Printability of flexographic printing aqueous inks

In the 1990s, it was a period of rapid development of flexographic printing. According to information provided by the FTA, at the end of the 20th century, 33% of the printing industry and 55% of products in packaging printing were flexible. Printed to complete. Since 1995, in China's packaging, decorating and printing industry, the proportion of flexographic printing has been increasing. More than 100 flexo production lines have been introduced and distributed in more than 20 provinces, cities and autonomous regions across the country. Major printed trademarks, folding cartons, cigarette cases, wine boxes, cosmetics boxes, health products boxes, stationery, liquid packaging, toilet paper, napkins, paper cups, wallpaper and other printed matter. First, the characteristics of the flexographic printing ink transfer The modern flexographic printing, refers to the use of high resolution, high elasticity, plate thickness of about 1.7mm in the flexible version, with anilox roller transmission of low viscosity, solvent volatile dry ink Printing method. Figure 1 Ink transfer system of flexographic printing presses At present, high-speed, high-quality flexographic printing presses mainly use the anilox roller and squeegee inking system shown in Figure 1 for ink transfer. The blade of the squeegee points at an obtuse angle to the linear velocity of the surface at the pressure contact of the squeegee and is a reverse squeegee (a positive squeegee for gravure printing). Flexo printing ink transfer is very simple, low viscosity and high fluidity ink, filled into the fine ink hole in the anilox roller, excess ink scraped by the squeegee, leaving the ink in the ink hole in the anilox roller, followed by Transferred to the graphic relief of the flexographic version, when the ink area on the printing plate touches the substrate, a “kiss” is formed, and the transfer of the ink is completed by light pressure kiss printing. Although the printing mechanism of the flexographic printing is very simple, the material of the anilox roller, the shape of the ink hole, the viscosity of the ink, and the drying property directly affect the transfer of the ink. In flexo printing, environmentally friendly water-based inks dominate. Water-based ink consists of colorants, binders, adjuvants and other ingredients. The colorant is a coloring substance of an aqueous ink and gives a certain color to the ink. In high-level flexographic printing, high-line anilox rollers are used to transport the ink, so that the amount of transferred ink is small, and the printed ink layer is thin. In order to obtain colorful prints, colorants for water-based inks generally use Yankee, which has good chemical stability and high strength coloring power. The binder consists of water, resins, amines and other organic solvents. Resin is the most important component of water-based ink, directly affect the ink adhesion, drying speed, anti-viscosity and heat resistance, etc., but also affect the gloss and the ink transfer, commonly used acrylic resin. The role of amines is to maintain the pH of aqueous inks alkaline, because acrylic resins provide better printing results in alkaline media. The role of water or other organic solvents is to dissolve the resin, adjust the viscosity of the ink and the drying speed. Flexographic printing is a high-speed printing. A small amount of ink is fixed on the anilox roller. Only the use of low-viscosity ink, the anilox roller can achieve the desired effect. The auxiliary agent mainly adjusts the ink's pH, dryness, etc. Commonly used adjuvants for water-based inks include: stabilizers, retarders, dilutants, defoamers, and the like. Stabilizers can adjust the pH and can also be used as a diluent to reduce the viscosity of the ink. Blockers are used to suppress the drying speed of aqueous inks, prevent the ink from drying on the anilox roller, and reduce the stencil. Diluter is used to reduce the color of freshwater ink, but has no effect on the viscosity and drying speed of the ink. The diluting agent is also a brightener that can increase the brightness of aqueous ink. Defoamers can suppress and eliminate bubbles and improve ink transfer performance. Second, the pH value of the ink viscosity and drying effect The biggest feature of aqueous ink is that the ink can be mixed with water before drying, once the ink is dry solid, it can not be dissolved in water and ink, that is, the ink has water resistance. Therefore, special attention should be paid when printing, do not let the ink dry solid on the anilox roller, so as not to block the anilox roller ink hole, hindering the quantitative transmission of ink caused by poor printing. On the other hand, it is also necessary to pay attention to the fact that during the printing process, the flexographic printing plate must always be kept wet by the ink, so as to avoid the blockage of the text patterns on the plate after the ink is dried. Water-based inks produced by the ink factory are generally shown in Table 1. Table 1 General properties of aqueous inks Color Viscosity (s) Fineness (μm) Adhesion (Tack) pH Yellow 40353.59 Red 75153.39.5 Black 801039.5 Gold 62 4.69.1 From the data in Table 1, it can be seen that aqueous ink is weak Alkaline, the pH is about 9 or so. In fact, the printability of the pH value of aqueous ink is mainly related to the viscosity and dryness. The relationship between the pH and viscosity of aqueous inks is shown in Figure 2. Figure 2 Relationship between the pH and Viscosity of Aqueous Inks Figure 2 shows the viscosity values ​​tested with a 4# coating cup. The pH was tested with a WS pocket digital display pH meter. The curve shows that the viscosity of the aqueous ink decreases with the decrease of the pH value. The relationship between the drying properties of aqueous ink and pH is shown in Figure 3. The drying test method in the figure is: place a small amount of ink on the scraper fineness meter at 100μm, quickly scrape off with a scraper and open the stopwatch at the same time. After 30s, align the lower end of the paper with the zero mark of the scraper. Attach the groove to the palm of your hand, press it with your palm quickly, peel off the paper, and measure the length of the unsqueezed ink. Expressed in millimeters, this is the initial dryness (dryness in Figure 3). The shorter the unsqueezed ink trace, the slower the drying. The curve in Figure 3 shows that with the gradual increase in pH, the water-based ink is less dry. Figure 3 Relationship between pH and Dryness of Aqueous Inks The pH of aqueous inks is mainly maintained by amines. However, due to volatilization of amines during the printing process, the pH value decreases, which will increase the viscosity and transferability of the ink. The change is worse, and the drying of the ink is accelerated, the anilox roller is clogged, and the stencil failure occurs. To maintain the stability of the ink properties, on the one hand, to avoid the leakage of amines as much as possible, such as covering the upper cover of the ink tank. On the other hand, the stabilizer should be added to the ink tank periodically and quantitatively. According to production experience, the pH of general water-based inks is maintained between 8.0 and 9.5. When the pH is too high, the ink dries slowly and there is a problem that the back side of the print is dirty and poor in water resistance. Third, the adjustment of water-based ink printing Flexographic printing presses, the vast majority of printing press printing materials rotary printing press. The new type of flexographic printing press has a compact structure, and it has a variety of on-line post-press processing functions from roll feeding to printing, glazing, rolling tooth line, slitting, crosscutting, and single-sheet finished or box-making, forming a package. One-stop production line for printing and processing. In the printing process, which part has a problem will cause a lot of waste. The printability of the ink is an important factor in ensuring that the flexographic printing press works normally and prints high quality products. The viscosity and dryness of the ink printability are even more critical. In northern China, there was a packaging and decoration printing factory. In 1996, a flexographic printing production line was introduced, mainly printing cigarette packets. At that time, the best water-based ink imported from the United States was used. When printing gold ink, it was stopped every 5 minutes. Workers used a brush to clean up the dried ink residue in the ink hole of the anilox roller before printing. It was extremely low. At that time, ink experts in the United States could not do anything. Such failures had never occurred in the United States. Later, the factory's workers and the Chinese printing process experts jointly searched for the cause and found that the malfunction was caused by the ink drying too quickly. The original ink imported from the United States, the drying speed is designed at a printing speed of 150 revolutions per minute. Due to the low tensile strength of the paper, the speed of the die cutter is limited, and the speed of the imported flexographic printing production line can only be reached. 85 revolutions per minute, so that the ink dried solid on the anilox roller and blocked the ink roller, in addition to the grain size of the gold ink itself and the printing process, the amine material leakage, the pH value decreased, the drying speed is even faster. For the cause of the failure, an appropriate amount of a retarder composed of water, ethylene glycol, butyl ether, isopropyl alcohol, and 1830 emulsion was added to the aqueous gold ink, and ethanol and ethylene glycol amine were added to the ink tank at regular intervals. Water and 1830 emulsion stabilizers keep the pH of the ink at around 9.5. During the printing process, the ink does not dry on the anilox roller.