Much effort has been spent optimizing sheet surface roughness for improved pressworking performance. However, comparatively little attention has been given to the problems associated with stamping coated steel sheets. It has been assumed that the surface roughness of the substrate dominates. Consequently, problems with sheet coatings have been mainly addressed on the press shop floor with lubricants and die surface treatments.
An extensive study on the effects of lubricants and lubricant additives on zinc-coated sheet revealed anomalous behavior particularly with the soft zinc coatings that could not be readily explained with tribological theory, namely Coulomb friction and the Stribeck curve. A later study examined the effects of lubricant weight on press performance with various zinc-coated steels. Once again anomalies appeared that were repeatable yet defied explanation.
This paper examines these anomalies and proposes an explanation for differences in stamping performance of zinc-coated sheet. A model of the sheet surface is presented to support this explanation. This model combines sheet roughness, sheet hardness, and dynamic fill volume as factors to consider. Conclusions are made with respect to the application of this model, and future work is outlined.