Multi-material 3D printing: The relevance of materials affinity on the boundary interface performance

Multi-material extrusion in 3D printing is gaining attention due to a wide range of possibilities that it provides, specially driven by the commercial availability of a large variety of non-conventional filament materials. As a result, one can print models that are not limited to aesthetics purposes but can now also provide larger functionality, and therefore with mechanical performance tuned according to their purpose. With this in mind, this paper addresses the mechanical performance of multi-material printed objects, specially focused on the interface zone generated between the different materials at their geometrical boundaries. Tensile test specimens were designed and printed in three types: (A) a single-material specimen printed by a single extrusion head; (B) a single-material but multi-section specimen printed in a zebra-crossing structure by two extrusion heads; and (C) a multi-material specimen printed with two materials in a zebra-crossing pattern. The materials considered were PLA, TPU and PET. The comparison of the mechanical performance between Type-A and -B specimens demonstrated the negative influence of the presence of a geometrical boundary interface between the same material. On the other hand, the comparison between Type-B and -C demonstrated how the previous performance loss was yet more drastic when the lack of chemical affinity between the materials was present. The methodology proposed to assess the quality of the pairs of materials selected is innovative, and enabled to depict the importance of the boundary design in multi-material printing techniques.