Study on short fiber reinforcement mechanism for mechanical performance improvement of 3D-printed short-continuous carbon fiber reinforced thermoplastic composites

3D printing of continuous carbon fiber reinforced thermoplastic (C-CFRTP) composites is a promising fabricating process to achieve complex lightweight structures within a short time; nevertheless, low mechanical performance between printed layers is always a limitation. Adding short carbon fibers during C-CFRTP fabrication could achieve short-continuous carbon fiber reinforced thermoplastic (S/C-CFRTP) composites with enhanced mechanical properties. However, the underlying reinforcement mechanism is not well understood, which hinders the optimization of 3D printing process for S/C-CFRTP. This paper investigates reinforcing behavior of short carbon fibers and impregnation behavior of the matrix (short carbon fiber reinforced PA6) under various 3D printing conditions. The results indicate that promoting both reinforcing and impregnation behaviors during S/C-CFRTP 3D printing can improve mechanical performance. As short carbon fiber content increases, the mechanical performance initially experiences enhancement caused by the reinforcing behavior of short fibers, followed by a decline attributed to the insufficient impregnation of the molten matrix. Higher nozzle temperatures facilitate the impregnation behavior of the molten matrix, resulting in superior mechanical performance. Moreover, by leveraging the reinforcement mechanism, the optimal parameter combination is proposed to improve tensile, flexural, and interlaminar shear strengths of the 3D-printed S/C-CFRTP to approximately 368.27, 319.80, and 20.09 MPa, respectively.