Investigation of 3D printing parameters on cooling rate, crystallinity, and tensile properties of recycled polyethylene terephthalate cooling rate, crystallinity, and tensile properties of recycled polyethylene terephthalate

Abstract

The successful implementation of fused filament fabrication (FFF) 3D printing using recycled plastics requires a deep understanding of the thermal behavior of the plastics throughout the printing process. This study investigated the influence of wall thickness of the printed sample, nozzle temperature, and cooling fan speed during 3D printing on the cooling rate, crystallinity, and tensile properties of recycled polyethylene terephthalate (rPET). The experimental process commenced with the collection of discarded rPET bottles, followed by thorough cleaning and washing to remove any adhesives and contaminants. Afterward, the bottles were cut and ground into flakes and then converted into filaments using a single-screw filament extrusion process. In-situ thermal analysis was conducted by integrating an infrared (IR) thermal camera into the 3D printing setup to monitor real-time temperature changes during the printing process. Results revealed that cooling rates increased markedly with reduced wall thickness, rising from 17.53 °C/min for the 3.6 mm wall thickness to 62.92 °C/min for the 1.2 mm wall thickness. Nozzle temperature exhibited a non-linear influence, with the highest cooling rate of 65.47 °C/min recorded at 240 °C, while enhanced cooling fan speed (100%) further accelerated cooling to 45.00 °C/min. Differential scanning calorimetry (DSC) and Raman spectroscopy confirmed that a slower cooling rate generally promoted crystallinity, which was observed in thick-walled and low-cooling speed prints. Tensile testing demonstrated a strong correlation between crystallinity and tensile performance, with ultimate tensile strength (UTS) reaching 55 MPa at 240 °C and 54.8 MPa at 25% cooling fan speed, outperforming previously reported rPET values. The use of rPET in FFF and the findings of this study contribute to the further exploration of rPET's potential in sustainable additive manufacturing practices.