A brief analysis of the characteristics and printa

A brief analysis of the characteristics and printability of water-based inks (Part 2)

2. Thixotropy

the property of inks that causes changes in flow properties under the action of external forces is called thixotropy of inks. On the printing machine, due to the function of the ink roller, the fluidity and ductility of the ink during transmission also increase. Until it is transferred to the printing sheet, the external force disappears, its fluidity and ductility decrease, and the ink thickens from thin, so as to ensure the accuracy and clarity of the imprint and dot. When the ink has good thixotropy, it is conducive to the smooth and uniform transfer of the ink and improve the transfer rate of the ink. The factors that determine the thixotropy of ink are as follows. (1) Properties of pigments. After the pigment with strong surface adsorption is made into ink, its thixotropy is large. Like a type of carbon black pigment, its surface is oxidized with ozone to reduce its surface adsorption. The ink made of it has less thixotropy than the ink made of untreated carbon black

(2) amount of pigment. Large dosage, thixotropy is also large, because the pigment molecules attract each other and flocculate

(3) pigment particle shape. The thixotropy of inks with needle and flake pigments is greater than that of granular inks

(4) wettability of pigments. The wetting ability of pigment and binder is poor, and the thixotropy of ink is large, on the contrary, it is small

if the thixotropy of water-based ink is too large, due to the short ink path of flexographic printing, the ink supply will not be smooth, and even the ink supply will be interrupted, which cannot ensure a uniform and accurate ink supply. If the thixotropy of water-based ink is too small, the dot will increase and the text and lines will become thicker due to the infiltration and over spreading of ink on the paper. Different types of flexographic printing have different requirements on the thixotropy of ink. Generally, the thixotropy of ink is required to be slightly larger for line version, text version and line version; The large field version is slightly smaller

3. Adhesion

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

(1) the adhesion of water-based ink is less than that of paper. When the adhesion of water-based ink is large, the ink separation is difficult, and the ink extension on the printing machine is uneven. When the ink layer is separated between the paper and the printing plate, if the resistance exceeds the binding force of the paper, the paper will be roughened or even peeled off

(2) the adhesion of the first color ink should be larger, and the adhesion of the following colors of ink should be gradually reduced. Otherwise, the post printing ink will stick away the pre printing ink

4.ph value

the water-based ink is generally weakly alkaline, and the normal range of pH value is 8.5 ~ 9.5. At this time, the printing performance of the water-based ink is good, and the printing quality is stable. The influence of pH value on the printability of water-based ink is mainly reflected in the viscosity and drying of the ink

when the pH value is higher than 9.5, the alkalinity is stronger, the viscosity of water-based ink decreases, the drying speed becomes slower, and the water resistance becomes worse; When the pH value is lower than 8.5 and the alkalinity is weak, the viscosity of the water-based ink will increase. The ink is easy to dry and fix on the printing plate or roller, which is easy to cause dirt on the plate and produce bubbles

the pH value of water-based ink is mainly maintained by ammonia compounds. However, due to the characteristics of the printing ring stiffness testing machine, the volatilization of ammonia substances in the process and the decrease of pH value will increase the viscosity of the ink and deteriorate the transferability. At the same time, the drying speed of the ink is accelerated, blocking the inking hole of the roller and causing paste. To maintain the stability of ink performance, on the one hand, it is necessary to avoid the leakage of ammonia substances as far as possible and cover the upper cover of the ink tank; On the other hand, ink stabilizer should be added to the ink tank regularly and quantitatively

precautions

1. Before printing, the temperature of all colors of inks should be stabilized at the temperature of the printing workshop, 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, so it is necessary to improve the machine speed; When the temperature is low, the evaporation of water is slow, and the drying of water-based ink is also slow. You can add a drying device or reduce the speed of the machine

2. The quality and performance of paper have a great impact on the drying speed of water-based ink and the quality of printed matter. Therefore, before reducing the weight and printing, we must fully understand the performance of the paper used, especially the absorption of ink. If the ink absorption of the paper is poor, and the ink cannot be absorbed by the paper soon after it is transferred to the paper, the wet ink will be transferred to the back of the previous paper, causing dirt. However, if the paper absorbs the ink too much, it will penetrate excessively and cause through printing, or seriously tarnish the surface of the printed product. If the printing is coated paper, the binder will all penetrate into the paper fiber, and the rapid penetration may cause the pulverization of the ink, so that the pigments left on the surface cannot be protected

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

4. the waste foamed plastic gradually swells up and must be fully stirred before use. The diameter of organic pigment particles in water-based ink is generally 2 ~ 5 μ m. However, the adhesion of pigment particles can make its diameter reach 10 ~ 25 μ m. Therefore, if the pigment is not evenly dispersed, it will affect the ink transfer performance, so it must be fully stirred before use

5. When the viscosity is high, it is necessary to dilute it with water or special solvent, especially pay attention to the fact that the viscosity of water-based ink is easy to decline when it is diluted. If the viscosity is too low, it is best to adjust it with raw ink, and use less or no thickener as far as possible. When the color concentration is high, it should be diluted appropriately with special diluent first, and then adjust the viscosity

6. In the process of use, when more foam appears in the ink, a small amount of defoamer can be added

7. At the end of printing, the stripper roll should be cleaned as soon as possible, otherwise the pigment in the ink will dry at the bottom of the inking hole of the stripper roll, affecting the ink delivery quality of the stripper roll

a comprehensive understanding of the printing of water-based ink is mainly to make the majority of employees full of enthusiasm for the company, which is very beneficial to better master the process of printing water-based ink. Only by constantly summarizing and exploring the printing suitability and experience, can we achieve a more ideal water-based ink printing

information source: printing technology, College of paper engineering, Shaanxi University of science and Technology (chenyongchang)