Know the ink

First, the ink: water-based ink, is a mixture of pigments, binders and water as a solvent mixed by a certain ratio, produced by a special processing technology. It is just one category in many printing inks and has a certain amount of fluidity. It is mainly suitable for the printing of flexographic printing presses and corrugated packaging boxes.

Second, the composition of ink:

1, pigment (6% - 30%): divided into pigments and dyes. Pigments are divided into organic pigments and inorganic pigments. They are insoluble in water and other solvents, and they are not soluble in resins. They are always present in solid ink in inks. Its role is to provide color and color density, can not be directly attached to the paper.

2, the link material (15% - 35%): commonly known as the heart of ink. It determines the type, quality level and printing characteristics of the ink. The pigment is evenly dispersed in the binder and transferred to the surface of the substrate in an appropriate manner. Resin particles are coated on the printed material so that the pigment is fixed on the substrate. It provides the hardness and viscosity of the ink. The resin usually exists as a solid but dissolves into a liquid at pH values ​​above 8.

3, Solvent (10% - 40%): Dissolve the resin in the ink, dissolve the solid resin into liquid, help to control the viscosity and drying speed, and make it have a certain degree of fluidity. Solvents are divided into true solvents, co-solvents, and spurious solvents. True solvents refer to organic amines such as ammonia, monoethanolamine, diethanolamine, triethanolamine, and the like. The main role is to neutralize the acidity of the resin, making it soluble in water. The co-solvent generally refers to alcoholic solvents that help the resin to be miscible with water, such as ethanol, isopropanol, ethylene glycol, etc., which can promote better dissolving of the resin in water and can adjust the drying speed of the ink. The spurious solvent mainly refers to the water dilution effect.
4. Auxiliaries (0.5% - 10%): An important auxiliary component for regulating the quality of ink. Such as defoamers, anti-wear agents, preservatives and so on.

All water-based inks contain the above ingredients, and for certain printing needs, some of the ingredients can be changed as "printability".

Third, the viscosity of ink:

1. The viscosity of ink is not equal to the color density;
2, high viscosity or low viscosity ink can have the same color concentration;
3, the viscosity of the ink can be adjusted according to the ink metering system made by the printer;
4. High-viscosity inks can have better results due to higher cohesion. When the pH value and surface tension of ink are well controlled, the performance of high-viscosity inks will be very good.
5, the traditional two-roller ink supply system must use lower viscosity ink, but can not be diluted with water, water and water can only damage the ink structure, reduce color intensity, make the color gray, not bright.

Fourth, the transfer of flexible printing ink:

Ink transfer during the flexographic printing process is very simple, low-viscosity, high-flow ink is filled into the fine mesh of the anilox roll, and then the ink remaining in the anilox roll mesh is transferred to the flexographic relief image. When the ink on the printing plate comes into contact with the substrate, it forms a “kiss”. Light pressure on the kiss print completes the transfer of the ink. The printing mechanism of the flexographic plate is simple, but the material of the anilox roller, the shape of the mesh, and the viscosity of the ink Dryness directly affects the transfer of ink.

Fifth, the effect of pH on ink viscosity and drying:

The biggest feature of water-based ink is that the ink can be mixed with water before drying. Once the ink is dried, it can no longer dissolve in water and ink, that is, the ink is water-resistant. Therefore, special attention should be paid when printing, and do not allow the ink to dry on the anilox roller, so as not to block the mesh of the anilox roller, hindering the quantitative transmission of ink and causing poor printing.

On the other hand, it is also necessary to pay attention to the fact that during the printing process, the flexographic printing plate must always be kept wet by the ink, and the ink drying should be avoided in the mesh of the anilox roller.

In general, the pH of the ink should be in the range of 8.0-9.5.

Problems related to pH: When the pH value of the ink is too high, it will cause printing problems, such as unsuitable drying, poor water resistance, image packing on the print, odor and so on. If the PH value is too low, the ink will be agglomerated, dirty, printed on the plate, and anilox roller mesh plugged. In the process of using the ink, the amine will volatilize from the ink and reduce the pH value when the ink is transferred from the rubber roller to the anilox roller, the anilox roller to the printing plate, and the printing plate to the cardboard. When the pH value begins to decrease by approximately 8.0, more and more resin tends to dry out because the amine volatilizes out of solution, causing the color concentration to decrease and the ink to be a viscous solid state. As a result, more and more paper dust and fiber will be formed. Stick on the plate and have to stop cleaning the plate to make more amines volatilize. Due to the increased viscosity, it is often mistaken for water to evaporate and to add a large amount of water to dilute, but it also reduces the pH so as to cause insufficient color concentration and damage the resin structure.

The key to ink painting is to understand a few points:

1. The pigment is always in a solid state in the ink and is insoluble in any solvent;

2. The resin exists in solid state at pH = 7 and dissolves in amine or emulsify at pH > 8.0, thereby coating the pigment;

3. When the ink is wet, it is easy to be dissolved by the ink, that is, the moisture of the ink; after drying, it is not easily soluble in water, that is, the water resistance of the ink.

4. When the ink is used for a period of time, the amine will volatilize and the resin will be released. The ink will thicken. Add new ink and amine instead of relying on water to dilute. Adding water will only destroy the structure of the ink and make the color darker. Brightness and color saturation decrease.

5, ink and water will also make the drying speed faster, so that the ink fills the mesh of the anilox roller.

In the case of multicolor printing, the control of the viscosity and acidity of the ink is even more important. Since the resin dissolves in an alkaline state, if the pH value of the latter color ink is greater than the previous color, when the latter color ink is superimposed on the previous color ink, the previous color ink will be dissolved by the later ink, resulting in “bleed "The phenomenon (that is, the previous color of ink will dissolve on the printing plate). In order to improve this phenomenon, the pH value of the latter color ink is usually slightly lower than the previous color ink 0.2, and the viscosity is slightly higher than the previous color ink 1-2 seconds.
E.g:

First color pH 9.0
Second color pH 8.8
Third color pH 8.6
Fourth color pH 8.4

When the overprinting effect of the ink is poor, in addition to adjusting the pH value and the viscosity of the above ink, if the effect is not good, the ink supplier may be required to increase the drying speed of the previous color ink, increase the surface activity of the latter color ink, and increase the overprint as much as possible. The distance between the colors can further improve the overprinting effect of the ink, which is the water resistance and moisture regain of the utilized ink (see the first floor post).

The drying of water-based ink on cardboard in general water-based printing presses depends mainly on its penetration in the paperboard fibers, and the volatilization process is relatively slow. When overprinting, the former color often occupies most of the paper fiber holes, and the latter color will have a relatively slow penetration speed when printing. In addition, the surface tension of the ink (34 dyne) is lower than the surface tension of the paper (42-50 dyne). After the previous color is printed, the surface tension of the print is bound to decrease, making the latter color printing more difficult than the previous color. The surface tension of the latter color is reduced by adding the surfactant to improve the color printing effect.

In addition, there are many kinds of viscosity cups for ink, such as: DIN cup, Zhan cup, Ford cup, Rigosha cup, coated cup, etc. Each cup is divided into 1, 2, 3, and 4 models. Cups with the same number of seconds, such as the DIN No. 4 cup, Zhan No. 4, and Rigosha No. 4 cups, have different absolute viscosities. Each cup has its applicable range, but no matter which one is used, try to control it at 10-60. Between seconds.

Source: Qingdao Runhai Packaging Materials Co., Ltd.