Influence of layer thickness and extrusion ratio on strand morphology, porosity, surface roughness, and anisotropic mechanical properties in FDM

Fused deposition modeling (FDM) technology is widely used in the areas of rapid prototyping, education, automobile and health care. However, the disadvantages of high porosity, low surface quality, and significant anisotropic mechanical properties hinder the applications in high-demand fields. In this paper, numerical model and experimental tests are combined to study the effects of layer thickness and extrusion ratio on the printing quality and mechanical properties of FDM structures. The computational fluid dynamics (CFD) and volume of fluid (VOF) method are adopted to investigate the flow behavior of melt and the cross-section of strand. The porosity characteristics, surface roughness and anisotropic mechanical properties of printed part are researched. The results show that, when the layer thickness is less than the diameter of nozzle, the squeezing of nozzle bottom face causes the melt climbing at the rear side of nozzle, which forms the bulges at the top corners of strand. Lower layer thickness helps to decrease the porosity and surface roughness. The proper increment of extrusion ratio can effectively reduce the porosity and improve the mechanical properties of the structures, yet large extrusion ratio may cause excessive squeezing of strand melt which will worsen the surface roughness and mechanical properties. This research contributes to a deeper understanding of FDM technology and provides guidance for the process optimization and structure design.