Waste plastic bags (WPB) from high density polyethylene (HDPE) have been reprocessed for many applications. However, it is limited to be used as wood plastic composites (WPC) for building applications as the decrease of mechanical properties. This research examined the optimal composition of WPB that can be added to WPC and complied with Indonesian Standard (SNI) 8154 -2015. Samples were produced according to extreme vertices mixture design using twin screw extruder (TSE). The materials were

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... bounded in the range of 0 to 20 wt%, sawdust at 50 to 70 wt%, neat HDPE at 30 to 50 wt%, and HDPE grafting maleic anhydride (PE-g-MA) at 0 to 10 wt%. The compositions were optimized to obtain desired flexural modulus and strength as required on SNI standard. The optimized formula was then reproduced and characterized to analyze its water content, density, swelling, and formaldehyde content. The results showed that the optimum formula was predicted at 3 wt% of WPB, 50 wt% of sawdust, 37 wt% of neat HDPE, and 10 wt% of PE-g-MA, giving 2111 MPa and 27.3 MPa of predicted values that above the minimum requirement for WPC material (2000 MPa for modulus, 20 MPa for strength). Meanwhile, the modulus and strength of verification sample were 2173 MPa and 25.7 MPa. Further analysis on the other properties showed 0.6 ± 0.1% of water content, 1.7 ± 1.4% of swelling, and 0.015 ± 0.000 mg/L of formaldehyde that lower than maximum specifications, and 1.13 ± 0.01 g/cm 3 of density that above minimum the requirement.