The recycled high-density polyethylene (rHDPE) has the potential to be used as a sustainable printing material for fused filament fabrication (FFF). The main challenges producing 3D printable filament from recycled plastic include maintaining a consistent filament-like diameter and ovality, ensuring the printability of the material and achieving adequate mechanical strength in the printed part. This study compares the printability, filament geometry, melt flow index (MFI) and tensile properties of printed specimens using rHDPE and commercial HDPE filaments. The research successfully produced 3D printing filament from rHDPE at 220°C, which was compatible with FFF printing. However, rHDPE filament exhibited significantly higher diameter deviations and filament ovality compared to commercial HDPE filament. During printing, both rHDPE and commercial HDPE specimens experienced warping at the bottom layers, a common challenge in 3D printing high-crystalline thermoplastic materials. rHDPE had a lower MFI value than commercial HDPE, allowing it to be printed at a lower temperature. For optimal tensile strength and modulus, both materials performed best when printed at 230°C. However, printed specimens made from rHDPE showed lower tensile strength and modulus than those made from commercial HDPE filament. This study highlights the feasibility of using rHDPE as a feedstock for FFF printing while addressing the challenges related to filament geometry, warpage and mechanical properties. © Penerbit Universiti Sains Malaysia, 2025. This work is licensed under the terms of the Creative Commons Attribution (CC BY) (http://creativecommons.org/licenses/by/4.0/).

Discarded expanded polystyrene (EPS) poses significant environmental challenges due to its accumulation in landfills and resistance to degradation. Upcycling EPS into 3D-printable recycled polystyrene (rPS) filament for use in fused filament fabrication (FFF) shows a promising sustainable solution. This study explores the mechanical recycling of EPS into rPS resin, which is then extruded into 3D-printable filament using a 3DEVO filament maker. A key focus is on studying the filament’s properties and the properties of its printed specimen. Filament diameter was monitored in real-time during extrusion using DevoVision software. The results showed that the rPS filament achieved an average diameter of 1.75 mm with a standard deviation of ±0.03 mm when extruded at 200 °C. Extrusion at temperatures above 200 °C resulted in filament diameters exceeding 1.75 mm. Specimens printed with rPS filament exhibited higher tensile strength, tensile modulus, and elongation at break, but lower impact strength compared to those printed with commercial high-impact polystyrene (HIPS) filament. Micrographs show that HIPS is a more ductile material, which contributes to its better impact strength. However, due to poor layer adhesion during printing, it exhibited lower tensile properties. The onset and maximum decomposition temperature of rPS did not show a significant change during the recycling process from EPS to filament. The onset decomposition temperature was above printing temperature, thus it did not limit the use of rPS for FFF printing applications. Despite HIPS displaying greater thermal stability, rPS still demonstrates strong potential as a sustainable printing material, and its applications include printing prototypes and serving as a support material in 3D printing, since it can be dissolved in acetone. © 2025, Universiti Malaysia Perlis. All rights reserved.