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FLEXO Magazine : November 2010
Technologies & Techniques It was reported that this might have contributed to an increase in contact area between the printing plate and the paperboard. Engström and Morin (1999) showed that the compression of base sheet of a coated paper increased with increasing moisture content. The researchers also reported that the permanent compression was greater at higher mois- ture content. When the paper is compressed, two important paper prop- erties, surface roughness and pore volume, which both influ- ence the ink transfer and ink immobilization, are affected. The surface roughness decreases as the compression increases (Bristow 1980; Heikkilä 1997). The pore volume decreases with increasing compression. Hsu (1961b) reported that the pore size of the paper is linearly related to the square root of the applied pressure. The decrease in surface roughness is also an effect of the decrease in the pore volume (Mangin and Geoffery 1989). An essential characteristic of the flexographic printing process—in contrast to conventional letterpress—is that the printing plate is deformable and, in the case of multilayer forms, also compressible. At the same time, as the paper is compressed in the print- ing nip, the printing plate thus exhibits deformation in the compression mode. The deformation within the plate and mounting system is affected by the E-modulus and the thick- ness of the components and the total compression is shared between the mounting foam and the printing plate (Cusdin 1997). The compression of the photopolymer plate and plate mounting materials used in the flexographic printing of liner board has been studied by Johnson et al. (2004), who showed that the top layer of the soft coat sleeve was the most compli- ant material and proposed that most of the deformation dur- ing the nip passage occurred in the sleeve. With regard to the compression of plate images, Cusdin (1997) reported that the image on the print surface is spread by lateral deformation of the printing plate material during the impression with both the anilox and the substrate, due to the Poisson’s effect. The deformation of printing plate dots involves two mechanisms; expansion of the dot surface and barreling of the dot shoulder (Bould 2001). Dot barreling was reported to be the dominant effect and low coverage dots had the highest gain relative to their size due to low dot stability, resulting from a small dot area and large dot depth. When the printing plate and the paper are brought into contact in the printing nip, both macro- and micro-defor- mations occur. The macro-deformation relates to larger homogeneous components e.g. the photopolymer, while the micro-deformation relates to the asperities on the surface. The place where the macro-deformation arises depends on the compressibility of the printing plate components and of the substrate. Together, the printing plate and the substrate, which can be made of several layers, can be considered as a sandwich construction made of different components. The E-modulus, E eff , of a sandwich construction under con- stant stress can be calculated (Dowling 1998) according to: where Eeff is the E-modulus of the sandwich construction [N/m2], E1, 2, 3,..., n are the E-moduli of the components [N/m2], l1, 2, 3,..., n are the thicknesses of the components [m] and l is the thickness of the sandwich construction [m]. Using these equations, it is possible to calculate the deformation in each component of the printing plate and the substrate under a given compressive stress at small deforma- tions. The equation predicts that the main deformation will occur in the most compliant layer. Although, this equation assumes constant stress, which is not the case in a printing plate in the printing nip, it gives a good indication of the defor- mation in each component. Nip MechaNics & iNk TraNsfer Several investigations have found that increasing con- tact pressure and increasing dwell time in the printing nip increase the total ink transfer to the substrate (Hsu 1961a; Hsu 1961b; Frøslev-Nielsen 1962; Schaeffer et al. 1963; Cozzens et al. 1967; De Grâce and Mangin 1984). With higher ink viscos- ity, the influence of printing pressure and printing speed was less (Hsu 1961a). Jensen (1989) reported that a rougher surface needed a higher pressure in order to fill the surface profile with ink. The printing pressure needed to achieve a uniform solid print increased linearly with increasing PPS roughness. Prediction of the contact performance in the printing nip using numerical modeling has been the topic of several investigations. Mirle and Zettle- moyer (1988) proposed a model which takes into account the ink hydrodynamics and the viscoelas- tic behavior of the photopolymer plate in the print- ing nip. The model gave results which correlated well with their experimental data. The maximum nip pressure was obtained in the middle of the printing nip for each of the printing plates investi- gated, and the different printing plates generated different maximum pressures. Forward roll coating with deformable rolls has been studied using an elastohydrodynamic model (Coyle 1988). The model predicts the flow rate, 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- figure 10. illustration of the increasing number of point contacts at asperities on the surface as an effect of increasing load. 86 FLeXO november 2010 www.flexography.org FLX_Nov10_mech.indd 86 11/1/10 2:26 PM
Sustainable Fall 2010