Viscoelastic properties of paint films and formability in deep drawing of pre-painted steel sheets

Damage of paint films on pre-painted steel sheets (PCMs) in a deep drawing process is influenced by the mechanical properties of the paint films. We reported that paint films, which exhibit high elongation and low elastic strain energy (ESE) have good formability in a deep drawing process. However, examination of the paint films from a rheological viewpoint is needed for further clarification of the formability of PCMs. In this study, the viscoelastic properties of polyester/melamine paint films widely used for PCMs were studied concerning dynamic viscoelasticity, stress relaxation and creep. The effects of the viscoelastic properties of the films on the formability of PCMs were examined. It is confirmed that paint films, which exhibit good formability in deep drawing, have higher glass transition temperature ( T g) than the temperature at which drawing was performed, also they have both low rubbery modulus and low creep viscosity. Furthermore, the rubbery modulus and the creep viscosity of such paint film decreases with the increase of strain. These results show that such paint films have low cross-linking density and that the cross-linking networks are easily destroyed by large deformation. We conclude that the paint films, which exhibit high plasticity in a large deformation process, have good formability in deep drawing.