A review on material extrusion (MEX) of polyethylene - Challenges, opportunities, and future prospects

Polyethylene (PE) is a widely used thermoplastic valued for its low cost, chemical resistance, recyclability, and good mechanical properties. PE makes up about one-third of the global plastic production. Recent advances in the field of additive manufacturing, especially material extrusion (MEX), have initiated interest in incorporating PE. However, using PE in such applications is challenging due to the polymer's semicrystalline nature, which causes significant shrinkage when cooled and limits chain diffusion, affecting the adhesion between the deposited layers. PE's non-polar character can also reduce the adhesion of the 3D-printed part to the printing bed. This paper analyzes the current progress in using PE in MEX, attempts to correlate PE's thermo-rheological properties with its 3D printability, and discusses the challenges associated with PE within the nozzle, post-deposition on the bed, and in the stand-off region between them. Additionally, the paper highlights the strategies to overcome these challenges to improve PE's printability in MEX by modulating the physicochemical properties of PE via incorporating fillers and nanofillers and blending with other polymers. Finally, future research directions are proposed to aid the design of PE-based feedstocks, which could also enhance plastic recycling.