A Parametric Study of Part Distortions in Fdm Using 3d Fea

We developed a finite element model to simulate the fused deposition modeling (FDM) process. The model considers the coupled thermal and mechanical analysis and incorporates the element activation function to mimic the additive nature of FDM. Due to repetitive heating and cooling in the FDM process, residual stresses accumulate inside the part during the deposition. The model is also used to evaluate the part distortions, revealing distortion features such as vaulting shapes and distortion-core shifting. A parametric study, three factors and three levels, was performed to evaluate the effects of the deposition parameters on residual stresses and part distortions. Prototype models with larger sizes were fabricated, measured, and compared with the simulations. The simulation results show that (1) the scan speed is the most significant factor to part distortions, followed by the layer thickness, (2) the road width alone is insignificant, however, the interaction between the road width and the layer thickness is significant too, and (3) there are other two-way and three-way interactions that are of secondary significance. Residual stresses increase with the layer thickness, and increase with the road width, to a less extent though, yet largely affected by the layer thickness. The FDM part distortions from the experiment show a similar trend as in the simulations, but no quantitative correlation. Introduction FDM is one of widely used solid freeform fabrication (SFF) systems because of the inexpensive machinery and durable part materials. In FDM, the thermoplastic material is heated to a semi-molten state and then extruded as an ultra-thin filament. While the extrusion nozzle is moving according to the toolpath defined by the part cross-sectional boundary, the material is deposited atop the previous layer and heat dissipation by conduction and forced convection causes the material to quickly solidify with the surrounding filaments. The bonding between filaments encompasses the local re-melting of the previously solidified material and diffusion [1]. Part geometry in SFF has been a frequently studied topic. Recently, Mahesh et al. used a benchmark part to evaluate several SFF processes and the measured tolerances such as flatness and symmetry were compared [2]. The authors noted that the residual-stress induced distortions (e.g. warpage and delamination) are prominent. As other SFF processes, in FDM, the heating and rapid cooling cycles of the work materials will aggravate non-uniform thermal gradients and cause stress build-up that consequently results in part distortions. Qiu et al. studied the toolpath effects in FDM and proposed an algorithm to match the toolpath with the extrusion speed so to eliminate voids and to correct overfill and underfill defects [3]. Pennington et al. conducted an experimental study to investigate factors significant to dimensional accuracy in FDM. The authors reported that the part size, the location and the envelope temperature have dominant effects [4]. Jiang and Gu also studied the extrusion phenomenon in FDM and reported that process parameters are critical to the part accuracy [5]. Reviewed, accepted September 14, 2006