AN IN-LINE RHEOMETER FOR MOLTEN PLASTICS - DESIGN AND STEADY-STATE PERFORMANCE-CHARACTERISTICS

A novel in-line rheometer for use with molten plastics has been developed and evaluated. It is a process sensor designed to be installed in a melt flow stream, for example at the exit of an extruder. The entire process stream flows through the rheometer, so there is no output signal delay due to the flow of melt in a sampling line and gear pump. A small fraction of the melt entering the rheometer is drawn into a small gap between a rotating drum and an outer cylindrical wall. The shear stress acting on a small area of the outer wall is monitored by a shear stress transducer. A technique for taking into account the small pressure flow that is superposed on the drag flow in the shearing gap is described. The major problems that must be addressed in designing an in-line sensor arise from the fact that the temperature, pressure and flow rate of melt entering the rheometer are subject to the fluctuations and upsets that are ubiquitous in industrial processes. The new rheometer is able to operate in this environment and yield reliable viscosity values. However, the output signal of the shear stress transducer was found to be affected by the pressure and contained a low frequency noise component that limited its accuracy. An improved version of this transducer is currently under development.