Dynamics and Control of Surface and Mold Temperatures in Injection Molding

Abstract

Abstract Mold temperature control in injection molding is very important for control of production rate and product quality. However, mold temperature is a complex function of machine design, process variables, machine settings and ambient conditions. Data were obtained regarding the variability in both space and time (in-cycle and cycle-to-cycle) of mold metal and mold surface temperatures and heat flux. In view of this variability, it is necessary to define variables which may be useful for control purposes. The following variables were evaluated: cycle average temperature, peak temperature, and partial cooling time. Dynamic models 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 temperature as the possible manipulated variables. In view of the dynamic studies, the cycle average surface temperature was selected as the controlled variable for control system design and simulation. The regulating abilities of PI, PID, and Dahlin controllers were evaluated. The results indicate that Dahlin control caused the controlled variable to follow the set-point closely, even when the process parameters vary, thus showing good robustness. However, the Dahlin algorithm achieves this advantage via large variations in the manipulated variable. This may cause difficulty in some circumstances.