Commercially available three-dimensional (3D) printers for continuous-fiber composites feature twin nozzles (one for short fibers (or resin) and the other for continuous fibers). When designers use nonuniformly curved patterns for continuous fibers, inevitable gaps between printing paths cause initial defects. To address this challenge, we developed 3D printer hardware with coaxially arranged nozzles for continuous and short-fiber filaments. This arrangement allows the adjustment of the amount of the short-fiber (or resin) along the continuous-fiber path, preventing print path gaps. The short-fiber nozzle uses a screw and pellets instead of filaments, with a hole at the center of the screw for printing continuous fibers. After modifying the printing conditions, we fabricated unidirectional specimens using the proposed 3D printer and conducted tensile tests, which confirmed its effectiveness for printing continuous-fiber composites.

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Snap-lock fasteners are very simple for locking two parts. However, because their configuration is complex, snap-lock fasteners are usually made of polymer materials. This implies that the material is usually weak. In this study, we propose a novel snap-lock fastener printed using a 3D printer that prints continuous fiber composites. The novel fastener designed in this study can be easily attached to and detached from fastened parts. Based on the tensile test results, continuous glass fibers were placed at the elastic hinges to strengthen the weak elastic hinges. The reinforced snap-lock specimens were tested under tensile loading. Therefore, the novel snap-lock fastener with continuous glass fiber was able to bear 1.7 times higher load than that without continuous glass fibers.

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The present study experimentally investigated the compressive strength of 3D-printed continuous carbon fiber composites. Two types of specimens were fabricated: type A was printed with the specimen thickness direction as the layup direction, and type B was printed with the specimen width direction as the layup direction. In previous research, these specimens were printed without surface layers (such as walls, floors, and roofs) using short carbon-fiber composite filaments, resulting in approximately 20% lower compressive strength compared with the Markforged datasheet. In this study, the specimens were printed with surface layers. The compressive strength showed results almost identical to those in the Markforged datasheet. To investigate the effect of the surface layers, the fiber waviness of the specimens was examined using X-ray computed tomography. The results indicated that the surface layers exhibited lesser waviness, leading to higher compressive strength.

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This paper deals with the bearing strength of bolted joints. To reduce structural weight, low-infill-rate structures are usually adopted for robot arm structures. Although many researchers have reported the bearing strength of laminated composites, the bearing strength of low-infill-rate 3D-printed structures has not been reported. This paper presents a new bearing strength test method applicable to thick structures. Using the new jigs, the effects of the thickness, twin hole effect, and rotation of the print axis on the plastic yielding strength were experimentally investigated using short carbon fiber/PA-6 composites. As a result, the thickness of the specimens had no effect on the strength, the twin hole reduced the strength by 36%, and the rotation of the print axis increased the strength by 30%.

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In recent years, it has been expected that conventional parts used in industrial robots can be replaced with 3D printed materials to reduce weight and improve performance. However, it is not easy to produce large parts in one piece using a 3D printer. This is due to the problem of thermal shrinkage and the fact that the size of the part depends on the bed size of the 3D printer. Therefore, it would be highly versatile if small parts could be joined easily and without loss of strength to form large parts. We proposed a new joining method by mating and bonding square pyramidal concavo-convex parts, and evaluated its strength by three-point bending tests. As a result, strength of about 140% was obtained compared to bulk materials of the same size. Comparisons were also made with different shapes and materials to demonstrate the effectiveness of the proposed joining method.

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A new 3D printer hardware with coaxially arranged twin nozzles was developed to print continuous and short-fiber filaments. The short-fiber nozzle used a screw and pellets instead of filaments, and a hole was drilled at the center of the screw to print continuous fibers. The new coaxial 3D printer showed a 7% void area and low strength in the lay-up direction. Thus, in this study, a novel nozzle cap was designed to improve the performance of the aforementioned 3D printer. The nozzle cap transferred heat from the heat block to the printed path by extending the rim area of the nozzle edge. Printing tests were performed, and the void ratio was reduced to 2%. Interlaminar shear test results indicated that the interlaminar strength improved by 2.3 times.

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