Effect Of Ultrasonic Vibration On Viscous Dissipation In Micro Injection Molding Process

With the rapid development of Micro Electro Mechanical System (MEMS) technology and the continuous optimization of production process, the application of micro components in electronics manufacturing, aerospace, biomedical and many other areas have been growing. It requires a high demand for the forming quality and dimensional accuracy. Micro injection molding technology, an important production method of micro parts, has been paid more attention by scholars because of its simple molding process, low production cost, high processing efficiency and stable quality.Compared with the traditional injection molding technology, the size of plastic parts in micro injection molding process is smaller with larger value of the ratio of channel area to volume and length to diameter, resulting in less heat taken by polymer melt into mold cavity. The micro scale effect coming from this, such as the viscous dissipation, wall slippage, convective heat and surface tension, etc., will affect the melt rheological behavior in the micro channels, and all of that will make it hard to meet the requirements of the plastic molding quality, processing efficiency, etc. Based on deeply analyzing of the characteristics of micro injection molding technology and the effect mechanism of ultrasonic on polymer melt, viscous dissipation on micro injection molding was studied, in which combined methods of mathematical modeling, numerical simulation and experimental measurement on the basis of polymer materials, viscous fluid mechanics, polymer rheology, ultrasonic related theories had been applied. Meanwhile, we did a research on the regularity and mechanism of coupling effect of ultrasonic vibration frequency, power and process parameters on melting viscous dissipation.Firstly, the mathematical model of viscous dissipation was established based on the basic equations of polymer melt flow in conventional injection molding process, combing with the characteristics of micro injection molding flow. And the model of ultrasonic vibration’s effect on viscous dissipation was also established by means of periodic change in pressure instead of ultrasonic vibration. Secondly, outlet temperature rise caused by viscous dissipation when PP and HDPE melt flow through micro channels at different process parameters was simulated using FLUENT software. Finally, outlet temperature rise caused by viscous dissipation was measured by self-designed micro injection molding platform with ultrasonic vibration, at various process parameters and ultrasonic parameters.Experimental and simulation results show that viscous dissipation effect weakened with the increase of inlet temperature and injection velocity, and decrease of the cross-section size, when micro channel length is constant. At the same process parameters, outlet temperature will raise with the viscous dissipation increases by improving the ultrasonic vibration power and frequency when polymer-melt flows in the micro channel. Besides, we can see that outlet temperature rise is the result of combination of viscous dissipation and channel wall heat conduction, when melt flows through micro channel. And the actual viscous dissipation calorific value is higher than the experimental measurement. Experimental measurements after considering the convective heat transfer and numerical simulation results and theoretical calculation results have good consistency, thus it proves the rationality of the viscous dissipation theory model.