May 27, 2020

The characteristics of water-based ink and the problems to be paid attention to in application

Flexo is a flexible, high-resolution, warpable relief. In recent years, flexo printing has developed rapidly in the printing field, and its development prospects are generally optimistic. The flexographic printing machine transfers the appropriate amount of ink to the substrate through a unique anilox roller. The appropriateness of the use of ink is directly related to the printing production operation and the quality of printing. Generally, the ink used in flexographic printing is very thin and belongs to the liquid type of ink. From the rheological point of view, it is an ink close to Newtonian fluid. Good fluidity, viscosity is only 0.1-1Pa / s. At present, three types of inks are mainly used in flexographic printing: organic solvent inks, water-based inks and UV-curable inks. Among them, water-based inks have always occupied a large part in flexographic printing due to their environmental protection and economic advantages. Here mainly discusses the printing suitability of water-based ink and its use in flexo printing.

1. Composition and characteristics of water-based ink

Like the general printing ink composition, the water-based ink usually also has colorant, binder, auxiliary and other components. The colorant is a coloring substance of the water-based ink, which gives the ink a specific color. In flexo printing, in order to make the printing color bright, the colorant is generally selected to have good chemical stability and high coloring power; the binder is composed of water, resin, amine compounds and other organic solvents. Resin is the most important component in water-based inks. Usually water-soluble acrylic resin is used. The binder component directly affects the ink's adhesion function, drying speed, anti-sticking performance, etc. It also affects the ink's gloss and ink transfer properties. Maintain the alkaline pH value of water-based inks to make acrylic resins provide better printing effects. Water or other organic solvents mainly dissolve resins to adjust the viscosity and drying speed of inks; auxiliary agents mainly include: defoamers and retardants , Stabilizer, diluent, etc. Since the water-based ink is a soap composition, it is easy to generate bubbles during use, so silicone oil should be added as an anti-foaming agent to suppress and eliminate bubbles, while improving the transfer performance of the ink. The retarder is used to inhibit the drying speed of water-based ink, prevent the ink from drying on the anilox roller, and reduce paste. The stabilizer can adjust the PH value of the ink, and can also be used as a diluent to reduce the viscosity of the ink. Dilution agent is used to reduce the color of fresh water-based ink, and can also be used as a brightener to improve the brightness of water-based ink. In addition, water-based inks need to add some wax to increase its wear resistance.

2. PH value of water-based ink

The PH value of the water-based ink is directly related to the printability of the water-based ink, such as the viscosity and drying properties. Experiments show that the viscosity of aqueous ink increases linearly with the decrease of PH value, and its drying property decreases linearly with the gradual increase of PH value. The PH value of water-based inks is mainly maintained by amine compounds. The volatilization of amine substances during the printing process lowers the PH value, causing the viscosity of the ink to rise, poor transferability, faster drying, clogging of the anilox roller, and paste plate failure. Therefore, to keep the PH value of the water-based ink stable, on the one hand, it is necessary to avoid the leakage of amine substances, such as covering the upper cover of the ink tank. On the other hand, stabilizers should be added regularly and quantitatively. Generally, the pH value of water-based ink should be maintained between 8.0-9.5. If the pH value is too high or too low, it will affect the printing quality. When the pH value is too low, it will affect the printing performance of acrylic resin. When the pH value is too high, the ink will dry slowly. It is easy to have the problems of sticky dirt on the back of the printed product and poor water resistance.

3. Problems in the use of water-based inks

The water-based ink can be mixed with water before drying. Once the ink is solidified, it is no longer soluble in water and ink. Therefore, the water-based ink must be fully stirred before use to keep the ink composition uniform. When adding ink, if the residual ink in the ink tank contains impurities, it should be filtered out first, and then used with new ink. When printing, do not let the ink dry on the anilox roller, so as not to block the ink hole. Impeding the quantitative transmission of ink causes unstable printing. In the printing process, the flexographic plate should always be wetted by the ink to avoid clogging the text pattern on the printing plate after the ink is dry. In addition, it is found that when the viscosity of the water-based ink is slightly larger, it is not appropriate to add water casually, so as not to affect the stability of the ink, and an appropriate amount of stabilizer can be added to adjust.

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