Investigation and analysis of the physical properties of wood–plastic composites (WPCs) reveal a complex interaction between weight ratio, grain size and wood type. The role of fibers in modifying the properties of pure high-density polyethylene (HDPE) and the potential improvement of shrinkage and warpage properties using fibers as a filler are met with significant challenges, namely moisture absorption, which can occur in wood composites due to wood's hygroscopic nature. The results showed that wood particles markedly reduced the shrinkage and warpage, and the pure HDPE sample was most susceptible to shrinkage and warpage. When analyzing the moisture absorption, it was found that wood fibers are the main factor in increasing moisture; the highest rates in terms of water absorption and thickness swelling reached 4.16% and 6.44%, respectively. In contrast, the pure HDPE sample showed almost complete resistance compared to the WPC samples. The lowest shrinkage and warpage were observed in sample 6, which recorded values of 2.31 ± 1.14% and 0.086 ± 0.040 mm, respectively, while the highest shrinkage and warpage were observed in sample 7, which recorded values of 6.07 ± 1.05% and 0.220 ± 0.036 mm, respectively. The optimum values of the influencing variables were determined to achieve the best balance between improving dimensional stability and reducing water absorption and thickness swelling by applying the Taguchi methodology for designing and analyzing experiments. The optimal value of the signal-to-noise ratio for minimum shrinkage and warpage is A2B3C1, and the optimal conditions for minimum water absorption and thickness swelling at both 2 and 24 h include Platanaceae wood type, a weight ratio of 25% and particle size of 300–600 μm. Adding wood fiber enhances the structural performance of the polymer. Microstructural analysis indicated that while samples with smaller wood grain sizes exhibit better homogeneity and fewer voids, leading to improved composite properties, those with larger grains show increased heterogeneity and voids that negatively impact performance; however, the effective binding material used ensures good adhesion between wood fibers and polymer matrix.
