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18 3 3 2 3 0.57 0.53 0.53 5.293 0.543
Fig.7. Plots of control factor effects
The goal in the spherical grinding process is to minimize the surface roughness value of the ground specimen by determining the optimal level of each factor. Since −log is a monotone decreasing function, we should maximize the S/N ratio. Consequently, we can determine the optimal level for each factor as being the level that has the highest value of η. Therefore, based on the matrix experiment, the optimal abrasive material was pink aluminum oxide; the optimal feed was 50 mm/min; the optimal depth of grinding was 20 μm; and the optimal revolution was 18 000 rpm, as shown in Table 3.
The optimal parameters for surface spherical grinding obtained from the Taguchi’s matrix experiments were applied to the surface finish of the freeform surface mold insert to evaluate the surface roughness improvement. A perfume bottle was selected as the tested carrier. The CNC machining of the mold insert for the tested object was simulated with Power MILL CAM software. After fine milling, the mold insert was further ground with the optimal spherical grinding parameters obtained from the Taguchi’s matrix experiment. Shortly afterwards, the ground surface was burnished with the optimal ball burnishing parameters to further improve the surface roughness of the tested object (see Fig. 8). The surface roughness of the mold insert was measured with Hommelwerke T4000 equipment. The average surface roughness value Ra on a fine-milled surface of the mold insert was 2.15 μm on average; that on the ground surface was 0.45 μm on average; and that on burnished surface was 0.07 μm on average. The surface roughness improvement of the tested object on ground surface was about (2.15−0.45)/2.15 = 79.1%, and that on the burnished surface was about (2.15−0.07)/2.15 = 96.7%.
Fig.8. Fine-milled, ground and burnished mold insert of a perfume bottle
5 Conclusion
In this work, the optimal parameters of automated spherical grinding and ball-burnishing surface finishing processes in a freeform surface plastic injection mold were developed successfully on a machining center. The mounted spherical grinding tool (and its alignment components) was designed and manufactured. The optimal spherical grinding parameters for surface grinding were determined by conducting a Taguchi L18 matrix experiments. The optimal spherical grinding parameters for the plastic injection mold steel PDS5 were the combination of the abrasive material of pink aluminum oxide (Al2O3, PA), a feed of 50 mm/min, a depth of grinding 20 μm, and a revolution of 18 000 rpm. The surface roughness Ra of the specimen can be improved from about 1.6 μm to 0.35 μm by using the optimal spherical grinding conditions for surface grinding. By applying the optimal surface grinding and burnishing parameters to the surface finish of the freeform surface mold insert, the surface roughness improvements were measured to be ground surface was about 79.1% in terms of ground surfaces, and about 96.7% in terms of burnished surfaces.本文研究了注塑模具钢自动研磨与球面抛光加工工序的可能性，这种注塑模具钢PDS5的塑性曲面是在数控加工中心完成的。这项研究已经完成了磨削刀架的设计与制造。 最佳表面研磨参数是在钢铁PDS5 的加工中心测定的。对于PDS5注塑模具钢的最佳球面研磨参数是以下一系列的组合：研磨材料的磨料为粉红氧化铝，进给量500毫米/分钟，磨削深度20微米，磨削转速为18000RPM。用优化的参数进行表面研磨，表面粗糙度Ra值可由大约1.60微米改善至0.35微米。 用球抛光工艺和参数优化抛光，可以进一步改善表面粗糙度Ra值从0.343微米至0.06微米左右。在模具内部曲面的测试部分，用最佳参数的表面研磨、抛光，曲面表面粗糙度就可以提高约2.15微米到0 0.07微米。