Measurement of crimp percentage:
To measure the yarn crimp two values are must be known, the cloth length from which yarn is removed and the straightened length of the thread. In order to straighten the yarn, some amount of tension must be applied just sufficient to remove all the crimps without stretching the yarn. The standardized tensions as per British standards.
The principle of yarn crimp determination is very simple. With a fine pen and rule, lines are drawn on a piece of cloth at a known distance. Some of the threads raveled out the yarns are straightened without stretching and the stretched length is noted and from that, the crimp is calculated. The difficulty lies in the straightening of the yarn without stretching it. To do this, the following three methods are available:
Straighten by hand: This is inaccurate since we do not know the force applied.
Straighten by standard weight: This is satisfactory if we know the weight to use.
Determine the straightened length from the load-elongation curve: This is the most accurate method.
Specimen Preparation:
Rectangular stripes are carefully marked on the cloth and each strip is cut in the form of a flap as shown in the fig. below. From each strip, 10 threads will be removed. Normally 3 strips in warp way and 3 strips in weft way are cut.
Removal of the Threads from the Flap:
The central part of the first thread is separated from the flap by means of a needle, but the two extreme ends are left secured. One end is then removed and placed in the clamp of the tester and the other end is removed and placed in the second clamp. By this method, there will be no loss in the twist of the yarn and also due to minimum handling, there will be no stretch in the yarn.
Several crimp testers such as W.I.R.A., Shirley, and the manra are available and here Shirley's crimp tester is taken for discussion.
Shirley Crimp Tester:
The instrument consists of a scale fixed on the base, V grooves are provided to support the balancing head, and a mirror at the other end. At one end of the balancing head, a fixed clamp is provided and at its other end on the frame, index lines are marked. Another movable clamp is provided on the base and can be slid over the scale. Tension weight is provided on the balancing head to change the tension according to the yarn count. The tension scale is marked in two ranges 0-3 gms and 0-175gms.
Procedure:
The counts of the warp and weft yarns are first determined and the correct tension is calculated. The sliding weight on the balancing head is adjusted to the required tension. The yarn sample is prepared as above and one end of the yarn is carefully inserted in the clamp such that the end of the yarn is in line with the rear edge of the clamp.
With the movable jaw set to a length somewhat less than the estimated length of the yarn, the other end of the yarn is inserted into it. The movable jaw is then moved slowly to the right until the index marks on the balancing head and the frame are in line. Then the length of the yarn corresponding to the red mark on the moveable grip is noted from the base scale.
Then, the crimp can be calculated as,
Long–Elongation Curve Method:
Parallel ink lines are marked on a piece of cloth P distance apart. Five yarns are unraveled and one is tested at a time. The yarn is clamped at one of the ink spots and the yarn is allowed to hang vertically in front of a scale. A small clamp of known weight is hung on the yarn at the other ink spot. At this load, the elongation is read. Successively small loads are applied and the elongation at each new load is noted. Then a curve is drawn by taking the elongation values on the x-axis and the load values on the Y-axis, as shown in fig.
In this curve, there is a curved portion OB and a straight-line portion BC. The region OB represents the removal of the crimp. The region BC represents the stretch of the yarn. If there were no crimp, the curve would be all the straight-line DC. Therefore, the distance OD represents the elongation of the yarn due to the removal of the crimp. Then the original length P, plus the value OD is the length of the yarn before weaving L.
i.e., L = P + OD
then the Crimp percentage can be calculated as follows:
Crimp% = (L - P)/ P X 100
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