Figure 3 shows representative graphs from the tensile tests conducted during the present study. Specifically, in Figure 3a, typical stress (MPa) to strain (mm/mm) curves are shown for PC material for the 10 mm/min strain rate. The layer thickness of the specimens, in this case, was 0.2 mm and curves shown are for the three different 3D printing temperatures studied in this work (one graph per temperature). Through this comparison, it can be observed that increasing the 3D printing temperature to 270 °C improved the mechanical performance of the material, probably due to increased inter and intra layer fusion. In Figure 3b, PC stress (MPa) to strain (mm/mm) typical curves for the different layer thicknesses studied are shown for the elongation speed of 10 mm/min and 3D printing temperature of 260 °C. The figure shows that varying the layer height had little effect on the strength of the specimen, as the difference was less than 10%. This difference could be plausibly attributed to the anisotropy of the process followed (AM). Figure 3c shows the effect of the strain rate on the PC polymer, at three different elongation speeds (10 mm/min, 100 mm/min and 300 mm/min), for specimens built with a 0.20 mm layer thickness at 260 °C. As it is shown, the strain rate had an effect on the typical stress to strain curve of the PC polymer, although the maximum strength developed on the specimens was similar in the three cases shown.

In Figure 4, the corresponding results are shown for the TPU polymer. In Figure 4a, a strong effect of temperature on the behavior of the materials is observed. Results shown could lead to the selection of the optimum 3D printing temperature among the three temperatures studied in this work. The temperature increase had an opposite effect on the TPU polymer, when compared to the PC polymer. When the temperature increased from 215 °C to 225 °C, a decrease of approximately 40% in the tensile strength was observed. It should be mentioned that these sharp drops shown by the stress graphs are caused by local breaks on the specimen, which occurred, during the test, either in its shell or in the infill structure. At higher 3D printing temperatures, TPU material developed smoother curves during the tensile tests. This could be probably attributed to a more isotropic behavior in this case. A better inter and intra layer bonding was possibly achieved, which had a clear effect on the properties of the material, as the specimen became stiffer.

The effect of layer height on the tensile stress behavior is shown in Figure 4b. In this figure, typical stress (MPa) to strain (mm/mm) curves of the TPU specimens tested at 10 mm/min strain rate, with a 215 °C nozzle temperature are shown for the three different layer heights studied. Layer height exhibited a strong effect on the deformation (especially in the non-elastic area) and the tensile strength of TPU material, with lower layer heights developing higher tensile strength and deformation values. The 0.15 mm layer height developed approximately 30% higher tensile strength and almost 25% higher strain values than the 0.20 mm layer height. Figure 4c shows indicative graphs for the TPU specimens from tensile tests with three different strain rate values. Specimens in these three cases were 3D printed with a 0.2 mm layer thickness at a temperature of 215 °C. The strain rate had a strong effect on the brittleness of the material. The increase in the strain rate significantly increased the stiffness of TPU material. The tensile strength, on the other hand, was not affected in the same way, as the difference among the developed values for the three different strain rates was not higher than 10%.

Figure 5 shows a comparison between the parameters studied in this work. Specifically, Figure 5a shows, for PC material, the average tensile strength values and their deviation for the different strain rates and 3D printing temperatures studied in this work, for a 0.2 mm layer height. The nozzle temperature created a different trend regarding the tensile strength for the various strain rate values. This was probably due to the different inter and intra layer fusion of the material caused by the different 3D printing nozzle temperatures. The lower sensitivity of the strain rate to the tensile strength was observed for the highest nozzle temperature studied in this work. A similar trend was observed in PC material regarding the sensitivity of the material and the strain rate, when altering the layer height. In Figure 5b, it is shown that the tensile strength for the lowest layer height had a more consistent behavior at the different elongation speeds, when compared to the calculated tensile strengths for higher layer thicknesses. The calculated tensile modulus of elasticity (MPa) is shown in Figure 5c for the different 3D printing temperatures studied and, in Figure 5d, the corresponding values are presented for the different layer heights, for the five elongation speeds tested herein. It can be further assumed that 3D printing temperature constitutes an important parameter affecting the tensile properties of the material.