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FLEXO Magazine : November 2010
©2010 To learn more, call 704.588.3371 or To ll Free 800.438.3111 At Harper Corporation we do more than sell anilox rolls. We deliver confidence that comes with a 100% performance guarantee, backed by a team of graphic experts, and supported by a full compliment of products and accessories. With anilox roll manufacturing and restoration facilities in Charlotte, NC, Green Bay, WI, Herford Germany and Bangkok, Thailand, Harper Corporation delivers dependable consistency no matter where you print. ANILOX ROLL DIVISION HARPERIMAGE.COM Americas • Europe • Asia ©2010 Harper Anilox Roll with built-in support. Performance guaranteed. From four locations across the globe. fluting tips (Netz 1996). Netz (1996) pre- sented a method for the measurement of washboarding. It was reported that the irregularities of the board surface were in the range of ± 40 μm. Wendler (2001) also demonstrated a method for the measurement of washboarding, and reported amplitudes between 7 and 56 μm on several different grades. This washboard-structure often generates print quality problems (Pedraza 1993; Zang and Aspler 1995; Crouch 1998; Jansen 1999; Holmlund 2002; Bonawandt 2003). The problem is more pronounced for the grades hav- ing a greater flute height and a longer flute wavelength (Kilhenny 2002). The manner in which the washboarding is influenced by manufacturing factors has been studied by Netz (1996), who reported that the magnitude of the washboarding decreases with decreas- ing amount of adhesive between the liner and fluting, increasing grammage of the liner and increasing relative hu- midity. A liner with a higher grammage is less able to bend around the flutes than a lower grammage liner (Jansen and Breakspeare 2001). Washboarding can also be accompa- nied by optical effects (Netz 1996) due to the uneven reflection of the wash- boarding structure and this depends on the liner properties, particularly its gloss, and on the angle from which it is viewed. Another quality problem observed by corrugated board manu- facturers is that the surface on the tips of liners, particularly of coated liners, has a higher gloss than the surface in the valleys. A possible explanation of this may be that the surface on the tips is “polished” during production and during the printing process. Banding on Printed Corrugated One major print quality problem generated by the washboarding is banding. As much of the work reported in this article deals with this subject, the nature of print banding merits closer inspection. Banding on printed corrugated board is a systematic type of print mottle and is a well-known phenomenon in the corrugated board industry (Zang and Aspler 1995; Netz 1996; Neumann 1998; Fulton 2003). The print defect has been known to have many names: washboarding, striping and fluting are among the most common. In this article, it is referred to as banding. The term was chosen since it is used in the field of image analysis with regards to the texture of images. Print banding arises mainly from a non-uniform contact between the print- ing plate and the board, which leads to a non-uniform ink transfer and ink distribu- tion. Banding is most common on post- printed corrugated board and it shows up in both halftone and in the full-tone areas. In the halftone areas, the band- ing effect is due to a difference between the dot sizes on the fluting tips and in the fluting valleys. In the full-tone areas, the banding effect is due to insufficient ink coverage or to a lower print density in the fluting valleys than on the fluting tips. Banding is more easily detected on white than on brown liners (Wendler 2001), because the print density varia- tion is greater when a white liner is used as the substrate. The easiest way to reduce or eliminate the banding effect is to reduce the washboarding by in- creasing the liner grammage. However, this will rapidly raise the material cost. thin-Plate teChnology The development and increasing use of thin-plate technology has been im- portant within the field of post-printing (Arimond and Koss 1995; Jansen 1996; Harris 1999). The technology was first presented in the early 1980s (Fulton 2003) and these types of printing plates have been reported to give a better quality than the earlier plates, espe- cially in halftone work (Jansen 1996; Neumann 1998; Arimond 2002; Kilhenny 2002) by being “more forgiving for the press operator” (Bonawandt 2003) and thus preventing distortion when they are wrapped around the printing cylinder. Printing can therefore be performed with less impression and less build up of ink on the plates (Fulton 2003). The thin-plate technology often uses printing plates consisting of four compo- nents: a plate cushion, a mounting foil, a double-sided adhesive tape and the printing plate. Other ways of combining the com- ponents are also used (Jansen and Stebani 2002a) and the “cushion back plate” has been reported to improve the print quality (Fulton 2003). Among these four components, the plate cushion, the mounting foil and the printing plate each has a specific function in the combined printing plate, due to their different material properties. Figure 1. Schematic illustration of the manufacture of double-face corrugated board from single-face corrugated board, and its final construction. www.flexography.org november 2010 FLEXO 79 FLX_Nov10_mech.indd 79 11/1/10 2:26 PM
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