Print quality of flexographic printed paperboard related to coating composition and structure, TAPPI Journal January 2018
ABSTRACT: Ink transfer and setting influence the achievable print quality and visual appearance. The pressure in the printing nip and the porosity of the substrate regulate the amount of ink that penetrates into a porous coating structure. The purpose of this study was to understand how print quality aspects could be related to ink penetration of water-based flexographic ink into coatings of differently engineered structures: calcium carbonate (GCC) of various particle size distribution (PSD), coatings with different amounts of latex binder, and coatings with various blends of GCC and kaolin.
Calcium carbonate with broad pigment PSD resulted in a lower print density compared to coatings of narrowly distributed particle sizes. Coatings of larger pore volume and greater dominating pore radius showed a higher amount of z-directional ink penetration. A high ratio of uncovered areas (UCA) could be detected for samples with high amount of latex. However, increased printing force eliminated these artefacts.
Increased printing force increased the print density to a higher degree than did a reduced ink viscosity for coatings with pure GCC. For coating layers containing both GCC and kaolin clay, decreased ink viscosity had a stronger impact on the print density than increased printing force. Print density was also affected by ink penetration, suggesting that the optical response is sensitive to the ink-substrate interaction layer. The result presented in this work also suggests that the print gloss decreases with increased amount of penetrated ink due to a higher rate of ink vehicle removal.
Application: By fine-tuning the coating composition and structure and by a proper selection of printing parameters, it was demonstrated that print quality defects can be addressed by fundamental studies of ink transfer and subsequent ink setting. The detailed understanding of mechanisms of ink-substrate interactions is a requirement for maintaining competitiveness in the packaging industry based on renewable materials.
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