Understanding mechanical properties in fused filament fabrication of polyether ether ketone

Using dynamic mechanical analysis (DMA), we investigate differences in the mechanical properties of a single-filament wall of polyether ether ketone (PEEK) constructed using fused filament fabrication (FFF) under a range of different printing conditions. Since PEEK is a semi-crystalline polymer, we employ a non-isothermal quiescent crystallization model, informed by infra-red (IR)-imaging measurements, to understand our findings. We propose that, under typical FFF cooling conditions, the weld region between filaments remains amorphous. In contrast, the core of the filament has increased time above the glass transition temperature allowing for a significant crystal fraction to develop. We correlate the predicted crystal fraction to a storage modulus using the Halpin and Kardos model. With only a single model fitting parameter we can make reasonable predictions for the perpendicular and parallel storage moduli measured via DMA over a range of printing conditions. This work provides a foundation for optimizing crystallization for the mechanical performance of the FFF printed PEEK.