The most suspicious and common quality problem in cheese dyeing is inner-middle-outer color difference. Here, regardless of the inner-middle-outer chromatic aberration caused by the fiber quality, the main reason for the inner-middle-outer chromatic aberration in terms of process and equipment is insufficient dye liquor flow. It can be analyzed from the following 12 directions.

1. Insufficient flow of main pump

The essence of inner, middle and outer chromatic aberration is that the amount of dye dyed on the inner, middle and outer yarns is different. Here, we can imagine a special situation: if the main pump is not turned on during the cheese dyeing process, that is, if the cheese is immersed in a static dyeing tank, the inner and outer layers of the cheese are mixed with the dye. The contact part will be dyed, the color will become lighter toward the middle, and the middle part may still be the original color. This is similar to the "white core" phenomenon in the axial flow pump dyeing machine in the 1970s due to belt slippage and insufficient pump speed. Obviously, the dye solution is not in contact with the yarn in the middle layer of the tube. Although the inner yarn becomes wet due to capillary action, the dye has been absorbed by the first contact yarn due to the adsorption of the yarn along the way, so the middle yarn can be dyed without dye.

It can be seen from the above that the main pump has no forced circulation, which will inevitably cause color difference between the inside and outside of the tube.

From the analysis of the theoretical model of cheese dyeing, it can be seen that cheese dyeing has an obvious characteristic, that is, when the dye liquor circulates in the cheese, the dye concentration will decrease along the way (dc/dx, concentration difference along the way ), only when the circulation speed of the dye liquor is large enough to supplement the absorption and refinement of the dye, so that the concentration of the dye liquor in contact with all the inner, middle and outer yarns is consistent, there will be no color difference along the way.

In other words, in order to prevent inner-middle-outer chromatic aberration, the main pump must provide sufficient dye liquor flow rate-flow. Therefore, in combination with other factors, the specific flow rate and circulation frequency mentioned in Chapter 5 are obtained. Theory and practice have proved that as long as the flow rate is sufficient, even if only positive circulation is used, the dyeing can be uniform, so that satisfactory color difference effect of inner-middle-outer can be achieved (in fact, due to the flow line of the inner-middle-outer ring The density is different, the inner-middle-outer ring will have color difference.

2. Insufficient effective flow

The flow of the main pump is not equal to the flow that actually penetrates the cheese layer (effective flow), but the ratio of the two is the effective flow (effective flow = effective flow of the main pump/actual flow). The reason for the difference between the two flows is the leakage of some flows, such as poor sealing between pipes. As mentioned above, the effective flow rate of different barrels ranges from 60% to 90%. Nearly half of the flow rate of the main pump of the stainless steel cone barrel sealed by the partition is invalid flow rate. In addition, "eye opening" or "breaking belly" caused by some reasons will cause the dye solution to "short circuit", which will also significantly reduce the effective flow rate.

3. The head of the main pump is insufficient

The head of the main pump is used to overcome various resistances (along path resistance, local resistance, yarn filtering resistance, etc.). ) During the dye liquor circulation. If the system resistance is large and the head of the main pump is insufficient, problems will occur. It can be seen from the high-quality characteristic curve of the pump that although the dye solution will not circulate, as the high-quality increases, the operating point will shift and the flow rate will decrease, resulting in inner-middle-outer chromatic aberration.

4. The winding density is too large and the cheese layer is too thick

As we all know, it is necessary to prepare loose cheese for cheese dyeing. It is not only uniform in density, but also looser than traditional cheese—with a lower density. Regardless of the winding density of the dyeing package, there will be "filter resistance" during the package dyeing process. High density results in high resistance, but low resistance. Filtering resistance is a part of the resistance of the cylinder dyeing system, it will change the shape of the system resistance curve, resulting in the deviation of the main pump operating point, that is, the resistance is large and the flow is small. Therefore, the winding density of the bobbin should also match the dyeing machine. When the pump capacity is not enough, the winding density should be smaller.

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

5. Positive and negative circular arrangement

From the inner layer to the outer layer, the radius of the dyeing tube gradually increases, and the yarn capacity of each layer also gradually increases, but the effective flow rate of the main pump for each layer does not increase. In other words, the flow rate of the dye liquor through the yarn level under different radii is the same. When the dye liquor circulates positively from the inside to the outside, the area of ​​the yarn along the path gradually increases, and the dye liquor passing per unit area increases with the increase of the radius. Decrease (ie, streamline density decreases). Although the velocity of the fluid is large enough to compensate for the decrease in concentration along the way, because there are fewer dyes and more yarns, there will also be problems of inner depth and outer lightness.

When the dye liquor circulates in the reverse direction (from the outside to the inside), the density of the dye liquor flowing along the way becomes larger and larger, and there is more (dye) material, less yarn, and the dye concentration decreases along the way, which is beneficial to reduce Color difference between inside and outside.

Therefore, for dyeing machines with insufficient main pump capacity, a certain amount of reverse circulation time should be considered. Therefore, some people design the anti-cycle time to be longer than the positive cycle time.

However, the author does not approve of frequent commutation, especially for some dyeing machines with slow commutation. Because commutation can shorten the circulation time of the dye solution, it will also cause problems such as increase in yarn hairiness and milling.

6. The main circulation direction of the dye liquor during the feeding process

Usually, when the dyeing machine feeds, the prepared material is pumped to the inlet of the main pump through a syringe pump, and then the main pump is mixed with other dye liquor in the main tank and enters the yarn forced circulation system through the outlet. There are positive and negative circulation directions.

In the positive circulation, the dye solution at the outlet of the pump leads directly to the inner layer of each tube. At this time, the substance concentration in this part of the dye bath is greater than the substance concentration in the entire bath, and the amount of yarn is the smallest. In the reverse cycle, the dye bath at the outlet of the pump first enters the cylinder of the dye vat to mix with the entire bath, and then is pressed (or sucked) from the outer surface of the spool to the inside of the spool. In the reverse cycle, on the one hand, because the dye solution is diluted, the concentration difference is not as large as at the beginning of the positive cycle. On the other hand, the number of yarns changes from large to small, so the concentration difference along the way is much smaller, which is also conducive to reducing the tube Deposits inside the tube. The author of this book suggests that when injecting materials, the main loop should be reversed, which helps reduce the inner-middle-outer chromatic aberration.

7. The color difference between inside and outside caused by changes in packing density

Different fibers have different degrees of swelling during the yarn dyeing process. 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 unwinding the bobbin, the diameter increases in water, which causes the yarn to become thicker, while the length increases very little (1% ~ 2%), which increases the winding density. Swelling is more serious in alkaline solutions. The length of synthetic fibers will shrink in hot water, which will also increase packing density. Some elastic yarns (such as spandex core-spun yarn, core-spun yarn) also shrink sharply under humid and hot conditions. All of these will destroy the original uniformly wound spool, and on the one hand, the density will increase, on the other hand, it will become uneven.

As mentioned above, the higher the packing density, the higher the filtration resistance, the lower the dye flow rate, and the worse the inner-middle-outer color difference.

8. The influence of spool diameter and its penetration rate

All data and a lot of practice show that a large bobbin diameter is good for leveling, and a high penetration rate is good for leveling, because:

(1) The bobbin diameter is large and the yarn layer is thin: under the same winding amount, the bobbin diameter is large and the yarn layer is thin, which is an obvious fact. When the yarn layer is thin, the filtration resistance of the dye liquor permeation circulation is small, and the flow rate is large, so the color difference between the inside and outside is small.

In addition, the thinner the yarn layer, the smaller the difference in winding diameter, streamline density and color difference between the inner and outer layers. The thinner the yarn layer, the shorter the dye penetration time, the smaller the dye chromatic aberration along the way, and the smaller the inner and outer chromatic aberration.

(2) The bobbin has a high penetration rate and low resistance: the bobbin is covered by holes, and the dye solution can only be redistributed to the inner layer of the yarn through these holes. For the dye solution, these holes form local resistance, which is part of the system resistance. High penetration rate and low resistance. On the contrary, the resistance is high and the flow velocity is reduced, which will cause inner-middle-outer chromatic aberration.

9. Dyeing performance and dyeing process

Whether it is the nature of dyeing chemicals or the setting of dyeing process parameters, it will affect the dyeing rate (especially the initial dyeing rate) and the transfer rate. When the dyeing speed is too fast and the transfer rate is too low, it will cause uneven dyeing, that is, inner-middle-outer color difference.

10. The impact of improper use of softener after dyeing

Generally speaking, the dyed yarn will have astringency and hairiness, which makes the subsequent process difficult, especially for dark colors. For this reason, many manufacturers have increased the "oiling" process, usually with a smooth surfactant treatment, some with silicone treatment. However, silicone has a "deepening" effect due to its low refractive index. If the "oiling" is uneven, that is, the deepening is uneven, and it will also cause inner-middle-outer color difference.

11. Water quality impact

When dyeing the bobbin, the yarn on the bobbin acts like a filter material. In the entire dyeing process, the pre-treatment, dyeing, soaping and cleaning solution must be filtered thousands of times. When the water quality is poor and the impurity content is too high, there is rust and dirt in the pipeline equipment or the pretreatment process, the cellulose symbiotics are deposited in the pretreatment solution, and some impurities are deposited on the yarn due to uneven dispersion. Yellow (yellow-brown) inner layer. Rust (Fe2+, Fe3+) can be removed by treatment with oxalic acid and acetic acid. Therefore, soft water should be used for drum dyeing, and the equipment pipes must be flushed.

12. Uneven packing winding density

When the bobbin is wound, the radial density distribution is not uniform, the inner layer is too loose, and the axial flow of the dye solution increases, causing the inner depth and outer shallow.