The increasing demand for sustainable materials in additive manufacturing has encouraged the utilization of recycled polymers and waste-derived reinforcements; however, limited studies have systematically developed and optimized HDPE–GTR composite filaments specifically for 3D printing applications using multi-criteria decision-making approaches. In this study, recycled high-density polyethylene (HDPE) reinforced with ground tyre rubber (GTR) particles (600–150 µm) is used to develop eco-friendly composite filaments through twin-screw extrusion. The HDPE: GTR compositions ranging from 90:10 to 50:50 are comprehensively evaluated in terms of physical (density, water absorption, melt flow index), mechanical (tensile strength, impact strength, hardness, flexural modulus), and thermal (melting temperature) properties. The results reveal that increasing GTR content enhances impact strength, melt flow behavior, and thermal stability, while tensile strength, hardness, and flexural modulus show a decreasing trend due to the elastomeric nature of GTR. To identify the optimal balance among these conflicting properties, the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is employed as a multi-criteria decision-making framework. The 80:20 (HDPE: GTR) composition is identified as the optimal formulation, offering a balanced combination of processability, mechanical integrity, and thermal performance. The novelty of this work lies in the integration of recycled HDPE and waste-derived GTR for filament fabrication, combined with a systematic TOPSIS-based optimization strategy tailored for additive manufacturing applications. The developed filament demonstrates improved printability and reduced warping, highlighting its potential for sustainable and cost-effective 3D printing.
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