Striping on flexo post-printed corrugated board

Abstract: Striping is the most common print non-uniformity on corrugated board. It is defined as periodic print density and/or print gloss variations parallel to the flutes. Corrugated boards are mainly printed with flexography, making striping a major concern for the flexographic post-printing industry. In spite of its long history, the basic mechanisms of striping have not been fully understood, and no concrete solution has been provided. The objective of this thesis is to obtain an understanding of the fundamental mechanisms behind and a solution to striping. Both experimental and numerical approaches have been taken in thiswork. Nonlinear finite element models have been constructed in both corrugated board and halftone dot scales to determine the pressure distributions in the printing nip. Ink transfer experiments have been performed to determine the print density vs. pressure relations. Parametric studies have been done for the effects of printing system variables and deformations. The results showed that striping is predominantly print density variations caused by pressure variations in the printing nip. The pressure variations are inherent to the corrugated board structure. Washboarding was shown to play a minor part in causing print density variations, but might contribute to gloss striping. A new printing plate design has been proposed to eliminate the pressure variations and hence the print density striping.