Outline how to control the printing viscosity of inks in plastic gravure
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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 control is appropriate is directly related to most of the quality problems such as pigment transfer, dirty plate, print gloss brightness, pigment adhesion fastness, static electricity, etc. Product yield and work efficiency. Then, in the plastic gravure printing, how to control the ink printing viscosity, in order to balance the various aspects, to ensure the quality of the printed product. This requires us to first understand the two concepts of ink printing viscosity and viscosity. According to the relevant textbooks or materials, the relationship between the two is usually vague, and some even regard the two as a concept. In fact, there is a clear difference between the two. The difference and connection with stickiness.
1. Define different
If the flow velocity of the ink layers is different due to some external reasons, especially when there is a force and a reaction force between the liquid layers having different flow velocities at the two contact surfaces, the force is called the internal friction of the ink, which is expressed. The nature is called the viscosity of the ink. The physical quantity that measures the viscosity of an ink is called viscosity.
2. Differences in performance
Viscosity occurs when resisting separation of the ink film, and the fluid exhibits viscosity only when flowing, and does not exhibit viscosity in the stationary fluid. The effect of viscosity is to block the relative sliding inside the fluid, thereby blocking the flow of the fluid, but this retardation can only delay the process of relative sliding and cannot stop it. This is an important property of fluid viscosity, and it is a rheological parameter that indicates the internal adhesive properties of the fluid. The viscosity is a measure of the internal friction generated between molecules when the liquid is moved by an external force. Viscosity is a property of fluids that always exist regardless of whether the fluid is at rest or flowing.
3. Detection methods and units are different
Viscosity is measured using a rotary viscous meter. The amount of resistance generated by the separation of the ink or the film layer is measured by a viscous meter, which is numerically expressed as a viscosity value. Sticky has no unit.
There are many ways to measure viscosity. There are generally capillary viscometers, small-bore viscometers, rotary viscometers, and rotating cone and plate viscometers. In general, a rotary viscometer is commonly used to measure the viscosity of an ink. When the rotor is subjected to the viscous drag of the liquid, the balance spring generates torque, competes with the viscous resistance, and finally reaches equilibrium, when the pointer connected to the balance spring indicates a certain reading on the scale disc (ie, the torsion angle of the balance spring). The viscosity of the liquid is obtained by multiplying the reading by a specific factor. It can be seen that the viscosity and viscosity differ in units.
4. Different influencing factors
The factors affecting the viscosity are: the diameter of the elastic roller, the hardness and the modulus of elasticity, the nature 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 ink or the binder, etc. The influence, the condition of the elastic roller during the cleaning process, the nature of the sample to be tested, and the like.
The factors affecting viscosity are: viscosity is closely related to its temperature, concentration of component particles, 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 converted to each other.
From the above four points, we can see that there is a big difference in ink printing viscosity and viscosity, but there is still a connection between the two. The main performance is that the viscosity is the physical quantity that measures the viscosity of the fluid. The dynamic viscosity of the fluid is determined by the inherent physical properties of the fluid. The value is a direct measure of the viscosity of the fluid. It is also the resistance of the fluid to shear deformation during motion. A measure of strength. Under the same environmental conditions, a large viscosity means that the viscosity is large, and vice versa.
Before we understand the relationship between ink printing viscosity, viscosity and these quality issues, we will next discuss how to control ink printing viscosity in high speed rotogravure.
The ink is composed of an organic solvent, a binder, a pigment, an additive, a binder, and the like. When these raw materials have been determined, the degree of processing, the method, and the composition of each component have been determined, and the quality of the pigment transfer is mainly related to the printing viscosity of the ink. Practice has proved that the ink printing viscosity has a certain range (11-24S, using the Zahn viscosity cup No. 3), the greater the viscosity of the ink printing, the worse the effect of pigment transfer. Since the action of the solvent is to dissolve the resin or the additive and the auxiliary agent, the fluidity is imparted to make the pigment easy to disperse. When the viscosity of the ink printing is too large, the entire ink system is in an excessively saturated state, and the fluidity of the pigment and the like is poor, and it cannot be uniformly dispersed, but is formed into a cluster and is easily piled up, so that the pigment cannot smoothly enter and exit the mesh. When the ink printing viscosity is too large, the pigment can not even enter the mesh at all, and it is even more difficult to transfer. This is what is commonly referred to as blocking. Therefore, we only need to make the colloidal system composed of resin, pigment and other organic solvents not in an excessively saturated state, but in a saturated state or a non-saturated state, so that pigments and the like can be well dispersed therein to form a uniform and fine colloidal system. So that the pigment enters and exits the mesh smoothly, and the problem can be solved. Some ink manufacturers recommend ink printing viscosity between 15-18S (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 good transfer effect, and long-time printing, improve efficiency, ink printing viscosity is generally 11-15S ( Look for the ideal state point between the Zahn Viscosity Cup No. 3).
Dirty problems often occur in plastic gravure printing. The phenomenon is that the non-printing surface of the drum is also stained with a layer of resin (the pigment is entrained in the resin) and transferred to the substrate, so that the substrate is stained in a sheet form or appears as a linear stain. The root cause of the dirty plate phenomenon is mainly related to the printing viscosity of the ink, in addition to the hardness of the blade, the blade pressure, the contact angle of the blade, the quality of the plate, and the like. As described above, when the printing viscosity is too large, the resin pigment and the like have poor fluidity, are not uniformly dispersed, and are easily piled up together. Under the relatively high-speed movement of the blade and the plate, the blade and the plate are easily damaged, thereby being easy. Linear contamination occurred. Equivalently, because the resin is also in a state of supersaturation, a large amount of resin is agglomerated together, and the affinity of the resin to the drum, under the pressure of the blade, the resin is tightly adhered to the drum (the pigment is entrained in the resin). This will result in flaky dirt. In general, the ink printing viscosity is small, and the dirty plate phenomenon is not easy to occur.
However, if the ink printing viscosity is too small, it means that the organic solvent content in the ink is large, and the components such as resin and pigment are relatively small, so that a smooth film layer cannot be formed when dried, and the printed product will be whitened and 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 printed matter. need.
At the same time, when the ink is printed on the plastic and becomes an ink film, the wrapped pigment can be firmly adhered to the surface of the plastic, and the resin is the main material for firmly adhering to the plastic. Therefore, the degree of pigment adhesion is also related to the printing viscosity of the ink. This indicates that the ratio of the resin and the pigment in the ink should not be too large, nor should it be too small, but should be within the normal range, and the pigment bonding firmness is large.
Under the normal ambient humidity, the ink printing viscosity is above 16S (Zahn No. 3 cup), and static electricity generally does not occur. The ink printing viscosity is below 16S (Zahn No. 3 cup). As the viscosity becomes smaller, electrostatic phenomena such as whisker, striated, edge repulsion, flying ink, poor transfer, and extremely irregular water stains will follow. Occurs and aggravates.
In view of the above reasons, the industry believes that the ideal state of ink printing viscosity is generally found between 11-17S (Zahn No. 3 cup). If static electricity occurs in this range, in principle, the method of improving the ink printing viscosity is generally not necessary to eliminate static electricity, but by maintaining normal environmental humidity, changing the composition of the diluent component, installing a static eliminating brush, and using static electricity. A method such as an anti-static agent eliminates static electricity. In this way, we can improve the efficiency and yield while ensuring the quality, and lay the necessary foundation for the printing enterprises to obtain profits.