## 2 Materials and Methods
### Evaluation of the Surface Texture of the Samples and Their Initial Weight
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{3}{c}{**Surface Roughness Parameter**} & \multicolumn{1}{c}{**Initial Weight**} \\ \hline
**Set No.** & **Type of PP** & **Ra, \(\mu\)m** & **Rz, \(\mu\)m** & **Wo, g** \\ \hline
**1** & **CS** & 1.32 \(\pm\) 0.24 & 6.74 \(\pm\) 1.08 & 41.04 \(\pm\) 0.27 \\
**2** & **HT** & 8.24 \(\pm\) 2.97 & 48.19 \(\pm\) 13.03 & 41.09 \(\pm\) 0.30 \\
**3** & **ER** & 0.21 \(\pm\) 0.05 & 1.73 \(\pm\) 0.39 & 43.78 \(\pm\) 0.62 \\
**4** & **SC** & 1.87 \(\pm\) 0.39 & 7.40 \(\pm\) 1.58 & 41.91 \(\pm\) 0.35 \\
**5** & **WBC** & 1.49 \(\pm\) 0.27 & 7.05 \(\pm\) 1.21 & 41.98 \(\pm\) 0.38 \\
**6** & **WPP (rPET-G)** & 10.92 \(\pm\) 4.13 & 62.29 \(\pm\) 18.81 & 41.06 \(\pm\) 0.32 \\
**7** & **WPP (PET-G)** & 10.68 \(\pm\) 3.94 & 61.51 \(\pm\) 17.90 & 41.15 \(\pm\) 0.29 \\ \hline \hline \end{tabular}  
* \(\eta\)—Post-processing; CS—Chemical smoothing; HT—Heat treatment; ER—Epoxy resin; SC—Synthetics coating; WBC—Water-based coating; WPP—Without post-processing.  
\end{table} 
Table 2: Initial roughness and weight of the samples (arithmetic mean \(\pm\) standard deviation). the measured values was also recorded. This is due to the different roughness of the individual walls of the sample. In the case of the rPET-G standard, for example, the following roughness was found: Upper part (final print layer) Ra = 5.89 \(\pm\) 0.70 \(\upmu\)m, Rz = 39.85 \(\pm\) 5.06 \(\upmu\)m; side walls of the samples Ra = 15.77 \(\pm\) 1.02 \(\upmu\)m, Rz = 83.91 \(\pm\) 6.23 \(\upmu\)m; bottom wall that adjoins the printing pad Ra = 10.97 \(\pm\) 0.87 \(\upmu\)m, Rz = 63.22 \(\pm\) 5.19 \(\upmu\)m. The value shown in Table 2 is then calculated from all these measured parameters. A statistically lower roughness was then proven in the case of the heat treatment. However, the resulting roughness is rather close to the parameters measured on the standard. For this reason, it is clearly the least suitable technology for smoothing the surface. However, this fact indicates that the material's glass transition temperature (Tg) has been exceeded, which is a must for this type of post-processing. The minimal deformation of the samples further indicates not only the optimal annealing temperature but also the appropriate exposure time in the annealing furnace [34]. Furthermore, it can be stated that, statistically, almost identical values were achieved by the chemical smoothing and application of synthetic and water-based coatings. However, slightly lower values of the Ra or Rz parameters were achieved by the chemical smoothing. This fact is caused by the etching of the surface of the samples, which leads to their leveling. The statistically lowest values were then achieved by applying the epoxy resin, which is due to its excellent self-leveling ability. In contrast, however, the disadvantage here is the low toughness and its susceptibility to crack formation [35]. The appearance of the individual samples after the implementation of the relevant post-processing adjustments is evident from Figure 2. Photographic documentation was made on a binocular microscope produced by the company Meopta (Pferov, Czech Republic) in combination with a Canon EOS 1100D (Tokio, Japan). Due to the resulting roughness parameters (samples without post-processing treatments), a satisfactory degree of adhesion of the applied coatings to the substrate can be predicted. An experiment published in [36] proved that when the Ra parameter exceeds the value of about 2 \(\upmu\)m, there is almost no increase in the coating adhesion. However, it must be added that the resulting adhesion also depends on the level of chemical bonding between the substrate and the coating [37]. However, by choosing high  
Figure 2: Appearance of the test samples: (**a**) Standard—Upper part (rPET-G); (**b**) Chemical smoothing; (**c**) Heat treatment; (**d**) Epoxy resin; (**e**) Synthetic coating; (**f**) Water-based coating. quality types of coatings and the technology of their application, both of these conditions should be met.