(9c) Characterization of Poly(ether imide) Towards the Development of a Fused Filament Fabrication (FFF) Process Model

Authors: 
Gilmer, E. L., Virginia Polytechnic Institute and State University
Mansfield, C. D., Virginia Tech
Baird, D. G., Virginia Tech
Bortner, M. J., Virginia Polytechnic Institute and State University
Fused filament fabrication (FFF) is the most common method of AM that relies on thermoplastic extrusion. As the AM field is transitioning from prototype visualization to functional prototypes and end product development, one of the greatest hurdles is the inherent z-axis (interlayer) weakness of the processed parts resulting from weak adhesion between deposited layers, producing anisotropic mechanical properties. This poor adhesion stems from insufficient polymer inter-diffusion between the layers, referred to as “self-healing.”

To better understand the evolution of part strength during the FFF process, we are developing a model to describe the self-healing phenomenon that couples the complex thermal and geometric profiles with diffusion at the layer interface. This model is strongly dependent on the transient behavior of the polymer governed by the processing and environmental conditions present during the printing process. Additionally, we are examining the effect of adding a continuous composite fiber with anisotropic thermal conductivity. Our preliminary work has developed a realistic three-dimensional model of a printed part which will accurately represent the interface between individual layers. Previous models have utilized idealistic models based on circles and ovals of equal size. Our results prove the individual layers are flattened ellipsoids that vary in diameter depending on their height from the heated bed. We have also analyzed the complete re-entanglement time of the neat polymer to quantify the extent of self-healing, combined with characterizing the thermal conductivity of the composite filaments. The corresponding mechanical strength is driven by interlayer adhesion and characterized as a function of processing conditions. Our preliminary rheological findings of a poly(ether imide) amorphous thermoplastic indicate that times for complete re-entanglement range from 47 seconds to more than 22 minutes for isothermal temperatures of 375 °C and 345 °C, respectively. Since the cooling process is non-isothermal, and the layers remain above their glass transition temperatures for less than 3 seconds, this would limit the amount of chain re-entanglement at the layer interface and corresponding bond strength between layers of a printed part, and suggests that a modified printing process is required to create stronger parts with this material. The inclusion of the composite fiber does increase the time at which the temperature of the polymer is elevated and should help increase the amount of self-healing that occurs during the printing process.

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