Flexographic Post-Printing of Corrugated Board : Contact Mechanics and Print Quality
Sammanfattning: As the corrugated board industry strives to produce products with a higher print quality in the converting stage, there is a need for a greater understanding of the factors influencing the final print quality and how we perceive the print. In this work, a method for quantifying the print quality of printed corrugated board is presented as well as methods that quantitatively characterize the mechanical interaction between the printing form and the corrugated board in the printing nip. The results of investigations addressing the impact of the mechanical properties of the printing form on ink transfer are also presented.A band-pass image analysis method adapted to the spatial wavelength of B-flute corrugated board was used to quantify the print quality on the board in terms of print banding. The findings were substantiated by a perceptual evaluation which showed a high correlation to the instrumental quantification.The main cause of print banding is a higher local contact pressure on the fluting tips than in the fluting valleys, which leads to a higher ink transfer to the fluting tips. A greater difference between the local contact pressure on the fluting tips than in the fluting valleys generated a higher degree of banding. The difference in local contact pressure as well as nip force variations could be measured using the proposed methods to characterize the interaction between the printing form and the corrugated board. The difference in local contact pressure was highly dependent on the mechanical properties of the components of the printing form. An increase in the stiffness in compression mode and in the bending stiffness of the printing form led to a smaller contact area in the printing nip and consequently a higher external pressure on the ink, resulting in an overall higher ink transfer but also a higher degree of banding.The influence of the mechanical properties of the printing form components was supported by the results of numerical simulations of the contact between the printing form and the corrugated board. Contact area, pressure level and texture of the local contact pressure were compared with measurements of local contact pressures for different printing forms and the results showed a high degree of agreement. Numerical simulations could thus predict the print quality achieved with different printing forms under different printing conditions.
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