(259c) Postprint Microwaves Processing to Enhance Mechanical Performance of Carbon-PEEK Composites

3D printing of plastics using filaments, e.g. Material Extrusion (MatEx), tends to lead to parts with inferior mechanical properties that are strongly anisotropic. Significant efforts have focused on the optimization of print conditions, but this does not lead to properties that are consistent with traditional manufacture such as injection molding. As the performance of the feedstock is increased, the challenges with achieving high performance parts also increases. One of the more promising high performance engineering plastics is PEEK, but the volume change on crystallization can lead to difficulties in dimensional accuracy. Fiber composites provide a route to minimize the shrinkage from crystallization while enhancing the mechanical strength. Here, we illustrate the printing of carbon fiber composite of PEEK using MatEx. The composite is readily printed, but the mechanical properties remain quite anisotropic, which we attribute to the poor interface formation. Heating to temperatures greater than Tg but below the melting point of the PEEK could yield improvements in mechanical properties from crystallization of amorphous PEEK across the interface. However, the mobility is rather low due to the reasonable crystallinity (ca. 30%) of the printed PEEK and the presence of the carbon fibers. From x-ray tomography, we find that the alignment of the carbon fibers in the print direction is high with a Herman's orientation parameter of 0.886. This orientation of the fibers suggests that similar orientation of the PEEK chains may be limiting the formation of a strong interface between printed roads. Unfortunately, higher temperatures for annealing would lead to a loss in the dimensional accuracy of the parts. To overcome this issue, we exploit the strong coupling of microwaves with the carbon fibers to provide rapid and efficient heating of the composite. Cessation of the microwave energy leads to a rapid cooling of the composite. Low power microwave (100 or 200 W) for 10-20 s is found to mostly preserve the printed structure, but can dramatically increase the Young's modulus by more than a factor of 3 in some cases. Interestingly, the efficacy of the microwaves in improving the mechanical properties is dependent on the print orientation. We attribute this effect to the coupling of the microwaves with the aligned carbon fibers at the scale of the printed filament roads. Long straight roads leads to larger improvements in the mechanical performance. The improvement in the mechanical performance must be associated with the changes at the interface between the printed roads as (1) the crystallinity of the specimens decreases as the mechanical properties are improved with microwaves and (2) the void fraction is increased in the parts with the largest improvements. These changes in the characteristics of the printed parts would generally lead to a decrease in the mechanical performance, but are counteracted by improvements in the interface development. Moreover, the decreased crystallinity indicates that there is some melting that occurs with the microwave processing, but the loss in crystallinity does not lead to bulk flow on the timescale for the microwave processing. These results suggest that postprint microwave processing may be a route to rapidly improve the mechanical performance in 3D printed carbon fiber composites.