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FLEXO Magazine : June2010
Technologies & Techniques 30 FLeXO june 2010 www.flexography.org Ultraviolet (UV) and electron beam (EB) curing tech- nology are well established in several printing and packaging applications. Each technology has its own advantages and disadvantages. A comparison of some key properties of each technology is given in Table 1. These inherent advantages and disadvantages have driven the evolution of current UV/EB printing processes. For example: • Sheetfed offset printing is almost exclusively UV, since it is challenging to inert and shield EB sheetfed processes. • EB is well established for web offset package printing be- cause paste inks may be wet trapped and cured with a single EB unit at the end of the press. EB is also preferred for food safety aspects of the process. An exciting technology known as WetflexTM has expanded the possibilities for EB package printing. The Wetflex process involves wet trapping of flexographic inks without interstation dryers and allows curing with a single EB unit at the end of the press. 1 Limitations of Wetflex include: • Limited to central impression (CI) press configurations, as the wet ink must not contact idler rolls needed for in- line press configurations. • Inability to wet trap over first-down white opaque ink lay- ers due to difficulty in applying subsequent colors to the heavy wet white layer. • The need to nitrogen inert the EB curing process. In spite of these limitations strong interest is expected with this solvent-free high quality printing technology. Printers may weigh the advantages and disadvantages and make a decision between using UV or EB technology. Another less common option may be to use some combination of UV and EB together. An early reference to dual UV-EB curing appeared in U.S. Patent 4,070,497 by Marco Wismer et al.2 They found that acrylate-based ink or coating layers could be partially cured to a “gelled” state. Cure could then be com- pleted by EB irradiation. This patent also described curing of multiple ink layers using UV to gel each layer as it is applied. This is followed by final EB exposure to complete the cure of all layers together. They further recognized that pigmented materials could be more easily cured by EB compared to UV. Table 1. Comparison of uV and eB Technology in Printing and Packaging Applications UV EB Equipment Size Compact – Well suited to interstation installations Large – Best suited to installation after last station Throughput Limited – multiple lamps needed for high speeds Very Fast – Typical ink cure dose easily delivered up to 1200 ft/min Inerting Most systems designed to cure in air Nitrogen inerting required for curing Food packaging Concerns with photoinitiator migration and uncured materials No photoinitiator required. High conversion minimizes uncured materials Consistency Lamp output decreases with age. Decrease varies across the width and spectral output of the lamp. Beam output is very consistent with age and over the width of the web. Heat Management High heat output must be managed. Heat increases for higher power input lamps. Relatively cool curing process Through Cure Challenge to cure through high density ink layers Easily penetrates high density ink layers A more recent reference specifically describes the ad- vantages of a dual UV-EB curing approach for flexographic printing.3 The inventors recognized challenges associated with using only UV for curing the ink layers. These challenges included: Dual Ultraviolet and electron beam Curing of Printing Inks early study Reveals Benefits of Combined Technologies By stephen C. lapin, Ph.D. and steve lundahl Study Summary: • Combined UV and EB curing of flexographic printing inks was investigated. • Initial UV curing produced a partially cured ink layer. • Cure was effectively completed by subsequent EB irradiation without nitrogen inerting. • Advantages of dual UV-EB curing include: - Ability to cure high density ink layers which are challenging to cure by UV alone. - Ability to dry-trap multiple ink layers using rela- tively low power interstation UV curing. - Assurance that all ink layers will be fully cured upon EB irradiation after the final print station. - Potential use in food packaging using low levels of migration resistant photoinitiators. FLX_June2010_mech.indd 30 6/10/10 9:39 AM