Optimization of the control of polymer extrusion processes

For a given type of barrel, screw and die, optimization is to select extruder heating zones, to distribute temperature-taking points, to establish set-point values, and to provide control algorithms and settings for zone temperature controllers. In the present new extrusion quality criterion (eqn 1) partial dimensionless criteria serve to couple the degrees of final thermal degradation (D-Dk) and final homogenization of the polymer (D-Hk) with its outlet temperature (V-k) These partial criteria allow to establish optimum thermal conditions for (a) the maximum extrusion output and (b) for a selected production range. Digitalized description of polymer property data for the polymer leaving the extruder (eqn 6) is used in the mathematical model of polymer homogenization (eqns 7, 8) and thermal degradation (eqns 9, 10). Conditions are sought to procure optimum temperature distribution in the polymer (eqns 11-26). The hierarchical zone temperature control system provides for (i) preheating the polymer from the starting (stocking) temperature v(p) to the total plastification temperature V-PL; (ii) maintaining the polymer at v(PL) until final heating starts up; (iii) heating the polymer from v(PL) to the final temperature v(k); and (iv) keeping the polymer at v(k) for a preassigned period of time tau(k) In the two-level temperature control system developed to optimize extruder's heating zones, e.g. to stabilize the quality of insulation coatings for wires (Fig. 12), the superior level determines the set-points for the temperature controllers that are acting at the inferior level in terms of the time-averaged temperature distribution rt presenting an optimum distribution for a specific extruder product (eqn 27). Computer simulations were used to verify the new technique by modeling the thermal field of a single-screw extruder and of the plastic being extruded. The present optimization results obtained at various thermocouple immersions are contrasted (Fig. 13) with conventional results.