standard deviation, deviation is squared as shown in 3rd column of table and the aveage of square of deviation is determined. Its square root is determined as shown below.
  = √(x_i - x_m)²)/(n - 1))
• Coefficient of Variation, CV%
  Coefficient of Variation, CV% = (σ/x_m)100. CV is a better measure of irregularity than MD as it gives greater weightage to portions moved far away from mean. Later models of Evenness testers are therefore equipped with quadratic integrator which measures CV. If irregularities are random CV ≈1.25 MD. As normal yarns contain extra irregularities, CV = 1.35 to 1.4 times MD. If irregularities are perfectly sinusoidal, CV = 1.11MD. Original models of Uster integrators were determining MD(U%) and CV with a certian amount of approximation. Later with improvements in electronics, later models were designed to determine CV precisely. But U% is still estimated with a certain amount of approximation. So the relationship between CV and U depends upon the model of Uster Evenness tester and type of irregularity in yarn as shown in Table 3 below.

  Table 3

  Type of irregularity	Model of Evenness tester	CV/U
  Symmetric with a single peak	GGP (very old model), Uster1,2 and 3	1.25
  Symmetric with 2 or more peaks	GGP	1.25
  	Uster 1,2 and 3	> 1.25
  Assymetric	GGP, Uster 1, 2 and 3	> 1.25 with a higher conversion factor for Uster 1,2 and 3

• Exceeding Frequency
  This is defined as the frequency f with which a particular lenier density is exceeded in a unit length of yarn and is given by
  f = A/L
  where A= number of times a specified value of CV% is exceeded in a given length L of yarn.
  Number of end breaks in subsequent processes is related to exceeding frequency.
• Fraction Defective
  This estimates the percentage of material that lies beyond certain limits away from the mean on either side. Weights removed far away from the mean are mainly responsible for end breaks, defects and rejections. Estimation of variability by assessment of the percentage of yarn which is beyond a certain weight has therefore several merits. For example one of the conditions stipulated in purchase of yarn is that not more than 5% of bobbins will have a count beyond mean ± specified percentage. Supplies that do not meet this specification are liable to rejection. The following examples show that yarns with a lower variability will be able to meet more stringent requirements. Example
  Count=30^s
  Mill A - CV of count = 2.5%
  Standard Deviation, σ = 0.75
  % bobbins falling outside 30 ± 2σ = 5 (From Table 4 below)
  Mill B - CV of Count = 5%
  SD σ = 1.5
  % of bobbins with count outside 30 ± 2σ = 31.7.(Table4)
  Mill A which has a lower CV% also has fewer % bobbins falling beyond ± 1.5 of mean. This shows the utility of checking the % of bobbins falling outside certain limits on either side of mean.

  Table 4:Proportion of values beyond a limit with Normal distribution

  Number of SD(σ) on each side of mean	Proportion of values lying outside limits%
  0	100
  0.5244	60
  1	31.7
  1.645	10
  1.96	5
  2	4.56
  3	0.27

• Irregularity Testers
  
  • Cut and weigh MethodYarn is wound on a drum and pieces of known length are cut with the help of a template. The cut pieces are weighed on a sensitive balance and CV% is estimated.This is a time consuming method and is adopted only in special research program. Moreover accuracy has to be maintained in cutting and weighing. Only advantage is it is a direct method and does not involve any assumptions.
  • Mechanical Type tester
    Tongue and Groove tester
    This is used mainly for slivers and rovings. This consists of two rollers one with a tongue and another with a groove, in between which the sliver is passed. Tongue roller moves to and fro depending upon the weight of sliver. The roller movement is amplified and integrated to find the CV%.Groove width ranges from ½ in for coarse, ¼ in for fine and 1/16 in for fine slivers. This is also time consuming. Further errors are introduced because of variation in compactness of sliver.
    • Shoe type tester
      This is mainly for yarns. Yarn is passed between a hard steel shoe and a a half round ball fitted on a light lever suitably fulcrumed in the middle. The diameter variations in yarn cause the lever to move up and down which is amplified by means of a light falling on a mirror fitted on the lever. From this CV% is estimated by an integrator
  • Photo Electric tester
    The yarn is passed in front of a slit. A light source is made to fall on yarn, running in front of the slit. The image of yarn is made to fall on a photo cell. The amount of light falling on photo cell varies depending upon diameter of yarn. The current generated by photocell is amplified and integrated.
  • Capacitance type tester
    The material is passed between two plates of a condenser at a known speed. Capacity of condenser varies depending upon weight variations in the material. The voltage generated is amplified and a continuous record of variations is recorded on a recorder. An integrator is used to determine MD%(U%) or CV%. Uster and Fielden walker testers are based on this prinicple.Merits of this system is it attempts to measure a quantity proportional to weight per unit length of material. Condenser slots of different sizes are used for slivers, rovings and yarns.The results are affected by
    • Fringe Effect - The fields of force between the plates are curved at the edges and as a result the material is detected before it enters the space between the condenser plates.This affects linearity of mass voltage relationship leading to errors in measurement. This can be overcome by incorporating a guard plate prior to normal condenser.
    • Filling proportion (λ) - This is the ratio between material and air space and is given by λ =(d/D)100. This should be kept much lower than 1. Otherwise relation between change in capacitance and mass of material becomes non linear. The change in capacity of a condenser as a result of material of dielectric constant ε is given by
      1/C =1/C₁ + 1/C₂ where C₁ = C₀εD/(D-d), C₂ = C₀εD/d. Since ε₀ = 1 and δC = C - C₀
      δC/C₀ = (ε - 1)/((ε((1/λ) - 1) + 1). When ε is much lower than 1, δC/C₀ is close to λ.Relation between chnage in Capacitance and dielectric constant is shown in Fig below.. At relatively high dielectric constants (above12) and low filling proportion, capacitance of condenser (and instrument reading) is primarily affected by weight per unit length variation of material and is not much affected by variations in dielectric constant, caused by variations in blend proportion and moisture content. Textile material usually have a dielectric constant above 12
    • Inadequate conditioning and variations in relative humidity during testing.The error due to uneven moisture content can be kept down if following precautions are taken
      • If testing room humidity is below 67%, its variations should not exceed 16% during conditioning and if it is above 67%, the variation should be below 8%.
      • Yarn and roving bobbins should be conditioned in testing room with a free circulation of air from all sides of each bobbin. Conditioning for yarn may be about an hour, for roving about 2 - 3 hours. Slivers should be tested immediately without conditioning, after discarding a few layers on the top. If sliver is conditioned, irregularity increases because of uneven penetration of moisture into inner sliver layers. A typical study showed how sliver irregularity increases with time(TABLE 5)

        Table 5: Increse in U% of finisher drawing sliver with conditioning time.

        Conditioning time	U%	CV(1m)%
        Without conditioning	2.5	1.31
        1hr	2.99	1.42
        4hr	3.27	1.51
        8hr	3.85	1.49
        24hr	3.88	1.39

    • Shape factor of material - Fibre assembly should be as close to cylindrical as possible and should not be asymmetric. A slight twist is given to multi filament yarns prior to testing for this purpose.
    • Material should be kept away from both plates or kept in touch with one of the plates.
  • Principle of operation of Uster Integrator
    In simple terms, the operation of Uster integrator is based on the use of resistance - Capacitance circuits. The integrator consists of two such circuits as shown in Fig below
    
    A voltage f_t is developed upon insertion of material in between measuring plates of the condenser in the main instrument.This is impressed upon R₁C₁ circuit. R₁ and C₁ are very large and as a result, condeser C₁ charges slowly to the mean value of fluctuating voltage f_t. The voltage across R₁ at any time is equal to the deviation of f_t from the mean value. The voltages across R₁ are impressed across R₂¹ without taking sign into account in the case of linear integrator. In the case of Quadratic integrator, the deviations are squared and impressed across R₂^'. The voltage across C₂ charges slowly to the mean value of deviation(in linear integrator) and mean value of square of deviation (in case of Quadratic integrator) and is displayed as irregularity value. Thus the equipment measures CV(8_mm, kv/α₁) where v is material speed α₁ is time constant of R₁C₁ circuit and k is a constant. When service selector of the instrument is switched to 'inert' test, an additional resistance R₀ and C₀ are introduced in the circuit. The voltage across C₀ is average voltage over a time of kv/α₀. The average of this is the voltage developed across C₁, and the deviations from average are the voltage across R₁. The mean value of the deviations (or square of deviations) are estimated by the voltage across C₂.
  • Pneumatic Method
    Yarn is passed through a narrow tube into which a stream of air is forced. Air flow rate is then measured. Air flow rate is affected by the mass of yarn in the tube. Variation in air flow rate is therefore a measure of irregularity.
  • Acoustic Meethod
    Yarn is moved through a sound field between a sound generator and a pick up device. Time taken for sound waves to move across the gap is measured electronically. Transit time of sound is dependent upon the weight of yarn in the gap.
• Causes of Irregularity
• Random Fibre Arrangement
  In order to produce a yarn without any irregularity, as soon as fibre ends, another should be placed in its place, as shown in Fig. With such a fibre arrangement in yarn, number of fibres in yarn cross- section will be the same throughout. Weight per unit length will also be the same throughout. But there is no mechanism in currently available spinning systems to ensure this arrangement. The best that can be done is arrange fibres randomly. In Blow room and Carding, fibres are randomly placed and so the best possible arrangement in card sliver will be one with random arrangement. Even if it were possible to produce a card sliver with fibres arranged one behind another, this arrangement will be destroyed by doubling in drawframes, as slivers are fed side by side without regard to position of fibre ends. In other words, doubling at drawframe randomises fibre arrangement. A certain amount of minimum irregularity is likely to be present due to random fibre arrangement. Since fibre length is very small compared to yarn length, the probability that a particular fibre is present in a certain cross-section is very small and the probability of its non occurrence in the cross-section is very large. Under such conditions, the number of fibres in cross-section will vary as per Poisson distribution. For a Poisson distribution, standard deviation, σ is equal to square root of Mean. If n= mean number of fibres in a cross-section
  σ=√n.
  CV%=(σ/n)100
  =100/√n. A certain amount of irregularity is also caused by variations in weight per unit length along the length of a fibre and between fibres. If CV_f is the irregularity due fibre weight variations then
  CV_r = √((100/√n)² + (CV_f/√n)²). A good deal of variation exists between variety/material to variety/material in variability of fibre weight per unit length but roughly CV_f may be taken as 30% for cotton, 50% for wool and 10% for man made fibres. Upon putting these values in the above equation
  CV_r =106/√n for cotton
  112/√n for wool and
  102/√n for man made fibres. This means that that irregularity due to random fibre arrangement will increase as n decreases. If C= Yarn Count(Ne) and M= Micronaire value of fibre, D= Denier of fibre
  n=15000/N×M
  =5314.5/N×D CV_rwill therefore be very low with Drawing sliver and increases slightly at roving and becomes noticeable in yarn. In fine counts CV_r will be a substantial part of the yarn irregularity. This also means that CV_r can be brought down by using finer fibres in place of coarser fibres.
  Index of Irregularity
  Index of irregularity(I) is the measure used to find out the extent to which actual irregulaity deviates from that due to random.
  Index of Irregularity=CV_a/CV_r where CV_a is actual measured irregularity. A higher value means that there is more scope for improving the processes. Table6 below shows how irrgularity due to random arrangement and Index of irregularity vary with count.

  Table:6 Variation of CV_r and I with count

  Count	CVr	CVa	I
  20 Crd	7.7	17	2.20
  30 Crd	9.26	17.5	1.88
  40 Cbd	10.5	14	1.33
  60Cbd	12.5	15.5	1.24
  100 Cbd	15.2	18	1.18

  It will be seen that while Irregularity due to random arrangement increases Index of irregularity reduces with increase in count. CV_rreduces from yarn to roving and further from roving to yarn because of the increase in number of fibres in cross-section. Index of irregularity on the other hand increases from yarn to roving and then from roving to yarn Typical results are given in Table7 below.

  Table 7:Variation of CV_rand I with stage of processing

  Material	CVr	CVa	I
  Yarn 20s	7.7	17	2.20
  Roving 1Ne	1.7	6	3.52
  Drawing sliver	0.6	4	6.68
  Yarn 100s	15.2	18	1.18
  Roving(100s) 2.8Ne	2.54	5	1.96
  Drawing sliver(100s) 0.19Ne	0.66	2.5	3.79

• Drafting Wave
  Drafting wave is another major source of irregulaity in yarns. Setting between drafting rollers has to be kept slightly higher to minimise fibre breakages and avoid spewing of material. As cotton has a variable staple diagram, a good proportion of fibres will have length shorter than effective length, with some of them being much shorter than roller setting. Even in man made fibres there will be fibres shorter than roller setting because of fibre breakages presence of hooks and disorientation. The shorte fibres will be out of control of both back and fron drafting rollers for part of their journey through drafting zone. They are therefore termed as 'floating fibres'. The velocity and movement of floating fibre is guided by the number frictional contact it makes with back roller held and front roller held fibres. Initially, it be surrounded by back roller held fibres and so will move at back roller speed. After some time, it will start making more and more contacts with front roller held fibres and fewer contacts with back roller held fibres. When the frictional hold with front roller held fibres becomes higher than that with back roller held fibres, floating fibre will accelerate to front roller speed and will reach front roller nip ahead of its scheduled time. As more and more floating fibres get thus accelerated, the chances of acceleration of other floating fibres also increase as their contact with fast moving fibres increase. This leads to the formation of a thick place in outgoing material. But the process cannot go on like this forever, as after a period of time there will be fewer floating fibres in drafting zone. Contacts a floating fibre makes with fast moving fibres also diminishes, which culminates in a thin place. Thus premature acceleration of floating fibres causes a succession of thick and thin places in the delivered material, which is termed as 'Drafting Wave'. The drafting wave is however not a periodic wave in the strict sense as both the amplitude and wavelength of the wave are affected by random disturbances. The disturbance arises from variation in fibre entanglements, disorientation, hooks, compactness of strand, presence of neps, trash etc; and variability in fibre characteristics. Drafting wave is therefore a damped harmonic wave with a wave length equal to 2-3 times the fibre length.

  Fibre Movement in drafting
  Movement of floating fibres in drafting zone has been studied in great detail by many authors by introducing coloured or radio active tracer fibres into the material. Geiger counters and high speed photography have been used to determine the velocity of fibres.The following conclusions emerge from the studies.

  • Apart from back roller and front roller speeds, fibres also move at intermediate speed
  • Fibre travels for short periods at front roller speed, these periods being alternated with longer periods at or near to back roller speed
  • Time spent at back roller speed by shorter fibre is much greater in apron drafting than in roller drafting thereby confirming the extra control provided by aprons.
  • Average speed in floating zone is nearly independent of fibre length and draft
  • All fibres of the same length do not accelerate at the same point. Only a percentage of fibres accelerate and this percentage increases as front roller nip is reached. The number and type of contact a floating fibre makes with slow moving and fast moving fibres determines its velocity