An intrinsic route to melt fracture in polymer extrusion: a weakly   nonlinear subcritical instability of viscoelastic Poiseuille flow
  As is well known, the extrusion rate of polymers from a cylindrical tube or slit (a ``die'') is in practice limited by the appearance of ``melt fracture'' instabilities which give rise to unwanted distortions or even fracture of the extrudate. We present the results of a weakly nonlinear analysis which gives evidence for an intrinsic generic route to melt fracture via a weakly nonlinear subcritical instability of viscoelastic Poiseuille flow. This instability and the onset of associated melt fracture phenomena appear at a fixed ratio of the elastic stresses to viscous stresses of the polymer solutionte 
M. Meulenbroek, C. Storm, V. Bertola, C. Wagner, D. Bonn and W. van   Saarloos