Micro-Polymer Parts Molded By Micro-UPM Process And Its Plasticization Mechanism

Polymer micro plastic parts, which feature good oxidation resistance, corrosion resistance, low melting point, low conductivity and good impact resistance, have gradually replaced metal micro parts and become transmission parts and execution components of MEMS devices. Some polymer materials are also known for their good transparency, biocompatibility and biodegradable absorbency. However, micro injection, micro hot embossing and micro extrusion are quite different in terms of production cycle, manufacturing cost, energy consumption, material applicability, mold structure and molding equipment. None of these three technologies is likely to meet all the requirements of low production cost, low energy consumption, high production efficiency and high performance. To cope with this problem, ultrasonic vibration in plastics molding process has attracted more and more attention from home and abroad in recent years.In this dissertation, the author proposes a novel polymer micro ultrasonic powder molding(micro-UPM) process. In this process, polymer powder within certain particle size is first filled in a micro-cavity(including micro-cavity inserts). Next, under the pressure of ultrasonic sonotrode, the powder particles are quickly plasticized by ultrasonic vibration and fill the whole micro cavity rapidly. Micro plastic parts are successfully fabricated after being cooled and solidified for amount of holding time. Following that, a set of micro combined mold with micro cavity but without runners is built and different types of micro mold inserts are made to fabricate EVA, i PP and UHMWPE micro plastic parts by micro-UPM. At the same tome, a visualized ultrasonic plasticization platform with a temperature measuring module is set up to carry out qualitative analysis and measurement of molding temperature curve in different areas of the micro plastic part. From the temperature curves, it is found that only by maintaining the melt molding temperature between the melting point and the degradation temperature could micro-UPM parts of good quality be fabricated. The whole dynamic plasticization process of i PP and UHMWPE parts is captured by the high-speed camera module.The dissertation also investigates the influence of molding parameters, including ultrasonic duration time and plasticization compression on the flash thickness of micro-UPM i PP parts. When ultrasonic plasticization compression remaines unchanged, the flash becomes thinner and thinner as the ultrasonic duration time increases. When the duration time surpasses the critical value, the flash will be automatically separate from the plastic part. According to the analysis of differential scanning calorimeter(DSC), the melting point of micro-UPM i PP parts is 4 ℃ higher than that of i PP material, close to the melting point of i PP parts. The crystallinity of micro-UPM i PP parts, on the other hand, increases dramatically too. According to the analysis of the X-ray diffraction(XRD), it is the α instead of β crystal form that can be found on different regions of the micro-UPM i PP parts. Their crystal orientation is less obvious too, compared with the parts traditionally molded by micro-injection in which “skin-core” structure can be commonly seen. According to the research on a series of micro-UPM i PP parts molded in the sequence of ultrasonic duration time, the crystallinity decreases gradually and the melting point as well as the initial crystallization temperature goes down. The grain size is reduced and the crystallization as well as the density of internal organization aggregation is improved. Once the ultrasonic duration time surpasses the critical value(1.2 s), the crystallization as well as the density of internal organization aggregation begins to reduce and porous structure inside the micro plastic part occurs too. In the tensile test of the micro-UPM i PP samples before and after annealing, their tensile strength and fracture elongation shows the tendency of going from better to worse. The mechanical properties are dramatically improved after annealing test but a certain degree of degradation occurs with the increase of annealing time. Therefore, it could be concluded that annealing does not improve mechanical properties of micro-UPM i PP parts.The dissertation also investigates the thermal property, the forming principles of two-phase structure, the microstructure and the micro mechanical property of micro-UPM UHMWPE parts by scanning electron microscope(SEM), polarized light microscope(PLM), DSC, single-crystal XRD and universal tester. In addition, the plasticization mechanism of micro-UPM UHMWPE parts is investigated by the temperature curves and the whole dynamic video of the plasticization process. The plastic parts molded by micro-UPM is extremely small in size as the high-frequency vibration, reaching 20 k Hz, generates "chain expansion" between particle interfaces and dramatically promotes the diffusion and interpenetration of chain segments, whose combined effects produce strong cocrystallization links by high-speed diffusion interpenetration. The molding quality of micro plastic parts depends on the the fraction of melt-recrystallized materials.PCB micro-hole array with micro mold inserts are fabricated, using high speed drilling. On the micro-UPM experimental platform, two types of micro-cylinder array plastic parts with diameters 0.15 mm and 0.20 mm respectively are fabricated according to design parameters. Compared with the micro-hole of the same specification, the average diameter of the micro-cylinder plastic part is about 10.0 μm smaller and the lateral surface of the micro-cylinder is rougher than the micro-hole wall. But their dimension in height is almost the same. DSC and nanoindentation tests results show that when polymer powder is not preheated, two types of micro-UPM micro-cylinder plastic parts generally consist of two phases: nascent phase and melt-recrystallized phase. The local elastic modulus Er of the micro-cylinder and its base part ranges from 0.7 GPa to 1.2 GPa. However, when polymer powder is preheated to 85 ℃, only single-phase melt-recrystallized phase could be seen in two types of micro-cylinder array plastic parts, whose crystallinities are 54.7% and 55.2% respectively and the Er ranges from 0.6 GPa to 0.8 GPa. As single-crystal XRD experiment shows, if the material powder is preheated, the crystallinity of the base part is more than that of the micro-cylinder part; and the grain size of(110) crystal face is bigger than that of(200) crystal face.