Fused deposition modeling (FDM) is the most common method for additive manufacturing of polymers, which is expanding in various engineering applications due to its ability to make complex parts readily. The mechanical properties of 3D printed parts strongly depend on the correct selection of the process parameters. In this study, the effect of three important process parameters such as infill density, printing speed and layer thickness were investigated on the tensile properties of polylactic acid (PLA) specimens. Taguchi design of experiment method is applied to reduce the number of experiments and find the optimal parameters for maximum mechanical properties, minimum weight and minimum printing time. Experimental results showed that the optimum process parameters for the modulus of elasticity and ultimate tensile strength were infill density of 80%, printing speed of 40 mm/s and layer thickness of 0.1 mm, while for the failure strain were the infill density of 80%, printing speed of 40 mm/s and layer thickness of 0.2 mm. Finally, the accuracy of the Taguchi method was assessed for prediction of mechanical properties of FDM-3D printed specimens.

Polylactic acid (PLA) foams were introduced as a good substitute for oil-based foams. Much research has been done on the autoclave of PLA foam, but discontinuous processes are not attractive for industries. In order to reduce the distance between the PLA foaming process and industrialization, it is necessary to introduce continuous processes. In this work, two solutions to improve the foaming of PLA are presented: using the extrusion process and adding cellulose nanofibers. So, in this study, a twin screw extruder was used by blowing agent, CO 2 , and in order to better dissolve the gas, a static mixer was used before the die. The molecular weight of the original PLA after extrusion has reduced in the range from 20%-28% due to the hydrolysis of the ester bond in the PLA. With the cellulose nanofiber loading, nucleation and cell density have enhanced. Scanning electronic microscope (SEM) microscopic images for the nanocomposites containing 1.5 wt% of cellulose nanofiber showed the highest cellular expansion and for the nanocomposites containing 2.0 wt% of cellulose nanofiber showed the most uniform cell distribution and it confirmed the density results and the calculated expansion ratio. Finally, by examining the dynamic-mechanical thermal analysis, it can be stated that the composition of cellulose nanofibers improves the mechanical and dynamic properties of cellulose nanofiber-reinforced PLA.

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