On the critical role of part thickness of PLA+flax composites and infill strategy on mechanical performance and energy efficiency in fused filament fabrication

Abstract Composite structures are increasingly being used across a wide range of industries for applications that require optimised mechanical performance and reduced weight. As the demand for innovative materials grows, research into composite structures manufactured through additive manufacturing is gaining increasing attention, especially with the fused filament fabrication (FFF) technique. In this context, PLA+flax composites printed by FFF are gaining attention for their enhanced mechanical properties and sustainability under specific circumstances. A critical factor affecting the mechanical performance of PLA+flax composites is part thickness, particularly in terms of low-velocity impact resistance. Thicker parts generally offer higher impact resistance and structural integrity, but the challenge for FFF parts lies in optimizing thickness alongside other variables, such as infill strategy. Although several studies have investigated the influence of infill topology or mechanical behaviour of FFF biocomposites, fewer works have simultaneously examined the combined effects of thickness, infill architecture and printing energy consumption for PLA/flax systems. This study demonstrates that increasing thickness does not necessarily improve mass-normalised mechanical performance nor energy-normalized efficiency in FFF PLA+flax composites unless coupled with infill architectures capable of efficient shear transfer. Among the investigated configurations, the 6 mm gyroid structure exhibited the highest penetration energy (16.07 J), slightly outperforming the fully solid configuration despite a lower material fraction. Furthermore, the volumetric specific energy consumption decreased from approximately 66.9 kJ/cm 3 for 2 mm specimens to 25.9 kJ/cm 3 for 6 mm specimens, indicating a substantial improvement in manufacturing efficiency with increasing thickness. Overall, gyroid and hexagonal architectures provided the most favourable balance between impact resistance, lightweight efficiency and energy consumption.