Surface Texture Modification
The flattening of microscopic surface irregularities occurs when paperboard is subjected to pressure and heat during calendering or printing. This micro-asperity smoothing improves the gloss and ink receptivity of the sheet by creating a highly uniform surface. It is a critical step in preparing high-end packaging substrates for demanding print applications.
Smoothing Mechanism Action
Calendering rolls apply heavy compression and elevated temperatures to the moving web, softening the surface fibers and coating pigments. An micro-asperity smoothing occurs as the peaks of the surface roughness are pushed down into the valleys, reducing the overall micro-roughness. This action increases the specular gloss of the board, as a flatter surface reflects more light in a single direction.
However, excessive compression can reduce the bulk and stiffness of the board, requiring a careful balance of roll pressure.
Print Quality Outcome
Printers rely on a smooth surface to ensure complete contact between the printing plate or blanket and the paperboard. When this smoothing is insufficient, the ink transfer is uneven, leading to microscopic voids and mottle in the printed solids. Modern mills utilize soft-nip calenders with elastic roll covers to achieve this smoothness without crushing the internal structure of the board.
Measuring the surface topography before and after the calender section verifies that the desired surface profile has been achieved.
