A brief analysis of the characteristics of water-based ink and printability (below)

2. Thixotropy The property of the ink that causes changes in flow properties under the action of an external force is called thixotropy of the ink. In the printing press, due to the action of the ink roller, the fluidity and ductility of the ink are also increased, and after the transfer to the printed sheet, the external force disappears, the fluidity and ductility of the ink are reduced, and the ink is thickened from the thin, thus Guarantee the accuracy and clarity of prints and dots. When the ink has a good thixotropy, it is beneficial to the smooth and uniform transfer of the ink and the transfer rate of the ink. The factors that determine the thixotropy of ink are as follows.

(1) The nature of the pigment. After the ink with strong surface adsorption is made of ink, the thixotropy is large, like a type of carbon black pigment, the surface of which is oxidized with ozone to reduce the surface adsorption, and the ink made is better than untreated carbon black. The resulting ink is less thixotropic.
(2) The amount of pigment. With a large amount of thixotropy, the pigment molecules attract and flocculate each other.
(3) Shape of pigment particles. The needle-like and flaky inks of the pigments have a greater thixotropy than the granular ones.
(4) Wettability of pigments. The wetting ability of the pigment and the linking material is poor, and the thixotropy of the ink is large. On the contrary, it is small.
If the thixotropy of the water-based ink is too large, since the ink path of the flexographic printing is short, the ink supply may not be smooth, and even the ink supply may be interrupted, and the uniform and accurate ink supply amount cannot be guaranteed. If the thixotropy of the water-based ink is too small, the text and lines will become thicker due to the ink's infiltration on the paper and the excessive dots in the exhibition. Different types of flexographic printing have different thixotropy requirements for inks. In general, the screen version, text version, and line version require the thixotropy of the ink to be slightly larger; the large-area field version is slightly smaller.

3. Adhesion

The ability of the ink to hinder the peeling of the ink film is called the adhesiveness of the ink. It is extremely important for the smooth progress of printing. The requirements for ink adhesion of flexographic printing mainly include the following points.

(1) Adhesion of aqueous ink is less than the binding force of paper. When the adhesion of water-based ink is large, ink separation is difficult, and the ink spread on the printing press is not uniform. When the ink layer separates between the paper and the printing plate, if the resistance exceeds the binding force of the paper, a phenomenon such as pulling and even peeling of the paper occurs.
(2) The adhesiveness of the first color ink is larger, and the adhesiveness of the back color ink is gradually reduced. Otherwise, there will be cases where the post-printing ink sticks the ink first.

4. pH value

Water-based inks are generally weakly alkaline, and the normal pH range is 8.5-9.5. At this time, the printing performance of water-based inks is better, and the quality of printed products is stable. The effect of pH on the printability of aqueous inks is mainly reflected in the viscosity and drying of the ink.

When the pH value is higher than 9.5, the alkalinity becomes stronger, the viscosity of the water-based ink decreases, the drying speed becomes slower, and the water resistance deteriorates. When the pH is lower than 8.5, the viscosity of the water-based ink increases when the alkaline is weak. The ink is easily dried and fixed on the printing plate or anilox roller, which can cause the plate surface to be dirty and generate bubbles.

The pH value of water-based inks is mainly maintained by ammonia compounds. However, due to volatilization of ammonia during the printing process, the PH value will drop, which will increase the viscosity of the ink and cause poor transferability. At the same time, the drying speed of the ink will increase, and the watermark will be blocked. Roller ink hole, paste version. To maintain the stability of the ink performance, on the one hand, it is necessary to avoid leakage of ammonia as much as possible, and to cover the upper cover of the ink tank; on the other hand, it is necessary to add the ink stabilizer to the ink tank regularly and quantitatively.

Precautions

1. Before printing, the temperature of each color ink used should be stabilized at the temperature of the printing shop, and the printing speed should be adjusted according to the specific conditions such as temperature. When the temperature is high, the water evaporates quickly and the water-based ink dries quickly. Therefore, the speed of the machine must be increased. When the temperature is low, the water evaporates slowly and the water-based ink can dry slowly. The drying device can be added or the machine speed can be reduced.

2. The quality and performance of paper have a great influence on the drying speed of water-based inks and the quality of printed matter. Therefore, it is necessary to fully understand the properties of the paper used, especially the ink absorption, before printing. If the paper absorbs ink poorly, the ink cannot be quickly absorbed by the paper after it has been transferred to the paper. Then, the ink that has not dried will be transferred to the back of the previous paper, causing it to become dirty. However, if the paper absorbs the ink too much, it will infiltrate excessively and cause print-through, or cause the surface of the print to lose its luster. If a coated paper is printed, all the binder material penetrates into the paper fibers, and rapid penetration may cause the powder to chalk, leaving the pigments on the surface unprotected.

3. According to the condition of the printing plate, the printing material and the printing speed determine the viscosity of the water-based ink.

4. Must be fully mixed before use, the organic pigment particles in water-based ink is generally 2 ~ 5μm in diameter, but the pigment particles can make the diameter of its adhesion to 10 ~ 25μm, so if the uneven pigment dispersion, will affect the ink transfer performance, so before use Must be well-mixed.

5. When the viscosity is high, it is necessary to dilute it with water or a special solvent. Special attention should be paid to the viscosity of the water-based ink when it is diluted. If the viscosity is too low, it is better to use the original ink for adjustment and use as little or no thickening as possible. The agent, when the color concentration is high, should first use a special diluting agent to properly dilute and then adjust the viscosity.

6. During use, a small amount of defoamer may be added when there is more foam in the ink.

7. At the end of printing, the anilox roller should be cleaned as soon as possible, otherwise the pigment in the ink will dry at the bottom of the ink roller hole of the anilox roller and affect the ink quality of the anilox roller.

A comprehensive understanding of the printability of water-based inks is very beneficial for a better understanding of the process of printing water-based inks. Only by continuously summarizing and exploring the printability and experience can we achieve more ideal water-based ink printing.

Source: Printing Technology School of Papermaking, Shaanxi University of Science and Technology (Chen Yongchang)