On the left and right sides of the silk, there are small, buckled weft threads, which feel rough like tiny knots when touched by hand. This is caused by the weft threads being woven into the silk surface in a kink or small loop shape. This is what we Often referred to as “weft shrinkage”. The main reason for weft shrinkage is that the water jet loom relies on water jets to insert the weft, and the weft is in a free flying state, while the twisted silk thread has the torque to return to its original shape, that is, it has a tendency to untwist. Although it has been shaped, the torque is “fixed” “But not dead”, the weft yarn is twisted and woven into the silk surface, giving it a rough knot-like shape.
So what are the specific causes of weft shrinkage? How to prevent it? Try to analyze it from the following points.
(1) Shaping: The silk threads of the crepe fabric are twisted and rotated around the axis. The long-chain polymers are twisted in the twisting direction, but the molecular arrangement inside the silk threads is not destroyed. , a stress is generated inside the wire to return to its original shape, resulting in twisting. In order to solve this problem, we often use shaping methods to temporarily stabilize the torque of the silk thread under the action of heat and moisture to facilitate weaving (during scouring, the twisting torque of the silk thread is re-released to form a good crepe effect) . The shaping mechanism of synthetic yarn is completely different from that of real silk. It mainly relies on the secondary transformation temperature of the yarn, that is, when the yarn is in a hot and humid state, the internal crystallinity of the yarn changes, thereby temporarily eliminating the torque of the yarn. Therefore, the setting temperature, humidity, and setting time are appropriate or not, which is the key to preventing weft shrinkage. Generally speaking: the setting temperature is too low, the time is too short, and the setting effect is not good (that is, the torque is not set enough). If the setting temperature is too high and the time is too long, the crystallinity of the silk thread will change, and the torque will be difficult to recover during refining, and the silk surface will not show a crepe effect. The appropriate setting temperature is 95 ~ 100℃, the humidity is 100%, and the time is 90 ~ 110 minutes.
(2) Rewinding: It is mainly a tension problem. If the processing tension is too small, it will easily cause the weft tension to relax and shrinkage will occur more easily. The appropriate tension is 0.19~0.28cN/dtex, so that The weft yarn can remain straight under this processing tension and be smoothly ejected from the nozzle. (3) In weaving, the tension of the weft wire during flight can be slightly larger, which can reduce the buckling of the weft wire and reduce the occurrence of weft shrinkage. Mainly by adjusting the front and rear positions of the yarn guide on the yarn guide frame, the air at the bell mouth of the weft feeder can be adjusted. Turn the circle small to reach. (4) Master the water spray pressure: If the water spray pressure is too high, the weft will be easily broken. If it is too small, it will easily cause shrinkage. Pay special attention to the water pump being used for too long, parts will be damaged, and poor water spray will easily cause shrinkage. Generally speaking, once the 3-year service period is over, some parts should be updated and replaced, and the fluctuation error of the nozzle water output cannot exceed 30mm. (5) Usage time after shaping: If the silk thread is left for too long after shaping, the weft shrinkage is likely to occur due to the gradual emergence of the torsion of the silk thread. Therefore, it is better to use the silk thread as soon as possible once it is shaped. (6) Others: It is necessary to control the amount of water discharged in advance, pay attention to the consistent height of the heald frame lifting and not shaking, and the consistent lifting and lowering of the warp sheet tension, etc. <br
