04.02.2016
Enhance your Screen Printing magazine experience by exploring our interactive digital edition. Editor's note: This article was updated from an original version that appeared in the January 1996 issue of Screen Printing magazine. 275°F 15 min * Not recommended for outdoor exposure Epoxy, enamel, acrylic, and polyester inks need toe cured at high temperatures, for long periods of time, or both. The unit is now read into the program as a 3D model, in order to simulate the speed profile of the flow of air and the thermal load of the air, reflectors and housing components alongside the UV unit. But do you know which type of ink is best suited for your product and the type of metal you're printing? The static receiver levels of the radiation are representative surfaces for UV drying and are displayed at various distances. The definitive factor is the lamp axis and the typical distances from the substrate or coating material.
Ray tracing and thermosimulation considerably shorten development times, as the design of both reflectors and housing components can be modified immediately in the 3D CAD system following an optimisation process. Do you know why a conventional solvent-based ink seems to perform where a UV ink seems to fail? Thanks to these simulation programs, time-consuming test set-ups in the laboratory are reduced to a minimum. This article will attempt to answer these questions and explore the use of UV screen-printing inks on metal versus solvent-based inks. These ink systems are generally fast curing and may have good outdoor durability, depending on the formulation.
The ray tracing of the UV system is subject to different requirements depending on the application. Both the cationic and free-radical UV ink systems are chemically reactive and can produce hard ink films with chemical, scuff, and flexibility properties similar to baked solvent-based inks.
During work with particularly temperature-sensitive substrates, parabolic reflectors that produce a beavertail ray are employed.
While circuit printing does fall under the "metal" umbrella, the special requirements for this application make it a unique product specialty. Any screen-printing ink will have to exhibit certain properties to satisfy the requirements for coated-metal printing. The ink may have to provide a hard film to resist scuffing, scratching, blocking, and solvents, but be flexible enough to meet bending, diecutting, and embossing requirements.
The radiated power per surface is measured and displayed as watts per square millimetre in a 3D graph. If the end product is for interior use, epoxies would be suitable in addition to other resin systems. Ray tracing is used to calculate and optimise the UV efficiency of the unit, the homogeneity of the radiation and the distance characteristics. If both solvent resistance and outdoor durability are important criteria, most air-dried inks and any epoxy-based inks are not suitable; conventional baked inks or free-radical UV inks could be. In any case, the most important parameter is that the ink film adheres after proper drying or curing.
But if the coating is thermoset (and in most screen-printing applications it is), printing with UV, air-dried, or baked inks is more difficult. This hardness depends on the resin system of the coating, the method of coating application, and the curing (usually baking) conditions of the coating.


Usually, the higher the temperature and the longer the cure, the harder the surface becomes. This surface hardness is the root of the problems that screen printers encounter in trying to get free-radical UV inks to adhere to coated metals as compared to solvent-based inks. Depending on the formulation, a free-radical UV ink deposit may shrink up to 50% during curing due to polymerization and crosslinking of resin components. With conventional baked inks, shrinkage is based on solvent content and tends to be more limited than with UV inks. After printing a coated metal with a conventional baked ink, the printed product must be baked at high temperatures for long periods of time. This will soften the surface of the metal coating to some degree and allow the ink to "wet" the surface better to achieve good adhesion. As the metal and the coating cool, the ink film also cools, and any shrinking occurs in slow stages, allowing for good adhesion. When ordering coated-metal substrates from your supplier, make sure you ask what type of coating is on the metal (acrylic, polyester, etc.) and whether any slip agents or other surfactants have been added to the coating that may make it more difficult to print. Printing UV Inks on Metal The most successful inks currently used for coated-metal decorating are solvent based. UV printing has one system, cationic, that exhibits very little shrinkage, but is slow curing and based on cycloaliphatic epoxides. Since little shrinkage is involved, the cationic ink system tends to adhere better to the thermoset-coated metals than the free-radical systems. Cationic inks continue to cure over long periods of time, however, and care must be taken to prevent intercoat adhesion problems.
Solvent-based ink Conventional solvent-based inks can be broken into two categories, based on how they cure: 1. A free-radical UV ink, on the other hand, reacts rapidly during the curing cycle, cools quickly, and shrinks on a surface that is hard and nonshrinking, such as thermoset-coated metal. The cured inks are normally epoxies, enamels, and polyesters or acrylics modified with a crosslinking resin, such as melamine. The metal and coating are not affected--they don't heat up so they neither expand nor shrink, and the substrate surface remains hard. When compared to a chemically reactive or "baked" ink film, the surface of the air-dried ink film will be softer and susceptible to scratching and scuffing. This puts stress on the adhesion of the ink to the substrate, to the point that the ink film may pop off the coated metal. Softer ink films could also lead to blocking if not dried properly, and because metal is heavy, blocking in the stack could be a problem. The coatings vary considerably in base chemistry and crosslink density, relating to surface hardness variables that often go beyond a UV ink's ability to wet the surface (that is, the surfaces are too hard for the ink to adhere to).
The epoxies, acrylics, or polyesters, which are chemically reactive, need to be cured at high temperatures, often for long periods of time (Table 1). While one batch of metal may process well, the next could be too hard, exhibiting poor adhesion. After curing, these inks generally exhibit very hard ink films, which are more resistant to solvents, gasoline, and cleaning solutions, as well as scratching and scuffing.
To increase your chances of success with these commercially available metals, you should conduct a cross-hatch test on each batch before the production run. These baked inks tend to be less flexible than air-dried inks, but they may be suitable for embossing or forming.


Additionally, surface-tension testing may be helpful in detecting any high levels of surfactants in the coating that could cause adhesion problems. However, the best chance you will have for successfully processing UV inks on coated metal surfaces is to control the coating and its resultant cure density, or hardness, yourself.
This is done by working with your metal supplier and your ink supplier to get a coating that is more receptive to UV inks. As a matter of fact, they are winning Golden Image Awards." You, of course, would be right.
Some printers have had success with free-radical UV inks on metals whose coatings have been qualified for the window of performance required. In most of these instances, the metal supplier has worked with the printer and ink manufacturer to develop a softer coating that meets printing requirements. Most of these applications have been more in point-of-purchase decorative signage (not exterior qualified), short-term display, and indoor signage. As a rule, a baked conventional ink (except an epoxy) will exhibit three- to five-year durability, and a clear coating will further enhance exterior performance. UV inks (not epoxy based) will exhibit two- to three-year durability, although a UV clear coat will further enhance exterior performance of these inks, as well.
The market Yes, both cationic and free-radical UV inks can be used for screen printing on coated metal. Free-radical UV inks are successfully being used as halftone inks for decorative signage and are being used with some success for nameplates and appliance trim. Raw materials will eventually allow manufacturers to produce a UV ink with properties comparable to those of a solvent-based baking ink. While you'll find no definitive answer to this question, the fact remains that the coated-metal printing market is shrinking. More and more, plastics are being specified for signs, nameplates, appliance trim, and automotive trim.
Plastic sheets can be stacked higher with less weight, which means that blocking problems are less probable. The baking of inks for coated metals consumes high energy levels and requires long curing-time cycles, both of which slow production.
With these concepts in mind, it will be interesting to see if UV inks for coated-metal printing ever grow to the size or ability of conventional baked inks.
Conclusion I have attempted to present an open-minded view of the status and future of UV inks in the coated-metal market.
Seeing the positives and the negatives, I have tried to explain this information in an easily understandable form and hope I have been successful. Blanco holds a bachelor of science degree in chemistry and an MBA from Fairleigh-Dickinson University. He is a frequent speaker at industry events and a contributing author to several industry journals.



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