| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 95 === ABSTRACT
This dissertation is concerned with the injection molding characteristics and demolding behavior of polymer micro- and sub-micron-structures with high-aspect-ratios. The filling behavior of the polymer injected by a traditional injection molding machine into the microcavities was investigated first. Then a new injection molding strategy was proposed along with the development of a novel mold insert learned from the drawbacks of the traditional injection molding process in fabricating microstructures.
In the filling of the microcavities, distinctive mold-filling behaviors and resulting defects were observed for various types of polymers such as PMMA, PC, PP and HDPE. Here, sacrificial molds were used to overcome the demolding problems in order to prevent the microstructures from direct damage as demolding. Experimental results revealed that injection molding of micro- and sub-micro-structures are more difficult than that of products with common dimensions. If the mold temperature is lower than the glass transition temperature (Tg) of the molding polymer, the filling resistance of the polymer in micro injection molding will be markedly high, and therefore, the polymer melt can only fill up the mold cavity of the base part of the micro-structure products and can not fill into the microcavities. To fill up the micro- and sub-micron-cavities successfully, the mold temperature must normally be above the Tg of the polymer used. Moreover, the main injection pressure and the main injection time substantially affect the achievable aspect ratio of the micro- and sub-micron-structures. As for the shrinkage control, both a mold temperature, which is above the Tg and a higher holding pressure can decrease the shrinkage. In addition, the shrinkage curve becomes linear with the increase of the holding pressure. However, the griping force due to the difference of shrinking ratio between the polymer and the mold insert can not be released, which results in the demolding defects. Therefore, a special in-mold process of stress relief is strongly necessary for solving the demolding problem to guarantee high micro-molding quality.
This thesis, in accordance with the shrinking behavior of the injection molding of microstructure, presents a silicon-based heat-generable mold insert for micro injection molding to solve the problem of demolding destruction, as mentioned earlier. Design of this mold insert not only has micro-channels constructed on the silicon wafer as the microcavities but also includes micro electrical heating lines embedded in the wall of the microcavities. These electrical heating lines with specified resistance were fabricated by doping phosphorus ions precisely into the surface of the silicon cavity wall using the ion implanting process. The performance of the novel mold insert was studied and then the mold insert was applied to the injection molding of micro-structures with high aspect ratios for possibly resolving the demolding problem. Experimental results indicated that silicon-based electrical heating lines embedded in the novel mold insert can provide stable heating power. By heating the cavity wall and the nearby plastic with appropriate timing and sufficient power in the cooling stage, we could effectively reduce the griping force between the patterned plastic microstructures and the micro-channels of the mold insert. Consequently, defect-free demolded products with high-precision were obtained. This novel mold insert provided extremely high thermal efficiency in heating and temperature control when it was applied to the micro injection molding in practice. The above mentioned advantages can in turn result in power saving and cycle time shortening, and thus, enhancing industrial applications.
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