Physically-based dynamic model for the control of cavity pressure in thermoplastics injection molding.

摘要

The injection molding process, due to its versatility, cost effectiveness, and ability to produce precise complex articles is widely used in plastics processing. Mold cavity pressure is a good indicator of the processes taking place in the cavity and plays an important role in determining the quality of the molded articles. The dynamic modeling and control of cavity pressure, based on a physically-based approach, is studied in this research project. The work deals with the filling and packing phases.; A lumped physically-based model was developed in order to study the behavior of the system. The model is derived from conservation laws and incorporates a physical understanding of the process. The whole system was divided into subsystems including the hydraulic system, ram-screw, barrel, and polymer delivery system. It was found necessary to account for polymer melt elasticity as well as non-Newtonian behavior of the polymer melt flow. Consideration of the growing solid skin in the polymer delivery system was found to be necessary.; The dynamics of the cavity pressure during the filling phase were investigated and found to be non-linear and time-varying in relation to the hydraulic servo-valve opening which is the manipulated variable. The dynamic behavior of the cavity pressure is approximated by piece-wise linearization of the non-linear governing equations to derive a transfer function using the physically-based model which is of fifth order. Adaptive PI, PID, and IMC controllers were designed and tested for the control of the cavity pressure. Various tuning techniques, along with changes in set-point, were used to determine conservative settings for the PI and PID controllers.; A similar approach was used to study the dynamics of the cavity pressure during the packing phase. A sixth order transfer function, with piece-wise linearization, was derived to approximate the non-linear and time-varying behavior of the cavity pressure during packing. The adaptive PI, PID, and IMC controllers were successfully applied into the packing phase. The transition of the filling-to-packing was selected to be detected by the derivative of the cavity pressure and adaptive controllers were successfully used for this phase.; Two commonly used injection molding grade thermoplastics, polyethylene and polystyrene, were used in experimental part of this work for model validation and controller testing.