| Summary: | 博士 === 國立交通大學 === 機械工程學系 === 100 === Glass offers better anti-thermal, anti-environmental, corrosion resistance, and optical properties than polymer materials. Therefore, glass can be used in demanding environments such as high-energy laser systems or severe weather conditions. Precision glass molding (PGM) techniques can be used in rapid fabrication of highly accurate optical components. However, key aspects of PGM techniques, such as the hard coating for mold protection and the operating parameters, are kept confidential by manufacturers. Because various types of optical glass have different characteristics, the initial testing period of the PGM process is time consuming. Therefore, researchers are difficult to follow the optimal molding parameters from previous studies. Consequently, this thesis focuses on the analysis of glass materials and the optimization of molding parameters.
In order to understand the molding mechanism for producing microstructures and precision curves components, the open-type and close-type PGM approaches were studied in this thesis. According to experimental results, the thermal absorptivity of glass in open-type PGM processing is better than the close-type due to glass can be heated by thermal convection of nitrogen in open-type PGM processing. Therefore, open-type PGM processing is suitable for forming microstructures. However, it is difficult to align two surfaces of components in this type of processing. To fabricate components with precision curves, closed-type PGM processing is needed. This thesis studies open-type PGM processing of microstructure arrays such as micro cylindrical lens arrays and one- or double-sided microlens arrays and close-type PGM processing of planar-integrated micro-optical components and high-precision aspheric lenses. Finally, a Fresnel lens, which combines microstructure and precision curve, was used to verify the experimental results of above experiments.
Precision diamond grinding technique was usually used to fabricate the mold of PGM processing on tungsten carbide (WC) material. But Precision diamond grinding technique was not easily to generate the complex and tiny microstructure. Hence, this thesis used not only diamond grinding technique but also wire electrical discharge machining technique on WC and laser machining on silicon carbide and stainless steel to fabricate the mold of glass molding. Besides, the electro polishing has been used to change the shape of micro structure on mold. The processing methods using in this thesis offers the various ways to manufacture molds.
This thesis obtains, the fracture of glass components can be avoid with increasing molding temperature, decreasing molding force and decreasing cooling rate in the PGM processing. Furthermore, decreasing molding temperature can reduce adhesive phenomenon. Besides, the thermal absorptivity of mold and glass in nitrogen atmosphere are better than vacuum environment, the deformable level of glass can be increased. But molding in vacuum environment can avoid bubbles product on glass. Therefore, increasing space of upper and under mold, temperature holding time and molding temperature are effective to form glass components in vacuum environment. If size of component larger than 15 mm, temperature holding time longer than 180 s is useful to uniform temperature and avoids fracture of glass. Moreover, slower cooling rate reduces the variation of refractive index. For small lens, 18.5 °C/min in cooling rate obtains the stable quality.
In this thesis, the wire electrical discharge machining process, which is suitable for machining hard materials such as WC, was used to fabricate a microgroove array on a WC mold in the open-type PGM approach. N-FK5 glass was used, and a micro cylindrical lens array 20 mm in diameter and 5 mm thick was successfully obtained. Cylindrical lenses 234 μm in height and 3.42 mm in curvature were fabricated in the array. In addition, a laser ablation approach, which offers the advantage of noncontact machining, was adopted to generate a microstructure array on a silicon carbide mold, and a double-sided microlens array 20 mm in diameter and 1.2 mm thick was obtained on soda-lime glass. The microlens components were 851 μm in curvature, 460 μm in width, 52 μm in height, and 700 μm in pitch. Furthermore, electro polishing was used to remove the recasting layer used in laser ablation, and a stainless steel mold was used. A microlens array 58.1 μm in curvature and 39.3 μm in height on soda-lime glass and a micro cylindrical lens array 79.8 μm in curvature and 37 μm in height were obtained. Both these components were 10 mm × 10 mm in size.
WC molds were successfully fabricated by a precise diamond grinding technique using in close-type PGM processing. The optimal molding temperature was 750 ?aC. N-BK7 glass was used to obtain planar-integrated micro-optical components 53 mm in diameter and 6 mm in height. K-VC79 glass was used to obtain a high-precision aspheric lens with a surface accuracy (peak to valley) of less than 0.5 λ (0.316 μm) at an optimal molding temperature of 585 ?aC. With an optimal molding temperature of 560 ?aC, a Fresnel lens 15 mm in diameter was fabricated from K-CSK120 glass, and a fill rate of 99% was achieved.
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