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Chapter 4

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Transcript

  • 1. CHAPTER FOUR:
    • SELVEDGE
  • 2.  
  • 3. 4.1 Requirements of Selvedges
    • The basic function of selvedge is to lock the outside threads of a piece of cloth and so prevent fraying.
    • It is easy to achieved if the weft yarn is inserted continuously from one package, such as a pirn on a conventional shuttle loom, because, each time the package carrying shuttle reverse its direction, a hairpin of weft yarn will form round the last thread in the cloth provided that this end changes its position between picks.
    • Requirement for selvedge:
    • Strong enough to withstand the strains during finishing process.
    • The selvedge should have a neat and uniform appearance.
    • Uniform to allow garment manufacture to line-up one edge of the fabric; layer upon layer so that many layers can be cut simultaneously and accurately.
  • 4. 4.2 Types of Selvedges
    • 4.2.1 Conventional Selvedge
    • Produced on a conventional shuttle loom.
    • Required strength for the post-weaving process to withstand the rubbing-action of the shuttle as it enters and leaves the warp shed.
    • Strength is also necessary to counteract the pulling of the weft thread as the shuttle traverses the loom, and this function must be performed by the outside ends of the fabric.
  • 5. Conventional Selvedge
    • Uniformity of appearance may be a problem if the outside ends in the fabric do not change position every pick.
    • Happens in twill and satin-type weaves.
    • If the appearance of the selvedge is unacceptable when one of these weaves forms the ground structure, then it will necessary to use some special technique of threading up or to introduce a plain-weave selvedge for a width of up to 1 cm at each side of the fabric.
    • Alternatively, it may be quite satisfactory to make only the outside end in the fabric change position after each pick and it is known as a catch cord or catch end.
  • 6. Conventional Selvedge
    • The fabric surface must be absolutely uniform from one side of the cloth onto the other.
    • It is thus not possible to vary the construction of the fabric in any way in the selvedge.
    • Under these circumstances, a thread of strong monofilament yarn or a length of flexible wire is used as the catch cord.
    • This thread, which is fastened to a point on the frame at the back of the loom, changes its position after each pick.
    • However, it extends for only 1-2cm into the fabric.
    • As the cloth is drawn forward, it will slide of the thread to leave a small loop of weft at the side of the fabric, but there will not be an increase in thickness.
    Small Loop of Weft at the Side of the Fabric
  • 7. 4.3 Tuck and Fringe Selvedges
    • There are several instances in which special selvedge consideration become necessary because the yarn is severed, namely:
      • at the centre of a wide fabric that is cut at the loom so that several narrow fabrics are produced side by side.
      • at both of a fabric in which the weft is cut after each pick length.
      • at one side of the fabric in which the weft is cut after every two picks.
      • Tuck and fringe selvedges are most commonly used under these circumstances.
  • 8. 4.3.1 Tuck Selvedge
    • Probably the more effective in preventing fraying and yet achieving the necessary standards in strength and appearance.
    • A trail of yarn, which extends beyond the last selvedge end, is pulled into shed formed for the next pick.
    • Provided that the last end changes position after each pick to catch loop, a selvedge is produced that is acceptable for a wide range of uses.
    • In many instances, it is necessary to reduce the number of ends/cm in the selvedge to compensate for the increase in weft density.
  • 9. 4.2.2.2 Fringe Selvedge
    • The most successful in preventing fraying and providing strength but as successive picks are beaten-up the weft tails are made to point up and down alternately by the crossing end so that the fringe does not appear.
      • Leno Structure with the Outside Ends
  • 10.
    • The simplest fringe selvedge requires only a chain, illustrated below:
    • The standard end is held in a fixed raised position by a loop. And the crossing end passes through the eye of a heald on the back shaft and also through a ring mounted on the end of the chain.
    • When the back heald is down, the chain is pulled round the standard end, but, on the other pick, when the heald is raised, the chain pulls the crossing end to the opposite side of the standard end.
  • 11.
    • Another method, requires an additional mechanism, uses wires with holes in their upper end as the diagram below:
    • These methods control the crossing ends that are in the raised position when the weft is inserted.
    • Between picks, they fall and then rise again in the same vertical plane by means of cam operation.
    • In the meantime, the standard ends that are permanently in the down position are given a side traverse by a second cam when the crossing ends are in their lowest position.
    • The direction of the side traverse is reversed after each pick.
  • 12.
    • Leno structure by twisting the outside ends around one another
    Leno Structure by Twisting the Outside Ends Disc Carrying Two Bobbins Disc, Fell and Cloth
  • 13.
    • Leno structure by locking the outside ends with an additional fine monofilament thread, which is stitched into the edge of the fabric:
    • Requires an additional mechanism and it’s unpopular.
    • The sewing needle must oscillate once every one or two picks depending on the openness of the weft set.
    • The locking needle oscillates vertically with the same frequency from a second cam to catch and hold the sewing thread.
    • The fringe selvedge is basically untidy, and a neater appearance is generally essential.
    • Such an improvement can be achieved if all the protruding weft threads are cut off to the same length and as near to the body of the fabric as possible.
  • 14.