Current large-scale additive manufacturing extruders use short reinforcing fibre-filled thermoplastic pellets, which marginally improve the mechanical performance but are too short to exploit the strength of the fibres fully. Long fibre pellets have much higher potential strength, and the development of higher strength materials could create stronger parts and/or reduce the mass of products, lowering their energy consumption in the case of vehicular parts of heated tooling.

By developing long-fibre thermoplastic pellet processing, this thesis works toward improving the mechanical performance of fused granulate fabricated parts. Extreme die-swell was encountered when extruding long glass-fibre polypropylene pellets. This die-swell is theorised to be caused by energy storage in an entangled network of long fibres, which is released upon extrusion. This issue was solved by developing a mix of two different material pellets, which was used with the tactical use of temperature zones to create a hetero-phasic blend inside the extruder. This technique and material blend enable controlling the melting of the long fibre pellet resin, lubricating the pellets to promote macro-alignment, and reducing heating through shear friction. These effects delay the dispersion and entangling of fibres.

This method eliminated the problem encountered; reducing porosity by 82%, increasing strength by 960% over the original swollen material and achieving specific strength 16% higher than the short fibre compound currently being used. This research presents a new method to make previously un-processable long fibre thermoplastic pellets useable with a 25 mm diameter screw extruder. It contributes to the development of unprecedented high-performance parts 3D printed at a large scale.