Printing UV inks on metallic materials

(Printing UV Inks on metal) Why some UV inks can be printed on coated metal surfaces and some not? If it is necessary to print the image on metal by screen printing, solvent ink or UV ink can be selected. So what type of ink is suitable for what type of product and metal type? Why can I sometimes use UV ink to print successfully and sometimes not? Why are the advantages of solvent inks sometimes more prominent? This article will answer these questions as much as possible and compare the difference between UV screen inks and solvent inks when printing on metallic materials. Explain from the following aspects: Substrate (coated); 2. Ink; 3. Part of the performance parameters; 4. Market (past, present and future). First, the coated metal printing market is positioned. This area mainly includes the following types of products: decorative products, signs, decorative vessels, and printed circuit boards. Although printed circuit boards do not belong to the metal category, special needs make it a special product. The following description from four aspects. Substrate When it comes to metal printing, what exactly is it? Is it printed directly on untreated sheet metal? In most cases, the answer is no. In general, metal screen printing is not performed on the original metal material but is printed on the coated metal. Below we look at the type of metal coating. There are two types of metal coating: 1. Thermoplastic type; 2. Thermosetting resin type; Thermoplastic coating is a coating that does not chemically react during the drying process. Drying may be accomplished by evaporation or thermal melting. It should be noted that the thermoplastic coating, after drying, usually does not have the characteristics of high temperature resistance and strong solvent corrosion resistance. At high temperatures, the thermoplastic coating can be remelted and dissolved when subjected to solvent attack. Thermosetting resin type coating is a coating method of curing by polymerization or cross-linking chemical reaction. When the thermosetting resin type coating is properly cured, it has heat resistance, does not melt at high temperatures, and has strong solvent resistance and abrasion resistance. Polymerization or cross-linking is accomplished by baking at high temperatures for a long period of time. The resin system used to coat the metal may be a vinyl (plastisol or solution system), polyester, or acrylic. The vinyl coating is thermoplastic, dried by solvent evaporation, and dried by thermal curing using plastisol. Polyesters and acrylics are the most commonly used resins in thermosetting resin types. Both thermoplastic and thermoset types have disadvantages. The softer thermoplastic coating may be dissolved by the ink, while the harder cross-linked thermosetting coating makes it difficult to adhere the ink. Although the thermoplastic coating is very soft, the ink is easy to adhere. Thermoplastic coating is not resistant to all chemicals and corrosive substances, which makes it unsuitable for many types of screen printing. As a result, many screen printers use a thermosetting resin type coating to coat a metal surface. Therefore, when printing on metal surfaces, it is necessary to understand what kind of coating is used, and whether or not a slip agent or other surfactant is added, which may hinder the adhesion of the ink. The best ink for inks currently printed on metal surfaces is solvent-based inks, but UV inks have also begun to be used, but only a small part of the market. Why is this so? Let us first look at how these two inks work and what their working mechanism is. Traditional solvent inks can be divided into two types, which are how to cure the classification: 1. Air drying (evaporative drying); 2. Crosslinking (chemical hardening by baking) cures. Air-drying inks are vinyl, acrylic or paint. Curable inks are epoxy type, enamel type and polyester type or acrylic materials modified with a cross-linking resin such as a trihydramine. (Note: Do not use epoxy trees for outdoor work.) Air-drying solvent-based inks, which do not dry through chemical reactions, have weak or no ability to resist solvents, oxidation, or detergents. Compared with cross-linking curable inks, the ink layers of air-drying inks are soft, easily rubbed off and abraded, and can clump if not adequately dried, and because the metal is heavy, agglomerates may be formed. It is a big problem. Most air-drying inks have good adaptability and are suitable for outdoor operations. Epoxy, acrylic or polyester are chemically reactive and require prolonged cure at elevated temperatures. After curing, these inks produce a very hard ink layer with solvent resistance, oxidation resistance, and detergent properties that are not easily removed and worn. However, this ink is less adaptable than the air-drying ink, but is more suitable for embossing or stencil making. Properly curing and cooling before drying, they will not clump. There are two chemical cures for UV ink UV inks: 1. Cationic; 2. Free radicals. Cationic curing system is a chemical reaction based on epoxy resin. These ink systems cure slower than traditional free radical or acrylic systems. The free radical system is a system based on acrylic polymers such as polyester or urea grease. These ink systems are capable of rapid curing and have good outdoor durability. Both cationic and free radical UV ink systems perform a chemical reaction and may create a hard ink layer that has the same chemical resistance, abrasion resistance, and adaptability as a cured solvent ink. Performance parameters Any screen printing ink must have certain properties that are required to print on a coated metal surface. The ink needs to provide a harder ink layer to resist abrasion, erasing, agglomeration and dissolution, but it must also have certain adaptability to meet the needs of bending, die cutting and embossing. If the final product is for indoor use, the epoxy type is suitable, in addition to the resin-based system. If the product does not require solvent resistance, air-drying ink inks are sufficient. If both anti-dissolution and outdoor durability are important, most air-drying inks and epoxy-based inks are not suitable, and conventional curing inks or free radical UV inks can meet the requirements. In any case, the most important parameter is ink adhesion after proper drying or curing. Then why are some conditions suitable, and some conditions are not suitable? To know the answer, you must go back to the metal coating stage. If the coating is a thermoplastic, many inks can adhere to it, but if the coating is a thermoset type, UV inks, air-dried inks, or baked inks may be difficult to print. This is related to the hardness of the metal thermoset coating. This hardness is related to the coating resin system, the coating method, and the coating curing conditions. In general, the higher the temperature, the longer the curing time, and the greater the surface hardness. Surface hardness is the root cause of problems encountered by screen printers printing on free metal inks on coated metal. The most important thing is to consider the shrinkage factor of the ink layer. The deposition of free radical UV ink can be reduced by 50% in polymerization and cross-link curing. With conventional curing inks, the shrinkage is based on the solvent composition and is less than the UV ink shrinkage. With conventional curing inks printed on coated metal surfaces, printed products must be baked at high temperatures for long periods of time. This softens the surface of the metal coating to a degree that will allow the ink to adhere well to the surface of the coating. As the metal and the coating cool, the ink ink layer also cools, any shrinkage occurs slowly, and good adhesion can be achieved. In this way, the ink ink layer does not peel from the metal coating. UV inks have a system that is cationic and exhibits less shrinkage but exhibits slower cure and cycloaliphatic epoxides. If these inks do not cure well, agglomeration can occur. Due to the smaller shrinkage, the cationic ink system is more prone to adhere to the heat-cured coating metal surface than the free radical system. Cationic inks cure after prolonged curing, but care must be taken to prevent the adhesion of the intermediate coating. On the other hand, free-radical UV inks react faster during cooling, cooling faster and harder to shrink or shrink on hard surfaces. This reduces adhesion because the free radical UV ink is cured in a UV curing unit. Metals and coatings are not affected because they are not heated so they neither expand nor shrink and the surface of the substrate remains rigid. However, UV ink deposition shrinks very quickly, and has a significant relationship with the ink adhesion of the substrate. In this regard, the ink ink layer may peel from the coated metal surface. Because most commercially available coating metals are made through a series of screw coaters, these coating metals were not originally intended for printing purposes (eg, for commercial buildings), so they are often not well suited for screen printing. Due to the difference in chemical reaction and cross-linking density, the coating layer is also different, and also related to the surface hardness parameter, which often exceeds the ability of the UV ink to wet the surface (ie, the surface is not conducive to ink adhesion). It is possible that one metal handle is good, while the next one is very hard and shows poor adhesion. In order to be able to perform perfect printing on such metal surfaces, a test should be performed on each product before production. Surface tension tests, for example, can help detect surfactants in any coating that can cause adhesion problems. However, the best way to successfully print on a coated metal surface is to control the coating and its final cure density or hardness. This can be done in cooperation with the metal supplier or the ink supplier to detect the degree of affinity of the UV ink. There are now some free radical UV inks that are more suitable for coating metals and, in fact, have achieved good results. Some printing commercial free radical UV inks have been successfully printed on metal, and these metal coatings meet the requirements of the parameters. In response to this situation, metal suppliers have worked with printers and ink manufacturers to develop a softer coating to meet the printing needs. Most applications are not just about decorative signs, short-term displays and interior decorations. In general, traditional curing inks (except epoxy inks) will have three to four years of durability, and clear coating will further enhance the external parameters. UV inks (non-epoxy) will have two to three years of durability, although UV clear coatings will also increase the external parameters of these inks. The market, whether cationic or free radical UV inks, can be used for screen printing on coated metal. Free radical UV inks have been successfully used for half-tone ink signage printing and are also successfully used for signage and decorative products. However, for long-term outdoor signage, solvent-based inks are still controlling the market. The technology sometimes eliminates the printing obstacles of UV inks. Compared to solvent-based curing inks, manufacturers will eventually produce UV inks for raw material considerations. The question is whether market opportunities will benefit R&D investment? Before you get the exact answer to this question, first explain the situation. At present, the market for coated metal printing is shrinking, and plastic products are used more and more for signs, signs, decorative products and mobile phone decoration. There are several reasons for moving from metal to plastic, including: Plastic is lighter than metal. Plastic sheets can be stacked higher, which means it will not cause any problems. At the same time, it is more economical than metal. The curing of coated metal inks requires high energy and requires long curing times. As the characteristics of UV inks can meet the needs of printing plastic materials, it will certainly see the UV inks grow into another ink that can compete with traditional curing inks in the near future. Conclusion UV inks are widely used inks in the future, but they are not illusory. They will play an important role in some areas. To master them, you must constantly learn them, understand them, and use them.