This study investigates the effect of repeated recycling processes on the properties of PLA matrix 3D printed filaments mixed with 20 wt.% Fe₃O₄ magnetic particles. The recycling process of the composite was simulated by using extrusion and manual cutting to achieve multiple closed-loop recycling processes. The research studied the 1st, 3rd, 5th, and 6th cycles, while the initial batch virgin materials (0 cycles) was used as a baseline.

Visual observations revealed that surface defects increase with each cycle, along with diameter inconsistencies and brittleness. The analysis showed that the 5th cycle had the highest diameter variation (range = 2.37 mm, standard deviation = 0.44 mm). The 6th cycle was unusable filament with a disoriented shape and chaotic flow behavior, which made it impossible to inject or print into the 3D printing machine. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis indicated significant morphological changes, agglomeration of magnetic particles, and content reduction along cycles. GPC analysis confirmed that the molecular weight of the PLA decreased and that the polydispersity increased, while NMR identified the absence of dichloromethane as a solvent, confirming that the degradation indicated by PLA molecular weight reduction was solely caused by thermomechanical reprocessing effects.

The findings suggest that PLA/Fe₃O₄ filaments can be processed up to five times, but the practical usability of the reprocessed filament is limited due to surface defects and poor filament diameter consistency. During the first cycle, the filament frequently stuck in the 3D printer feeder due to diameter inconsistencies, and by the third cycle, attempts to print resulted in improperly shaped parts, highlighting significant processing challenges. Additionally, repeated processing cycles lead to loss of magnetic particles, significantly affecting the filament’s functionality. Characterization using the Vibrating Sample Magnetometer (VSM) demonstrates a reduction in magnetic properties, with a 28.62% decrease in Cycle 3, a 51.41% decrease in Cycle 5, and a 73.91% decrease in Cycle 6 compared to Cycle 1. Moreover, the thermal stability of PLA is also compromised, as evidenced by the decrease in degradation temperature recorded in the Thermogravimetric Analysis (TGA), highlighting the declining performance of the filament after multiple recycling cycles. Further improvements in processing methods, material quality, and testing are recommended to enhance recyclability and ensure consistent print results in future applications.