1.1 reasons for yarn fracture
Yarn breaking is caused by a part of fiber breaking and a part of fiber slipping. That is to say, the number of fiber slippage in the section is reduced, the strength of single fiber at the section is low, and the unevenness is large; The reason why the fibers in the yarn slip off is mainly because the fiber length is short, the lint is accumulated, the fiber is less curled, the fiber flexibility is poor, and the holding force between the fibers is poor
1.2 factors affecting yarn strength
(1) Raw material properties, such as length, linear density, breaking length, etc;
(2) The influence of spinning process on the properties;
(3) Yarn formation structure, such as fiber straightening and parallelism, arrangement and distribution of the yarn, twist of the yarn, etc;
(4) Yarn evenness, such as weight unevenness, evenness unevenness and twist unevenness. Because fracture always occurs at the weakest point.
1.3 direction of improving yarn strength
To improve the yarn strength, we should choose raw materials reasonably, reduce the mixing difference, improve the quality of semi-finished products, improve the semi-finished products and the yarn structure, improve the yarn evenness and reasonably select the twist coefficient.
Yarn strength is also a system engineering, and the important thing is to solve the weak ring of yarn strength. To solve the strong weak ring, it seems to be simple, but it is actually a relatively difficult thing to do. It covers too many aspects. In a word: details determine success or failure! The strength characteristics of the original yarn include average strength, strength unevenness, average elongation and elongation unevenness. Generally, the average strength of warp yarn is 15cn / tex, the strength CV% is about 9-10%, and the average strength of weft yarn is 12cn / tex, and the strength CV% is about 9-10%. The average breaking elongation is generally controlled within 2%. The important strength index of air-jet loom stopping is the strength weak ring, i.e. the minimum strength. It is generally believed that when the yarn fineness is only 40% of the normal yarn, this kind of detail weak ring will inevitably lead to broken ends and stop. If the strength weak ring is 4cn / tex and the elongation is less than 2%, it will inevitably lead to broken ends. In addition to detail defects, other weak twists, poor joints, and miscellaneous coarse knots will also cause stop between warp and weft, accounting for about 39%.
There are three types of details on the original yarn: one is short pile details, one is at the end of thick and thin knots, and the other is long details or several short details connected together will cause broken ends. In order to reduce the strong weak ring and provide good raw yarn for the downstream process, corresponding measures must be taken. For example, the new roving frame with the four roller drafting form of four unit drive eliminates the details caused by switching on and off, on the other hand, improves the drafting evenness and reduces the defects of coarse and fine nodes, and also strengthens the matching of the driving teeth and keys of the drafting part to avoid looseness and ensure the synchronization of the drafting process, It is also one of the important methods to reduce details. In addition, the reasonable configuration of the process and the selection of good drafting parts, such as rubber rollers, rubber rings, upper and lower pins, and pressure mechanisms, are also factors that reduce details.
2、 Relationship between raw materials and yarn strength
2.1 relationship between fiber length and evenness and yarn strength
Table 1 Relationship between fiber length and evenness and yarn strength
2.2 relationship between fiber linear density yarn and yarn strength
Under the same density, the finer the fibers used for spinning, the more fibers in the yarn cross section. The fibers with fine linear density are generally softer, and the chances of internal and external transfer during twisting are increased, which is conducive to the increase of fiber contact area, thus improving the fiber holding force and friction force. When the yarn is stretched and broken, the number of slipping fibers will be reduced, and the yarn strength will also be improved. The fiber linear density has a great influence on the yarn strength of fine and special yarns, but has no obvious influence on the yarn strength of coarse and special yarns. But maturity is related to fiber linear density. Although the linear density of the fibers with poor maturity is fine, the strength of the single fiber is low, and the strength is reduced when spinning with this fiber.
2.3 relationship between fiber breaking length and single strength and strength
When the fiber breaking length is large, the fiber linear density must be fine or its strength is high, which is undoubtedly beneficial to the yarn strength. Therefore, fiber breaking length is an important factor affecting yarn strength. When other conditions are the same, the single fiber strength is high, and the yarn strength is also high. When the strength of single fiber is particularly poor, the fiber is easy to break and form short fibers during spinning, and is easy to be kneaded into granules, which deteriorates the yarn evenness, thus reducing the yarn strength.
3、 Influence of spinning technology on yarn strength
3.1 evenness of raw material mixing and yarn strength
It is necessary to reduce the difference rate of raw materials and enhance the mixing effect of pre spinning on raw materials to improve the yarn strength. The uniform mixing of raw materials and the uniform distribution of each raw material component in the yarn can obtain the best yarn structure, so that the strength of each fiber in the drawing process of the yarn can be reasonably and fully utilized to improve the yarn strength. To improve the mixing uniformity of raw materials, the following points should be done well:
3.1.1 reduce the shape difference of mixed raw materials
No matter what kind of fiber, in order to prevent the mixing unevenness caused by the difference of raw material characteristics, the shape difference of various fibers in the mixed components must be reduced as much as possible, especially the difference of fiber linear density, length, initial modulus and the difference of packaging density and size. When the two fibers are mixed, attention should be paid to the design of the blending ratio, because the strength of the blended yarn will be the lowest if the blending ratio is not properly designed.
3.1.2 enhance the mixing effect of the process.
Improve the carding effect of the carding machine and enhance the mixing opportunity between single fibers. Properly increase the number of combs and drawing processes, and pay attention to the improvement of mixing methods, so that the raw material components can be fully mixed before entering the spinning.
3.2 pre spinning process and yarn strength
In the blowing and carding process, attention shall be paid to avoid excessive impact as much as possible under the condition that the raw materials are fully opened, so as not to break the fiber or damage the original strength of the fiber. On the premise of full loosening, lint and impurities shall be eliminated as much as possible to reduce the chance of new grain formation, and the original grain formation shall be eliminated as much as possible, because these are the main factors affecting the strength.
3.2.1 reasonably configure blowing and carding process
The mechanical action of the opening and cleaning process is intense, so the process parameters should be reasonably selected. Improve the opening efficiency of each machine and fully eliminate large impurities and harmful defects; Comb instead of beating or reasonably reduce the striking point to avoid striking the fiber too violently, so as to prevent damage to the fiber, breaking the fiber and damaging the original strength of the fiber.
The carding process should give full play to the carding function, eliminate short fibers and knots, and damage the fibers. Increasing the speed of the carding machine can improve the carding degree and reduce the knots, but it is easy to damage the fibers if the speed is too high. In addition, different tooth shapes and tooth densities can be selected to improve the combing effect and reduce fiber damage.
Table 2 Effect of sliver short fiber ratio on yarn strength
Sliver short fiber rate (%); yarn breaking strength (CN / Tex); yarn strength variation coefficient (%); yarn breaking rate (root / 1000 spindles)
3.2.2 reasonable configuration and rough process
The task of the drawing process is to reduce the unevenness of the medium and long segments, improve the fiber straightness, and avoid or minimize the short segment unevenness of the sliver. If self leveling is adopted. It has a significant effect on reducing yarn strength unevenness. Reasonable drafting form and drafting process are the necessary conditions to ensure the effective control of fibers in the drafting process. Otherwise, good evenness of sliver cannot be guaranteed.
Properly increase the temperature and humidity of roving process, increase the moisture regain of roving, and make the fiber easy to straighten. Increasing the relative humidity and reducing the drafting distance can stabilize the fiber movement and improve the evenness of the sliver. In addition, when the moisture regain is high, the internal stress of the fiber is easy to disappear after drawing, which can maintain the state of straight and parallel fibers. However, if the moisture regain is too high, the product quality will be degraded due to abnormal production (such as rubber roller and roller).
In actual production, it is necessary to strictly control the weight of final merging and reduce the table difference and eye difference. Reduce the spindle difference of roving winding elongation caused by the weight difference of fiber sliver, and reasonably control the roving elongation. Relationship between yarn strength and roving unevenness: the yarn strength decreases with the increase of roving short segment unevenness, and the roving periodic unevenness makes the decrease of yarn strength more obvious.
3.3 spinning process and yarn strength
3.3.1 yarn evenness and yarn strength
Practice proves; If the evenness of the yarn is good, the single strength unevenness of the yarn will decrease. There are many factors that affect yarn strength, such as draft ratio, roller spacing, roving twist coefficient, etc. However, the main factors affecting the evenness of the spinning sliver are the process and friction boundary arrangement in the spinning front area, especially the use of the front draft area collector, the length of the floating area and the pressure of the apron nip.
3.3.2 yarn weight unevenness and yarn strength
The weight unevenness of 100 meters of spinning yarn is an important factor that causes the strength unevenness between tube yarns. In general, the 100 meter weight unevenness of spun yarn must be stable within 2.5%, so as to avoid the sudden standard single strength unevenness exceeding the standard. There are some reasons that the single strength unevenness of the finished yarn is high, but the 100 meter weight unevenness is not shown, because the yarn segment with strength is very short. Therefore, it is required to reduce the unevenness of the long segments of the spinning yarn and pay attention to reducing the unevenness of the segments about half a meter. In addition, attention should be paid to reducing the quality of semi-finished products before spinning. For example, the unevenness of the cleaning roll shall be within 1.% and the short fiber rate of the combed sliver shall be within 10%. The combination number is accurate, and the tension of the roving is within 1.5 to prevent the floating head and adhesion of the roving.
3.3.3 whisker structure and yarn strength
During spinning twisting, the final strength of the yarn is mainly affected by the tension of the whiskers, the width and tightness of the whiskers, the fiber arrangement and the straightening state. The present gathering spinning (also known as compact spinning) is a good method to improve the structure of the whisker before twisting, reduce the hairiness of the yarn and increase the strength of the finished yarn.
3.3.4 spinning coarse details, granulation and yarn strength
Pure spinning is easy to break at the place of details and large knots, and mixed spinning is easy to break at the place of coarse knots or the inflection point of coarse and fine knots. Most of the large neps of pure cotton are composed of untwisted fibers, while the cotton fiber content at the coarse node of cotton blended yarn is generally larger than the designed blending ratio. The coarse knots of the yarn lead to uneven twist distribution, small twist at the coarse place, and low relative fineness. During stretching, the transmission of twist is far slower than the stretching speed, and the transmitted twist is pulled off before it reaches the coarse node, which is the reason why large knots and coarse nodes become the weak link of yarn strength and break. Therefore, it is necessary to strengthen the operation and operation management, and pay attention to the key points.
3.3.5 spinning twist coefficient and yarn strength
The twist coefficient and twist of spinning yarn have influence on the yarn strength. The relationship between twist and yarn strength is as follows: with the increase of twist, the yarn strength gradually increases, but after reaching a certain value, the twist continues to increase, and the twist angle of the fiber also increases, so that the effective component of the fiber strength in the axial direction of the yarn is indeed reduced. The excessive twist will increase the uneven stress distribution of the inner and outer fibers of the yarn, aggravate the different timing of fiber breaking, and reduce the yarn strength instead, The twist corresponding to the highest strength is called the spinning critical twist.
4、 Relationship between temperature and humidity in the workshop and yarn strength
Improper or unstable adjustment of temperature and humidity will affect the instability of moisture regain of roving and spinning, thus affecting yarn strength and increasing broken ends.
Because the cross-section expansion and extensibility of the fiber increase after moisture absorption; With the increase of fiber kneading, adhesion and friction coefficient, the fiber is easy to be controlled by the drafting mechanism, resulting in the uniform yarn and the improvement of fiber parallelism and straightness, thus increasing the holding force and friction force between fibers, which is the main reason for the improvement of yarn strength.
Increasing the moisture regain of roving can not only improve the strength of spinning, but also improve the evenness and appearance of spinning. According to the analysis of relevant data, when the moisture regain of cotton yarn is above 7.2%, the yarn is full, smooth, round and hairy; Good gloss; When the moisture regain is below 6.0%, the yarn is soft, hairy, poor gloss and low yarn strength. However, if the moisture regain is too large, it will be easy to wrap the top roller and roller, which will increase the broken ends in spinning, and also reduce the yarn strength. Therefore, the moisture regain of roving should be properly increased so that the temperature and humidity conditions in the spinning workshop can ensure that the yarn is in the moisture release state during processing.
Generally, the moisture regain of spinning is required to be 7.2-8.0%, the workshop temperature is 26-30 ℃, and the relative humidity is 55% - 60%. At the same time, the temperature and humidity in each area of the workshop shall be evenly distributed as much as possible in management to ensure that the temperature and humidity in the workshop are appropriate and stable.
5、 Conclusion
It is a systematic project to improve the yarn strength. The effect can be achieved only by comprehensive consideration of reasonable selection of raw materials, reduction of mixing differences, improvement of the quality of semi-finished products, improvement of semi-finished products and yarn structure, improvement of yarn evenness, reasonable selection of twist coefficient and control of temperature and humidity.
source:https://mp.weixin.qq.com/s/bFGeVPO5ikALWhx1Y7cKtA