Evenness is the uniformity of thickness or quality within short segments of yarn, sliver or roving along the axial direction. The quality of textiles is closely related to yarn evenness. If the spun yarn is not evenly dry, the strength of the yarn will be reduced and affect the strength of the fabric. When weaving with uneven spun yarns, various defects and bars will appear on the fabric, affecting the appearance quality.
01 Impact on Textile
When the uniformity of semi-finished products decreases When the yarn is even, the uniformity of the spun yarn is also reduced accordingly; if the spun yarn is not evenly dried, the strength of the yarn will be reduced and affect the strength of the fabric. When weaving with uneven fine yarns, various defects and bars will appear on the fabric, affecting the appearance quality. The requirements for the uniformity of fine yarns in knitting production are generally more stringent than in woven production. In knitting processing, uneven spun yarn or yarn defects will damage the normal knitting process and sometimes cause needle breakage. In the manufacture of tire cord, excessive unevenness of the spun yarns will cause adjacent yarns to become entangled with each other, thus affecting the processing process and product quality. In addition, uneven spun yarn will increase the breakage rate in spinning and weaving, and even reduce labor productivity.
02 Reasons for uneven dryness
1) Differences in the properties of fiber raw materials
Uneven yarns are caused by differences in the properties of fiber raw materials. Various natural fibers have inhomogeneities in length, linear density or other properties. Processing by the same machinery and technology will cause differences and cause unevenness.
2) Random arrangement of fibers
Uneven yarns occur due to random arrangement of fibers. According to the principle of short fiber spinning, the ideal sliver can be assumed to be composed of randomly arranged fibers. This randomly arranged sliver has a certain unevenness, which is called random unevenness. The value is the same as the average number of fibers in the sliver cross section. is inversely proportional to the square root of , and random unevenness will increase when the number of fiber roots is small.
3) Poor selection of spinning process parameters
Uneven yarn slivers occur due to poor selection of spinning process parameters. For example, improper selection of process parameters such as drafting mechanism spacing and pressure will lead to poor fiber movement control, which will cause problems with details and thick spots, resulting in uneven thickness.
4) Spinning mechanical defects
Uneven yarn sliver caused by spinning mechanical defects. For example, roller or rubber roller eccentricity, gear defects, etc. will cause obvious periodic unevenness in the yarn, often showing undulating wavy changes in thickness. Shorter wavelengths are called short segment unevenness, and long lengths are called long segment unevenness. Generally, the short segment unevenness produced by the pre-spinning machine is due to the longer draft, and long segment unevenness appears in the yarn; the unevenness produced by the spinning machine is the short segment unevenness.
03 Determination method of evenness
Uneven evenness commonly used in spinning production There are three main methods for determining the rate: cutting and weighing method, blackboard dry visual inspection method and instrumental detection method. The cutting and weighing method can be used for each semi-finished product and spun yarn; the blackboard dry visual inspection method is mainly used for spun yarn; the instrumental detection method can be divided into capacitive detection and mechanical detection. Capacitive detection is suitable for slivers, rovings and spun yarns, while mechanical detection is only suitable for slivers and rovings.
1) Cutting and weighing method
The cutting and weighing method is to press the yarn strips into Cut sections into specified lengths and weigh them individually, and then calculate the unevenness. The length and number of fragments taken depend on the actual production situation and test accuracy requirements. The disadvantage of this method is that it takes a lot of time, and it is only suitable for measuring the weight unevenness of longer segments on pre-spun semi-finished products with a relatively coarse number.
2) Blackboard evenness visual inspection method
Blackboard evenness visual inspection is commonly used in production to check and evaluate the level of spun yarn evenness Methods. By winding the spun yarn evenly around a rectangular (or trapezoidal) blackboard at equal intervals, you can visually analyze the uneven composition of the spun yarn and rate the evenness of the spun yarn against the standard sample. Grading standards and sampling assessment methods vary from country to country. The blackboard visual inspection method has certain requirements for blackboard specifications, lighting during inspection, and observation distance.
3) Instrument detection method
The capacitive uniformity tester is suitable for testing slivers and slivers spun from various short fibers. Evenness of roving and fine yarns. For filaments, a false twisting device must be installed to eliminate the “cross-section effect” of the yarn (that is, the detection error caused by the variation of the cross-sectional shape of the yarn between the detection capacitor slots). The test method is to pass the yarn through the middle of the capacitor plate. The thickness of the yarn segments causes capacitance changes, and the unevenness of the yarn is calculated through the electronic circuit. When applying a capacitive uniformity tester, samples with high humidity or uneven humidity should be avoided to avoid excessive test errors. The mechanical evenness tester is suitable for testing the evenness of slivers and rovings. Feed the sliver or roving into a groove of a certain specification, apply a certain pressure on it, and measure the thickness change of the sliver. </p
