To get a better understanding of the non-linear viscoelasticity, we return to the injection moulding example. Above, a simplification of the polymer flow in the mould is given.
If a material element is to flow over the line of symmetry, the material does not experience any deformation until it reaches the flow front. There it will experience elongation due to the fountain flow effect.
A material element that flows away from the symmetry line, experiences a shear flow up to reaching the flow front. In the front, the material experiences a combination of shear and elongation.
Both these types of deformation, shear and elongation, will stretch and orient the polymer chains. The amount of stretch and orientation depends on the deformation history that the material experiences and is greatly influenced by the flow path or streamline the material is following.
Given that the temperature difference between the polymer melt and the mould wall is high, the polymer molecules will be frozen in into the solidified layer at the mould wall in a stretched and oriented state.