Dividers Roland

The Ins and Outs of UV-Cure Systems

Bill Schiffner is a freelance writer/editor based in Holbrook, N.Y. He has covered the imaging industry for 25 years and has reported on many evolving digital imaging technologies including wide-format printing and newer electronic digital signage. He was the editor for a number of imaging publications and websites. He can be reached at bschiffner@optonline.net.

UV-cure digital printing continues to be a fast-growing segment in the wide-format printing market. InfoTrends—an industry research firm based in Weymouth, Massachusetts—predicts that the total system revenue from wide-format UV-curable inkjet printing systems—which includes hardware and ink—will grow from just over $2.7 billion in 2015 to about $4.3 billion in 2020, for a compounded annual growth rate of about 10 percent.

One of the key reasons for the rise of UV-cure inkjet technology is that it offers the widest range of printable substrates, which allows shops the option to expand their application offerings to their client base.

The beauty of UV systems is that the ink remains uncured until exposed to UV light. Experts say this helps to keep the inkjet nozzles open and reduces the need for maintenance and maintenance cycles. The process can save ink and increase use time compared to that of solvent ink chemistry.

Let's take a look at the current state of UV-curing systems in wide format production inkjet printers, how they work and where this technology is heading.

Versatile and Effective

Ken Hanulec, vice president of marketing, EFI Inkjet Solutions, Meredith, New Hampshire, reports that UV- and UV-LED curing are among the most efficient, versatile and effective methods for wide- and superwide-format imaging. “Digital UV inkjet ink formulas include reactive agents that cure the ink when exposed to a UV lamp mounted on the printer," he says, "UV-LED inkjet works in a similar way, but with reactive agents in the ink that respond to the narrower range of UV light waves emitted by an LED-based UV lamp.”

Both UV and UV-LED systems give users the advantage of instantly dry prints, Hanulec continues. These can be immediately finished and/or delivered to customers right after printing. “Since this work often involves very large graphic prints," he says, "having instantly dried prints presents an extra benefit because it eliminates the tremendous amount of space that printed graphics might otherwise need to sit out to dry.”

He adds that the other big benefit to both UV and UV-LED inkjet is the fact that they can adhere to a wide range of substrates. “Where some print technologies only work with flexible media, UV and UV-LED can be used with many different substrates, including specialty substrates such as glass and wood.”

How Inkjet UV-Curing Works

“The ink consists mainly of photoinitiators, monomers and oligimers; and when together in an ink formula and they are exposed to UV light, it triggers a chemical (cross linking) process that transforms the ink into a solid film that tightly adheres directly onto the substrate’s surface," says Roberts. "This all happens within a fraction of a second."

Heather Roden, product marketing manager, FujiFilm North America Corporation, Graphic Systems Division, Hanover Park, Illinois, puts it this way: “Photoinitiators in the ink react to UV light, which triggers the cross linking, or polymerization—the monomers and oligimers in the ink come together to form a dry film," she says.

“Perhaps you’ve heard the term ‘Instantaneous Cure’ inks,” adds Jay Roberts, product manager, UV Products at Roland DGA Corp., Irvine, California. "This is the precise difference between the UV-curable and solvent ink families. UV-curable inks are ‘cured’ once they are exposed to wavelengths of UV light

Conventional UV-curing Vs. UV-LED

Roberts points out the two main types of UV lamps being employed in large-format UV-curing printers include mercury vapor UV lamps and the LED-based UV lamps which rely entirely on LED lamps for the source of UV radiation.

“Unlike most mercury vapor UV lamps, LEDs come to full brightness immediately without need for a warm-up time or ‘burn in’ time as we call it in the printing world," Roberts says. "The other dramatic difference is the UV wavelength produced by the lamps. In digital printing, the mercury-vapor UV lamps produce light waves in the 365nm spectrum (Type D lamps), while the LED-based UV lamps produce wavelengths at about 390nm.”

This means that the intensity of the radiation is dramatically less with LED-based systems. He says it can be illustrated two ways: First, the LED lamp produces less heat and second the LED uses less energy to illuminate. The ink that is used to print with mercury vapor lamps is quite different than the UV-LED system's inks as well. The chemical make-up directly corresponds to the light intensity (wavelength) and as such, the inks cannot be intermingled. They are developed specifically for the light wavelength of either mercury vapor or UV-LED that the particular system employs.

Technical Similarities and Differences

“As discussed, the mercury-vapor UV lamps use a greater intensity light wavelength. With this greater intensity, there are the two major drawbacks. The lamps use much more power and they are significantly larger than the LED lamps, says Roberts.

 “The mercury-vapor lamps are powered by 220-volt power, while the UV-LED lamps are powered under 110 volts. While the more powerful mercury-vapor lamps produce better adhesion on some materials, they may sometimes blister and warp other materials which might be sensitive to the heat the vapor lamps generate," he says. "The average mercury vapor lamp produces temperatures up to 200 degrees Fahrenheit, while UV-LED produce 104 degrees Fahrenheit.” He points out that this temperature difference allows UV-LED inkjets to print on sensitive films and generally a wider range of substrates, even without adhesion promoters, which are used with both lamp sources.

An Integrated Approach

But not all perspectives on cure systems are alike. Christopher Guyett, sales and marketing coordinator at printer manufacturer Durst Image Technology U.S., LLC, Rochester, New York, points out that Durst’s approach to LED has been to integrate it where it can solve a problem or advance the technology. “Some industrial Durst innovations have integrated LED to some extent," he says. "The strongest advantage to LED is the reduction of heat when compared to high-energy UV. In cases where excess heat has an adverse effect on the substrate being used and the underlying hold-down method is insufficient to overcome the problem, LED-based lamps can be a solution.

"The disadvantage is that specially formulated inks are then required [that can react to the narrow wavelengths produced by the LED] and may not perform on as wide an array of materials. Further, as production speeds increase, there is reduced ability to obtain the cure level necessary for some materials.”

He adds that Durst also has been on the leading edge of one “big” trend—a progression to digital imaging based on aqueous inks, as another option to conventional UV-curable and solvent-based inks. “The name of the program is Durst Water Technology," Guyett says. Development of the system, "is largely driven by two factors," Guyett continues. "New governmental 'green' directives (such as the European REACH regulations) and further growth into the interior-print, point-of-sale packaging, and P.O.P. segments.”

New Developments

At the same time other printer manufacturers are building on UV-LED technology and incorporating other advancements—such as a nitrogen gas blanket built into the printing zone—to develop a truly high-speed LED-based UV-cure printer. With this system the nitrogen blanket inhibits the presence of oxygen which hampers curing—allowing the ink to fully cure and at much faster speeds. Such is the case with EFI's new VUTEk GS5500LXr Pro, which offers printing speeds of 2,000-4,000 square feet an hour.

“LED inkjet used to be limited in speed capabilities," Hanulec says. "Printer manufacturers had not devised a way to cure ink fast enough using LED-based UV systems compared with other curing methods,” says Hanulec. “Our engineers created a system that removed oxygen from the surface where ink and substrate meet, replacing it with nitrogen, which allows the ink to cure more thoroughly and at faster speeds. This ‘nitrogen blanket’ process was a breakthrough for us in terms of bringing LED inkjet imaging into the high production space. But in the time since we first introduced production-speed LED inkjet printing to the marketplace, our engineers have also established ways to still maintain high quality and production speeds without a full nitrogen blanket process.”

He points out that this process enabled them to also create a lower-cost production LED inkjet device, the EFI H1625 LED printer. “Today that model is a top seller among smaller businesses seeking to offer the versatility of LED as an entry point into the growing digital print display graphics space,” he says.

EFI Rolls Out New Inkjet Technology

As for future developments, Hanulec says that EFI has recently unveiled its own new water-based inkjet platform that also employs UV lamps. “Our AquaEndure inkjet technology is a water-based inkjet platform that really addresses what signage and graphics companies need to do in digital printing," he says. "Because it is economical and offers high-quality imaging with strong color fidelity. Part of the advantage of the new ink is that it has a low-energy, which means it can be used with a wide range of substrates. It also has a very eco-friendly profile and no odor.”

He explains that starting out, AquaEndure will address the roll-to-roll market for grand-format interior décor applications, wall coverings and more. “But we intend to expand this technology to many future generations of printers, including hybrid roll/flatbed printers," Hanulec adds. "Ultimately, the ink will be an ideal fit for just about anything that is currently printed today with UV, LED or latex across the signage and graphics and packaging spaces.”

EFI did a live demonstration of its AquaEndure ink system at the recent drupa show this year, where it also debuted the company's new Nozomi C18000 single-pass LED press. The high-volume, high-speed flatbed printer employs a UV-LED curing system and is aimed at high-productivity corrugated board work. But future versions of the unit will be aimed at the sign and graphics markets, Hanulec says, adding that "in the future our AquaEndure inks will run in the Nozomi platform."

“AquaEndure extends the benefits even further than traditional UV-LED," he continues. "It will have a cost advantage over several other types of imaging technologies where printing companies are finding that ink prices are really cutting into margins. AquaEndure will also have some applications in food packaging that are not possible today with UV or UV-LED inks, so that creates a whole new opportunity for a range of applications where high-quality and value are important,” he concludes.

Durst Water Technology

Guyett says that Durst’s water-based Functional Aqueous (WT Inks), and its related printers, produce a litho-like print quality at production speeds. “These systems offer odorless operation and are more eco-friendly than existing solutions," he says. "Printers include the Rhotex 322 and Rhotex HS soft-signage printers, and the new Rho WT 250 HS and Rho 180 TR aqueous printers. The Rho WT 250 HS and Rho 180 TR are not yet available in every region where Durst does business.

“Durst's Water Technology utilizes a new proprietary Quadro Array piezo inkjet printhead technology, that generates minimal heat and features a circulating ink system. It prevents the vaporization of the water molecules within the ink, eliminates premature ink curing and clogging of nozzles to ensure operational readiness. Durst’s aqueous-ink systems can print on coated and uncoated materials, both rolled and rigid substrates.”

He adds that Durst Water Technology is not seen as a replacement technology—at least not for the foreseeable future. But as technologies advance and demand for more “friendly” print processes grows, aqueous-based imaging will take a more prominent place in the industry.

“Durst’s Functional Aqueous (WT Inks) have a more complex formulation and dry quicker than latex ink systems, which results in considerably higher productivity," Guyett adds. "These inks, which are priced similarly to Durst’s other UV-curable inks, combine the benefits of UV-reactive molecules—adherence, durability, flexibility, low migration—with the advantageous properties of water. However, they are not subject to labeling requirements regarding environmental or health impact,” he explains.