The speed and pressure of the injection ram have a great influence on the quality of injection molded products. Many parameters of the control system composed of a microcomputer, a servo valve and servo controller, and various interfacing elements should be adjusted to produce good quality of molded parts. To study the effects of discretization and limitation of the microcomputer based process control, a closed loop ram speed control system is analyzed with linearly approximated governing equations of an injection molding system.
A machine language level program is developed to control the ram speed and pressure. This program is applied to an injection molding machine to verify effects of controller gain, feedback gain and input signal magnitude. The results of the experiment are compared with the analysis to yield the characteristics of the microcomputer application to an injection molding machine.
It is found that the feedforward gain of the servo controller is the most important factor of a closed loop ram speed and pressure control, and that the feedback gain of A/D converter should be determined with the relation of maximum controlled variable to the number of bits per a word of the microcomputer. Mold filling pattern and the relationships between the ram position, speed and the melt flow front velocity are also studied experimentally. With these results, the effect of flow front velocity on the part quality is studied quantitatively and principle of ram speed control setting is suggested.
