Studies On Polishing Media Movement Affected By Ultrasonic Cavitation Based On CFD

With the extensive application of modern mold industry, stiff market competition and continuously growing demand for improved product performance has led to increasingly precise surface treatment for molds. Polishing process is a vital process in the mold manufacturing and directly results in product performance. Many materials possess properties like high strength and stiffness at elevated temperatures, extreme hardness, high brittleness, high corrosion and oxidation resistance, chemical inertness et al. Although these properties would bring about superior product performance, their precise shaping or machining can be difficult. The primary machining of a mold, especially a mold with shaped cavity is mostly relied on conventional grinding. The quality of mold products is determined by handwork of operators. Consequently, the process results in not well accepted concerning the reliability of the polished surface, degradation of some useful properties and low efficiency. As a result, an effective automated polishing process is desired to fulfill complex curved surfaces, grooves and blind holes polishing so as to improve polishing efficiency and acquire reliable product quality.This dissertation will study the basic theory in a novel polishing process for dies and molds termed ultrasonic polishing. Ultrasonic cavitation generated by high-frequency ultrasonic transducer is investigated by theoretical analysis and numerical simulation. The complex and microscopic phenomena in ultrasonic polishing, including the distribution of acoustic pressure field and vapor fraction, microscopic abrasive movement and the impact of each factor on ultrasonic cavitation are all difficult to be directly obtained by experimental approaches. Therefore, numerical method based on CFD will be employed to solve these key issues in the dissertation. The primary contents and innovations are as follows:(1) We apply mixture model and turbulence model to simulate acoustic cavitation for the first time. The energy generated by ultrasonic cavitation is discussed and it can enhance the abrasive polishing for the mold shaped cavity, resulting in more intense material removal.(2) Additionally, DOE in MINITAB is used for orthogonal design of polishing tests and the interaction between the various impact factors is achieved through it.(3) Moreover, there is no selectivity of cavitation. The distribution of cavitation is relatively stable, that is concentrated in the vicinity of the bottom center of the mold shaped cavity, while small around the inner wall and the edge of the area. The corresponding change in the velocity of the abrasive occurs according to the distribution of cavitation in the mold shaped cavity.(4) To achieve a more precise machining effect in ultrasonic polishing, the influence of cavitation on abrasive material removal rate should be suppressed in order to strengthen the machining of abrasive itself to the workpiece.(5) Acoustic cavitation in the ultrasonic polishing process can be changed by altering the ultrasonic frequency, amplitude, liquid viscosity and the depth of the tool head. A reasonable range of the values for these parameters and the interaction between them can be acquired in accordance with orthogonal design based on DOE.