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Extrusion width, the width of printed filaments, affects multiple critical aspects in mechanical properties in material extrusion additive manufacturing: filament geometry, interlayer load-bearing bonded area and fibre orientation for fibre-reinforced composites. However, this study aims to understand the effects of extrusion width on 3D printed composites, which has never been studied systematically.

Four polymers with and without short-fibre reinforcement were 3D printed into single-filament-wide specimens. Tensile properties, mechanical anisotropy and fracture mechanisms were evaluated along the direction of extruded filaments (F) and normal to the interlayer bond (Z). Extrusion width, nozzle temperature and layer height were studied separately via single-variable control. The extrusion width was controlled by adjusting polymer flow in the manufacturing procedure (gcode), where optimisation can be achieved with software/structure design as opposed to hardware.

Increasing extrusion width caused a transition from brittle to ductile fracture, and greatly reduced directional anisotropy for strength and ductility. For all short fibre composites, increasing width led to an increase in strain-at-break and decreased strength and stiffness in the F direction. In the Z direction, increasing width led to increased strength and strain-at-break, and stiffness decreased for less ductile materials but increased for more ductile materials.

The transformable fracture reveals the important role of extrusion width in processing-structure-property correlation. This study reveals a new direction for future research and industrial practice in controlling anisotropy in additive manufacturing. Increasing extrusion width may be the simplest way to reduce anisotropy while improving printing time and quality in additive manufacturing.

This study/paper aims to develop fundamental understanding of mechanical properties for multiple fibre-reinforced materials by using a single-filament-wide tensile-testing approach.

In this study, recently validated single-filament-wide tensile-testing specimens were used for four polymers with and without short-fibre reinforcement. Critically, this specimen construct facilitates filament orientation control, for representative longitudinal and transverse composite directions, and enables measurement of interlayer bonded area, which is impossible with “slicing” software but essential in effective property measurement. Tensile properties were studied along the direction of extruded filaments (F) and normal to the interlayer bond (Z) both experimentally and theoretically via the Kelly–Tyson model, bridging model and Halpin–Tsai model.

Even though the four matrix-material properties varied hugely (1,440% difference in ductility), consistent material-independent trends were identified when adding fibres: ductility reduced in both F- and Z-directions; stiffness and strength increased in F but decreased or remained similar in Z; Z:F strength anisotropy and stiffness anisotropy ratios increased. Z:F strain-at-break anisotropy ratio decreased; stiffness and strain-at-break anisotropy were most affected by changes to F properties, whereas strength anisotropy was most affected by changes to Z properties.

To the best of the authors’ knowledge, this is the first study to assess interlayer bond strength of composite materials based on measured interlayer bond areas, and consistent fibre-induced properties and anisotropy were found. The results demonstrate the critical influence of mesostructure and microstructure for three-dimensional printed composites. The authors encourage future studies to use specimens with a similar level of control to eliminate structural defects (inter-filament voids and non-uniform filament orientation).