Performance of Polymer Coatings Under Forming Conditions

Prepainted metal sheets being environment friendly and cost effective as compared to postpainted metal sheets, are widely used in construction, packaging, transportation and automotive industries. One of the key requirements for prepainted coatings is to retain its surface quality and properties during forming process. During forming process, major surface damage occurs when the coated sheet is bent and un-bent around the die corner. To reduce surface damage of coatings, proper control of the parameters during forming and detail study of the surface conditions is required. In the present study, influence of forming parameters such as die radius, lubrication and specimen material are investigated. The influence of these parameters on friction, surface damage and properties of polymer coatings are evaluated. Experiment set-up is built to conduct bending under tension test. This test gives a better way to evaluate coating performance, as it closely simulates the die region of real forming process and considers bending effects. Experimental results show increase in friction and surface damage with decrease in die radius. Moreover, with decrease in die radius hardness of the coating decreases and strain in the specimen increases. Lubrication has some effect on coefficient of friction, but the influence is not as significant as that of die radius. This is attributed to the fact that, the polymer coating itself acts as a solid lubricant in the test. Material effect was studied, polypropylene coating being the softer material compared to PVDF coating shows more surface damage in the form of scratches. Numerical simulations were performed using Finite Element Analysis package (FEA) Abaqus. A 2D model was built, exploiting the plane strain condition for bending under tension test. Numerical simulations indicate that maximum contact pressure and von Mises stress are concentrated at the beginning of the drawing edge. Apart from the location, the value of contact pressure was higher for smaller die radius. Thus, experiments help in studying the effect of forming parameters on coating performance and numerical simulations provide more insight into the critical areas where stresses are high. Numerical simulations also provide a scope to study the effect of material and geometric parameters on performance of coatings without running experiments