The control indexes of water ink mainly include viscosity, PH value and dryness.
1. Master the viscosity of ink. Ink viscosity refers to the ability of ink to hinder the movement of other molecules under the influence of its own internal force. Generally use a certain amount of ink, flow through the capillary at a fixed temperature (generally 20 ℃), record the time required to determine the viscosity.
The viscosity of the ink is the main factor that determines the ink's transfer performance, print fastness, penetration and gloss. The viscosity of water ink is too high, the color becomes darker, the consumption of ink increases, and the drying speed slows down; the viscosity is too low, the color changes, and the dots expand, resulting in a decline in printing quality.
In flexographic printing, the viscosity of water ink is not solely determined by the viscosity of the ink itself, but by all factors that affect the viscosity of the ink, such as the anilox roller, image conditions, and other factors. The actual viscosity should be able to be easily circulated to the doctor blade by the pump, and can be transferred to the printing plate and substrate in equal amounts, so that the density of the printed product and the image dots will not change significantly, and the printing quality should be maintained. The consistency of the ink must be consistent.
When printing screen version and field version, the ink viscosity should be different. In general, when printing a screen, the viscosity should be slightly lower, and the printing speed of the printing machine also has an effect on the ink viscosity; when the printing speed is high, the ink viscosity is lower, and when the printing speed is low, the ink viscosity is higher. Generally, after the anilox roller and the ink transfer roller are added with a doctor blade, if the stripes of ink can be observed, it means that the viscosity is suitable; if the droplet-shaped ink dots appear, it means that the viscosity of the ink is too large; if the ideas are not seen at all , Indicating that the viscosity is too small.
2. Adjustment of ink viscosity. The adjustment and control of ink viscosity should start with the control of ink temperature and the amount of thinner. In order to ensure the consistency of printing quality, the consistency of ink viscosity must be maintained. The following table gives the amount of thinner:
It can be seen from the table that temperature has a greater influence on the amount of diluent added. Dilute ink has a great influence on the final density of the printed matter. For example, the temperature of the ink before printing is 10°C and the viscosity is 25S. After a short period of operation, the temperature of the ink increases to 20°C and the viscosity decreases to 18S. As a result, the ink layer is diluted and the density decreases. Therefore, in actual production, the temperature of the ink used should be stabilized at the temperature of the printing workshop. For the increase in ink temperature and the change in viscosity due to friction during printing, cooling can be considered to solve it.
Ink is particularly sensitive to the addition of thinner, so you must be careful and precise in adjusting the viscosity. Avoid excessive (not more than 5%) to prevent the ink viscosity is too low. Bubbles are formed due to surface tension, and generally hard water produces more foam than soft water. If reducing bubbles, defoamers should be added. Do not use paraffin-based defoamers, otherwise the printing durability of the plate will be affected. Silicone-based defoamers must be diluted with water and added to the ink under continuous stirring. Silicone-based defoamers with high undiluted concentration will be added to the ink, which will form pinholes in the printed matter. Deinking has occurred.
3. PH value of ink. Ink and ink generally control PH value of about 9, slightly alkaline. If the PH value floats down too much, it will cause the ink erosion (a property that the ink becomes thin when it is subjected to external force and returns to its original state after standing), making the ink thicker. Ink will flow into the inside of the scraper or rubber roller, resulting in dry ink. Insufficient amount of ink on the anilox roller, such as white leakage during printing. If the PH value is too high, the viscosity drops, and the drying is slow. The method of simply adding a small amount of water to reduce the viscosity of the ink to adjust the fluidity of the ink will not be satisfactory. For this reason, alkaline substances should be added to adjust the ink appropriately. PH value.
4. Dry. The drying of ink and wash has always been troublesome. To solve the drying of ink and wash, comprehensive adjustment is needed. For example, the comonomer composition, neutralizing agent, co-solvent and auxiliary agent of water ink are screened and improved in many aspects. The drying speed of the ink will be greatly improved to meet the requirements of the existing production speed. If the drying system of the existing equipment can be adjusted and improved accordingly, the drying of the ink will be more ideal.
5. We will gather all the previous views on the use of ink and wash, but nothing more than the following: Shake them well before use or sampling, and confirm the viscosity under the same test conditions; according to the design status of the printing plate and the printing Speed, determine the machine viscosity of the water-based ink, when the viscosity is high, it should be properly diluted with water or a special solvent, especially pay attention to the viscosity of the water-based ink when it is diluted easily.
If the viscosity is too low, it is best to use the original ink to adjust, and use as little or no thickener as possible. When the color density is high, you should first use a special diluent to properly rinse and then adjust the viscosity; during the printing process, pay attention to the change of viscosity and check the viscosity to make a record; during use, a small amount of foam can be added The defoamer (usage is generally controlled within 0.2%); at the end of printing, the relevant mechanical equipment should be cleaned as soon as possible; unfinished ink should be returned to the barrel (but not mixed with the original ink), kept in a sealed and cool place; restart It is best to use a 90-mesh or above net to filter before use.
In recent years, traditional inks have the disadvantages of slow drying, poor gloss, and easy paper stretching. With the improvement of the application of new materials and related conditions of printing equipment, there have been obvious improvements. The application of domestically produced flexographic high-precision ink and wash shows that the drying speed can reach 200m/min and the resolution has exceeded 150 lines.