Plastic screen printing ink application requirements

1 Introduction

Plastic screen printing is a kind of reprocessing of plastic products. Due to the fact that the plastic itself has a relatively single dyeing property and the color has poor light fastness, in order to make up for these disadvantages and improve the appearance of plastic products, secondary processing is required. The ink used for screen printing has a lot to do with the quality of screen printing products. General thermosetting ink is not suitable for plastic screen printing, because the curing temperature of the ink is mostly above 100°C, and the plastic is already deformed at this time, so the solvent type ink can only be selected according to the solubility of the plastic, that is, the volatile dry type ink.

2. Requirements for Selection of Screen Printing Inks for Plastics

It is more difficult to formulate plastic printing inks. Since the substrate is a thermoplastic plastic, if a strong solvent is used, the surface of the plastic will be “overdissolved”. Therefore, it is necessary to use a solvent of a slightly soluble plastic, and similar to the structure of the resin of the printed plastic ink, and their swelling. The coefficient can be approximated. Under the effect of the solvent, the intermediate layer can be formed between the ink and the plastic, so that the ink has high strength and wear resistance after film formation.

2.1 Printing inks and solvents for plastics

When performing plastic printing, the ink performance should be tested first, and the printing should be carried out with troubleshooting. The printing ink must be selected according to the materials used and the purpose and requirements of the finished product. Polyethylene plastics are very good in terms of water resistance, chemical resistance, electrical insulation, etc., and their cost is low, so their use is very wide. Therefore, the proper ink and solvent must be selected according to the different uses of the finished product. Polypropylene plastics have excellent mechanical strength and weather resistance, so they are used in the printing of high-end commercial signs and lighting signs. There are many high precision in the prints, so choose pigments and solvents carefully. The weather resistance of the signboard is very important, but as the time passes, the solvent will change color, so we must pay attention to this point. In the synthetic resin, some resins decompose and degrade under the action of sunlight, so processing such as lamination, spraying, lamination, etc. is added in the post-processing process. Before printing, we must also understand the conditions of use of the plastic, the degree of bending, bonding properties and the required resistance to drugs and friction properties. At the same time, it must also know what processing methods are used for plastic post-processing.

2.2 Surface tension and fixing performance of ink

Screen printing on plastic sheets and films is difficult because the ink is more difficult to fix on polyvinyl chloride (PVC), polyester, polypropylene, polyethylene and other plastics. This is because the surface tension of the ink does not match the surface energy of the plastic. When screen printing is performed on various plastic sheets and films, surface tension and surface energy are two extremely important factors that affect screen printing quality. The surface energy and surface tension of the solid are measured in dynes/cm and can be used as an indicator of the ink fixing performance and wettability. In order for the ink to be successfully secured to the printing plate or film, the surface tension of the oiler must be equal to or less than the surface energy of the printing plate or film. In general, when the difference between the surface tension of the ink and the surface energy of the plastic increases, both the wettability and fixation properties of the ink are improved.
(1) Surface tension of the ink In actual application, the surface tension of the ink can be regarded as a fixed value. Some ink suppliers provide surface tension data, while other suppliers prefer to recommend smaller surface energy data for better results. This allows the supplier to ensure that the difference between the surface tension of the ink and the surface energy of the sheet and film is not too great. The surface tension of the ink can be changed by changing the solvent or adding a surfactant and a silicone organic resin. However, improperly changing the surface tension of the ink may cause defects such as unevenness in the print and white spots.
(2) Surface Energy of Solids All solids have a normal surface energy, but we are concerned with the surface conditions at the time of printing, the so-called apparent surface energy. Solid surfaces that are contaminated with grease, fingerprints, plasticizers, and mold release agents have significantly lower apparent surface energy than the base value. By increasing the roughness of the plastic surface or performing surface treatments (plasma treatment, corona treatment, and flame treatment), the surface energy value of the plastic can be significantly increased. Plastics such as polyvinyl chloride (PVC), polyester, polypropylene, and polyethylene have lower surface energy due to plasticizers (softness) and release agents (prevention of mucous membranes) after film formation. Continue to move to the surface, greatly reducing the surface energy. Determining whether the dissolution of the additive is the cause of the decrease in surface energy, the best experimental method is to wipe the test piece with the alcohol, the film surface. First determine the surface energy of the plastic, and then wipe the surface of the substrate with ordinary alcohol and a clean cotton cloth. If the surface energy value changes (usually larger), there is probably contamination.

2.3 Ink Drying

After the printing is completed, it is generally desired that the ink 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, screen printing ink development trend

While promoting environmental protection printing, environmental protection inks have also become an important development trend. The environmental protection ink has the following requirements: (1) The ink should be non-toxic and harmless to the human body and living organisms, and 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 inks (1) UV inks are suitable for various substrates such as paper, coated paper, wood products, synthetic paper, polyvinyl chloride (PVC) 1 propylene (PMMA), thermosetting resins, etc. (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

Among the volatile drying inks, water-based inks developed in the 1980s have received attention because they are environmentally friendly. 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. For example: (1) Screen printing water-based ink drying. 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 inks are both thermoplastic and cold stamped to ensure that they are technologically advanced. 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.