To understand why the choice of ink is so important for metal printing, you must go back to the metal coating process. If the coating on the metal is thermoplastic, many inks will adhere to it. However, if the coating is thermosetting (all thermosetting in most screen printing), printing with any type of screen printing ink is difficult and one should monitor the entire production process closely and regularly.
The difficulty in printing on metals with thermosetting coatings is that these coatings have higher hardness characteristics. The hardness of the thermosetting coating will be affected by the resin system, the coating method and the coating curing method. The higher the temperature of the curing process and the longer it lasts, the harder the surface of the coating will be. The harder the surface, the harder it is for the ink to adhere to the coating. The higher surface hardness is the reason why it is difficult for screen printers to print UV inks on metallic materials.
The shrinkage characteristics of UV inks, and in particular the type of free radicals, are the main reasons why these inks are difficult to adhere to hard thermosetting coatings. Depending on the formulation, an ink film containing a free-radical UV ink may gradually shrink (sometimes more than 50%) as the cross-linking reaction of the ink progresses during curing. The ink film will cure and shrink in a matter of seconds, while the metal substrate and the coating on it will not change substantially. Since the UV ink shrinks very quickly, it exerts a certain pressure on the adhesion point between the ink and the substrate. If this pressure is large enough, the ink film can be easily detached from the metal substrate.
UV-radical-containing inks react rapidly, cool, and shrink during curing. It should be noted that if there is no heating during the curing process, the metal and coating will not expand or shrink, as does the UV ink. The shrinkage of the ink film can also cause the ink to fail to adhere. On the contrary, when people use traditional thermosetting solvent inks to print on coated metal, this product must be baked at high temperatures, so that the surface of the metal coating can be softened so that the ink is The surface of the coating is better adhered. When the metal and the coating are cooled down, the ink film is cooled down. All the slow shrinkage that occurs during this process will not affect the adhesion of the ink.
Cationic UV inks are less likely to shrink than free radical inks, but cationic formulations usually cure slowly. Since the shrinkage of the ink film is not so severe, the ink system can usually be firmly attached to a metal material having a thermosetting coating. However, cationic inks require a lot of energy to cure, and they will continue to cure after being exposed to UV light, which makes them more prone to ink build-up. When using this type of ink, you must be careful to fully cure it, thereby avoiding problems with ink adhesion.
Another issue that you need to keep in mind is that the coating metals that are now common in the market are made by Coil Coatings. Their initial market has nothing to do with the printing industry, but more to the commercial construction industry. Or other manufacturing areas. Therefore, the metals provided by these companies are not suitable for screen printing. These coatings have considerable differences in chemical composition and cross-link density, which tends to limit the adhesion of UV inks. There may be a lot of metal that can handle well, but the next batch may be very bad and have poor adhesion.
In order to increase the adhesion of inks to metals commonly found in the market, one must determine the hardness of the surface before using the substrate for production, and test its adhesion (using cross-hatching and related tests). In addition to this, tensile testing also helps people to discover abnormalities on the coating and avoid poor ink adhesion.
If you can make good control of the coating process and ensure that the hardness of the coating can reach a certain level, then you can get the ideal metal printing effect. Work closely with your metal suppliers and ink suppliers to achieve the desired hardness of the coating and continuously test the introduced materials to meet your requirements. Now, there are many free radical and cationic UV inks that can be used on metal substrates. Metal suppliers should also work with printers and ink manufacturers to develop a softer coating that can be used to meet printing needs. Products that often need to be printed on metal include: point-of-sale advertising, decorative signage, short-term exhibition boards, and indoor images.
In general, thermosetting solvent inks printed on metallic materials (except for those containing epoxy groups) generally have three to five years of durability. If you add a protective coating to the print, its performance will be more outstanding. Products printed with UV inks have durability of two to three years, but one can also extend its durability through a protective coating.
Are you ready for metal printing?
Although plastic occupies the absolute dominant position in today's image and signage printing market, metal substrates used for screen printing still maintain a strong momentum of development. For those screen printers who want to increase their screen printing capabilities, the good news is that they will no longer be limited to the use of solvent-based inks.
Due to the continuous improvement of raw materials, the range of UV-cured inks for metal printing has also been expanded unprecedentedly. They bring many benefits to the metal printing process, such as: high resolution, durability, fast processing and so on. Whether you use UV inks or solvent inks, if you can work closely with ink and substrate suppliers to learn more about product performance and use these materials to optimize the printing process, then you can achieve it on metal. Unrivalled printing results.
Source: Dongguan Printing Network
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