Optimization of the control of polymer extrusion processes
Polimery 1997, No 6, 386
For a given type of barrel, screw and die, optimization is to select extruder heating zones, to distribute temperature-taking points, to establish set-pointvalues, 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 (DDk) and final homogenization of the polymer (DDk) with its outlet temperature (vk).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 vp to the total plastification temperature vPL; (ii) maintaining the polymer at vPL until final heating starts up; (iii) heating the polymer from vPL to the final temperature vk; and (iv) keeping the polymer at vk for a preassigned period of time tk. In the two-level temperature control system developed to optimize extruder'sheating zones , e.y., 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 in ferior level in terms of the time-averaged temperature distribution representing 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.