The mechanical performance of fibre-reinforced 3D-printed composites is highly dependent on slicing and processing parameters, yet a systematic understanding of these effects remains limited. This study aims to systematically evaluate the influence of infill pattern, infill density, bed orientation, and layer thickness on the tensile properties of short carbon fibre-reinforced Nylon (Onyx) and unreinforced Nylon fabricated using Markforged 3D printers. Tensile testing was conducted to assess tensile modulus, ultimate tensile strength, elongation at break, and Poisson’s ratio across varying slicing parameters. Various raster angles (0°, 90°, 45° and −45° relative to the x-axis), three infill patterns (rectangular, triangular, hexagonal), and multiple densities (17 %–92 %) were examined to assess their influence on mechanical behaviour. Scanning electron microscopy (SEM) and fibre volume fraction ignition testing were used to quantify void content and fibre distribution in Onyx composites. Furthermore, the Rule of Mixtures (ROM) was applied and demonstrated strong agreement with experimental results, providing a predictive framework for tensile performance across different infill densities. The findings of this study contribute to the optimisation of fibre-reinforced additive manufacturing by identifying key parameters that enhance mechanical properties, supporting structural applications in aerospace, automotive, and lightweight engineering systems.
