yarn evenness_testing_by abubakkar marwat

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Textile Testing-II (TS-333)

By:

Abu Bakkar Marwat

05-NTU-05 Section A

Semester VI NTU Faisalabad

0313-6660505 [email protected]

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Table of contents: Introduction 01 Types of irregularity 01 Causes of irregularity 02 Denoting unevenness or irregularity 03 Types of variations 04 Importance of yarn evenness 04 Measuring and assessing evenness 05 1-Visual examination 05 2- Gravimetric method 06 3- Capacitive method 07 3.1- Uster evenness tester 07 4- Mechanical method of roving/sliver evenness measurement 13 5- Optical method (Zweigle G580) 15 6- Pneumatic method 15 7- Acoustic method 16 7.1- Impulse acoustic method 16 Index of irregularity 18

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⇒⇒⇒⇒ Introduction:

Non-uniformity in variety of properties exists in yarns. There can be variation in twist, bulk, strength, elongation, fineness etc. Yarn evenness deals with the variation in yarn fineness. This is the property, commonly measured as the variation in mass per unit length along the yarn, is a basic and important one, since it can influence so many other properties of the yarn and fabric made from it. Such variations are inevitable, because they arise from the fundamental nature of textile fibres and from their resulting arrangement.

The spinner tries to produce a yarn with the highest possible degree of homogeneity. In this connection, the evenness of the yarn mass is of the greatest importance. In order to produce an absolutely regular yarn, all fibre characteristics would have to be uniformly distributed over the whole strand (yarn). However, that is ruled out by the inhomogeneity of the fibre material and by the mechanical constraints. Accordingly, there are limits to the achievable yarn evenness.

⇒⇒⇒⇒ Types of Irregularity:

1) 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 proportional to fibre number i.e.; number of fibres crossing a section of yarn. Variations in fibre number are the factor influenced by drafting. So any improvement in drafting or spinning will first reflect in improvement in variability of weight per unit length. 2) 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. 3) Twist:

Twist variation is important because of its influence on performance of yarn and fabric dye ability 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 certain amount of variation is also present along the chase of cop. 4) 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.

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5) 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 disturbs warp shed movement in weaving and results in breaks, stitches and floats. Among other factors, worn out rings and travelers, vibrating spindles, excessive ballooning and variation in humidity in spinning room cause variations in hairiness from bobbin to bobbin

6) Colour:

Variations in colour of yarn cause batch to batch variation 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 minimize large variations in colour. HVI testing equipments have therefore a module for checking colour.

⇒⇒⇒⇒ Causes of irregularity: 1) Irregularity caused by raw material:

The natural fibres have variable verities. They have no true fixed length, fineness, shape of cross-section, maturity, crimp, etc., which have effect on yarn properties specially evenness. These variations are due to different rates of cell development due to changes in environmental conditions (nutrients, soil, and weather).

In man made fibres, variations in mass/unit length occurs due to changes in polymer viscosity, roughness of spinneret orifice, variation in extrusion pressure and rate, filament take-up speed, presence of delustrant or additives, which can modify the particular shape and fibre surface geometry.

2) Irregularity caused by fibre arrangement:

Textile fibres are not rigid. Their manipulation during conversion into yarn is an immensely complex combination of mechanical movement which usually requires some degree of compromise. The desirable results of relocating large number of fibres at high speed and arranging in well ordered form tend to be difficult. Fibres

assembled into the form of a twisted strand constitute a yarn.

Fibres are not precisely laid end to end, and gaps are present between them. As a result of yarns twist, fibres arrange in spiral form in a series of folds, kinks, and doublings. 3) Effect of fibre behavior: Fibres shape directly affects yarn regularity. The fibres section, arrangement of fibre section and space between the fibres will vary from yarn section to section. Hence the mass of each section will differ.

A thin place in yarn will have lower mass and less strength. In thin regions, yarn twist tends to be higher since resistance to deformation is lower.

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4) Inherent shortcoming of machinery: In many engineering processes the units from which the final product is

assembled are positively controlled by hand or machine and positioned with only a few thousandths of an inch tolerance. In spinning it is surprising how often the individual fibres are only negatively controlled-at times they are carried forward by air currents or jostled along by surrounding fibres, or they are held in position by friction and twist.

Fibre manipulation by rollers, aprons, gills, and other machine parts is hampered by fibre variation, and the machines can only be set to give the best results within the limitations imposed by the material.

The drafting wave is one example of irregularity due to the inability of a drafting system to control each fibre. Where roller drafting is used, the distance from one nip to the other is greater than the length of the shorter fibres. These short fibres ‘float’ in the drafting zone and move forward in an irregular but cyclical manner which results in the drafted strand having thick and thin places. The wavelength of this type of

irregularity is about 2-5 times the mean fibre length but it is not necessarily constant for a particular strand. In addition to a varying wavelength, the amplitude of the drafting wave is also variable. 5) Mechanically defective machinery:

Since machines even in good condition produce irregular yarns, it is reasonable to assume that defective machinery will increase the amount of irregularity. The implementation of an efficient maintenance system is essential if the level of irregularity is to be kept within bounds. Machines drift out of adjustment, bearings become worn, components get damaged, and lubrication systems clog and dirt works its way into the mechanism. Faulty rollers (top roller eccentricity) and gear wheels usually produce periodic variation.

⇒⇒⇒⇒ Denoting "UNEVENNESS" OR "IRREGULARITY" :

The mass per unit length variation due to variation in fibre assembly is generally known as "IRREGULARITY" or "UNEVENNESS". It is true that the diagram can represent a true reflection of the mass or weight per unit length variation in a fibre assembly. For a complete analysis of the quality, however, the diagram alone is not enough. It is also necessary to have a numerical value which represents the mass variation. The mathematical statistics offer 2 methods:

1. The irregularity U%: It is the percentage mass deviation of unit length of material and is caused by uneven fibre distribution along the length of the strand.

2. The coefficient of variation C.V.%

In handling large quantities of data statistically, the coefficient of variation (C.V.%) is commonly used to define variability and is thus well-suited to the problem of expressing yarn evenness. It is currently probably the most widely accepted way of quantifying irregularity. It is given by

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Coefficient of variation (C.V.%) = .

100.

S D

avg value where S.D = Standard deviation

• The irregularity U% is proportional to the intensity of the mass variations around the mean value

• The U% is independent of the evaluating time or tested material length with homogeneously distributed mass variation.

• The larger deviations from the mean value are much more intensively taken into consideration in the calculation of the coefficient of variation C.V. %.

• C.V.% has received more recognition in the modern statistics than the irregularity

value U%.

• The coefficient of variation C.V.% can be determined extremely accurately by electronic means, whereas the calculation of the irregularity U% is based on an approximation method.

• It can be considered that if the fibre assembly required to be tested is normally distributed with respect to its mass variation, a conversion possibility is available between the two types of calculations

C.V.% = 1.25 x U%

⇒⇒⇒⇒ Types of variations: short, medium and long term:

Variations in the fibrous strand are classified according to their wavelength with respect the fibre length used to form that particular strand. 1) Short term variation: These variations are of the wave length 1-10 times of fibre length. Amplitude of these variations is greater than long term variations. These result due to faulty processing at the last machine. Such variations if excessive produce a fabric of objectionable appearance. 2) Medium term variations: These variations are of the wave length 10-100 times the fibre length. Such

variations do not cause a pattern as it hides into the adjacent warp yarn. In weft it will appear as a thick line again hidden by adjacent weft. However excessive variations give the cloth a streaky appearance. 3) Long term variations: These variations are of the wave length 100-1000 times the fibre length. Such variation cause periodic faults known as diamond bars or block bars in the woven

fabric along the weft direction. A weft yarn to cause a diamond bar pattern must have a long term periodic variation of wave length less than twice the pick length.

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⇒⇒⇒⇒ Importance of yarn evenness:

�� Irregularity can adversely affect many of the properties of textile materials. The most obvious consequence of yarn evenness is the variation of strength along the yarn. If the average mass per unit length of two yarns is equal, but one yarn is less regular than the other, it is clear that the more even yarn will be the stronger of the two. The uneven one should have more thin regions than the even one as a result of irregularity, since the average linear density is the same. Thus, an irregular yarn will tend to break more easily during spinning, winding, weaving, knitting, or any other process where stress is applied.

�� A second quality-related effect of uneven yarn is the presence of visible faults on the surface of fabrics. If a large amount of irregularity is present in the yarn, the variation in fineness can easily be detected in the finished cloth. The problem is particularly serious when a fault (i.e. a thick or thin place) appears at precisely regular intervals along the length of the yarn. In such cases, fabric construction geometry ensures that the faults will be located in a pattern that is very clearly apparent to the eye, and defects such as streaks, stripes, barre, or other visual groupings develop in the cloth. Such defects are usually compounded when the fabric is dyed or finished, as a result of the twist variation accompanying them.

�� Twist tends to be higher at thin places in a yarn. Thus, at such locations, the penetration of a dye or finish is likely to be lower than at the thick regions of lower twist. In consequence, the thicker yarn region will tend to be deeper in shade than the thinner ones and, if a visual fault appears in a pattern on the fabric, the pattern will tend to be emphasized by the presence of colour or by some variation in a visible property, such as crease-resistance controlled by a finish.

�� Other fabric properties, such as abrasion or pill-resistance, soil retention, drape, absorbency, reflectance, or luster, may also be directly influenced by yarn evenness. Thus, the effects of irregularity are widespread throughout all areas of the production and use of textiles, and the topic is an important one in any areas of the industry.

⇒⇒⇒⇒ Measuring and Assessing Evenness:

1) Visual examination: (Seri Plan Method)

Yarn to be examined is wrapped onto a matt black surface in equally spaced turns. The black boards are then examined under good lightening conditions using uniform non-directional light. A.S.T.M. has a series of Cotton Yarn Appearance Standards which are photographs of different counts with the appearance classified in four grades. The test yarn is then wound

on a blackboard approximately 9.5 x 5.5 inches with the correct spacing and compared directly with the corresponding standard.

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Motorized wrapping machines are available: the yarn is made to traverse steadily along the board as it is rotated, thus giving a more even spacing. It is preferable to use tapered boards for wrapping the yarn if periodic faults are likely to be present. This is because the yarn may have a repeating fault of a similar spacing to that of one wrap of yarn. By chance it may be hidden behind the board on every turn with a parallel-sided board where as with a tapered board it will at some point appear on the face. 2) Gravimetric Method (Cut and weigh method): This is the simplest way of measuring in mass per unit length of a yarn. The method consists of cutting consecutive lengths of the yarn and weighing them. For the method to succeed, however, an accurate way of cutting the yarn to exactly the same length is required. This is because a small error in measuring the length will cause an equal error in the measured weight in addition to any errors in weighing operations.

One way of achieving accurate cutting to length is to wrap the yarn in around a grooved rod which has a circumference of exactly 2.5 cm and then to run a razor blade along the groove, leaving the yarn in equal 2.5 cm lengths. The lengths so produced can then be weighed on a suitable sensitive balance.

If the mass of each consecutive length of yarn is plotted on a graph as in Fig., a line showing the mean value can then be drawn on the plot. The scatter of the points about this line will then give a visual indication of the unevenness of the yarn. The further, on average, that the individual points are from the line, the more uneven is the yarn.

There are two ways of expressing this unevenness:

1. The average value for all the deviations from the mean is calculated and then expressed as a percentage of the overall mean (%age mean deviation, PMD). This is termed U% by the Uster.

2. The standard deviation is calculated by squaring the deviations from the mean and this is then expressed as a percentage of the overall mean (CV %).

The two values are related by the following relation:

CV % = 1.25 PMD � This method is slow and laborious.

Example: Suppose a yarn is cut into short consecutive lengths (2.5 cm), and then each length weighed:

S.No Weight

“X”

Deviation from

Mean (X- X ) (X- X )2

1 20 0.5 0.25

2 18 1.5 2.25

3 19 0.5 0.25 4 17 2.5 6.25

5 22 2.5 6.25

6 21 1.5 2.25

7 18 1.5 2.25

8 20 0.5 0.25

9 19 0.5 0.25

10 21 1.5 2.25

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10

Series1

19.5x mg=

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X 19.5 ( )x x

n

−∑

= 13

22.5

9 = 22.5

Mean Deviation =

( )x x

n

−∑

= 13/10 = 1.3

Percentage Mean Deviation (P.M.D) =

.

.

mean deviation

avg value x 100 =

1.3100

19.5×

= 6.66 %

Standard deviation S.D =

2( )

1

x x

n

−−

∑

=

22.5

9 = 1.58

CV% =

.100

.

S D

Avg value =

1.58

19.5100×

= 8.10 % Irregularity in the yarn is 6.66 %.

3) Capacitive method: The measuring device of an electronic capacitance tester is a parallel plate

capacitor. Under certain conditions, the effect of introducing a non-conducting material such as a sliver or yarn into the space between the plates is to change the capacity of the capacitor, the change being proportional to the weight of material present. If, therefore, the material is drawn through the capacitor continuously, the changes in the capacity will follow the variation in the weight per unit length of the strand, the unit length being the length of the capacitor. If is necessary to detect the changes in capacity and to translate them electronically into meter readings which indicate the coefficient of variation. At the same time a trace of the variation should be made on a pen recorder if required. Some of the design problems may be described:

1. Changes in capacity are linearly related to the weight of material present, the material thickness should not exceed 40% of the distance between the capacitor plates. Hence interchangeable capacitors are necessary to test a range of materials from sliver to fine yarns.

2. The length of the capacitor should be as short as possible so that the variations in weight are measured over short lengths.

3. Shape of the cross-section of the tested strand affects the change in capacity. So it is necessary that the strand maintains its shape during its passage through the capacitor.

4. The moisture in the material affects the magnitude of the change in capacity, a higher moisture content giving a greater change in capacity.

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3.1) Uster evenness tester: Schematic diagram of this capacitance based evenness tester is shown. Two

oscillators A & B have equal frequencies when there is no material in the measuring capacitor C. When the two frequencies are superimposed the difference in frequency is zero. The presence of material (yarn) in the capacitor causes its capacity to change and so alter the frequency of the oscillator A. There will then be a difference between the two frequencies which varies according to the material (strand thickness) between the capacitor plates. Suitable circuits D translate these frequency differences into signals which (1) are indicated on the meter M, (2) drive the pen of the recorder, and (3) are fed into the integrator which indicates the average irregularity either as PMD or CV% according to the model used. The actual tester is illustrated which shows the following external features:

i) the ‘comb’ of eight measuring capacitors of different sizes ii) the creel and guides to control the material iii) the traverse rollers which can control the material speed over a range from 2-100 yd/min iv) the control switches v) the meter on the main unit which indicates the momentary variations in the material vi) the integrator which indicates the P.M.D. or C.V. vii) the high-speed pen recorder whose chart speed can be varied between 1 & 40 in./min. 3.1.1) Mass evenness, measuring principal: i) The sensor for measuring the evenness of slivers, rovings or yarns is a capacitive measuring sensor:

ii) A high-frequency electric

field is generated in the sensor slot between a pair of capacitor plates. If the mass between the capacitor plates changes, the electrical signal is altered and the output signal of the sensor changes accordingly.

iii) The result is an electrical signal variation proportional to the mass variation of the test material passing through.

iv) This analog signal is then converted into a digital signal, stored and processed directly by the USTER® TESTER 5 computer.

v) The capacitive measuring principle is very reliable and has good signal stability. With this measuring principle not only yarns, but also rovings and slivers can be tested.

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The Uster is adjusted and balanced after a preliminary warming-up period. Several points are noticeable:

3.1.2) Material speed: In yarns, drafting wave variations have a wavelength of about 2-2.5 times the

mean fibre length. So the testing speed must be chosen so that frequency of these fluctuations lies within the capacity of the recording pen to follow them. For example, with a yarn speed of 50 yd/min and a drafting wavelength of about 3” the pen must be capable of responding accurately to 10 fluctuations/sec. The recorder of Uster can cope up with a yarn speed of 100 yd/min. 3.1.3) Chart speed and chart contraction:

The yarn speed & chart speed ratio, i.e. the chart contraction, is chosen to suit the type of variation being examined.

• For short term variation a ratio of 8-20 is recommended.

• For medium term variation a ratio of 40-160 is recommended.

• For long term variation a ratio of 200-1000 is recommended.

3.1.4) Choice of measuring capacitor: In order to achieve a change in capacity which is linearly related to the amount of

material between the capacitor plates the thickness of the material relative to the size of the capacitor must not exceed certain limits. In the Uster instruction manual a Table states which capacitor in the ‘comb’ must be used for the different hanks and counts. 3.1.5) The imperfection indicator:

The introduction of the imperfection indicator in place of the ‘Hy-Lo’ offers the quality control department greater opportunities for detailed analysis of faults in yarns. Signals from the main units are fed to the imperfection indicator which simultaneously measures neps, thick and thin places. A count of all three types of fault is made and the results shown of separate counters. It is possible to adjust the sensitivity of the system so that a chosen size of fault is counted and smaller faults ignored. In 1964, a new set of ‘Uster’ yarn standards were introduced, the yarn parameters considered being evenness, neps, thick places and thin places.

Yarns spun from staple fibres contain "IMPERFECTIONS”. They are also referred to as frequently occurring yarn faults.

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3.1.6) Properties of the diagram: The diagram is an important part of evenness testing. It contains a large

amount of information which cannot be provided by the spectrogram, by the U/CV-value or by the counting of imperfections. The diagram is especially valuable for the detection of the following faults: • Seldom-occurring events • long-term variations • periodic mass variations with wavelengths which are longer than 100 m and therefore cannot be confirmed by the spectrogram • extreme thick and thin places • randomly-occurring thick and thin places which tend to occur in batches • slow changes of the mean value • with periodic faults, it can be determined whether the fault is permanently present or occurs only in batches • with measurements "within" a bobbin, seldom-occurring events can be found and changes in the mean value taking place over a number of kilometers can be confirmed • with unusual measured values the diagram can often be used to demonstrate whether these refer to a faulty or to a correct measurement. 3.1.6.1) Normal diagram

The mass diagram directly shows the mass variation of the test material in graphical form. A normal diagram contains the entire information from which the other test results (CV, Spectrogram, IPI, etc.) are derived.

The vertical axis shows the (+) and (-) mass deviations of the test material. The

zero line along the middle represents the average value of the material. This average value is determined over the first meters of test material running through the sensor. The horizontal axis represents the length of the material (in meters or yards) which has been tested.

Fig. 1-2 Normal diagram

The mass diagram shows if there are any significant random mass deviations or increased variation along the tested material. 1.4 Application examples of diagrams Example 1:

Spun yarn, cotton 100%, count Nm 85 (Nec 50). The periodic fault in Fig. 1-4 was caused by a defective front roller of the ring spinning machine. The defect was located on one side of the front roller. The cause was careless grinding of the cylinder. Therefore, the fault occurs periodically with a distance of 60 m as a result of the reversal mechanism.

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Fig. 1-4 Diagram, mass variations, normal test, scale ± 100% Example 2:

Spun yarn, cotton 100%, count Nm 50 (Nec 30). This yarn (see Fig. 1-5) is characterized by long-term random mass variations as a result of a non-regulated draw frame.

Fig. 1-5 Diagram, mass variations, normal test, scale ± 100% Example 3:

Filament yarn, Polyamide 6.6, dtex 22f7 dtex. Mass variation (Fig. 1-6), which is not strictly periodic, caused by turbulent air streams in the quench air duct.

Fig. 1-6 Diagram, mass variations, normal test, scale ± 5%

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Neps-a fault length of 1mm having cross-section 200 % of the average value

Thick place-a fault length of approximately the fibre staple length having a

cross-section of 50 % more than the average value

Fig:USTER® TESTER 5 allows the following sensitivity thresholds for thick places: +35%/+50%/+70%/+100%. Every time the selected limit is exceeded, a thick place is counted. Thin place-a fault length of approximately the fibre staple length, having a cross-section approx. 50 % less than the average value

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The reasons for these different types of faults are due to raw material or improper

preparation process. A reliable analysis of these imperfections will provide some reference to the quality of the raw material used.

Imperfection indicator record imperfections at different sensitivity levels: Thin place -30% : yarn cross section is only 70% of yarn mean value -40% : yarn cross section is only 60% of yarn mean value -50% : yarn cross section is only 50% of yarn mean value -60% : yarn cross section is only 40% of yarn mean value

Thick place +35% : the cross section at thick place is 135% of yarn mean value +50% : the cross section at thick place is 150% of yarn mean value +70% : the cross section at thick place is 170% of yarn mean value +100%: the cross section at thick place is 200% of yarn mean value

Neps

400%: the cross section at the nep is 500% of the yarn mean value 280%: the cross section at the nep is 380% of the yarn mean value 200%: the cross section at the nep is 200% of the yarn mean value 140%: the cross section at the nep is 140% of the yarn mean value

Thick places and thin places which overstep the minimum actuating sensitivity of +35% and -30% , respectively, correspond to their length to approximately the mean fibre length. Medium length or long thick and thin places are to be considered as mean value variations and are not counted by the instrument.

The standard sensitive levels are as follows Thin place : -50% Thick place : +50% Neps : 200% ( 280% for open-end yarns)

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� Neps can be divided, fundamentally, into two categories:

� -raw material neps � -processing neps

The raw material neps in cotton yarn are primarily the result of vegetable matter and immature fibres in the raw material. The influence of the raw material with wool and synthetic fibres in terms of nep production is negligible.

Processing neps are produced at ginning and also in cotton, woolen and worsted carding. Their fabrication is influenced by the type of card clothing, the setting of the card flats, workers and strippers, and by the production speeds used. 3.1.6) Uster yarn standards:

Advances in spinning technology and improvements in quality control systems, have demanded standards on the quality of the yarns rather than the characteristics of the intermediate products. Yarns from all over the world were sampled and tested, the results analyzed and the standards prepared. The representation of the standards

is in graphical form. Charts for combed and carded cotton, blends and worsted-spun yarns are available, but for the purpose of explanation we shall consider the charts for combed cotton yarns.

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In figure 1, the yarn count is on the logarithmic horizontal scale and the C.V. percentage on the vertical scale. Suppose the yarn considered is a 40s (15 tex). Locating the 40 position on the count scale we travel vertically until meet the 50% line; reading across to the vertical scale we read off the value C.V. 16.2%. This means that of all the 40s combed yarns spun, 50% of them will have a C.V. of 16.2 or worse, and 50% of them better (lower) than 16.2%. The spinning technologist can therefore compare his own product with the products of other spinners. If our own 40s combed yarn has a C.V. of 13.2% we would be amongst the spinners of good quality yarns as the intercept on the 5% line corresponds to a C.V. of 13.5%. In other words, only one spinner in twenty spins 40s combed are evenly as ourselves. Standard charts for thick and thin places, and neps, are similarly constructed, the example shown in fig.2 being the nep count chart. For our 40s combed cotton yarn the number of neps per 1000 m, for the 50% line, is 54. (On the carded yarn chart the corresponding value for 40s is about 600)

3.1.7) Extreme imperfections: With the selector switch set to ‘Extreme Imperfections’ the thick place, which are more than double the average cross-section, can be counted. For worsted spinners two limits have been fixed, +33% and 100% and the two counters register the number of times that these limits have been exceeded.

4) Mechanical method of sliver/roving evenness measurement:

Principle of mechanical method of evenness testing is compression of fibrous strand. Wool Industries Research Association has developed W.I.R.A sliver, roving levelness tester. It can provide a continuous record of the test performed.

Main features of the tester:

• Positively driven roller R1, with a rectangular groove

• Negatively driven top roller which has flange type shape and can be positioned into the groove of roller R1

• R2 is mounted on an arm (lever) which is pivoted at P

• Recording pen and graph paper

Test procedure:

In order to check different types of materials several pair of rollers is available. For example, rollers with groove width of 1/32”, 1/16”, 1/8” and 1/4 “.

• Suitable pair of rollers are mounted at the equipment

• Magnification lever system is adjusted to get adequate pen movement

• Pen movement can be adjusted from 20 times to 400 times of the top roller up & down movement to get properly magnified trace

• Pen to and fro movement is adjusted to get a trace between

11

2 inches on the trace paper

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• Material speed to chart speed can be varied from the gear box for material to chart speed ratio of 2” to 100” of material speed inches/min

It is possible to derive the co-efficient of variation by measuring the height of the chart at large number of points, and carrying out the statistical calculation.

Assessment of result: The irregularity trace obtained from the mechanical tester (WIRA) is recorded on

chart paper having 30 divisions, each 1/10 inches wide. Redraw trace will be centered at line 15 following procedure is adopted to determine standard deviation and CV%

of mass.

Step1 = choose centre line at 15 Step2 = mark two lines 16, 18 above centre line and two lines at 12, 14 below centre line Step3 = mark distance a,b,c on a plain paper and measure sum up distance (a+b+c) for line 18, 16, 14, & 12. Step4 = record sum up lengths in table form Step5 = in this trace 70 inches sliver had been checked to obtain 7 inches trace calculate % length of trace on each line for line 18: 0.88/70 x 100 = 12.6 Subtract 12.6 form 100 for length = 100-12.6 = 87.4 Step6 = prepare a sheet of probability paper by marking from 12,13,14,15,16,17,18 on vertical side. Mark horizontal scale (in %age) for the worked out length % plotted. Step7 = draw the best straight line through the plotted points Step8 = from straight line read ordinate against values of horizontal line of 15.9%, 5%, and 84.1%. These ordinates would be 11.95%, 14.72%, 17.47% for this mentioned test. Step9 = work out standard deviation S.D = max. Ordinate value – min. ordinate value/2

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Reference receiver

Receiver

Absolute yarn dia.

Yarn

Infra-red light

=

17.47 11.95

2

−

= 2.76 Step10 = work out co-efficient of variation of mass CV% = S.D/mean chart height x 100

=

2.76

46.72 = 5.9 % 5) Optical method (Zweigle G580):

This instrument uses optical method of determining the yarn diameter and its variation. In the instrument an infra-red transmitter and two identical receivers are arranged as shown in Fig. the yarn passes at speed through one of the beams, blocking a portion of the light to the measuring receiver. The intensity of this beam is compared with that measured by the reference receiver and from the difference in intensities a measure of yarn diameter is obtained. The optical method measures the variations in diameter of a yarn and not in its mass. For a constant level of twist in the yarn the mass of a given length is related to its diameter by the equation: Mass = CD2 where C = constant, D = diameter of yarn However, in practice the twist level throughout a yarn is not constant. Therefore the imperfections recorded by this instrument differ in nature from those recorded by instruments that measure mass variation. Advantages: it is very accurate like human eye. Because of the way yarn evenness is measured, this method is not affected by moisture content or fibre blend variations in the yarn. Disadvantage: twist through the yarn is not same thereby changing the diameter and hence results are not very accurate. 6) Pneumatic Method:

In this method the yarn to be tested is passed through a narrow tube, into which a stream of air is being forced. The air flow rate is then measured, usually by pressure change or some associated phenomenon. In theory, the air-flow in an empty tube is impeded, as yarn is inserted, by an amount related in some way to the mass of material present in the yarn. Clearly, careful calibration is a necessary procedure in using equipment of this type and the relationship may not necessarily be a linear one. In addition, the effects of such factors as temperature and atmospheric pressure, as well as relative humidity, will have to be controlled more carefully than usual.

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6.1) Procedure: Two air streams are directed towards each other, and a bundle of filaments is

guided to meet the resulting jet at right angles. The filaments are directed towards the stream of higher intensity, which thus causes pressure fluctuations in both, and these changes are measured in the stream of lower intensity to generate a signal proportional to the mass of yarn interfering with air-flow. The method is still relatively untried and unproved. 7) Acoustic Method:

Sound waves are used to measure the evenness of yarn. 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. 7.1) Impulse Acoustic Method:

A new method was elaborated to determine the inner (transversal) unevenness of multifilament chemical yarns and its defectiveness in the yarn’s cross-section by means of the impulse-acoustic spectrograms, both separately and in combination with an analysis of the full stretching diagram. This method was applied for different chemical yarns. There are no other methods of obtaining such information, including all the standard testing methods currently used. 7.1.1) Principles of the new impulse acoustic method of yarn control:

� The impulse-acoustic emission-recording method is based on stretching the untwisted multifilament structures (parallel fibres or bundles of filaments) and accumulating the filaments’ deforma-tion energy to rupture step by step the structural elements, i.e. the separate filaments. The deformation energy of every stretched filament

comes free at rupture, and gives returning impulses owing to the contraction process. Therefore, at the moment of rupture, the energy of contraction is transformed into to the energy of acoustic oscillation. These impulses, their intensity, arrangement etc. are recorded, and produce the acoustic spectrum, which is the picture of deformation and the multi-element material destruction (of the multifilament yarn) at the time of its deformation. It therefore represents the picture of distribution of the yarn rupture. The impulse-acoustic spectrum is shown in the coordinates of deformation/impulse intensity.

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Figure.4. Clamp; scheme of sensor position; 1 - sensor, 2 – clamp, 3 – rod, 4 – connecting cable, 5 – yarn.

Figure 5. Form of acoustic impulse corresponding to single filament rupture; U – amplitude of signal, t – time.

� The impulse acoustic spectrum consists of a series of separate impulses located at the axis of elongation or the time of deformation. Great differences exist among the acoustic spectra of various yarns. The spectra differ in the impulse intensities and their positions in relation to the elongation. These data give an excellent reflection of individual filaments’ breaking arrangements.

� Higher unevenness and defectiveness of yarn correspond to a broader width of the spectrum. The intensity and position of the first impulse, or a small group of impulses, give information about more defective filaments. It is necessary to stress that the information concerned with the first impulse is perceived only by the impulse-acoustic method. This information cannot be perceived in a standard stretching diagram such as that commonly carried out by dynamometric tests (load-deformation curve).

� At present, together with the impulse-acoustic tests, a traditional load-deformation diagram is also recorded, as it was established that the best information and interpretation of the test results can be obtained by combination of the impulse-acoustic spectrum with the full deformation diagram (up to the point of the yarn rupture). The acoustic spectrum and the full stress-strain curve both permit every impulse coordinate to be found. 7.1.2) Equipment for tests:

The testing device for the impulse-acoustic method consists of a low noise and vibration level electronic dynamometer with an additional acoustic system. A scheme of the testing device is shown in Fig. 3. The FPZ-10/1 Tiratest testing machine (dynamometer) from Thőringisches Prőfmaschinenwerk GmbH (Germany) was applied for our research. The measuring system includes standard deformation and load sensors and an addi-tional acoustic measuring chain composed of a sensor, amplifier, and amplitude detector. The acoustic piezo-sensor is placed on the dynamometer clamp, as shown in Fig. 4.

Every filament break while stretching the yarn gives an acoustic returning impulse which is registered by the measuring system. The form of the acoustic signal is shown in

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Figure 5. All signals from the three sensors are sent to an analogue-digital transformer, and next to a computer for registration, processing and indication. The resulting information includes the impulse-acoustic spectrum combined with the full stretching diagram stored in the computer memory and visible on the monitor. According to the constant amplification factor, the height of each amplitude is proportional to the energy impulse returning from the filament ends. In this way, all the impulses of filament breaks as well as the full stretching diagram are reordered (Fig.6).It should be stressed that an appropriate sensitivity and minimal response time of the electronic system recording every impulse are necessary to separate each acoustic impulse while testing yarns consisting of a high number of filaments. Standard and special software were applied to analyze the test results. 7.1.3) Sample preparation & testing procedure: The yarn samples are initially untwisted to zero twist; in this manner the bundle of parallel filaments is prepared. To fix the yarns, its ends are glued between two pieces of paper (with polyvinyl acetate glue, or another with a short drying time). The samples are conditioned at standard conditions (relative air humidity of 65 ± 2% and temperature of 20 ± 2 °С). The velocity of the clamp movement guarantees obtaining a time to sample break of 20 ± 2 sec. If another procedure was necessary it would be used.

The clamping length is within the range of 30 mm to 100 mm. This length should be correlated with the friction forces between filaments. If the friction between filaments is high, a short length is necessary. However, the best length for standard tests is 100 mm. The number of tests for one kind of sample should be 5 (or 10 in the case of filaments of high unevenness). Index of Irregularity:

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Index of irregularity (I) is the measure used to find out the extent to which actual irregularity deviates from that due to random. Index of Irregularity=CVa/CVr where CVa is actual measured irregularity. A higher value means that there is more scope for improving the processes. Table below shows how irregularity due to random arrangement and Index of irregularity vary with count.

Count CVr CVa I

20 card 7.7 17 2.20

30 card 9.26 17.5 1.88

40 combed 10.5 14 1.33

60 combed 12.5 15.5 1.24

100 combed 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. CVr reduces 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 Table below:

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 (2.8) Ne 2.54 5 1.96

Drawing sliver(100s) 0.19Ne

0.66 2.5 3.79

yarn evenness_testing_by abubakkar marwat

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