Improvement in build-direction thermal conductivity in extrusion-based polymer additive manufacturing through thermal annealing

While additive manufacturing offers significant advantages compared to traditional manufacturing technologies, deterioration in thermal and mechanical properties compared to properties of the underlying materials is a serious concern. In the context of polymer extrusion based additive manufacturing, post-process approaches, such as thermal annealing have been reported for improving mechanical properties based on reptation of polymer chains and enhanced filament-to-filament adhesion. However, there is a lack of similar work for improving thermal properties such as thermal conductivity. This paper reports significant enhancement in build-direction thermal conductivity of polymer extrusion based parts as a result of thermal annealing. Over 150% improvement is observed when annealed at 135 degrees C for 96 h. The effect of annealing temperature and time on thermal conductivity enhancement is investigated through experiments. A theoretical model based on Arrhenius kinetics for neck growth and a heat transfer model for the consequent impact on inter-layer thermal contact resistance is developed. Predicted thermal conductivity enhancement is found to be in good agreement with experimental data for a wide range of annealing temperature and time. The theoretical model may play a key role in developing practical thermal annealing strategies that account for the multiple constraints involved in annealing of polymer parts. This work may facilitate the use of polymer extrusion additive manufacturing for producing enhanced thermal conductivity parts capable of withstanding thermal loads.