Facile manufacturing strategy for origami structure of thermoplastic elastomer foam with gradient stiffness by 3D printing

A facile in-situ foaming technology for 3D printing was proposed based on thermoplastic polyurethane (TPU) /thermally expandable microsphere (TEM) systems, which resulted in the formation of both macroscale origami structures with gradient stiffness and uniformly distributed microscale cells in the TPU. The precise fabrication of the printed TPU/TEM foam was investigated by adjusting the printing process parameters, such as printing speed, printing temperature, and extrusion flow rate, with reduced layer-to-layer pore size and porosity owing to in situ expansion and interlayer bonding. With the addition of 4 % TEM, the foam exhibited a microstructure with an average cell size of 76.7 μm, cell density of 5.6 × 10⁶ cells/cm³, and overall printed foam density of 0.32 g/cm³. More interestingly, the 3D printed foamed TPU/TEM system with hierarchical structure exhibited higher densification strain and energy absorption efficiency than the general 3D printed structure. The introduced microscopic cells and their yield deformation after full densification of the macroscopic structure could be the dominant factors for the enhanced energy absorption performance. Based on in situ foaming additive manufacturing, the origami structure with gradient stiffness of TPU/TEM systems can be easily designed by controlling the layers with different TEM contents. Further investigation of the compressive energy-absorption behavior of the origami structures with gradient stiffness showed multi- energy absorption plateaus corresponding to the strength of the printed material in the order of material strength from low to high, thereby achieving controlled yield deformation behavior. Furthermore, the TPU/TEM foam with a hierarchical structure also has the benefit of low thermal conductivity, which was only 0.037 W∙m⁻¹∙K⁻¹ at room temperature. Combined with the advantages of 3D printing personalization, thermoplastic elastomers with thermally expandable microspheres can be proposed as a facile strategy for manufacturing energy-absorbing structures with multi-designability, hierarchical structure, gradient stiffness, and tunable deformation behavior.