In this study, walnut shell is used as an additive in wood-plastic composite production to explore their potential in manufacturing processes. Different percentages of Polycarbonate (PC) (0, 7.5, 15 wt %) are incorporated with high-density polyethylene (HDPE) as the base material in various proportions to evaluate its impact on the final product. The study employs the Taguchi technique of the design of experiment (DOE) in injection molding to analyze the data. Moreover, a method involving multi-criteria decision-making (MCDM) is used to select the optimal model through practical experiments. The mechanical properties, including tensile strength, flexural strength, and impact strength, are thoroughly analyzed. The results indicate that composite L1, which contains 0 wt % PC and 20 wt % walnut shell with a particle size of 1.5–1 mm, exhibited an 8% increase in tensile strength but a 34.89% decrease in impact strength. In contrast, composite L6, comprised of 7.5 wt % PC and 40 wt % walnut shell, demonstrated a 30.19% increase in flexural strength. To choose the best conditions using the MCDM method, preference percentages are assigned to each result. Considering the importance of these properties for strength, very close ratios (i.e., 34% for flexural strength, 33% for tensile strength, and 33% for impact strength) are allocated. The findings indicate that 0 wt % PC, 20 wt % walnut shell and the particle size ranging from (1.5 to 1) mm is the most effective compound among all the conditions.
