## 3 Results and Discussions
### Structure of the Specimens  
To explain the dependencies described above, the surface and the fractured section of the tensile specimens were examined by light microscopy, as described in Section 2. Figure 6: The variation of the UTS [MPa]: **(a)** UTS vs. color, **(b)** UTS vs. printing scenarios. As one can observe in Figures 7-10, the color-dependent behavior of the specimens, regarding both the dimensional accuracy and the tensile strength is directly related to their mesostructure, where aspects ranging from over-extrusion (Figure 7a-c, red PLA and Figure 8a-c, black PLA) to under-extrusion (Figure 9b,c, grey PLA and Figure 10a-c, natural PLA) can be observed. The above presented structural differences are obviously caused by the distinctive thermal behavior of the different colored polymers, although printed with the same process parameters. As shown in previous research [17], the glass transition temperatures of colored PLA filaments are significantly influenced by the addition of coloring agents (the \(\mathrm{T_{g}}\) values for natural, green and black PLA filaments, manufactured by 2M3D(r) Company (Novo Hamburg, Brazil) were found to be equal to: 61.13 \({}^{\circ}\)C, 67.83 \({}^{\circ}\)C and respectively 70.29 \({}^{\circ}\)C, that means that up to 9.16 \({}^{\circ}\)C color-dependent differences were recorded). On the other hand, as shown by the results presented in Section 3.1 _\"Variation of temperature on the specimen surface\"_ of the present work, the temperature of the upper layers during deposition of the black PLA varied between 49 \({}^{\circ}\)C and 64.4 \({}^{\circ}\)C (in dependence of the printing scenario and the specimen position on the printing plate), that means very close around the glass transition temperature indicated by the filament producer to be equal to 58 \({}^{\circ}\)C [24] for all filament colors. Therefore, it can be assumed that the same deposition temperature, for different filament colors might have been situated closed below or closed above their glass transition temperatures during the printing process. Aiming to point out the differences regarding the thermal behavior of the four PLA filaments, printed with the same process parameters, also the breaking surface of one individually printed and in liquid nitrogen embrittled PLA-specimen for each color was analyzed by means of a field emission scanning electron microscope (FE-SEM). Representative images are presented in Figure 11.  
As one can observe, the FE-SEM-images clearly demonstrate that, although the same printing parameters were applied for the printing of the different colored specimens, their mesostructures show characteristic aspects ranging from pronounced over-extrusion (Figure 11a, red PLA) to evident under-extrusion (Figure 11d, natural PLA). Regarding the overall dimensional accuracy, evaluated by means of the specimens' cross-sectional area, as presented in Figures 3-5, regardless of the printing scenario and position on the build plate, the red specimens exhibited the largest dimensional deviations, whereas the best dimensional precision was obtained in case of the grey PLA filament. As visualized by the images presented in Figure 7, the selected process parameters have produced, for all printing scenarios (individually or simultaneously printing of three, respectively five specimens) to over-extrusion for the red PLA filament and by this to specific voids, such as non-uniform filament roads and oozing. In case of the grey PLA filament, the same process parameters ensured proper conditions for individual printing  
Figure 10: Top view (**up**) and fractured section (**down**) of natural PLA tensile specimens printed: (**a**) individual, (**b**) three specimens simultaneous, (**c**) five specimens simultaneous. (Figure 9a), but increasing the number of simultaneously printed specimens reduced to the appearance of under-extrusion defects (larger gaps between the filament roads and delamination of successive layers, especially in the vicinity of the wall lines, Figure 9b,c).