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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After machining, the samples were subject to a surface roughness measurement, to classify / assess the influence of the machining parameters on the machined surface quality (this procedure will be described in detail in subsection 2.3). Not all samples were selected in order to avoid an excessive / redundant information and also due to the time consuming polishing procedure. The Taguchi method was employed (using the Minitab software) considering not only the surface quality but also the required machining time, allowing for the selection of representative samples.

2.2. Polishing experimental testing

A parametrized methodology for polishing the previously machined samples was build from the typical sequence of manual polishing. This was achieved by employing an automatic polishing machine (Metkon DIGIPREP 251), as illustrated in figure 2a, typically used in metallographic sample preparation. As such, the manual polishing procedure was recreated (according to mold manufacturers expertise) and adapted to an automatic and reproducible process (independent of operator skills) that was easier to control. The equipment consists of a rotating polishing head in which samples (maximum of 6, simultaneously) are held against polishing discs / cloths mounted on a platen. Furthermore, there is constant water jet / lubricant that is applied in polishing zone. Figure 2b shows a previously machined sampled mounted on resin allowing polishing procedure.

Fig. 2. Polishing setup and methodology: (a) automatic polishing machine; (b) example of polished steel sample mounted on resin; (c) schematic representation of the polishing steps, showing the parameters of each.

The polishing procedure for the mould parts consists of 4 sequential steps (2 grinding steps and 2 polishing steps), as illustrated in figure 2c). The grinding steps are performed using grinding papers with silicon carbide abrasive particles (SiC). Two di ff erent grits are used for this step, an 800-grit grinding paper followed by a 1000-grit grinding paper. For the finishing steps, two combinations of fibre cloth and diamond paste were used, a synthetic fibre flock cloth was used with a 6 µ m monocrystalline diamond particle paste, suspended in a water-based medium. For the finer step, and second one of the polishing steps, a woven cotton cloth was used alongside another monocrystalline diamond paste, having now an average particle size of 3 µ m.

Fig. 3. Influence of polishing time on Ra and Sa obtained from milling initial condition for each grinding (a) and finishing (b) polishing steps.