Process variations and the transient behavior of extruders

To explain output fluctuation problems in extrusion, a transient model of material flow through an extruder is developed and used to study the transmission of external disturbances through the extruder. This model can be implemented for any extrusion geometry for which a pressure-throughput model exists. Simulation results show that every extrusion process has a characteristic critical frequency. All disturbance frequencies above this critical value are damped out and the lower frequency components are transmitted through the process with little amplitude attenuation. These low frequency disturbances are of most concern since they are not damped by the extrusion process. Extrusion screw configurations are designed to satisfy steady-state performance requirements, and nonsteady-state performance is rarely considered. The findings indicate that a two pronged approach would be required combining screw design (passive control) to handle higher frequencies and closed-loop control (active control) to detect and remove lower frequency disturbances.