Mitigating visual highlighted bands in automotive black metallic coatings: Bernoulli effect and hemming gap optimization

This study aims to address the problem of visually highlighted bands at the end of the hemmed edge of the automotive black metal top-coat of door-covers, and proposes a solution by combining the Bernoulli effect and the optimization of the hemmed edge gap. By establishing a physical model, the correlation between the formation of the highlighted band and parameters of the hemmed edge gap (gap size, edge angle, and length) was revealed. Experiments showed that adjusting the gap size of the hemmed edge can prevent the formation of a concave surface with an upper right curvature radius, thereby eliminating the highlighted band. Meanwhile, controlling the optical reflection focusing area to be within twice the size of the gap opening can further suppress this phenomenon. Microscopic analysis (SEM) revealed that Ti enrichment in the coating enhanced the highlighted band effect. This study innovatively integrates fluid mechanics, optical reflection, and surface topography analysis, providing theoretical guidance for the design and process optimization of black metal top-coats on automotive door-covers and pointing out the direction for the industry to systematically solve such problems.