3D printing for manufacturing medical devices

Abstract

Additive manufacturing (AM) enables the fabrication of complex, customised components for high-value applications in aerospace and biomedical engineering. However, the use of high-performance polymers (HPPs) such as polyetheretherketone (PEEK) in fused filament fabrication (FFF) remains limited by rapid crystallisation during cooling. This crystallisation is expected to restrict interlayer chain diffusion, promote anisotropy, and compromise mechanical reliability. Addressing these challenges requires strategies that balance crystallisation kinetics with molecular mobility during deposition.

This thesis investigates the potential of blending semi-crystalline PEEK with amorphous polyetherimide (PEI) to suppress crystallisation, enhance interlayer bonding, and improve printability in FFF. The primary objective is to establish how blend composition and processing conditions interact to control crystallisation behaviour, microstructural development, and mechanical performance. A systematic composition sweep of PEEK/PEI blends was combined with a focused processing study on a transitional composition (PEEK40PEI60), enabling separation of composition-driven and process-driven effects.

PEEK/PEI blends were compounded, extruded into filaments, and fabricated into test specimens using FFF. Crystallisation kinetics and structural ordering were characterised using differential scanning calorimetry (DSC) and X-ray diffraction (XRD), supported by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Mechanical performance was evaluated by tensile testing, with top-layer temperature systematically varied for the PEEK40PEI60composition to probe the influence of thermal history during deposition.

The results demonstrate that increasing PEI content progressively suppresses nucleation and crystal growth, extending the diffusion window during deposition and promoting improved interlayer bonding. XRD and DSC confirm reduced crystallinity and delayed crystallisation kinetics in PEI-rich blends relative to neat PEEK. Mechanically, blending enhances strain-to-failure and reduces the tendency towards brittle mechanical behaviour, with PEEK40PEI60 achieving significantly higher strain-to-failure than pure PEEK. Top-layer temperature tuning further reveals a critical processing window near 200 °C, where partial crystallisation reinforces stiffness while sufficient chain mobility is retained to preserve weld integrity.

This work establishes a diffusion-dominated versus nucleation-dominated framework as a mechanistic basis for interpreting PEEK/PEI behaviour in FFF. Through a systematic investigation of blend composition and processing conditions, relationships between crystallisation behaviour, interlayer bonding, and mechanical performance were identified across a broad range of PEEK/PEI systems. The findings demonstrate how thermal management can be used to influence the balance between crystallisation and molecular diffusion during deposition, providing both conceptual insight and practical guidance for process optimisation. Collectively, the results support the development of composition- and process-optimised PEEK/PEI systems for safety-critical aerospace and biomedical applications and highlight pathways for future research into ternary blends, in-situ characterisation, and long-termperformance validation.

Date of Award	Aug 2026
Original language	English
Sponsors	Department of Education (Northern Ireland)
Supervisor	Krzysztof Rodzen (Supervisor) & Adrian Boyd (Supervisor)