Crystallization and microstructure effects on interlayer performance and ductility of Polyetherketoneketone in open-chamber fused filament fabrication

This study investigates the key parameters governing interlayer bonding in Fused Filament Fabrication (FFF) of the high-performance thermoplastic Polyetherketoneketone (PEKK C). Results show that maintaining the top-layer temperature below the crystallization threshold enhances ductility, whereas excessive crystallization induces brittleness due to insufficient chain entanglement at the layer interface. Top-layer temperature was regulated via a halogen heater positioned near the nozzle, reducing the cooling rate and directly influencing the thermal history and mechanical properties of printed parts. For the first time, this work demonstrates that an open-chamber printer can tailor mechanical performance through halogen heater power adjustment, challenging the prevailing assumption that high-performance semicrystalline polymers require enclosed systems. This approach offers a more sustainable and accessible alternative, achieving mechanical properties comparable to, or surpassing, those obtained with enclosed systems. Five distinct thermal histories were examined, supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) with topology reconstruction, and optical microscopy. Correlation with mechanical performance was established through differential scanning calorimetry (DSC), including crystallization kinetics to assess diffusion and nucleation limits. Preserving ductility is critical for replacing conventional manufacturing methods. The proposed approach achieved an ultimate tensile strength of 93.9 ± 5.5 MPa in printed PEKK parts.