Fused filament fabrication of phase change materials in high density polyethylene with expanded graphite for shape-stabilization and thermal conductivity enhancement

• Novel FFF of HDPE/Expanded Graphite composites for PCM applications. • Melting range of 50–60 °C with a latent heat capacity of 70 J/g. • Thermal conductivity enhanced up to 0.8 W/m·K via expanded graphite addition • Excellent leakage control, shape stability, and enhanced mechanical stiffness. • Demonstrated suitability for thermal management in electronic devices. Organic Phase Change Materials (PCMs) are essential for passive cooling, yet their poor shape stability and low thermal conductivity limit their integration into advanced manufacturing techniques. Addressing this, we report a novel, high-performance functional composite filament developed for the first time for Additive Manufacturing (AM) via Fused Filament Fabrication (FFF). The composite leverages a non-microencapsulated PCM (fatty acid mixture, T m = 55 °C) confined within a high-density polyethylene (HDPE) matrix, chosen for its low processing temperature (140–160 °C) to prevent PCM degradation. Expanded graphite (EG) was preliminarily vacuum-impregnated with the PCM and subsequently melt-compounded with HDPE, serving as a multifunctional filler to enhance both thermal transport and structural stability. The resulting filament was successfully processed into 3D-printed geometries, achieving a high PCM loading of 36 wt% and a melting enthalpy of 70 J/g. Comprehensive analysis revealed a significantly enhanced thermal conductivity, reaching 0.8 W/m·K, and a Figure of Merit of 0.71 · 10 4 W/(m 2 ·K 0.5 ·s 0.5 ), effectively balancing thermal response with energy density. Crucially, the synergistic architecture formed by the HDPE matrix and the EG functional filler ensures exceptional PCM shape stability and robust leakage resistance, while retaining superior mechanical integrity across the entire temperature range of the phase transition. This work demonstrates a viable and scalable manufacturing route for producing multifunctional, high-loading PCM composites with exceptional potential for thermal management in demanding applications like battery and electronic cooling.