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YARN TESTING

 

INTRODUCTION:
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 etc.

SAMPLING:
In order that the results obtained are reproducible and give reliable information about the material,
the sampling must be true and representative of the bulk lot. The sampling procedure should be designed
to take account of and to minimise the known sources of variability such as the variation between
spindles, the variation along the length of the bobbin, etc. The procedure for sampling and the number
of test carried out are given under each characteristic.

AMBIENT CONDITIONS FOR YARN TESTING:


Some textile fibres are highly hygroscopic and their properties change notably as a function of the moisture
content. Moisture content is particularly critical in the case of properties, i.e yarn tenacity,
elongation, yarn evenness, imperfections, count etc. Therefore conditioning and testing must be carried out
under constant standard atmospheric conditions. The standard atmosphere for textile testing involves a
temperature of 20+-2 degree C, and 65+-2% Rh. In tropical regions, maintaining a temperature of 27+-2 degree C,
65+-2%RH is legitimate. Prior to testing, the samples must be conditioned under constant standard
atmospheric to attain the moisture equillibrium. To achieve this it requires at least 24 hours.

TWIST:
"Twist is defined asthe 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:

    1. English count(Ne)
    2. Metric count(Nm)
    3. French count(Nf)

    INDIRECT SYSTEM:

    1. Tex
    2. Denier
    1. Ne : No of 840 yards yarn weighing in One pound
    2. Nm : No of one kilometer yarn weighing in One Kilogram
    3. Nf : No of one kilometer yarn weighing in 0.5 kilogram
    4. Tex : Weight in grams of 1000 meter(1 kilometer) yarn
    5. 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
    1. single end strength testing
    2. 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
    1. Tensile strength -load is applied gradually
    2. 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.

 

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