In plastic gravure printing, how to control the printing ink viscosity is a very important problem. Especially in high-speed rotary gravure printing, the ink viscosity control is directly related to the transfer of pigment, dirty plate, printing luster brightness, pigment bonding strength, static electricity and other quality problems, which has a great impact on the product yield and work efficiency.
Factors affecting ink viscosity
The factors affecting the stickiness include the diameter, hardness and elastic modulus of the elastic roller, the properties of the roller, the rotational speed of the roller, the temperature of the roller, the temperature and humidity of the environment, the temperature of the sample, the thickness of the ink film, the condition of the elastic roller during the cleaning process, and the properties of the tested sample.
The factors affecting the viscosity are that the viscosity is closely related to the temperature, the concentration and the particle size of the component particles, but not to the humidity and air pressure. The measurement accuracy and measurement unit of different measurement methods are different and cannot be converted to each other.
From the above four points we can see that the ink printing viscosity and viscosity have a great difference, but there is still a relationship between the two. The main performance is that the viscosity is a physical quantity to measure the viscosity of a fluid. The dynamic viscosity of a fluid is determined by the inherent physical properties of the fluid itself. Its value is a direct measure of the viscosity of the fluid, and also a measure of the strength of the resistance to shear deformation of the fluid in motion. Under the same environmental conditions, high viscosity means high viscosity, and vice versa.
How to control ink viscosity in high speed rotary gravure printing

Ink is composed of organic solvents, binders, pigments, additives and auxiliary materials. When these raw materials have been determined, the processing degree, method and composition of each component have been determined, the quality of pigment transfer is mainly related to the printing viscosity of the ink. Practice has proved that the printing viscosity of ink has a certain range (11 ~ 24s, using chaen viscosity cup No. 3). The higher the ink printing viscosity is, the worse the effect of pigment transfer is. Because the role of solvent is to dissolve resin or additives and additives, etc., to give its fluidity, so that the pigment is easy to disperse. When the printing ink viscosity is too high, the whole ink system is in the state of supersaturation, and the pigment and other substances have poor fluidity and can not be evenly dispersed. Instead, they appear in groups and are easy to pile up together, so that the pigments can not enter and leave the mesh smoothly. When the ink printing viscosity is too high, the pigment can not even enter the mesh, let alone transfer. This is commonly known as the blocking phenomenon.

Therefore, as long as the colloidal system composed of resin, pigment and organic solvent is not in the state of supersaturation, but in the state of saturation or unsaturated, so that pigments and other substances can be well dispersed in it to form a uniform and delicate colloidal system, so that the pigment can enter and leave the mesh smoothly, and the problem can be solved. Some ink manufacturers suggest that the ink printing viscosity is between 15 and 18S (chaen viscosity cup No. 3). But in practice, especially in high-speed gravure press (printing speed is 100-260m / min), in order to ensure good transfer effect, and to print for a long time and improve efficiency, the ink printing viscosity is generally between 11 and 15s (chaen viscosity cup No. 3).
In plastic gravure printing, there are often dirty plates. The phenomenon is that the non printing surface of the drum will also be stained with a hazy layer of resin (pigment is carried in the resin), and transferred to the substrate, making the substrate appear flake dirt or linear dirt. In addition to the soft hardness of the scraper, the pressure of the scraper, the contact angle of the scraper and the quality of the cylinder, the dirty plate phenomenon is mainly related to the printing viscosity of the ink. As mentioned above, when the printing viscosity is too high, the resin pigment and other substances have poor fluidity and can not be evenly dispersed, which is easy to pile up together. Under the relatively high speed movement of the scraper and cylinder, the scraper and cylinder are easy to be damaged, and thus prone to linear dirt. At the same time, due to the resin is also in the state of supersaturation, a large number of resins are gathered together, and the resin has affinity to the roller. Under the pressure of the scraper, the resin will adhere tightly to the drum (the resin contains pigment), which will lead to sheet dirt. Generally speaking, ink printing viscosity is small, dirty plate phenomenon is not easy to occur.
However, if the printing viscosity of the ink is too small, it means that the content of organic solvents in the ink is more than that of resins and pigments, so that a smooth film can not be formed when it is dry, and the printed matter will turn white and become dull and lack of luster. Therefore, if the print needs to have a better gloss, generally consider using a larger ink printing viscosity (13 ~ 19S, chaen No. 3 cup), that is, resin, pigment composition is relatively more, to meet the needs of the printed matter.
At the same time, when the ink is printed on the plastic and becomes an ink film after drying, it can firmly adhere the pigment wrapped on the plastic surface, and resin is the main material for firmly adhering to the plastic. Therefore, the pigment adhesive strength is also related to the printing viscosity of the ink. This shows that the proportion of resin and pigment in the ink should not be too large or too small, but should be in the normal range, the pigment bonding strength is large. Under normal environmental humidity, the printing viscosity of the ink is above 16S (chaen No. 3 cup), and the electrostatic phenomenon generally does not occur. The printing viscosity of the ink is below 16S (chaen No.3 cup). With the decrease of the viscosity, the electrostatic phenomena such as whiskers, stripes, edge exclusion, flying ink, poor transfer, and irregular water stains of pigments will occur and aggravate.
In view of the above reasons, people in the industry believe that the ideal state point of ink printing viscosity is generally found between 11 and 17s (chaen No. 3 cup). If static electricity occurs within this range, in line with the principle of mutual consideration, it is generally not necessary to improve the printing viscosity of the ink to eliminate static electricity, but to maintain normal environmental humidity, change the composition and structure of diluent, install electrostatic brush and use electrostatic preventer. Only in this way can we ensure the quality, improve the efficiency and yield, and lay the necessary foundation for the printing enterprises to obtain profits.
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