Applying the constrained minimum variance control theory on in-line viscosity control in the extrusion molding process

In this study, the constrained minimum variance (CMV) control strategy was applied on the extrusion process. An in-line viscometer was designed and used to measure the viscosity of the polymer melt between the screw end and the die. Under constant temperature, the screw speed was treated as the plant control input, and the viscosity of the polymer melt was treated as the output. The system transfer function and the disturbance dynamic model of the plant were obtained experimentally. Comparing the CMV control strategy with the open loop control method, the variance decreased 39.1% in the simulation, and it also decreased 24.3% in experimental results. This shows that the CMV control method is effective on reducing the fluctuation of viscosity in an extruder. Improved stabilization of the viscosity of the polymer melt in the barrel was thus achieved.