Important Yarn parameters and

Yarn diameter and CountRelationship between yarn count and diameter is dependent upon specific volume of yarm. Specific volume, v, is the ratio of the volume of yarn to that of the same weight of water. Specific volume of yarn depends upon the raw material, type of spinning system, twist factor and spinning parameters. Ring spun yarns have a lower specific volume than rotor spun yarns. Acrylic and woolen yarns have a higher specific volume than cotton yarns. Specific volume reduces with increase in twist factor. If d is diameter in inches of yarn of count C v = 858Cd2. (1/d) = 29.3√C. Specific volume of yarn is usually 1.1 to 1.2. If speific volume is assumed as 1.1(1/d) = 28√C.            If d1 is diameter in cm and N is the count in tex units then            d1=√N/267.3.

Twist FactorTwist factor indicates the amount of twist put into the yarn and determines among other things the strength, elongation, specific volume, liveliness of yarn. If twist factor is k and t is twists per inch then,k = t/√C. If fibres follow helical path, Twist factor is equal the angle surface fibres make with axis of yarntan x = √Πdt=√Π(√v)k/29.3=0.107k(√v) When v = 1.1tanx = k/9.

TenacityTenacity denotes intrinsic strength of yarn. Strength of yarn is dependent on count and its intrinsic strength. It is usual to refer to tenacity by the term Breaking length. This is the length whose weight is equal to the breaking strength of yarn. If lea strength is used to denote strength, intrinsic strength is given by CS where C is count and S is breaking strength. This is commonly known as CSP or count strength product. As CS reduces with count, corrected CSP is determined by correcting for the deviation of actual count from nominal. If C1is the actual and C nominal count, corrected CSP = CS - (C1 - C)18. If strength is by single thread then tenacity is given by S/N where S is breaking strength and N is count in tex.

IrregularityIrregularity is an important characteristic of yarn that determines appearance, sale value of yarn and fabric. It also has considerable influence on strength realisation from fibre and performance of yarn. The best possible regularity that can be achieved by current spinning is that with random arrangement of fibres. Irregularity due to random arrangement of fibres, CVr, is given byCVr = (√(1002 + CVf

2))/n whereCVf = Coefficient of variation of weight per unit length of fibre n = Number of fibres in yarn crosssection.CVf for cotton varies within and between bolls, between lots, stations but has a value roughly equal to 30%. In the case of wool it is around 50% and for synthetics around 10%. Upon putting this figure in the above formulae,CVr = 106/√n for cotton=112/√n for wool=102/√n for synthetics.

Index of IrregularityIndex of Irregulaity = Actual CV of yarn/CVr. Index of irregularity indicates the scope for improving the spinning processes for getting better regularity.

ImperfectionsThese represent outlayers of variations in yarn which have a profound influence on appearance of yarn and fabric and performance of yarn. Uster imperfection tester measures three types of imperfections viz

Thin places, Thick places, Neps. Nep is differentiated from a thick place by the length of the defect. The instrument evaluates nep as a thick place whose length is shorter than 4mm and longer thick places are evaluated as thick. 4 classes of each of these faults are measured as indicated in Table below.

Thin(%) Thick (%) Nep (%)

-30 +35 +140

-40 +50 +200

-50 +70 +280

-60 +100 +400

Thin places are influenced by fibre properties combing and drafting conditions. Thick places are influenced by fibre properties, blow room, carding and combing and drafting conditions. Neps are influenced by fibre properties and blow room, carding and combing conditions.

FaultsFaults are seldom occuring defects as agaist imperfections which are frequent occuring defects. Faults show prominently in fabric and lead to rejections. They also affect the performance of yarn in weaving and knitting. Uster Classimat has facility to detect faults as per their size and length. Longer and thicker size faults show up prominently and are termed as objectionable

o Short Length faults16 categories of short length faults are measured by Classimat.

Short Length Faults

Size of Fault A - 0.1 to 1cm B - 1 to 2cm C - 2 to 3cm D - above 4cm

+100 to +150% A1 B1 C1 D1

+150 to +250% A2 B2 C2 D2

+250 to +400% A3 B3 C3 D3

above +400% A4 B4 C4 D4

o

Very short thick places are caused by the presence of seed coats, broken seeds, trash in the case of cotton and cutterfibres in synthetics. Medium size short thick places are caused by embedded fluff, loose lint among others. Faults of 4cm and above are caused by slubs, undrafted ends, bad piecings etc.

o Long Thick Faults5 catergories of long thick faults are measured by classimat.E - Above 8cm length and  above +100% crosssection size.

Size of fault 8 - 32cm above 32cm

+45 - +100% F G

o

Long thick places of E category are known as 'Spinner's double' and are caused by lashing of an end with adjoining end at roving or ring frame. F and G faults are due to drafting defects at earlier stages, sliver splitting in creel at roving and long overlap of sliver at the time of sliver break at drwaframe.

o Long Thin Faults4 types of thin places are measured

Size of Fault 8 - 32cm above 32cm

-30 to -45% H1 I1

-45 to -75% H2 I2

o

Long thin faults are due to raw material defects, drafting faults in roving and drawing, split sliver in creel of roving, mal functioning of stop motion at drawframe.

o CorrelogramIf r(l) is the (Auto)correlation coefficient of thickness( or weight per unit length)of yarn at points 'l' apart, then the plot of r(l) against l is Correlogram. Correlogram is a useful tool for detecting the presence of periodicities in yarn.Let t1 and t2 denote thickness(or weight per unit length) of yarn at points separated by 's' distanceAuto Correlation Function r(s) = (N(∑titi+s - ∑ti∑ti+s)/∑{(N∑ti

2 -∑(ti)2)(N∑ti+s2 -∑(ti+s)2)}. For

lengths below fibre length, correlogram is determined by the fibre length distribution of cotton. Since part of the fibres are common to both crossections, there will be a positive auto correlation coefficient. Corrleation coefficient between points x aprt,

                  

r(x) = I/la (l - x) f(l)dlwhere la is mean fibre length and f(l) is frequency of fibres of length l and lm is maximum fibre length

o Variance Length curveVariance length curve is a graph relating variance of weight of length of yarn and the length. This is a very useful tool in characterising the various types of irregularities in yarn and will assist in locating processes which require improvement. Irregularity in a yarn is given byVariance(x L).Here L denotes the total length of yarn taken for study and x denotes the length of pieces cut from the yarn and weighed. If x is varied keeping L constant, variance length curve obtained is known as Between length curve or BL curve. On the other hand, if x is kept constant and L is varied, the curve obtained is known as Within length curve or VL curve. At lengths shorter than longest fibre length, BL curve is influenced by fibre length distribution of fibre as some fibres will be common to both sections. Variance length curve is related to correlogram of yarn as indicated below

B(x) = B(O)(2/L2) (x-u)r(u)du where r(u) denotes auto correlation coefficient of thickness between points u distance apart, B(x) = Variance of weight of lengths x long. When x is less than lt, where lt is half the length biassed mean fibre length of fibre,lc

B(x) = B(O)(1 - (x/3la)) Where lais mean fibre length. For lengths longer than longest fibre length,B(x) = B(O)lc/x,  if there are no other irregularities( no extra auto correlation coefficients) where lc is length biassed mean length of fibre. This enables one to find out how much the actual variance length curve deviates from the ideal. Variance length curve shows the amount of short, medium and long term variations present in a yarn. While short term variations come from ring frame, medium term variations arise from roving frame and long term variatons from drawframe.

o SpectrogramSpectrogram is a fourier analysis of variations present in the material. The amplitude of the variations are sorted as per their wavelength and plotted as a amplitude vswavelength curve. The spectrogram of a yarn due to random fibre arrangement has a "hill" whose maximum wavelength lies in the region of 2.5 to 3 times fibre length. On the top of it, waves introduced by drafting waves is superimposed. Wavelength of drafting wave is also 2.5 to 3 times fibre length and so in normal yarn the "hill" is pronounced depending upon the amplitude of drating wave. With cut staple fibres peak value of spectrogram lies at 2.7 times fibre length. A shorter but a smaller

amplitude wave will also be found at half the fibre length and this is a lower harmonic. In between these two peaks, there will be a valley. In cotton, wollen and other material with variable staple diagram, the spectrogram has a hump like shape with maximum around 2 to 3 times mean length. Thus spectrogram of cotton yarns has a maximum at 6 to 8cm, of woolen yarn at about 20cm. OE rotor yarns have a peak at a slightly lower length than ring yarns because fibres are curled with hooks leading to a lower projected length on yarn axis. When periodic variation is present, spectrogram will show a sharp peak at the point corresponding to wavelength of periodicity. So spectrogram is a useful tool for detecting the periodicities in the material. It also gives their wavelength which can be used to trace the cause of periodicity and rectify it.

o Blend VariabilityBlend variation is an important parameter to be controlled in blends in view of their influence on appearance, fault incidence, grade, and sale value of fabric. Two types of blend variability have to be minimised.

1. Variations in blend proportion of the component fibres in cross section along the length of yarn

2. Inadequate intermixing of components within a cross section.

Longitudinal blend variationIf black and white fibres are randomly mixed, white fibres in cross section will have a mean equal to np and a variance equal to npqwhere n is the avearge number of fibres in yarn cross section, p proportion of white fibres and q is proportion of black fibres in yarn. This represents the minimum blend variability that will be present in yarn with the best possible mixing of components. Index of blend variability is a quantity used to assess the extent of departuture of actual blend variation from random mixing. Index of blend irregularity = √((1/N)∑((wi - p ni)2/pqni)where wi, bi and ni denote the number of white, black and total number of fibres in a crossection and N total number of yarn cross sections examined. IBI compares the observed deviation in blend proportion in each section against theoretically estimated value of that section and determines the avearge of this over a number of sections.

Intimacy of lateral mixingTo assess intimacy of mixing the ribbon of fibres emerging from front roller nip is examined.Two measures of intimacy mixing are used1.Index of mixing,Π = proportion of white fibres that have a white right hand neighbour.2. Measure of mixing, g = number of groups of white fibres in the strand. Π = (w - g)/g. If mixing is randomΠ= p.g = npq. Degree of mixing is the ratio gactual/grandom

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