Superfinishing of hardened steel for the moulding industry of automotive lighting parts

Achieving a mirror-like finish surface quality in plastic injection molds and dies is necessary for the manufacture of optical components in the automotive industry. Any defects present on the mold surface will be evidenced on the injected part, compromising the functionality of the final product. Moreover, the typically employed finishing techniques (milling and polishing) in mould steels can be excessively time-consuming. The identification of machining and polishing combinations that meet the demanding surface requirements while minimizing time and material resources is therefore essential for ensuring the efficiency and cost-effectiveness of the mould-making process. In this work, an experimental study was conducted on the milling of hardened tool steel using a 6 mm two flute ball nose end mill. Distinct configurations of axial and radial depths of cut as well as feed per tooth were considered and their impact on the pre-polishing surface roughness was analysed. A methodology for the parametrization of polishing conditions was developed, ensuring consistency of speed and pressure. Further tests were conducted to evaluate the relation between post-polishing surface quality and the total duration of the combined finishing approach. The polishing stage consisted on submitting the milled samples to SiC paper (800 and 1000 grit) and diamond cloth (6 and 3 µm) polishing. The method provided enhanced control of the polishing operation and revealed milling/polishing improved sequences for attaining surface roughness technical requisites.