Comparative Effects of Thermal Annealing and UV Treatment on the Tensile Strength of 3D-Printed PLA-Carbon Fibre Composites: A Design-of-Experiments Approach

Abstract

Fused Deposition Modelling (FDM) offers substantial advantages in terms of manufacturing flexibility, material efficiency, and cost; however, its broader use in load-bearing applications remains constrained by process-induced voids and weak interlayer bonding. Short carbon fibre-reinforced polylactic acid (PLA-CF) can partially address these limitations, although the mechanical performance of printed composites may be further modified through appropriate post-processing. Despite increasing interest in post-processing strategies, direct experimental comparisons between thermal annealing and ultraviolet (UV) treatment under controlled printing conditions remain limited, particularly for short-carbon-fibre-reinforced PLA. This study therefore compared the effects of thermal annealing and UV treatment on the ultimate tensile strength (UTS) of FDM-printed PLA-CF containing 10 wt.% short carbon fibre. Two independent 3² full-factorial experiments were conducted using ASTM D638 Type IV specimens fabricated under identical printing conditions. The effects of the post-processing parameters were evaluated using descriptive statistics, analysis of variance (ANOVA), and regression modelling. Untreated specimens exhibited a mean UTS of 31.55 ± 2.43 MPa. Thermal annealing at 120 °C for 60 min produced the highest UTS of 36.53 MPa, corresponding to a 15.78% improvement over the untreated condition, with annealing temperature exerting a statistically significant effect on UTS (p < 0.001). UV treatment achieved a maximum UTS of 35.01 MPa at an exposure time of 40 min and a lamp distance of 8 cm, representing a 10.97% improvement, with both exposure time and lamp distance significantly affecting the tensile response (p < 0.01). Annealing produced a numerically higher and less variable UTS than UV treatment at their respective best-performing settings. However, the direct difference of 1.52 MPa was not statistically significant, indicating that independent confirmation with a larger sample size is required.. These findings demonstrate that post-processing route and treatment parameters play distinct roles in controlling the tensile performance of FDM-printed PLA-CF and provide a quantitative basis for selecting post-processing conditions for mechanically demanding printed components.