Crystallization Behavior of Recycled Semi-Crystalline Polymers in 3D Printing: Progress, Challenges, and Opportunities

In recent years, plastic recycling has emerged as a critical concern in environmental protection and waste management. Among the various techniques for repurposing plastic waste into valuable products, extrusion of filaments for 3D printing has proven to be a highly effective method. A thorough understanding of the crystallization behavior of recycled plastics used in 3D printing is essential, as it significantly influences their final performance. This review provides an in-depth analysis of the crystallization behavior and crystallinity of recycled semi-crystalline polymers, with particular emphasis on recycled commodity plastics such as recycled polyethylene terephthalate (rPET), recycled polypropylene (rPP), and recycled high-density polyethylene (rHDPE). Recent research published between 2015 and 2025 was systematically synthesized and provides information on sources of plastic waste, additives employed, and recycling processes involved, with the findings summarized in a table that highlights their effects on polymer crystallinity. Furthermore, the key factors impacting the crystallinity of 3D-printed recycled plastics were examined, including the influence of additives, multiple processing cycles, printing parameters, and thermal treatments. Research gaps and the challenges faced during the printing process were also identified and discussed. By consolidating recent findings, this review provides a comprehensive understanding of the crystallization behavior of recycled plastics in 3D printing, thereby providing guidance for future research and developing strategies to optimize the performance of these materials.