Urethane drawstrips: A new method to control sheet metal flow during forming

Abstract

Urethane drawstrips offer a new method for controlling sheet metal flow into dies. Since urethane drawstrips do not deform sheet metal, they can be placed on the die binder inside the trim line of a sheet metal blank, allowing a smaller blank size with commensurate cost savings. Experiments showed that urethane drawstrips, which operate entirely by friction, generate less restraining force than conventional steel drawbeads which combine friction and deformation to generate restraining force. The effects of potential variables on the restraining force of urethane drawstrips were investigated and analyzed in terms of Coulomb’s Friction Law. The restraining force varied directly with parameters which affect the coefficient of friction, μ, such as surface roughness, surface contamination, zinc-base coating and lubrication. For conditions of constant μ, the restraining force also varied directly with changes that affected the urethane-sheet metal pressure, such as contact area, urethane hardness and compression. The results are discussed in relation to possible applications of urethane drawstrips to replace or augment conventional drawbeads.