Ultraviolet curing of inks on substrates has been around for a good many years — nearly 40. It is surprising, therefore, to hear equipment and ink suppliers tell of the many converters and printers who have yet to take advantage of the process.

“It is surprising,” says Mark Hahn, director of sales and marketing for AAA Press International, Arlington Heights, Ill. “We see a great many converters using UV curing systems out West. In the East, however, many are still not using even UV varnish.”

Five years ago, the industry estimate was that about 5 percent of narrow web converters were making use of UV curable inks. A good many — impossible to estimate — were already using UV curing, and that included all of the big names in the business. Today the estimate ranges from 15 percent to 25 percent, depending on who’s being asked.

Some see the market as having reached a plateau — growing but flattening. “My perspective is that the rate of growth has slowed,” says Pete Kershner of EYE Ultraviolet, Wilmington, Mass. “UV flexo is still a healthy part of the market, though. In the mid-1990s the growth was really surging. It has found its plateau at this point.

UV curing is an aspect of printing that has made significant strides in the past few years, both in the equipment area and in the ink technologies. “Cool UV” came to the marketplace recently, offering less heat on the substrate. Today many equipment companies offer this type of lamp and system for both narrow and wide applications.

One reason for the surprise that UV has not penetrated further is that use of energy-curable inks produces no volatile organic compounds (VOCs). Curing of solvent inks, which are still around, releases many compounds into the surrounding air.

Another advantage is the overall quality of the finished product. UV curable inks can be laid down heavily, and cured instantly, producing deeper and more pronounced colors and tones.

Quick lamp changes
UV curing involves installation in the press of an assembly that houses UV lamps, plus a power supply. That arrangement hasn’t changed overall, but some modifications have come into vogue. One such assembly is the “interstation” model, which can be installed between print stations in the press.

“What press manufacturers have done is to let operators make quick changes and have flexibility,” says Hahn. “What we have done with our LIGHTouch interstation model is allow them to hard-mount a web box between the stations, and they can put the lamps in there when they need them.”

For example, he says, a converter might have a six-station press and wants UV capability, but is unwilling to make a substantial investment in six UV curing units. Instead, they can purchase a four-lamp system and two extra web boxes, which gives them the ability to move the lamp assembly from station to station within a few minutes. “You can have UV on stations one, two, five and six, for instance, and move lamps to three and four when you need them. All the pneumatic, exhaust and electrical are quick disconnect.”

Such a setup is also beneficial to the converter who wants to enter the UV varnishing area. “Those folks don’t want to spend a lot of money,” Hahn says. “Instead of spending $60,000 they can spend $20,000 and go from station to station to station.”

Cool UV
Ultraviolet inks are cured by exposure to light in the ultraviolet band of the spectrum, between 200 and 400 nanometers. Curing times differ for inks of different colors and compositions. Darker colors, for example, require more UV exposure; they absorb UV light, and black absorbs most. The light must be strong enough to penetrate to all of the photoinitiators within the ink. Each pigment has a window in the UV spectrum that allows the UV to penetrate and cure.

The process produces heat. There is some heat from the UV light itself, but nowhere near as much as from infrared (IR), which is near UV in the spectrum. This is why some press manufactures construct chill drums to bring the temperature of the web down prior to application of another coat of ink. The temperatures can be quite high, and the chill drums lower them to around 100°F or so.

Heat has varying impacts on different types of substrates. “On the one hand film material has gotten a lot more press-stable,” says Kershner. “A lot of the original difficulties with in-line presses running those materials is tapering off, because the materials are more stable. The next challenge, though, is to see if they can deal with the thermal issues. The downsizing of film materials is going to continue — they will be lighter over the next two to three years,” he says.

One of the recent developments aimed specifically at the heat issue is “cold UV.” This technology utilizes various heat filters and dichroic reflectors to disperse a substantial amount of the IR heat.

Kershner says that in EYE Ultraviolet’s “Cold Mirror” technology, dichroic reflectors replace standard aluminum reflectors, “reducing temperatures between 60 and 70 percent between the bulb and the web.”

EYE offers IR Cut Filters for extremely heat sensitive applications, on substrates such as styrenes, PVC, polypropylene and polyethylene with low heat distortion temperatures. The filter shields the web from IR rays and allows the UV to pass through to the substrate.

Cool UV is penetrating the industry. Starna Industries and its associate company, SY Technologies, of England, offer a system that promises a 40 percent reduction of infrared heat.

AAA Press International’s cool UV package has a “hot mirror” positioned between the lamp and the web, allowing UV light through and keeping a majority of the IR out, says Hahn. “The other source of heat is reflected light,” he adds. “We put a UV optimize coating directly on the reflectors to absorb the reflected IR. We also use variable speed blowers to bring the temperature down.”

Hahn says he sees a number of converters opting for 600-watts-per-inch lamps recently. These produce higher amounts of heat per inch, but they enable the operator to run the press at higher speeds. “We’re seeing presses running at 600 to 700 feet per minute,” he says, “such as the Roto Press. With some of the coatings that are being used out there, 600 watts will give you a bigger bang.”

UV curable inks are improving daily, say industry observers. The ink is more expensive, certainly, because it costs more to make. The utilization rate, however, is lower. “It might cost two times as much, but you will get three times as much yield,” says Hahn.

Virtually every company that manufactures inks for narrow web flexo is producing a series of UV curable inks, because they see it as a strong growth market. Still, practitioners report that one company’s blue is not the same as another’s, so printers should experiment and find the combinations that work best for them.

Still others report that some ink manufacturers are trying to save money in production by cutting back on the photoinitiators in the ink. Those are the chemicals that react to the UV light and begin the chain of chemical reactions that convert the ink to a hard, dry solid. If a formula is altered so that curing time is longer, the problem is probably not with the lamps.

“There are variations out there,” says Hahn. “Some manufacturers are recommended by suppliers, and some are not. The printer should test a variety of inks. If you buy a system and it doesn’t work right, you should examine the inks. The lamps most likely are putting out the proper wavelength.”