Screw extrusion additive manufacturing of elastomers and high-performance thermoplastics

Abstract

Fused granulate fabrication (FGF) is an emerging material extrusion technology for additive manufacturing which offers a lower cost and more versatile solution over the established fused filament fabrication (FFF) techniques. FGF uses granulates instead of filament which area cheaper and enable the use of softer materials which would not be processable with FFF. Despite this, no small-scale FGF systems exist which are capable of reliably extruding high-performance thermoplastics and research on very soft TPEs remains limited. The aim of this thesis was to address this gap by developing a lightweight, simple, and small-scale screw granulate extrusion system intended for additive manufacturing of niche elastomeric and high-performance thermoplastic materials. Extruder development involved four iterations with Extruder 1 successfully 3D printing ABS and then subsequent iterations optimised extruder performance to 3D print thermoplastic polyolefin (TPO), TF3ZG0-LCNT, polyetherimide (PEI), and polyether ether ketone (PEEK) (Extruders 2-4). The research also aimed to establish optimal processing parameters to additively manufacture products using these materials and then to test and investigate the resulting material properties and case study part’s performance. The methodologies used in this study involved material characterization using multiple spectroscopy techniques, thermal analysis, mechanical testing, and imaging. Results showed that TPO is a good alternative to the popular thermoplastic polyurethane (TPU) as it does not require pre-drying which simplifies material handling for 3D printing. TF3ZG0-LCNT, a super soft TPE was successfully 3D printed, setting a record as the softest TPE yet, with uniaxial strains exceeding 3365% and biaxial strains up to 590%. PEI, an amorphous high-performance polymer demonstrated viability for rapid tooling, producing injection molding (IM) inserts capable of 14 IM cycles, albeit part complexity influenced performance. The mechanical properties of PEEK products obtained via FGF were improved by using high molecular weight grades and elevated processing temperatures. This achieved improved layer bonding including a stress at failure of 72.4 MPa in the XY plane. The system’s effectiveness was also validated by producing specialized parts including a child orthotic insole (TPO), elastic inflatable actuators (TF3ZG0-LCNT), injection mould inserts (PEI), and mechanical components (PEEK). This research advances FGF technology, enabling the additive manufacturing of specialized materials using a small-scale system, thus reducing associated costs and making the technology available to a wider user base and application field.