Water-based ink is a uniform paste-like substance composed of binders, pigments, additives and other substances. The binder provides the necessary transfer performance of the ink, and the pigment gives the ink its color. The binder of water-based ink is mainly divided into two types: water-diluted and water-dispersed. There are many types of resins that can be used in the former, such as maleic acid resin, shellac, maleic acid resin modified shellac, urethane, water-soluble acrylic resin and water-based amino resin. The water-dispersed type of vehicle is obtained by polymerizing emulsified monomers in water. It is a two-phase system, in which the oil phase is dispersed in the water phase in the form of particles. Although it cannot be dissolved by water, it can be diluted by water, so it is also It can be considered as an oil-in-water emulsion type.
viscosity
Viscosity is a property that prevents fluid flow. It is a measure of the ability of fluid molecules to interact with each other to hinder the relative movement of its molecules, that is, the resistance of fluid flow.
Viscosity is the most important control index in the application of ink, which directly affects the transfer performance of ink and the quality of flexographic printing. The viscosity is not only related to the viscosity and density of the resin in the binder, but also related to the type and particle size of the colorant. In printing, low-viscosity ink transfers quickly, which will cause disadvantages such as light color, large dot expansion, high-gloss dot deformation, and uneven ink transfer; high viscosity and slow ink transfer will affect the transfer performance of the anilox roller and produce ink color. Unevenness, sometimes the color is not printed deeply, dirty version, paste version and other disadvantages.
If the viscosity is low, it can be adjusted by blending with new ink; if the viscosity is high, it can be diluted with water or water and ethanol (50% each), or adjusted with an ink stabilizer. In addition, during the printing process, the temperature also has a great influence on the viscosity of the ink. Usually, the viscosity decreases when the temperature rises, and vice versa, the viscosity increases. Therefore, in the printing process, in order to keep the density of the printed product consistent, the temperature of the printing workshop should be kept constant.
PH value
The linking material used in ink is mainly alkali-soluble acidic resin, so the control of PH value is very important. Usually, the PH value should be controlled at 8.5-9.5. At this time, the printing performance of ink is the best, and the quality of printed products is the most stable.
Since the amine is constantly volatilizing during the printing process, and the operator has to add new ink and various additives from time to time, the pH value of the ink is changing at any time. When the pH value of the ink is higher than 9.5, the alkalinity is too strong, and the viscosity of the ink will decrease, resulting in slower drying speed and poor water resistance; when the pH value is lower than 8.5, the alkalinity is too weak, and the viscosity of the ink will increase. If the temperature is high, the drying speed will become faster, and the printing plate and anilox roller will be easily blocked, causing the layout to be dirty and air bubbles will be generated.
Since the influence of PH value is directly related to the viscosity, it is recommended to control the two in actual printing, especially in chromatic printing, more attention should be paid to this problem. Usually, in addition to the viscosity cup, a simple PH value indicator should be prepared for detection at any time. When the pH value of the ink is low, a pH stabilizer or a small amount of alkaline substance can be added; when the pH value is high, a solvent or thinner can be added for dilution.
Thickness
The thickness of the so-called ink refers to the degree to which solid powdery substances such as pigments and fillers mixed in the ink are dispersed.
The thickness of the ink is also a very important quality indicator, because it is not only related to the application performance of the ink (including tinting strength, hue, stability, etc.), but also related to the rheological properties and economic benefits of the ink. If the ink is too thick, it will cause problems such as piled ink and paste, and the coloring power and hue of the ink will also deteriorate; if the ink is too fine, the fluidity will be great, and it will flow after printing. This flow will cause the dot to expand and affect Print quality. In addition, the thickness of the ink will also affect its dispersibility. When the dispersibility of the ink is not ideal, its printing performance will not be satisfactory. It is manifested that the surface of the printed matter is not smooth, smooth, and soft, and it will produce a "blocky" feeling, thus greatly reducing the printing effect.
In actual printing, inks of different thicknesses can be selected according to specific requirements. When printing printed products with relatively thin screen lines and relatively high precision, the fineness of ink should be relatively high; when printing printed products that do not require high precision or solid printed products, the thickness of ink can be appropriately reduced, which is also for Reduce costs and improve economic efficiency.
dryness
In addition to the performance of the ink itself, the drying speed of the ink is also related to the printing speed, the drying capacity of the drying equipment and the performance of the printing material. This article only considers the factors of the ink itself. The principle of selecting ink dryness is usually: the ink should be able to fully dry under the existing conditions of the printing machine before the next printing and before rewinding or stacking, but it should not dry on the printing plate.
Ink ink uses water as a solvent and alkali-soluble acidic resin as a linking material. It is mostly used in the printing of corrugated boxes and paper bags. Therefore, the drying methods are mainly osmotic drying and a small amount of volatile drying. Since water is used as a solvent, its volatilization speed is not as fast as other solvents, so the drying of ink is slower than that of solvent-based inks. Therefore, it is usually necessary to add a small amount of alcohol to the solvent to increase the drying speed of ink. In addition, under the condition that the solvent is determined, the choice of different types of resins will also affect the drying properties of the ink. Because different resins have different curing speeds on the substrate and have different release characteristics to solvents, the drying properties of the inks are also different.
In practical applications, the dryness of the ink is also related to its viscosity and pH value. The ink is dry but the viscosity may be too high, which can be solved by further dilution; the ink dries quickly, and the pH value may be lower than 8.5, which can be adjusted by adding a stabilizer.