UV LED Curing
A recently developed technology for curing inks is attracting attention, but it still has a way to go to penetrate the label industry.
The FireFly family of UV LED curing arrays from Phoseon.
Over the past couple of years, a new technology has moved into view. It has not made a lot of waves, at least not yet, and discussions about its use in the narrow web industry are generally quiet. The technology is UV LED curing, and it poses a challenge – perhaps – to conventional ultraviolet curing of inks and coatings. An LED is a light-emitting diode, and its light, called electroluminescence, is produced by semiconduction. Traditional UV curing is accomplished by bulbs filled with various gases.
The process made its debut in 2008 at Drupa, where it was used primarily to cure inkjet inks. But inks can be manufactured to work with UV LED in flexography or any other print process.
Some proponents of LED curing say that it’s the next big thing, that it could replace the technology that is in wide use today. Others maintain that its time is yet to come. A couple of things are fairly certain about UV LED curing, however. It uses less power than conventional UV, and the LED array has a longer life. But it’s slower. Or maybe not.
“We are having some early success in the label sector, largely because labels want to go faster and faster, and thinner and thinner,” says Chad Taggard, director of marketing for Phoseon Technology, a manufacturer of LED curing systems based in Hillsboro, OR, USA. “The advantage of LED technology is that it doesn’t generate heat, so it won’t damage those thin labels, those heat sensitive substrates.”
The energy required to illuminate the LED array – there are many LEDs in one unit – is significantly lower than that used to power a gas filled UV bulb. Because of that difference, the UV LED has to be positioned quite near to the material. “We need to be close to the substrate: 5 mm to 10 mm away, up to 25 mm,” says Taggard. UV lamps use reflectors and have to be 2.1” away from the material, focused down into a single light, Taggard adds. “LED is a floodlight – it expands. Therefore the farther away from the substrate, the less light you get.”
“LED curing speed is faster,” Taggard maintains, a claim that is contrary to the opinions of others. “LED has two advantages over the classic UV lamp. The first is that you can turn LEDs on and off, because they are solid state. Run a label through the beam, turn it off in microseconds. It can be flashing. You are not using up energy and cost by generating heat when you don’t want to; the LED goes on only when it’s required to. The second advantage,” he adds. “is that you can ramp up speed to 150 meters per minute.” That’s meters, not feet.
Phoseon, a venture backed company, was launched in 2002. “This is what we do: UV LED,” says Taggard. “A lot of other companies have LED as well as traditional. There are applications where LEDs aren’t applicable. Our primary market is in the inkjet printing industry, but we are moving rapidly into the adhesives area, and coatings: laminations, floorings, wood coverings.”
Taggard says that LEDs are just part of the system that Phoseon produces. “They make up 10 percent of our total system performance. The rest comes in the ability to build the LEDs, put them in a system that you can keep cool, and in which you can increase the power over time.” The LED array is housed in a system that has a printed circuit board and a cooling system – either by air or water. “LEDs themselves put out a tremendous amount of heat,” he notes. “You have to extract that heat in some way.
UV curable inks – which technically are solids, not liquids, in their uncured state – are “dried” via a polymerization process that occurs when ultraviolet light stimulates photoinitiators to form links with other particles in the mix to form a solid. Traditional UV curing makes use of the full ultraviolet spectrum, which ranges from approximately 200 nanometers (nm) to 450nm. LEDs, however, focus only on a narrow band of the UV spectrum: usually between 395nm and 405nm. “The bulk of the inks available today are centered around 395nm, though there are some that cure at 365nm as well,” Taggard says. “It’s the UV-A range. Most of the industry is around 395, which is the sweet spot for curing.”
“We have some inks for the UV LED market, but there’s not much interest yet,” says Tony Bean, product manager, energy curing, for ink manufacturer Sun Chemical, Parsippany, NJ, USA. “The attention today is in the jet inks, and screen inks. For general graphic arts applications this may not be the perfect answer. Certainly there are positives to LED curing, but from an ink maker’s point of view the technology creates some problems.”
Phoseon’s FireLine LED array
“With a standard lamp you have a broad spectrum from 200nm to 400nm and other energies you don’t necessarily want,” Bean adds. “But we take advantage of that. Different photoinitiators do different things, and they do what we want them to do. Second, pigments in inks transmit energy. Some will cure better than others.
“Beyond inks, there are coatings. These are typically applied for some kind of surface effect. The type of energy that the LED lamps produce do not cure surfaces well. It’s difficult to get a good UV coating with LED lamps. Another problem is that the photoinitiators that give the surface cure don’t absorb the energy very well.”
Still, Bean acknowledges that the new technology might have a place in the future: “I believe that somewhere down the line the technology will improve to receive broader acceptance.”
As for curing speed, Bean has this to say: “The LED people say that speed is not necessarily a limitation, but how many of the units do you want to put on a press? The power is relatively low, so you need a lot of them. As you increase power, you have to cool the LED electronics. You don’t have as simple an application as you once had. You have to ask yourself what is the overall economic impact, and does it really make sense to make more?
As with standard UV curable inks, the cost will be higher. “We have to put more photoinitiators into the inks, which typically are more expensive items. If you’re pounding a lot of inks and coatings through a press, that can actually add up,” says Bean.
James McCusker is president of Honle UV America, based in Marlboro, MA, USA, which manufactures UV curing systems, the conventional kind as well as UV LED products. He sees some interest in the LED process out in the marketplace, but says it is not strong.
“The industry is growing in its ability to cure digital inkjet inks with LED,” says McCusker, “but speed is a limitation. Some users are asking it to perform well on a lot of inks, plus clearcoating.”
Honle UV America, a division of Honle AG in Germany, supplies UV curing equipment for EFI Jetrion’s digital inkjet label press. EFI, which manufactures its own inkjet inks, does so for traditional UV curing as well as for UV LED curing. “Jetrion is quite focused on UV,” McCusker says. “They are using LEDs to cure the white ink, and the conventional UV on the other colors.” (The Jetrion system lays down white first, followed by CMYK process color.) White is composed of titanium dioxide, a large molecule, and along with black is considered a challenge to cure using any energy system. Jetrion’s use of LED for the white is known as pinning, in which the energy brings the ink to a higher state of viscosity, but not quite to a full cure. “The pinning makes printing on the white a lot easier,” McCusker adds.
The heat generated by a UV LED system comes from the back side of the array,” he notes. “As a result, it has to be cooled, either by air or by water. The cooling also controls the output of the LED.” One industry observer speculated that the energy required to operate a water cooling system could dampen the claim that LED curing uses less power, but system manufacturers say that the power consumption of the cooling system comes nowhere near the amount required for conventional systems.
Most ink manufacturers are working on UV LED ink systems, or have already brought some to market, such as Siegwerk and INX International. At Print 09 in Chicago, INX Digital displayed a short-run narrow web UV LED press, featuring single-pass output at a cure speed up to 80 feet per minute. Recently, the company exhibited its LED-INX, a UV-curable pigmented inkjet ink that is in the development phase.
|Both the EFI Jetrion (left) and CSAT digital inkjet presses are making use of UV LED curing systems.|
Jonathan Graunke, vice president, energy curable technologies, for INX International says that the UV LED market has been moving reasonably well. “It appears that over time, more people are learning about it and willing to try something new.”
At the 2010 Digital Label Summit in Barcelona, Spain, Adrian Lockwood, CEO of UK-based Integration Technology Ltd. speculated that LED curing could replace conventional UV lamps. “Advantages in greater energy efficiency and lower maintenance requirements equate to savings in cost, in addition to environmental compliance, that spells the end of less efficient mercury-based bulb technology,” he said. The company’s curing units included the LEDZero Solidcure, which has a small footprint and light weight, and the LEDZero Pincure for precision pinning.
In addition to the Jetrion inkjet press, EFI manufactures VUTEk wide format UV inkjet systems. The company announced recently that it has chosen Phoseon Technology to supply the curing equipment for VUTEk machinery. “The introduction of the EFI VUTEk GS3250LX UV-curing printer sets a new standard for performance and capability,” says Ken Hanulec, EFI’s vice president of marketing. “The UV LED lamps provide unique capabilities we could not have delivered to our customers with traditional mercury-based lamps.”
“The successful application of Phoseon’s FireJet product to superwide format printers requires that the UV LED system supplier meet stringent technical, reliability and performance challenges,” according to Tom Molamphy, vice president, sales and marketing at Phoseon. “Working as a technology supplier to EFI VUTEk allowed Phoseon to provide a custom solution tailor-made to EFI’s specifications.”
Another wide format inkjet manufacturer, Roland DGA, has introduced the 64” VersaUV LEJ-640 UV-LED hybrid inkjet printer. Based on the company’s VersaUV technology, the LEJ-640 prints CMYK, white and clear on most substrates, from roll media to half-inch thick board stocks for custom graphics with special effects and finishes. “As our newest addition to the VersaUV line, the LEJ-640 takes package prototyping and specialty printing to the next level with support for thick media, including corrugated cardboard, foam board and other rigid substrates,” says Hiroshi Ono, group product manager for Roland DGA. “The LEJ-640 also prints and simulates embossing on virtually any flexible packaging media, as well as on a wide variety of materials used for press proofs, membrane panels, wide-format signage, POP, window displays and interior décor items. Safe-to-use UV-LED lamps are energy efficient and require no warm up time.”
Heraeus Noblelight, Hanau, Germany, has unveiled its UV LED system, called NobleCure. “At a wavelength of 395nm, NobleCure has an irradiance of at least 4 watts per square centimeter,” says Harald Maiweg, head of the optoelectronics division in the special light sources business division of Heraeus. “As the area of the emission window is almost 40 square centimeters, the resulting total output is about 200 Watts. This effective radiation capacity is supported by the use of state-of-the-art micro-lens technology. The lenses enable the shaping of the beam that increases the uncoupling efficiency of the UV LED system by more than 50 percent. Irradiance remains relatively stable and drops just slightly even at different distances to the material to be cured. Therefore, NobleCure can be used at different working distances to the substrate without the need to retool the system.”
It can be implemented in the wavelength range of 365nm to 405nm, Maiweg adds, and can therefore be used also for various adhesive curing and digital printing applications.
UV LED has a longer lifespan than do conventional UV bulbs. “It’s fantastic in terms of longevity,” says Taggard of Phoseon. “A couple of companies have gotten upwards of 20,000 hours, running constantly, 24/7, and still going. We all claim 10,000 hours, and we are now saying 15,000.”
Will this system take off in the narrow web space? Observers seem optimistic. “On a flexo press you might use it for variable information printing,” says Jim McCusker of Honle UV America. We see a lot of complementary use of LED in the future, in digital, flexo, and across the board.”