Additive manufacturing by fused filament fabrication (FFF) is widely used for producing polymer components, but its layer-wise deposition typically results in surface roughness that accelerates fatigue crack initiation and reduces durability. Improving surface quality is therefore essential for extending the service life of printed parts. This study investigates how the ironing process, an integrated smoothing operation in FFF, affects surface roughness and tensile fatigue performance of polylactic acid (PLA), and whether ironing parameters can be optimized to enhance fatigue life. PLA fatigue specimens were fabricated according to ASTM D7791, and three ironing parameters were varied: ironing speed (20, 50, 80 mm/s), ironing line spacing (0.1, 0.2, 0.3 mm), and ironing flow (5, 10, 15 %). A Taguchi L9 orthogonal array was used to design experiments. Surface roughness (Ra) was measured using a contact profilometer, tensile strength was obtained from static tests, and high-cycle tensile fatigue tests were conducted at 10 Hz with loads ranging from 225 to 2250 N. ANOVA and signal-to-noise (S/N) analyses were performed to determine factor significance and identify the optimal parameter combination for minimizing roughness. Measured Ra values ranged from 0.75 to 6.30 µm, demonstrating a strong influence of ironing conditions on surface quality. ANOVA revealed that ironing speed contributed ∼71 % of total variation in Ra, followed by line spacing (∼11 %) and flow (∼4.6 %). Specimens with smoother surfaces exhibited superior fatigue behavior, with fatigue life ranging from approximately 10,700–20,400 cycles. A negative correlation between Ra and cycles-to-failure (R² ≈ 0.60) confirmed that reduced roughness delays crack initiation and extends fatigue life. Overall, the study shows that optimized ironing is an effective, low-cost method to improve the fatigue resistance of PLA FFF parts. The work quantifies the relationship between ironing parameters, surface quality, and fatigue performance, providing practical parameter guidelines and contributing to fatigue-based design strategies for additively manufactured polymers.
