Applications of statistical tools in various processing stages of textile production


Fiber Production

Measures of central tendency like process average gives an idea about average staple length of fibre produced in a continuous or batch wise process. Coefficient of variation (CV) of the process signifies about the process control. On the other hand, analysis of time series is helpful in estimating the future production based on the past records. Measures of dispersion such as standard deviation and CV are useful in comparing the performance of two or more fibre-producing units or processes. Significance tests can also be applied to investigate whether significant difference exists between the batches for means or standard
deviations. Analysis of variance can be applied for studying the effect of parameters of fibre production and methods of polymer dissolving.

Textile Testing of Fiber Yarn and Fabrics

Results analysis in textile testing without the applications of statistical tools will be meaningless. In other words every experiment in textile testing include the use of statistical tools like average calculation, computation of SD, CV and application of tests of significance (t-test, z-test and f-test) or analysis of variance (one way, two way or design of experiments). Populations can be very well studied by normal or binomial or Poisson’s distributions. Random sampling errors are used in studying about the population mean and SD at 95% and 99% level of confidence. Application geometric mean for finding out the overall flexural rigidity or Go has an important role in fabric selection for garment manufacture.

A special mention is made in determination of fibre length by bear sorter where all the measures of central tendency and dispersion (mean length, modal length quartile deviation, etc., in the form of frequency distribution) are computed to understand about the cotton sample under consideration for testing its potential in yarn manufacture. On the other hand ball sledge sorter uses weight distribution from which mean and SD are computed. In the case of cotton fibres, the development of cell wall thickening commonly referred as “Maturity” concept can be very well determined using normal distribution and confidence intervals. Several properties are tested for different packages produced from the same material or from the same frame by applying significance tests. Effect of instruments and variables for different types of samples can be
very well studied by using ANOVA. All the fabric properties tested on a single instrument or different instrument can be understood by using design of experiments. In one of the research applications, which include the testing of low stress mechanical properties for nearly 1000 fabrics are studied by ‘Principle Bi component analysis or Bi plot’. Measures of dispersion like coefficient of variation and percentage mean deviation are very much used in evenness measurement.

Yarn Production

There are several stages involved in the cotton yarn production. When fibres are mixed and processed through blow room, within and between lap variations are studied by computing mean, SD and CV lap rejection, and production control are studied by p and x charts. Average measure is used to find the hank of silver in carding, draw frame, combing and average hank of roving in roving frame and average count at ring frame. Generally the spinning mill use ‘average count’ as the count specification if it is producing 4–5 counts. On the other hand the weaving section uses ‘resultant count’ which is nothing but the harmonic mean of the counts produced. Control charts are extensively used in each and every process of yarn production (for example, the process control with respect to thin places, neps, etc.). Application of probability distributions like Poisson, Weibull and binomial for various problems in spinning is found very much advantageous to understand the end breakage concept. In ring spinning section several ring bobbins are collected and tested for CSP and difference between the bobbins and within the bobbins is studied using ‘range’ method. In cone winding section the process control can be checked either by using control chart for averages or chart for number defectives.

Fabric Production

Design of experiments such as latin square design or randomized block design can be used to identify the effect of different size ingredients on wrap breakages on different looms in fabric formation. Most of the suiting fabric constructions involve the use of double yarn which is nothing but the harmonic mean of different counts. Poisson’s and normal distribution can be applied for loom shed for warp breakages. Using statistical techniques the interference loss can also be studied in loom shed. Various weaving parameters such as loom speed, reed and pick can be correlated with corresponding fabric properties and are interpreted in terms of loom parameters. Control charts are used to study the control of process/product quality in fabric production also. For example, selection of defective cones in a pirn winding from a lot (fixed population) or in a production shift n p and p charts are used. The width of the cloth and its control can be understood by x and defectives per unit length and their control is understood by c charts. The testing process includes determination of average tensile strength (and single thread strength also) and the corresponding CV%.

Chemical processing and Garment Production

The scope of statistics is unlimited. For example the effect of n number washes (identical conditions) on m fabrics on a particular fabric property can be easily found by either tests of significance or analysis of variance. Similarly the effect of different detergents on fabric types can be investigated by two-way analysis of variance. Similarly different types of fabrics and the effect of sewing conditions can be studied by ANOVA.
In garment production the control of measurements and its distribution can be well understood by control and polar charts.

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Electronic autoleveller in Spinning


Electronic autoleveller is used for achieving an automatic adjustment with two different criteria of speed variation:

  • feed rate variation, for all autolevelling standard applications
  • • variation of the delivery speed when the machine requires steady feed rate like in case of linkage with other machines with the same throughput speed (for example, in the after-card drawframe combined with a set of cards).

image The first system, previously analysed, is most frequently used in this process stage. Its operation is schematised in Figure: a mechanic feeler detects the thickness of the material fed, the variations are transformed into electric signals and sent to a control unit which, with a suitable delay corresponding to the passage of the material from the feeler to the drawframe, determines the variation of the feed rate and therefore of the draft. The electronic autoleveller does not set definite limits to the possibility of adjustment but in relation to the correct detection and to the speed limit of the intersecting comb head, the suitable adjusting range applicable varies between – 25% and + 25%. It is also possible to store the maximum and minimum drawing limits beyond which the machine no longer complies with the technological operating conditions allowed for each material.

Spinning standards for mill planning


Spinning table

Staple length

In inch

Lap weight oz/yd

Spinnability (warp) Ne

Lap hank oz/yd

7/8

16

<20

0.00119

7/8 to 15/16

15

20-30

0.00126

15/16 to 1

14

30-40

0.00136

1 to 1(1/8)

13

40-50

0.00146

1(1/8) to 1 (1/4)

12

50-60

0.00158

1 (1/4) to 1(1/3-/8)

11

60-70

0.00173

1(1/3-/8) to 1(1/2)

10

70-80

0.00190

1(1/2) to 1(9/16)

9

80-100

0.00211

Sliver weight and sliver hank table

Count

Sliver weight

Sliver hank

Sliver weight

Grains/yd

g/yd(ktex)

Ne

g/yd

Upto 15

65

4.60

0.1284

4.21

15-20

65

4.60

0.1284

4.21

20-30

60

4.24

0.1392

3.88

30-50

55

3.89

0.1518

3.56

50-70

50

3.54

0.1668

3.24

70-90

45

3.19

0.2022

3.92

90-120

35-40

2.48 to2.83

0.260-0.2279

2.27-2.59

TM values for various staples:

If the staple length of fibre is 7/8” than TM value would be 4.75 and if staple length increases by 1/16” than TM value goes down by 0.1.

 

Machine its draft, doubling and waste%

Machine

Draft

Doubling

Waste%

Blow room

According to the count to be spun

Card

90-120

According to the count and between 3.5-5.5%

Draw frame

6-8

6-8

0.5

Lap former

1.2-1.7

16-24

0.5

Super lap former

2-5

24-60

0.5

Sliver lap

1.3-1.7

16-24

0.5

Ribbon lap

6

6

0.5

Comber

Old 40-70

New 40-90

6

4

<6 scratch comb

6-10 semi comb

10-18 regular comb

18-25 double comb

Simplex

8-12

If required 2

0.5

Slubber

3-5

0.5

Inter

4-6

0.5

Roving

5-8

If required 2

0.5

Ring frame

Conv.

10-20

2

Modern

20-50

2

Rotor

Conv.

100-150

2-3

Modern

100-250

2-3

Winding

1

M/c and its production capacity in terms of speed

1. Blow room

Production capacity in terms of speed Efficiency %
With chut feed system= 300-500 Kg 90
With 2 scutcher per blow room line and lap roll dia. 10” and rpm 7(+/-)1. The production varies from 180-240 Kg/hr/line 90

2. Card

Production capacity in terms of speed
Type Doffer rpm Efficiency
Conv. 5-15 90-92%
SHP 10-20 90-92%
HP 20-40 90-92%
VHP 40-120 90-92%

3. Draw frame

Production capacity in terms of speed
Type Efficiency No of delivery Mpm
Conv. 70-75% 4 20-30
SHP 70-75% 3 200
HP 70-75% 2 250-500
VHP 70-75% 1 750-1000

4. Lap former and super lap former

Delivery speed-50-70 mpm

Efficiency-70-75%

5. Comber

Feed per nip- 3.5 to 7mm

Type Neps/min No of heads No of delivery Efficiency
Conv. 60-70 6 1 90
HP 15-225 8 2 90
VHP Upto 400 8 2 90

6. Speed frame

Type Spindle speed No of spindles Efficiency
Slubber 500-550 72 85
Inter 55-600 138 85
Roving 600-1000 168 90
Simplex 1200-1500 132 85

7. Ring frame

Ring dia Count Spindle rpm Efficiency
50 <20 12000 90
45 20-40 13500 90
42 40-60 14500 90
42 60-80 15500 90
38 >80 16500 90
TM values for speed frame
machine Staple length
7/8” 1” 1 (1/4)” 1(1/2)”
Slubber 1.2 1 0.95 0.7-0.8
Inter/simplex 1.2 1.1 0.95 0.7-0.8
Roving 1.2 1 0.8-.9
The value for PET and VR for length of sliver and devices 2 or 3 is 0.6

8. Winding machine specification

Delivery rate 1000-2000 mpm

Efficiency- 90%

No. Of spindle- 60

Maintenance allounce
machine % maintenance allounce
Blow room
Card 10% of total no of cards
DF
LF
Comber
Simplex
RF 2% on number of spindle
Rotor -do-

9. Rotor specification

Rotor rpm 80000-100000 rpm

Efficiency- 90%

END PACKAGES PRODUCED ON WINDING M/CS


Most of the common packages on which the yarns are wound can be divided in to two groups.

(1)Parellal wound packages

(2)Cross wound packages.

(1)Parellal wound packages:-

These are double flanged bobbins,also known as warper’s bobbins on which yarn is wound in such a way that the coils of yarn are laid parellel sided or barrel shaped,Flanges are needed on either sides to support parellely laid coils.If flanges are not provided then coils at the two ends will COLLAPSE.  To withdraw the yarn from these packages,package has to be rotated by pull of yarn.Hence high unwinding speed will lead to excessive unwinding tension & yarn will break.Also as the unwinding is stopped the package continues to rotate due to its inertia,hence yarn may continue to come out from package.So this package is not suitable for the process taking place at high speed.

(2)Cross wound packages:-

image

In this case the yarn is wound on cylindrical tubes or conical tubes.The yarn is laid on this in form of helices at the two extremes. In this type of winding the yarn wraps cross one another hence these packages are called cross wound packages.Because of laying in cross fashion there is no possibility of yarn coils collapsing at the two extremes.Hence these packages do not need flanges. The cylindrical cross wound package is known as CHEESE & the conical one as CONE. The yarn can be withdrawn from cone & cheese overend (& side ways unrolling also).The over end withdrawal allows unwinding at high speed without extreme increase  in tension.Rotation of package for unwinding is not essential hence the unwinding from package stops almost at the same instant when withdrawal is stopped. For some special cases yarn is required to be withdrawn side ways also.

Bobbins may be made of card or plastic, the latter being perforated if the yarn is to be package dyed. Parallel-sided cheeses have tubular bobbins. For cones, the bobbin is of a conical form, i.e., a truncated cone; the angle of taper — the semivertical angle — depends on the end use for the resulting package. Table 1 lists four common tapers. The wound cone package may have a fixed taper, which gives it flat ends, in which case the package is referred to as straight-ended. Cones may also have an accelerated taper, where the taper of the package is greater than the bobbin, resulting in a concave end at the top (the nose) and a convex end at the bottom (the base) of the package. These are called dished ends.

Table :- Common Tapers for Random-Wound Cones

Cone taper
(semivertical angle)
End uses
3°30′ General purposes
4°2′ Wet processing (e.g., dyeing)
5°5′ Weft knitting: at final diameter taper may increase to 10°
9°1′ Weft knitting: at final diameter taper may be 14° to 18°

Comparison of cross and parallel wound package

Sr no. Cross wound Package Parallel wound package
1 Self supporting Package Flanges are required to support the yarn
2 Overhead Unwinding Side-end Unwinding
3 Package is Stationary during unwinding Package rotates during Unwinding
4 The yarn stops immideatly the unwinding Stops The yarn doesnot stop unwinding as the  package continues to rotating due to inertia
5 Suitable for High speed unwinding Not suitable for high speed unwinding
6 yarn is laid at an angle to each other The yarn is laid parallel to one another
7 eg., Warper’s Bobbin Eg, Cone, Cheese & Spool

Compact Spinning for Improved Quality of Ring-Spun Yarns


Momir Nikolić,
Zoran Stjepanovič*,
Franc Lesjak**,
Andrej Štritof**

Abstract

A new impulse in the field of ring spinning technology is offered by compact-condensed spinning. The article presents the comparison of two chosen spinning systems for the production of compact ring yarns. We have analysed and compared the physical, mechanical and morphological properties of conventional and compact yarns, spun at the same technological and kinematical parameters from the same cotton, cotton/PES and cotton/viscose roving. The construction specificities of the Suessen and Zinser compact ring spinning frames, on which the comparative spinning was performed, are described within this work. The purpose of the study was to determine the influence of differences in compacting systems on yarn quality, and to compare the compact and conventional yarns produced.
Key words: ring spinning, compact spinning, compact yarns, physical/mechanical properties of yarns.

Introduction and Motivations
In spite of modernisation and rapid technological development in the field of ring spinning, the mechanism ring-traveller spindle has remained almost the same until now. Furthermore, ring spinning remains the dominant spinning technology even today. The producers of modern spinning frames have been developing the machines with improved construction of different working elements and optimal spinning geometry, with a ring diameter of 36 mm, a tube length of 180 mm and spindle speed of up to 25,000 min-1. All serving and transport functions have already been automated. A high linking level of spinning and winding, and even of the winding and twisting technological processes, has been achieved using the elements of computer-assisted automation and control. Besides the conventional functions (spindle speed, delivery speed, productivity, twist, draft, machine efficiency), computer-based systems control and enable the optimisation of spinning conditions (formation of bobbins, position of ring rail, automated doffing and setting of empty tubes, cleaning and oiling of main machine parts). Construction improvements of different working elements of the ring-spinning frame and optimised spinning geometry of the continuous form of fibres (roving or sliver) enable increased productivity, better yarn quality, as well as flexibility and profitability of the process.
All these optimisations and improvements of the ring spinning frame, however, have not enabled the reduction of the spinning triangle, which can be defined
as the most problematic and weakest spot in the yarn formation process using the ring-traveller system [1,2]. The spinning triangle that occurs while the yarn is formed is the cause of many fibres leaving the drafted roving, or being partly spun into the yarn with one end only. This causes greater waste of fibres, lower exploitation of fibre tenacity in yarn, poorer appearance and greater hairiness of the spun yarn. The newest research in the field of ring spinning has shown that modification of a three-cylinder drafting equipment with tow aprons in a region after front drafting rollers enables ring spinning to proceed with a minimised spinning triangle, or even without it at all. This modified process is called compact or condensed spinning [1,3,4].
The purpose of the study presented within this article was to produce, analyse and compare the yarns using two different systems for the production of compact and conventional ring yarns, offered by two well-known producers of ring spinning machines. In Predilnica Litija (Litija Spinning Mill), one of the Slovenian short-staple spinning mills, a need exists to modernise the existing ring spinning frames for medium-fine yarns produced from cotton fibres and blends consisting of cotton and chemical fibres, mainly PES and viscose. Today, the main goal of the company is to achieve improved yarn quality that will ensure better competitiveness and higher yarn prices. Therefore, a decision was made to compare the quality of conventional and compact yarns and (also taking into account the production costs), to explore whether the quality parameters of compact yarns had been improved significantly enough to justify the purchase of new machines, or the adaptation of drafting equipment of the existing ring spinning frames.
For this reason, it was decided to use the same roving with a linear density of 588 tex produced by the Litija Spinning Mill, and to produce a certain amount of yarns using the Suessen and Zinser ring spinning frames equipped with compact and conventional drafting systems under comparable technical and kinematical conditions.
The tests were directed and supervised by the leading technical personnel of the Litija Spinning Mill, together with the specialists of Suessen and Zinser workshop spinning mills, where the production of yarn samples was carried out over approximately the same time period. 20 kilograms of each yarn type (one compact and one conventional from each ring spinning machine producer) was produced in order to ensure sufficient yarn quantity for testing purposes. Yarn testing was done by both machine producers in the laboratories of Suessen and Zinser, using valid standard methods and procedures that guaranteed the statistical significance of test results. The information on yarn quality was then sent to the Litija Spinning Mill, where the data was analysed and compared.

Conventional Versus Compact Ring Spinning Technology
The twist that is transmitted to the yarn in the ring spinning process originates along the curve between the traveller and front drafting rollers. Transmission of twists is opposite to the yarn movement in this area. The traveller transmits twists to already drafted fibres as close as possible to the clamping point after the front rollers. However, the twists never reach the clamping point, because after leaving the front rollers the fibres tend to direct towards yarn axis. The different length of the path of the inner and outer fibres that form the yarn cause a so-called spinning triangle in ring spinning [5]. The length of the spinning triangle depends on spinning geometry and twisting intensity [6,7]. The form and dimensions of the spinning triangle significantly influence the structure, surface characteristics, physical and mechanical characteristics of spun yarn. Not all fibres that are placed at the external edges of the triangle can be spun into the yarn structure, and can leave the drafting equipment without having been spun into the yarn. Such fibres also increase yarn hairiness.
The gradual transmission of twists with the traveller along the yarn balloon causes a certain tension in the fibre bundle that forms the spinning triangle, a tension
which is not distributed symmetrically in the yarn cross section. It is greatest in fibres that are positioned at the edges of the spinning triangle, and smallest in fibres lying in the middle of the triangle. This asymmetric distribution is the reason for fibre breakage according to their position in the spinning triangle during subsequent processing [4,6,8,9]. Furthermore, the fibres gradually take over the external axial yarn loading; therefore, they also break one after another. The consequence is lower yarn strength and poorer utilisation of the fibre tenacity (35 to 50%).
Much has already been done to minimise the influence of the spinning triangle in the ring spinning process. Different mechanical devices such as condensers have been used in the past to retard the widening of the roving [10]. However, these measures were only partly successful. The length between the mechanical condenser in the main drafting area and clamping point between the front rollers was too long to ensure the condensing effect. As soon as the condensed fibre bundle left the mechanical condenser, the fibres were relaxed and again formed an undesirably wide fibre bundle.
Minimisation or even elimination of the spinning triangle, enables almost all fibres to be incorporated into the yarn structure with maximum possible length and pre-tension of the fibres, irrespective of their position in the spinning triangle. The uniform pre-tension of the majority of fibres enables more synchronic breakage of the majority of the fibres, which contributes to higher yarn strength and better utilisation of the fibre tenacity (from 65% up to even 80%).
All compact yarns, whether produced of short-staple fibres (cotton, cotton-type chemical fibres and their mixtures) or long-staple fibres (wool, wool-type chemical fibres and their mixtures) represent a whole new range of yarns as regards their quality and appearance. When compared with conventional ring-spun yarns, compact yarns have significantly higher tenacity and elongation, work to break, and abrasion resistance. In addition, their surface smoothness, elasticity and softness are much better thanks to the almost ideal structure of compact yarns. To achieve tenacity comparable with conventional ring-spun yarns, a lower number of turns per metre can be used, which enables higher productivity of the spinning machine, as well as better
elasticity and softer hand of different flat textile products.
The better use of the fibres’ tenacity in compact yarns enables the use of cheaper raw material. Yarn singeing is not required because of minimal secondary hairiness, caused by the fibres exceeding the length of 3 mm. One should also mention savings in the sizing process of up to 50% compared to the conventional yarns. In some cases, sizing is even not required [11]. Lower primary hairiness (hairs with a length of 1 to 2 mm) and significantly lower secondary hairiness (hairs with a length of 3 mm and more) result in less prominent pilling in yarns and in the finished textile products.
Lower yarn hairiness enables the production of flat textiles with better appearance and more explicit, sharp contours, for example in jacquard-woven and printed fabrics. When smooth surface, high gloss and durability of the end product is required, compact yarns should be used for production in spite of the slightly higher price. In spite of the difficult situation of the whole textile sector, there are still some spinning mills in Slovenia that continue production and successfully compete on the national and international markets. Most of them use conventional ring and rotor spinning [12]. The most successful yarn manufacturers are already testing different compact spinning machines and searching for the most appropriate way to modify the drafting equipments of their existing ring spinning machines.

Comparison of the Two Compacting Principles

The drafting equipment of the Fiomax E1 compact spinning machine (Figure 1) consists of a pair of delivery rollers (1- 11), a double aprons area (2-21), a pair of front rollers (3-31) and a condensing zone (S1-S4). The condensing unit consists of a profile tube (S), the lattice apron (G) and the delivery top roller (4).
Regarding the configuration of the drafting unit (1-11) to (3-31), it is a standard three-cylinder system with two aprons, and enables the processing of a wide spectrum of raw materials. The tube (S) has a built-in slot in order to create negative pressure in the area (S1-S4). Drafted roving comes into the condensing field (S1-S4), where the fibres are condensed up to the clamping point (4-S4) consisting of the top roller (4) and sucking tube (S).

Picture

The delivery top roller (31) is connected with the top roller (4) by the gear wheel. The top rollers (31) and (4) are driven by the delivery drafting cylinder (3). Using friction, the top roller (4) drives the endless lattice apron, which slides over the profile tube (S) that is not moving. To guarantee a slight axial tension of fibres in the condensing zone (S1-S4), the roller diameter (4) is slightly larger than that of the top roller (31). In this way, a small drafting of fibre bundle is ensured in the condensing zone, which enables optimal axial tension and fibre orientation.
The profile tube (S) has a small slot in the area (S1-S4) and is closely embraced by a lattice apron. The porosity of the apron and the negative pressure in the slot area result in a condensed fibre bundle that is transported up to the zone (4-S4). The oriented fibres remain completely condensed and closed up to the delivery clamping and twist insertion line (4-S4) because of the slot length. Therefore, no spinning triangle is formed, which enables literally all the fibres to be wound into the yarn and optimal yarn structure. The slot in the profile tube (S) can be positioned in the direction of fibre movement or at an incline to the direction of fibre flow, for instance when processing shorter fibres, such as carded cotton. This ensures the firm incorporation of outer fibres into the yarn because of a transverse force on the fibre band during the fibre transport and the rotation of fibres around their axis. The lattice apron is made of the cotton fabric in plain weave, and has more than 3000 holes per square centimetre. The drafting unit of the Fiomax E1 spinning machine enables the fibres to stay condensed up to the clamping and twist insertion line, which results in a minimised spinning triangle. The result is spun yarn with maximum strength and minimal hairiness. Besides the condensing effect, a light tensioning of the fibre bundle during condensing is also crucial for this process.
The drafting equipment of the Zinser RM 700 spinning machine (Figure 2) consists of the standard three-cylinder drafting unit with two aprons (1-11-2-21-3-31) and condensing unit (4-41).
The top roller (41) is covered by the endless apron with a set of holes in the middle. The apron runs over the profile tube (H), which has a suction slot in the zone (H1-H2). The drafting unit construction in zones (1-11), (2-21) and (3-31) is a modern three-cylinder drafting system
that enables the processing of a wide range of fibre lengths. The final drafting occurs between the zones (2-21) and (3-31). The fibre bundle is condensed under suction on a perforated surface in the zone (H1-H2). In a short zone between (H2) and (4-41), the fibre bundle is not under suction and therefore loses some of its hitherto gained condensed form. Therefore, in the zone (4-41) the spinning triangle is not reduced to the minimum, which negatively influences the quality of spun yarn. This undesired effect is more obvious when processing shorter fibres. The suction slot is directed in the fibre bundle axis in the area (H 1- H2). It is not possible to set it under a certain incline regarding the fibre bundle axis. The drafting system construction enables light axial tension acting on fibres in a condensing zone between (3-31) and (4-41), which has a positive effect on increased adhesion between fibres that are incorporated into the yarn. The technological data of the Suessen and Zinser compact and conventional ring spinning machines used for production of yarn samples is given in Table 1.

Click to Enlarge

compact spinning.html_Picture4

compact spinning.html_Picture5

Direction of Yarn Samples’ Production and Raw Material Characteristics

The production of yarn samples was directed and supervised by the leading technical personnel of the Litija Spinning Mill together with the specialists from Suessen and Zinser. Since we wanted to compare the compact and conventional yarns produced on at least two different spinning machines types, the production of yarn samples was carried out at the Suessen and Zinser workshop spinning mills at approximately the same time period. 20 kilograms (20 bobbins with roving) of each yarn type (one compact and one conventional from each ring spinning machine producer) was pro-
duced in order to ensure sufficient yarn quantity for testing purposes.
The following fibrous material was used for the production of yarn samples:

compact spinning.html_Picture3

A standard spinning preparation and modern machinery were used to produce the roving with a linear density of 588 tex from each fibre blend at the Litija Spin ning Mill using the same fibre lots fo each blend. After that, conventional and compact yarns with a linear density o 20 tex were produced under comparable technological and kinematical condition on the Suessen and Zinser ring spinning frames.

Quality Properties of Produced Yarns

After production, the quality of yarns was tested in the laboratories of the Sues sen and Zinser machine producers, where the yarn samples were spun using valid standard methods and procedures tha guaranteed the statistical significance o test results. Ten bobbins of each compac and conventional yarn were tested. The information on yarn quality was then sent to the Litija Spinning Mill, where the data was analysed and compared. The following physical, mechanical and mor phological properties of the compact and conventional yarns produced were tested

and compared: real fineness, twist, breaking force, elongation at break and work to break, Uster properties, hairiness and length distribution of hairs on 100 m of a yarn. An Uster Tester 3 was used for testing the hairiness of the produced yarns. The results are given in Tables 2-4.

Discussion
Based on the researched and compared mechanical, physical, morphological and Uster values of the conventional and compact ring yarns spun on the Zinser and Suessen spinning machines, the following conclusions can be drawn:
Properties of compared yarns made of 100% cotton fibres
The breaking force of the compact yarn with a nominal linear density of 20 tex and spun on a Zinser ring spinning machine is 18.88% higher than the conventional ring spun yarn, produced on the same machine but without the condenser unit. The breaking force of the compact yarn spun on the Suessen ring spinning machine is up to 29.48% higher when compared with the conventional ring spun yarn, produced on the same machine but without the condenser unit. A higher difference in breaking force between the compact and conventional yarns produced on the Suessen ring spinning machine can be explained with the
construction of the drafting system that enables maximum fibre condensation all the way up to the clamping line, which is not the case in Zinser’s drafting system. A greater breaking force was measured in the yarn produced on the Zinser spinning machine.
Elongation at break of compact yarns is 7 to 8% higher compared to conventional yarns. The tenacity of a compact yarn produced on the Zinser spinning machine surpasses the conventional yarn by 17%, while this value is higher by up to 23.24% in the yarns spun on the Suessen spinning machine. A slightly higher value is noted in the yarn produced on the Zinser spinning machine.
The work to break of a compact yarn spun on a Zinser spinning machine is 21.82% higher than in conventional yarn. In the compact yarn produced on the Suessen spinning machine, the work to break is 32% higher than that of the conventional yarn. the higher absolute value of work to break was determined in the compact yarns produced on the Zinser spinning machine. The physical and mechanical properties of the compact and conventional yarns are represented in Figure 3.
No significant changes regarding Uster properties (Uster CV%, number of thin,
thick places and neps) in the conventional and compact yarns were determined. This can be explained by the use of the same three-cylinder drafting equipment, which is proven to be the major influence on these yarn properties.
The Uster hairiness (H) of compact yarns is significantly lower when compared with the hairiness of conventional yarns (Figure 4). Conventional ring spun yarn produced on the Suessen spinning machine has an Uster hairiness H=5.80, and the yarn spun on Zinser spinning machine has an Uster hairiness H=5.54. A lower value of Uster hairiness, H=3.80, was determined in compact yarn produced on the Suessen spinning machine, while that spun on the Zinser spinning machine has an Uster hairiness H=4.64. The reason for this is the construction of the drafting equipment, as explained above.
The morphology of the yarn, defined as the number of hairs of different length per 100 metres, shows the significantly lower primary hairiness (1 to 3 mm) and secondary hairiness (4 to 12 mm) of compact yarns. Better results and significant improvements were achieved with the Suessen spinning machine, which can also be explained by the special construction and elements of the drafting unit.
Properties of compared yarns made of 50% CO/50%PES fibre blend
When comparing the physical and mechanical properties of conventional and compact yarns produced of 50% CO/50% PES fibre blend, we found no significant differences. This can be explained by the greater bending rigidity of polyester fibre component, which reduces the fibre condensing effect and its contribution to better physical and mechanical properties of compact yarns.
The analysed Uster properties of conventional and compact yarns are very similar, which confirms the fact that the condensing effect significantly influence neither the yarn irregularity nor the number of yarn faults.
The Uster hairiness (H) of compact yarns is significantly lower when compared with the hairiness of conventional yarns. A slightly better hairiness value was determined in yarn spun on the Zinser spinning machine (H=3.26) when compared with the yarn produced on the Suessen spinning machine (H=3.20).
The primary and secondary hairiness of a compact yarn made from this mixture

and spun on the Zinser spinning machine are better than in yarn produced on the Suessen ring spinning machine. Both compact yarns have significantly improved primary and secondary hairiness when compared with conventional ring yarns.
Properties of compared yarns made of 87% CO/13% CV fibre blend
The breaking force of the compact yarn with a nominal linear density of 20 tex and spun on a Zinser ring spinning machine is 18.32% higher than the conventional ring spun yarn produced on the same machine but without the condenser unit. The breaking force of the compact yarn spun on the Suessen ring spinning machine is up to 32.30% higher than the conventional ring spun yarn produced on the same machine but without the condenser unit.
Elongation at break of compact yarns is 4 to 11 % higher compared to conventional yarns. The tenacity of a compact yarn produced on the Zinser spinning machine surpasses the conventional yarn by 15.90%, while this value is higher at up to 28.87% in yarns spun on the Suessen spinning machine. A higher absolute value of tenacity is determined in yarn produced on the Suessen spinning machine.
The work to break of a compact yarn spun on the Zinser spinning machine is 20.87% higher than in conventional yarn. In compact yarn produced on the Suessen spinning machine, the work to break is up to 41.88% higher compared to the conventional yarn. A slightly higher absolute value of Work to break was determined in compact yarns produced on the Suessen spinning machine.

The analysed Uster properties of conventional and compact yarns have very similar values. Conventional ring spun yarn produced on a Suessen spinning machine has an Uster hairiness of H=5.20, and the yarn spun on the Zinser spinning machine has an Uster hairiness of H=4.72. A lower value of Uster hairiness, H=3.40, was determined in compact yarn produced on the Suessen spinning machine, while the yarn spun on the Zinser spinning machine has an Uster hairiness of H=3.60, which can be explained by the inability of Zinser’s drafting system to keep thoroughly condensed fibres up to the clamping line. The Uster hairiness (H) of compact yarns is significantly lower when compared with the hairiness
of conventional yarns, irrespective of the machine system.
Both the primary and secondary hairiness of a compact yarn made of this fibre
blend and produced on the Zinser ring spinning machine are lower when compared with the yarn spun on the Suessen spinning machine. Both primary and secondary hairiness of compact yarns

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are significantly lower than in conventional yarns, irrespective of the machine system. The improvement is more obvious when comparing conventional and compact yarns spun on the Suessen ring spinning machine, with or without a condenser unit.

Conclusions
The aim of the study presented herein was to analyse and compare the yarns using two different systems for the production of compact and conventional ring yarns from the producers Suessen and Zinser. The same roving produced by the Litija Spinning Mill with a linear density of 588 tex was used to produce 20 kg of yarns from cotton, cotton/PES and cotton/viscose fibre blends under comparable technical and kinematical conditions. The tests were directed and supervised by the leading technical personnel of the Litija Spinning Mill together with the specialists of the Suessen and Zinser spinning mills, where the production of yarn samples was carried out over approximately the same time period. Yarn testing was carried out by both machine producers in laboratories using valid standard methods and procedures that
guaranteed the statistical significance of the test results. An analysis of results obtained within the comparative research into the quality properties of conventional and compact ring yarns produced at the Suessen and Zinser companies led to the following conclusions:compact spinning.html_Picture9

§ Compact yarns can be regarded as completely new ring spun yarn types as regards their morphological, physical and mechanical properties. With regard to fibre straightening, light axial tension and condensing of the fibrous bundle that form compact yarn, the new yarn structure can be defined as near-optimal.

§ The compact yarns have the following advantages when compared to the conventional ring yarns: significantly reduced primary and secondary hairiness, smooth surface, high gloss, improved mechanical and physical properties (with the exception of compact yarn produced from 50% CO/50% PES fibre blend), similar Uster properties, better resistance to rubbing, softer touch, and lower pilling effect in woven and knitted fabrics.

§ It is obvious that in the future compact yarns will be used as referential samples and benchmarks, based on which the quality of different types of spun yarn will be estimated.

§ Because of the numerous advantages of compact spinning, it can be assumed that the new spinning technique represents a promising impulse for ring spinning and spun yarn production.

§ If the spinning mills’ customers – producers of woven and knitted fabrics – require high quality spun yarns and are ready to pay approximately a 10% higher price for them (because of the higher cost of the compact ring spinning machine and the slightly higher energy costs), then the compact spinning has a promising future because of the higher production and improved quality of compact yarns.

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Viscose Rayon


Viscose is a viscous organic liquid used to make rayon and cellophane. Viscose is becoming synonymous with rayon, a soft material commonly used in shirt, coats, jackets, and other outer wear.

Viscose fibres are made from regenerated cellulose. The process of dissolving pulp (the very viscose solution of pulp gave the name to the fibre product) was an achievement of the industrial revolution of the 19th century. And this innovation triggered the discovery of full synthetic fibres during the 20th century. Many other cellulose dissolving and regeneration processes like cuprammonium rayon, which was the first process made available for manmade fibres and was already developed in the 1850s, or cellulose derivatives (like acetate) were in competition with the viscose process, but viscose proved to be superior based on process and product performance. Its booming period ended after World War 2 with the introduction of the synthetic competitive products. In the last decade, the production has stabilised at approximately 2.7 million tonnes worldwide (Europe
600000 tonnes).

It is preferably supplied to market end-uses (textile products and nonwovens) where the hydrophilic (moisture absorbing) properties of the material, for instance in direct contact with the skin or with mucous membranes, are relevant.

Currently (2005), about 85 % of the total viscose fibre production is produced as staple fibres and about 15 % as filaments.

It has to be mentioned that a substantial amount of regenerated cellulose in the form of films (cellophane) based on the viscose process are still applied in sausage encasings and other packaging foils.

In recent history in Europe, textile viscose filament end-uses are receiving increased competition (resulting in phasing out of capacity) by cheaper competitive yarns based on polyester and polyamide, whereas viscose staple fibre and viscose tyre cord keep a strong position.

Viscose currently is becoming less common also because of the polluting effects of carbon disulphide and other by-products of the process, forcing some factories to close.

  • Processes (staple fibres and filament yarn)

When producing viscose fibres, the pulp – which is mainly cellulose from wood – is dissolved and subsequently precipitated under controlled conditions. The most important process worldwide is the so-called ‘viscose process’ where the alkaline pulp is treated with carbon disulphide (CS2) and dissolved by adding sodium hydroxide solution. A viscous orange-brown solution called ‘viscose’ is formed which is ripened, degassed and then pressed through spinnerets into a highly acidic spinning bath. Here, the cellulose precipitates when CS2 and the by-product H2S is released. After this, the cellulose is stretched, washed and then undergoes further processing.

At this point, a distinction has to be made between staple fibres and filament yarn:

• Staple fibres are cut into short pieces after the spinning bath. These short fibres, which are each approximately 4 cm long, are spun into textile yarns or processed into ‘non-woven’ products later on.
• In contrast, filament yarns are spun into endless fibres which can be used immediately.

Viscose products for textile usage with certain improved product characteristics are called ‘modal fibres’.

Regular rayon (or viscose) is the most widely produced form of rayon. This method of rayon production has been utilized since the early 1900s and it has the ability to produce either filament or staple fibers. The process is as follows:

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Figure 1.  gives a schematic view of both staple fibre and filament yarn production.

Cellulose: Production begins with processed cellulose

Immersion: The cellulose is dissolved in caustic soda: (C6H10O5)n + nNaOH —> (C6H9O4ONa)n + nH2O

Pressing: The solution is then pressed between rollers to remove excess liquid

White Crumb: The pressed sheets are crumbled or shredded to produce what is known as “white crumb”

Aging: The “white crumb” aged through exposure to oxygen

Xanthation: The aged “white crumb” is mixed with carbon disulfide in a process known as Xanthation, the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulfide under controlled temperature (20 to 30°C) to form cellulose xanthate: (C6H9O4ONa)n + nCS2 —> (C6H9O4O-SC-SNa)n

Yellow Crumb: Xanthation changes the chemical makeup of the cellulose mixture and the resulting product is now called “yellow crumb”

Viscose: The “yellow crumb” is dissolved in a caustic solution to form viscose

Ripening: The viscose is set to stand for a period of time, allowing it to ripen:

(C6H9O4O-SC-SNa)n + nH2O —> (C6H10O5)n + nCS2 + nNaOH

Filtering: After ripening, the viscose is filtered to remove any undissolved particles

Degassing: Any bubbles of air are pressed from the viscose in a degassing process

Extruding: The viscose solution is extruded through a spinneret, which resembles a shower head with many small holes

Acid Bath: As the viscose exits the spinneret, it lands in a bath of sulfuric acid, resulting in the formation of rayon filaments:

(C6H9O4O-SC-SNa)n + ½nH2SO4 —> (C6H10O5)n + nCS2 + ½nNa2SO4

Drawing: The rayon filaments are stretched, known as drawing, to straighten out the fibers

Washing: The fibers are then washed to remove any residual chemicals

The basic process ends here. Depending on the desired product it is continued optional by

Cutting: The filaments are cut down when producing staple fibers

Spinning: Filament yarns are spun into endless fibres Figure

  • Production of filament yarns

Until the spinning step, the process is very similar to the production of staple fibres

Long fibred pulp is used as the raw material. For the first step, it is treated with diluted sodium hydroxide solution (approximately 15 %). Afterwards, the liquid is removed by pressing and then it is recycled back into the process together with fresh NaOH. Next, the pulp sheets are defibrated, pre-ripened and put into CS2 for chemical conversion to xanthate. After the addition of aqueous NaOH, the viscose emerges which is ripened and degassed in vacuum prior to spinning.

Depending on the quality of the fibres, the spinnerets have different numbers of holes ranging from 30 to more than 2000. The spinning bath is sulphur acidic and contains high concentrations of sodium sulphate (Na2SO4) and zinc sulphate (ZnSO4).

Three different spinning methods are used:

  1. pot spinning – the viscose is pressed directly into the spinning bath. This is possible for threads from the size of 67 to 1330 dtex*
  2. continuous spinning – the viscose is pressed through the spinneret into a spinning tube where the flowing spinning bath picks up the coagulating fibre. This is again possible for threads from the size of 67 to 1330 dtex
  3. bobbin spinning – this process is similar to continuous spinning, but the fibre is fully coagulated. In order to achieve this, it is let into a second spinning bath where the coagulation is finished. This technique is possible for threads from the size of 1220 to 2440 dtex.

After spinning, the fibres are washed, finished, dried and spooled.
Currently, there are installations with integrated as well as batch washing

DREF Spinning


  • Introduction

Friction (DREF) spinning system is an Open-end and or Core sheath type of spinning system. Along with the frictional forces in the spinning zone the yarn formation takes place. The DREF spinning system is used to produce yarns with high delivery rate(about 300mpm). Still it has to gain its importance with the growth along with technical textiles in India. Amongst the spinning systems, DREF provides a good platform for production of core spun yarns due its spinning principle.It offers less spinning tension to the core and core will be positioned exactly at the centre of the yarn.
Development of DREF core-spun yarns unveils a path for new products including high performance textiles, sewing threads and in the apparels due to its exceptional strength, outstanding abrasion resistance, consistence performance in sewing operation, adequate elasticity for the stretch requirements, excellent resistance to perspiration, ideal wash  and wear performance and permanent press.

  • Principle of Friction (DREF) spinning Systems

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The friction spinning system consists of opening & individualization of fibres from slivers, reassembling of individualized fibres, twisting and winding of yarn. The figure 1 describes the DREF spinning principle where the opened fibres made roll with an aid of a mechanical roller for reassembling and twisting. Due to separate yarn winding and method of twist insertion, it has capability to go for high production rate.

  • DREF 1

DREF-1 friction spinning system was developed in 1973 by Dr.Fehrer.A.G. of Austria.The schematic diagram of DREF 1 spinner is shown in the figure 2.The fibres were opened with an opening roller and allowed to fall on a single perforated cylindrical drum slot ,which has negative pressure for fibre collection.The rotation of the drum impart twist to fibre assembly .

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The ratio of perforated drum to yarn surface is very large, hence the drum speed can be kept relatively low, even if one takes the unavoidable slippage into account. Due to the absence of positive control over the fibres assembly, slippage occurred between the fibre assembly and perforated roller, which reduced twist efficiency. Hence this development could not be commercialized.

  • DREF-2image

This is the development with earlier machine. DREF-2 was exhibited in the year 1975 at ITMA exhibition. The feasibility of using two perforated rotating cylinders, (as fibre collecting means), while at the same time the spinning-in of fibres into yarn occurred. It operates on the basis of mechanical/aerodynamic spinning system with an internal suction and same direction of drums rotation. The schematic diagram of the DREF-2 friction spinner is shown in the figure3. Drafted slivers are opened into individual fibres by a rotating carding drum covered with saw tooth type wire clothing. The individualized fibres are stripped off from the carding drum by centrifugal force supported by an air stream from the blower and transported into the nip of two perforated friction drums where they are held by suction. The fibres are sub-sequentially twisted by mechanical friction on the surface of the drums. Suction through the perforations of the drums assists this process besides helping in the removal of dust and dirt, thereby contributing to production of cleaner yarn. The low yarn strength and the requirement of more number of fibres in yarn cross-section(minimum 80-100 fibres) were restricted the DREF-2 spinning with
coarser counts (0.3-6s Ne).

  • DREF-3

The DREF-3 machine is the next version of DREF 2 for improving the yarn quality came to the market in the year 1981.Yarns up to 18s Ne. can be spun thro this system.

This is a core-sheath type spinning arrangement. The sheath fibres are attached to the core fibres by the false twist generated by the rotating action of drums. Two drafting units are used in this system, one for the core fibres and other for the sheath fibres. This system produces a variety of core-sheath type structures and multi-component yarns, through selective combination and placement of different materials in core and sheath. Delivery rate is about 300 m/min. DREF 3 schematic diagram is shown in the figure 4.

  • DREF-5

It was developed by Schalafhorst, Suessen and Fehrer Inc. The range of count to be spun from this system is from 16’s to 40’s Ne.Production speed is up to 200m/min.The schematic diagram of the DREF 5 is shown in the figure 5. The individualized fibres from a single sliver are fed through a fibre duct into the spinning nip at an angle to the yarn axis, so that they are stretched as far as possible, when fed into the nip[7]. This spinning system was not commercialized due to some reasons.

  • DREF-2000

It is the latest development in friction spinning demonstrated in ITMA 99. DREF-2000 employs a rotating carding drum for opening the slivers into single fibres and a specially designed system being used for sliver retention. The fibres stripped off from front the carding drum by centrifugal force and carried into the nip of the two perforated spinning drums. The fibres are subsequently twisted by mechanical friction on the surface of the drums, which rotates in the same direction. The process assisted by air suction through the drum perforations. Insertion of twist in ‘S’ and ‘Z’ direction is possible without mechanical alterations to the machine. Yarns upto 14.5s Ne can be produced at speeds of 250 m/min.

  • DREF-3000

In the ITMA 2003, the first public appearance of the DREF 3000 was made. The yarn can be spun form 0.3Ne to 14.5Ne.The features of DREF 3000 includes a drafting unit and opening head with infinitely variable drive control, spinning units with two infinitely variable suction spinning drums, take-off and winding units with infinitely variable speeds and filament guide with monitoring device. The drafting unit can handle all types of synthetic fibres, special fibres such as aramid, FR and pre-oxidized fibres, polyimides, phenol resin fibres (e.g. Kynol), melamine fibres (e.g. Basofil), melt fibres (e.g. PA, PES, PP), natural fibres (wool, cotton, jute, linen, flax, etc.), as well as glass fibres in blends with other materials. The DREF 3000 processes these fibres in the form of slivers composed of one type of fibre, or using slivers with differing fibre qualities at one and the same time. Slivers with a homogenous fibre mixture can also be employed. DREF 3000 core yarns offer high output, breakage-free spinning and weaving mill operation and thus up to 95% efficiency can be achieved.

  • Yarn formation in Friction spinning system

The mechanism of yarn formation is quite complex. It consists of three distinct operations, namely: Feeding of fibres, Fibres integration and Twist insertion.

  • Feeding:

The individualized fibres are transported by air currents and deposited in the spinning zone. The mode of fibre feed has a definite effect on fibre extent and fibre configuration in yarn and on its properties. There are two methods of fibre feed 1) Direct feed and 2)Indirect feed.

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In case of direct feed, fibres are fed directly onto the rotating fibre mass that outer part of the yarn tail. In indirect feed, fibres are first accumulated on the in-going roll and then transferred to the yarn tail. Figure 7 (a) and (b) are showing the above methods of fibre feed.

  • Fibres Integration:

The fibres through feed tube assembles onto a yarn core/tail within the shear field, is provided by two rotating spinning drums and the yarn core is in between them. The shear causes sheath fibres to wrap around the yarn core. The fibre orientation is highly dependent on the decelerating fibres arriving at the assembly point through the turbulent flow. The fibres in the friction drum have two probable methods for integration of incoming fibres to the sheath. One method, the fibre assembles completely on to perforated drum before their transfer to the rotating sheath. In the other method, fibres are laid directly on to rotating sheath.

  • Twist insertion:

There has been lot of deal with research on the twisting process in friction spinning. In friction spinning, the fibres are applied twist with more or less one at a time without cyclic differentials in tension in the twisting zone. Therefore, fibre migration may not take place in friction spun yarns. The mechanism of twist insertion for core type friction spinning and open end friction spinning are different,which are described below.

Twist insertion in core-type friction spinning:
In core type friction spinning, core is made of a filament or a bundle of staple fibres is false twisted by the spinning drum. The sheath fibres are deposited on the false twisted core surface and are wrapped helically over the core with varying helix angles. It is believed that the false twist in the core gets removed once the yarn is emerged from the spinning drums, so that this yarn has virtually twist less core. However, it is quite possible for some amount of false twist to remain in the fact that the sheath entraps it during yarn formation in the spinning zone.

Twist insertion in Open end type friction spinning
In open end type friction spinning the fibres in the yarn are integrated as stacked cone. The fibres in the surface of the yarn found more compact and good packing density than the axial fibres in the yarn. The Figure 8 shown the arrangement of fibres in the DREF-3 yarn as stacked cone shape .

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  • Structure of the yarn tail:

The yarn tail can be considered as a loosely constructed conical mass of fibres, formed at the nip of the spinning drums. It is of very porous and lofty structure.The fibres rotating at very high speed. Lord and Rust have been studied a number of short-duration photographs of the yarn tail during the yarn formation. In these photographs, they located an appendage protruding from the open-end of the yarn tail and called it as the tip of the tail. Observing through the perforated drums, they found this tip to be very unstable, flickering about like a candle flame a draught. With the help of the photographs, they have concluded that the yarn tail is enlarged and torpedo-shaped being squashed by the nip of the perforated drums and the fibres on its surface are loosely wrapped. Moving away from the tip, these wrappings have been shown to
become tighter. They have further added that the surface structure of the tail consists of outstanding fibres, which stand out almost radically.

  • Spinning Tension for DREF yarns

Figure 9 explains that the Friction spun yarns have less spinning tension during the yarn formation. Due to less tension during the spinning the core component can be placed exactly at the centre of the yarn.

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  • Friction Spun Yarns Properties:

Friction spun yarns (DREF) yarns have bulky appearance (100-140% bulkier than the ring spun yarns).The twist is not uniform and found with loopy yarn surface. Friction spun yarns with high %age of core have high stiffness. Friction spun yarns are usually weak as compared to other yarns. The yarns possess only 60% of the tenacity of ring-spun yarns and about 90% of rotor spun-yarns. The increased twist and wrapping of the sheath over the core improve the cohesion between the core and sheath and within the sheath.

The breaking elongation ring, rotor and friction spun yarns have been found to be equal. Better relative tenacity efficiency is achieved during processing of cotton on rotor and friction spinning as compared to ring spinning system.

Depending on the type of fibre, the differences in strength of these yarns differ in magnitude. It has been reported that 100% polyester yarns, this strength deficiency is 32% whereas for 100% viscose yarns, it ranges from 0-25%. On the other hand, in polyester-cotton blend, DREF yarns perform better than their ring-spun counterparts. A 70/30% blend yarn has been demonstrated to be superior in strength by 25%. The breaking strength of ring yarns to be maximum followed by the rotor yarn and then 50/50 core-sheath DREF-3 yarn.

DREF yarns have been seen to be inferior in terms of unevenness, imperfections, strength variability and hairiness. DREF yarns occupy an intermediate position between ring-spun and rotor spun yarns as far as short hairs and total hairiness s concerned. For hairs longer than 3mm, the friction spun yarns are more hairy than the ring spun yarns. Rotor spun yarns show the least value in both the values. DREF yarns are most irregular in terms of twist and linear density while ring spun yarns are most even.

Chattopadhyay and Banerjee have studied the frictional behaviour of ring, rotor, friction spun yarns of 59 and 98.4 Tex spun from cotton, polyester, viscose fibres, with varying levels of twist. The yarn to yarn and yarn to guide roller friction was measured at different sliding speeds and tension ratios. However for polyester fibres, the rotor spun yarn showed highest friction, followed by friction and ring spun yarns.

  • Advantages of Friction spinning system

The forming yarn rotates at high speed compare to other rotating elements. It can spin yarn at very high twist insertion rates (ie.3,00,000 twist/min). The yarn tension is practically independent of speed and hence very high production rates (up to 300 m/min) can be attainable. The yarns are bulkier than rotor yarns.

The DREF II yarns are used in many applications. Blankets for the home application range, hotels and military uses etc. DREF fancy yarns used for the interior decoration, wall coverings, draperies and filler yarn. Core spun yarns thro this friction spinning are used in shoes, ropes and industrial cable manufacturing. Filler cartridge for liquid filtration also effectively made with these yarns. Secondary backing for tufted carpets can be produced with waste fibres in this spinning system .Upholstery, table cloths, wall coverings, curtains, hand-made carpets, bed coverings and other decorative fabrics can be produced economically by DREF Spinning system. Heavy flame-retardant fabrics, conveyor belts, clutches and brake linings, friction linings for automobile industry, packets and gaskets are some examples were the DREF yarns can be effectively used.

The DREF-3 yarns made fabrics used in many applications like backing fabrics for printing, belt inserts, electrical insulation, hoses, filter fabrics and felts made from mono-filaments core. Hot air filtration and wet filtration in food and sugar industries these yarns made fabrics are used. It also used in clutch lining and brake lining for automotive industries.he multi-component yarns manufactured using DREF 3000 technology are mainly employed for technical textiles of the highest quality. They provide heat and wear protection, excellent dimensional stability, outstanding suitability for dyeing and coating, wearer comfort, long service life , as well as a range of other qualitative and economic advantages. These include cost savings due to the use of less expensive materials, special fibres and wires as yarn cores. Apart from their strength, DREF 3000 yarns are also notable for their good abrasion-resistance,uniformity and excellent Uster values compare to previous systems.

  • Limitations of Friction spinning system

Low yarn strength and extremely poor fibre orientation made the friction spun yarns very weak.The extent of disorientation and buckling of fibres are predominant with longer and finer fibres.Friction spun yarns have higher snarling tendency. High air consumption of this system leads to high power consumption.  The twist variation from surface to core is quite high; this is another reason for the low yarn strength. It is difficult to hold spinning conditions as constant.  The spinning system is limited by drafting and fibre transportation speeds.

Website: http://drefcorp.com

Fiber Migration in yarn structure


Yarn structure plays a key role in determining the yarn physical properties and the performance characteristics of yarns and fabrics. The best way to study the internal structure of the yarns is to examine the arrangement of single fibers in the yarn body, and analyze their migration in crosswise and lengthwise fashions. This requires visual observation of the path of a single fiber in the yarn. Since a fiber is relatively a small element some specific techniques have to be utilized for its observation. In order to perform this task, two different experimental techniques have been developed by previous researchers.

a. Tracer fiber technique: This technique involves immersing a yarn, which contains a very small percentage of dyed fibers, in a liquid whose refractive index is the same as that of the original undyed fibers. This causes the undyed fibers to almost disappear from view and enables the observation of the path of a black dyed tracer fiber under a microscope. Dyed fibers are added to the raw stock before spinning to act as tracers. This technique was introduced by Morton and Yen .

b. Cross sectional method: In this method first the fibers in the yarn are locked in their original position by means of a suitable embedding medium, then the yarn is cut into thin sections, and these sections are studied under microscope. As in the tracer fiber technique, the yarn consists of mostly undyed fibers and a small proportion of dyed fibers such that there is no more than one dyed fiber in any yarn cross-section.

Fiber Migration

Fiber migration can be defined as the variation in fiber position within the yarn. Migration and twist are two necessary components to generate strength and cohesion in spun yarns. Twist increases the frictional forces between fibers and prevents fibers from slipping over one another by creating radial forces directed toward the yarn interior while fiber migration ensures that some parts of the all fibers were locked in the structure.

It was first recognized by Pierce that there is a need for the interchange of the fiber position inside a yarn since if a yarn consisted of a core fiber surrounded by coaxial cylindrical layers of other fibers, each forming a perfect helix of constant radius, discrete layers of the yarn could easily separate. Morton and Yen discovered that the fibers migrate among imaginary cylindrical zones in the yarn and named this phenomenon “fiber migration.”

Mechanisms Causing Fiber Migration

Morton [42] proposed that one of the mechanisms which cause fiber migration is the tension differences between fibers at different radial positions in a twisted yarn. During the twist insertion, fibers are subjected to different tensions depending on their radial positions. Fibers at the core will be under minimum tension due to shorter fiber path while fibers on the surface will be exposed to the maximum tension. According to the principle of the minimum energy of deformation, fibers lying near the yarn surface will try to migrate into inner zones where the energy is lower. This will lead to a cyclic interchange of fiber position. Later Hearle and Bose  gave another mechanism which causes migration. They suggested that when the ribbon-like fiber bundle is turned into the

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Apart from the theoretical work cited above, several experimental investigations have been carried out during 1960’s to find out the possible factors affecting fiber migration. Results showed that the fiber migration can be influenced mainly by three groups of factors:

q fiber related factors such as fiber type, fiber length, fiber fineness, fiber initial modulus, fiber bending modulus and torsional rigidity;

q yarn related factors, such as yarn count and yarn twist ; and

q processing factors such as twisting tension, drafting system and number of doubling.

Methods for Assessing Fiber Migration

To study fiber migration Morton and Yen introduced the tracer fiber method. As explained in the previous section, this method enables the observation of the path of a single tracer fiber under a microscope. In order to draw the paths of the tracer fibers in the horizontal plane, Morton and Yen made measurements at successive peaks and troughs of the tracer images. Each peak and trough was in turn brought to register with the hairline of a micrometer eyepiece and scale readings were taken at a, b, and c as seen in Figure 22. The yarn diameter in scale units was given by c-a, while the offset of the

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peak or trough, the fiber helix radii, was given by

The distance between

adjacent peaks and troughs was denoted by d. The overall extent of the tracer fiber was obtained from the images, as well. Morton and Yen concluded that in one complete cycle of migration, the fiber rarely crosses through all zones of the structure, from the surface of the yarn to the core and back again, which was considered as ideal migration.

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Later Morton [42] used the tracer fiber method to characterize the migration quantitatively by means of a coefficient so called “the coefficient of migration.” He proposed that the intensity of migration i.e., completeness of the migration, or otherwise, of any migratory traverse could be evaluated by the change in helix radius between successive inflections of the helix envelope expressed as the fraction of yarn radius. For example intensity of migration in Figure 23 from A to B was stated as

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where rA and rB are helix radius at A and B, respectively and R is yarn radius.

In order to express the intensity of migration for a whole fiber, Morton used the coefficient of migration, which is the ratio of actual migration amplitude to the ideal case. The coefficient of migration was given by

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Merchant [ 1 ] modified the helix envelope by expressing the radial position in terms of (r / R) in order avoid any effects due to the irregularities in yarn diameter. The plot of (r / R) along the yarn axis gives a cylindrical envelope of varying radi
us around which the fiber follows a helical path. This plot is called a helix envelope profile. Expression of the radial position in terms of (r / R) involves the division of yarn cross sections into zones of equal radial spacing, which means fibers present longer lengths in the outer zones. Hearle et al. [18] suggested that it is more convenient to divide the yarn cross sections into zones of equal area so that the fibers are equally distributed between all zones. This was achieved by expressing the radial position in terms of (r / R)2, and the plot of (r / R)2against the length along the yarn is called a corrected helix envelope profile which presents a linear envelope for the ideal migration if the fiber packing density is uniform (Figure 24). The corrected helix envelope profile is much easier to manage analytically.

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In 1964 Riding [52] worked on filament yarns, and expanded the tracer fiber technique by observing the fiber from two directions at right angles by placing a plane mirror near the yarn in the liquid with the plane of the mirror at 45° to the direction of observation. The radial position of the tracer fiber along the yarn was calculated by the following equation:

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where x and y are the distances of the fiber from the yarn axis by the x and y co­ordinates; and dx and dy are the corresponding diameter measurements.

Riding also argued that it is unlikely that any single parameter, such as the coefficient of migration will completely characterize the migration behavior due to its statistical nature. He analyzed the migration patterns using the correlogram, or Auto-correlation Function and suggested that this analysis gives an overall statistical picture of the migration. Riding calculated the auto-correlation coefficient, rs from a series of

values of r / R for a separation of s intervals and obtained the correlogram for each experiment by plotting rs against s. Later a detailed theoretical study by Hearle and Goswami showed that the correlogram method should be used with caution because it tends to pick up only the regular migration.

Hearle and his co-researchers worked on a comprehensive theoretical and experimental analysis of fiber migration in the mid 1960’s. In Part I of the series Hearle, Gupta and Merchant came up with four parameters using an analogy with the method of describing an electric current to characterize the migration behaviors of fibers.

These parameters are:

i. the mean fiber position, which is the overall tendency of a fiber to be near the yarn surface or the yarn center.

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ii. r.m.s deviation, which is the degree of the deviation from the mean fiber position

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iii. mean migration intensity, which is the rate of change in radial position of a fiber.

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iv. equivalent migration frequency, which is the value of migration frequency when an ideal migration cycle is formed from the calculated values of I and D.

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r is the current radial position of the fiber with respect to the yarn axis;

R is the yarn radius;

n is the number of the observations; and

Zn is the length of the yarn under consideration

By expressing the migration behavior in terms of these parameters, Hearle et al. replaced an actual migration behavior with a partial ideal migration which is linear with z (length along the fiber axis) but has the same mean fiber position, same r.m.s deviation, and the same mean migration intensity.

Later Hearle and Gupta [20] studied the fiber migration experimentally by using the tracer fiber technique. By taking into consideration the problem of asymmetry in the yarn cross section they came up with the following equation:

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where

r1 and r2 are the helix radii

R1 and R2 are the yarn radii at position z1 and z2 along the yarn.

In 1972 Hearle et al. carried some experimental work on the migration in open-end spun yarns, and they observed that migration pattern in open-end yarns was considerably different from that of ring spun yarns. They suggested that this difference was the reason for the dissimilarity between mechanical and structural parameters of these two yarns.

Among numerous investigations of migration, there have been some attempts to develop a numerical algorithm to simulate yarn behavior. Possibly the most promising

and powerful approach was to apply a finite element analysis method to the mechanics of yarns.

One of the most recently published researches on the mathematical modeling of fiber migration in staple yarns was carried out by Grishanov, et al. They developed a new method to model the fiber migration using a Markov process, and claimed that all the main features of yarn structure could be modeled with this new method. In this approach the process of fiber migration was considered as a Poisson’s flow of events, and the fiber migration characteristics were expressed in terms of a transition matrix.

Another recent study was done by Primentas and Iype. They utilized the level of the focusing depth of a projection microscope as a measure of the fiber position along the z-axis with respect to the body of the yarn. Using a suitable reference depth they plotted the possible 3-dimensional configuration of the tracer fiber. In this study they assumed that yarn had a circular cross section and the difference between minimum and maximum values in depth represented the value of the vertical diameter, which was also equal to horizontal diameter. However, the yarn is irregular along its axis, and its cross section deviates from a circle. Besides, it is questionable that the difference between minimum and maximum values in depth would give the value of the vertical diameter. As these researchers stated this technique is in the “embryonic stage of development.”

Basic Structural Features of Spun Yarn


The end product of the cotton fiber-to-yarn conversion system is a spun yarn or a staple-fiber yarn, which is suitable for making numerous end products from knit apparels to woven fabrics, from towels to sheets, and from carpets to industrial fabrics. The diversity of yarn-based products results in different views of what constitutes a yarn quality. Indeed, different textile manufacturers often express different views of yarn quality depending on the particular end product produced and the type of downstream processing used.

In general, the spinner may define yarn quality as an index of appearance, strength, uniformity, and level of imperfections. However, the spinner is much more concerned about how the yarn user views yarn quality.The knitter may have more detailed criteria of yarn quality. These may include:

  • A yarn that can unwind smoothly and conform readily to bending and looping while running through the needles and sinkers of the knitting machine. This translates to flexibility and pliability.
  • A yarn that sheds low fly in and around the knitting machine. This translates to low hairiness and low fiber fragment content
  • A yarn that leads to a fabric of soft hand and comfortable feeling. This translates to low twist, low bending stiffness, and yarn fluffiness or bulkiness.
  • A yarn that has better pilling resistance. This translates to good surface integrity.

The weaver may have a different set of yarn quality criteria:

  • A yarn that can withstand stresses and potential deformation imposed by the weaving process. This translates to strength, flexibility, and low strength irregularity.
  • A yarn that has a good surface integrity. This translates to low hairiness and high abrasion resistance.
  • A yarn that can produce defect-free fabric. This translates to high evenness, low imperfection, and minimum contamination.

In light of these different and often conflicting views of yarn quality, the spinner must customize the yarn to meet its intended purpose. This can be achieved through integration of yarn quality into the overall specification of the end product. This requires establishing appropriate values of fiber attributes and optimum machine settings. In an ideal Fiber-to-Yarn Engineering (FYE) program, the translation of yarn quality into an acceptable end product performance is based on a well-defined design approach in which all phases of FYE are integrated to produce a yarn that is consistent and reflective of the end user requirements. This calls for an in-depth knowledge, not only of the general yarn  haracteristics, but also of the structural features of yarn.

  • Basic Structural Features of Spun Yarn

Before proceeding with the discussion on yarn characteristics, it will be important to discuss the basic structural features of spun yarn. Understanding these features provides an insight into the interpretation of yarn behaviour during processing or in the end product. The basic structural features of spun yarn are: yarn density, bulk integrity, and surface profile. These features are discussed below.

In a yarn structure, fibers represent the main component. The other component is air pockets created by the technology forming the structure. Accordingly, the yarn bulk density should be determined by the packing fraction, 0, as defined by the following equation:

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The packing fraction is an indication of the air spaces enclosed by the fibers. For example, a packing fraction of 0.5 indicates that there is as much space taken by air as by fiber. Most spun yarns have packing fraction well above 0.5.

The importance of packing fraction lies in its powerful effects on many yarn and fabric properties. It is indeed one of the major design parameters
of textile fabrics. For a given fiber material, a yarn of very high packing fraction is likely to be stiff and probably weak. On the other hand, a yarn of very low packing fraction is likely to lack the bulk and surface integrity required to hold the yarn structure together during processing. In relation to fabric performance, yarn density plays a major role in determining many of the performance characteristics of fabric. One of the major fabric characteristics influenced by yarn density is fabric comfort. In general, fabric comfort is viewed in terms of two main aspects [El Mogahzy, 1998]: neurophysiological and thermo-physiological comfort. The neuro-physiological aspect deals with the fabric/skin physical interaction, and the thermo-physiological aspect deals with moisture and heat transfer through fabric. A high packing fraction will likely produce a highly compacted yarn that is likely to produce a stiff fabric and result in a greater true contact between the fabric and the human skin. These two features typically result in neuro-physiological discomfort. The thermo-physiological effect can be explained on the ground that air is the best heat insulator of all materials. On average, the thermal conductivity of air is more than eight times less than that of fibers (thermal conductivity of air = 6×10-5 cal.sec -1.cm -1. deg -1 C). The air pockets in the yarn assists in creating an entrapped or still air in the fabric and this can greatly enhance the thermal insulation of the human body against changing environmental conditions. Yarn density also influences other characteristics such as dimensional stability, strength, extensibility, flexibility, fabric cover, air permeability, and absorption characteristics.

Staple fiber yarns and textured yarns normally have lower density than continuous filament yarns made from the same fiber material.  Different spinning techniques produce different degrees of yarn density as a result of the different patterns of fiber compactness imposed by yarn twisting and spinning tension. For instance, a ring-spun yarn will typically exhibit higher degree of compactness than a comparable rotor-spun yarn due to the true twist the high tension used in ring spinning. The extent of fiber compactness can also be altered within the same spinning system. For instance, a higher rotor speed in open-end spinning is likely to produce higher fiber compactness in the yarn due to the higher centrifugal force applied on the fibers inside the rotor.

In theory, yarn density has approximately linear relationship with the product (twist.tex1/4) of spun yarn (Neckar, 1998). Fiber properties that influence this product will also influence fiber compactness or yarn density. These include: fiber diameter, cross-sectional shape, fiber length, fiber resiliency, and fiber density. For a given yarn count, and a given twist level, fine and long fibers will normally result in higher yarn density than coarse and short fibers.

  • Yarn Bulk Integrity

Yarn bulk integrity is determined by the fiber arrangement in the yarn structure. Fiber arrangement is expected to have significant effects on many yarn and fabric characteristics including yarn liveliness, fabric dimensional stability, yarn appearance, yarn strength, and fabric cover. The bulk integrity of a spun yarn largely reflects the impact of the spinning process on yarn structure. In general, different spinning techniques provide different forms of bulk integrity through providing different fiber arrangements. Obviously, the simplest fiber arrangement can be found in a continuous filament yarn where fibers (or continuous filaments) are typically arranged in parallel and straight form. As shown in Figure 1 a slight deviation from this arrangement can be caused by slightly twisting the filaments or through deliberate distortion in the filament orientation as in the texturizing process.

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Fig.1 :- fiber arrangement in continues filament yarn

In staple fiber yarns, fiber arrangement is quite different from the simple arrangement discussed above. The discrete nature of staple fibers makes it impossible to fully control the fiber flow in such a way that can produce a well-defined fiber arrangement. For this reason, a spun yarn typically exhibits some irregularities along the yarn axis. In addition, no spun yarn can be free of fiber ends protruding from its surface as shown in Figure 2. Different spinning systems produce different forms of bulk integrity or fiber arrangements. The general features of fiber arrangement produced by four different spinning systems are shown in Figure 3 shows. In chapter 9, we will discuss how these  pinning techniques produce these structural arrangements.

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Fig.2;- Fiber arrangement in staple yarn

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Fig 3:- different fiber arrangement of different spinning technique

  • Yarn Surface Profile

The surface profile of a spun yarn may be described by three basic parameters: the overall surface appearance of yarn, surface integrity, and surface irregularities. The importance of yarn surface profile lies in the fact that a yarn is initially judged by its surface appearance. As the yarn goes through the weaving or the knitting process, surface integrity (abrasion resistance and hairiness) becomes the most critical factor determining yarn performance. As the yarn is finally woven or knitted into a fabric, surface irregularities (thick and thin places, and yarn neps) are typically the most noticeable defects in the fabric.

As expected, yarn density (or fiber compactness), and bulk integrity (or fiber arrangement) will greatly influence yarn surface profile.Accordingly, different spinning techniques will produce yarns of different surface profiles. Within a given spinning system, the main factors influencing yarn surface profile are:

  • The drafting mechanism (roller drafting or aerodynamic drafting)
  • The consolidation mechanism (twisting or wrapping)
  • The surface roughness of the spinning component (e.g. the traveler/ring contact in ring-spinning, the navel surface in rotor-spinning, and the condensation surface n compact spinning)

Material-related factors that can influence yarn surface profile include:

  • Short fiber content
  • Fiber neps
  • Fiber rigidity (flexural and torsion rigidities)
  • Fiber contaminants

In general, high levels of short fibers can result in excessive surface hairiness particularly in ring-spun yarns. The fact that short fibers flow under minimum or no control in the textile process can result in many surface disturbances of yarns spun on any spinning system. Many of the random thick and thin places in the yarn can be attributed to short fibers. Fiber neps that are not removed by the textile process will be presented in the yarn either in the bulk or on the surface. In both cases, surface disturbances will appear as short thick places or yarn neps. Fibers of high bending or torsion rigidity will not conform to the manipulation process exerted during textile processing. These fibers are likely to act in an unpredictable fashion leading to many surface disturbances (hairiness or irregular fiber wrappers). Many of the long-fiber hairiness can be attributed to high rigidity. On the other hand, fibers of extremely low rigidity may tend to entangle and form neps. Extraneous materials typically exhibit different shapes, colors, and sizes from fibers. The chance of these contaminants to appear on the yarn surface is much greater than to be incorporated in the yarn bulk. In either case, these contaminants, if not removed in the early stage of processing, can alter the yarn surface profile substantially.

  • Yarn Twist

Twisting is the primary binding mechanism of spun yarns. In general, twist is defined as a measure of spiral turns given to a yarn in order to hold the constituent fibers together. In practice, yarn twist is described using three main parameters: (a) twist direction, (b) twist level (turns/unit length), and (c) twist factor.

Twist Direction

Twist may be performed in the following two directions:

S-Direction: A single yarn has “S” twist if, when it is held in the vertical position, the fibers inclined to the axis of the yarn conform in direction of slope to the central portion of the letter S.

Z-Direction: A single yarn has “Z” twist if, when it is held in the vertical position, the fibers inclined to the axis of the yarn conform in direction of slope to the central portion of the letter Z.

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fig.:-6 twist direction and twist level in idealised twist geometry

Twist Level

The amount of twist in the yarn is commonly expressed by the number of turns per unit length. In order to understand the meaning of twist and its relation to other yarn parameters, we will use the classical idealized helical geometry of a circular yarn (Hearle et al, 1969) shown in Figure 7.6. In this geometry, the yarn is assumed to be built-up of a series of superimposed concentric layers of different radii in each of which the fibers follow a uniform helical path so that its distance from the center remains constant. Based on this model, the length of one turn of twist, h, is given by:

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Where TPC is turns per cm, and TPI is turns per inch.
In practice, equation 7.11 is commonly used to determine the twist multiplier of yarn for a given yarn count and a given twist level. It simply indicates that the twist multiplier is an expression of the twist level adjusted for yarn count. The Importance of Yarn Twist In practice, the importance of twist direction is realized when two single yarns are twisted to form a ply yarn. Ply twist may be Z on Z, or S on Z depending on appearance and strength requirements of the ply yarn. Recall that in determining the yarn count of a plied yarn, we had to account for the possible contraction or increase in length resulting from twisting. Normally, the Z on Z twist will result in a contraction of the plied yarn, while the S on Z twist will result in an increase in length. This amount of contraction or expansion will depend on the amount of twist inserted. When the yarn is woven or knitted into a fabric, the direction of twist influences the appearance of fabric. When a cloth is woven with the warp threads in alternate bands of S and Z twist, a subdued stripe effect is observed in the finished cloth due to the difference in the way the incident light is reflected from the two sets of yarns. In twill fabric, the direction of twist in the yarn largely determines the predominance of twill effect. For right-handed twill, the best contrasting effect will be obtained when a yarn with Z twist is used; on the other hand, a left-handed twist will produce a fabric having a flat appearance. In some cases, yarns with opposite twist directions are used to produce special surface texture effects in crepe fabrics.Twist direction will also have a great influence on fabric stability, which may be described by the amount of skew or “torque” in the fabric. This problem often exists in cotton single jersey knit where knitted wales and courses are angularly displaced from the ideal perpendicular angle. One of the solutions to solve this problem is to coordinate the direction of twist with the direction of machine rotation. With other factors being similar, yarn of Z twist is found to give less skew with machines rotating counter clockwise. Fabrics coming off the needles of a counter clockwise rotating machine have courses with left-hand skew, and yarns with Z twist yield right-hand wale skew. Thus, the two effects offset each other to yield less net skew. Clockwise rotating machines yield less skew with S twist. The amount of twist inserted in the yarn can influence many yarn characteristics. As will be shown in chapter 9, twisting is the primary mechanism to bind fibers in both ring and open-end spinning. Twisting is a unique binding mechanism that many engineers outside the textile field are not familiar with. It is, perhaps, the only binding mechanism that allows the structure to retain a great deal of its flexibility (as compared to glue or adhesive chemicals which result in more stiff structures). The relationship between yarn strength and twist level is well recognized among textile technologists and engineers. This relationship is generally illustrated in Figure 7. Initially, as the twist level (number of turns per unit length) increases, yarn strength will also increase. This effect holds only up to a certain point beyond which further increase in twist causes the yarn to become weaker. Thus, one should expect a point of twist at which yarn strength is at its maximum value. This point is known as the “optimum twist”.

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figure 7 Effect of twist on spun yarn strength

Many investigators made various attempts to explain the strength-twist relationship (e.g. Hearle et al, 1969, and Lord, 1981). In practical terms, the strength-twist relationship may be explained on the ground that at zero twist, fibers are more or less oriented along the yarn axis but without any binding forces (except their interfacial contact). As twist slightly increases, the contact between fibers will increase due to the increase in traverse pressure, and the force required to stretch the yarn must first overcome the inter-fiber friction. Further increase in twist will result in further binding between fibers and an increase in the number of cross-linking points between fibers. This provides an opportunity for many fibers to be held at some points along their axis by other fibers. When this happens, the fiber strength begins to play a role in resisting the force required to stretch or rupture the yarn. Eventually, fiber strength will play a greater role than interfiber friction in tensile resistance. However, the discrete nature of fibers will always necessitate inter-fiber cohesion. The trend of increasing strength with twist will continue until some points where the fibers become so inclined away from the yarn axis that the contribution of fiber strength will decrease. This will result in a reduction of yarn strength with the increase in twist. In light of the above interpretation, one can see that there are two effects governing the strength-twist relationship. The first effect is an increase in yarn strength with twist resulting from the increase in the cohesion of fibers as the twist is increased. The second effect is a decrease in yarn strength with twist resulting from a decrease in the effective contribution to the axial loading of the yarn due to fiber obliquity. Thus, the curve shown in Figure 7.7 may be divided into two sections (Figure 8):

(i) a low twist region in which the effect of fiber cohesion outweighs that of obliquity, giving rise to an increase in strength, and

(ii) a high twist region in which further increase in cohesion no longer produces an increase in strength because of the overwhelming effect of fiber obliquity.

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figure 8:- interpretation of strength-twist relationship

The twist level used can influence a number of fabric characteristics. These include: fabric hand, and skew. High or low levels of twist may be required depending on the type of fabric produced and its desirable characteristics. Highly twisted yarns are “lively” and tend to untwist (or snarl). Consequently, fabrics made from these yarns will possess a lively handle. This effect is utilized in producing crepe yarns (TM = 5.5- 9.0), which are used to produce crepe surface cloth. When soft fabrics are desirable (e.g. knit shirts), a low level of twist is required. Low twist level is also required to minimize fabric skew. In general, the higher the level of twist in the yarn the greater the tendency for the knit fabric to skew or torque.

  • Yarn Diameter

The use of linear density to express the yarn fineness provides a convenient and a practical approach for characterizing this important characteristic. All machines in the fiber-to-yarn conversion system are set on the basis of the linear density of fiber strands. In certain applications, however, yarn fineness expressed in diameter or thickness provides more useful information. For example, determining the structural features of a fabric (e.g. cover factor, yarn crimp, etc.) requires a prior knowledge of yarn diameter. It is important, therefore, to measure yarn diameter or to provide an estimate of its value. In this section, we discuss methods for estimating yarn diameter.Theoretically, equation 7.8, introduced earlier, provides a general expression of yarn radius as a function of the linear density and the volumetric density of the yarn. For direct count system (say, tex), this general relationship will be as follows:

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For indirect systems (say, cotton count), the general expression of yarn diameter is as follows:

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The above expressions indicate that the value of yarn diameter mainly depends on the linear density or yarn count, tex or Ne, and the volumetric density of yarn, p. As indicated earlier, volumetric density describes the degree of compactness of fibers in the yarn structure. This means that yarn twist will have a significant effect on yarn diameter.

Yarn Diameter Formula: In practice, yarn diameter is typically estimated using empirical formula. One of the most commonly used expressions for estimating yarn diameter is that developed by Peirce in 1937 (see Table 7.4). In this expression, yarn density was assumed to be 1.1 g/cm3. In a recent study, El Mogahzy et al (1993) developed empirical expressions for estimating the diameters of ring-spun, rotor-spun, and MJS airjet spun yarns. These expressions (also given in Table 7.3) were developed based on extensive microscopical testing of actual yarn thickness of the three yarn types using a wide range of yarn count, and twist levels. The formulae shown in Table 7.3 indicate that yarns made from different spinning systems and of equal nominal count will exhibit different values of yarn diameter. This is a result of the difference in fiber arrangement and fiber compactness of different yarn types. For example, a ring-spun yarn and a rotor-spun yarn of cotton count 20’s will have estimated diameters of 0.253 mm, and 0.275 mm, respectively. The higher value of rotor-spun yarn diameter indicates that it is bulkier than the ring-spun yarn.

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We should point out that the formula for ring-spun yarn developed by El Mogahzy et al (1993) tend to produce a value of yarn diameter that is slightly higher than that estimated by Peirce equation. As shown in Figure 7.9, the difference between the two estimates decreases as the yarn becomes finer. The main reason for the difference was due to discrepancy in the value of yarn density, particularly in the coarse to medium range of yarn count. Using a combination of capacitive and optical measures of different yarns, we found that the density of cotton ring-spun yarns can range from 0.85 to 1.2 g/cm3 depending on the spinning system, fiber characteristics, and structural parameters (count and twist).

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The Importance of Yarn Diameter :- As indicated above, yarn diameter is used to estimate fabric structural parameters such as width, and cover factor. Since thousands of ends or wales are presented side-by-side in the woven or the knit fabrics, a slight change in yarn diameter can result in a substantial change in the overall cover factor of fabric. The effect of yarn diameter on the geometrical features of fabric structure can be realized through examination of the equations developed by Peirce to determine the cover factor of woven fabric (Peirce, 1937), or the equations developed by Munden (1963, 1967) to determine the tightness factor of plain weft knitted structures. In the context of fiber-to-yarn engineering, yarn diameter is certainly a major design criterion. Factors affecting yarn diameter are essentially those that affect yarn density or fiber compactness. As we indicated earlier, fiber properties that are expected to influence fiber compactness include: fiber fineness, fiber stiffness, fiber length, and fiber crimp. In general, coarse and stiff fibers will result in bulkier or thicker yarn than fine and flexible fibers (Stout, 1958). In other words, as the fiber becomes coarser (higher denier, or millitex), yarn density becomes smaller, leading to an increase in yarn diameter, although the count of yarn remains unchanged. Zurek (1961), one of the leading scientists in yarn structure, explains the above phenomenon on the ground that coarser or more rigid fibers have higher resistance to bending, while twisted into yarns, than finer or more flexible fibers; hence, the radius of their curvature is longer. Only movement of the fiber away from yarn axis can cause the increase of radius. On the same ground, fiber length also affects yarn density and consequently yarn diameter. For a given yarn count and at the same twist factor, the larger the fiber length, the higher the yarn density, and the smaller the yarn diameter. In theory, fiber compactness may be characterized by two main categories of fiber arrangement in the yarn cross-section (Hearle et al, 1969):

(i) the open-packed structure, andimage

(ii) the closed packed
structure. These are illustrated in Figure 7.10. In the opened-packed structure, fibers lie in layers between successive concentric circles. The first layer is a single core fiber around which six fibers are arranged so that all are touching; the third layer has twelve fibers arranged so that the fibers first touch the circle that circumscribes the second layer; additional layers are added between the successive circumscribing circles. In the closed-packed structure, all fibers touch each others which give rise to a hexagonal array of fibers in the yarn cross section. In practice, fiber packing may deviate largely from these idealized forms. This deviation may be attributed to a number of factors including: non-circularity of fibers, dimensional variability, the relaxation and coherence of fibers in the yarn structure, and the effect of twist. The last factor is explained on the ground that twist causes the development of tangential and radial forces, which result in fiber migration and binding of fibers together.

  • Yarn Strength :-

Yarn strength is considered as one of the main criteria characterizing yarn quality. Indeed, no other yarn characteristic has received more investigative attention than yarn strength. Most of the studies dealing with yarn strength focused on developing models characterizing yarn strength as a function of structural parameters and fiber attributes. Many of these models revealed a great deal of information about the complex nature of yarn strength. In fact, the interpretation of the strength-twist relationship discussed earlier stems from existing models describing the effect of twist on yarn strength. In recent years, interest in modeling yarn strength with respect to relevant fiber attributes has increased as a result of the revolutionary development of fiber testing and information technology, and the introduction of new spinning technologies. Despite the numerous studies of yarn strength, no universal model exists today that can fully explore or predict the mechanical behavior of staple-fiber yarn under tensile loading, from the progressive fiber assistance to the rupture mode. This is primarily due to the overwhelming stochastic nature of spun yarns making it very difficult to achieve a complete resolution of the different factors influencing yarn strength. Our interest in modeling yarn strength stems from the fact that fiber-to-yarn modeling is a basic phase of fiber-to-yarn engineering. Empirical models for predicting yarn strength and other yarn characteristics can be developed within the boundaries imposed by a given textile process. These models can be verified not only through a sound database, but also on physical basis.

Practical Parameters Describing Yarn Strength: – In chapter 6, we discussed the concept of load-elongation (or stress-strain) curve and the different parameters that can be derived from it. This concept basically holds for any material subject to tensile loading including textile yarns. Accordingly, the parameters associated with the curve (e.g. breaking stress, toughness, modulus, etc) can be used for characterizing yarn strength. The shape of the curve, however, varies widely depending on many factors including: yarn type (ring, rotor, or airjet), twist level, and yarn texture.
In practice, the strength of staple fiber yarn is commonly described using the following parameters:
· Skein strength
· Count-strength product (CSP)
· Single-end strength
· Strength irregularity (C.V strength%)

Yarn Skein Strength :- The skein strength is typically measured by winding a 120-yard skein on a wrap reel. The yarn is then removed from the reel and tested in the form of several revolutions of parallel threads using a pendulum tester at a constant rate of traverse. When the specimen is subjected to tensile loading, all threads will resist the loading until a break occurs in one of the threads (the weakest point). The remaining unbroken threads will then support the skein until a second thread breaks. This process continues through a succession of thread breaks until a total failure occurs. It is believed, therefore, that the skein strength test provides a combined measure of the strength of a composite specimen of yarns and the inter-yarn friction. The parameter obtained from this test is called the skein or lea strength expressed in pounds. The skein strength test is commonly accompanied by a yarn count test in which the same test specimen is weighted to determine the cotton count. The count-strength product known as the CSP provides a strength measure commonly known as the skein-break factor (Ne.lb). In practice, this measure is used more commonly than the absolute value of skein strength. Typical values of skein break factor for different yarns are given in Table 7.4. These values are based on yarn data corresponding to cotton U.S. crops 1990 and 1991.The wide range of CSP values is, therefore, a result of the wide range of values of fiber characteristics used in the make of the yarns.

Single-End Strength : The single-end strength represents a more fundamental parameter than the skein strength. Using modern tensile testers (e.g. Uster TensoRapid® ), strength parameters can be obtained at a constant rate of extension of 5 m/min and a gauge length of 50 cm. These parameters include: breaking load, breaking elongation, load-elongation (or stress-strain) curve, yarn tenacity, yield stress and strain, specific work of rupture, and tensile modulus. Typical values of strength parameters of different types of cotton yarns and at different values of yarn count are listed in Tables 7.5 through 7.9. These tables are modified from the Uster statistics®, 1997. Another tensile tester, also developed by Uster, is called the TensoJet®. This tester operates at a very high rate of extension (400 m/min). Using this tester, up to 30,000 tests per hour can be performed. This tester allows measuring strength variability from a large number of breaks. Strength Irregularity (C~V~trength %) Similar to count variability, strength irregularity is commonly defined by the coefficient of variation of yarn strength:

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The importance of strength irregularity lies in the fact that during processing (warping, dyeing, weaving or knitting), the incident of breakage often occurs at the weakest points of the yarn. Knowledge of the extent of variability in yarn strength will permit estimation of the strength of the weakest points. For example, suppose that the mean strength of a ring-spun yarn of count 20’s is 18 cN/tex, and the C.V% of yarn strength is 8%. From the above equation, the standard deviation of strength is 6 = 8×18/100 = 1.44 cN/tex. Typically, yarn strength as a variable follows a normal distribution. One of the basic features of the normal distribution is that the total relative frequency (or the area under the curve) between μ ± 36 is about 99.74%. As shown in Figure 7.11, it follows that this yarn will have weak points of strength values as low as 13.7 cN/tex, which is only 76% of the mean strength.

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The Importance of Yarn Strength :

The importance of yarn strength can be realized in all stages of processing from spinning to finished fabric manufacturing. In any spinning technique, yarn strength represents a crucial parameter, which determines the performance of spinning. For instance, an ends down in ring spinning is often a result of the failure of the yarn to withstand a high peak of spinning tension. This failure results from a weak portion of the yarn. The strength-twist relationship is considered to be a characteristic curve of the spinning performance that must be established to produce a high strength yarn. As indicated earlier, fiber properties such as strength, length, fineness, and friction play a vital role in determining this relationship. During yarn preparation for weaving, the yarn is subject to continuous tension as a result of the repeated winding and unwinding necessary for weaving preparation. This tension should be within the elastic boundaries of the yarn to avoid permanent deformation. During dyeing or sizing, the yarn is subjected to chemical treatments that can alter its mechanical behavior. For example, the sizing process results in an inevitable reduction in yarn elongation and yarn flexibility. It is important, therefore, to examine the modulus and the elongation profiles of yarn during weaving preparation. During the weaving process, thousands of yarns are simultaneously subject to continuous cyclic loading, which is a basic necessity for the interlacing actions required to make cloth. Weaving peak tension may reach levels exceeding 35% of the average breaking force of the yarn. Both tension variation, and yarn strength variation are expected. A single yarn may break when it exhibits a level of strength that is lower than the weaving tension at some points of the yarn. When a maximum tension coincides with a minimum strength point of the yarn, failure of yarn to withstand the tension will occur. This failure may result in an end breakage and a complete stop of the weaving process. During knitting, the yarn is subject to tension, which may reach levels of more than 30% of the average breaking force of the yarn. Again, both knitting tension and yarn strength exhibit variability. Accordingly, the failure of yarn to withstand knitting tension may occur in the same fashion described for weaving. A yarn break during knitting will have an adverse effect not only on the machine efficiency but also on the fabric quality. The stress-strain behavior of yarn is a critical factor in determining the mechanical behavior of fabric under different modes of deformation (e.g. tension, bending, and shear). In general, a strong yarn will make a strong fabric, and a stiff yarn will result in a fabric of poor comfort characteristics. An optimum combination of strength and flexibility can be achieved through many options including a proper level of twist, and a judicious choice of fiber attributes. Yarn Evenness and Imperfections The evenness, or regularity of a fiber strand (e.g. sliver, roving, or yarn) is a measure of the extent of uniformity in the strand thickness along its length. Imperfections represent abnormal incidents exceeding in their forms the expected variation in the thickness of a fiber strand. As shown in Figure 7.12, these include thin places, thick places, and neps. The reference method of evenness and imperfection analysis is obviously the microscopic method. However, the large sample of yarn required to obtain reliable microscopic information makes this method time-consuming, particularly in a practical environment. Alternatively, we may take a long fiber strand, cut it into portions of equal length, and weigh each portion. The thickness variation can then be determined from the variation in the weight per unit length as shown in Figure 7.13. This method is called the “cut and weight” method and it is used as the basis for the more advanced capacitive method commonly used by most textile mills.

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  • Methods of Evenness Testing :

There are many methods that can be used for testing the evenness of a fiber strand. These include (Slater, 1986, Walker, 1950,
Townsend et al, 1951):
· The capacitive method
· The optical method
· The pneumatic method
· The acoustic method, and
· The mechanical method
The capacitive method utilizes a capacitor (or an electrode). When a non-conductive material (such as a fiber strand) enters the field of the electrode, a capacitance change occurs. The variability in capacitance is used to indicate the variability in the mass of the fiber strand. The main element in the capacitive method is the detecting electrode. This consists of a pair of metal plates, acting as an air-spaced capacitor. The capacitive method is utilized in the popular Uster® evenness tester. A critical assumption underlying the use of the capacitive method is that the relationship between mass and capacitance change is linear. If the fiber/air ratio is increased beyond a certain limit, the electrode becomes overloaded and this relationship becomes non-linear. In this regard, a fiber/air ratio of 40% or less is recommended. Other limiting features of the capacitive method include the requirement of a rounded fiber strand, the necessity of keeping the strand well away from both plates or be in constant contact with one of them, and the high sensitivity of the method to relative humidity. In the optical method, a light source is directed onto a fiber strand, and the mass per unit length of the strand is detected by either optical extinction or optical reflection. In case of the optical extinction, the shadow cast is taken to be proportional in area to the mass of the fiber strand in the test zone. In case of optical reflection, the fiber strand is directly illuminated; when a normal strand is in the test zone, no reflection is detected; abnormalities such as fluffs, loops and protruding fibers reflect light, which can be measured electrically. Optical principles are utilized in the Uster® tester, and the Zewigle EIB system. The primary limiting factor of the optical method in measuring the evenness of a fiber strand is its sensitivity to the geometrical profile of the strand. Irregular cross sections are likely to be presented to the light source in preferential direction of alignment.In the pneumatic method, the fiber strand is passed through an orifice or a narrow tube, into which an air stream is being forced. The evenness of the fiber strand is then measured by the variation in the rate of airflow resulting from mass variation. Limiting factors of this technique include the non-linear relationship between the airflow rate and the mass of fiber trand, and the high sensitivity to atmospheric conditions (humidity and temperature). This method has been used in association with autolevelling systems (evenness control system) of fiber strands during carding.
In the acoustic method, the fiber strand moves through a sound field between a generator and a pick-up device. The time taken for sound waves to move across the gap is measured electronically. The change in this transit time is believed to correspond to the change in the cross-sectional dimensions of the fiber strand. This method has the advantage of being insensitive to moisture change. Some instrument developers have used this principle for measuring sliver uniformity during carding and drawing.In the mechanical method, the irregularity of a fiber strand is detected using a mechanical feeler, which senses the mass variation of a fiber strand as it passes through a pair of drafting roller. It is normally utilized in conjunction with autoleveling systems.Another important irregularity parameter is the socalled “limiting irregularity”. This parameter theoretically provides an irregularity measure of a fiber strand in which fibers are arranged in a completely random fashion. In practical terms, it implies irregularity under best machine conditions. The limiting irregularity, C.V%limit , is simply defined by:

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Equation 7.14 indicates that as the number of fibers per yarn, or strand, cross-section increases, the limiting irregularity decreases. This may be explained on the ground that the increase in the number of fibers creates a compensating or a doubling effect that reduces the irregularity in yarn cross-section.

In practice, the concept of limiting irregularity can be used to estimate the partial effect of process-added variability on the overall irregularity. In this regard, the Uster evenness tester can provide the so-called “irregularity index” defined by the following equation:

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The concept underlying the utilization of an irregularity index is that every process will inevitably add variability to the fiber strand. This added variability is a result of the limited capability of existing processes to maintain a perfectly random distribution of fibers in the strand cross-section, and along the strand length. In addition, mechanical defects such as improper fiber control and draft roller eccentricity adds a periodic component to the variability in the fiber strand. The irregularity index, I, compares the limiting variability to the total measured variability of the fiber strand. In the ideal situation where no process-added irregularity exists, both the measured and the limiting irregularities will be theoretically equal; in this case, the irregularity index will be equivalent to unity. In actual processing, however, the measured irregularity will exceed the limiting irregularity, and a value of I greater than one will be expected.

Yarn Imperfections :

Staple-fiber yarns usually exhibit 3 main types of imperfections: thin places, thick places, and neps. In the USTEO evenness tester, thin and thick places refer to imperfections that are within the measuring sensitivity range (f 100% with respect to the mean value of yarn cross-sectional size). Figure 7.15 shows a relative frequency diagram showing the yarn sensitivity range. Typically, thin and thick places can be of up to one-inch length. Neps are classified as the yarn imperfections, which may exceed the f 100% limit. They are typically of 3 to 10 mm length. Typical values of Uster imperfections are listed in Tables 7.12 through 7.14. Thick places exceeding the 100% limit are determined using the so-called Classimat method.

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  • Yarn Surface Integrity

The critical importance of yarn surface integrity stems from the fact that despite the advanced spinning technology that we witness today, the yarn as spun can not be woven or knitted without some form of treatment to enhance its surface integrity. Millions of dollars are spent every day to apply chemicals to the yarn surface so that it can flow smoothly through the weaving process. These chemicals provide a temporary function and are later disposed or partially recycled. The cost of these chemicals and their byproduct environmental effects clearly justify extensive research in the area of yarn surface to seek ways to improve the inherent surface structure of spun yarns. In practice, yarn surface integrity is typically characterized by two main parameters: abrasion resistance and hairiness. Abrasion is generally defined as the wearing away of any part of the material by rubbing against another surface. Accordingly, the measuring principle of abrasion resistance is normally based on placing a number of parallel threads under a predetermined initial tension, and subjecting these threads to an abrasive solid surface moving (or rotating) at a constant speed. This will exert a constant abrasive force, which continues to act on the yarn surface until the yarn is finally worn out. The abrasion resistance is commonly expressed by the number of abrasive cycles required to break the yarn. Testing of abrasion resistance of staple fiber yarns is often associated with a lack of repeatability of test results. This is largely attributed to the complex variable nature of yarn surface and to the presence of fiber loops and hairs protruding from the surface. Yarn hairiness may generally be defined as the extent of hairs protruding from the yarn body. Two methods are currently used for measuring yarn hairiness:

(i) the hair count method, and

(ii) the hair length method.

In the first method, fibers protruding from the yarn surface are counted by projecting the fiber shadow onto phototransistors. This method is utilized in the Zweigle® hairiness measuring device, which provides values of the number of hairs per meter; hairs extending over lengths from 1 mm to 25 mm can be counted. Obviously, the maximum number of hairs will be detected at the closest distance to the yarn body (1 mm).In the second method, the measuring field is formed by homogenous rays of parallel light; if a yarn lies in this field, only those rays of light scattered by the fibers protruding from the yarn body are detected. This method is utilized in the USTE~® evenness tester. Hairiness in this case is defined by the index H, which is defined as the total length (in cm) of all protruding hairs with reference to a sensoring length of 1 cm. For example, a hairiness value H = 5 will correspond to a total protruding fiber length of 5 cm per 1 cm sensing length. Typical values of Uster hairiness are shown in Tables 7.15 and 7.16.

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