In High-Speed Rotogravure Printing, How Can Ink Viscosity Reach an Ideal State?

In plastic gravure printing, how to control ink printing viscosity is a critical issue. Especially in high-speed rotogravure printing, whether ink printing viscosity is properly controlled is directly and greatly related to the vast majority of quality problems such as pigment transfer, plate scumming, print gloss and brightness, pigment adhesion fastness, static electricity, etc., and affects the finished product rate and work efficiency.

Factors affecting ink viscosity

Factors affecting tackiness include the diameter, hardness and elastic modulus of the elastic roller, the properties of the roller, the rotational speed of the roller, the roller temperature, ambient temperature and humidity, sample temperature, ink film thickness, the influence of the ink or vehicle on the elastic roller, the condition of the elastic roller during cleaning, the properties of the sample being tested, etc.

Factors affecting viscosity: viscosity is closely related to its temperature, the concentration of component particles, particle size, etc., but has little to do with humidity and air pressure. The measurement accuracy and measurement units of different measurement methods are different, and they cannot be converted into each other.

From the above four points we can see that ink printing viscosity and tackiness are very different, but the two are still connected. This is mainly manifested in that viscosity is a physical quantity measuring the magnitude of fluid tackiness; the dynamic viscosity of a fluid is a quantity determined by the inherent physical properties of the fluid itself, and its value is a direct measure of the magnitude of fluid tackiness, as well as a measure of the fluid's ability to resist shear deformation in motion. Under the same ambient conditions, high viscosity means high tackiness, and vice versa.

How to control ink printing viscosity in high-speed rotogravure printing

Ink consists of organic solvents, vehicle, pigments, additives, auxiliaries, etc. Once these raw materials are fixed, and the degree of processing, methods and composition of each component are fixed, the quality of pigment transfer is mainly related to the printing viscosity of the ink. Practice has proved that ink printing viscosity has a certain range (11～24S, using Zahn cup No. 3); the greater the ink printing viscosity, the worse the pigment transfer effect. This is because the role of the solvent is to dissolve the resin or additives and auxiliaries, 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, the fluidity of pigments and other substances is poor, and they cannot be uniformly dispersed but appear in clusters and easily pile up together, so the pigment cannot smoothly enter and exit the cells. When the ink printing viscosity is too high, the pigment may not even enter the cells at all, let alone transfer. This is what is commonly called plate clogging.

Therefore, as long as we keep the colloidal system composed of resin, pigment, etc. and organic solvents not in an oversaturated state, but in a saturated or unsaturated state, so that pigments and other substances can be well dispersed in it to form a uniform and fine colloidal system, then the pigment can smoothly enter and exit the cells, and the problem can be solved. Some ink manufacturers recommend an ink printing viscosity of 15～18S (Zahn cup No. 3). But in practice, especially in high-speed gravure presses (printing speed 100～260m/min), in order to ensure a good transfer effect while allowing long-term printing and improving efficiency, the ink printing viscosity is generally set at an ideal state point found between 11～15S (Zahn cup No. 3).

In plastic gravure printing, plate scumming also frequently occurs. Its phenomenon is that the non-printing surface of the cylinder is also stained with a hazy layer of resin (with pigment entrained in the resin), which is transferred to the substrate, causing the substrate to show patchy stains, or linear stains. The root cause of plate scumming, apart from the hardness and softness of the doctor blade, blade pressure, blade contact angle, and the quality of the plate cylinder, is mainly related to the printing viscosity of the ink. As mentioned above, when the printing viscosity is too high, the fluidity of resin, pigment and other substances is poor, they are not uniformly dispersed and easily pile up; under the relative high-speed movement between the doctor blade and the plate cylinder, the blade and cylinder are easily damaged, thus linear stains easily occur. At the same time, because the resin is also in an oversaturated state, a large amount of resin aggregates together, and due to the affinity of the resin for the cylinder, under the blade pressure the resin will firmly stick to the cylinder (with pigment entrained in the resin), thus producing patchy stains. Generally speaking, when the ink printing viscosity is low, plate scumming is unlikely to occur.

However, if the ink printing viscosity is too low, it means the ink contains more organic solvent and relatively less resin, pigment and other components; thus it cannot form a smooth film layer upon drying, and the print will turn whitish, become dull and lack gloss. Therefore, if the print needs good gloss, generally a higher ink printing viscosity should be considered (13～19S, Zahn cup No. 3), that is, relatively more resin and pigment components, to meet the needs of the print.

At the same time, when the ink printed on plastic dries to form an ink film, it can firmly attach the encapsulated pigment to the plastic surface; the resin is the main material for firm adhesion to plastic. Therefore, pigment adhesion fastness is also related to the ink printing viscosity. This shows that the proportion of resin and pigment in the ink should be neither too large nor too small, but within a normal range, for pigment adhesion fastness to be high. Under normal ambient humidity, when the ink printing viscosity is above 16S (Zahn cup No. 3), static electricity generally does 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 extremely irregular water-stain-like pigment patterns will occur and worsen.

In view of the above reasons, industry insiders believe that the ideal state point for ink printing viscosity is generally found between 11～17S (Zahn cup No. 3). If static electricity occurs within this range, on the principle of mutual accommodation, one generally does not eliminate static electricity by increasing ink printing viscosity, but rather by maintaining normal ambient humidity, changing the composition structure of the diluent, installing static-eliminating brushes, using antistatic agents, etc. Only in this way can efficiency and yield be improved while ensuring quality, laying the necessary foundation for printing enterprises to gain profit.