In-line monitoring flow in an extruder die by rheo-optics

A new modular slit die with optical windows in two different positions and three pressure transducers flush-mounted along the wall was built and coupled to the exit of a twin-screw extruder. Thus, the birefringence and the pressure drop of polystyrene were monitored inline during extrusion. Two experimental procedures were tested: steady-state and cessation of extruder feeding. The latter proved to be very useful in the case of polystyrene since the ratio between the birefringence and the pressure drop can be quantified for a wide range of steady-state conditions with a single experiment. In fact, down to relatively lower values of pressure drop, the birefringence proved to be a function of shear stress at the wall only, depending neither on the initial feeding rate nor on the aspect ratio of the slit die, for W/h down to 5, at least.