Determination of crimp percentage in warp and weft of a given woven fabric.
When warp and weft yarns interlace in fabric they follow a wavy or corrugated path. Crimp percentage is a measure of this waviness in yarns. Warp and weft crimp percentages are two of the eleven structural elements in fabric construction discussed by Peirce. The relationships between the geometry of a cloth structure and its physical behavior in use are complex. Although much pioneer work has been done there are many unresolved problems still to be investigated.
- To measure the crimp percentage in warp and weft of a given woven fabric.
Percentage crimp is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in the cloth, expressed as a percentage. From the definition of crimp two values must be known, the cloth length from which the yarns is removed and the straightened length of the thread. In order to straighten the thread, tension must be applied, just sufficient to remove all the kinks without stretching the yarn. In practice it is seldom possible to remove all the crimp before the yarn itself begins to stretch.
The standardized tensions recommended in the B.S. Handbook are given below:
|Cotton||Finer than 7 tex||0.75 tex|
|Coarser than 7 tex||0.20 tex + 4|
|Woolen & Worsted||15 to 60 tex||0.20 tex + 4|
|60 to 300 tex||0.07 tex + 12|
|All man-made continuous filament yarns||All counts||Tex / 2|
From those two values we can calculate the crimp percentage with the following formula:
1 – p
c = ———- x 100
Where, c = crimp, l = uncrimped length and p = crimped length.
Five groups of threads selected for test are two warp way and three weft way groups. The mean crimp percentage is calculated warp way and weft way. Rectangular strips are carefully marked on the cloth and each strip cut into the form of a flap. From each strip ten threads will be removed. Removal of threads is as follows: the central part of the first thread is separated from the flap fringe by means of a dissecting needle, but the two extreme ends are left secured. One end is then removed and place in the grip of the tester, and the other end is removed and placed in the second grip. In this way the thread is transferred from the cloth to the crimp tester without loss of twist and with a minimum handling. Several crimp testers are available, Shirley crimp tester is one of them.
- Temperature – 25oC and relative humidity – 67%
- Standard atmosphere: temperature – 20oC and relative humidity – 65%.
- Crimp tester
- Fabric sample
Sample: Cotton woven fabric. Length = 10″.
- At first we have to select the warp or weft way of the fabric. Then we should select the test length of the yarn. Here it is 10″.
- According to test length we will cut the flap of fabric.
- Now a single yarn is to remove from the flap of fabric carefully as discussed in theory.
- One end of the yarn is gripped in the fixed gripper of the machine and the other end is gripped in the other setting the test length.
- Now the tension for the sample is found out from its count and it is set in the machine.
- After that we will apply tension along the yarn length with hand by taking away the other end of yarn far from the first end.
- As soon as the white marl on the tension bar is on the same line of its both sides white mark, we will stop far away the other end.
- The length of the yarn after applying tension is taken from the scale.
- Now from this two lengths crimp percentage is calculated from the given formula.
- In this way at least 10 crimp percentage for warp and 10 for weft is taken and average crimp percentage is calculated from them.
|S/n||Warp Yarn||Weft yarn|
|Crimped length (p)||Uncrimped length (l)||Crimp percentage (c)||Mean||Crimped length (p)||Uncrimped length (l)||Crimp percentage (c)||Mean|
The count of yarn tested is 20 Ne both for warp and weft way. Therefore,
Tex = —————
From the table of standard tension we find required tension = 0.2 x 29.53 + 4 = 9.9 gm. Suppose l = 10.7″. Then,
………………………………..10.7 – 10
Crimp Percentage, c = ———————– x 100
= ——– x 100
- Average warp crimp percentage 6.7% and
- Average weft crimp percentage 10.5%.
We found that crimp% for warp is less than weft. It is because the warp yarns are kept in tension during weaving. Besides they are stronger and better yarn than weft. So they do not extend more. On the other hand weft yarns are kept in low tension and low in quality. So they can extend more. As a result their crimp percentage is more. We should notice that variation in crimp can give rise to faults in fabrics, e.g. reduction in strength, bright picks and diamond barring in rayons, strips in yarn dyed cloths and so on. So we should control it which is also necessary for design fabric to give required extensibility. Since crimp is related to length, it affects the amount of cloth as well as cost of production.
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