Viscosity is the size of ink cohesion, is the most important control index in the application of ink. In the process of printing, a certain viscosity of ink is a necessary condition to maintain its normal transmission and transfer: if the viscosity of ink is too low, it will cause light color, large dot expansion, distortion of highlight points and uneven ink transfer. Ink viscosity is too high, it will affect the transfer performance of the mesh roller, the color is uneven, the color is sometimes not deep, and it is easy to cause dirty version, paste version, foaming, dry and other ills. The factory viscosity of ink painting, depending on the manufacturer or variety, is generally controlled within the range of 30-60s / 25 ° C (with 4 paint cups). It is better to adjust the viscosity between 40-50s when using.
 
For offset printing ink, the ink can be evenly coated on the printing plate through the reconciliation between the ink rollers, and the flexographic ink is only filled in the mesh roller mesh hole by its own fluidity, adhesion and ink to the plate, and the ink must be filled in a very short time, only the lower viscosity is thinner ink has this property.
 
 
If the viscosity is too large, it will cause the following defects:
 
Ink transfer, transfer difficult. Because the flexo printing machine is very fast, the first color printing is finished to the second color printing, and the interval is only a few seconds to a few tenths of a second, which requires the ink viscosity is small, easy to separate, fast transfer, and fast drying; Easy to cause paper hair, depowder phenomenon, and even cause paper stripping; The ink is difficult to fill the mesh hole, and the scraper is difficult to smoothly scrape away the ink on the mesh wall. Practice has proved that under the same conditions, the split state of the ink layer with small viscosity is more favorable to the transfer of ink than that with large viscosity.
 
Therefore, flexographic ink generally has a smaller viscosity, and its range is: 0.15-0.3Pa-s. However, the ink viscosity can not be too small, otherwise it is easy to imprint the ink in the dot is easy to deform, so that the reproduction of the picture is poor. It will cause the ink body to emulsify, or make it unable to transfer and transfer normally, and gradually accumulate pigments on the roller and plate, when the accumulation reaches a certain degree, it will cause paste.
 
Generally, for smooth substrates, when the ink star is sufficient, the transfer rate is higher, so the viscosity of the ink is required to be slightly higher; For paper with soft structure, the viscosity of the ink used should be slightly lower. In actual production, when using a press with an ink scraper, the ink star of the ink is less stable when the temperature is too low than when the temperature is slightly higher. The increase in ink temperature will reduce the viscosity, and the result is that the printing density is reduced and the ink layer is diluted.
 
To maintain the consistency of printing quality, it is necessary to keep the ink viscosity consistent. The printer must attach great importance to the impact of ink temperature, and the temperature of the ink used should be stabilized at the temperature of the printing workshop before printing. This measure is very important, otherwise the ink density in the printing process will have a large change.
 
 
Factors affecting viscosity of water-based ink
 
1. The influence of temperature on the viscosity of ink
The effect of workshop temperature on printing quality, relative humidity in 60%~85% is better. The viscosity is most sensitive when the temperature difference is large in winter and summer. When the temperature is high, the water evaporates quickly and dries quickly. During operation, it is necessary to pay attention to extending the drying time or improving the machine speed; When the temperature is low, the water evaporates slowly, and the ink drying is also slow. During operation, the drying speed of the ink can be increased or the drying device can be added. When used in winter, because the ink is easy to freeze below 0 ° C, if the ink is frozen, it can be placed in a room with a high temperature, let it dissolve naturally, and can continue to use after mixing evenly.
 
2. The influence of thixotropy on the viscosity of ink
Thixotropy means that the fluidity of the ink increases under the action of external agitation, and the fluidity gradually decreases after the agitation is stopped and the performance of the original state is restored. After the ink is placed for a long time, some ink with poor stability is easy to precipitate and layer. There are also false thickening phenomena. At this time, it can be fully stirred, and after a certain time of stirring, the above problems will disappear naturally. When using fresh ink, be sure to mix well in advance and then make dilution adjustment. Or it is expected that after a certain amount of ink is applied to the machine, the viscosity will decrease, because the water content in the machine and the thixotropy of the ink can improve the fluidity of the ink.
 
3. the effect of PH value on ink viscosity
The PH value of another indicator that needs to be controlled in ink application, and its normal range is 8.9.5, then the printing performance of water-based ink is the best, and the quality of printing is the most stable. When the PH value is higher than 9.5, the alkaline is too strong, the viscosity of the water-based ink is reduced, the drying speed is slow, the water resistance is poor, and the paste is easy to occur. When the PH value is lower than 8.5, that is, the alkaline is too weak, the viscosity of the water-based ink will increase, the ink is easy to dry, and the ink is blocked to the plate and the roller, causing the page to be dirty, and the air bubbles will be produced.

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