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FLEXO Magazine : November 2010
Technologies & Techniques brought into contact, the first contact is at a single asperity, point contact, and as the load increases, an increasing num- ber of asperities, come into contact due to the deformation of the asperities which were first in contact. The real contact area between non-conforming bodies is generally small compared to the nominal contact area, since contact is limited to asperities. This leads to a highly concen- trated stress in the contact regions (Lo 1968; Kagami et al. 1986; Adams and Nosonovsky 2000; Mihailidis et al. 2001). The magnitudes of the real contact area and of the pres- sure at the contact areas are affected by several factors: • Geometry and Topography of the bodies Shape of the bodies. Smoothness of the bodies, size, shape and numbers of asperities. • Material Properties of the bodies E-modulus (E) or Young’s modulus, describing the stiff- ness in compression and tension mode. Poisson’s ratio (ν), describing the strain relations. Shear modulus (G), describing the stiffness in shear mode. Viscoelasticity, describing the combination of viscous and elastic behavior of the material properties, taking into account the time-dependency of the material properties. Friction between the bodies. • Conditions Load applied on the bodies. Time of contact. Temperature of the bodies and the surroundings. Relative humidity of the surrounding air. Compression of paper and printing plate. In mechani- cal printing processes, non-conforming contact is dominant due to the asperities of the paper surface, the heterogeneous topographical structure. In order to obtain satisfactory ink transfer, the non-conforming contact must be minimized. This is naturally done by compression of the printing plate and/or the paper in the printing nip. The compressibility of the printing plate and of the paper has been shown to influence the paper- printing plate contact and the subsequent ink transfer (Miller and Poulter 1961; Bristow 1980; Jensen 1989; Heikkilä 1996; Se- rafano and Pekarovicova 1999; Jansen and Breakspeare 2001; Johnson et al. 2003; Provatas and Uesaka 2003; Endres 2004). Paper is compressible, exhibiting a non-linear strain- hardening behavior in z-direction compression. The strain hardening is due both to an increase in contact area and to the irreversible compaction of the fibre network (Rättö 2005). Several factors influence the compressibility of the paper. Hsu (1963) found that the density of the paper has a great influence on its compressibility. This is closely related to the results presented by Mangin et al. (1993) who found that the compressibility of the paper increased with increasing initial pore volume of the paper surface. It was shown, when comparing TMP (Thermo Mechanical Pulp) and kraft fibres, that the stiffer TMP fibres present more residual compressibility. This can be explained in terms of the fibre flexibility which influences the number of bonds between the fibres and the strength of the bonds on the strength of the formed paper (Torgnysdotter and Wågberg 2004). The TMP fibres are stiffer resulting in fewer and weaker bonds between the fibres than with kraft fibres. The topography of the paper also affects the compressibility. Heikkilä (1997) presented a non-linear viscoelastic model for coated paper contact smoothness, expressed as percentage contact area, as a function of pressure and time. In this model, the paper surface compression is divided into elastic and viscoelastic components. Rättö (2005) showed that the surface roughness has a greater effect on the com- pressibility of papers of low grammage than that of papers with high grammage. Pawlak and Keller (2004) studied the local tangent modulus of handsheets using an indentation instrument (Pawlak and Keller 2003). They reported that the apparent density determines the mean value of the tangent modulus, and that variations in local tangent modulus are created by other factors. The local thickness, local variations in roughness, local variations in sheet structure and distribu- tion of fibres morphologies were factors that may influence the local tangent modulus. The compressibility of paper in terms of the dynamic contact smoothness which is related to the compression of the paper during the printing nip passage has been studied by Blokhuis and Kalff (1976). They found that the contact area between a roll and several grades of papers increases with increas- ing pressure. Another study addressing the dynamic contact smoothness of paper has been performed by Heikkilä (1996), who reported that the paper surface compressibility was visco- elastic and that the viscoelasticity varied greatly between differ- ent types of papers, as well as spatially in the same paper. The viscoelasticy affected, not only the magnitude of the contact area, but also the texture of the smoothened structure. Further investigations by Mangin and Geoffery (1989) and by Mangin et al. (1993) have shown that the printing rough- ness decreased with increasing printing pressure and that the papers with a higher roughness were more compressible than less rough papers. The nip dwell time also influenced the compressibility of the paper in the printing nip. These results are in agreement with the theories regarding the compress- ibility of paper later presented by Heikkilä (1996) and by Rättö (2005). Johnson et al. (2003) found, studying the interaction be- tween water and paperboard in a flexographic printing press, that the surface compressibility on paperboard increased as an effect of pre-treatment of the surface by water. Figure 9. Percentage film transfer versus ink film on a film transfer form divided into regions i, ii and iii. Redrawn from nordström and grön (1998). www.flexography.org novembeR 2010 FLeXO 85 FLX_Nov10_mech.indd 85 11/1/10 2:26 PM
Sustainable Fall 2010