Study On Mechanism Of Ultrasonic-assisted Micro-injection Molding

Ultrasonic-assisted micro-injection molding is a novel method for fabricating micro plastic parts.In this method,the ultrasonic vibration energy directly enhances the fluidity and filling performance of the polymer melt,and improves molding quality of the micro part.This enhancement effect of ultrasonic vibration in this novel method has been confirmed by experimental research.However,the mechanism of the filling behavior and the aggregation behavior in molding stage is still not clear.In view of the unknown mechanism in the process of ultrasonic-assisted micro-injection molding,the rheological properties,wall slip behavior and aggregation behavior of the plolymer melt during the molding process are investigated.The theoretical models of rheological properties,wall slip behavior and non-isothermal crystallization behavior under the effect of ultrasonic vibration are established and analyzed.The experimental systems are built and experiments are carried out to study the molding mechanism.The mechanism of the filling and aggregation process in the novel method is discussed within the results of numerical simulation and experiments.The results show that:(1)In the filling process of ultrasonic-assisted micro-injection molding,ultrasonic vibration changes the rheological properties of the polymer melt through the aspects of energy storage and energy loss function.This effect reduces the melt viscosity,improves the uniformity of melt viscosity,increases the melt flow rate,and then achieves a better molding quality of the micro plastic part.(2)In the filling process of this novel method,ultrasonic vibration promotes the detachment of absorbed polymer chains from the wall and the disentanglement of polymer chains from a thin layer at the interface,which lead to the decrease of the critical shear stresses in the weak and strong wall silp behaviors.The more susceptible of the wall slip behavior under the effect of ultrasonic vibration reduces the melt viscosity,improves the melt fluidity and filling performance in filling process of the micro cavity,especially in the flow front and the edge region.(3)The ultrasonic vibration enhances the melt filling performance in micro cavity and improves the molding quality of micro plastic parts by two aspects: changing the rheological properties and promoting the wall slip behavior of the melt.In the low shear rate region during mold filling process,ultrasonic vibration changes the rheological properties of the melt and improves its filling performance.In the high shear rate region during mold filling process,ultrasonic vibration changes the rheological properties,and at the same time,promotes the wall slip behavior,further enhances the filling performance of the melt and improves the molding quality.(4)Ultrasonic vibration affects the aggregation behavior of semi crystallinepolymer in the aspects of improving the crystal growth rate and damage crystal structure.There exists an optimal excitation voltage of ultrasonic vibration which could obtain a highest crystallinity,hardness and elastic modulus of the molded micro plastic part.Experimental results show that,in ultrasonic-assisted micro-injection molding method,the crystal morphology of the polymer material has a significant influence its material properties,compared to the amorphous morphology,the crystalline morphology under the effect of ultrasonic vibration plays a more significant effect on the material properties.