Water-based ink and its application in flexo printing

The flexo is a flexible, high resolution, warped relief. In recent years, flexo printing has developed rapidly in the printing industry, and its development prospects are generally optimistic. Flexo printing press is a unique anilox roller, the right amount of ink is transferred to the substrate. The proper use of ink is directly related to the printing production operation and the quality of printed products. In general, the ink used in flexographic printing is very thin and belongs to liquid ink. From the perspective of rheology, it is a kind of ink that is close to Newtonian fluid and has very good ink. Good fluidity, viscosity is only 0.1-1Pa/s. At present, three types of inks are mainly used in flexo 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, economy, and other advantages. This article mainly discusses the printability of aqueous ink and its use in flexo printing.

First, the composition and characteristics of water-based ink

Like the composition of general printing inks, water-based inks usually have components such as colorants, binders, and auxiliary agents. Colorants are color rendering materials for aqueous inks, giving the ink a specific color. In the flexo printing, in order to make the print colorful, the coloring agent is generally selected from pigments with good chemical stability and high coloring power; the binder is composed of water, resin, amine compound and other organic solvents. Resin is the most important component in water-based inks. Usually water-soluble acrylic resin is used. The components of the binder directly affect the adhesion function of the ink, the drying speed, and the anti-sticking properties. It also affects the ink luster and the ink-transmitting property. The main amines are amine compounds. Maintaining the alkaline pH of water-based inks allows the acrylic resin to provide better printing effects. Water or other organic solvents mainly dissolve the resin and adjust the ink viscosity and drying speed. The auxiliary agents mainly include: defoamers and retarders. , Stabilizers, dilutants, etc. Since the aqueous ink is a soap composition, bubbles are easily generated during use, so silicone oil is added as an anti-foaming agent to suppress and eliminate bubbles and improve the ink transfer performance. Blockers are used to suppress the drying speed of aqueous inks, prevent the ink from drying on the anilox roller, and reduce the stencil. Stabilizers can adjust the pH of the ink, and can also be used as a diluent to reduce the viscosity of the ink. Diluter is used to reduce the color of freshwater ink, but also as a brightener to improve the brightness of water-based ink. In addition, water-based inks need to add some wax to increase their wear resistance.

Second, the pH of water-based ink

The pH of the water-based ink is directly related to the viscosity of the water-based ink, printability such as drying. Experiments show that the viscosity of water-based ink increases linearly with the decrease of PH value, and its drying rate decreases linearly with the gradual increase of PH value. The pH value of water-based ink mainly depends on the maintenance of amines. Volatilization of amines during the printing process causes the pH value to drop. This causes the viscosity of the ink to rise, the transferability to be poor, the drying to speed up, the anilox roller to clog, and the stencil failure. Therefore, to maintain the pH value of water-based inks, on the one hand, it is necessary to avoid the leakage of amines as much as possible, such as covering the upper cover of the ink tank. On the other hand, it is necessary to regularly add stabilizers. Usually the pH value of the water-based ink should be maintained between 8.0-9.5, the PH value is too high too low will affect the printing quality, PH value is too low, affect the printing performance of acrylic resin play; PH value is too high, the ink dry slowly, Easy to appear on the back of the printed product is dirty and poor water resistance.

Third, the problem of the use of water-based ink

Water-based inks can be mixed with water before drying. Once the ink is dried, it is no longer soluble in water and ink. Therefore, the aqueous ink must be fully stirred before use to keep the ink composition uniform. When ink is added, the remaining ink in the ink tank should be filtered out first and used with new ink. Do not allow the ink to dry on the anilox roller during printing so as not to clog the ink hole. Obstructing the quantitative transfer of ink causes printing instability. During the printing process, the flexographic plate must always be wetted by the ink, so as to avoid the blockage of the text pattern on the plate after the ink is dried. In addition, when waterborne inks are found to have a slightly higher viscosity, it is not appropriate to add water indiscriminately so as not to affect the stability of the ink. An appropriate amount of stabilizer can be added to adjust. â– 

Author unit: Printing Engineering, Jiangnan University