Interfacial weaknesses in multi-material FFF sandwich structures with TPMS cores: a comparative study of PLA and PLA-CF facesheets

This study examines the mechanical performance and interfacial behavior of 3D-printed sandwich structures with gyroid TPMS cores, comparing single-material panels made entirely of PLA to multi-material counterparts featuring carbon fiber-reinforced PLA (PLA-CF) facesheets. All specimens were fabricated using fused filament fabrication (FFF) with multi-material capabilities, enabling the fully integrated fabrication of core and facesheets in a single process. Mechanical characterization included three-point flexural tests on the sandwich structures and uniaxial tensile tests on the facesheet materials, supported by optical microscopy. Unexpectedly, the PLA sandwiches exhibited approximately 25% higher flexural stiffness (1.20 GPa vs. 0.96 GPa) and peak stress (25.7 MPa vs. 19.7 MPa) than the PLA-CF ones, despite the higher intrinsic stiffness reported for the reinforced polymer. Tensile tests revealed that PLA exhibited a higher elastic modulus (+ 19%), as well as greater tensile strength (+ 35%) and ductility (+ 14%), than PLA-CF, confirming that short-fiber addition led to reduced effective load transfer and increased brittleness. Microscopic analysis revealed well-fused interfaces in the PLA samples, while PLA-CF structures displayed interfacial defects such as voids and fiber pull-out, which acted as crack initiation sites under load. These results demonstrate that weak interfacial bonding in multi-material FFF structures can negate the expected benefits of fiber reinforcement. The findings underscore the critical role of interface quality in multi-material additive manufacturing and provide insight into the limitations of single-process fabrication strategies when combining dissimilar polymers. Improving interfacial adhesion is essential to unlock the full mechanical potential of fiber-reinforced composites in lightweight structural applications.