Fused filament fabrication (FFF) is one of the most common additive manufacturing/3D printing techniques where continuously extruded semi-molten filaments are deposited in a layer-by-layer manner. The quality of the manufactured part depends critically on filament-filament contact and adhesion as well the void fraction. Filament-to-filament adhesion affects the part strength under transverse load. In our earlier work, we studied the effect of mechanical loading of the deposited filament by an in situ roller ball on the quality of adhesion, and subsequently, on the thermal and mechanical properties of the printed parts. Under an optimized set of experimental conditions, a significant improvement in material toughness and tensile strength was measured. Here, we have developed an integrated theoretical model that predicts the impact of in situ compression rolling on filament-filament contact during deposition. Rolling parameters such as ball weight, ball temperature, filament temperature are explicitly included in the model. The effect of these parameters on the part height, void fraction and filament adhesion are studied. Based on the JKR (after Johnson, Kendall and Roberts) contact theory and the theory of elasticity, our mathematical model predicts the evolution of filament-to-filament contact width and corresponding void fraction and part height in a representative volume element of the printed part. Our prediction is in good agreement with experimental measurements. The theoretical model is used to optimize the filament temperature during the rolling process. Specifically, we find that isothermal contact between filaments results in the best adhesion.We have also found that the resulting structure always contains some percentages of void. This study suggests that applying pressure by rolling immediately after deposition decreases the void fraction significantly. The study also shows that rolling can provide good control over the part thickness, reduce the effect of die-swelling, and prevent poor adhesion and part warping. Applying pressure on soft filaments results in reshaping the filament cross-section to look more like an elliptical shape. This new shape has longer contact width, which gives more area for polymer chain diffusion and thus higher interlaminar strength. &nbsp;&nbsp;&nbsp;Our model predicts the printed part height for both rolling and no rolling cases with very small errors. Through this finding, the final height of the printed structures could be predicted. This study also gives a future recommendation for a new configuration of preheating and post-rolling, which further improves the interlaminar 3D printed parts strength. This work shows that the JKR contact theory, along with the theory of elasticity, can be used to predict the final height of 3d printed parts along with their mesostructure.

Effect of in Situ Filament Compression Rolling on the Mechanical Behavior in Fused Filament Fabrication (Fff)

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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