As in the screen printing industry, inkjet printing for outdoor applications is moving from solvent inks to inks that can be cured using an intense ultraviolet (UV) light for wide-format applications. Though most wide-format UV-cure printers currently use arc lamp type UV curing systems borrowed from screen printing, the industry is now developing newer, more sophisticated and efficient systems using ultraviolet light emitting diode (LED) technology. The promise of LED technology in this application is longer bulb life, higher energy efficiency and much cooler cure temperatures. Here we will look at this evolution of this technology and some of the advantages and challenges it presents to the wide-format print provider. 

 

Roland’s VersaUV LEC series of printer/cutters employ LED curing lamps and offer broad capabilities for specialty label and packaging applications.

Gerber’s CAT UV and Solara ion flatbed printer series are unique in that employ proprietary “cold cure” lamp technology (not from LED lamps) and cationic UV-curable inks.

UV INKJET CHALLENGES

In the early 2000s, the first flatbed UV-curing inkjet printers were brought to the market, first in Europe by companies like Inca Digital, Durst and Zünd, and then in the U.S. by Leggett & Platt (now Polytype America), Raster Printers (now EFI-Rastek) and others. Initially inks were developed using chemistry similar to that used in UV-curable screen printing inks along with UV lamps to cure them. The long term objective is to replace screen printing for rigid substrates. 

 

The first UV-cure printers were slow and expensive and had limited application. Short term they could eliminate the need for mounting and laminating so they could be cost effective in some applications, reducing the extra materials, labor and floor space. Solvent inks produced volatile organic compound (VOC) emissions that posed environmental and worker safety concerns as well. UV inks eliminated much of the VOC concerns but worker eye safety, ozone generation and a lot of heat from the UV lamps provided additional concerns. The problem with the heat from conventional UV-cure lamps is its negative effect on certain heat-sensitive substrates. 

 

With the development of UV-LEDs, there is the potential for curing without the heat and ozone. The expectation has been building that UV-LED could provide inkjet printing with a much better solution if the output energy could be increased and the cost could be reduced to match the benefits. Initially the power generation capability of the UV-LEDs was much too low to fully cure the ink at reasonable printing speeds, so they were used by some printer manufacturers to “pin” the ink—stopping it from bleeding and coalescing as it was printed—then fully cured by traditional UV lamps. 

 

Since the UV light emitted by the LEDs is from the narrow set of bands of the UV light spectrum compared to traditional wide-spectrum conventional UV lamps, the photoinitiators in the ink formulations had to be “dialed in” to accommodate specific wavelengths. One advantage for the UV-LED lamps was a narrower emission spectrum so that more of the energy emitted from the light source would be effective in curing the ink and not wasted in heat or light of the wrong wavelength. 

 

Mimaki has increased the speed (by up to 80 percent) of its JFX Plus series of LED-based UV-cure printers by adding a post-curing unit that exposes the ink to a second dose of UV radiation.

Russia-based Sun Innovations developed a line of LED-based UV-curing hybrid printers, the NEO UV-LED-Evolution series, and has also developed their own inks specifically for their systems.

UV-LED MORE BROADLY ADOPTED

Today, several UV-LED manufactures have met the challenge with increased power and cost effectiveness. These curing systems have now been implemented by several printer manufacturers to take advantage of the benefits while their ink providers have developed new inks tuned to the wavelengths of the UV-LED emissions. 

 

Roland DGA is offering its Roland VersaUV LEC series of printer/cutters, which employ LED curing lamps, and offers broad capabilities for label, packaging and wide-format graphics applications. 

 

Mimaki is offering an improved speed version of its JFX Plus series of flatbed UV-LED printers. They claim to have increased their speed by up to 80 percent by using a post-curing unit that exposes the ink to a second dose of UV radiation. The fact that a post-curing unit is necessary suggests that the UV-LED energy is insufficient to run at production speeds. 

 

Sun Innovations is a Russia-based organization that showed its NEO UV-LED-Evolution hybrid UV-cure printers at a recent trade show. Although just beginning to present its products in North America is has placed many units in Europe. In addition to developing its own UV-LED curing systems, it has developed its own inks specifically designed for its systems. 

 

Gerber Scientific, with its Solara ion and CAT UV flatbed printer series (no longer available) employs proprietary “cold cure” lamp technology (not from LED lamps) with its cationic UV-curable ink system. This system is unique, but the cooler cure temperatures allow the unit to print onto an exceptionally wide spectrum of substrates.

 

Although the improvements in UV-LED light sources’ corresponding UV-curable inks have been significant, more work is needed to increase the power available so that printer manufacturers can provide printing speeds more in line with traditional UV light sources.

 

WHAT’S NEXT?

The future path is clear. Better inks in clear, white and process colors along with higher powered, lower cost UV-LED curing systems are coming, and the performance of wide-format UV-cure printers will continue to improve. With the safety benefits over solvent inks, improved curing resulting in better durability, adhesion and increased printing and curing speeds, the future for UV-LED printers is good. The beneficiaries are both the print provider and graphics customer with lower cost, superior graphics and a lower impact on the environment.