## 3 Results and Discussion
### Measurement of the Bending Strength  
The results of the bending test, including the basic descriptive statistics, are shown in Table 5. Moreover, Figure 5 expresses the graphical dependence of the bending strength on the number of degradation cycles or on the type of post-processing. It is clear from the results of this test that the 60 testing cycles implemented did not bring about significant changes compared with the initial state of the samples based on the comparison of standard deviations (before and after the degradation test). This statement also applies to both standards (in contrast to the tensile test). This fact can be attributed to the several times larger cross-section of the sample (225 mm\({}^{2}\)) compared with the tensile test (18 mm\({}^{2}\)). For this reason, it can be stated that when used in undemanding applications where the product is exposed to moisture only in rare cases, only the standard without post-processing modifications can be recommended. Figure 4: Nature of the sample failure after the degradation test (tensile test): (**a**) Chemical smoothing (39.27 MPa); (**b**) heat treatment (45.02 MPa); (**c**) epoxy resin (42.81 MPa); (**d**) synthetic coating (41.44 MPa); (**e**) water-based coating (40.97 MPa); (**f**) standard-recycled rPET-G (39.01 MPa); (**g**) standard-virgin PET-G (41.15 MPa). However, everything also depends on the strength requirements for the given component. Increasing the resistance against moisture absorption can be solved in the case of the standard, for example, by choosing a higher number of perimeters or by adjusting other 3D printing parameters [12; 13; 14], e.g., by setting a lower thickness of the printing layer, the surface roughness parameter can be further optimized [54; 55]. The advantage of this solution clearly lies in its speed and simplicity, since after the 3D printing is finished, the part is immediately ready for assembly. In addition, the surface of the component is not contaminated by the coating, which is also very advantageous in terms of the ease of recycling at the end of the product's life cycle [24]. An example of the failure of individual samples during a bending test is shown in Figure 6. Figure 6c concretely proves the very low toughness of the epoxy coating, which is manifested by extensive cracks relatively far from the fracture area of the base material. Figure 6e further shows the delamination of the water-based coating, which  
Figure 5: Flexural strength before and after the degradation (arithmetic mean \(\pm\) standard deviation): CS—Chemical smoothing; HT—Heat treatment; ER—Epoxy resin; SC—Synthetics coating; WBC—Water-based coating; rPET-G—Recycled (without post-processing); PET-G—Virgin plastic (without post-processing). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Quantity**} \\ \hline  & \multicolumn{6}{c}{**Testing Phase**} \\ \hline  & \multicolumn{2}{c}{**AM, MPa**} & \multicolumn{2}{c}{**D, MPa2**} & \multicolumn{2}{c}{**SD, MPa**} & \multicolumn{2}{c}{**CV, \%**} \\ \cline{2-9}
**Type of PP** & **Before** & **After** & **Before** & **After** & **Before** & **After** & **Before** & **After** \\ \hline
**CS** & 59.26 & 58.40 & 3.3124 & 7.1824 & 1.82 & 2.68 & 3.07 & 4.59 \\
**HT** & 64.51 & 63.39 & 3.8416 & 5.9536 & 1.96 & 2.44 & 3.04 & 3.85 \\
**ER** & 59.98 & 59.32 & 2.5600 & 4.3264 & 1.60 & 2.08 & 2.67 & 3.51 \\
**SC** & 59.04 & 58.50 & 1.2100 & 3.7636 & 1.10 & 1.94 & 1.86 & 3.32 \\
**WBC** & 59.48 & 58.69 & 2.4336 & 5.0176 & 1.56 & 2.24 & 2.62 & 3.82 \\
**WPP (rPET-G)** & 58.67 & 57.63 & 3.0276 & 8.8804 & 1.74 & 2.98 & 2.97 & 5.17 \\
**WPP (PET-G)** & 60.79 & 59.87 & 2.8900 & 6.3504 & 1.70 & 2.52 & 2.80 & 4.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Evaluation of the flexural test (before and after 60 degradation cycles). occurred as a result of external influences or due to the loss of adhesion of the coating to the base substrate. Figure 6f also shows an example of delamination of the print layers in the standard.