PSI - Issue 53

Bruno Sousa et al. / Procedia Structural Integrity 53 (2024) 291–298 Author name / Structural Integrity Procedia 00 (2023) 000–000

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1. Introduction

The constant development in technology, design and performance in the automotive sector frequently sets challenges in the manufacturing routes of related components. Automotive lighting parts stand out as an example of those challenges, not only relevant for the aesthetics of modern vehicles but also in ensuring road safety. The very high surface quality of lighting parts (typical obtained by plastic injection) implies very demanding surface quality of the mould itself (Pina et al., 2018). Such technical requisites commonly lead to the combination of finishing machining and polishing techniques on hardened steel. Starting from raw material CAM processing is employed in the shaping of a CAD geometry, commonly through milling operations. Still, the outcome surface roughness of this process falls short on the typical technical requisites, which typically impose mirror-like surfaces. For that reason, final abrasive polishing steps are performed up to removal of scallop height (Grandguillaume et al., 2015). Due to commonly being a manual process (Boerret et al., 2008), the final polishing steps are labour-intensive, relatively unpredictable and time consuming (Rebeggiani et al., 2012), therefore increasing the mold cost. With regards to milling and polishing (within mold manufacturing), di ff erent studies can be found in literature focusing on the improvement of each independent process. Focusing on the mitigation of dimensional errors on the milling of sculptured surfaces, Salgado et al. (2005) and Uriarte et al. (2007) have performed a study of the process machine and tool sti ff ness, based on force estimation and deflection calculation through numerical approaches. Even though micromilling approaches may enable adequate surfaces for mould injection, those are long-winded; plus, the mechanics of the process is quite di ff erent from the macro process (i.e. need for better vibration control, specific tooling and machine); lastly, the need for post-processing polishing (even if to a smaller extent) hinders the adoption of micromilling. O¨ ktem et al. (2006) has developed and experimentally validated an optimization method (Taguchi) for obtaining low surface roughness, improving the surface quality by approximately 5%. With the goal of increasing material removal rate while meeting dimensional and surface quality Oliaei and Karpat (2014) have investigated the e ff ect of micro milling process parameters in the creation of circular pockets on Stavax steel for micro mold fabrication. The authors have noticed the significant influence of the tool path strategy on tool wear, which in turn may compromise surface quality. In order to reduce manual polishing (or even replace) di ff erent approaches have been developed. Despite the limitations in tool handling and positioning, the usage of collaborative robots has been experimentally employed in complex surfaces (Almeida et al., 2018; Wang et al., 2019) capable of surface flatness correction by up to 70% as well as higher e ffi ciency, improved reliability and robustness scenarios. Moreover, With the aim of predicting material removal rates and eliminating trial and error approaches, Almeida et al. (2017) have developed an abrasive wear numerical model focusing on hardened steel. Laser polishing has additionally been employed as a viable method in mould applications. Laser polishing is based on the vapourisation / melting of a microlayer that only a ff ects to the peaks of the surface topography (Lamikiz et al., 2007). Lim et al. (2021) have analysed the surface condition of AISI4140 mold steel obtained through pulsed laser polishing. Despite the good results in termes of surface roughness, tensile residual stresses are generated, as opposed to the negative residual stresses obtained from mechanical polishing. Dimensional and geometrical accuracy as well as surface roughness are ensured by correct sequences of the milling and polishing steps. The improvement of the mold manufacturing process should be seen as a combined approach given the interlink between the two operations. Despite the bigger focus (on literature) of independent process improvement, some authors have analysed linked milling and polishing approaches. Grandguillaume et al. (2015) focused on milling and polishing combinations in order to reduce overall processing time of aluminium for blowing mold process applications. de Souza et al. (2014) has shown that the tool path strategy plays a significant influence not only on milling time, surface roughness and manual polishing time. The results show that the right choice of the tool path can save 88% of the time and 40% of the finishing costs, if compared to the less appropriate options. In this paper a methodology is developed for the evaluation of di ff erent milling and polishing operational conditions on the final roughness of hardened mold steel, further enabling a improved understanding of the combined processing, with particular focus to time consumption and surface quality compromises.