In China, because flexographic printing has only emerged in recent years, printing workers either use inertial thinking or do not know where to start when controlling ink and wash. The result is often a lot of wasted manpower, material resources, and financial resources, and heavy losses . Therefore, it is particularly important to correctly understand the technical control indexes of ink and wash. This article will discuss the main technical indicators such as viscosity, PH value, thickness, and dryness of flexo ink.
Viscosity
Viscosity is a property that prevents fluid flow. It is a measure of the interaction between fluid molecules that hinders the relative movement of molecules, that is, the resistance to fluid flow.
Viscosity is the most important control index in ink application, because it directly affects the transfer performance of ink and the quality of flexographic prints. In addition to the viscosity and density of the resin in the linking material, the viscosity is also related to the type and granularity of the coloring material. In printing, low viscosity and fast ink transfer will cause light color, large dot expansion, high gloss dot deformation, uneven ink transfer and other disadvantages; high viscosity and slow ink transfer will affect the transfer performance of the anilox roller and produce ink color Non-uniformity, sometimes the color is not printed deep, and the dirty, paste and other disadvantages.
Low viscosity can be adjusted by mixing with new ink; while high viscosity, it can be diluted with water or water and ethanol (50% each), or adjusted with water and ink stabilizer. In addition, in the printing process, the temperature also has a greater impact on the viscosity of the ink, usually the viscosity increases when the temperature increases, otherwise, the viscosity increases. Therefore, in the printing process, in order to maintain the consistency of the printed matter, the temperature of the printing workshop should be kept constant.
PH value
The binder used in ink is mainly alkali-soluble acid resin, so the control of PH value is very important. Generally, the PH value should be controlled at 8.5 to 9.5. At this time, the ink printing performance is the best and the printing quality is the most stable.
Because the amine continues to volatilize during the printing process, the operator must add new ink and various additives from time to time, so the PH value of the ink changes 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 High, causing the drying speed to become faster, easy to block the printing plate and the anilox roller, causing the plate surface to be dirty, and bubbles will be generated.
Because the influence of PH value is directly related to viscosity, it is recommended to control the two in actual printing, especially when register printing, this problem should be paid more attention to. In addition to the viscosity cup, a simple PH indicator should be prepared for testing 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 diluent can be added for dilution.
Thickness
The so-called ink thickness refers to the degree of dispersion of solid powdery substances such as pigments and fillers mixed in Lianke.
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 coloring power, hue, stability, etc.), but also related to the rheological properties of the ink and economic benefits. If the ink is too thick, it will cause ink stacking, paste, etc., and the coloring power and hue of the ink will also deteriorate; if the ink is too fine, the fluidity will be large, 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 dispersion. 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 product is not smooth, smooth and soft, and it will produce a "block" feeling, thus greatly reducing the printing effect.
In actual printing, different thickness inks can be selected according to specific requirements. The fineness of ink is higher when printing printed materials with finer screens and higher accuracy; when printing prints or field prints that do not require high accuracy, the thickness of ink can be reduced appropriately, which is also for Reduce costs and increase economic benefits.
Dryness
The dryness of the ink is an important technical indicator. The drying speed of the ink is not only related to the performance of the ink itself, but 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 drying is usually: the ink should be able to be fully dried before the next printing and before rewinding or stacking under the existing conditions of the printing press, but it cannot be dried on the printing plate.
Ink is made of water as a solvent and alkali-soluble acidic resin is used as a binder. It is mostly used for corrugated boxes and paper bag printing. Therefore, the drying method is mainly permeation drying and a small amount of volatile drying. Because water is used as a solvent, its volatilization rate is slower than other solvents, so the drying of water ink is slower than that of solvent-based ink. Therefore, a small amount of alcohol is usually added to the solvent to increase the drying speed of water ink. In addition, under the condition that the solvent is determined, the selection of different types of resin also has an effect on the drying properties of the ink. Because different resins have different curing speeds on the substrates, and the release characteristics of the solvents are also different, 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, it may be that the PH value is lower than 8.5, and stabilizers can be added to adjust. In addition, according to the actual printing speed, a quick-drying agent or a slow-drying agent can be added to the ink to appropriately change the drying speed of the ink to obtain a stable printing effect. The amount of fast drying agent or slow drying agent is generally 1% -2% of the total ink volume.
There are many quality indicators that need to be controlled in the application of ink and wash. In addition to the main indicators mentioned in this article, they also include: color, gloss, adhesion, and stability. In actual printing, in order to comprehensively improve the printing quality, all aspects should be considered comprehensively. index.
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