3D printing of continuous carbon fiber reinforced thermoplastic composites has gained significant attention for its flexibility in fiber path design and mold-free manufacturing. However, weak interfacial bonding remains a key constraint on mechanical performance. This research examines the in-situ shear behavior of co-extruded carbon fiber/polyamide (PA) composites using three binder materials: PA6/PA66 blend, short carbon fiber-reinforced PA6, and short glass fiber-reinforced PA6, which were co-extruded with continuous carbon fiber/PA6 prepreg to produce single-bead specimens. The study analyzes the effects of printing temperature, speed, layer thickness, and prepreg volume fraction on interfacial failure mode and shear strength. Results demonstrate ductile failure with plastic deformation. Short fibers enhanced interfacial properties by inhibiting crack propagation. Increasing printing temperature, reducing speed, and raising prepreg content from 7% to 16% all improved shear strength, peaking at 35.13 ± 1.36 MPa with 0.6 mm layer thickness. These findings provide valuable insights for optimizing material selection and process parameters in 3D printed composites.
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