Yarn Evenness

Non-uniformity in variety of properties exists in yarns. There can be variation of twist.,bulk, strength, elongation , fineness etc.

Yarn Evenness deals with the variation in yarn fineness. This is the property, commonly measured as the variation in mass per unit length along the yarn, is a basic and important one, since it can influence so samy other properties of the yarn and of fabric made from it. Such variations are inevitable, because they arise from the fundamental nature of textile fibres and from their resulting arrangement.

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

Yarn Testing

Yarn occupies the intermediate position in the manufacture of fabric from raw material. Yarn results are therefore essential, both for estimating the quality of rawmaterial and for controlling the quality of fabric produced. The important characteristics of yarn being tested are:

• Yarn Twist

• Linear Density

• Yarn Strength

• Yarn Elongation

• Yarn Evenness

• Yarn Hairiness 

Yarn Twist

Twist is defined as the spiral disposition of the components of yarn, which is generally expressed as the number of turns per unit length of yarn, e.g turns per inch, turns per meter, etc.

• Twist is essential to keep the component fibres together in a yarn.

• The strength, dyeing, finishing properties, the feel of the finished product etc. are all dependent on the twist in the yarn.

• With increase in twist, the yarn strength increases first , reaches a maximum and then decreases.

• Depending on the end use, two or more single yarns are twisted together to form "plied yarns" or "folded yarns" and a number of plied yarns twisted together to form "cabled yarn".

• Among the plied yarns, the most commonly used are the doubled yarns, wherein two single yarns of identical twist are twisted together in a direction opposite to that of the single yarns.

• Thus for cabled and plied yarns, the direction of twist and the number of turns per unit length of the resultant yarn as well as of each component have to be determined for a detailed analysis. Direction of twist is expressed as "S"-Twist or "Z"-Twist.

• Direction depends upon the direction of rotation of the twisting element.

• Twist take up is defined as, The decrease in length of yarn on twisting, expressed as a percentage of the length of yarn before twisting. 

Linear Density or Count of Yarn

The fineness of the yarn is usually expressed in terms of its linear density or count. There are a number of systems and units for expressing yarn fineness. But they are classified as follows

 

Direct System

• English Count (Ne)

• Metric Count (Nm)

• French Count (Nf)

Indirect System

• Tex

• Denier

English Count (Ne)

No of 840 yards yarn weighing in One pound

Metric Count (Nm)

No of one kilometer yarn weighing in One Kilogram

French Count (Nf)

No of one kilometer yarn weighing in 0.5 kilogram

Tex

Weight in grams of 1000 meter(1 kilometer) yarn

Denier

Weight in grams of 9000 meter(9 kilometer) yarn

• For the determination of the count of yarn, it is necessary to determine the weight of a known length of the yarn. For taking out known lengths of yarns, a wrap-reel is used. The length of yarn reeled off depends upon the count system used.

• Another factor which determines the length of yarn taken for testing is the type of balance used. Some balances like quadrant balance, Beesley's blanace have been specially designed to indicate the yarn count directly from tests on specified short lengths of yarn and are very useful for determining the counts of yarn removed from the fabrics. The minimum accuracy of balance required is 0.001mg

• One of the most important requirements for a spinner is to maintain the average count and count variation within control. The term count variation is generally used to express variation in the weight of a lea and this is expressed as C.V.%. This is affected by the number of samples and the length being considered for count checking. While assessing count variation, it is very important to test adequate number of leas. After reeling the appropriate length of yarn, the yarn is conditioned in the standard atmosphere for testing before it's weight is determined.

• The minimum number of sample required per count is 20 and per machine is 2.

Yarn Strength and Elongation

Breaking strength, elongation, elastic modulus, resistance abrasion etc are some important factors which will represent the performance of the yarn during actual use or further processing. Strength testing is broadly classified into two methods single end strength testing skein strength or Lea strength

Tensile Strength of Single Strands of Yarn

• During routine testing, both the breaking load and extension of yarn at break are usually recorded for assessing the yarn quality. Most of the instruments record the load-elongation diagram also.

• Various parameters such as initial elastic modulus, the yield point, the tenacity or elongation at any stress or strain, breaking load, breaking extension etc can be obtained from the load-extension diagram

• Two types of strengths can be determined for a yarn

Tensile Strength

Load is applied gradually

Ballistic Strength

Applying load under rapid impact conditions

• Tensile strength tests are the most common tests and these are carried out using either a single strand or a skein containing a definite number of strands as the test specimen.

• An important factor which affects the test results is the length of the specimen actually used for carrying out the test. The strength of a test specimen is limited by that of the weakest link in it.If the test specimen is longer, it is likely to contain more weak spots, than a shorter test specimen. Hence the test results will be different for different test lengths due to the weak spots.

• The amount of moisture in the yarn also influences the test results. Cotton yarn when fully wet show higher strength than when dry, while opposite is the case with viscose rayon yarns. Hence, to eliminate the effect of variation due to moisture content of the yarn, all yarn strengrth tests are carried out, after conditioning in a room where the standard atmospheric condition is maintained.

• The rate of loading as determined by the "time-to-break", which is the time interval between the commencement of the application of the load and the rupture of the yarn, is an important factor , which determines the strength value recorded by using any instrument. The same specimen will show a lower strength when the time-to-break is high, or higher when the time-to-break is low.

• The instruments used for determining the tensile strengh are classified into three groups, based on the principle of working.

CRT - Constant Rate of Traverse

CRE - Constant Rate of Extension

CRL - Constant Rate of Loading

• In the instruments of CRE type, the application of load is made in such a way that the rate of elongation of the specimen is kep constant. In the instruments of the CRL type,the application of load is made in such a way that the rate of loading is constant througout the duration of the test. This type of instruments are usually preferred for accurate scientific work. In the CRE and CRL types of instruments, it is easy to adjust the "time-to-break" while this adjustment is not easy in the CRT types of instruments.

• The uster Tensorapid applies the CRE principle of tensile testing. Constant Rate of Extension describes the simple fact that the moving clamp is displaced at a constant velocity. As a result, the specimen between the staionary and the moving clamp is extended by a constant distance per unit of time and the force required to do so is measured. Apart fron single values, this instrument also calculates mean value coefficient of variation and the 95% confidence range of maximum force, tenacity,elongation and work done

• The total coefficient of variation describes the overall variability of a tested lot, i.e the within-sample variation plus the between-sample variation. If 20 individual single-end tensile test are performed on each of ten bobbins or packages in a sample lot, the total coefficient of variation is calculated from the pooled data of the total number of tests that were carried out.

• In tensorapid, the breaking tenacity is calculated from the peak force which occurs anywhere between the beginning of the test and the final rupture of the specimen. The peak force or maximum force is not identical with the force measured at the very moment of rupture. The breaking elongation is calculated from the clamp displacement at the point of peak force. The elongation at peak force is no identical with the elongation at the very moment of rupture(elongation at rupture).

• The work to break is defined as the area below the stress/strain curve drawn to the point of peak force and the corresponding elongation at peak force. The work at the point of peak force is not identical with the work at the very moment of rupture.

• To compare tensorapid test results with other results,

a) A measurement must be performed according the CRE princple

b) Testing speed must be exactly 5 m/min

c) The gauge length or the length of the specimen should be 500 mm

d) The pretension should be 0.5 cN/tex

• There are two fundamental criteria which affect the compatibility between different measurements of tensile yarn properties.

a) Testing conditions, i.e the testing principle(CRE,CRL),Testing speed, gauge length, and pre-tensioning.

b) The second criteria,which also affects the magnitude of the differences, relates to the specific stress/strain characteristic of the yarn itself, which is determined by the fibrous materials, the blend ratio, and the yarn construction.

Skein Strength or Lea Strength

The skein breaking strength was the most widely used measure of yarn quality in the cotton textile industry. The measurement of yarn quality by this method has certain drawbacks. Firstly, in most of the subsequent processing, such as winding, warping or weaving, yarn is used as single strand and not in the form of a skein except occasionally when sizing ,bleaching, mercerising or dyheing treatments are carrried out on hanks. Secondly, in the method used for testing skein strength, the rupture of a single strand at a weak place affects the result for the whole skein. Further, this method of test does not give an indication of the extensibility and elastic properties of a yarn, the characters which play and important role during the weaving operations. However, since a large size sample is used in a skein test as against that in a single strand test, the sampling error is less. The skein used for strength test can be used for determination of the linar density of the yarn as well.

• In addition to the factors influencing the yarn strength, the size of the skein(lea) will affect to a large extent the strength recorded. The usual practice is to use a lea(120 yards) of yarn prepared by winding 80 turns on a wrap-reel having a perimeter of 1.5 yards(54 inches), so that during a test, there are 160 strands of 27 in.(") length. There are different systems in use. But the actual breaking strength recorded on the machine would depend on the type of skein used as both the number of strands and test length may differ. The instruments most commonly used for this test is CRT type, where the bottom hook moves at 12 inches per min.

• After findingout skein strength, broken skeins are also weighed to determine the linear density. The most common skein used is the lea and the results of lea strength tests are expressed as C.S.P., which is the product of the linear density(count)of the yarn in the English system (Ne) and the lea breking strength expressed in lbs. In view of the fact that C.S.P. is much less dependent on yarn count than on strength, especially when count diffferences are small, C.S.P. is the mostg widely used measure of yarn qauality.

Yarn Hairiness

Yarn hairiness is a complex concept, which generally cannot be completely defined by a single figure. The effect of yarn hairiness on the textile operations following spinning, especially weaving and knitting, and its influence on the characteristics of the product obtained and on some fabric faults has led to the introduction of measurement of hairiness.

Facts About Yarn Hairiness

• Hairiness occurs because some fibre ends protrude from the yarn body, some looped fibres arch out from the yarn core and some wild fibres in the yarn.

• Torsion rigidity of the fibres is the most important single property affecting yarn hairiness. Other factors are flexural rigidity, fibre length and fibre fineness.

• Mixing different length cottons-No substantial gain in hairiness. Although the hairiness of a yarn could be reduced to some extent by the addition of a longer and finer cotton to the blend. The extent of reduction is not proportional to the percentage of the longer and finer component. This is probably due to the preferential migration of the coarser and shorter component, which has longer protruding ends, from the yarn body. The addition of wastes to the mixing increases the yarn hairiness; the effect of adding comber waste is greater than that of adding soft waste.

• Blending-not a solution to hairiness. The blended yarns are rather more hairy than expected from the hairiness of the components; a result similar to that found in cotton blends. This may be due to the preferential migration of the shorter cotton fibers; a count of the number of protruding ends of both types of fiber shows that there is more cotton fiber ends than expected, although the difference is not very great.

• The number of protruding ends is independent of twist, whereas the number of loops decreases when the yarn twist increases because of a greater degreee of binding between hte fibres owing to twist. The number of wild fibres decreases only very slightly with twist because of their position on the yarn periphery.

• The proportion of fiber ends that protrude from the yarn surface, counted microscopically has been found to be about 31% of the actual number of ends present in the yarn.

• If the length of the protruding fibre ends as well as that of the loops is considered, the mean value of the hairiness increases as the cross-sectional area increases and decreases with the length of the loops. The hairiness is affected by the yarn twist, since an increase in twist tends to shorten the fibre ends

• Wild fibres are those for which hte head alone is taken by the twist while the tail is still gripped by the front drafting rollers.

• Fibre length influences hairiness in the sense that a greater length corresponds to less hairiness.

• Cotton yarns are known to be less hairy than yarns spun from man-made fibres. The possible reason for this is the prifile of the two fibres.Because of taper, only one end, the heavier root part of the cotton fibre, tends to come out as a protruding end in a cotton yarn. With man-made fibres, both ends have an equal probability of showing up as protruding ends.

• Combed yarn will have low yarn hairiness, because of the extraction of shorter fibres by the comber.

Different Methods Used to Measure Hairiness

Uster Hairiness Index

This is the common method followed in India. The hairiness index H corresponds to the total length of protruding fibres within the measurement field of 1cm length of the yarn.

Zweigle Hairiness Index

This zweigle hairiness measurement (S3) gives the number of protruding fibres more than 3 mm in length in a measurement length of one meter of the yarn.

From the above you can infer that Uster hairiness index give the total length of hairs whereas zweigle hairiness testers give the absolute number of fibres. Though the later measurement is more accurate, most of the Indian spinners are still following Uster hairiness index only.

The factors effecting hairiness can be sub divided into 3 major components.

a) The fibre properties

b) Yarn parameters

c) Process parameters

a) The Fiber Properties

Fibre length, Uniformity ratio, Micronaire and short fibre content are the properties exerting high influence on hairiness. Among the above the length and short fibre content exerting major influence. For a particular count, higher length of fibre leads to lesser hairiness and high short fibre content leads to high hairiness.

b) Yarn Parameter

Hairiness is dependent on the number of fibres present in the cross section of the yarn. Hence coarser yanrs have more hairiness compared to finer yarns.

The yarn twist is another major factor and higher twists lead to less hairiness up to a certain extent. This is the main reason while hosiery yarns normally have high hairiness compared to warp yarns.

However in a mill condition, the fibre parameters and yarn parameters cannot be adjusted. Hence the next topic, process parameters, assumes very high significance, as this is the only available option at the mill level to reduce the hairiness.

c) Process Parameter

The preparatory machines do not have a big influence on hairiness. The Speed frame, Ring frame and the Cone winder are the only machines to be attended for reduction in hairiness.

Ribbon Yarn

Ribbon or Tape Yarn is a kind of novelty yarn. It is made of ribbon, but generally not the kind of ribbon used in sewing and millinery. Rather, they are ribbons made especially for knitting or crocheting, with some in a tubular form, some woven flat, and some similar in appearance to bias tape. Ribbon yarns can be composed of many materials, from synthetics to silk to plant fibers.

Ladder Yarn

Ladder Yarn or Train tracks yarn is a type of novelty yarn. It is constructed like ladders, with a horizontal stripe of material suspended between two thinner threads, alternating with gaps. Sometimes a contrasting strand is fed through the gaps to produce another look.

Flamme Yarn

Flamme Yarns are a kind of novelty yarn. It is generally a loose or untwisted core wrapped by at least one other strand. The extra element can be a metallic thread, or a much-thicker or much-narrower strand of yarn, or yarn that varies between thick and thin. Some companies have come to put twin yarns on the market to show off combinations of one regular yarn and novelty yarns in assorted colors or even two different types of novelty yarns.

Eyelash Yarn

Eyelash Yarn is a type of novelty yarn. It appears as a thread base, with several long strands spaced at even intervals that jut out at an angle from the main strand. The long strands, or hair, can be metallic, opalescent, matte, or a combination of types. The hair can be curly or straight and can sometimes be two different lengths. Prominent types are composed of 100% polyester with a straight and relatively short hair. Because of its thinness, eyelash is normally carried along with another, plainer yarn to add visual interest to the primary yarn.

The texture and composition of such yarns have been explored by many companies, and there are innumerable variations of eyelash yarns, such as pigtail or ponytail, which have a thicker base and what appear to be flags tied onto the base strand at even intervals, or fur, in which the base has a more frequently-occurring or thicker grouping of hairlike strands which, in the finished fabric, will be hairy and have the general aspect of faux fur.

Novelty Yarn

Novelty Yarns are yarns with an interesting texture or other notably unusual features that distinguish them from ordinary yarn like cotton and wool. Typically these involve at least one or two strands of regular yarn twisted together with something else to make an interesting texture, and are frequently made from synthetics such as nylon, but can also be composed of natural fibers.

Very often, Novelty Yarns will involve frequent color change. Most often these will be obtained through the print process, in which a fiber will have different colors through a dyeing process. Sometimes the color will come through the sequence in which different colors are spun together. In some yarns the same process is used, but at the same time the color repeats are long enough to enable a self-striping feature. If the proper number of stitches is cast, then stripes will appear as the yarn is knitted into a garment. Sock yarn companies have evidently taken a great interest in self striping yarn. Such yarns have a wide array of different effects that can be obtained by knitting the yarn in the round over the number of stitches normally cast for a sock.

Texturized Yarn

Texturized Yarns are made by a process of air texturizing (sometimes referred to as taslanizing), which combines multiple filament yarns into a yarn with some of the characteristics of spun yarns.

Filament Yarn

Filament Yarns consists of filament fibers twisted together. Thicker monofilaments are typically used for industrial purposes rather than fabric production or decoration. Silk is a natural filament, and synthetic filament yarns are used to produce silk-like effects.

Overview of Yarn

Yarn is a long continuous length of interlocked fibers, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and ropemaking. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or other lubricants to withstand the stresses involved in sewing. Embroidery threads are yarns specifically designed for hand or machine embroidery.

Structure of Yarn

Spun Yarn is made by twisting or otherwise bonding staple fibers together to make a cohesive thread. Twisting fibers into yarn in the process called spinning can be dated back to the Upper Paleolithic, and yarn spinning was one of the very first processes to be industrialized. Spun yarns may contain a single type of fiber, or be a blend of various types. Combining synthetic fibers (which have high strength, artificial lustre, and fire retardant qualities) with natural fibers (which have good water absorbance and skin comforting qualities) is very common. The most widely used blends are cotton-polyester and wool-acrylic fiber blends. Blends of different natural fibers are common too, especially with more expensive fibers such as angora and cashmere.

Measurement of Yarn

Yarn quantities are usually measured by weight in ounces or grams. In the United States, Canada and Europe, balls of yarn for handcrafts are sold by weight. Common sizes include 25g, 50g, and 100g skeins. Some companies also primarily measure in ounces with common sizes being three-ounce, four-ounce, six-ounce, and eight-ounce skeins. These measurements are taken at a standard temperature and humidity, because yarn can absorb moisture from the air. The actual length of the yarn contained in a ball or skein can vary due to the inherent heaviness of the fiber and the thickness of the strand; for instance, a 50 g skein of lace weight mohair may contain several hundred meters, while a 50 g skein of bulky wool may contain only 60 meters.

Color of Yarn

Yarn may be used undyed, or may be colored with natural or artificial dyes. Most yarns have a single uniform hue, but there is also a wide selection of variegated yarns:

• Heathered or Tweed : Yarn with flecks of different colored fiber

• Ombre: Variegated yarn with light and dark shades of a single hue

• Multi-Colored : Variegated yarn with two or more distinct hues (a "parrot colorway" might have green, yellow and red)

• Self-Striping : Yarn dyed with lengths of color that will automatically create stripes in a knitted or crocheted object

• Marled : Yarn made from strands of different-colored yarn twisted together, sometimes in closely-related hues