Editor's note: This article will give you an idea of â€‹â€‹how ordinary coating on metallic materials can be used to visualize the final printing effect, and why you need to carefully select the ink before proceeding to production.
Among all the features of screen printing, the durability of screen printing ink is the most important one. Moreover, screen printing is also a popular decoration method. Both solvent-based and UV-curable inks can impart rich colors to prints while also helping them to avoid being damaged during handling, as well as being subject to wear, fading, and other environmental factors. The use of screen printing inks with these functions in combination with highly durable substrates can bring us excellent prints with outstanding performance.
Many high-performance screen printing products have the characteristics of printing on metallic materials such as steel and aluminum. Metal logos, signage, appliance signs and decorations, containers and many other printed products, these products generally have many images and logos printed with screen-printing inks. But the production of such prints is indeed a huge challenge for screen printers that do not have the experience of printing on metal.
When you want to print on metallic materials, your homework must be done well. To understand the nature of the coatings on metallic materials, the production tolerances of these materials, and the best combination of ink/substrate, this plays a decisive role in the success or failure of printing. To make effective use of these materials, printers must first understand the main properties of these metal materials. In addition, screen printing inks suitable for different materials and different image requirements are also to be found.
Understanding metal substrates
Normally, when we talk about printing on metal, the metal we refer to is actually not an unprocessed metal material. Moreover, most screen printing processes involving metals are printed on coated or treated metal materials. The coating on metal is roughly divided into two types: thermoplastic materials or thermosetting resins.
Thermoplastic coating does not undergo chemical reactions during the drying of the substrate. The drying of the thermoplastic coating is completely achieved by evaporation or heating. During the heating process, the coating material can be fused with the metal. In the thermoplastic coating of metallic materials, it must be remembered that this coating does not resist the corrosion of high temperatures and aggressive solvents. In other words, the coating on the metal melts again if it encounters a high temperature or aggressive solvent during screen printing.
Thermosetting resin coatings are generally cured by chemical reactions called "polymerization" and/or "crosslinking." The coating can be dried by heating the coated metal with a high temperature for a long period of time. Thermoset coatings are resistant to high temperatures and re-melting, but also have high resistance to solvent corrosion and wear.
Resin systems that can be used as metal-coated base materials are ethylene (plastisol or solution systems), polyester or acrylic, and the like. The vinyl coating is thermoplastic. Although it is softer than the thermosetting coating and it is easier for the ink to adhere, the thermoplastic coating has little resistance to chemicals or abrasion, which makes them in multiple screen printing applications. It is restricted. Therefore, screen printers prefer to use metal substrates with thermosetting coatings.
Polyester and acrylic are the most common resins in thermosetting coatings. Thermosetting coatings, unlike flexible thermoplastic coatings, do not dissolve screen-printing inks, but instead have a stronger cross-linking effect, making it difficult for ink to adhere to the coating.
When ordering coated metal materials from a supplier, you must ensure that the materials you ordered are suitable for use on metals (acrylic, polyester, etc.), and make sure that the coating material contains The ink is adhering to a slip agent or other surfactant. If such surfactants are included, suppliers should recommend to printers cleaners that can safely remove these contaminants before printing.
Metal printing ink
Both solvent-based and UV-curable inks can be used to decorate coated metal materials, but the use of solvent-based inks is more common. Although advances in technology have allowed UV-curable inks to be used on a variety of metallic materials, the chemical composition and curing characteristics of the inks still limit its function, making it appear to be inferior to solvent-based inks.
Solvent-based inks for metal printing usually have two dry forms, one is air-dry (by evaporation) and the other is heat-cured (chemically hardened or cross-linked). Air-drying inks are generally formulated with vinyl, acrylic or lacquer effects. These inks do not react chemically during the drying process, so they are easily scratched and abraded, and have almost no resistance to solvents, gasoline, and/or aggressive cleaning solutions. If they are not completely dry, the ink film they produce will have the problem of ink agglomeration. Because metal is a relatively heavy substrate, if the phenomenon of ink agglomeration occurs on the metal, it will bring about certain problems. At present, many air-dried inks give prints good flexibility and excellent outdoor durability.
Solvent-based inks that use thermosetting drying instead of evaporative drying usually contain some form of epoxy, enamel, polyester, or acrylic (epoxy components are well suited for use in thermosetting inks that require high outdoor durability). All of these inks require long drying times at higher temperatures. After curing, the ink films they form are very hard, so they are stronger than air-dried inks. They not only resist the corrosion of solvents, gasoline and cleaning solutions, but also avoid scratches and wear. Of course, these inks are less flexible than air-dried inks, but they are suitable for hot embossing or deformation. If a proper curing and cooling process is performed prior to stacking, prints produced with thermosetting solvent inks will not exhibit agglomeration of the ink.
UV curable inks for metal printing are also divided into two types: cationic or free radical (acrylate). Cationic UV-curable inks contain an epoxy-based chemical composition. The curing speed of this ink system is slower than that of a conventional free radical system, which is generally based on acrylic resin. If properly processed, both ink systems can provide good outdoor durability for prints. Both cationic and radical UV curable inks are chemically reactive. During curing, they produce a stiff ink film. These ink films not only resist abrasion and chemical attack, but also have similar properties to thermoset solvent inks. The flexibility.
Whatever type of ink you choose for metal printing, this ink should be able to meet the specific performance requirements of the job. For example, the ink film formed by silk screen printing ink not only has a certain hardness to resist abrasion, scratches, ink accumulation and solvent erosion, but also has enough flexibility to allow people to bend and die-cut prints. And pressure bumps. There will not be an ink and a metal substrate that will satisfy all your production requirements. Instead, you will need to handle different coating materials while matching the ink system with different coating materials and the end use of the product.
For example, if the product is used internally, it is very suitable for epoxy type inks, and resin thermosetting inks are also available. If the product does not need to be solvent-resistant, air-drying solvent-based inks are sufficient. If prints need to have both corrosion resistance and outdoor durability, air-drying and epoxy-based solvent inks are not ideal, but other traditional thermoset or UV-curable inks can be used for this purpose. field. Your primary task is to ensure that the ink film adheres to the substrate after drying or curing, and that different inks are selected according to different materials.
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 improve the adhesion of inks to the common metals on the market, people
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