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FLEXO Magazine : November 2010
Technologies & Techniques Plate Cushion Today, the plate cushion is often a cellular solid material. A cellular solid is made up of an interconnected network of solid struts forming a three-dimensional polymer foam. The plate cushion material used in the printing plates of thin- plate technology is usually made of polyurethane or polyeth- ylene, where polyurethane has an open cell structure and polyethylene a closed cell structure (Kilhenny 2000; Jansen and Breakspeare 2001). The essence of the polymeric foam is its ability to convert kinetic energy into strain energy through elastic or visco-elastic deformation. In the literature regarding the post-printing of cor- rugated board, this phenomenon is described by Harris (1999) and Jansen and Stebani (2002b), who say that a large part of the excess printing pressure is absorbed using a plate cushion. Cusdin (1997) described it by saying that the foam will allow for the additional impression required to overcome the variable printing nip without excessive deformation of the print surface. Jansen and Breakspeare (2001) have also addressed the phenomenon in physical terms saying that the plate cushion offers the ability to absorb energy when a pressure is applied. This extraordinary energy-absorbing capacity of the foam is due to its low relative density. The relative density is defined as: Relative density = ρ* ρS where ρ* is the density of the foam [kg/m3] and ρS is the density of the solid from which the cell walls are made [kg/m3]. The relative density of polymer foams ranges approximate- ly from 0.002 to 0.05 (Gibson and Ashby 1997). The relation between relative density and porosity, p, of a foam is given by: p=1– ρ* ρS These features, the relative density and porosity, enable the interconnected network of solid struts to convert energy via elastic buckling during loading in the linear-elastic regime (Gibson and Ashby 1997). A high degree of pressure confor- mity can then be achieved during the ink transfer when the corrugated board and the printing plate are in contact. The firmness of the foam is controlled primarily by its density, and closed-cell polyethylene foams are in general more resilient than open-cell polyurethane foams (Kilhenny 2000). Manufacturers classify the mechanical properties of their foams using terms such as soft, medium and firm. From a solid mechanics perspective, this is unfortunate, since stiffness relations between stress and strain are described in terms of Young’s modulus or E-modulus. The plate cushions currently in use for post-printing are ap- proximately 1mm to 3mm thick. Cusdin (1997) stated that the foam thickness should be no more than half the plate material thickness and Jansen (2002) has reported that the plate foam should not be thicker than the photopolymer plate used, to prevent any possible instability in the system. A mounting foil is a homogeneous film usually made of some type of polyester with a typical thickness between 0.2 and 0.4mm. Since the mounting foil is stiff it seems reason- able to assume that it probably has a great influence on the bending stiffness of the printing plate. The bending stiffness affects the degree of deformation and stability of the printing plate when it is in contact with the corrugated board. The sta- bility controls the ability of the printing plate to distribute a line load at the moment of printing. A rule of thumb is that a stiffer material leads to a narrower load distribution. Print Quality evaluation A consensus definition of print quality is hard to find within both the research community and in the industry, because there are so many factors that contribute to print quality (Barros 2004). It is, however, possible to make accurate physical mea- surements on a print that are found to correlate well with rigor- ous subjective evaluations of print quality (Fahlcrantz 2005). In the paper and graphic arts industries, print density, dot gain and print mottle are often used as measures of the print quality for process control. These three measurements have also been used in the investigations included in this thesis, but before these measures of print quality are discussed it is use- ful to obtain an overview of the link between printed objects and the human visual system. Print density. The optical print density is a measure of contrast. It is calculated as the logarithm of the ratio of the reflectivity of the substrate to the reflectance factor of the printed substrate. The optical print density for solid-tone, DS, and halftone, DH, are defined as: where RS is the reflectance value of the printed solid-tone, RH the reflectance value of the printed halftone and R∞ the reflectivity of the unprinted substrate (opaque). The dependence of the solid-tone print density, DS, on the ink amount on the substrate can be described by the equa- tion proposed by Tollenaar and Ernst (1961): ) 1( m y S e D D − ∞ − = where DS is print density, D∞ the print density maximum or saturation print density, y ink amount on substrate [g/m2] and m a parameter for the steepness of the print density curve. Dot gain. This is a measure of the increase in the diameter of a printed halftone dot compared to the diameter of the printing plate relief. This increase in dot size occurs when the ink is transferred to the substrate and is due to both a mechanical gain and an optical gain. The mechanical gain is an increase in the physical diameter because the ink is spread out sidewise when it is exposed to the pressure between the printing plate and the substrate (Lagerstedt and Kolseth 1995). Mechanical dot gain can also occur when the dot is transferred to the paper and setting is achieved by ab- sorption. This takes place in the vertical direction, into the substrate, but it may also occur sideways, contributing to an increase in the dot diameter. Optical dot gain is a result of the fact that light is scattered within the paper and some of the light is trapped below the halftone dots and is absorbed by the ink (Yule and Nielsen 1951). This loss of light means that the halftone print appears darker and the tone value is increased. Optical dot gain depends on the opacity and on the surface reflection of the printed substrate. Overall dot gain, a collective effect of physical and optical dot gain, FD, can be estimated using the Murray-Davies equation (Murray 1936): and where AD is the optically effective area of the dots [ percent], FD the overall dot gain [ percent], AR the nominal area of the halftone [ percent], DH the halftone density and DS the solid-tone density. Print mottle. This term describes opti- cal heterogeneity, unevenness in optical print density and print gloss. It appears in solid-tones or smooth image regions. Print mottle appears stochastically or systematically (Rosenberg et al. 2001; Fahlcrantz 2005). The causes of print mottle are closely related to uneven ink transfer and absorption which is affected by substrate and ink properties as well as by printing press conditions. This will be discussed later. Print mottle can be evaluated using instrumental measurements methods, often by image analysis. The results obtained by the in- strumental measurements are validated against subjective quality ratings by human observers. Figure 2. Flute wavelength and flute height of double-face corrugated board. 80 FLeXO november 2010 www.flexography.org FLX_Nov10_mech.indd 80 11/1/10 2:26 PM
Sustainable Fall 2010