Characteristics of water-based inks and problems to be paid attention to in application
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The flexo is a flexible, high resolution, warpable relief. In recent years, flexo printing has developed rapidly in the field of printing, and its development prospects are generally optimistic. The flexographic press uses a unique anilox roll to transfer the right amount of ink onto the substrate. The proper use of ink is directly related to the printing production operation and the quality of printed products. The ink used in general flexographic printing is very thin. It is a liquid ink. From the rheological point of view, it is a kind of 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 ultraviolet-curing inks. Among them, water-based inks have always occupied a large part in flexographic printing because of their environmental protection and economy. Here, we mainly discuss the printing suitability of water-based inks and their use in flexo printing.
First, the composition and characteristics of water-based ink
As with the general printing ink composition, the aqueous ink is usually composed of a coloring agent, a binder, an auxiliary agent and the like. The colorant is a coloring substance of the aqueous ink, giving the ink a specific color. In the flexo printing, in order to make the imprint bright, the coloring agent generally uses a pigment with good chemical stability and high tinting strength; the binder is composed of water, a resin, an amine compound and other organic solvents. Resin is the most important component in water-based inks. Water-soluble acrylic resin is usually used. The binder component directly affects the adhesion function of the ink, drying speed, anti-sticking performance, etc., and also affects the gloss and ink transmission of the ink. Maintaining the alkaline PH value of the water-based ink enables the acrylic resin to provide a better printing effect. The water or other organic solvent mainly dissolves the resin, adjusts the viscosity and drying speed of the ink; the auxiliary agent mainly includes: defoaming agent, retarder , stabilizers, thinners, etc. Since the aqueous ink is a soap composition, bubbles are easily generated during use, so silicone oil is added as a defoaming agent to suppress and eliminate bubbles while improving the ink transfer performance. The retarder is used to suppress the drying speed of the aqueous ink, prevent the ink from drying on the anilox roll, and reduce the paste. The stabilizer can adjust the pH of the ink and can also be used as a diluent to reduce the viscosity of the ink. Dilution agent is used to lighten the color of water-based inks, and can also be used as a brightener to improve the brightness of water-based inks. In addition, some waxes are added to the water-based ink to increase its abrasion resistance.
Second, the pH value of water-based ink
The pH of the aqueous ink is directly related to the viscosity of the aqueous ink, printability such as dryness. Experiments show that the viscosity of aqueous ink increases linearly with the decrease of PH value, and its drying property decreases linearly with the increase of PH value. The pH value of the water-based ink mainly depends on the maintenance of the amine compound, and the evaporation of the amine substance in the printing process causes the pH value to decrease, thereby causing the viscosity of the ink to rise, the transferability is poor, the drying is accelerated, and the anilox roll is clogged, and a paste failure occurs. Therefore, to maintain the pH value of the water-based ink, on the one hand, try to avoid the leakage of amines, such as covering the upper cover of the ink tank. On the other hand, it is necessary to add a stabilizer in a quantitative manner. Generally, the pH value of the water-based ink should be kept between 8.0 and 9.5. If the pH value is too high or too low, the printing quality will be affected. When the pH value is too low, the printing performance of the acrylic resin will be affected. When the pH value is too high, the ink will dry slowly. It is prone to the problem of dirty and poor water resistance on the back of the print.
Third, the problem of the use of water-based inks
The aqueous ink can be mixed with water before drying, and once the ink is dry, it is no longer soluble in water and ink. Therefore, the aqueous ink must be thoroughly stirred before use to keep the ink composition uniform. When adding ink, the residual ink in the ink tank should be filtered first, and then used with new ink. Do not allow the ink to dry on the anilox roller during printing to avoid clogging the ink holes. Blocking the quantitative transfer of ink causes printing instability. During the printing process, the flexographic plate should always be wetted by the ink to prevent the ink from clogging the text on the printing plate after drying. In addition, when the viscosity of the water-based ink is slightly larger, it is not advisable to add water casually so as not to affect the stability of the ink, and an appropriate amount of stabilizer may be added to adjust.