Ink thixotropy is one of the properties of the printing process, it has a greater impact on the quality of printed products. The so-called thixotropy refers to a phenomenon that the ink becomes thinner or thicker and the fluidity changes under the action and influence of external forces 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 appear "thixotropic" during the printing process, affecting the stability of the product quality. Therefore, to correctly understand the thixotropy of ink, take some measures to overcome and make up from the process and technology, and reduce the degree of thixotropy in ink printing, in order to better ensure the printing quality of the product.
Understanding the thixotropy of ink
In the production process practice, we found that the general ink has a certain thixotropy, which is mainly manifested as the ink becomes thin after agitation and friction, which increases the fluidity, and after leaving it for a period of time, the ink again Will return to the original thick state. For example, the ink with higher viscosity, under the agitation of external force, its solidified state is destroyed, and it has certain fluidity. In the static state, with the increase of time, the solidification state gradually increases, when restarting, it will affect the normal flow of ink in the ink fountain, and even appear to be unable to transfer. On the other hand, the ink on the printing press ink fountain and rubber roller, after rotating friction, its temperature increases, so the fluidity and ductility also increase, until it is transferred to the printed sheet, due to the disappearance of external force, its fluidity, The ductility is reduced, and then it becomes thinner and thicker, thus ensuring the clarity of the printing ink color. In addition, when the temperature is low, the ink appears thick; otherwise, the ink becomes thinner and the fluidity increases. When the machine is stopped for a long time, the ink in the ink fountain will gradually return to its original state from thinning to thickening. These are normal phenomena of ink thixotropy.
Generally, in the printing process, most of the ink will be thickened and thinned. The reason is that the temperature changes, such as machine temperature increase, ink roller temperature increase, and plate temperature increase. When the ink has a certain thixotropy, it is conducive to the smooth and uniform transfer and printing of the ink.
In addition to the above factors, the thixotropy of the ink is mainly determined by the composition characteristics of the ink. For example, an ink made of a pigment with strong surface adsorption has a large thixotropy; the pigment in the ink is needle and plate The thixotropy of the spherical particles is larger than that of the spherical particles; the larger the amount of pigment, the greater the thixotropy, which is due to the pigment molecules attracting and flocculating; the poor wetting of the pigment and the binder, the thixotropy of the ink is also It is large; if the resin molecular weight is large, the ink thixotropy is also large. In addition, the thixotropy of pigment-based ink is relatively large, the reason is that the pigment content in the ink is too much and the quality of the pigment is poor, such as poor wettability, low relative density and low coloring power of the ink. This problem also occurs when there are too few binders in the ink and the viscosity is too low.
The effect of ink thixotropy on printing quality
It can be said that the thixotropic discomfort of the ink has a great influence on the printing 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 be transported smoothly, the ink supply is abnormal, and the ink color of the printed product is getting lighter. Affects the stability of printing ink color, which is the main manifestation of poor thixotropy of ink. In this regard, we must pay attention to constantly stirring the ink in the ink fountain, so that it maintains continuous and uniform fluidity, and ensure the ink color quality of the product.
Some inks have a too high thixotropy during the transfer process, and its viscosity is high before the thixotropy occurs, and it is easy to cause ink blocking and paste-up failures during printing. Therefore, some printers are equipped with a stirrer in the ink fountain to stir the ink to make it "thixotropic" to improve the flow of the ink and reduce the viscosity of the ink, so as to prevent the phenomenon of ink blocking and paste. The thixotropy of the ink is too large, the ink in the printing process is getting thinner and thinner, the ink color of the printed product is lighter and lighter, and it is almost impossible to adjust. This ink is not suitable for printing solid full-page printed products. this problem.
Technological measures to control ink thixotropy
Printing requires that the ink has proper thixotropy. If the ink thixotropy is too large or too small, it will adversely affect the printing. The thixotropy of ink is mainly determined by its material properties, but as mentioned earlier, printing conditions will also affect the thixotropy of ink to a certain extent. Therefore, when the inherent thixotropy of the ink cannot achieve normal printing, corresponding control can be carried out through technological measures. For example, when 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, due to the small thixotropy of the ink, the fluidity is also small, which is not conducive to uniform ink transfer and inking. At this time, the ink can be thinned appropriately to meet the requirements of the printing process.
In order to prevent the excessive "thixotropy" of the ink and the adverse effects on printing, it is necessary to control the printing process conditions so that the thixotropy of the ink is controlled within the printability range. First, try to carry out in a constant temperature and humidity environment Printing, installing air conditioning equipment in the production workshop. If the conditions are limited, when the climate is dry and the wind is strong, the doors and windows of the workshop can be closed to prevent the ink from thickening and sticking; the second is to control the printing speed as much as possible when the ink itself is too thixotropic In the lowest range, in order to reduce the friction coefficient of the machine, the temperature of the ink is increased, and the ink is thinned; thirdly, the use of the ink roller on the machine should be reduced as much as possible without affecting the uniformity of the layout and sufficient ink The number of branches to reduce the friction coefficient during printing; Fourth, adjust the position of the ink roller and water roller on the machine to prevent the contact between the ink roller and the ink roller, the ink roller and the plate surface, or the water roller and the plate surface from being too tight, the friction coefficient It will cause abnormal temperature rise and make the ink thinner. Fifth, control the stroke when the ink fountain roller rotates. If the ink fountain roller has a large rotation stroke, the temperature rise will be high, and the ink will also become thin. 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 requirements; Sixth, the letterpress (including the flexible version) ) In the case of large printing plates, the printing speed should be controlled lower to reduce friction and heating; Seventh, the printing ink should be selected reasonably. The printing screen version requires the thixotropy of the ink to be slightly larger, while the printing of large-area solid plate ink The thixotropy is slightly smaller, so as to improve the printing quality.
In summary, the ink thixotropy has a certain effect on printing, and the factors that affect the ink thixotropy are many. Therefore, correctly understand the thixotropy of ink and its related factors, take appropriate technical measures to properly control the printing process, so that the ink maintains a relatively stable state during the printing process, and effectively avoids one or other printing quality disadvantages The emergence of printing, improving printing efficiency and product printing quality are of great significance.
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