Meaning
The process by which a wet coating flows under gravity or surface tension to eliminate application marks defines the final aesthetic and functional smoothness of a film. Proper low-shear leveling occurs after the material is applied but before it dries or cures. It is the phase where the fluid must have enough mobility to heal ridges left by the die or the roller.
This behavior is governed by the viscosity of the fluid at very low deformation rates.
Surface Uniformity
Achieving a mirror-like finish requires the fluid to stay wet long enough for the peaks of the coating to flow into the valleys. If the solvent evaporates too quickly, the application marks will be frozen into the dried layer. This defect is particularly visible in high-gloss automotive coatings or optical films.
Gravity Flow
Thick coatings are more prone to sagging if the low-shear leveling is too aggressive. The fluid must be engineered to have a yield stress that stops flow once the surface is smooth. This prevents the material from running down vertical surfaces or pooling at the edges of the substrate.
Coating Finish
Measuring the surface roughness of the cured film provides a quantitative assessment of the leveling quality. If the low-shear leveling was insufficient, the film will show a texture known as orange peel. Industrial processes often use leveling agents to reduce surface tension and promote a flat profile.
The low-shear leveling rate must be balanced against the drying speed of the line.
