Quantifying Micro Surface Burnishing in Overprint Varnishes under Sled Shear
Surface burnishing under sled shear converts diffuse reflectance to specular gloss through silica displacement, controlled by matrix crosslinking and particle size.
Surface Asperities
Microscopic Roughness
Microscopic irregularities and peaks on the surface of paper and board affect their printability and coating holdout. The concentration and height of surface asperities determine the overall roughness of the substrate, influencing how inks and coatings are absorbed. High-quality graphic boards are calendered to compress these peaks and create a smoother, more uniform surface.
Minimizing these irregularities is necessary for achieving high-resolution print reproduction. Papermakers monitor these microscopic features to ensure that downstream processing runs smoothly without friction defects on the production line.
Ink Coverage
During the printing process, prominent asperities can prevent complete contact between the printing plate and the paperboard surface. This poor contact results in micro-voids, speckling, and uneven ink transfer, especially in gravure and flexographic printing. Heavy coating applications are often used to fill these microscopic valleys and provide a level surface.
Maintaining uniform surface smoothness across the roll ensures consistent print quality.
Barrier Continuity
Extrusion coatings and liquid-applied barriers require a smooth base sheet to form a continuous, pinhole-free layer. If surface asperities are too high, they can puncture thin polymer coatings, creating pathways for moisture to penetrate. Pre-coating or intensive calendering reduces the height of these peaks before barrier application.
This preparation optimizes performance.