On Print | UV LED Is Beyond the Tipping Point

Anywhere ultraviolet (UV) cure is being considered, light-emitting diode (LED) should be seriously investigated.

UV LED is over the tipping point for adoption. In several major applications, it has matched mercury bulb cure, the primary reason holding back adoption. For example, in sheetfed offset printing, LED not only out performs mercury vapor, it is enabling conversion of presses with traditional ink to UV cure. The “numbers” now add up. Anywhere UV cure is being considered, LED should be seriously investigated.

LED UV lamps have real advantages over mercury (Hg) vapor lamps (see Table below). They should have about 10 times longer life, 20,000 hr, so output is very consistent for long periods. They are easy to operate and control, and they use significantly less energy. They emit pure UV, without infrared (IR), so they are friendly to heat-sensitive materials. And they have small profiles, so they are especially good for retrofitting to traditional ink presses.

Until 2015, UV LED wasn’t commonplace except where it permitted “things that couldn’t be done before” or its advantages, such as small format, were compelling. There was always a “but”: “it is not as fast,” “needs inerting,” “leaves the surface tacky,” “more costly,” and so on. The numbers did not add up, and so conversion was not justified. That is changing.

 

Comparison of UV LED and Mercury Vapor Lamps

LED Lamps Mercury Vapor Lamps
Lamp Life Time to 5% energy loss: at least 10,000 hr, commonly 20,000 hr, with test systems exceeding 40,000 hr, or 1½ to 3 years for a 24/5 machine. 15% to 30% loss at 2,000 hr
Consistent Output Continuously aging vapor lamps need continual measurement and adjustment. Inadequate cure has a real probability. These are real production costs and potential hassle.
Simple Operation Immediate on/off — no cool down or warm up period. Variable power by electronic control (think of a dimmer).
Clean No ozone, so no extractors. No mercury. Less effort to maintain EHS standards.
Lower Energy 30% of power is UV, 70% is heat—removed through the back of the diode. 3% to 5% is UV.
Cool Light Heat is removed from the back of the diode by conduction to circulating air or water. Little IR emitted with the UV. Good for temperature-sensitive substrates. IR has the same path as the UV, so it is directed to the cure zone. Deflection and reflecting are inefficient.
Small Footprint Compact, modular, cool and lightweight. Lamps and supporting frames have cross-section dimensions of inches. Readily fits between press stations or after coaters. Large, bulky, heavy, and hot.
Pricing Approaching parity with HG vapor.

 

What has been the catch?

First, LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched to the lamp, monomers, speed, and applications. To get good cure, LED requires different photoinitiators, and in turn, some different monomer and oligomers in the formulations.

Secondly, the power of LED lamps, especially peak energy, has been much lower than Hg bulbs. If just in the bulk, greater exposure time and high photoinitiator levels are effective. But it has been at the surface. Oxygen interferes with cross-linking. It stops the chain reaction. If the rate of cross-linking is very high, oxygen in the fluid is consumed before it is replenished from the air. But if cure is slow, oxygen wins. Many systems have been installed with inerting—the air in the cure zone is displaced by nitrogen, or the combustion products of air and gas, basically nitrogen and carbon dioxide.

Hg bulbs typically have reflectors that focus the rays so the light is most concentrated at the ink or coating layer. This greatly raises peak power and swamps any competing reactions. Early LED lamps were not focused.

What has changed recently?

  • First, formulating changes and materials have been developed and the knowledge has been shared. Many chemists now know how to reformulate to match the lamps.
  • Second, lamp power has increased. Diodes designs are improved, cooling is more efficient so diodes get packed more closely. That, in turn, raises lamp power, measured in watts per unit area at the lamp face, or better, at the fluid.
  • Third, lenses on lamp assemblies are focusing the power so peak irradiance is higher.

The combination is making LED directly competitive with Hg bulb in many applications.

LED is well suited for converting sheetfed offset printing to UV cure. For a start, this ink needs to be cured only at the end, not at each station. Secondly, the cure unit can fit in the small space between the last station and the stacker, for example. Third, the cure units have the power. Fourth, printers buy ink from a small number of large manufacturers. These companies can devote the resources needed to carry out thorough reformulation.

UV cure is very common on narrow web flexo presses. Cure is necessary after each application, so there may be six to ten lamps on a line. The waste of time and substrate with Hg bulb warm-ups adds up; their power supplies and air handling have large footprints. We also believe that the simplicity and consistency of LED complements “Lean” activities, including running extended gamuts, instead of spot colors. Here too, formulating is by the ink manufacturers’ experts.

LED cure best complements inkjet. On reciprocating printheads, hot and heavy Hg bulbs require massive scanning system frames, not necessary with LED. Fixed head machines have the print heads assembled in modules and installed in overlapping rows. Again, the compact cool UV lamp fits nicely attached to a head module. Also, digital printing is short run with frequent stops, so immediate on/off means higher productivity and more revenue potential. Recent ink developments and LED power enhancement are eliminating inerting. Again, ink formulating is by an ink manufacturer, which should have experts. LED is the default for all new UV inkjet developments.

There is a commonality to these applications: The formulating is in the hands of a group of experts in well-equipped laboratories, with expert field technicians looking over their shoulders. The ink is not released until it has been proven functional first by the manufacturer, and then with field trials. Additionally, the learning from one customer can lead to improvements for all. Any printer adopting gets the benefit of the accumulated expertise.

Contrast this with facilities that make their own formulations. For example, a specialty or custom coater with one machine and a dozen or 15 families of product assembled over a decade or more, by a rotating technical department. Replacing Hg bulbs with LED is becoming very appealing to the production manager, but it will be a nightmare if the formulations are not ready first. Of course, with its low profile, the LED system might be mounted adjacent to the Hg lamps in a converting line so the transition can be gradual.

Printing expert Dene Taylor, PhD, founded Specialty Papers & Films Inc. (SPF-Inc.), New Hope, PA, in 2000 for clients seeking consultation for technical management, new product design, development, commercialization, and distribution, as well as locating/managing outsourced manufacturing. Contact him at 215-862-9434; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.spf-inc.com.

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To learn more about UV and EB curing technologies, applications, and formulating, and to meet the industry vendors and experts, attend RadTech 2016 Technology Expo & Conference in Rosemont, IL, May 16-18, 2016.


 

 

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