Guidelines for selection of yarn part 2 12.02.14

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Guidelines for selection of yarn part 2 12.02.14

  1. 1. Yarn Technical Specifications & selection guidelines
  2. 2. Technical Specifications
  3. 3. 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
  4. 4. Yarn Twist  The amount of twist inserted in the yarn can influence many yarn characteristics.  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).
  5. 5. Yarn Twist  Highly twisted yarn is “Lively” and tends to twist upon it and produce “Snarls”.  Fabrics made from highly twisted yarns will possess a lively handle.  In general, the higher the level of twist in the yarn the greater the tendency for the knit fabric to skew or torque.  An increase in the amount of twist produces an increase in the yarn strength, if yarn strength is increase, the fabric strength will be increased.
  6. 6. Yarn Twist  The warp yarns are the yarns with highest twist.  The twist of weft yarns is approximately 4 - 5 % below the twist of warp yarns.  The twist of hosiery yarns is approximately 12 – 15 % below the twist of warp yarns.  A fine yarn requires more twist than a coarse yarn for the same application.
  7. 7. Twist Multiplier ( TM)  The twist multiplier is an expression of the twist level adjusted for yarn count.  The commonly used formula to determine the twist multiplier of yarn for a given yarn count and a given twist level is: TM = TPI / √ Ne  The twist factor is also named twist multiplier in some countries.
  8. 8. Twist &Twist Multipliers  The threshold between weaving and knitting yarn has been determined to be the following twist multipliers: • Combed cotton yarn αe = 3.7 • Carded cotton yarn αe = 3.9  Yarns with twist multipliers below these values have been classified as knitting yarns.  This is the standard adopted by Uster
  9. 9. Twist &Twist Multipliers  The cloths were woven from the 10s cotton yarn. ( W – Warp, F – Filling, and the TM)
  10. 10. Twist Direction
  11. 11. Twist Direction  When the yarn is woven or knitted into a fabric, the direction of twist influences the appearance of fabric and the hand feel.
  12. 12. Twist Direction  Fabric A will be more lustrous than Fabric B, because light reflected by fibres in the warp and weft is in the same direction.  Fabric A will be softer while fabric B firmer.
  13. 13. Twist Direction & Spirality  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.
  14. 14. Twist Direction & Spirality  One of the solutions to solve the problem of Spirality is to coordinate the direction of twist with the direction of machine rotation.  With other factors to give less counterclockwise.  Fabrics coming off the needles of a counterclockwise 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. being similar, yarn of Z twist skew with machines is found rotating
  15. 15. Twist Direction & Strength  No consistent differences in strength for the cloths made from yarns with S-twist and Z- twists yarn
  16. 16. Yarn Twist & Cover Factor  Since thousands of ends or wales are presented side-byside 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.  Differences in yarn twist lead to deviations in yarn diameter. A reduction is yarn twist increases the yarn diameter and decreases the density  The yarns with low twist numbers are bulkier and softer, the fabric produced has a higher cover factor and softer hand.
  17. 17. Yarn Twist & Cover Factor  Knit fabric strength is depends upon yarn strength and stitch (loop) length.
  18. 18. Yarn Twist & Cover Factor  Higher TPI yarn is more rigid and have higher resistance to bending while loop forming in knitting, than Low twist yarn; hence, the radius of their curvature is longer. Hence, we can not have small loop length.  An optimum combination of strength and flexibility can be achieved through many options including a proper level of twist.
  19. 19. Twist & Elongation  Increasing twist factor will leads to an increase of yarn elongation, which in turn the increase of the knitted fabrics elongation.  Good elongation values in the yarn will reduce fabric holes.  Also, the increased twist fabric causes the increase in seam elongation
  20. 20. Yarn Hairiness  The reduction of twist increases the hairiness because the number of protruding fibers increases.  Consequently the pilling tendency is also affected.
  21. 21. The effects of loop length and yarn twist factors on seam performance  Seam elongation was positively affected by loop length and yarn twist factor.  By contrast, seam strength and seam efficiency were negatively affected by loop length and yarn twist factors .  i.e. As the loop length and yarn twist factor increases, seam strength and efficiency decreases.  Increasing yarn twist factor will increase the strength of constituent yarns, which in turn increases the knitted fabric strength. Thus, the increased twist factor will lower seam efficiency.
  22. 22. Yarn twist CV%  In handling large quantities of data statistically, the coefficient of variation (C.V.%) is commonly used to define variability and is thus well-suited to the problem of expressing twist variation.  It is currently probably the most widely accepted way of quantifying irregularity.  It is given by : coefficient variation (C.V.%) = (standard deviation/average) x 100
  23. 23. Yarn twist CV%  The variation of yarn twist CV should not exceed 3.5% to avoid quality problems which can be recognized by the human eye.
  24. 24. Yarn twist for a specific end use  Alpha e is for English count
  25. 25. Summary  Cotton spun yarns for knitting should exhibit good hand or softness, therefore need less twist.  This lower twist leads to softer yarn and fabric.  Yarn Torque or liveliness should be at a minimum to help prevent excessive fabric shrinkage, skew, and torque.
  26. 26. Summary  The formation of spirality may be prevented by various yarn-related methods.  Some of these methods include : a) low-twist-lively yarns; b) balanced plied yarns can be preferred, c) and S-twist and Z-twist single yarns are used at alternate feeders, respectively
  27. 27. Yarn Twist We will discuss other parameters in next edition. Until then, Bye Bye. Note: This is a Training material prepared for internal training purpose.

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