Common quality problems in package dyeing—color difference between inside and outside

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 true 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. The main pump head is insufficient
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.
4. The winding density is too high and the cheese layer is too thick.
As we all know, package dyeing requires a loose package, which not only has a uniform density, but is also looser and less dense than conventional packages. Regardless of the winding density of the dyeing bobbin, "filtration resistance" will be generated during bobbin dyeing. The greater the density, the greater the resistance, and vice versa. The filter resistance is part of the resistance of the tube dyeing system, which will change the shape of the system resistance curve, causing the main pump working point to shift, that is, high resistance and low flow. Therefore, the winding density of the package should also match the dyeing machine. When the pump capacity is not too sufficient, the winding density should be smaller. ​

If the yarn layer wound on the bobbin is too thick (too much yarn), the filtration resistance will also increase. It is generally believed that the thickness of the yarn layer should not exceed 50mm.

5. Forward and reverse circulation arrangements
The radius of the dyeing bobbin gradually increases from the inner layer to the outer layer, and the yarn capacity of each layer also increases, but the effective flow rate of the main pump does not increase on each layer. That is to say, the dye liquor flow rate passing through the yarn layer at different radii is the same. When the dye liquor makes a positive circulation from the inside to the outside, the area of the yarn along the way gradually increases, and the dye liquor passing through the unit area increases with the increase of the radius. Reduction (i.e. streamline density decreases). Although the velocity of the fluid is large enough to compensate for the decrease in concentration along the way, due to less (dye) material and more yarn, the problem of deep inside and shallow outside will also occur. ​

When the dye liquor is reversely circulated (from the outside to the inside), the density of the dye liquor streamlines along the flow direction becomes larger and larger, and the situation becomes more (dye) material and less yarn. The dye concentration along the way decreases, which is conducive to reducing the internal-intermediate flow rate. —Outer color difference. ​

Therefore, a certain reverse circulation time should be considered for dyeing machines with insufficient main pump capacity. As a result, some people design the reverse cycle to take longer than the forward cycle.

But the author does not agree with frequent reversal, especially for some dyeing machines with slow reversal. Because reversing direction will reduce the circulation time of the dye liquor, and some may also cause problems such as increased yarn hairiness and felting.

6. The direction of the main circulation of the dye liquor during feeding
Generally, when feeding a dyeing machine, the injection pump pumps the prepared material to the main pump inlet, and then the main pump mixes it with other dye liquor in the main kettle and enters the yarn forced circulation system through the outlet. There are positive and negative directions of circulation. ​

During positive circulation, the dye liquor at the pump outlet directly leads to the inner layer of each package. At this time, the material concentration of this part of the dye bath is greater than that of the entire bath, and the amount of yarn is the least. During reverse circulation, the dye bath at the outlet of the pump first enters the dye vat and is mixed with the entire bath, and then is pressed (or sucked) from the outer surface of the package to the inside of the package. During reverse circulation, on the one hand, because the dye liquor is diluted, the concentration difference is not as large as at the beginning of the forward cycle, and on the other hand, the amount of yarn changes from large to small, so the concentration difference along the way is much smaller. This has a negative impact on the package. The reduction of inner layer sediments is also beneficial. The author of this book suggests that when injecting materials, the main circulation should be reversed, which will help reduce the inner-middle-outer color difference.

7. Changes in package density cause inner-middle-outer color difference
During the yarn dyeing process, the swelling degree of different fibers is inconsistent. For example, in water, the diameter of cotton fiber increases by 20%, the diameter of wool increases by 15%, the diameter of viscose fiber increases by more than 35%, and the diameter of Tencel increases even more. Since the yarn is dry when winding, the diameter increases in water, causing the yarn to become thicker, while the length increases very little (1% to 2%), making the winding density larger. Swelling in alkaline solutions is even more serious. The length of synthetic fibers will shrink in hot water, which will also increase the density of the package. Some elastic yarns (such as spandex core-spun yarn and wrapped yarn) will also shrink violently in hot and humid conditions. All of these will destroy the original evenly wound bobbin. On the one hand, the density will become larger, and on the other hand, it will become uneven.

As mentioned above, an increase in the package density will increase the filtration resistance, resulting in a smaller dye liquor flow, and aggravating the inner-middle-outer color difference.

8. The influence of bobbin diameter and penetration rate
All data and a lot of practice show that a large bobbin diameter is conducive to leveling, and a high penetration rate is conducive to leveling. This is because:

(1) The diameter of the bobbin is large, and the winding yarn layer is thin: For the same winding amount, the diameter of the bobbin is large, and the yarn layer is thin. This is an obvious fact. The yarn layer is thin, the filtration resistance of the dye liquor penetration cycle is small, and the flow rate is large, so the inner-middle-outer color difference is small. ​

In addition, the thinner the yarn layer, the smaller the difference in winding diameter of the inner and outer yarns, the smaller the difference in streamline density, and the smaller the color difference between the inner and outer layers. The thinner the yarn layer, the shorter the time for the dye solution to penetrate, the smaller the dye degree difference along the process, and the smaller the inner-middle-outer color difference. ​

(2) The bobbin has high penetration rate and low resistance: the bobbin is full of holes, and the dye solution can only be redistributed to the inner layer of the yarn through these holes. For the dye liquor, these holes form local resistance and are part of the system resistance. The penetration rate is large and the resistance is small. On the contrary, the resistance is large and the flow rate is reduced, which will lead to inner-middle-exterior color difference.

9. The influence of dye properties and dyeing process
Whether it is the nature of the dyeing material itself or the setting of the dyeing process parameters, it will affect the dyeing rate (especially the initial dyeing rate) and the dye migration rate. When the dyeing rate is too fast and the dye migration rate is low, unevenness will be caused. Dyeing means inner-middle-outer color difference.

10. Effects of improper use of softener after dyeing
Generally, the yarn will be 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 on water quality
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 the 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 winding density of the bobbin is uneven
When the bobbin is wound, the radial density distribution is uneven and the inner layer is too loose, causing the axial flow of the dye liquor to increase, causing the inside to be deep and the outside to be shallow.