Mechanical Property Improvement of 3D-Printed rPP and vPP through Heat Treatment and Sustainable Filament Reuse

This study investigates the mechanical property enhancement of 3D-printed virgin and postconsumer recycled polypropylene (vPP and rPP) structures through systematic heat treatment. ASTM D638 specimens were subjected to cryogenic cooling (0 °C), room temperature, isothermal heating at 60 and 80 °C, and ramped heating between 60 and 110 °C to assess the influence of thermal postprocessing on porosity, tensile strength, and failure mechanisms. Results revealed a consistent decrease in porosity with increasing temperature. Yield strength demonstrated a bell-shaped trend, with maximum yield strength recorded at 60 and 80 °C for vPP and rPP, respectively. Beyond these temperatures, thermal degradation led to a decline in mechanical performance. Ramped heating further enhances stress relaxation and interfacial bonding, improving structural integrity without inducing embrittlement. Heat treatment led to a transition from brittle to ductile failure, while uncertainty analysis validated the repeatability of mechanical behavior, particularly for rPP. This work contributes to the circular economy by enabling the reuse of postconsumer recycled PP in moderately loaded, noncritical components, semistructural applications, or eco-efficient applications where moderate strength is sufficient, thereby reducing environmental impact and material dependency.