Additively manufactured implants, surgical guides, and medical devices that would have direct contact with the human body require predictable behaviour when stress is applied during their standard operation. Products built with Fused Filament Fabrication (FFF) possess orthotropic characteristics, thus, it is necessary to determine the properties that can be achieved in the XY- and Z-directions of printing. A concentration of 10 wt% of hydroxyapatite (HA) in polyetherketoneketone (PEKK) matrix was selected as the most promising biomaterial supporting cell attachment for medical applications and was characterized with an Ultimate Tensile Strength (UTS) of 78.3 MPa and 43.9 MPa in the XY- and Z-directions of 3D printing, respectively. The effect of the filler on the crystallization kinetics, which is a key parameter for the selection of semicrystalline materials suitable for 3D printing, was explained. This work clearly shows that only in situ crystallization provides the ability to build parts with a more thermodynamically stable primary form of crystallites.

The enthalpy of fusion (ΔH _f ) is a critical parameter for determining the degree of crystallinity in semi‐crystalline polymers such as polyetherketoneketone (PEKK). Despite its increasing application in high‐performance and biomedical fields, the widely accepted theoretical ΔH _f value of 130 J/g for 100% crystalline PEKK is based on assumptions drawn from related polymers like PEEK. Furthermore, this value may not accurately reflect the behavior of PEKK composite materials. Hydroxyapatite (HA) fillers are increasingly being used in biomedical applications to enhance the bioactivity of normally bioinert materials. However, the presence of HA may influence the crystallization kinetics of the polymer. In this study, the standard ΔH _f ^100% value was re‐evaluated by implementing differential scanning calorimetry (DSC) and X‐ray diffraction (XRD) measurements on 3D printed PEKK samples subjected to controlled thermal treatments, to maximize the crystalline fraction. The findings suggest a refined theoretical value of 155.24 ± 5.39 J/g for the ΔH _f ^100% for the PEKK 7003 matrix and offer an indication of how this value transfers to PEKK‐HA composites. This work offers a more accurate basis for the thermal and morphological characterization of PEKK, with implications for both structural and biomedical contexts.

This study investigates the influence of blending polyether ether ketone (PEEK) with amorphous polyetherimide (PEI) across 11 compositions to modulate crystallisation behaviour, thermal stability and mechanical performance during fused filament fabrication (FFF). Using an open‐chamber printer, the effects of PEI content on crystallisation suppression, segmental diffusion and molecular mobility were systematically examined through an integrated characterisation approach, including differential scanning calorimetry (DSC), X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and tensile testing. The findings demonstrate that high‐performance additive manufacturing (AM) can be achieved via diffusion‐limited mechanisms, with optimised processing conditions enabling ductile performance for the selected composition, PEEK60PEI40, which exhibited a tensile modulus of 1749.7 ± 54.8 MPa, ultimate tensile strength (UTS) of 109.4 ± 3.9 MPa and strain at break of 15.3 ± 3.7%. Results from nine compositions clearly indicate that theoretical nucleation‐limited deposition is not suitable for AM. Furthermore, the study highlights that optimisation of 3D‐printed parts can be realised only through precise control of the top‐layer temperature during printing. The temperature must facilitate the formation of a physical network of entangled polymer chains between the previously deposited layer and the newly extruded material. This process ensures strong interlayer bonding and enhances the mechanical performance of the printed part while preserving its ductility. This has been demonstrated for neat matrices and composites, and now this approach is being extended to polymer blends.