Abstract: In this paper, the actual printing, due to the problems caused by the viscosity of the gravure ink for a brief exploration, and its reasons for how to adjust the instructions to be described.
Keywords: Viscosity, Pattern fog, Ink flying, Printing speed, Gloss, Surface tension value
The ink used in gravure printing is an extremely volatile solvent ink that is dried by the evaporation of gravure paper and solvents of high quality ink absorption. Because gravure inks require high fluidity and low viscosity for their own viscosity, the requirements for drying are adapted to the speed of the printing machine, the length of the travel path, the quality of the printing materials, the corrosion of the layout, and the indoor temperature and humidity for real-time adjustment. Once the operator is negligent or improperly controlled, various problems can easily arise.
Viscosity plays a very important role in this. In this paper, we will make a brief exploration of the problems caused by the viscosity of gravure ink in actual printing, and explain why the reasons for this change are as follows.
First, version fog problem
In plastic intaglio printing, the non-printing surface of the printing platen roller is often stuck with a layer of ink. When transferred to the printing substrate, it often appears as a sheet or line-like stain. This is the dirty plate. , Also known as the fog version. Ingravure printing, the cause of the dirty version of the ink is due to the squeegee ink, in particular, is not scraping the ink during the scraping process of non-graphic part of the ink, the blank part of the ink scraping is not clean due to the main mechanical and ink components problem.
Mechanical factors: The first reason is that the squeegee wears badly. During the normal use of the squeegee, it eventually wears out. To solve this problem, it is possible to check the direction of the squeegee along the gravure roll to ensure that the squeegee is evenly pressed against the gravure roll. At the same time, make sure that the squeegee itself is free of wear and dirt, which can affect the squeegee's squeegee effect. If these unresolvable problems occur, it is best to replace the squeegee again; the second reason is that if the surface of the copper plated and chrome plated layers of the gravure platen is rough, then the squeegee blade will not be clean. , Sometimes changing the angle of the squeegee can solve this problem. If the adjustment of the squeegee cannot overcome the roughness on the gravure roll, then the plate roll needs to be polished.
Ink component factor: If the squeegee is frequently replaced during the production process, it is likely that the colorant in the ink has a strong abrasive property. It should be considered whether to replace another ink. In addition, improper use of solvents or excessive use of solvents in the ink can result in flocculation of the colorant in the ink. The flocculation causes the color material to remain under the squeegee, causing a dirty plate or abrasion of the squeegee. In addition, the lubricating properties of the ink is also a cause of the dirty version. The lubricity of the ink is related to the resin in the ink composition. Some gravure ink manufacturers will add a small amount of dark asphalt to the ink components to improve the ink squeegee performance.
In a word, the main reason for the fog problem is that a large amount of resin precipitates and coalesces, or the selected resin has affinity for the printing plate cylinder. Under the pressure of the blade, the resin will stick tightly to the roller.
The cause of the smudge problem has a great relationship with the printing viscosity of the ink in addition to the ink formulation itself, the pressure and the contact angle of the doctor blade, and the quality of the printing plate. If the ink printing viscosity is too high, the viscosity of the ink becomes larger. In the high-speed movement of the printing plate, the ink will have a great impact impact on the squeegee, so that the squeegee is difficult to smoothly scrape off the ink of the non-graphic part, and thus the printing plate is also generated. If the pigment in the ink is unevenly distributed in the ink system, or the resin in the ink fails to wrap the pigment particles well, the pigment particles will re-aggregate. Formation of pigment groups, which is often referred to as the phenomenon of back-coarseness, can easily result in the formation of linear or stripe-shaped cutting lines in printing. In general, the lower the printing viscosity of the ink, the less likely it is to cause smearing problems. Therefore, in the actual printing process, special attention must be paid to changes in the viscosity of the printing ink, and the printing viscosity should be adjusted in time to improve the quality of the printed matter.
Second, high speed printing ink problem
When the gravure printing speed exceeds 20m/min or more, if the gravure printing ink uses a chlorinated polypropylene resin with a wide molecular weight distribution, the viscosity of the printing ink is too low, which may cause the problem of “flying inkâ€.
The reason for this is that the viscosity of the printing ink is too low. The cohesion of the ink is greatly reduced, and when the ink is transferred to the printing film, the low molecular weight resin is easily separated, and a small “ink spot†can be seen in the blank part of the print. To solve the problem of "flying ink", we can generally start from the following two aspects: - using a chlorinated polypropylene resin with a narrow molecular weight distribution; second, according to the printing situation, properly increase the printing viscosity of the ink.
Third, the viscosity of the color, transfer rate and color saturation, etc.
The gravure ink is transferred to the printing substrate by means of the ink of the gravure recessed dot. In contrast, the volume of the printing dot is substantially fixed. As the viscosity of the printing ink increases, the viscosity of the ink also increases, and the more ink is transferred from the dots, the higher the color density and saturation of the printed material, but this has a certain limit.
When the viscosity of the ink continues to rise to a certain degree, the ink transfer rate also increases synchronously. If the viscosity of the ink continues to rise, the viscosity of the ink becomes stronger and the ink becomes difficult to enter. In the outlets, the number of inks in the outlets is getting smaller and smaller. The transfer rate of inks begins to gradually decrease, and the color density of printed materials begins to decline. Ink manufacturers recommend the printing viscosity of gravure ink is generally 13 to 22 seconds (3 # Zahn Cup), which is only an empirical value, in the actual printing process, in order to achieve a good printing transfer effect, to carefully adjust the printing viscosity.
Fourth, the viscosity of the printing speed
Existing gravure printing presses have printing speeds from 30 m/min to 300 m/min, and typical printing presses have printing speeds of 80 to 200 m/min. In actual printing, the viscosity is selected according to the printing speed of the printing press. As printing speed increases, its printing viscosity value will decrease.
1, even if the same printing speed, due to ink formulation design differences. The print viscosity will also vary. Therefore, in the actual printing process, it must be adjusted according to the effect of printing.
2, affected by the seasonal temperature. The same printing inks have different viscosities in summer and winter, and many ink manufacturers have been working hard to reduce the influence of temperature on ink viscosity.
3, but also according to the actual conditions of the printing plate roller to select the most suitable ink printing viscosity.
Fifth, the viscosity of the impact of the print gloss
When more solvent is added to the original ink, the viscosity of the gravure ink will be lower, and the content of the resin and the pigment in the printing ink will be reduced. After the ink is transferred to the printing substrate, it will not easily form a smooth ink layer and the white ink will appear. The phenomenon directly leads to lack of luster in the printed matter. Therefore, in order to obtain excellent gloss, it is necessary to ensure that the printing ink has a suitable resin and pigment content. Therefore, there are many ink manufacturers repeatedly asked the printing workers not to simply flush the solvent in the ink, but to choose the matching supply of the varnish or diluting agent, but also to ensure that there is a sufficient proportion of resin and pigment in the upper ink. .
Six, ink viscosity and static electricity problems
Under normal circumstances, OPP printing ink itself has limited antistatic ability and is more prone to static electricity than polyamide type surface ink and polyurethane system ink. For OPP printing inks, the higher the viscosity of the printing ink, the more likely it is to cause electrostatic problems. Under normal temperature conditions, the viscosity of the printing ink is more than 16 seconds (3# Zahn cup). Generally, static electricity does not occur. Printing viscosity less than 16 seconds / 3 # cup, there will be whiskers, markings, edge repulsion and other failures are prone to occur.
If the printing ink viscosity is already 15 seconds / 3 # cup, it is generally recommended not to try to increase the viscosity of the printing ink, because increasing the printing viscosity of the ink will not only increase the printing cost, but also bring about the difficulty of printing transfer, easy to produce Stain Print failures.
According to the specific situation of static electricity generation, take other methods to eliminate static electricity. For example, it can be adjusted by changing the proportion of solvent in the diluent. Insiders pointed out that the purpose of eliminating static electricity can be achieved by adding 5% isopropanol (which is a polar solvent with good conductivity) or adding a small amount of antistatic agent as appropriate.
Seventh, ink viscosity becomes bigger and gel problem
It is generally believed that the increase in viscosity of solvent-based liquid inks is due to insufficient adsorption of the resin on the surface of the toner particles. If the surface of the pigment particle crystal does not completely adsorb the polymer resin, the surfaces of the pigment particles without the adsorption resin will aggregate with each other to form flocculation.
From the technical point of view, the surface of the colorant is not completely surrounded by resin. Adsorption does not exist in isolation. It will cause the solvent to adsorb on the surface of the colorant particles. Although the solvent sometimes wets the colorant, the solvent does not affect the colorant. Dispersion cannot be in a stable position. Therefore, it appears that a solvent rather than a polymer resin surrounds the surface adsorbed on the toner particles. Over time, the surface of the pigment particles. Over time, the pigment particles slowly flocculate and the end result is a change in the fluidity of the ink.
Theoretically, the problem of solving the solvent-adsorption pigment particles is to use a resin having a high affinity with the pigment particles so that the affinity of the solvent with the surface of the pigment particles is lower than the affinity of the resin with the pigment particles. . Also, if the solubility of the polymer resin in the solvent mixing system is good, then the probability of flocculation of the toner particles can be minimized because the solvent/resin combination with the colorant/solvent is better.
Eight, ink surface tension value problem
If you need to measure the surface tension of the ink during work, then we can use a tensiometer to measure. However, this measurement method only measures the static surface tension of the ink (surface tension of the ink at rest). However, the ink will certainly receive dynamic forces on the press. Therefore, the really effective surface tension should be the surface tension measured in the presence of dynamic forces, and sometimes dynamic surface tension is not easily measured.
When printing with aqueous inks, surface tension is an important issue. This is because the surface tension of aqueous ink is higher than the surface energy of the substrate. In these aqueous inks, it is often necessary to add some additives to reduce the surface tension of the ink, so that the aqueous ink can be well wetted on the surface of the substrate.
For any particular aqueous ink formulation, the most important effect of surface tension on the surface is to affect the leveling of the ink on the surface of the substrate. The surface tension of solvent-based inks is generally low, so they can wet well on the surface of many substrates. However, good wettability does not mean good adhesion, but it just means that the printing quality is good. The combination of resin components in the ink and the surface of the substrate is the primary factor that affects the adhesion and composite strength of the ink. The composite strength is also related to the adhesion of the ink itself. The interplay of polymer resin in the ink and the physical effects between various raw materials in the ink,