In plastic gravure printing, controlling the printing viscosity of the ink is a very critical issue. Especially in high-speed rotary gravure printing, whether the control of ink printing viscosity is appropriate is directly related to most quality issues such as pigment transfer, dirty plates, gloss brightness of printed products, pigment adhesion strength, and static electricity, affecting the yield and work efficiency of the 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, roller speed, roller temperature, environmental temperature and humidity, sample temperature, ink film thickness, the impact of ink or binder on the elastic roller, the condition of the elastic roller during cleaning, and the nature of the tested sample.
Factors affecting viscosity are closely related to viscosity and its temperature, concentration of component particles, particle size, etc., and have little relationship with humidity and air pressure. The measurement accuracy and measurement units of different measurement methods are different and cannot be converted into each other.
From the above four points, we can see that there is a significant difference between ink printing viscosity and viscosity, but the two are still related. This is mainly reflected in that viscosity is a physical quantity that measures the magnitude of fluid viscosity. The dynamic viscosity of a fluid is determined by the inherent physical properties of the fluid itself, and its value is a direct measure of the magnitude of fluid viscosity, as well as a measure of the fluid's ability to resist shear deformation during motion. Under the same environmental conditions, a higher viscosity indicates a higher viscosity, and vice versa.
How to control ink printing viscosity in high-speed rotary gravure printing
Ink is composed of organic solvents, binders, pigments, additives, and auxiliaries. When these raw materials are fixed, and the processing degree, method, and composition of each component are determined, the quality of pigment transfer is mainly related to the printing viscosity of the ink. Practice has proven that the ink printing viscosity has a certain range (11-24S, using Zahn viscosity cup No. 3), and the higher the ink printing viscosity, the worse the pigment transfer effect. This is because the role of the solvent is to dissolve resins or additives and auxiliaries, giving them fluidity, making it easier for pigments to disperse. When the ink printing viscosity is too high, the entire ink system is in a state of excessive saturation, and the fluidity of substances such as pigments is poor, making it impossible to disperse evenly, but rather clumping together, which easily accumulates. In this way, pigments cannot smoothly enter and exit the mesh. When the ink printing viscosity is too high, pigments may not even be able to enter the mesh at all, let alone transfer. This is commonly referred to as the blocking phenomenon.
Therefore, we just need to ensure that the colloidal system composed of resin, pigments, and organic solvents is not in a state of excessive saturation, but rather in a saturated or unsaturated state, allowing pigments and other substances to disperse well within it, forming a uniform and delicate colloidal system. This way, the entry and exit of pigments through the mesh will be smooth, and the problem can be solved. Some ink manufacturers suggest that the ink printing viscosity should be between 15-18S (Zahn viscosity cup No. 3). However, in practice, especially in high-speed gravure printing machines (printing speed of 100-260m/min), to ensure good transfer effects while maintaining long printing times and improving efficiency, the ideal state point for ink printing viscosity is generally sought between 11-15S (Zahn viscosity cup No. 3).
In plastic gravure printing, the problem of dirty plates often occurs. The phenomenon is that the non-printing surface of the roller will also be covered with a hazy layer of resin (with pigments mixed in), which is then transferred to the substrate, causing the substrate to appear dirty in patches or lines. The root cause of the dirty plate phenomenon is not only related to the softness and hardness of the doctor blade, the pressure of the doctor blade, the contact angle of the doctor blade, and the quality of the plate cylinder, but is also mainly related to the printing viscosity of the ink. As mentioned above, when the printing viscosity is too high, the fluidity of substances such as resin and pigments is poor, and they do not disperse evenly, making it easy to accumulate. Under the relative high-speed motion of the doctor blade and the plate cylinder, the doctor blade and the plate cylinder can easily be damaged, leading to linear dirt. At the same time, since the resin is also in a state of oversaturation, a large amount of resin aggregates together, and due to the affinity of the resin for the roller, under the pressure of the doctor blade, the resin will stick tightly to the roller (with pigments mixed in), resulting in patchy dirt. Generally speaking, when the ink printing viscosity is low, the dirty plate phenomenon is less likely to occur.
However, if the ink printing viscosity is too low, it indicates that there is a high content of organic solvents in the ink, while the components such as resin and pigments are relatively low. This will prevent the formation of a smooth film layer during drying, causing the printed product to appear whitish, dull, and lacking luster. Therefore, if the printed product needs to have better gloss, it is generally necessary to consider using a higher ink printing viscosity (13-19S, Zahn cup No. 3), meaning that the resin and pigment components should be relatively more to meet the needs of the printed product.
At the same time, when the ink dries on plastic and forms an ink film, it can firmly adhere the encapsulated pigments to the plastic surface, with resin being the main material for firm adhesion to plastic. Therefore, the adhesion strength of the pigment is also related to the size of the ink printing viscosity. This indicates that the ratio of resin to pigment in the ink should not be too large or too small, but should be within a normal range for the adhesion strength of the pigment to be high. Under normal environmental humidity, when the ink printing viscosity is above 16S (Zahn cup No. 3), static electricity phenomena generally do not occur. When the ink printing viscosity is below 16S (Zahn cup No. 3), as the viscosity decreases, static electricity phenomena such as whisker-like, mottled, edge repulsion, ink flying, poor transfer, and irregular water stains of pigments will occur and worsen.
In view of the above reasons, industry insiders believe that the ideal state point for ink printing viscosity is generally sought between 11-17S (Zahn cup No. 3). If static electricity phenomena occur within this range, based on the principle of mutual consideration, it is generally not advisable to eliminate static electricity by increasing the ink printing viscosity, but rather to eliminate static electricity through methods such as maintaining normal environmental humidity, changing the composition structure of the diluent, installing static eliminators, and using anti-static agents. This way, quality can be ensured while improving efficiency and yield, laying a necessary foundation for printing enterprises to gain profits.