Issue 48

C.M.S. Vincente et alii, Frattura ed Integrità Strutturale, 48 (2019) 748-756; DOI: 10.3221/IGF-ESIS.48.68 752 R ESULTS AND DISCUSSION Mechanical properties of PLA uncoated samples he tensile tests average response values for each different specimen according with the experimental parameters defined on Tab. 1 are presented on Fig. 2. The best conditions for maximizing strength correspond to sample LVL1, with a layer thickness of 0.1 mm, raster angle of (0 o /90 o ) and extrusion temperature of 220 o C. The level of parameters that lead to a maximum ductility they correspond to sample LVL2, with a layer thickness of 0.2 mm, raster angle of (–45 o /45 o ) and extrusion temperature of 200 o C. The obtained results represent a variation in strength (more than 15%) and ductility (more than 20%) between samples, by simply changing the levels of the FDM process parameters. These results are consistent with the previous reported trends of the FDM process for strength and ductility of PLA parts [7,9,12,14–16]. Taking in account the mechanical design specifications, these mechanical properties for PLA, reinforces the idea that adapting the levels of parameters could lead to a reduction of raw materials and energy costs of the FDM process. Water absorption tests On Fig. 3a are shown the results obtained from the experiments of water absorption tests for PLA samples LVL1 and LVL2. Figure 3 : (a) Evolution of the WG per area of samples LVL1 and LVL2 (uncoated, coated with acrylic and coated with PU) as a function of the square root of the immersion time (grey shadowed areas correspond to the statistical errors). SEM images evidencing the roughness of samples with a layer thickness of (b) 0.2 and (c) 0.1 mm. Solid lines are auxiliary visual guidelines. The maximum weight gain per area of the cubes was reduced in average from 2.98×10 –3 g.cm –2 (uncoated) to 2.50×10 –3 g.cm –2 (acrylic coated) and 1.84×10 –3 g.cm –2 (PU coated), meaning that adding protective coatings of acrylic or PU on PLA helps on preventing the absorption of water by PLA, Tab. 2. The variation from uncoated to acrylic coated samples represents a reduction of water absorption of 16%, while the PU coating allow reduce water absorption by 38%. It was also observed that sample LVL1 absorbs more water per area than sample LVL2, with or without protective coatings. The open porosity follows the same trend as the weight gain per area in all samples meaning that it is one important mechanism for water absorption in PLA parts produced by FDM. The values of water absorption coefficient also reveal that the time needed to absorb a volume unit of water (proportional to K –1 ), increases when samples are coated, reaching its maximum with the PU coating. Sample LVL1 possess the lowest layer thickness (0.1 mm) and higher extrusion temperature (220 o C). The lower the layer thickness, the rougher the surface is expected to be [30], as it can be verified by the comparison of Fig. 3b and 3c. T