Mechanical performance optimisation of 3D printed PLA carbon fibre for ankle foot orthosis applications

Abstract Ankle–foot orthoses (AFOs) are widely used assistive devices that improve gait and stability in patients with lower limb disorders. However, conventional manufacturing methods are often time-consuming and offer limited flexibility for patient-specific designs. This study investigates the optimisation of fused deposition modelling (FDM) process parameters to enhance the mechanical performance and surface quality of polylactic acid carbon fibre (PLA-CF) for orthotic applications. A Taguchi L9 experimental design was applied to evaluate the effects of layer thickness (0.1–0.3 mm), nozzle temperature (200–220 °C), printing speed (30–90 mm/s), and raster orientation (0°, ± 45°, 90°). The optimal parameter combination consisted of a 0.2 mm layer thickness, 210 °C nozzle temperature, 60 mm/s printing speed, and ± 45° raster orientation, experimentally achieving a maximum compressive strength of 81 ± 1 MPa and a minimum surface roughness of 7.43 μm. Differential scanning calorimetry (DSC) revealed that crystallinity increased from 1.59% in the as-printed condition to 30.41% after annealing. Among the annealing conditions investigated, treatment at 100 °C for 2 h achieved the highest compressive strength of 91 ± 1 MPa. Finite element analysis (FEA) of AFO models with thicknesses of 4, 6, 8, and 10 mm identified the 8 mm design as the optimal configuration, providing the best balance between weight, cost, and mechanical performance, with a maximum stress of 11.39 MPa, deformation of 8.9 mm, and a safety factor of 3.68. These findings demonstrate that FDM optimisation can significantly improve PLA-CF orthoses, providing a cost-effective and customisable framework for personalised orthopaedic applications.