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FLEXO Magazine : November 2010
To learn more, call 704.588.3371 or To ll Free 800.438.3111 Quality, consistency and efficiency in a pressroom translates into lower production costs. And no company is more committed to helping you achieve these goals with unprecedented results than Harper. So much so, we devote an entire division to the cause. Call 800-438-3111 to get started and receive a copy of our “Seven Element Flexo” free. GRAPHICSOLUTIONS DIVISION HARPERIMAGE.COM Americas • Europe • Asia ©2010 We’re talking savings. 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. Image analysis methods are based on frequency analysis, for instance the STFI Mottling method (STFI 1999). In these methods, the optical signal reflected from a printed sample is de- composed into components at various spatial frequencies by a Fourier trans- form. A brief explanation of the concept of Fourier analysis is here presented. Figure 3. Topography of uncoated B-flute corrugated board, measured by means of white light interferometer. Left hand figure, a 3 dimensional representation of the surface, and right hand figure, the same surface represented in a 2 dimensional image. www.flexography.org novemBer 2010 FLEXO 81 FLX_Nov10_mech.indd 81 11/1/10 2:26 PM
Sustainable Fall 2010