Overview of Flexographic and Intaglio Printing Waterborne Inks(I)

First, what is water-based ink

The so-called water-based inks are mainly scientifically processed using water as a solvent. Compared with other printing inks, aqueous inks do not contain volatile organic solvents, so they are not harmful to the health of printer operators during the printing process. Impact, pollution to the atmosphere, no pollution to the print itself. Because the ink is non-flammable, it can eliminate the hidden dangers of flammability and explosiveness in the printing shop and is conducive to safe production. Of course, the scope of application of ink and ink is not the same. The ink is mainly offset, and the ink is mainly flexo and gravure. In developed countries such as Europe, America and Japan, ink has gradually replaced inks and become a special ink for printing methods other than offset printing. In the United States, 95% of flexographic prints use ink and 80% of gravure prints use ink.

In addition to environmental protection, ink inks widely used in foreign countries have excellent performance: black color stability, high brightness, strong coloring power, non-erosion plate, strong post-print adhesion, adjustable drying speed, and strong water resistance. Color overprint and spot color printing can be used and so on. The development and application of ink and wash ink in our country started late, but it has developed rapidly, especially in recent years, it has been developing at a rapid rate. With the increase in the demand for ink and wash, the quality of domestic ink and wash has also been improved. In the traditional sense, ink ink has the disadvantages of slow drying, poor gloss, impermeability to water, impractical printing, etc. It has now been significantly improved. Under the condition that the price of imported ink is generally high, cheap domestic ink and wash ink has gradually occupied the market.

Second, the composition and characteristics of water-based ink:

Water-based inks are made from water-soluble resins, high-grade pigments, solvents, and aided by scientific processing. The water-soluble resin mainly acts as a binder in the ink, which causes the pigment particles to be uniformly dispersed, so that the ink has a certain fluidity, and provides adhesion with the substrate material, so that the ink can form a uniform film layer after printing. . The hue in the ink is mainly determined by the pigment. The pigment is evenly distributed in the binder in the state of microparticles. The pigment particles can absorb, reflect, refract, and transmit light, and thus can exhibit a certain color. At first glance, the pigment is required to have a vivid color, a proper coloring power and hiding power, and a high degree of dispersion. In addition, depending on the purpose of use, it is also possible to have different resistance. The role of the solvent is to dissolve the resin, so that the ink has a certain degree of fluidity, can be successfully transferred during the printing process, and adjust the viscosity and drying properties of the ink. Water-based ink solvents are mainly water and a small amount of ethanol.

Commonly used additives in water-based inks are: pH stabilizers, slow-drying agents, defoamers, dilutants, and the like.

(1) PH stabilizer. The pH stabilizer is mainly used to adjust and control the pH value of the water-based ink, making it stable within the range of 8.0 to 9.5. At the same time, it can also adjust the viscosity of the water-based ink and dilute the ink. In general, a proper amount of pH stabilizer should be added at certain intervals during the Indo-Europe process so that the water-based ink can maintain a good printing state.

(2) Slow drying agent. The slow drying agent can suppress and slow the drying speed of the water-based ink, prevent the ink from drying on the plate or the anilox roller, and reduce the occurrence of printing failures such as blocking and stencil printing. However, the amount of slow-drying agent should be controlled generally between 1% and 2% of the total amount of ink. If the amount of slow-drying agent is too much, the ink will not dry completely and the printed matter will produce sticky or odor.

(3) Defoamer. Defoamers function to suppress and eliminate the generation of bubbles. In general, when the viscosity of the water-based ink is too high, the PH value is too low, or when the printing press operates at a high speed, air bubbles are likely to be generated. If the amount of air bubbles generated is large, there will be a leakage of white ink and uneven ink. It will affect the quality of prints.

Water-based ink complies with environmental requirements. The product has non-toxic, non-corrosive, non-irritating odor, non-flammable, non-explosive, good safety, convenient transportation, high concentration, low dosage, low viscosity, good printing adaptability, and performance. Stable, good adhesion fastness, fast drying, excellent water resistance, alkali resistance and anti-wear performance after drying; easy to adjust operation before printing, after printing and printing process, the printed matter appears full of prints, bright colors, high gloss, Rich and rich, printed complex patterns can also reach rich levels, can print bright and high-gloss colors and other characteristics.

Third, the application of water-based ink

The biggest feature of water-based ink is to reduce the emission of VOCs (organic volatiles) into the atmosphere, thereby preventing air pollution, improving the printing operation environment, and greatly reducing the risk of fire. It can completely eliminate the harmful effects of certain toxic substances in solvent-based inks on the human body and the pollution of packaged goods, and improve the overall environmental quality. It is particularly suitable for packaging and printing products that require strict sanitary conditions such as foods and medicines.

Water-based printing inks, the most important application areas at present are flexographic printing and gravure printing. According to data, 95% of the US narrow-format flexographic paper prints use water-based inks, and 80% of gravure prints use water-based inks. Domestic development in recent years is also relatively fast, but there is still a big gap between the number of applications and foreign countries.

The application of water-based ink is an important factor affecting the printing quality. The control index of water-based ink includes fineness, viscosity, drying speed, and pH.

In the flexographic printing, the transfer of water-based ink and the control of the amount of ink are borne by the anilox roller. Therefore, the fineness of the anilox roller must be selected according to the condition of the product. In the field printing, an anilox roller of about 300 lines is generally selected. The ink layer is relatively thick, and the fine-grained printing of 150 lines requires the selection of an anilox roller of 600 lines. The number of screen lines of an anilox roller is generally four times the number of printed lines in order to obtain better print definition and resolution.

Viscosity is the most important control indicator in water-based ink applications and is a property that prevents fluid flow. Since the viscosity directly affects the transferability of the ink and the quality of the printed matter, the control of the viscosity of the ink is very important. In the printing process, the viscosity of the water-based ink is too low to cause defects such as light color, dot enlargement, and uneven ink transfer. On the other hand, if the viscosity of the water-based ink is too high, the transfer performance of the ink is relatively poor, and a dirty plate is likely to appear. Paste and other ills.

Flexo-type water-based inks are generally used without dilution, and their viscosity ranges from 30′′ to 45′′/25° C. (coated with 4# cups). The viscosity of the flexo-based water-based inks for cartons can be lower, as long as water stabilizers are regularly added. The viscosity change can be basically controlled. The gravure water-based ink is generally diluted with a 1:1 diluent of ethanol and water according to conventional practice, and the viscosity is generally controlled at 15" to 25"<0>C.

Water-based inks are usually alkaline (mainly dependent on amines), and the pH of water-based inks should be controlled between 8.0 and 9.5. If the pH of the water-based ink is too high, it will affect the drying of the ink. Speed; If the PH value is too low, the viscosity of the ink rises, the drying speed becomes faster, and the printing plate and the anilox roller are likely to be clogged, causing the layout to be dirty.

Drying of ink and wash is mainly volatile drying and infiltration drying. The drying performance of ink refers to the time required for the ink to dry after being transferred to the substrate. This is one of the important indicators for measuring the quality and performance of the ink. Therefore, controlling the drying performance of the ink is an important step to obtain good printing results. means.

The drying speed of a flexographic water-based ink depends on the printing speed of the printing press, the drying capacity of the drying equipment and the properties of the substrate material, and the ink's own composition. The general principle for selecting the ink drying speed is that under the existing conditions of the printing press, the ink should be sufficiently dry before the next printing group and before rewinding or stacking, but it cannot be dried on the printing plate. If the ink dries too quickly, it will dry and build up on the plate and anilox roller, and it may clog the anilox roller, resulting in the loss or destruction of the halftone dots, and the leak in the solid part; if the ink has dried faster Slow, there will be print odor phenomenon, or in the case of multi-color overprint will cause the back of the dirty.

In fact, the ink viscosity and PH value will also have a certain influence on the drying performance of the ink. For example, if the viscosity is too high, the drying speed of the ink will be reduced. If the PH value is too low, the ink will be dried too quickly. Therefore, in the printing process, the viscosity and PH value of the water-based ink should be adjusted and controlled first, and then an appropriate amount of quick-drying agent or slow-drying agent should be added to the ink according to the actual printing speed to adjust the drying speed of the water-based ink.

The fineness of water-based ink refers to the size of the pigment particles in the ink and its uniformity of dispersion in the binder. It is also a very important quality indicator. The fineness of the ink will not only have a certain influence on the coloring power and the color equal performance of the ink, but also affect the rheology of the ink. Ink particles are too thick, it is easy to cause the edge of the network hair, paste, and other ills; However, if the ink particles are too thin, the ink flow is too large, it will cause the network to expand, will inevitably affect the quality of printed matter.