HipPEEK project is truly multidisciplinary. The main activities performed in the project and key achievements include:
- Exhaustive property mapping of Triply Periodic Minimal Surface (TPMS) based lattices. TPMS are surfaces with zero mean curvature and a periodic pattern which repeats itself in the 3D space. These geometries have been found in nature, as in the Rubi butterfly wings or in the sea urchin skeleton. TPMS based lattices have been found promising for multifunctional applications due to their curvy nature. In this project, we carried out the most thorough characterization so far, to the best of our knowledge, including Young, shear and bulk modulus, compressive and shear strength and sound absorption. This large-scale characterization, through experiments and numerical simulations, provides a comprehensive mapping of the design space for TPMS, which will serve as scafolds for our hierarchical PEEK structures.
- Optimization of PEEK fused filament fabrication (FFF) print parameters. It is well known that PEEK is challenging to print, due to its semicristalline nature, high melting temperature and fast crystallization kinetics. This results in warping, layer debonding and dimensional fidelity loss of additive manufactured PEEK parts. Thus, to harness PEEK's outstanding mechanical properties and translate them to HipPEEK's hierarchical cellular structures, it is fundamental to fully understand the effects of the different print parameters involved and optimize them. This was rigurously undertaken through a design of experiments coupled with deep microstructural characterization of PEEK printed specimens. Main ourcomes of this work are the determination of the contribution of each process parameter to the mechanical properties of FFF PEEEK, which has highlighted the importance of printing at environmental temperatures around the polymer's Tg, thus the importance of a heated chamber. A further outcome is a general model linking print parameters to mechanical properties. Moreover, it has been found that crystallinity alone does not fully explain the mechanical behaviour of PEEK - short range order of the amorphous phase appears to play a significant role.
- Quantification of the plastitization effect of CO2 in PEEK, as well as sorption and desorption kinetics. The fabrication of HipPEEK's hierarchical cellular structures involves the combination of physical foaming with CO2 and additive manufacturing through FFF. It is a two step process, which involves the saturation of PEEK filament with CO2 in an autoclave, and its in-situ foaming and printing in a single step. Thus, it is fundamental to determine the soprtion and desorption kinetics of CO2 in PEEK, in order to ensure its saturation and determine the saturated filament's shelf life. Further, in order to correctly establish the processing window for the combined foaming and printing step, the cuantification of the plastitization effect of CO2, in terms of depression of the transition and melting temperatures, is required. This was undertaken within the project.
-Design, Fabrication and Characterization of PEEK TPMS hierarchical lattices. Based on the ourcomes of the previous tasks, PEEK hierarchical cellular structures, with TPMS-based meso-structures, from which the struts are composed by foam microstructure.Three different TPMS geometries (IWP (+), OCTO (+) and SYxxx (+) ) were selected and fabricated, as:
- representative examples of rigid (IWP (+) , compliant (SYxxx (+) and mixed (OCTO (+) architectures in terns of compressive behaviour.
- representative of resilient (IWP (+)) and sacrifical (OCTO (+) ) in terms of penetration impact behaviour.
- representative of highly resonant and sound absorbing (OCTO (+) and highly tunable (SYxxx (+) and IWP (+) architectures in terms of their sound absorption behaviour.
It has been demonstrated that hierarchical porosity significantly enhances toughness and ductility of the lattices, paving the way for bio inspired damage tolerant PEEK lightweigth multifunctional materials.
