2.3. Drying of the ink After the printing is completed, it is generally desirable that the ink can be dried quickly to increase the efficiency and to save the drying rack. However, drying too quickly or too slowly will cause difficulties in printing. Too fast drying will cause the ink on the printing plate to change rapidly and appear on the printing plate, reducing the ink's fluidity and the ink's wetting effect on the surface of the printing material. The pattern printing is not firm. If it is done too slowly, it will cause difficulties for multi-color overprinting, increase the space for sheeting, or make the next version impossible to print. Therefore, the correct understanding of the drying speed is an issue that must be paid attention to in order to increase the printing efficiency and ensure the printing quality. The factors affecting the drying speed, besides room temperature, humidity and air flow conditions, the more important is that according to the specific drying conditions of the printing shop, the type and ratio of solvents should be reasonably selected because the drying speed of the ink depends on the volatilization rate of the solvent. . Below are the evaporation speeds of commonly used solvents ranging from fast to slow: toluene-acetone-ethyl acetate-ethanol-butyl acetate-cyclohexanone-xylene-n-butanol. In the choice of solvent, in addition to considering the drying speed, it must also pay attention to its compatibility with various plasticizers in plastics. For example, most organic solvents dissolve quickly with plasticizers in polyvinyl chloride films. If the solvent is not properly selected, when the ink is printed, the film will quickly swell and lose its flat appearance, making printing impossible. When this happens, adjust the solvent formulation or replace the thin film with less plasticizer.
3. Development Trend of Screen Printing Inks While promoting environmental protection printing, environmental protection inks have also become an important development trend. Environmental inks have the following requirements:
(1) The ink should be non-toxic and harmless to humans and organisms. It should not contain toxic elements such as halogens or heavy metals, or its content should be controlled below relevant standards.
(2) In the entire life cycle of packaging products, the ink should not cause pollution to the human body and the environment, causing public hazards. As early as in the late 1980s, some developed countries began mass production and use of environmentally friendly plastic inks. The main varieties were water-based plastic inks and UV inks.
3.1, UV ink
(1) UV inks are suitable for various substrates such as paper, coated paper, wood products, synthetic paper, polyvinyl chloride (PVC) 1 propylene (PMMA), and thermosetting resins.
(2) The performance of UV screen printing inks is strong, adapting to high quality, diversified products and special shaped products that require special processing.
1Physical: liquid crystal display screen, anti-corrosion negative welding; electrical insulation, magnetic, etc.: thermal temperature foaming, fluorescence, night crystal, thermal transfer and so on.
2 Chemical, medicinal, corrosive, ethanol peeling, etc.; biochemical anti-bacterial, anti-fouling, etc.; absorption, adsorption, reaction of water resistance, oxidation corrosion.
3 surface decoration: crystal glue, anti-metal etching, matte wrinkles, non-glare, mirror metal, etc.: three-dimensional, thick film printing, Braille, crystal pattern, wrinkles and so on.
3.2. Water-based ink In the volatile drying ink, the water-based ink developed in the 1980s was valued for its environmental benefits. However, due to its poor water stencil property, and the problem of wrinkling due to the absorption of moisture from the paper when printed on paper, it has not been popularized. After entering the 1990s, the above problems are gradually resolved. E.g:
(1) Drying of screen printing aqueous ink. The biggest disadvantage of water-based inks is the slower drying than solvent-based inks. Now the composition of water-based inks has been completely improved.
(2) Water-based ink is good for absorbent articles such as cloth (cotton, hemp, woven fabrics), paper (paper, paperboard), wood (wooden boards, woodware), and leather (leather, artificial leather), but hard to the surface. The fixing of the printing materials is difficult. Now because the fixing agent for water-based inks is polyester, polyester and resin, its all-round function is excellent for non-absorbent substrates, smooth and hard substrates, and difficult-to-fix objects. force.
(3) The water-based ink not only achieves full hydration, but also solves the problems of ink softness, vividness, and prevention of bleeding. In Europe, water-based UV inks have long been used to screen color large-format road signs and posters. The increasing use of water-based inks and water-based UV inks is mainly due to environmental regulations and requirements to improve the operating environment. Of course, there is another reason. Four-color screening printing is very good. Therefore, in order to develop the advantages of water-based inks, the rapid expansion of the water-based ink market is inevitable.
3.3, thixotropic screen printing ink Thixotropy screen printing ink is not only thermoplastic, but also cold printing, to ensure their own leading position in the technology. The thixotropic screen printing ink developed can be used for ceramic tiles, tableware, glass, and enamel products. Recently, a series of low-melting thermoplastic inks have been developed to reach the peak of research. Thixotropy inks have the same rheological properties as cold screen printing inks or thermoplastic screen printing inks. These properties have the following advantages in printing.
(1) Reducing waste Using traditional inks, restarting the printer often results in dirty prints or other defects. Since thixotropic screen printing inks have good rheological properties, the use of such inks helps to solve the above problems. When the blade scrapes the ink, the fluidity of the ink increases, so that the ink is uniformly deposited on the print. After the printing is completed, the fluidity of the ink is lowered, thereby preventing the ink from overflowing on the screen. It should be noted that the thixotropy screen printing ink after the squeegee has enough fluidity to meet the requirements of continuous high speed printing. At the same time, thixotropic screen printing inks are also suitable for semi-automatic screen printing machines with slower printing speeds.
(2) Improve Printing Accuracy Fine-grained pigments are formulated with thixotropic binders to form inks, which can be used for higher mesh screens. Thicker ink films can be printed with thixotropic screen printing inks. Use traditional inks when passing thicker screens. Because the ink is too thick, the sharpness of the image is reduced. With thixotropic inks, however, the screen will not be filled with ink, and the image will not be distorted by the paste. When printing high-precision multi-color images, the use of thixotropic screen printing ink directly printed on the object, the ink layer can maintain a certain thickness without affecting the fine level of the image.
4. Conclusion In summary, we see that in the future screen printing ink market, the traditional ink will slowly disappear. Water-based inks and UV inks and emerging thixotropic inks will replace them.