Owing to the superior mechanical properties and moldability of carbon fiber reinforced thermoplastics (CFRTP), they have been increasingly adopted in various applications. Injection molding is a typical method for fabricating CFRTP, however, this method requires a mold, and hence it is difficult to use this method to fabricate complex geometries. Meanwhile, 3D printing is suitable for fabricating complex geometries and for optimizing the structure. Recent advancements in 3D printing technology have enabled printing of composite materials that have arbitrarily curved continuous reinforcements. Previous studies have revealed that a fiber bundle becomes twisted in a curved section, and this might affect the mechanical properties of 3D printed CFRTP. In the present study, the mechanism of twisting a fiber bundle in a 3D printed CFRTP is investigated. In addition, fiber fracture in a curved section is evaluated by measuring the electrical resistance of a fiber bundle. As a result, it has become clear that the twisting of a fiber bundle is caused by the flattening of the fiber bundle, the adhesive force on the print bed, and tensile load from the print nozzle. Moreover, the electrical resistance measurements verified the fracture of fibers in a curved section.
