Flexo Printing on Paper: Achieving High-Resolution Graphics with Coated and Uncoated Papers
The surface characteristics of paper – smoothness, porosity, and coating – profoundly affect the print resolution, dot gain, and color gamut. Coated papers offer superior smoothness and can reproduce fine screens (150 lpi or higher), while uncoated papers are more challenging due to their rough and absorbent surface. This article provides guidance for optimizing flexo print quality on both types.
Coated papers (e.g., C1S, C2S) have a clay or pigment coating that fills the pores and provides a smooth surface (Bekk smoothness >300 s). The ink stays on the surface, reducing dot gain and allowing sharp edges. The resolution can reach 175 lpi with appropriate anilox (800-1000 lpi). However, the coating reduces absorbency, so the ink must dry by evaporation; thus, the drying system must be efficient. Adhesion is good, but the surface can be sensitive to scratches. UV inks are often used on coated papers for high-gloss applications.
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Uncoated papers (e.g., kraft, offset, bond) have a rough, porous surface. The ink penetrates the fibers, causing dot spread (higher dot gain). The resolution is limited to 80-120 lpi. To compensate, the plate's dot compensation is increased (smaller dots on plate). The anilox should have lower line screen (400-700 lpi) with higher volume to supply enough ink for density, but this increases gain. A balance is needed: use an anilox that gives sufficient density without excessive gain. The impression pressure must be higher to conform to the surface, but this also increases gain. Water-based inks with higher viscosity reduce penetration, improving sharpness.
Strategies for uncoated papers: Pre-calendering or sizing the paper improves smoothness. Using a primer (starch or clay) can fill the pores, reducing penetration. The plate should be made with flat-top dots to reduce gain. The press speed may need to be lower to allow ink to set. After printing, a varnish can be applied to enhance gloss and rub resistance.
Color reproduction: On uncoated papers, the color appears duller because the ink penetrates and the paper's roughness scatters light. The color gamut is smaller. A higher ink deposit (higher anilox volume) can increase density but at the cost of more gain. The printer may need to adjust the color targets to match the lower gamut. Coated papers give more vibrant colors and higher contrast.
Quality control: For both types, use a control strip with solids, tints, and gray balance. Measure density and dot gain; adjust the anilox and impression accordingly. For critical jobs, a test run with multiple anilox volumes is used to select the optimal combination. By understanding the differences, flexo printers can tailor their process to achieve the best possible quality on each paper grade, satisfying customer requirements for both economy and aesthetics.
