Magnetorheological finishing of electroless nickel-phosphorus-plated mold for ultraprecision injection molding

The ultraprecision machining of the mold surface is important because the performance of optical components produced using optical injection is related to the surface roughness and shape precision of the mold core. To process hard-to-cut mold core materials such as stainless steel, electroless nickel-phosphorus (NiP) plating is applied to the surface of the substrate, and ultraprecision processing is performed on the plating surface using diamond turning (DT) and magnetorheological finishing (MRF). Although MRF processing can remove tool marks caused by diamond turning and improve shape accuracy, it suffers from poor processability for NiP. In this paper, an ultraprecision processing method with a heat treatment process was presented to improve the MRF processability of electroless NiP. The plating was heat-treated under each condition and subjected to DT processing followed by MRF. The surface roughness and reflectance were measured and evaluated after processing. As a result, the surface roughness of NiP subjected to a 1-h heat treatment at 450 °C significantly decreased from Ra 7.0 nm to Ra 1.5 nm when compared to the specimen that did not undergo any heat treatment. An improvement in surface roughness confirmed that reflectance improved by more than 9% and 18%, respectively, compared to that for the diamond-turned and non-heat-treated specimen. These results confirmed that the MRF performance was improved through the NiP heat treatment process and contributed to quality improvements in ultraprecision injection optical parts. Furthermore, this technology can be applicable to various fields that used NiP plating, including the manufacturing of metal reflectors in the visible and ultraviolet regions.