Surface Response Optimisation of Milling Parameters in Wood Polymer Composite with Recycled and Virgin Polypropylene

This study aims to optimise milling process parameters for wood-plastic composites manufactured from virgin and recycled polypropylene (v-PP and r-PP) reinforced with teak wood. The investigated parameters were spindle speed, feed rate, and depth of cut. Specimens were produced by injection moulding, and surface roughness was measured in accordance with ASTM D7127. The mean peak-to-valley height (Rz) was used to characterise the surface quality of the specimens. Response surface methodology based on a Box-Behnken design was employed with 15 experimental runs, evaluating spindle speed, feed rate, and depth of cut at three levels each. The results show that machining parameters significantly influence surface roughness. Depth of cut was the most influential factor for v-PP composites, whereas spindle speed and the quadratic term of depth of cut were most significant for r-PP composites. The developed quadratic regression models showed good agreement with the experimental data, with adjusted R2 values of 86.19% for v-PP composites and 82.04% for r-PP composites. Optimisation identified the milling parameters that minimise surface roughness as 5,000 rpm spindle speed, 1,543.43 mm/min feed rate, and 1 mm depth of cut for v-PP composites, and 4,798 rpm spindle speed, 1,600 mm/min feed rate, and 1.39 mm depth of cut for r-PP composites. Although these results are specific to the materials studied, the methodology can be applied more broadly to optimise machining processes for other composite materials.