The ''sharkskin - effect'' in polymer extrusion.

A specific form of melt flow instabilities associated with surface defects for polymer extrudates and commonly referred to as the ''Sharkskin-effect'', is modelled. It is shown that the relaxation oscillation model of Molenaar and Koopmans developed to describe ''spurt'' defects can be expanded to include a description for the dynamics of surface defect appearance. By introducing a non-linear viscoelastic constitutive equation (Johnson-Segelman-Oldroyd or Kaye-Berstein-Kearsley-Zapas) into the relaxation oscillation model a boundary layer can develop which is unstable. The energy balance equation defines which solution of the many possible ones from the momentum equation will be selected by the polymer system.