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FLEXO Magazine : November 2010
Technologies & Techniques and printing plate, the thickness of the corrugated board and the maximum compression of the corrugated board within the linear (elastic) part of the FCT-curve without loss of strength. Cusdin (2000) described the pressure required to transfer sufficient ink to the corrugated board to avoid banding. This was done by first ascertaining the optimum printing pressure range that will result in good ink transfer to the liner. Then, by matching the elastic modulus of the printing plate materials to the expected deformation of the area between the fluting tips, the optimum pressure could be calculated. If a compressible plate cushion is used in the printing plate when corrugated board is post-printed, a large part of the excess printing pressure is absorbed and the print quality is improved (Harris 1999; Jansen and Stebani 2002a). The effects of three different types of thin-plate technology printing plates, using high and low nip impression, B -flute and C-flute board, and two types of anilox roller, on the print result have been studied by Kilhenny (2002). The following printing plates were used in the study: Plate and Vinyl Carrier consisting of, 3 .94 mm photopolymer plate, 0 .76mm vinyl carrier and 0.05mm adhesive; plate with cushion PET carrier consisting of, 3 .18mm photopolymer plate, 0 .05mm adhesive, 0 .25mm PET carrier and 1.27mm urethane cushion; reverse mount consisting of, 2.92 mm unbacked photopolymer plate, 0 .76mm urethane cushion, 0 .25mm PET carrier, 0 .76mm vinyl carrier and 0.13mm adhesive. The reverse mount is one type of “cushion back plate.” T he reverse mount was the most compliant printing plate. The plate with cushion PET carrier intermediate and the plate and vinyl carrier the least compliant, according to CFD-tests. The solid ink density obtained did not differ between the three printing plates. The plate and vinyl carrier showed the high- est dot gain, the plate with cushion PET carrier intermediate and the reverse mount the lowest. Banding was evaluated as the density difference between light and dark regions in 30 percent half-tone areas. The reverse mount showed the least banding in most of the printing conditions tested. The banding effect was more pro- nounced at the higher impression level and for C-flute board. It was concluded that each of the printing plates has its own specific field of application. However, the reverse mount gave a higher level of performance for corrugated printers. The most significant factor affecting the banding was the impression lev- el, where a higher impression led to greater banding. The next most significant factor was board type, where C-flute was more susceptible to banding than B-flute. It was also concluded that the CFD analysis yields a significant insight into on-press per- formance, even if it does not account for the dynamic behavior of the printing plate, which may influence the results. AdditionAl StudieS Five detailed studies were conducted examining various aspects of this phenomenon. Each will be presented in sub- sequent issues of FLEXO beginning in early 2011. From the work presented in these papers, the following conclusions are particularly interesting: • The proposed band-pass image analysis method showed a high correlation with perceptual evaluation and could quan- tify print banding on printed B-flute corrugated board. • The cause of print banding was that a higher local con- tact pressure arose on the fluting tips than in the fluting valleys, so that more ink was transferred to the fluting tips and an increase in the difference increase the banding. • Higher stiffness in compression mode and a higher bend- ing stiffness of the printing plate generated a smaller contact area in the printing nip. Consequently, a higher external pressure on the ink was developed and this resulted in an overall higher ink transfer, especially to the fluting tips, in turn increasing the degree of banding. • A printing plate containing a photopolymer plate with lower Young’s modulus complied better with the wavy surface of the corrugated board, due to a higher degree of strain and a more uneven strain distribution in the photopolymer plate. • The proposed methods for measuring nip force variations and local contact pressure variations during the printing process could be used for in situ study of the nip force and local contact pressure variations that arise when different corrugated boards and different printing plates are in contact during the printing process. • The numerical simulations of the contact between the printing plate and the corrugated board could be used in order to predict the contact area, pressure level and tex- ture of local pressure as a function of material properties and print conditions and thus predicting the subsequent print quality. n ABOUT THE AUTHOR: Erik Hallberg received his doctoral degree in flexographic printing technology in 2006. After receiving his doctoral degree, he joined 3M as technical spe- cialist for the Displays and Graphics Division. In 2008, Hall- berg moved back to the paper industry as business solutions manager at Mondi, Extrusion Coating Division. Today, he is senior development engineer at Crane Currency focusing on product development. Figure 12. Contact pressure distribution as a function of the position for a half wavelength according to the equation. The direction of the arrow shows the effect of increasing elastic modulus of the flat surface. This paper has been edited, and its References and Bibliography section omitted due to space restrictions. To read the complete doctoral thesis, please visit: http://www.t2f.nu/t2frapp_f _174 .pdf. www.flexography.org november 2010 FLeXO 89 FLX_Nov10_mech.indd 89 11/1/10 2:26 PM
Sustainable Fall 2010