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FLEXO Magazine : November 2010
Technologies & Techniques same reasons for insufficient ink transfer has been performed by Dickson (2004). He investigated the influence of surface structure on ink distribution on newsprint sheets by means of confocal laser scanning microscope and found that most unprinted regions occurred beneath a reference surface fitted to the mean sheet surface. Depressions that were unprinted beneath the reference surface were deeper and broader than those that were printed. Also, the edges of the unprinted depressions were higher. He also suggested, (Endres 2004), that the unprinted regions were clustered around structure elements, in this case large and prominent fibres at the paper surface. Pressure pulses with a maximum in the range of 1 to 1.6MPa, measured using a load cell, for printing on liquid packaging board in a central impression flexographic press have been reported by Johnson et al. (2004). In the same study, they in- vestigated how the dynamic nip pressure in the printing press was affected by the hardness of the printing plate. One printing plate with a hardness of 64̊ Shore A and another with a hard- ness of 75̊ Shore A were used. They reported that increasing the impression resulted in higher dynamic nip pressure but that neither press speed nor printing plate had any significant influence on the maximum dynamic nip pressure. The softer printing plate gave a higher print density in solid-tone areas, a higher ink transfer and less dot gain than the harder printing plate. It was suggested that the softer printing plate complied better with the paper surface and thus improved the contact between the paper and the printing plate. Neumann (1998) reported that a softer foam cushion reduces banding and gives a slightly higher dot gain than a firmer foam cushion. A further study addressing the effect on print performance of the physical properties of the cushion mounting tapes has been presented by Kilhenny (2000). The dot gain, solid print density and impression latitude could be predicted using Compressive Force Deflection (CFD), a static measurement of the force necessary to compress the material to an arbitrary impression, drop ball rebound, an energy absorption measurement, and impression latitude, a measurement of how much the impression can be increased without significantly affecting dot gain. It was reported that a softer cushion reduced the solid print density and dot gain. This reduction in dot gain is contradictory to what was found by Neumann (1998). In terms of contact mechanics in the post-printing process the surface irregularities, the washboarding profile of the corrugated board can be described on both a macro- and a micro-scale, as above. The macro-scale refers to the wash- boarding structure and the micro-scale to the surface rough- ness of the liner. The macro-scale contact can be generalized considering the contact between a flat and a wavy surface. When a flat and a wavy surface are brought into contact, the surfaces first touch along parallel lines at the tips of the waves. As the surfaces are pressed together, the lines expand into longitudinal strips reaching out from the tips of the waves. Westergaard (1939) investigated this type of contact and showed that the periodic pressure of the situation when one surface is flat and the other has a sinusoidal profile with an amplitude Δ and wavelength l (Δ << l) is given by: where 2/ width contact a= and p(x) is the pressure at position x [N/m2], E1 and E2 are the elastic moduli of the flat surface and of the wavy surface [N/m2], ν1 and ν2 are the Poisson’s ratios of the flat surface and the wavy surface, Δ is the amplitude of the surface with sinusoidal profile [m], l is the wavelength of the surface with sinusoidal profile [m] and a is the half contact width [m]. The load on a corrugated board in the thickness direction will be taken up by the fluting tips, so that the contact pres- sure will be higher on the fluting tips than on the fluting val- leys. As indicated earlier, a higher contact pressure increases the ink transfer and more ink will thus be transferred to the fluting tips than to the fluting valleys. This uneven ink transfer will lead to print banding. This is most pronounced on un- coated grades. Post-printing on coated grades has shown the reverse type of ink transfer, where higher amounts of ink have been transferred to the fluting valleys. This inverted banding can probably be explained if ink spreading and ink immobili- zation are studied. Netz (1998) investigated how the banding effect in post-print- ing is influenced by the corrugated board properties and print- ing conditions. He concluded that the flute structure was the most important factor, and that the print banding is generated by different local pressures in the printing nip. He also showed that banding increased with increasing nip pressure in halftone areas. In the full-tone areas, the relationship was the opposite, the banding decreased with increasing pressure. He suggest- ed that the value of the initial slope obtained in the flat crush test, FCT-stiffness, had a more pronounced influence on the degree of banding than the magnitude of washboarding. With increasing FCT-stiffness, the degree of print banding increased in halftone areas and decreased in solid-tone areas. Wendler (2001) found a linear relationship between washboard depth and solid tone print coverage. The gradient of the relationship varied depending on the surface treatment that the liner of the corrugated board had undergone. Netz (1998) also presented an equation, based on Hooke’s law, which can be used to calculate the minimum nip height allowed before the corrugated board will be damaged. The equation depends on the FCT-stiffness of corrugated board 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.” The 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. Figure 11. Macro-scale contact between a flat and a wavy surface. 88 FLeXO noveMber 2010 www.flexography.org FLX_Nov10_mech.indd 88 11/1/10 2:26 PM
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