Effect of ink thixotropy on print quality
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Ink thixotropy is one of the performances of the printing process, which has a greater impact on the quality of printed products. The so-called thixotropic refers to a phenomenon in which the ink becomes thinner or thicker and the fluidity changes under the influence and influence of external force and external environmental factors. For the printing process, stable product printing quality requires that the ink should maintain a moderate and stable fluidity. However, due to the inherent factors of the ink itself and the printing conditions, the ink will inevitably have a "thixotropy" phenomenon during the printing process, which affects the stability of the product quality. Therefore, to correctly understand the thixotropy of the ink, take some measures from the process and technology to overcome and make up, reduce the degree of thixotropic in ink printing, in order to better ensure the printing quality of the product.
Understanding of ink thixotropy
In the production process practice, we found that the general ink has a certain thixotropy, the main performance is that the ink becomes thin after being stirred and rubbed, so that the fluidity is increased, and after it is allowed to stand for a while, the ink is again Will return to the original thicker state. For example, if the viscosity of the ink is large, the solidification state is destroyed under the agitation of the external force, and the ink has a certain fluidity. In the static state, as time goes on, its solidification state gradually increases. When it is restarted, it will affect the normal flow of ink in the ink fountain, and even the phenomenon of non-transmission can occur. On the other hand, the ink is heated on the ink fountain and the rubber roller of the printing press, and its temperature rises, so that the fluidity and the ductility also increase, and after the transfer to the printed sheet, the fluidity is lost due to the external force disappearing. The ductility is reduced, and the thickness is thickened to ensure the clarity of the printed ink. In addition, when the temperature is low, the ink becomes thick; on the contrary, the ink becomes thinner and the fluidity increases. When the machine is in a state of shutdown for a long time, the ink in the ink fountain will gradually return to its original state from thin and thick, which is a normal phenomenon of ink thixotropy.
Generally, in the printing process, most of the ink will be thinned by thickening, which is caused by factors such as temperature rise, temperature rise of the ink roller, and temperature rise of the printing plate. When the ink has a certain thixotropy, it facilitates smooth and uniform transfer and printing of the ink.
Influencing the thixotropy of the ink, in addition to the above factors, is mainly determined by the composition characteristics of the ink. For example, an ink made of a pigment having a strong surface adsorption property has a large thixotropy; the pigment in the ink is a needle and a plate. The thixotropy of the spherical particles is larger than that of the spherical particles; the large amount of the pigment has a large thixotropy, which is due to the attraction of the pigment molecules to flocculation; the wettability of the pigment and the binder is poor, and the thixotropy of the ink is also It is large; the resin has a large molecular ratio and the ink thixotropy is also large. In addition, the pigment type ink is relatively thixotropy, which is caused by excessive pigment content and poor pigment quality in the ink, such as poor wettability, low relative density, and low ink coloring power. Also, this problem occurs when the amount of binder in the ink is too small and the viscosity is too small.
Effect of ink thixotropy on print quality
It can be said that ink thixotropy does not have a great influence on print quality. In the production practice, we found that in the printing process, especially in the case of low temperature and slow machine speed, the ink in the ink fountain often fails to convey smoothly, the ink supply is abnormal, and the ink color of the printed product becomes lighter and lighter. It affects the stability of the printing ink color, which is the main performance of poor ink touch denaturation. In this regard, it is necessary to pay attention to frequently mixing the ink in the ink fountain to maintain continuous and uniform fluidity and ensure the ink color quality of the product.
Some inks in the process of transmission, due to excessive thixotropy, its viscosity before the occurrence of thixotropic phenomenon, it is easy to cause ink blocking, paste failure. Therefore, some printing machines are equipped with a stirrer in the ink fountain to stir the ink to "thixotropy" to improve the flow of the ink and reduce the viscosity of the ink to prevent ink blocking and paste. If the ink is too thixotropy, the ink in the printing process becomes more and more dilute, and the ink of the printed product becomes lighter and lighter, which is almost impossible to adjust. This ink is not suitable for printing on the full version of the printed product. this problem.
Technical measures to control ink thixotropy
Printing requires the ink to have proper thixotropy, and too much thixotropy of the ink can have a negative effect on printing. The thixotropy of the ink is mainly determined by its material properties, but as mentioned above, the printing conditions will also affect the thixotropy of the ink to some extent. Therefore, when the inherent thixotropy of the ink cannot achieve normal printing, the corresponding technical control measures can be used for corresponding control. For example, during printing, the proper ink fluidity can be adjusted according to the characteristics of the printer and the temperature. When the printing speed is low and the temperature is low, since the ink is less thixotropy, the fluidity is also small, which is not conducive to uniform ink transfer and inking. At this time, the ink can be appropriately diluted to meet the requirements of the printing process.
In order to prevent excessive "thixo" of the ink, the adverse effects on printing, control the printing process conditions, so that the thixotropy of the ink is controlled within the range of printability, first, in a constant temperature and humidity environment Printing, installation of air conditioning equipment in the production workshop. If the conditions are limited, when the climate is dry and windy, the doors and windows of the workshop can be closed to prevent the ink from thickening and becoming sticky. Second, in the case of excessive thixotropy of the ink itself, the printing speed should be controlled as much as possible. In the lowest range, to reduce the friction coefficient of the machine, the ink temperature is increased, so that the ink is thinned; the third is to minimize the use of the ink roller on the machine without affecting the uniform layout and sufficient ink loading. The number of branches is used to reduce the friction coefficient during printing; the fourth is to adjust the position of the ink roller and the water roller on the machine to prevent the friction coefficient between the ink roller and the ink roller, between the ink roller and the plate surface, or between the water roller and the plate surface. The abnormal temperature rise causes the ink to become thinner; the fifth is to control the stroke when the ink fountain roller rotates. If the ink fountain roller has a large rotation stroke, the temperature rise is high, and the ink is also easily thinned. Therefore, in the case of high printing speed, the rotation stroke of the ink fountain roller should be adjusted as small as possible, and the gap between the ink fountain steel sheet and the ink fountain roller should be appropriately adjusted to meet the printing ink supply requirement; the sixth is the relief printing (including the flexographic printing plate). When the printing layout is large, the printing speed should be controlled to be lower to reduce the frictional temperature; the seventh is to select the printing ink reasonably, the printing screen version requires the ink to be slightly more thixotropy, and the printing of the large-area solid version of the ink. The thixotropy is slightly smaller, so that the print quality can be better improved.
In summary, the ink thixotropy has a certain influence on printing, and the factors affecting the thixotropy of the ink are various. Therefore, a correct understanding of the thixotropy of the ink and its related factors, in the printing, take appropriate technical measures to properly control, so that the ink remains relatively stable during the printing process, effectively avoiding one or the other printing quality ills The production, improving printing efficiency and product printing quality is of great significance.