| Parison length control in extrusion blow molding of plastic parts has been studied and attempted for the past quarter-century. Most methods used involve cycle-to-cycle control where the next part shows an improvement over the previous one using either weight measurement of the part or a simple photocell initiating the cutting of the parison. In the present thesis, plant models are developed using a control engineering technique called system identification. Since the process model thus determined is based on experimental data, sag and swell of the parison are implicitly taken into account. Furthermore, an in-cycle robust controller is designed and simulated where the die gap opening and the extrusion speed are adjusted as the parison is being extruded. The outlined control strategy focuses on controlling the parison length against process disturbances and machine drifts. The control system thus obtained is then reduced to two single-input single-output controllers for potential implementation on the extruder machine employed throughout the research. |
