Is LED theNext Generation of UV-cure?

The pros and cons of cationic UV inkjet printing were highlighted in an earlier article where I shared my doubts as to whether it would be able to meet the wide format community’s needs in the future (see Think Ink, DG, March 2008 issue). Now an alternative UV-cure technology has been demonstrated which to my mind may indeed be that future solution.

It shows promise at delivering UV inkjet technology that both solves many of the problems of free-radical inkjet technology as it is implemented today, but also avoids the pitfalls presented by cationic UV inkjet. It’s not a significantly new ink technology but a better UV cure-lamp technology coupled with an ink set that matches lamp output. We will explore this future technology and point out its advantages and disadvantages as compared to cationic UV and traditional free radical UV as we see them.

LED UV LIGHT SOURCES
LED stands for Light Emitting Diode. It is a microchip device that emits UV light. There are several manufacturers that provide these devices to UV inkjet printer manufacturers. Initially, LED lamps were only used to “pin” the UV ink with a quick partial cure right after printing to control drop spread and reduce penetration of the ink into porous substrates and fabrics. “Pinning” was all that could be done because the LED light sources provided insufficient energy to fully cure the ink at printing speeds. Traditional arc or microwave UV-cure lamps in addition to the LED UV sources were still required to fully cure the ink. Properly configured, this combination could help the printer manufacturer increase printing speeds and maintain high image quality.

Last year a Russian printer manufacturer, Sun LLC, introduced a UV-curing inkjet printer called Neo UV-LED equipped with LED UV cure lamps that provided a full cure. Their printer does not require the additional use of traditional UV lamps. You might ask why this is important. Well, first off, the LED UV lamps turn on and are ready for printing immediately. Traditional UV-cure arc lamps require a warm-up period to reach full power.

LED UV lamps are cold when lit, similar to the UV lamp on Gerber’s Solara ion printer (though Gerber’s lamp is not LED-based). LED lamps produce little or no ozone whereas traditional arc UV lamps generate a tremendous amount of both heat and ozone. So this results in a reduced need for ventilation. The lower heat reduces the potential for substrate warping and out-gassing from plastic substrates, which together with the ozone, really requires the removal of the air in the print area to avoid injury.

In addition, LED lamps consume much less energy and they don’t require replacement bulbs as traditional UV lamps do. The expected life of the LED UV light source is longer than that of the printer. So as the manufacturers of LED UV cure-light sources continue to improve the performance and cost of their products they become more and more desirable as an alternative for the printer manufacturer.

WHAT ABOUT THE INKS?
With the discussion of the Gerber Solara ion and its cationic UV inks we pointed out that it had the potential of overcoming the problem of achieving a complete cure, always an issue with free radical chemistry. This is one of cationic UV ink’s most important features. With the newest high-intensity LED UV light sources configured for inkjet printers and free-radical inks designed to match the output of these lamps, a complete “through cure” has been demonstrated at very high printing speeds.

It provides two important advantages. First, the curing is accomplished very quickly after the ink is jetted. This is important since we always have a race between the UV light reaching the photoinitiators in the ink before the oxygen in the environment can react with them destroying their effectiveness. It also allows the ink formulator to consider reducing the amount of photoinitiators in the ink because they don’t have to have an excess to compensate for oxygen depletion. This would reduce the liquid ink toxicity, cost less to produce and may improve shelf stability.

DEMO AT DRUPA
At the big printing tradeshow, Drupa, held every four years in Germany, companies show off their newest technology, even if it’s years off in the future before it is incorporated into real products. It’s sort of like the concept cars at the Detroit Auto Show. Well, at this year’s Drupa, UV cure-lamp maker Summit UV and ink maker Triangle Digital INX teamed up to demonstrate the power of Summit UV’s new Black Diamond 365 series high-energy LED UV light source with Triangle’s newly customized Evolve UV inkjet inks. The result was a demonstration of the ability of this system with one UV lamp to print with repeatable through-cure at speeds up to 80 feet per minute (in a single-pass label printer configuration). In my opinion, this is a great accomplishment in UV-cure printing technology, and once fully implemented, should obsolete the traditional UV light sources and also provide a superior solution to both traditional free radical and nascent cationic UV-curing inkjet printing systems.