Thermomechanical Behavior and Bonds-up Mechanisms of Polylactic Acid Layers in Multi-Color Fused Deposition Modeling under Different Printing Parameters

Multicolor fused deposition modeling has attracted growing interest in fabricating visually and functionally customized components; however, filament switching and color-transition interfaces may degrade interlayer bonding and mechanical performance. This study examines the tensile behavior of polylactic acid (PLA) parts produced by mono-color and two-color FDM printing and proposes an optimization strategy for improving multicolor strength. Tensile specimens were fabricated according to ISO 527-2 Type 1B using a Bambu Lab A1 printer. Experimental results show that multicolor PLA exhibits reduced stiffness and strength compared with mono-color PLA: the average tensile strength decreases from about 24,5 MPa to 20,5 MPa and the elastic modulus from roughly 1240 MPa to 1130 MPa, while elongation at break increases markedly, indicating modified deformation and failure mechanisms associated with color-transition interfaces. To enhance tensile strength, a Taguchi L27 (3⁴) design was employed to evaluate the effects of layer thickness, nozzle temperature, printing speed, and infill density. Signal-to-noise ratio analysis based on the larger-is-better criterion identified low layer thickness, high nozzle temperature, low printing speed, and moderate infill density as the optimal parameter combination. Analysis of variance revealed that layer thickness is the dominant factor, accounting for approximately 76% of the variation in tensile strength. In addition, a second-order regression model was developed, yielding a high coefficient of determination (R² ≈ 0,97) and demonstrating good agreement between experimental and predicted results. These findings provide practical guidance for improving the mechanical performance of multicolor FDM-printed PLA parts.