| Because of high efficiency, low cost, high volume automated production of micro parts, micro-injection molding is widely used in field of micro-electromechanical systems. But due to the tatal or partial tiny size of the micro-cavity, the filling flow of the polymer melt is very difficult. Especially when the structure of the parts is complicated, the flow resistance in the micro-cavity will grow exponentially, which can lead to incomplete filling of the micro-parts and seriously affect the molding quality of the micro-parts. In this paper, based on deeply understanding of the micro-injection molding technology and the applications of ultrasonic vibration in the polymer processing, the application of ultrasonic vibration to the micro-cavity molding is proposed. By the means of thermal and mechanical effects generated by the high frequency vibration of ultrasound field to reduce the viscosity of the polymer melt, which can improve the filling performance of the micro-cavity, and thus improve molding quality the micro-parts.Firstly, cell container with complex structure is choosen as the object of this paper. Micro-injection mold with ultrasonic vibration function was designed and manufactured. And the integration of micro-injection molding system and the ultrasonic vibration system was realized.Secondly, the orthogonal experiments and single factor short-shot molding experimental method were used to study the effects of different process parameters and ultrasound parameters on the filling performance of the micro-cavity. The results show that when the ultrasonic vibration is not applied, in addition to increasing the injection rate makes the filling rate of PP decrease, the increasing of the other process parameters can make the micro-cavity filling rate of HDPE and PP increase significantly. Because of the difference in material properties of HDPE and PP, the rate of increase the two materials are different; comparing with not applying of ultrasonic vibration, the applying of ultrasonic vibration makes the micro-cavity filling rate of the two materials increase significantly. The short shot Experimental results show that when vibration frequency is20KHz、25KHz、30KHz, the the filling rate of HDPE is increased as the increase of the ultrasonic power. But for PP, when the ultrasonic frequency is20KHz and30KHz, the filling rate of the micro-cavity is increased firstly and then decrease as the increase of the ultrasonic power. Due to the increase of ultrasonic power, the d crystallinity and reconciliation orientation degree of PP increase. This makes the contraction degree of the micro-parts increase and the filling rate of the micro-cavity decrease.Finally, the mechanism of the ultrasonie field is analysised. The impacting relationship of ultrasonic field on the cavity temperature and pressure is studied. By the test of X-ray diffraction, infrared spectroscopy scanning, polarizing microscope, the impacting relationship of ultrasonic field on the crystallization,orientation and degradation of the micro-parts is studied. The heat effect generated by the ultrasonic vibration enhance the activity and the thermal motion of the molecules, which makes the temperature of polymer melt increase. The force field of ultrasonic vibration is superimposed on the shear force field, which is equivalent to applying a pulsating impactforce on the melt, and accelerate the filling and flow of macromolecules, reduce the pressure loss of the melt in the flow process. The high-frequency shear vibration generated by the ultrasonic makes the macromolecules strand break, and the polymer produced irreversible chemical degradation. All of these will cause the decrease of viscosity, thereby effectively promote the filling rate of the micro-cavity.At last, the process parameters and ultrasonic power of the full filled micro-cavity are gained, as well as the complete cell container. |
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