Biopolyamide composites for fused filament manufacturing: impact of fibre type on the microstructure and mechanical performance of printed parts

As additive manufacturing (AM) becomes more widespread in ever more demanding applications, the performance demands on printed parts are increasing. Efforts are directed towards improving mechanical performance in all manufacturing directions, including requirements such as sustainability, economic viability, and weight savings. This work focuses on the systematic study of printed parts by manufacturing fused filaments fabrication (FFF) of a bio-based polyamide (PA11) reinforced with different types and amounts of fibres: short glass fibre (GF) and a needle-shaped nanofibre: sepiolite (SEP). The aim was to establish which of these two had the best balance between improving mechanical properties and forming intra- or interrater defects. The surprising results revealed that the different morphologies of these fillers induce two opposite stiffening mechanisms and defect microstructure. In the case of SEP, a change in the crystalline polymorph, a higher crystallisation rate and the elevated dispersion of high and constant surface area fibres increase the stiffness at a lower effective load. Additionally, nanocomposites possess lower percentage porosity with more isotropic and smaller average inter-raster pores compared to GF composites. The latter are stiffened only by the immobilisation effect of the confined polymer chains in a system with a high dispersion of fibre sizes with heterogenous and intrarasterised defects. By contrast, these morphologies provide the GF composites with a more effective energy dissipation mechanism in impact tests and higher thermal stability. Graphical abstract