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FLEXO Magazine : November 2010
Technologies & Techniques pressure and roll surface deflections in the nip between the rollers. It was reported that, at low loads, the roll radius, roll surface velocity, viscosity and line load governed the coated-film thickness, and that at high loads, the roll radius, roll surface velocity, viscos- ity, line load and Young’s modulus governed the coated-film thickness. The influence of roughness profiles on the nip performance of a lithograph- ic printing press has been numerically investigated by Bohan et al. (2002). They developed a model which described the change in pressure, film thickness and flow rate as a function of the rough- ness amplitude, roughness wavelength and fluid viscosity. They concluded, among other things, that the roughness amplitude had the greatest impact on the film thickness and that the rough- ness wavelength had no effect on the pressure profile. Jensen (1989) reported that a higher surface compressibility of coated white-top liner leads to a lower print quality, mainly because the compress- ibility has a positive correlation with the PPS-value. A more easily compressed uncoated paperboard surface, as a result of the pre-treatment of the surface by water, was interpreted to be the cause of decreasing print mottle of flexographic printed paperboard by Johnson et al. (2003). The water was re- ported to have a softening effect on the surface compressibility and thus to lead to an increase in contact area between the printing plate and the substrate. The compressibility of paper, in terms of the behavior of the paper/printing plate interaction has been simulated numeri- cally by Provatas and Uesaka (2003). The model describes paper compres- sion for a stochastic fibre network during printing. The effect of surface compressibility, surface roughness and local curvature on the paper-plate contact during printing was examined. They found that the surface compress- ibility might be more important than surface roughness and local curvature to describe the paper/plate contact. A dimensionless surface texture pa- rameter which characterizes the con- tact between a hard printing plate and the paper surface has been presented by Yan and Aspler (2003). The param- eter incorporates paper surface height and surface curvature terms, both of which can be determined from surface profilometry measurements. The sur- face texture parameter was found to correlate with the speckle area in the solid print areas of newsprints. Heikkilä and Oittinen (1995) found, studying the interaction of paper surface struc- ture, screening and print noise power spectra of solid offset prints, that the different scales corresponding to the width of fibres and the dimensions of the coating particles and their clusters had their own specific effects on the ink transfer and on the subsequent development of print noise. Endres (2004) investigated the origin of missing dots in gravure printing us- ing local contact and stress analyzer (Endres et al. 2005). She suggested that the missing dots occurred mainly in low- pressure areas near structure elements, fiber-crossings and pigment or filler agglomerations, in the paper that were exposed to high compressive stresses. Another investigation which suggests the www.flexography.org november 2010 FLeXO 87 Your package has only three to six seconds to grab the consumer’s attention... 800.346.8570 www.colorresolutions.com CRI inks speed up the process. Now you can achieve vibrant, true-to-life colors for corrugated and folding cartons and printed store displays. CRI can custom formulate flexographic inks and coatings with brilliant high-gloss colors and a durable scratch and rub-resistant finish. Combined with CRI’s unique TrueColorBalanceTM color management system for superior process control, your attention grabbing package will keep your customers coming back for more. Best of all, CRI water-based inks are among the most earth-friendly, with extremely low VOC’s and minimal environment impact. Contact a CRI specialist for fast response to your ink and color application questions. FLX_Nov10_mech.indd 87 11/2/10 9:55 AM
Sustainable Fall 2010