Inks geared to dedicated flatbed printers differ from those created for the applications typical of hybrid UV-curing printers. (Photo courtesy ESP Technologies Group)

When you go clothes shopping, you generally find the apparel is sized S, M, L or XL. From experience, you can be reasonably confident that you can chose one that fits. When you see “one size fits all,” you know you may be able to wear it, but it is unlikely to fit the way you would like it. This also is true for UV-curable inks. One size fits all does not always meet the needs of the application. For this reason, ink manufacturers have begun developing a range of inks with different property trade-offs, designed to better meet the needs of specific applications. Screen printers have understood this concept for years. When confronted by inkjet printing for the first time, the screen printers’ reaction is sometimes, “One ink—you have to be kidding!”

In this article, I will try to discuss some of the trade-offs and some of the efforts to develop inks formulated for specific applications.

FLATBED UV PRINTERS
Dedicated flatbed UV-curing printers are designed to print onto rigid materials. To get the durability–mar resistance, chemical resistance and good adhesion—the UV ink must be formulated with stronger, more brittle binders. If this ink were used on a flexible substrate, such as banner material, it would crack and delaminate during use.

On the other hand, if you are printing onto ceramic tiles, these properties are critical ones because the image on the tile is expected to perform without any top coat or laminate. So a specialized ink formulation is needed, not a general purpose UV ink. There is no one size fits all in UV-cure inkjet inks! Durability and flexibility are in opposition in the design of the inks. So the end use should dictate what ink should be used.

HYBRID PRINTER
Hybrid UV-curing printers present a dilemma for the ink chemist. Do I design for durability or for flexibility? The answer is both... and neither. A compromise is required. To achieve enough flexibility to avoid cracking and assure acceptable adhesion, durability is compromised. Marring and scratch-resistance will be poorer as will chemical resistance. The key here is to optimize the formula to enable “good enough” durability/flexibility properties for general purpose use. Most inks used today in wide-format UV-curing inkjet printers fall somewhere within this category.

If, however, the printer is used primarily for a specific application, it may be prudent to seek an ink that matches that need and not use a general purpose ink. Be weary of those that under-cure to gain flexibility. This is not a good solution, particularly on fabrics, where inadequate cure is likely. The ink may penetrate the fabric, and the fibers may shield the ink from the UV light resulting in under-curing. Over time this ink will fully harden and become brittle, making the fabric stiff.

And it is important to note that there are serious safety concerns with under-curing as well because the active ingredients of uncured ink will remain active and may cause harm to people touching the ink.

Specialty UV-cure inkjet applications, such as thermoforming, demand an extremely flexible ink set. (Courtesy Durst U.S.)

SPECIALTY APPLICATIONS
General purpose inks stretch only about 10 to 15 percent. If an application requires more stretch than this, for example, printing on leather or leather-like plastics, then 25 to 30 percent stretch is required to avoid cracking. Here the chemical resistance and mar resistance are significantly compromised, but completely cured cracking will be avoided, and adhesion should be acceptable.

Perhaps the most demanding application is in vehicle wraps. Here a dedicated roll-to-roll UV-curing printer would likely be used, and the stretch should be as much as 200 percent! UV-curing inks can be designed to stretch sufficiently, but the possibility remains of seriously compromising the ink’s chemical and mar resistance. In a normal application, this would not be an acceptable durability performance, but with wrap applications, the ink is sandwiched between two pieces of vinyl (printed vinyl and laminate), neither property is all that important. So, for this application, a “stretch ink” is definitely the right solution. Any of the others mentioned above would most certainly crack in the wrap application.

CONCLUSION
We have covered the issues around the design of UV-curable inkjet inks based on end use application. These are the same issues faced by the screen printer in his effort to provide a satisfactory printing solution to his customers. He chooses the ink chemistry using the same considerations. That is why he reacts to the one size fits all inkjet printing pitch, knowing full-well that the end users needs cannot be fully met by just one ink set.

As you explore the endless applications and substrates on which your UV-curing printer is capable of printing, keep in mind the trade-offs that are necessary to achieve optimum performance in your specific application, and challenge your ink providers to provide the optimum ink for your application.