Hot-end co-extrusion of multiple thermoplastic filaments for unblended composite additive manufacturing

Additive manufacturing (AM) techniques can print multiple materials to realise three-dimensional products with increased complexity not only in their shapes but also in their mechanical behaviour. Here, we study hot-end co-extrusion printing of multiple thermoplastic filaments. By co-extrusion printing of different filaments in bi- and tri-material configurations from a single printing platform, we demonstrate that versatile material behaviours and functionalities can be realised. In particular, rigid thermoplastic filaments can be mechanically boosted to exhibit fracture-free elongation via co-extrusion with a soft elastomer. For example, the widely used polylactic acid (PLA) and acrylonitrile butadiene-styrene (ABS) filaments can be co-axially extruded with thermoplastic polyurethane (TPU) filaments to enhance their toughness by 34-fold and 30-fold, respectively. During hot-end co-extrusion, the two materials do not mix but are still coupled by strong hydrogen bonds at their interface, thereby enabling mechanical behaviour not observed in conventional blended composites. We further utilise our hot-end co-extrusion 3D printer to manufacture and demonstrate customised products, such as fracture-free mechanical structures, personalised functionally graded body protection gears, and highly repeatable and rapidly actuatable soft actuators. The unblended, locally tuned pneumatic actuator can lift an object that weighs 12.6 times heavier than its own weight at 17.7% contraction ratio.