Extrusion width critically affects fibre orientation in short fibre reinforced material extrusion additive manufacturing

This study develops new understanding of fibre orientation distribution in material extrusion additive manufacturing of short fibre reinforced polymer composites. Short carbon fibre reinforced polylactic acid was 3D printed at several nozzle temperatures (Tn), printing speeds (V), and extrusion widths set in the GCode (Wset), to identify the effects of these parameters on fibre orientation. Microscopic characterisation of the top surface of 3D printed filaments allowed detailed analysis of fibre orientation including dependencies on extruded filament width and printing parameters. The printing parameters directly affected measured width of extruded filaments and a wider extrusion resulted from increased Tn, increased Wset, and decreased V. An assumption of normal distribution appropriately represented the planar fibre orientation distribution. A direct relationship was found between fibre orientation and extrusion width: fibres were more highly oriented along the printing direction when the extruded filament was narrower. This is logical because when extruded material spreads laterally to the printing direction - to achieve wider extrusions - the polymer-fibre-composite melt flow is less aligned to the printing direction. A consistent relationship between fibre orientation and extrusion width was found even when printing conditions varied several-fold (4-fold change in V; 2-fold change in Wset; 20 °C change in Tn), highlighting the dominant influence of the extruded-filament geometry. This relationship also existed for considerably different 3D printing hardware and a different polymer (acrylonitrile butadiene styrene). A case study for tensile modulus of three different short-carbon-fibre-reinforced polymers, each with three different extrusion widths, showed how the findings related to fibre orientation can explain variation in mechanical properties for multiple materials. The results of this study allow more in-depth understanding and analysis of processing-structure-property correlations. They also highlight the crucial role of extrusion width and fibre orientation as well as the importance of characterising both direct and indirect effects of printing parameters.