Visual Disturbance
Visual irregularities in multi-color offset printing often stem from how ink is transferred onto an already wet substrate. The term wet mottle describes the uneven density and blotchy appearance that occurs when subsequent ink layers are applied over a wet ink film. This print defect occurs in the printed areas of paper and paperboard.
Fluid Interaction
Differences in ink receptivity and absorption across the coating layer of the board are the main causes of this problem. In multi-station printing, the first ink layer must set at a rate that allows the next ink to transfer onto it evenly. If the paperboard has uneven surface porosity, the first ink layer will dry inconsistently, causing the next ink to transfer poorly and unevenly.
This poor transfer leads to the characteristic mottled or blotchy appearance.
Surface Modification
Prevention of this issue is achieved by optimizing the coating formulation at the paper mill to ensure highly uniform porosity and absorption rates across the board surface. Papermakers adjust the ratio of binders and pigments, such as fine clay and calcium carbonate, to create a uniform surface. This modification helps the board absorb the ink solvents at a consistent rate, ensuring that each ink layer behaves predictably during printing.
Controlling these surface properties ensures that high-speed, multi-color packaging presses can run smoothly, producing crisp, high-quality graphics with solid color areas that are free of any blotchiness or mottle.
