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FLEXO Magazine : November 2010
To learn more, call 704.588.3371 or Toll Free at 866.588.8686 Or order on our website. Nothing compares to the power of CeramClean IITM when it comes to removing dried ink without damaging the cell structure. Use it with UV, water-based and solvent inks—on anilox rolls, or ceramic, chrome and gravure cylinders. It’s a great stain remover. Available in original thick, gel and pourable liquid in a full range of sizes. HARPERSCIENTIFIC DIVISION HARPERIMAGE.COM Americas • Europe • Asia ©2010 We’re talking clean. equation is satisfactory for coated pa- pers but not for uncoated papers. According to Nordström and Grön (1998), the ink-substrate interaction can be divided into three regions, I, II and III, when film transfer is plotted versus ink film on the printing plate. In region I, the percentage ink transfer increases with increasing ink film thickness on the form, but the filling in of pores and the coverage of the surface is incomplete. In this region, the surface structure has the greatest influence on the ink transfer. In region II, the percentage ink transfer decreases with increasing ink film thickness. Within this region, the film immobilization, i.e . the absorptive and volumetric proper- ties of the substrate, determines the ink transfer. In region III, the percentage ink film transfer remains constant, and in this region the fluid properties, rheologi- cal and fluid dynamic force, determine the ink film transfer. It has been reported that the sur- face structure affects both ink transfer and print evenness and that a smooth surface is necessary to receive a uniform film of ink from the printing plate (Zang and Aspler 1995). Kapoor and Wu (1978) reported that ink transfer depends on the geometric properties of the paper, such as voids, non-contact areas, and sharpness of asperities. These voids and asperities cause small spots which have a color or texture different from the rest of the paper. Barros et al. (2004) have shown that uncovered areas in full-tone flexo-printed paperboard are associ- ated with depressions in the surface topography. Jensen (1989) found that the print density decreased with increasing Parker Print-Surface (PPS) roughness and that the surface roughness has an obvious influence on print mottle. The porosity and pore size distribu- tion of the substrate influence the setting of the ink and ink holdout. The spread- ing and absorption of the fluids depend on the pore size distribution, rather than on the porosity alone (Gane 2004). Aspler et al. (1992) reported that, at given printing conditions, more ink is transferred to those newsprints that are more absor- bent, leading to a higher print density. However, for a given amount of ink on paper, the print density decreases with increasing water absorbency. Their interpretation was that the print density per gram of ink on paper is lower for Figure 4. Schematic illustration of principle for letterpress printing plate with raised printing elements. It also illustrates the principle of screening. Figure 5. Compressive stress-strain curves for an elastic solid and a foam made of the same material. The energy absorbed per unit volume at a stress, σp, is marked as the area below the curves. W is the energy absorbed per unit volume, up to a strain, ε, and Es the cell wall modulus. Redrawn from Gibson and Ashby (1997). www.flexography.org novEmbER 2010 FLEXO 83 FLX_Nov10_mech.indd 83 11/1/10 2:26 PM
Sustainable Fall 2010