| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 94 === For the molds of optical lenses, ultra-precision machined mold inserts are required. Most of the mold inserts are machined by a single-point diamond (SPD) turning of electroless nickel deposit on tooling material. Since the preparation procedure makes the nickel deposit expensive and time-consuming, other materials for rapid tooling during development are demanded. In this study, Be-Cu alloy which can be SPD machined is explored.
This study is devoted to investigating the use of the Be-Cu alloy mold insert in plastic injection molds for aspheric optic lens. In the first part of this study, the effects of the molding conditions on the oxidation of mold inserts are experimentally investigated. The parameters include mold temperature, melt temperature and injection speed; the materials include PS, PMMA and COC. The oxidation degree and surface roughness of the Be-Cu inserts as function of injection counts are measured and analyzed. To reduce the oxidation, blowing nitrogen into mold cavity and depositing Ni-P-PTFE on the insert surface are attempted. In the second part of this study the effects of the conditions on the molding quality of aspheric optic lens using a Be-Cu insert in the injection mold. Taguchi’s method is employed to evaluate the significance of different processing condition on the quality of molded parts. Form errors of molded parts measured by the surface profiler and residual stresses measured by photo-elasticity are used to evaluate the molding quality.
The results indicate that melt temperature is the major factor to influence mold oxidation rate, and injection speed and mold temperature are next in importance. There is no obvious change of the oxidation behavior in Be-Cu mold insert when using different material in the same molding conditions. The differences in the change of surface roughness in the first 250 injection counts are less than 7~8 nm. When the injection counts exceeds 1000, micro crack can be observed in the mold surface. Thus the molded optic lens can not be used.
The XPS analysis show that Cu2O is the primary constituent on the surface of the mold insert. Blowing nitrogen onto cavity surface has no significant effect on reducing oxidation. On the other hand, Ni-P-PTFE deposit can prevent the oxidation perfectly.
The differences between the profiles in mold insert and molded parts are used to evaluate the molding precision. Mold temperature and packing pressure are found to be the principle factors affecting the molding precision. Higher mold temperature and packing pressure result in better dimensional precision. The molecular orientation and residual stress are compared with aid of birefringence observed with polariscope. Molded in a fan-gated mold, it is found that the lenses molded with higher mold temperature yield less orientation and residual stress. Molded in a rectangular-gated, lenses molded with higher meet temperature yield less residual stress. Over-packing causes drastic disturbance to molecular orientation and results in poor molding quality.
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