| Summary: | 碩士 === 國立高雄應用科技大學 === 機械與精密工程研究所 === 101 === The Blu-Ray Pick-Up Head has been using single aspheric plastic lens instead of multi-piece glass assembly lens so as to reduce the weight of the objective lens and the manufacturing cost in recent development. However, the steepness of lens is increased as the NA value is increased, so that the central thickness and the edge thickness of the lens have large difference. Therefore, during the injection molding process, the objective lens suffers the shrinkage problem which affects the optical quality of the lens eventually. In order to solve above problem, the injection molding experiment is applied for making the objective lens for Blu-Ray Pick-Up Head in this research and the Taguchi method and the mold compensation technique are implemented to reduce the deviation of the shape precision of lens so as to provide an optimization process for manufacturing objective lenses.
In this research, a plano-convex lens, 5 mm in diameter and 3 mm in total thickness, was designed by the ZEMAX optical analysis software. The specifications of the lens developed in this study are: the NA is 0.85, the spot size is 0.58 μm and the RMS_OPD is smaller and/or equal to 0.033 λ, these specifications meet the requirements of an objective lens of the optical pick-up head for reading Blu-Ray Disc. During the optimization of the design process, some facts were found: when the NA was high, the steepness of the lens surface was large; while when the total thickness was too thin, the edge of the lens was thinning, so that the lens could not capture the edge light beams to the main optical axis. The key point for correcting the optical aberration of a lens was applying aspheric lens at its two faces and optimizing the coefficient of the aspheric character; introducing a high order coefficient raised the collecting ability of the edge light beams for the lens so as to reduce the optical aberration; however, the edge thickness of the lens should be high enough to set the gate of the mold cavity. Before the injection molding experiment, the Moldex3D software was applied to predict the relationship between the melt filling and the molding parameters. In the experiment, the optical simulation result showed that the aspheric character is the main factor for affecting the optical efficiency of a lens. Therefore, this research applied the Taguchi method for investigating how much the injection speed, the melt temperature, the mold temperature and the packing pressure play on the shape precision of the aspheric character. The deviation of the shape precision is defined by the P-V value which is targeted as lower than 1 μm based on the fault tolerant analysis of a random shaped surface.
The experiment revealed that the packing pressure affected the shape precision of the aspheric character mostly, and contributions to the two faces were 37.9 % and 49.8 % respectively; P-V values of the test sample set with the best parameters were 3.84 μm and 6.85 μm respectively; finally, the mold compensation technique was introduced into the test sample set with the best parameters for the injection molding experiment, and the real P-V values of the shape precision were measured as 3.32 μm and 6.17 μm, which showed the reliability of the experiment.
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