This paper presents an overview of the measurement of the flow birefringence of molten polystyrene under shear stress in a thin slit die in an injection molding machine. The flow birefringence was determined in the direction normal to the shear plane (‹n11-n33›) by measuring the intensity of linearly polarized light using a crossed polarizer and analyzer. The temperature in the barrel of the injection machine and in the mold was adjusted to be equal, so that the molten polystyrene temperature was kept constant during the experiments.
The flow birefringence (n11-n33)w in the higher stress region of the slit wall was found to be nearly proportional to the shear-stress at the wall (σw).
In the low stress regions at low temperatures, the flow birefringence was found to be proportional to the square of the shear stress (σw)2 which was applicable to the stress optical law.
