UV Curing

July 19, 2005

The use of ultraviolet curing continues to grow as more printers discover its benefits.

The concept of using ultraviolet light to achieve rapid or instant drying is one which has been developed over many years. For some time now, the flexographic and narrow web printing industries have been extolling the many virtues of UV curing. Some of the areas commonly accepted as benefits of converting to UV curable inks, coatings and adhesives are color consistency, environmental issues, physical qualities, print quality, process printing and screen print affect.

UV (ultraviolet) and EB (electron beam) represent technologies that instantly “cure” or dry specially formulated inks, coatings, and adhesives. UV/EB technology offers increased production speeds, improved physical properties, and generates almost no air pollution compared with traditional methods.

Metal halide UV curing lamps from Primarc Technology.
Jeff Bade, sales manager for Primarc UV Technology, Phillipsburg, NJ, says, “Simply put, UV curing occurs when photoinitiators are energized by UV energy and initiate monomers, in an ink or coating, to polymerize and form a solid. Most of us recognize inks or coatings as liquids that ‘dry’ as their water or solvent ingredients evaporate. As the solvents evaporate, the ink’s pigments are left behind giving an image. UV curable materials go through a chemical reaction, or ‘cure’ to become solid, leaving an image of greater density.”

Curing Equipment
The equipment used to produce a cure in the previously mentioned applications may fall into one of several categories depending upon available space, cure speed and other considerations.

Curing systems typically consist of a high voltage power supply, a control panel and a curing head, which contains the components necessary to deliver the UV energy to the coating or ink that is on the media passing above or below it. According to Bade, a curing head typically contains the UV curing lamp (UV source), a reflector system for focusing the UV energy on the printing media, and a cooling system to maintain proper operating temperatures of the UV lamp.

Honle UV ACM system fitted to a Mark Andy 4150-10" press
Jim McCusker, president of Honle UV America, Marlboro, MA, identifies three types of lamps used in UV curing: mercury vapor lamps, metal halide lamps, or gallium lamps. “The mainstream UV lamp is a mercury lamp. It has three primary spectral outputs, it has outputs in the area of 254, 312 and 365 nanometers. The metal halide has a broader spectrum between the 350 and 450 nanometer wavelength. Mercury is used most in the business. But when using heavier laydowns, thicker inks, opaque inks, metal halide is required. The gallium lamp has a primary output slightly above the 400 nanometer wavelength.” Gallium lamps are used mostly for wood curing, not in web converting, he adds.

The medium pressure mercury vapor lamp (electrode type) is made in various lengths to suit the machine or application, and is a quartz tube about 1" or 25mm in diameter. An inert gas, such as argon or xenon, together with a small amount of mercury, is used to fill the tube, which also has an electrode installed at either end. When the lamp is connected to an appropriate power source an electrical arc passes between the two electrodes, vaporizing the mercury. The resultant energy emission from the lamp is primarily white light, infrared and ultraviolet light.

There are also electrodeless lamps. This type of lamp has a similar spectral emission to the electrode type but the method of operation is totally different. The lamp in operation is energized by microwaves generated by magnetrons rather than an electrical arc. The tube might also be filled with other materials depending on the spectral emission required for the application.

Janet Geyer, senior product manager, Fusion UV Systems, Gaithersburg, MD, says, “Microwave powered lamps maintain their stability from day one, whereas the arc lamps [meaning the mercury lamps] tend to lose intensity of output with use. The microwave lamps can last anywhere from 3,000 to 6,000 hours, depending on the bulb, and the output is consistent to day to day.” Fusion UV’s halide bulbs are warranted for 6,000 hours. Most mercury lamps are warranted for 1,000 hours.

Geyer also contends that microwave lamps are more expensive and therefore are not often used in the narrow web industry.

“Another difference between arc lamps [or mercury lamps]” says Geyer, “is that you can get them as long as you want. With our system the longest you can go is 10". If your press is wider, then you would add another system next to it. So your initial equipment investment can be a bit higher. But in the long run the quality of your product is better.”

Dan Naughton, director of sales and marketing, UV Research, Brea, CA, says the lamp itself is only one key component of a UV curing system. “The real component that really makes a UV curing system is the reflector design, which is what really gives you the intensity at your curing point. Without the reflectors, the bulb would have to be about a quarter of an inch from the substrate, which would burn it every time. This is why it is very important to keep the reflectors clean.”

Eleanor Midlik, president of Prime UV Systems, Carol Stream, IL, agrees that most UV systems have a finely focused UV reflector system. “The mirror finish reflector concentrates the UV energy on to a very narrow, finely focused point of a web,” she says. “This is what enables the high speed of UV curing.”

Naughton says reflectors come in three designs. “There are parabolic, elliptical — which is what most people use — and a focused elliptical reflector, where we have ellipses built into our reflector design in order to get more intensity. The bulb is smaller in diameter, and the little ellipses built into the focused reflector give you more direct rays, rather than being reflected. Anytime UV is reflected, you lose intensity.

“In a parabolic reflector, the UV light bounces around inside three or four times before it comes out to its focal point. So you’re going to lose maximum intensity at peak radiance.”

Naughton explains that the parabolic reflector is used mainly when speed is not an issue and when a large area must be cured. “When speed is not an issue, it’s not necessary to have the maximum intensity because you’re going by so slowly it will still absorb the UV light. Whereas with an elliptical reflector, you’re pointing that light at exactly where you want it to be because your substrate is going by so fast.”

Cold UV
UV lamp by GEW
A few years ago the industry started hearing about “cold UV.” David Horton, sales director of GEW Inc., Cleveland, OH, says that cold UV is a bit of a misnomer. “It isn’t really cold. You have a ‘cold’ reflector, and it’s not really cold, but it has a dichroic coating, which is a special light splitting coating, and that’s coated onto the reflector, which absorbs the heat and reflects the UV wavelengths.”

Naughton further explains, “Cool UV is the process in which you remove the IR away from the web or your substrate. This is done by dichroic coating the shutters — which is a NASA approved coating from years ago — and what that does is absorb the short wavelength or the IR wavelength from light. Additionally, we also use a dual cord barrier glass. Those are the pieces of cord between the bulb and the substrate. One of those pieces of barrier glass has what’s known as hot mirror cords coating which reflects the IR or the heat back in the irradiator housing and is evacuated out by blower, therefore not transferring the heat to the substrate.”

Cool UV is used mainly with film applications where heat sensitivity is an issue. Cool UV is generally more expensive. According to Naughton, there is usually about a 10 percent up-charge per unit.

UV Inks
Over the years a great deal of development work has been undertaken to improve the characteristics of UV cured products. Much time and effort was devoted to improving the inks to make them perform more like conventional water based inks. Technology has advanced to the point where it is now difficult to determine the difference between the two inks. In the not-too-distant past, the viscosity of UV curable inks was very high. Ink makers have made great strides to reduce that viscosity.

UV ink from Akzo Nobel
There is a difference, however. According to Mike Buystedt, director of market development at Akzo Nobel Inks, Plymouth, MN, one major difference is that UV ink cures, not dries. “It’s a totally different chemistry based on reactive chemistry,” he says. “When the photoinitiators hit UV light at a certain wavelength, it creates polymerization, or a chain reaction. UV ink cures with UV light.” UV cured inks remain open in the duct and on rollers. Curing occurs on the substrate only when ink is exposed to UV energy generated by the machine’s UV lamp array.

“The most commonly used technology is based on acrylate chemistry and cures through the free radical polymerization process,” says Tony Bean, marketing manager for energy curable paste inks at Sun Chemical, Northlake, IL. “Second, UV chemistry does not normally contain any solvents, so the hazards and environmental concerns associated with solvents are eliminated. Finally, the free radical polymerization process is induced by a light energy. This means that the curing starts and completes extremely quickly. It also means that darker and more opaque colors or coatings do not cure as rapidly as lighter, less opaque materials.”

Jeff Andrews, director of sales and marketing, Hanovia, Union, NJ, says that because UV ink is cured instantaneously, there is no worry about hazardous VOCs or odors that are released when laying down conventional inks.

What happens when inks and coatings cure, instead of drying? According to suppliers of UV curing systems, the whole process goes faster and converters save money.

Although UV ink is more expensive, Buystedt says the whole process of UV curing saves money because “there is less down time on the press, in flexo anyway, due to cleaner runs for longer periods of time.” He lists its benefits. “You can leave it in the press overnight. You don’t have to clean your plates between breaks. After a couple of impressions you’re running good material, with consistent color. In the long run it actually ends up being less expensive because you’re using a lot less material, because you’re wasting less, and you’re being more productive on the press.”

Buystedt says it is important to communicate with the ink supplier about the type of substrate being used so that the UV ink can be formulated properly for that substrate. Bean says, ‘The proper choice of ink is important in any process. All of the demands of the application process, as well as the end product use, must be met for the final product to be successful and acceptable. ‘Coca-Cola red’ cannot come out like ‘Pepsi blue’. Ink cannot come off on the reader’s hands. Inks and coatings must not alter the flavor of the chocolate. If the press runs at 1,000 feet per minute, an ink that cures at 800 feet per minute is unacceptable.”

Curt Hudson, general manager of UV products for Water Ink Technology, Lincolnton, NC, says it is important to differentiate between clear coatings and pigmented inks, as far as their needs with regard to UV curing systems. “Pigmented inks tend to respond much better when they have high attention to peak radiance and are coupled with the proper dwell time. Total energy is important, of course. You have to have the critical total minimum amount of energy, the total minimum amount of peak energy, the total minimum dwell time, a critical minimum temperature, and of course your distribution has to be married to the ink and coating. But in broad terms, inks rely more on peak radiance first and dwell time second, and total amount of energy third, if there is going to be an order there. Coatings tend to respond better to dwell time, then total energy, and then peak radiance. Of course, all of them have to have a critical minimum. What might be a good curing unit for clear coatings may not be the best one for pigmented inks.

“As a rule, there are some units that meet the needs for curing both clears and pigmented inks, but there are a lot of curing units out there that don’t do one or the other as well as the converter needs for them to.”

Dave McDowell, UV product manager at UV Technology, Cincinnati, says “UV is relatively widespread in the label and narrow web industry, and it’s growing especially as printers look to increase their markets beyond just water based capability. End users are looking for higher quality and better finishes. We see more and more label producers incorporating UV to give them the advantage of being able to run a broader range of application than they did before, especially in this current market place.”

Tom Becker, general manager at Aetek UV Systems, Romeoville, IL says that although there is some reluctance to changing over to UV mostly because of dealing with the unknown, there is still significant amount of growth in usage. He says, “It’s just a matter of the press operators learning the technology, and learning how to deal with the UV inks, which are more viscous and act a bit differently. But the advantages are there and those people who are using it have shown some tremendous gains in terms of what they can do with the process.”

Frank Simonetti, sales manager, Aetek UV Systems, says the advancements in ink have made it easier for the label industry to make the conversion to UV. In addition, most presses with UV stations also have hot air dryers. The end user still has the comfort of going back to water based inks and drying them with air, and on other projects to switch to UV. This has given him the flexibility to go both ways, and it has helped the industry to give a better understanding of the UV.”

AAA Press International's LIGHTouch UV Curing units: Top-Loading Interstation model (left) and Side-Loading Interstation model
Narrow web converters now can move their UV curing units from one station to another. AAA Press International, Arlington Heights, IL, manufactures such a unit. According to Sales and Marketing Director Mark Hahn, “Most printers are doing short runs, so having quick change lamp assemblies is imperative. For example, it allows printers to buy only four lamps for an eight-color press and have the ability to move them around on that press.

Jon Wilkie, president of Nicollet Technologies, Minneapolis, concludes that “The overall cost of UV is less once the printer reaches a point of sophistication in the application. If a converter can gain all of the elements that are required in an efficient manner, his cost of operation will be lower than for someone who isn’t using UV.”