Coating Defect
Small bubbles or craters that form in a cured ultraviolet light-sensitive topcoat indicate a failure of the film to stay flat and continuous. This uv varnish blistering often occurs when trapped gases or moisture are released from the board surface during the high-energy curing process. It ruins the aesthetic appeal of high-gloss packaging and can create weak spots where the coating might flake off.
Underlying Cause
Residual moisture in the board or the presence of unsealed ink can vaporize when exposed to the intense heat of the UV lamps. If the varnish cures too quickly, these gas bubbles are trapped before they can level out. In some cases, the problem is caused by a chemical incompatibility between the varnish and the underlying ink or coating.
This interaction creates high surface tension that prevents the liquid from wetting the board properly.
Remedial Action
Reducing the lamp power or increasing the press speed can sometimes lower the heat input enough to prevent gas release. Ensuring the board is properly conditioned and that the inks are fully set before varnishing provides a more stable foundation. Using a primer can also help seal the surface and provide a consistent interface for the varnish.
