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FLEXO Magazine : August 2014
addition to the bright white light, enabling UV ink to cure on press (Schiler). From an environmental standpoint, the lights contain traces of mercury, which must be disposed of properly. e mercury in the tube is a liquid at room temperature. In order for radiation to take place, the mercury needs to be vaporized and ionized, so the tube will conduct electricity. Very similar to uorescent tubes, the starter that is needed to create the light is contained in the lamp itself. A third electrode is mounted near one of the main elec- trodes and connected through a resistor to the other main electrode. In addition to the mercury, the tube is lled with argon gas at low pressure. Once the power is applied, the argon is ionized and strikes a small power area between the starting electrode and the main elec- trode. It discharges heat and eventually there is enough to create arcs between all the electrodes. Because of this, mercury vapor lamps take around seven minutes to start up. In turn, maximum brightness of these lights takes signi cantly longer to reach than most lights (Hugot, 1972). is is an important consideration in high speed print. Tradi- tional exo curing methods use mercury vapor UV to cure inks. When using UV ink curing systems, the UV spectrum of the light source is critical. e lamps are constructed to emit a wide band of UV light energy, from 365-nm. to 440-nm. on the spectrum. Ink com- position and activators must react to that speci c range of UV light. UV CURING: LIGHT EMITTING DIODE (LED) An alternative to mercury vapor lamps is light emitting diodes, or LEDs. ey emit light through a complex process called electrolumi- nescence via a semiconductive metal. A semiconductor is a metal that can be either conductive or nonconductive depending on the environ- ment it is in. A semiconductor with extra electrons is called N-type or negative material, since it has extra negatively charged electrons. A semiconductor with extra room for electrons is called P-type or posi- tive material because it has extra positively charged gaps called holes. In a P-N junction, free electrons are attracted to the positively charged areas and the negatively charged electrons move across a gap from one side to the other, resulting in the ow of electrons (Uchida, 1988). As an electron travels to a hole it carries energy, but in order to t into the hole it must release any extra energy and when it does, this extra energy is converted to photons. e UV LED emits UV radiation from 395-nm. to 400-nm. in the ultraviolet range. Again, this is an important consideration when using ink with photoinitiators that require a certain UV range to react and cure because if the wrong ink is used, the UV curing system won't cure the ink properly. UV INK FORMULATION Similar to water and solvent based ink formulation, the ink formula- tion for ultraviolet ink contains resins that assist with curing the ink. e resins that are in the vehicle contain oligomers and monomers. Photoinitiators are used to trigger a reaction with these two mole- cules. A high dose of ultraviolet light is a catalyst that triggers the photoinitiators present in the resin. ese oligomers and monomers couple together, polymerize and solidify. e photoinitiator ser ves as a method of absorption of the UV light from a wavelength of 200-nm. to 400-nm. (the UV spectrum). Once it is absorbed it causes a rapid polymerization of the ink on the substrate, adhering with a durable, strong bond. DIFFERENCES e main di erences between the conventional UV lamps and the LED UV lamps are the energy consumption and the wavelengths at which they emit UV. e energy usage is signi cantly higher with conventional UV than that of LED and the conventional UV power unit is signi cantly larger physically in comparison to LED lamps, taking up critical real estate in the pressroom. ere are also di erent photoinitiators used in the vehicles that react at di erent wavelengths. e UV wavelengths are a large factor in curing the inks. Each respective technology needs special ink that correlates with the energy output. LED lamps have less latitude, with a range of only 5-nm. for curing (395-nm. to 400-nm.). is could be potentially hazardous for precision curing because of the small band, in com- parison to the band of the conventional UV (365-nm. to 440-nm.). Issues could arise for non cured ink migrating its way onto the printed product, or worse, the food product. As long as the correct inks are used with their respective curing system, no issues should arise. During the "Emerging Trends" session at the 2013 Annual Forum in San Diego, CA, Vice President of Narrow Web North America at Flint Group Packaging & Narrow Web Mike Buystedet presented on exog- raphic UV LED inks and the UV LED curing system. Many tests were run using UV LED curing systems. In summarizing these ndings, the following results were shown: • e LED UV ink had high color strength, a larger expanded gamut and a much denser black. ey were able to print UV metallics and UV adhesives that are generally hard to print • Curing these inks on glossy, matte and product resistant coated substrate was possible, which is not a general standard in the exographic industry • In regard to the system, the heat sensitive material ran well and testers were able to increase the speed of the press, providing greater utilization. All applications were validated by running multiple tests, showing the many bene ts to the LED technology • e "instant on" nature of LED lamps was proven very e cient to the process and reduced makeready time • As a result of lower maintenance, yield time was improved. LED lights have no moving parts, no shutter failures and no mainte- nance is necessary • Once the press is running, the lights turn on and when the run is over, they turn o . e LED UV needs no time to warm up like conventional UV 32 FLEXO | AUGUST 2014