| Summary: | 碩士 === 中原大學 === 機械工程學系 === 85 === Gas injection molding process, being an innovative technology, has advantages of saving material, reducing warpage, shrinkage and cycling time, etc. To achieve these advantages, gas injection control system play an important key.
In this paper, experiments have been conducted to characterize the existing gas injection system. Then a PC based computer control system was then developed with man／machine control interface panel to conduct the closed-loop pressure control.
Currently, there are two kinds of pressure regulation. One is gas volume control system and the other is gas pressure regulating control system. Gas volume control system is simply a boost pressure control. The pressure during packing stage is not easy to hold and multiple stages of pressure settings are not easy to achieve either. Compared to the gas volume control system, pressure regulating control system could more precisely control the pressure even for multiple gas injection points. As present implementation, these two control systems are all integrated with programmable controller. The systems are lack of flexibility for multiple control purpose.
To improve the control capabilities of programmable controller based, a PC based control system is designed and characterized. To facilitate operating the machine, we designed multiple editing screens for the man／machine interface control. Except the main menu, sub-menus are modularized to simplify the operation for users. It is not only easy to operate and easy to learn for users, but also easy to maintain and revise.
From the experiment results, system show 0∼5 bars error to the set values (in the range of 150 bar) under the closed-loop pressure control. Consider the deviations base on the error of the pilot-pressure, the error will be 0.725％. This is comparative small. If the error for the gas penetration length is limited to 6％ all parts full fill the error criteria. This is another performance index which show the repeatability of the process.
Compared the pressure rise time with the Airmold system, pressure will rise to its set-value within 0.2 to 0.4 seconds. In addition to the pressure control system test, gas volume control system was also tested. Pressure rise time to the set-value at first stage is approximately 0.3 to 0.5 seconds, the rise time for the second stage depends on the flow rate of the booster.
The developed system and the Airmold system are all available to control the pressure under different set-values. Base on the experiments, it is found that the developed system has better performance than the Airmold system. As we mention previously, gas volume control system has poor performance when controlling pressure at different set-values.
Controller for the presently develop system and Airmold system are all PID structures. Cinpres system is purely a logic process controller controlled system which has limited applications. The developed system has better performance and flexible structure for the future applications.
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