Improving the punch and die wear behavior in tin coated steel stamping process

Abstract

Prestigious brands of cookies usually use metallic tins as packaging to distribute and sell their products, trying to impress the customer through the appearance of the packaging and simultaneously avoiding smashing or damaging of the cookies during logistics operations. These packages are commonly made in a tin coated (2.8g/m2) thin steel sheet (ARCELOR electrolytic Tin plate), which causes severe wear problems on both die and punch tool components during the stamping process at room temperature. The border of the package presents an inconsiderable deformation, despite its almost perpendicular orientation to the top surface, but this top is usually patterned, which also implies the flow of the sheet between the top and bottom die surfaces. Due to the softness of the Tin coating, it easily adheres to the die, generating premature wear and several other concerns in maintaining the required final shape of the tin lid. Lubrication would be an easy way to solve the problems referred above however, lubricated operations should be avoided as these kinds of packages are for food purposes.This study started by identifying the main wear mechanism developed in the main surfaces of the stamping tool, promoted by the Tin coated steel sheet used in the packages. Two advanced PVD coatings (B4C and Mo) were tested, leading to the improvement of the punch and die wear behavior under these working conditions. The transfer of Tin material from the metallic sheet to the punch and die was studied, as well as the friction coefficient of this sheet against some selected coatings, while also trying to minimize the Tin adherence to the surface of the tool. Tribological tests under medium loads were carried out in order to ascertain what kind of coating presents better wear behavior in the referred work conditions. Regarding the obtained results, some improvements will be applied to the coating structure to adjust the deposition parameters in order to go forward to industrial tests. Worn surfaces were studied by Scanning Electron Microscopy (SEM) and material transfer was analyzed by Energy Dispersive Spectroscopy (EDS).Results obtained with some of the tested coatings confirm that it is possible to minimize the Tin transfer from the covered steel sheet to the die and punch, ensuring a longer life of these parts, decreasing the tool maintenance operations and improving the Overall Equipment Efficiency (OEE) of that stamping process.