Technical control indicators that should be paid attention to in ink application
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Flexographic printing has developed rapidly in recent years. It has broken the traditional offset printing system of “unconventional world”, which can be “three-pointed” with offset printing and gravure printing, and has been recognized as one of the “best and most The way to develop the future. It is no exaggeration to say that these are mainly the credits of flexo ink. Because it replaces the traditional solvent-based ink, in addition to its unique choice in the choice of binders and solvents, it also has a wide range of printability and economic characteristics in performance, and more importantly, it is excellent in environmental protection. Performance, in full compliance with the development trend of modern printing, will become an important part of the future Chinese packaging products into the international market.
In China, because flexographic printing has only recently emerged, printers are accustomed to the control of traditional solvent-based inks. When controlling ink, they either use inertial thinking or do not know where to start. It is often a waste of manpower, material resources, and financial resources, and the loss is heavy. Therefore, for those engaged in flexo printing, it is especially important to understand the technical control indicators of ink.
Due to limited space, this paper only discusses the main technical indicators such as viscosity, pH value, thickness and dryness of flexo ink.
1. Viscosity
Viscosity is a property that prevents fluid flow. It is a measure of the interaction between fluid molecules that produces the ability to block the relative movement of molecules, that is, the resistance of fluid flow.
Viscosity is the primary control indicator in ink applications because it directly affects the transfer properties of the ink and the quality of the flexographic print. The viscosity is related to the viscosity and density of the resin in the binder, and also to the type and particle size of the pigment. In printing, the viscosity is low and the ink transfer is fast, which will cause defects such as light color, large dot enlargement, high-light dot deformation, uneven ink transfer, etc.; high viscosity and slow ink transfer will affect the transfer performance of the anilox roll and produce ink color. Uneven, sometimes the color is not printed, and the ills such as dirty plates and pastes.
The viscosity is low, and it can be adjusted by mixing with new ink. When the viscosity is high, it can be diluted with water or water and ethanol (50% each), and can also be adjusted by using ink stabilizer. In addition, in the printing process, the temperature also has a large influence on the viscosity of the ink, and generally the viscosity is lowered by the temperature rise, and vice versa, the viscosity is increased. Therefore, in order to keep the print density consistent during the printing process, the temperature in the printing shop should be kept constant.
2. PH value
The binder used in the ink is mainly an alkali-soluble acidic resin, so the control of the pH is very important. Usually the pH should be controlled at 8.5 to 9.5. At this time, the printing performance of the ink is the best and the quality of the printed product is the most stable.
Since the amine is constantly volatilized during the printing process, the operator also adds new inks and various additives from time to time, so the pH of the ink changes at any time. When the pH of the ink is higher than 9.5, the alkalinity is too strong, the viscosity of the ink will decrease, resulting in slower drying speed and poor water resistance; when the pH is lower than 8.5, the alkalinity is too weak, and the viscosity of the ink will rise. High, resulting in faster drying speed, easy to block the printing plate and anilox roller, causing dirty surface and bubbles.
Since the influence of pH value and viscosity are directly related, it is recommended to control the two in actual printing, especially in the case of color printing. 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 may be added. When the pH is high, a solvent or a diluent may be added for dilution.
3. Thickness
The thickness of the ink refers to the extent to which the solid powdery substance such as a pigment or a filler mixed in the linked group is dispersed.
The thickness of the ink is also an important quality indicator because it is not only related to the application properties of the ink (including tinting strength, hue, stability, etc.), but also to the rheological properties and economic benefits of the ink. If the ink is too thick, it will cause defects such as ink build-up and paste, and the tinting strength 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 also affects its dispersibility. When the dispersibility of the ink is not ideal, its printing performance is also unsatisfactory, which is manifested by the fact that the surface of the printed matter is not smooth, smooth, soft, and has a "block"-like feeling, thereby greatly reducing the printing effect.
In actual printing, different thicknesses of ink can be selected according to specific requirements. When the printed network cable is relatively thin and the precision is relatively high, the fineness of the ink is relatively high; when the printing is not very high precision printing or solid printing, the thickness of the ink can be appropriately reduced, which is also for Reduce costs and increase economic efficiency.
4. Dryness
The dryness of the ink is an important technical indicator. In addition to the performance of the ink itself, the drying speed of the ink is related to the printing speed, the drying ability of the drying equipment, and the performance of the printing material. This paper only considers the factors of the ink itself.
The principle of selecting the drying properties of the ink is usually that the ink should be sufficiently dried under the existing conditions of the printing machine before the next printing and before rewinding or stacking, but not on the printing plate.
The ink uses water as a solvent, and an alkali-soluble acidic resin is used as a binder. It is mostly used for printing corrugated boxes and paper bags, and thus the dry form is mainly osmotic drying and a small amount of volatile drying. Since water is used as a solvent, its volatilization rate is inferior to other solvents, so the drying of the ink is slower than that of the solvent-based ink. Therefore, it is usually necessary to add a small amount of an alcohol to the solvent to increase the drying speed of the ink. In addition, under the conditions determined by the solvent, different kinds of resins are selected, which also affects the drying property of the ink. Because the curing speed of different resins on the substrate is different, and the release characteristics of the solvent are also different, the drying property of the ink is also different.
In practical applications, the drying properties of the ink are also related to its viscosity and pH. The ink is dry but may be too viscous and can be solved by further dilution; the ink dries quickly, possibly with a pH below 8.5, and can be adjusted by adding a stabilizer. In addition, a quick-drying agent or a slow-drying agent may be added to the ink according to the actual printing speed, and the drying speed of the ink may be appropriately changed to obtain a stable printing effect. The amount of the quick-drying agent or the slow-drying agent is generally from 1% to 2% of the total amount of ink.
5. Conclusion
There are still many quality indicators that need to be controlled in ink application. In addition to the main indicators mentioned in this article, it also includes: color, gloss, adhesion, stability. In actual printing, in order to improve the printing quality comprehensively, we should comprehensively consider various items. index.