Analysis of 14 causes of color difference between interior and exterior
1. Insufficient flow of main pump
The essence of the color difference between the inner, middle and outer layers of the package is that the amount of dye dyed on the inner, middle and outer yarns is different. Here we can first imagine a special case: if the main pump is not turned on when dyeing the bobbin, which is equivalent to immersing the bobbin in a static dye bath, the inner and outer layers of the bobbin will be colored where they are in contact with the dye, and the color will become lighter toward the middle. , the center part may still be its original color. This is similar to the axial flow pump dyeing machine in the 1970s where belt slippage often occurred and the pump speed was insufficient, or the pump was not noticed even after it stopped, resulting in a “white core” phenomenon. Obviously, the dye liquor is not in contact with the yarn in the middle layer of the package at this time. Although the inner yarn is wet due to capillary action, the dye has been absorbed by the yarn that contacted first due to the adsorption of the yarns along the way. Therefore, the middle layer yarn can be dyed without dye.
It can be seen from the above that without the forced circulation of the main pump, it is inevitable to cause color difference from the inside to the middle to the outside of the barrel.
From the analysis of the theoretical model of package dyeing, it can be seen that package dyeing has an obvious characteristic, that is, during the cyclic movement of the dye liquor in the package yarn, the dye concentration will decrease along the way (dc/dx, concentration difference along the way). Only when the dyeing process The liquid circulation speed is large enough to supplement the adsorption and dilution of the dye, so that the concentration of the dye liquid contacted by the yarn from the inside to the middle to the outside is consistent, so that there will be no color difference along the way.
In other words, to prevent internal-middle-external color difference, the main pump must provide sufficient dye liquor flow speed – flow rate. From this and combining other factors, we have the specific flow rate, cycle frequency, etc. involved in Chapter 5. Theory and practice have proven that as long as the flow rate is sufficient, even if only positive circulation is used, level dyeing can be achieved to achieve a satisfactory effect of no color difference from the inside to the middle to the outside (in actual situations, due to the streamline density of the dye liquor in the inner, middle and outer circles) Different, there will also be inner-middle-outer color difference.
2. Insufficient effective traffic
The flow rate of the main pump is not equal to the flow rate that actually penetrates the package yarn layer (effective flow rate). The ratio between the two is the effective flow rate (effective flow rate = effective flow rate/actual flow rate of the main pump). The reason for the unequal flow rate between the two flows is that part of the flow rate leaks, such as poor sealing between the bobbins. As mentioned before, the effective flow rate of different bobbins ranges from 60% to 90%. Nearly half of the flow of the stainless steel tapered bobbin main pump sealed with spacers is invalid flow. In addition, due to some reasons, “eyes exposed” or “broken belly” will lead to “short circuit” of the dye liquor, which will also significantly reduce the effective flow rate.
3. Insufficient lift of the main pump
The lift of the main pump is used to overcome various resistances during dye liquor circulation (resistance along the way, local resistance, yarn filter resistance, etc.). If the system resistance is large and the lift of the main pump is insufficient, problems will occur. It can be seen from the H-Q characteristic curve of the pump that although the dye liquor will not stop circulating, as H increases, the working point shifts and the flow rate will become smaller, which will cause inner-middle-outer color difference.
Supplement: pump head flow, forward and reverse cycle time. When dyeing cheese yarn, the dye liquor must penetrate into the inner layer of the cheese yarn by the lift of the pump, and achieve dyeing balance through reciprocating internal and external circulation. When dyeing cheese yarn, the pump is required to have sufficient head flow and can be adjusted through the dye liquor bypass valve. If the lift flow rate is too large, part of the short fibers in the yarn will come out of the yarn twist, causing fluffing or even breakage; if the lift flow is too small, poor penetration of the dye liquor will occur, resulting in the phenomenon that the middle layer of the cheese yarn is shallower than the outer layer. Generally, the flow rate of cotton yarn is between 35 and 50L/(min·kg), and that of T/C yarn is between 30 and 40L/(min·kg). During the dyeing process of package yarn, the ratio of forward and reverse circulation times should be determined according to the lift flow rate of the pump and the type of yarn. Generally, when dyeing cheese yarn, the time of reverse circulation (outside-inside) is longer than the time of forward cycle (inside-outside). The cheese color has a light inner layer and a dark outer layer, which increases the positive circulation of the dye liquor; the cheese color has a dark inner layer and a light outer layer, which increases the reverse circulation time of the dye liquor. The adjustment of the forward and reverse circulation of cheese dyeing liquid flow has a great effect on color leveling. Because the dye liquor is sprayed outward from the center of the bobbin during positive circulation, the pressure is high. At this time, the bobbin is under positive pressure, and the center inside the bobbin is small. The liquid flow is concentrated and expands toward the outer layer of the bobbin, and the dye liquor passes through the yarn layer to create resistance. This resistance can cause the dye liquor to flow unevenly in the middle of the yarn layer, causing the flow rate to gradually slow down, and the dye liquor to be easily distributed unevenly in the bobbin. Therefore, if the positive cycle time is too long, it is easier to produce internal and external color differences. In reverse circulation, the dye liquor flows from the periphery of the bobbin to the center, and the bobbin is under negative pressure. Although there is still a certain resistance for the dye liquor to pass through the yarn layer, the diameter of the bobbin gradually becomes smaller, which has little effect on the flow rate and flow rate in the bobbin. The distribution is relatively uniform, and the contact concentration and time between the yarn layer and the dye liquor are relatively close. Therefore, more reverse circulation is beneficial to improving the color difference. At the same time, when reverse circulation is sucked from the outside to the inside, the bobbin will not be easily deformed. However, how to determine the specific forward and reverse circulation time ratio should be determined according to the actual conditions of each factory’s equipment.
<br9. The influence of the properties of dyeing materials and the dyeing process. Whether it is the properties of the dyeing materials themselves or the settings of the dyeing process parameters, they will affect the dyeing rate (especially the initial dyeing rate) and the dye migration rate. When the dyeing rate is too fast, , when the dye migration rate is low, uneven dyeing will occur, that is, inner-middle-outer color difference. .
10. The impact of improper use of softeners after dyeing. Generally, the yarn will become astringent and hairy after dyeing, making subsequent processes difficult, especially for dark colors. For this reason, many manufacturers have added an “oiling” process, which is usually treated with smoothing surfactants, and some are treated with silicone. However, silicone will produce a “deepening” effect due to its low refractive index. If the “oiling” is uneven, that is, the deepening is uneven, it will also cause inner-middle-outer color difference.
11. Impact of water quality. Cheese dyeing has high requirements on water quality. The general requirements are: free chlorine content <0.05mg/L; hardness <25mg/kg; turbidity <3°; iron content <0.1mg/kg. If the water quality contains high chlorine, the reactive dyes that are sensitive to chlorine will change color, resulting in a color difference between the outer layer and the inner layer of the cheese. Turbid water quality means that the water contains some suspended solids and other particles. During dyeing, the cheese yarn is like a "filter", and its impurities are adsorbed in the yarn layer, preventing it from flowing normally, which can lead to color difference between the inner and outer layers. Too much Fe2+ and Fe3+ in the water will cause iron embroidery yellow circles in the close contact between the two ends of the inner yarn and the package yarn of bleached and light-colored products. It will also play a catalytic role in the oxygen bleaching process, causing the package yarn to be brittle and damaged. . If the water hardness is too high, most dyes will agglomerate and cause dyeing defects.
When the package is dyed, the yarn on the package acts as a filter material. During the entire dyeing process, pre-treatment, dyeing, soaping, and cleaning fluids are penetrated and filtered thousands of times. When the water quality is poor, the impurity content is too high, there is rust and dirt in the pipeline equipment, or during pre-treatment, cellulose symbiotic organisms are in the pre-treatment liquid. Due to uneven dispersion, some impurities are deposited on the yarn, causing the inner layer to turn yellow. (Also available in yellowish brown). Rust (Fe2+, Fe3+) can be removed by treatment with oxalic acid and acetic acid. Therefore, soft water should be used for tube dyeing, and equipment pipes must be flushed.
12. The bobbin winding density is uneven. When the bobbin is wound, the radial density distribution is uneven and the inner layer is too loose, so that the axial flow of the dye liquor increases, causing the inside to be deep and the outside to be shallow.
13.Preshrinking of cheese yarn
If it is polyester/cotton yarn, it must be steamed at 130°C for 30 minutes before winding to untwist the yarn to a certain degree of shrinkage. However, the pre-shrinking process is often ignored by many dyeing factories: such as polyester/cotton yarn during scouring and When dyeing at high temperatures, severe shrinkage (5% to 10% shrinkage) will occur, and the inner layer of the yarn shrinks faster and more tightly than the outer layer, making it difficult for the dye solution to penetrate evenly. There is no shrinkage in pure cotton cheese yarn.
14. Develop a reasonable dyeing process.
For example, reactive dyes should be added in batches of salt and Yuanming powder to control the temperature rise. For disperse dyes, the pH value of the dye solution should be controlled, and the temperature must be strictly controlled between 90 and 130°C. After dyeing with vat dye, immediately heat up, add water, and remove foam to avoid color spots on the surface of the package. The operating requirements must be strictly followed.
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