Flexographic printing technology question and answer (5)
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Why use water-based inks to monitor pH and viscosity at the same time. Everyone knows that the change in viscosity of printing ink has a direct influence on the amount of ink on the printing plate. Therefore, if the viscosity of the ink changes, it will affect the printing quality. Therefore, it is generally necessary to monitor the viscosity of the ink during the printing process, which is particularly important in gravure printing, because the gravure printing uses a low-viscosity solvent-based ink, the solvent evaporates quickly, and the viscosity is liable to change. However, flexographic printing is very different from gravure printing. The above discussion of the composition of water-based inks has referred to "amines", which are solubilizing resins that determine the acidity of water-based inks. Generally, the pH range of water-based inks should be between 8.8 and 9.5. At this time, the printing performance of water-based inks is the best, and the quality of printed products is the most stable. Since the amine is constantly volatilized during the printing process, the operator will occasionally add new ink and various additives to the ink, and the pH of the ink may change at any time. When the pH value is higher than 9.5, the enthalpy is too strong, the viscosity of the water-based ink will decrease, the drying speed will be 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 weak. When it is raised, the ink is easy to dry, blocked on the plate and the anilox roll, causing stain on the plate surface and foaming. Therefore, the pH of water-based inks is regarded by today's flexographic printing industry as one of the most critical factors controlling the production process. In a sense, pH control is even more important than viscosity control. We recommend users to adjust the pH value. Viscosity is linked for monitoring. This is a new attempt, and it is best to prepare a simple pH indicator in addition to the viscosity cup. The operator must not only understand the pH values of the various ink additives used, but also their changes, and should strictly follow the technical data provided by the supplier when printing. As for the degree to which viscosity and pH should be controlled, we believe that the more accurate it is, the better it is. Because water-based inks have only one optimum match between pH and viscosity, water-based inks can be adapted to printers, anilox rolls, ink transfer systems, flexographic plates, substrates, drying systems, and shop floor environments. An external environment consisting of. Experience has shown that when overprinting another color over a color, the viscosity of the ink should be gradually increased and the pH of the ink gradually reduced. This helps the drying of the ink and prevents the ink that is printed back from drying the already dried ink that has already been printed to affect the quality of the print.
Incidentally, the pH range of some special water-based inks is different from ordinary water-based inks, such as gold and silver inks with a pH of 7.5 to 8.0 and fluorescent inks of 7.5 to 8.3.
Types and methods of use of commonly used water-based ink additives
1. pH stabilizer
It is well known that pH is important for the stability of water-based inks, so the printing plant should first be equipped with a pH stabilizer. The pH stabilizer can supplement the amine and water volatilized from the ink transfer system during the printing process, which helps the viscosity of the ink to be stable and ensures the printing quality. Depending on the substrate, the pH stabilizer is divided into a common type and a film type. When printing fine mesh wires, use a dedicated pH stabilizer. The general pH stabilizer is used to add 1~2% of the total ink amount every 30 to 40 minutes, or to add the stabilizer to the circulating ink pump.
2. Defoamer
Generally, an appropriate amount of antifoaming agent is added to the ink before starting the printing to prevent the ink from foaming, and an appropriate amount of defoaming agent may be added if necessary during the printing process. The amount of defoamer used does not exceed 0.5 to 1% of the total amount of ink.
3. Quick drying, slow drying agent
The quick-drying and slow-drying agent can be added to the ink according to the actual printing speed of the printing machine, and the drying speed of the water-based ink can be appropriately changed to obtain a satisfactory printing effect. The amount is 1~2% of the total ink amount.
4. Withdrawal agent
Under the premise of constant pH and viscosity, the addition of the lightening agent can reduce the color concentration of an ink. The amount should be determined on a case-by-case basis.
5. Thinner
The addition of a diluent to the ink reduces the viscosity of the thickened water-based ink without affecting the pH. The amount must never exceed 5% of the total ink.
6. Enhancer
The reinforcing agent can enhance the adhesion of the printing ink or the varnish to the printing material, and the amount is generally 1-2% of the total ink amount.
7. Remove static agent
The antistatic agent can effectively prevent the accumulation of static electricity on the printing machine, and the amount thereof is generally 0.5 to 1% of the total ink amount.
In view of the current situation that many brands of imported water-based inks and water-based inks produced by domestic manufacturers try to occupy the market, we suggest that users first familiarize themselves with the various properties of an authoritative brand of water-based inks and the use of corresponding additives. After trying other brands, you should avoid mixing the two brands of water-based inks or additives, 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 the imported water-based ink.
What should you pay attention to when printing non-absorbent materials with water-based inks?
When using a flexographic water-based ink to print non-absorbent substrates such as films and aluminum foils, first select the appropriate printing ink. Since the past gravure printing uses benzene ink or alcohol ink, the surface tension value of the solvent is below 28 dynes per cm, so for the general film, the adhesion of the gravure ink is not a problem, and some films do not need to be discharged beforehand. Processing can be printed. However, for water-based inks, the adhesion properties of the ink are critical. In order to avoid losses, it is generally necessary to conduct a large number of tests before the official printing in order to select an ink having better adhesion, and the surface tension value of the water-based ink or the UV ink with respect to the printing material should be maintained at 38 dynes per cm or more. A dyne value 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, draws a wet mark on the surface of the substrate, and then observes whether the wet mark will present a spherical oil film, and how long it takes to form a spherical oil film. In this way, the adhesion performance of the water-based ink on the printing material is judged. The disadvantage of this method is that the test pen is easily stained after being used for many times and affects its accuracy.
For flexo press users who use plastic film as the printing material, they should try to buy those discharged films, and a discharge processor (CORONATREATER) should be installed on the flexo press to discharge the film again before printing. Handle to ensure the highest surface tension value of the film. Because even if the user purchases the discharged film, when the film is stored for too long or improperly stored, the original treatment effect is weakened until it disappears, which affects the adhesion of the ink. If an undischarged film is purchased, it is difficult to make the film have good ink adhesion by a single discharge treatment before printing, and it is difficult to achieve an ideal printing effect.
In order to reconfirm the adhesion of the ink on the substrate, the following tests can be performed after proofing:
1. Scratch resistance test
The medium pressure is applied with the nail of the thumb, and the ink is repeatedly scraped a plurality of times at the ink layer which has been printed and dried on the film. If the ink layer is scratched or the ink is peeled off, the adhesion of the ink is not ideal.
2. Tape peeling test
Use 3M brand 610 or 810 adhesive tape to evenly adhere to the printed sample, and use your fingers to press it on the back of the tape to make it stick. Then grasp the ends of the tape and pull it at a moderate speed to observe the ink being stuck by the tape. Happening. As long as there is ink on the tape, the adhesion of the ink to the substrate is not good.
3. Dry friction test
The printed surfaces of the two prints are stacked one on another, and a pressure of 2 to 4 pounds is applied repeatedly to rub, and scratches or ink peeling indicate poor ink adhesion.
4. Moisture resistance test
The method is similar to the above dry rub resistance test except that a little water is added between the two prints.
5. Resistance to detergent test
Drop a drop or a few drops of the detergent on the print. After 15 seconds, wipe it off with a paper towel or a clean soft cloth, then wipe it with a medium pressure. If there is any change in the printed image, the ink is indicated. The adhesion is not good. After the above test, if it is found that an ink performance cannot meet the printing requirements of the printing material, the ink of other brands should be replaced and the test should be continued until a suitable ink is found.