Why use water-based ink to monitor pH and viscosity at the same time We all know that changes in the viscosity of printing ink have a direct impact on the amount of ink on the printing plate, so that if the viscosity of ink changes, it will also affect the printing quality. Therefore, the viscosity of the ink is generally monitored during the printing process. This is particularly important in gravure printing because the gravure printing uses a solvent-based ink with a low viscosity. The solvent volatilizes quickly, and the viscosity tends to change. However, flexographic printing is very different from gravure printing. When discussing the composition of water-based inks, we mentioned "amines," which are solubilizing resins, that determine the degree of acidity of water-based inks. Usually, the pH value of the water-based ink should be in the range of 8.8 to 9.5. At this time, the printing performance of the water-based ink is the best and the quality of the printing product is the most stable. As the amine is constantly evaporating during the printing process, the operator will occasionally add new inks and various additives to the ink. The pH of the ink may change at any time. When the pH value is higher than 9.5, the viscosity is too strong, the viscosity of the water-based ink will decrease, the drying rate will slow, and the water resistance will be poor. When the pH is lower than 8.8, the viscosity of the water-based ink will be too low. As the ink rises, the ink is easily dried and plugged onto the plate and the anilox roller, resulting in dirt on the plate and foaming. Therefore, the pH value of water-based inks is regarded by the flexographic printing industry as one of the most critical factors in controlling the production process. In a sense, pH control is even more important than viscosity control. We recommend that the user Viscosity is linked and monitored. This is a new attempt. It is best to prepare a simple pH indicator in addition to the viscosity cup. Operators should not only understand the pH values ​​of the various ink additives used and their changes, but also should strictly follow the supplier's technical data when printing. As for the extent to which the viscosity and pH should be controlled, we believe that the more accurate and better it is, the more feasible it is. Because water-based inks have only one optimum pH and viscosity match point, water-based inks can be adapted to printers, anilox rollers, ink transfer systems, flexographic plates, substrates, drying systems, and plant environments. The composition of an external environment. Experience has shown that when overprinting another color on one color, the viscosity of the ink should be gradually increased and the pH of the ink gradually reduced. This helps the ink to dry, preventing the post-printing ink from causing the pre-printed, dried ink to wet again and affecting the print quality. Incidentally, the pH range of some special water-based inks is different from that of ordinary water-based inks, such as gold-silver inks with pH values ​​of 7.5-8.0 and fluorescent inks with 7.5-8.3. Commonly used types of water-based ink additives and methods of use 1. pH stabilizers As we all know, the pH value is important for the stability of the water-based ink, so the printing plant should first be equipped with pH stabilizers. The pH stabilizer can supplement amines and water that are volatilized from the ink transfer system during the printing process, helping to stabilize the viscosity of the ink and ensuring print quality. According to the different substrates, pH stabilizers are divided into ordinary type and thin film type. Special pH stabilizers are used when printing fine mesh cables. The usage of the general pH stabilizer is to add 1~2% of the total ink volume every 30~40 minutes, or stir the stabilizer into the circulating ink pump. 2. Defoamers are generally added with proper defoamer to the ink before starting printing to prevent the ink from foaming. An appropriate amount of defoamer can also be added during the printing process if necessary. The amount of defoamer does not exceed 0.5 to 1% of the total ink volume at a time. 3. Fast-drying, slow-drying agent can be added to the ink according to the actual printing speed of the printing press, and the drying speed of the water-based ink can be properly changed to obtain a satisfactory printing effect. The amount is 1 to 2% of the total ink. 4. Withdrawal agent Under the premise of constant pH and viscosity, adding a lighter can reduce the color concentration of certain inks. Its amount should be determined according to the specific circumstances. 5. Diluent The addition of a diluent to the ink reduces the viscosity of the thickened water-based ink without affecting the pH. The amount used must not exceed 5% of the total amount of ink. 6. Enhancer The enhancer can enhance the adhesion of the printing ink or bright oil to the printing material, and the amount thereof is generally 1 to 2% of the total ink amount. 7. Destaticizing agent The destaticizing agent can effectively prevent static electricity from accumulating on the printing machine. The amount of static electricity is generally 0.5 to 1% of the total ink. In response to the current situation that many brands of imported water-based inks and water-based inks produced by domestic manufacturers are trying to occupy the market, we recommend that users first become familiar with the use of various properties of an authoritative brand of water-based inks and the use of corresponding additives, and the printing quality is stable. After experimenting with other brands, the use of water-based inks or additives of the two brands should generally be avoided because different brands of inks or additives have different formulations. We have encountered the phenomenon of precipitation reaction after adding a domestic slow drying agent to imported water-based inks. When printing non-absorbent materials with water-based inks, what should be taken care of? When using flexo-based inks to print non-absorbent substrates such as film and aluminum foil, first select the appropriate printing ink. Since gravure printing used benzene ink or alcohol ink in the past, the surface tension value of the solvent is below 28 dyne per cm. Therefore, for a general film, gravure ink adhesion is not a problem, and some films do not even discharge in advance. Processing can be printed. But for water-based inks, ink adhesion is critical. In order to avoid losses, a large number of tests are usually conducted before the official printing in order to select inks with good adhesion, and the surface tension value of the water-based ink or UV ink relative to the substrate should be maintained at 38 dynes per centimeter or more. A dyne test pen can be used to test the surface tension of the substrate. The dyne test pen is a colored pen with a felt tip. It draws a wet mark on the surface of the printing material, and then observes whether the wet mark will show a spherical oil film and how long it will take to form a spherical oil film. In order to determine the water-based ink on the printing material adhesion properties. The disadvantage of this method is that the test pen is easily blurred after it is used many times and affects its accuracy. For users of flexo printers using plastic film as substrate, discharge-treated films should be purchased as much as possible, and a CORONATREATER should be installed on the flexo printer to discharge the film again before printing. Treatment to ensure the highest surface tension value of the film. Because even if the user purchases a film that has been subjected to a discharge treatment, when the film is stored for a long time or when it is not stored properly, the original treatment effect is weakened until it disappears, which affects the adhesion of the ink. However, if you choose a non-discharged film, it is difficult to make the film with good ink adhesion only after a single discharge process before printing, and it is difficult to achieve the desired printing effect. In order to reconfirm the fastness of the ink on the printing material, the following tests can be done after proofing: 1. Scratch Resistance Test The thumb's fingernail was applied with moderate pressure, and was repeatedly scraped several times at the ink layer that had been printed and dried on the film. If the ink layer was scratched or the ink was peeled off, the ink adhesion was not satisfactory. 2. Tape resistance test Use 3M brand 610 or 810 adhesive tape to evenly adhere to the printed sample, press it on the back of the tape with your fingers to make it stick, and then use the two ends of the tape to pull up at moderate speed. Observe that the ink is stuck on the tape. As long as there is ink on the tape, it indicates that the ink has poor adhesion to the printing material. 3. Resistance to Dry Friction Test The two printed surfaces of the print samples were stacked one on top of the other and applied with 2 to 4 pounds of pressure repeatedly rubbed. Scratches or peeling of the ink indicated poor adhesion of the ink. 4. The wet rub resistance test method is similar to the above dry rub resistance test except that a little water is added between the two prints. 5. Detergent Test Drop a drop or a few drops of detergent on the printed sample. After 15 seconds, use a paper towel or a clean soft cloth to clean it. Then wipe it with medium pressure. Any change indicates poor adhesion of the ink. After the above test, if it is found that certain ink properties cannot satisfy the printing requirements of the printing materials, other brand inks should be replaced and the test should be continued until a suitable ink is found. (End of the article) (Liang Xinyu Witlon (Hong Kong) Limited) ("The Graphic Arts Society Monthly")
