How to control the printing viscosity of ink in plastic gravure

In plastic gravure printing, how to control the ink printing viscosity is a very critical issue. Especially in high-speed rotogravure printing, whether the ink printing viscosity is controlled properly is directly related to the vast majority of quality issues such as the transfer of pigments, dirty printing plates, glossiness of printed products, fastness of pigment bonding, and static electricity. Product yield and work efficiency. So, in the plastic gravure, how to control the ink printing viscosity, in order to take into account many aspects, to ensure the quality of printed products. This requires us to first understand the two concepts of ink printing viscosity and viscosity. According to the investigation of relevant textbooks or materials, the overview of the relationship between the two is usually very ambiguous, and some even see the two as a concept. In fact, there are obvious differences between the two. The following summarizes several points to illustrate the ink printing viscosity. The difference and connection with stickiness.

1. Different definitions

If the flow rate of each layer of the ink is different due to some external reasons, there is a force and reaction force between the liquid layers with different flow speeds at the two contact surfaces. This force is called the internal friction force of the ink. The property is called the viscosity of the ink. The physical measure of ink stickiness is called viscosity.

2. Differences in performance

Viscosity occurs when resisting separation of the ink film, and the fluid exhibits tack only when it flows, and does not exhibit tackiness in the stationary fluid. The effect of viscosity is to block the relative sliding of the fluid inside, thus blocking the flow of fluid, but this retardation can only delay the relative sliding process and cannot stop it. This is an important characteristic of fluid viscosity, and it is a rheological parameter that indicates the internal adhesive properties of the fluid. Viscosity is a measure of the internal friction generated between molecules as the fluid moves by external forces. Viscosity is a property of a fluid that always exists whether the fluid is stationary or flowing.

3. Detection methods and units are different

Tack is measured with a rotary viscometer. The stickiness meter is used to measure the resistance generated by the separation of the ink or web layer and expressed numerically as the stickiness value. There is no unit for stickiness.

There are several ways to measure the viscosity. In general, there are capillary viscometers, small-pore viscometers, rotary viscometers, and rotary cone-plate viscometers. In general, a rotating viscometer is used to measure the viscosity of the ink. When the rotor is subjected to viscous drag of the liquid, the balance spring generates torque, which counteracts the viscous resistance, and finally reaches equilibrium. At this time, the pointer connected to the hairspring indicates a certain reading on the dial (ie, the twist angle of the hairspring). Multiply the reading by a specific factor to get the viscosity of the liquid. This shows that the viscosity and viscosity are different in units.

4. Different influence factors

The factors that affect the viscosity are: elastic roller diameter, hardness and elastic modulus, roller properties, roller speed, roller temperature, ambient temperature and humidity, sample temperature, ink film thickness, ink or binder, etc. for the elastic roller Impact, the status of the elastic roller in the cleaning process, the nature of the tested sample, etc.

The factors affecting viscosity are: viscosity is closely related to its temperature, component particle concentration, particle size, etc., and has little relationship with humidity and pressure. The measurement accuracy and unit of measurement of different measurement methods are different and cannot be mutually converted.

From the above four points we can see that ink printing viscosity and viscosity are very different, but the two are still linked. Viscosity is the physical quantity that measures the viscosity of a fluid. The dynamic viscosity of a fluid is determined by the inherent physical properties of the fluid. The value is a direct measure of the viscosity of the fluid, and it is also the resistance of the fluid to shear deformation during movement. A measure of ability. Under the same environmental conditions, high viscosity means high viscosity, and vice versa.

Before we understand the relationship between ink printing viscosity, viscosity and these quality issues, we next discuss how to control the ink printing viscosity in high-speed rotogravure.

The ink consists of organic solvents, binders, pigments, additives, additives and the like. When these raw materials have been determined, the degree of processing, methods, and composition of the ingredients have been determined, the quality of the 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 the Zahn viscosity cup No. 3), the greater the ink printing viscosity, the worse the effect of pigment transfer. Because the role of the solvent is to dissolve the resin or additives and additives, etc., the fluidity is imparted so that the pigment can be easily dispersed. When the ink printing viscosity is too large, the entire ink system is in an oversaturated state. The fluidity of the pigments and other substances is poor and can not be uniformly dispersed. Instead, they appear in clusters and are likely to accumulate together, so that the pigments cannot smoothly enter or leave the mesh. When the viscosity of the printing ink is too high, the pigment cannot even enter the mesh at all, let alone transfer. This is what is commonly referred to as blockage. Therefore, we only need to make the colloidal system composed of resin, pigment, etc. and organic solvent not be in an oversaturated state, but saturated or unsaturated, so that pigments and other substances can be well dispersed therein to form a uniform and delicate colloidal system. , so that the paint goes in and out of the mesh smoothly, and the problem can be solved. Some ink manufacturers recommend ink printing viscosity between 15-18S (Zarn viscosity cup number 3). However, in practice, especially in high-speed gravure printing machines (printing speed is 100-260m/min), in order to ensure good transfer effect, but can print for a long time, improve efficiency, ink printing viscosity is generally 11-15S ( Seen between the Zahn viscosity cup No. 3) its ideal point.

In plastic gravure printing, dirty print problems often occur. The phenomenon is that the non-printing surface of the roller is also stained with a layer of resin (entrained in the resin) and transferred to the substrate to make the substrate stain-like, or appear as a linear stain. The origin of the dirty plate phenomenon is related to the printing viscosity of the ink, in addition to the hardness of the blade, the pressure of the blade, the contact angle of the blade, the quality of the plate cylinder, and the like. As mentioned above, when the printing viscosity is too high, the resin pigments and other substances have poor fluidity and cannot be evenly dispersed, and are likely to be stacked together. Under the relatively high-speed movement of the doctor blade and the plate cylinder, the doctor blade and the plate cylinder are easily damaged, and thus are easy to damage. Line stains occur. Equivalent to the resin is also in a state of supersaturation, a large number of resin agglomerates together, and the affinity of the resin to the roller, under the pressure of the blade, the resin will tightly adhere to the roller (the pigment is entrained in the resin), In this way, there is a flaky dirt. In general, the viscosity of ink printing is small, and the dirty printing phenomenon does not easily occur.

However, if the ink printing viscosity is too small, indicating that the organic solvent in the ink content, and resin, pigment and other relatively small components, it will not be able to form a smooth film when dry, the print will be white, become dull Light, lack of luster. Therefore, if the printed product needs to have a good gloss, it is generally considered to use a larger ink printing viscosity (13-19S, Zahn No. 3 cup), that is, the resin and pigment components are relatively more to meet the printing product need.

At the same time, when the ink is printed on the plastic and dried to become an ink film, the coated pigment can be firmly attached to the plastic surface, and the resin is the main material for the plastic to be firmly attached. Therefore, the firmness of the pigment adhesion is also related to the printing viscosity of the ink. This shows that the proportion of the resin and the pigment in the ink should not be too large, nor too small, but should be within the normal range, and the bonding strength of the pigment is large.

Under the normal environmental humidity, the printing viscosity of the ink is above 16S (the Zahn No. 3 cup), and static electricity generally does not occur. The printing viscosity of the ink is below 16S (Cahn No. 3 cup). As the viscosity becomes smaller, electrostatic phenomena such as beard-like, streak-like, edge-repelling, flying ink, poor transfer, and irregularly-formed water stains will follow. Occur and increase.

In view of the above reasons, people in the industry believe that it is generally desirable to find the ideal state of ink printing viscosity between 11-17S (the Zahn No. 3 cup). If static electricity occurs within this range, in accordance with the principle of mutual consideration, it is generally not necessary to increase the printing viscosity of the ink to eliminate static electricity, but by maintaining the normal environmental humidity, changing the composition of the thinner composition, installing the static brush, and using static electricity. Anti-static agents and other methods eliminate static electricity. Only in this way can we improve the efficiency and the finished product rate while ensuring the quality, and lay the necessary foundation for the printing enterprises to obtain profits.