Structural adhesive bonding between layers in additive manufacturing composites

• Pull-out tests confirmed that carbon fibre pre-impregnation is essential for improvement of fibre–matrix adhesion. • Two polymer types were determined in a composite sample, originating from two different parts of the manufacturing process: pre-impregnated polymer that covers carbon fibre and 3D-printing polymer that fills the space between carbon fibres in layers and between layers. • The adhesive joint between those two polymer types is crucial for the mechanical strength of the whole sample after manufacturing and exposure to temperature (sub-zero and elevated). • A post-processing of the samples can significantly increase the quality of the adhesive joints in the material, as after exposure to elevated temperature, no voids were observed in the material, and all spaces between carbon fibres were filled by the polymer. In additive manufacturing (AM) structures, the quality of adhesive bonding between consecutive printing lines and layers is crucial for the structural integrity and mechanical performance of the manufactured elements. This paper analyzes the quality of adhesive bonding in AM carbon fiber-reinforced polymer (CFRP) produced using a modified material extrusion process (MEX) developed by Kaunas University of Technology. The modification replaces two separate printing heads (for polymer and fiber filaments) with a single head to enhance structural integrity through improved interlayer adhesion, and it also introduces a dedicated fiber preparation procedure prior to printing that affects adhesive bonding quality. Scanning electron microscopy and terahertz spectroscopy were used to evaluate the composites both immediately after manufacturing and after exposure to temperature variations (elevated and sub-zero), and tensile testing was performed on the samples. The analyses focused on the quality of adhesive bonding between printing lines (impregnated carbon fibers) and consecutive layers. The results show that the pre-impregnation process substantially improves the quality of the adhesive joint between the fiber and the matrix. In addition, two polymer types were identified in the AM-produced CFRP: the first, originating from pre-impregnation, covers the carbon fiber, while the second originates from the 3D printing process.