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FLEXO Magazine : June2010
To learn more, call 704.588.3371 or Toll Free 800.438.3111 At Harper Corporation we do more than sell anilox rolls. We deliver confidence that comes with a 100% performance guarantee, backed by a team of graphic experts, and supported by a full compliment of products and accessories. With anilox roll manufacturing and restoration facilities in Charlotte, NC, Green Bay, WI, Herford Germany and Bangkok, Thailand, Harper Corporation delivers dependable consistency no matter where you print. ANILOX ROLL DIVISION HARPERIMAGE.COM Am ericas • Europe • Asia ©2010 Harper Anilox Roll Engineered for performance. Built to last. Guaranteed performance. From four locations across the globe. www.flexography.org JUNE 2010 FLEXO 31 • Migration of photoinitiator compo- nents in food packaging applica- tion • The effect of heat from high pow- ered lamps on certain film sub- strates • The effect of lamp heat on the CI drum. They describe a dual UV-EB flexo- graphic printing system that uses inks containing less than 10 percent pho- toinitiator and cured with interstation lamps operating at less than 300w/in. of input power. The system again utilizes a single EB unit after the last printing or coating station to complete the cure of all the layers. They also described advantages of improved ink adhesion to film substrates resulting in EB induced "grafting" of the ink to the substrate. Another reference also describing dual UV-EB curing in a flexographic printing process includes the use of UV light-emitting diode (LED) sources to provide interstation curing prior to final EB cure.4 This reference also includes the possibility that some of the ink layers may not need interstation curing prior to the final EB cure. The same authors also showed that inks could be effectively EB cured after initial UV curing with as little as 3 percent photoinitiator.5 In summary, the advantages of dual UV-EB curing for flexographic printing may include: 1. Improved print quality compared to solvent and water based ink due to the low dot gain achieved with 100 percent solids inks. 2. Low energy usage compared to thermal drying. 3. Elimination of solvents. 4. Elimination of thermal oxidizer equipment. 5. Limited lamp heat effects on sub- strates and equipment. 6. Use of ink chemistry based on stable well established UV flexo inks. 7. Use of robust UV dry trapping pro- cess which may be more forgiving than wet trapping. 8. UV dry trapping allows in-line or CI press configurations. 9. Final EB cure allows the use of high optical density inks that may be difficult to cure by UV alone. 10. Higher line speeds compared to UV curing alone. 11. Inks suited for food packaging; low levels of migration resistant photoinitiators may be used since only partial UV cure is needed prior to the final EB cure. 12. Assurance of complete cure due to consistent EB output and pen- etration. In spite of these advantages, dual UV- EB curing has only limited use in com- mercial applications. This may be due in part to existing patents and also the lack of available inks and press equip- ment for the demonstration and testing of these systems. The purpose of the paper is to demonstrate the performance of dual UV-EB curing systems in order to facilitate future growth of this technology. EXPERIMENTAL A set of cyan, magenta, yellow, and black (CMYK) process colors flexo inks were prepared. The ink formulations are shown in Table 2. The inks utilize a photo- initiator system that is designed for low migration and also complies with a list of materials recognized by a major global consumer food product company.6 Inks were applied with a Harper flexographic hand-proofer using a 900lpi (1.36bcm2) and also a 700lpi (2.27bcm2) anilox roll. This applica- tion method produces ink weights representative of commercial printing applications. An Avery Fasclear pres- sure sensitive film substrate was used for the testing. The optical densities of the ink films were measured with a model 939 X-Rite densitometer. Initial UV curing was conducted using an American Ultraviolet laboratory curing unit equipped with a variable speed conveyor and a 300w/in. medium pres- sure mercury arc lamp. The applied UV energy was measured with a UV Process Supply Compact Radiometer. Following UV curing the ink films were further cured with a BroadBeam EP series electron beam unit operating at 125kV. Samples were attached to a mov- ing carrier web and exposed with an EB dose of 30kGy. Duplicate samples were EB cured in air and also using nitrogen inerting (<200ppm oxygen). Ink curing was characterized after UV curing and also after subsequent EB cur- ing. The ink was wiped with a dry cloth to check for residual surface tack after curing. Ink adhesion to the substrate was determined using Scotch 810 tape. The approximate amount of ink remaining on the substrate after tape removal from a crosshatch area was recorded. Resis- tance of the cured ink films to methyl