Sunday, October 28, 2018

YARN PARAMETERS, YARN SPECIFICATION

 YARN PARAMETERS:

The different parameters of yarn are given below:

YARN COUNT (Linear density of yarn):

The yarn count (linear density of yarn) express the fineness of yarn. It is a number which indicates the length per unit weight or weight per unit length of yarn. There are two systems of expressing linear density or fineness of yarn.

Direct system:

In this system, length of the yarn remains constant and weight of yarn varies according to the fineness of yarn. Tex, denier, and lbs. are some examples of this system. As the count increases, the yarn becomes coarser in this system.

Indirect system:

In this system, the weight of the yarn remains constant and the length of the yarn varies according to the fineness of yarn. Ne, Nm, lea are some examples of the indirect system. As the count of yarn increases, the yarn becomes finer.

NOMINAL COUNT OF YARN:

The concept of the nominal count is very important. It can be understood by some examples. Suppose we have to spin 10s count of yarn. All the required parameters are followed to produce this count of yarn. But count resulting after spinning will not be equal to 10scount. Sometimes it may be slightly finer say 10.2s and sometimes it may be slightly coarser say 9.8s count. This resulting count will be written as 10s count. Therefore this count will not be an actual count of yarn. It is called nominal count of yarn. “Thus we can say that a nominal count is a whole number (count of yarn) which may slightly differ from its actual count”.

RESULTANT COUNT:

When the two or more plies of same or different counts are twisted together or grouped together, a new count of yarn results.  The term resultant count is used for these muli plies yarn. The  resultant count of yarn is calculated as follows:
If R1 and R2 are the counts of two different yarn( Yarn counts are in the indirect system). After doubling or grouping together the resultant count R will be:

If these yarn counts are in the direct system 
then the resultant count will be:

CORRECTED COUNT:

This is a very interesting and important factor in respect of double plies yarn or multiplies yarn. The concept of corrected count can be understood by example. Suppose we take a two plies yarn say 2/ 40s means two plies of 40s. Its resultant count should be 20s.  But the contraction in length of yarn occurs due to twisting during doubling. Thus weight per unit length of yarn increases and the resultant count becomes coarser. If we require 20s resultant count after doubling, the count of each ply should be more than 40s. It has been observed that almost 10% contraction in length occurs during doubling. Therefore each ply should be 10% finer than 40s. When the doubling of yarn is done, the contraction in the yarn length is measured and the count of single ply yarn is selected according to the required resultant count and contraction % in length during doubling.

Before placing the purchase order of any yarn, the customer should ask to send the samples of a yarn from two or more yarn suppliers. The yarn count test should we conducted in-house or outside lab. Now you can make a comparison of yarn count among these samples of yarn. The yarn showing minimum yarn count variation should be selected for purchasing.
Now ask your supplier for the test report and see the Count CV% (coefficient of variation percentage). Minimum count CV% means better the yarn. Less than 3 count CV% is considered as a good result.
If there is much count variation in the yarn, it will be reflected in the fabric, the GSM of fabric may vary due to count variation. The areas of thick and thin fabric may result in the same piece of the fabric. Lines of the thick end and thick pick may appear continuously in the fabric. The purchaser should be fully aware of this parameter.

 The appearance of the yarn:

The appearance of yarn is judged visually by naked eyes. It also plays a decisive role in the evaluation of yarn quality. How the yarn looks visually, it directly reflects in the fabric. The user should always be conducted with the appearance test of yarn to be purchased.
An appearance board winder machine is used to see the appearance of yarn. This board has black colour on one side and white colour in another side. The yarn is wrapped onto the board with the help of board winding machine in such a way that yarn coils should not be overlapped to each other. Now various type of defects are observed.
The defects to be observed to decide the yarn appearance are given below:
·       Neps
·       Slubs
·       Knots
·       Thin places
·       Thick places
·       Hairiness
·       Broken seeds
·       Immature fibres
·       Cleanness
·       Colour
If all above yarn regularities are minimum in the yarn, the yarn appearance will be the best. The purchaser can also compare two yarn of the same count on the basis of the yarn appearance and can select the best one.
The purchaser should always keep in his mind that effect of the yarn appearance directly reflected on the fabric surface.
 If the yarn appearance is poor, the fabric appearance will also be poor.
Since in the yarn dyed fabric, the yarn mercerizing is not done normally, therefore immature fibres present in the yarn result as undyed fibres on the yarn surface.
If there are more broken cotton seeds are present, it also affects the dyeing cost up to some extent.
The thick places, thin places and hairiness level of yarn directly reflect on the fabric surface. These yarn irregularities also affect loom efficiency and fabric quality. The hairiness of the yarn causes pilling on the fabric surface.

Tensile strength of the yarn:

It is the most important parameter of yarn. The tensile strength of the yarn is the force required to break the yarn. The tensile strength of the yarn is a very important property of yarn. It directly affects the performance of yarn during weaving and another process. Since yarn goes under tension during different processes of weaving and jerk applies to the yarn so that the yarn should have enough strength to bear this tension and jerk. There are two methods used to test the strength of the yarn:
·       Bundle strength
·       Single end strength

Bundle strength:

This method is used in spun yarn only. Bundle strength does not give correct value of yarn strength but it helps to compare two yarns of the same count. If we have to select the best one yarn between two same counts of yarn, bundle strength is very useful.
A skein or lea of 120 yards (having 80 wraps of 1.5 yards) is prepared. It is broken with the help of vertical lea strength tester and breaking force (tensile strength) is recorded in the pound, Kg or Newton. Now count strength product of yarn is calculated as follows:
C.S.P= Count of yarn (Ne) x tensile strength (breaking strength of lea in a pound)
The yarn having more CSP is better than the yarn of lower CSP for the same count of yarn.

Single end strength:

In this method, the exact tensile strength of yarn is measured. It expresses the force required to break the single strand of yarn. It is usually measured in grams or centi newton. The tensile strength of the yarn is reflected in the tensile strength of the fabric. The fabric woven from the yarn having good tensile strength shows good tensile strength. It directly affects the productivity and quality of the fabric.
 The CV% of the tensile strength of the yarn is determined. The yarn having CV% less than 3 is considered as good tensile strength. 

Yarn elongation:

Elongation percentage at the time of tensile strength testing is also recorded. Elongation of yarn plays a very important role during weaving. If the yarn has no elongation, it is almost impossible to weave the fabric by this yarn.
The difference between stretched length at the break after applying force to an original length of yarn expressed as a term of percentage is called elongation % at break. If a yarn has a higher elongation at break means it has better ability to bear the tension and jerk during weaving and other processes.

Tenacity: 

it is the tensile strength of yarn. The braking force per unit linear density of a yarn is called tenacity of yarn. It is expressed as grams-force per tex (gf//tex). It gives the real tensile strength of a yarn. As the tenacity increases, the strength of yarn becomes better.

Rupture per kilometer (R.K.M): 

The length of yarn in kilo-meters” at which yarn begin to break due to its own weight when the yarn is hung vertically. It is equivalent to breaking load in grams/ tex. The yarn having the value of RKM more than 20 is considered as the best yarn.

Degree of Hairiness:

The degree of hairiness tells the number of fibres protruding from the base of yarn. The hairiness H measurement unit of the Uster Tester determines the hairiness of approximately 1centimetre length of yarn.
This value is specified as an average value of hairiness over the total test length. If the yarn has 2.5 hairiness value, 2.5 centimeters fibres desist from the yarn at a yarn length of 1 centimeter of yarn. If the yarn has low hairiness value, it means it has lower chances to the generation of pilling. It will give a better performance during weaving and other processes.


Yarn twist (Degree of twist):

The yarn twist is a very important parameter. The amount of twist required to hold the fibers and yarns together depends on the diameter or size of yarns. The coarse yarn needs less amount of twist to hold the fibres together in the yarn and fine yarn needs more amount of twist to hold the fibres together in the yarn. The amount of twist or degree of twist seriously affects the performance of yarn. If the yarn has less twist than standard then fibre slippage may occur. The yarn having low twist posses low strength. There may be the chance of pilling formation due to low twist. It will give soft feel and touch. If the yarn has more twist than standard. The yarn will show good strength but it will give rough feel and touch. There may be a chance of snarling too. The amount of twist is expressed in turns/inch.
·       T.P.I. – turns per inch In the yarn.
Type of twist: it shows the direction of twist. There are two types of twist direction possible which are:



·       S-twist:

The clockwise twist of yarn is called s-twist. The inclination of fibres in yarn looks similar to the middle part of the English letter “S”.

·       Z-twist:

The anticlockwise twist of yarn is called z-twist. The inclination of fibres in the yarn looks similar to the middle part of English letter Z.

EVENNESS OF YARN:

Yarn evenness is a characteristic of yarn which denotes the level of variation in yarn linear density or mass per unit length of yarn. Evenness of yarn refers the yarn irregularities is respect of yarn count along its length. Evenness of staple spun yarn is judged because staple spun yarn has count variation due to many reasons. The continuous filament yarn has no variation in yarn count so that evenness is not an issue for continuous yarns. The evenness of yarn is a very important quality aspect of yarn because it directly affects the fabric to be woven. Count irregularities directly appear on the fabric surface. A yarn having poor evenness has more thick and thin places along the yarn length, while an even yarn has little variation in the count or linear density along the yarn length. Since twist tends to accumulate in the thin places in the yarn so that irregularity in yarn linear density also causes variations in twist along the yarn length. This twist variation also affects the yarn diameter. Following parameters are observed during testing of evenness of yarn:
·       Uster %
·       Neps
·       Thick places
·       Thin places
·       Total imperfection
Uster %: 

This is a measure of variation of linear density or count of yarn in term of percentage. It shows the total imperfections in percentage. If a yarn has lower uster %. It means this yarn has a lower imperfection. The quality of this yarn will be better.

Neps:  

A very short thick place in the yarn (a small yarn defect containing a length of 2milimeter, the diameter of 3 times or more at a standard setting of 200%) is called nep. It is made of unopened  fibres , broken seed coated by fibres or  a trash particle. It can be + 200% thicker than the average diameter of yarn. The increase for neps is calculated to a reference length of 1mm. They can be a bunch of entangled fibres commonly not bigger than pin ball head.

Thick place:  

A place in the yarn having yarn diameter in excess of +50% of the average yarn diameter and the length 8-12 mili meters is considered as the thick place. This yarn defect affects fabric appearance.
Thin place: 

A place in the yarn having yarn diameter -50% or more than average diameter and any length is considered as the thin place. This is a very serious defect. Any thin place causes end break during weaving. The number of thin places per 1000 meters should be in the range of 1- 2 .  Higher number of thin places creates serious troubles during weaving. The productivity of loom decreases. The fabric quality also influences.

Total Imperfections:
Imperfections is the total of thin, thick places and neps in 1000 metres of yarn. The yarn having total imperfection less than 100 is considered as the good quality of yarn.


A yarn user should always be aware of above yarn parameter. Before purchasing any yarn, the sample should be tested in-house or outside the lab. User should always ask to send the yarn sample to two or more yarn supplies along with detailed yarn test report and a comparison chart should be made before purchasing the yarn. Now the user can select the best option in available samples and can place his order.
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3 comments:

  1. Really nice info respect to yarn quality parameters.

    Here i am noticed need some correction to be done,as listed below.

    1.Count CV% - Technically & commercially yarn count C.V% should be less than 1.5% , that yield good result.

    2.Direction of twist denotes wrong,for S-twist yarn spun in counter-clockwise and Z-twist will be in clock wise.

    ReplyDelete
    Replies
    1. When the fibres strand is twisted in the clock wise direction, s - twist yarn is manufactured, in the z- twist yarn, the turns of twist inserted by rotating fibres strand in anticlockwise direction

      Delete

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