In plastic gravure printing, how to control the viscosity of ink printing is a very critical issue. Especially in high-speed rotary gravure printing, whether the viscosity of ink printing is properly controlled is directly related to most quality problems such as pigment transfer, dirty plate, gloss brightness of printed products, pigment bonding firmness, static electricity, etc., and affects the yield and work efficiency of printed products.
Factors affecting ink viscosity
Factors affecting viscosity include the diameter, hardness and elastic modulus of the elastic roller, the nature of the roller, the speed of the roller, the temperature of the roller, the ambient temperature and humidity, the temperature of the sample, the thickness of the ink film, the influence of the ink or the binder on the elastic roller, the condition of the elastic roller during the cleaning process, and the nature of the tested sample.
Factors affecting viscosity include viscosity and its temperature, the concentration of component particles, particle size, etc., which are closely related to humidity and air pressure. The measurement accuracy and measurement units of different measurement methods are different and cannot be converted to each other.
From the above four points, we can see that there is a big difference between ink printing viscosity and viscosity, but the two are still related. Mainly manifested in that viscosity is a physical quantity that measures the viscosity of a fluid. The dynamic viscosity of a fluid is a quantity determined by the inherent physical properties of the fluid itself. Its value is a direct measure of the viscosity of the fluid and a measure of the strength of the fluid's ability to resist shear deformation during motion. Under the same environmental conditions, a high viscosity indicates a high viscosity, and vice versa.
How to control the viscosity of ink printing in high-speed rotary gravure printing
The ink is composed of organic solvents, binders, pigments, additives, and auxiliary materials. When these raw materials are determined, the degree of processing, methods, and the composition of each component are determined, the quality of pigment transfer is mainly related to the printing viscosity of the ink. Practice has shown that the viscosity of ink printing has a certain range (11~24S, using Zahn viscosity cup No. 3). The greater the viscosity of the ink printing, the worse the effect of pigment transfer. Because the role of the solvent is to dissolve resins or additives and auxiliary agents, give them fluidity, and make the pigment easy to disperse. When the ink printing viscosity is too high, the entire ink system is in an oversaturated state, and the fluidity of pigments and other substances is poor. They cannot be evenly dispersed, but appear in clumps and are easily piled up together, so that the pigment cannot smoothly enter and exit the mesh. When the viscosity of the ink printing is too high, the pigment cannot even enter the mesh, let alone transfer. This is commonly known as the blocking phenomenon.
Therefore, we only need to make the colloidal system composed of resin, pigment, etc. and organic solvent not in an oversaturated state, but in a saturated state or an unsaturated state, so that the pigment and other substances can be well dispersed in it to form a uniform and delicate colloidal system, so that the pigment can enter and exit the mesh smoothly, and the problem can be solved. Some ink manufacturers recommend that the ink printing viscosity be between 15 and 18S (Zahn viscosity cup No. 3). However, in practice, especially in high-speed gravure printing machines (printing speed is 100-260m/min), in order to ensure a good transfer effect, while being able to print for a long time and improve efficiency, the ink printing viscosity is generally between 11 and 15S (Zahn viscosity cup No. 3) to find its ideal state point.
In plastic gravure printing, dirty plate problems often occur. The phenomenon is that the non-printing surface of the roller will also be stained with a hazy layer of resin (pigment entrained in the resin), and transferred to the substrate, making the substrate appear flaky or linear. The root cause of the dirty plate phenomenon is not only related to the hardness of the scraper, the pressure of the scraper, the contact angle of the scraper, the quality of the plate cylinder, etc., but also mainly related to the printing viscosity of the ink. As mentioned above, when the printing viscosity is too high, the resin pigment and other materials have poor fluidity and cannot be evenly dispersed. They are easy to pile up together. Under the relatively high-speed movement of the scraper and the plate cylinder, the scraper and the plate cylinder are easily damaged, and linear dirt is easy to occur. At the same time, because the resin and the like are also in an oversaturated state, a large amount of resin is aggregated together, and the resin has an affinity for the roller, under the pressure of the scraper, the resin will stick tightly to the roller (pigment entrained in the resin), so that flaky dirt appears. Generally speaking, the dirty plate phenomenon is not easy to occur when the ink printing viscosity is small.
However, if the ink printing viscosity is too low, it means that the ink contains more organic solvents, while the resin, pigment and other components are relatively less. In this way, it cannot form a smooth film when drying, and the printed product will turn white, become dull and lack luster. Therefore, if the printed product needs to have a better gloss, it is generally necessary to consider using a larger ink printing viscosity (13-19S, Zahn No. 3 cup), that is, the resin and pigment components should be relatively more to meet the needs of the printed product.
At the same time, when the ink is printed on the plastic and dries to form an ink film, it can firmly attach the wrapped pigment to the plastic surface. The resin is the main material for the firm attachment of the plastic. Therefore, the pigment bonding firmness is also related to the printing viscosity of the ink. This shows that the ratio of resin and pigment in the ink cannot be too large or too small, but should be within the normal range, so that the pigment bonding firmness is large. Under normal environmental humidity, the ink printing viscosity is above 16S (Zahn No. 3 cup), and static electricity generally does not occur. When the ink printing viscosity is below 16S (Zahn No. 3 cup), as the viscosity decreases, static electricity phenomena such as beard, streak, edge rejection, ink flying, poor transfer, and extremely irregular water stains of pigments will occur and worsen.
In view of the above reasons, industry insiders believe that the ideal state point of ink printing viscosity is generally found between 11 and 17S (Zahn No. 3 cup). If static electricity occurs within this range, in line with the principle of mutual consideration, it is generally not necessary to increase the ink printing viscosity to eliminate static electricity, but to eliminate static electricity by maintaining normal environmental humidity, changing the composition structure of the diluent, installing a static electricity removal brush, and using static electricity prevention agents. In this way, while ensuring quality, we can improve efficiency and yield rate, laying a necessary foundation for printing companies to make profits.