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 The preparation of plastic printing inks is difficult. Since the substrate is a thermoplastic plastic, if a strong solvent is used, the surface of the plastic will be “overdissolvedâ€. 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 When plastic printing inks and solvents are used for plastic printing, the ink properties should be tested first, and the printing should be carried out in case of 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 inks Screen printing on plastic sheets and films is difficult because inks are difficult to fix on plastics such as PVC, polyester, polypropylene, and polyethylene. 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.
