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FLEXO Magazine : November 2010
Technologies & Techniques Banding/Washboarding Analysis A study of Contact Mechanics on Flexo Post-Print Board By erik Hallberg In post-printed corrugated board, print banding—a striped pattern that occurs within the printed areas—is a common problem and one of the most detrimental print defects. This defect is often a result of the poor printability associated with corrugated board, due to its wavy surface topography. As the corrugated industry strives to achieve higher print quality, there is a great need for a better understanding of the factors influ- encing the final print quality and of how we perceive the print. The main goal throughout the work has been to elucidate, and thereby gain a better knowledge of, the factors that influence the interaction between the printing plate, the cor- rugated board and the printing press, as well as their impact on the print quality. Since print banding is one of the most frequent print defects on post-printed corrugated board, ef- forts have also been made to explain the cause and quantify it from a visual and an instrumental evaluation point of view. Influence of the mechanical properties of the components in the printing plate on the contact mechanics has been studied. These studies have been complemented by experi- mental work designed to clarify the role of contact mechanics on print quality. ConstruCtion And WAshBoArding Corrugated board is cost effective since it is strong, light and recyclable (Jönsson 1993; Nordstrand 2003; Berglund 2004). All these properties are achieved by the elegant construction of fiber-based material. Combining the plain liner layer and wavy fluting into corrugated board creates a structure which gives considerable rigidity combined with low weight. The air in the flutes also serves as an insulator, which provides protection for the packaged goods against tempera- ture variations. Corrugated board is manufactured in several grades, which differ in the types of liner and fluting, in the number of plane layers and fluting layers, and in the flute height and flute wavelength. Although corrugated board has a high potential on the packaging market, it suffers from a drawback compared to other packaging materials with regards to its often poor printability (Netz and Wennerblom 1993). The converting of the liner and the fluting into corrugated board often gener- ates a wavy topography associated with the fluting tips and valleys. This is known as washboarding and it can be felt by the fingers. Washboarding arises because the adhesive used to com- bine the liner and fluting introduces a contracting force be- tween the liner and fluting making the liner bend around the 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 corrugaTed • Banding on printed corrugated board is a systematic type of print mottle and is a well-known phenomenon in the corrugated board industry. • In terms of contact mechanics in the post-printing process, the surface irregularities, the washboarding profile of the corrugated board can be described on both a macro- and a micro-scale. • The load on a corrugated board in the thickness direction will be taken up by the fluting tips, so that the contact pressure will be higher on the fluting tips than on the fluting valleys. • The reverse mount was the most compliant printing plate. • The most significant factor affecting the banding was the impression level, where a higher impression led to greater banding. Figure 1. schematic illustration of the manufacture of double-face corrugated board from single-face corrugated board, and its final construction. 78 FLeXo noveMBer 2010 www.flexography.org FLX_Nov10_mech.indd 78 11/1/10 2:26 PM
Sustainable Fall 2010