DYNAMICS AND CONTROL OF SURFACE AND MOLD TEMPERATURES IN INJECTION-MOLDING

Mold temperature control in injection molding is very important for co ntrol of production rate and product quality. However, mold temperatur e is a complex function of machine design, process variables, machine settings and ambient conditions. Data were obtained regarding the vari ability in both space and time (in-cycle and cycle-to-cycle) of mold m etal and mold surface temperatures and heat flux. In view of this vari ability, it is necessary to define variables which may be useful for c ontrol purposes. The following variables were evaluated. cycle average temperature, peak temperature, and partial cooling time. Dynamic mode ls were developed, on the basis of experimental data, to determine the process gains, time constants and time delays for the above variables with regard to coolant temperature, coolant flowrate, and melt temper ature as the possible manipulated variables. In view of the dynamic st udies, the cycle average surface temperature was selected as the contr olled variable for control system design and simulation. The regulatin g abilities of PI, PID, and Dahlin controllers were evaluated. The res ults indicate that Dahlin control caused the controlled variable to fo llow the set-point closely, even when the process parameters vary, thu s showing good robustness. However, the Dahlin algorithm achieves this advantage via large variations in the manipulated variable. This may cause difficulty in some circumstances.