Substrate Holdout and Pinhole Formation in Inline Flexographic Coating
Substrate holdout and pinhole control require matching anilox coat weight delivery to sheet absorbency while regulating dynamic wetting and surface skinning.
Dewetting Fisheye
Defect Morphology
Formation of a small, circular void or crater with a raised droplet or thick rim at its center during the drying of a liquid film constitutes a common surface defect on coated substrates. This coating issue, referred to as a dewetting fisheye, occurs when a liquid coating pulls away from a point of low surface energy. The resulting spot remains unshielded and susceptible to moisture or physical damage.
Surface Tension
Contamination on the paperboard or film surface is the primary cause of this localized failure of the liquid to wet the substrate. Silicone droplets, grease, machine oils or dust particles create localized zones of extremely low surface tension. When the wet coating flows over these points, the liquid recedes to minimize its surface energy, forming a circular margin around the contaminant.
This phenomenon leaves the underlying substrate exposed, which can lead to printing gaps or failure of protective barriers in flexible packaging. Preventive measures include thorough web cleaning before the coating station or the addition of surface-active agents to the coating mixture to lower its dynamic surface tension.
Production Remedy
Liquid filtration and inline corona treatment are used to maintain high surface energy on the receiving web. These steps prevent the formation of localized non-wetting zones during continuous high-speed runs.