Low migration in UV label printing – Part 3

Low migration in UV label printing – Part 3

07.03.2016

After clarifying the relevance of low-migration printing in the first part of this series of articles, the second part highlighted the practical implications of this type of printing. In this third and final part of the series, we will now discuss some specific challenges associated with the use of low-migration inks. The focus will not be on scientific correlations, but rather on the implications for the everyday work of label printers.

What are low-migration UV inks?

First of all, it should be noted that there are no migration-free inks. Even the exclusive use of low-migration inks does not guarantee that the relevant regulations can be complied with. Furthermore, "low migration" is neither an international standard nor a protected term. It is merely a collective term for ink series with optimized migration behavior. Compared to physically drying inks (e.g., solvent-based inks), the drying process for chemically drying UV inks is always more delicate and susceptible to errors. However, chemically drying inks also have migration weak points, as demonstrated by the residual oil problem with oxidative drying offset inks.

Describing the detailed chemical structure of a low-migration ink in detail would go beyond the scope of an article that focuses on the practical application of these inks. However, there are interactions in the ink structure that have far-reaching consequences in everyday use on a printing press:

• Ink costs: As a rule, low-migration inks require purer and better-matched raw materials. Both of these requirements drive up raw material costs.
• Migration behavior: The raw materials used today to optimize migration behavior generally slow down the drying process. As a result, higher UV drying power is required compared to conventional inks, even at the same machine speed.
• Color adhesion: Almost all efforts to optimize the migration behavior of the color immediately result in a decrease in the adhesive strength of the color (color adhesion).

The ink manufacturer therefore strives to find an application-dependent compromise between the three factors of cost, adhesion, and migration. For label printers, this means that they must tailor their ink selection even more specifically to the respective area of application than before. This is the only way to achieve a sensible balance between cost and function in everyday use.

Processing low-migration inks on a printing press

Before converting an existing printing press to low-migration printing (see article 2), it is advisable to test the ink series in question in the various applications without production stress. As a rule, it can be observed that the printing process no longer reacts in the same way as with conventional inks. The following changes are often seen in practice:

• Reduced ink adhesion: Ink adhesion is often confused with insufficient drying, which is not always the case. It is advisable to use suitable test procedures to determine the cause. If it is definitely determined that the ink adhesion does not meet expectations, the suitability of the ink for the application in question must first be checked. If no improvement can be achieved with suitable ink series or alternative substrates, the only option is often to apply preliminary primer coats.
• Insufficient surface drying: If it is indeed a problem with surface drying, the first step is to determine whether the UV output can be increased or whether an improvement can be achieved by reducing the machine speed. If an improvement can be seen, the UV system must be checked to ensure it is operating correctly. "Blind" reflectors, UV tubes with excessive burn times, and any contamination of the quartz glass are often disruptive factors that only become apparent during low-migration printing.
• Insufficient deep drying: If the boundary layer between the substrate and the ink film is not completely dry, this indicates that too much ink has been applied. Ink manufacturers generally specify the maximum permissible area application. Reducing the ink application is a quick way to determine whether this improves the result. Here, too, it is important to check that the UV system is operating correctly. However, the use of doped UV tubes with better penetration can also be useful here.

Only when the behavior of the applications to be produced is known and satisfactory solutions have been found for any problems should an existing machine be converted to low-migration printing. If the results are still unsatisfactory, the drying system may need to be reevaluated. Existing UV drying systems can often be upgraded to more powerful UV drying systems or completely replaced. When re-evaluating printing machines, a UV drying system with nitrogen inerting can sometimes also be considered. Replacing the oxygen atmosphere in the printing gap with a nitrogen atmosphere (inertization) significantly improves drying efficiency. It is always worth consulting with the machine manufacturer, as the addition of additives to low-migration inks should generally be avoided.

Interactions in the production of low-migration printing inks

Effect of standard coatings (typical overall distribution: 30% UV light, 15% visible light, 55% infrared)

Summary

Low-migration printing in UV label printing is still in its infancy. With increasing experience in dealing with the relevant regulations and implementing them in everyday practice, safety and efficiency in application will also improve. In addition, low-migration UV printing inks are expected to evolve and become easier to process in the future.

While migration regulations for packaging currently apply exclusively to food products, it can be assumed that hygiene and cosmetic products will also be subject to stricter regulations in the future. It is therefore definitely worthwhile to look into low-migration UV label printing now.

Major brand manufacturers and retail chains already strictly adhere to existing regulations and will likely continue to do so in the future, with even stricter enforcement. Image problems and the resulting costs associated with contaminated food are an absolute worst-case scenario in the global trend toward "clean food." Branded goods manufacturers and retail chains are therefore primarily concerned with minimizing risk. Despite all the challenges of low-migration printing, this also presents label printers with an opportunity to differentiate themselves in the market in the long term.

At Gallus, we are convinced that our products and consulting services give label printers a decisive competitive advantage when it comes to low-migration printing. Gallus is happy to support its customers with material recommendations, training, service, and technical assistance.