## 4 Discussion  
The aims of this study were (1) to determine how print layer thickness affected the surface roughness of FDM PEEK before and after mechanical polishing; and (2) to assess the effectiveness of polishing PEEK with SiC sandpaper and compare it to polishing PMMA with the same technique. This study demonstrated that printing PEEK specimens with a 0.3 mm print layer thickness produced inferior surface roughness both before and after SiC sandpaper polishing. When compared to the gold standard (PMMA), after polishing, the PEEK specimens had the same roughness in Group A (0.1 mm impression layer thickness) and lower roughness in Group B (0.3 mm impression layer thickness). The advantages of 3D FDM printing include minimal time to produce items, a limited amount of material, and low cost. However, it has two disadvantages: (1) the need for postprocessing treatment to remove the support structures and polish the surface, and (2) a variable behavior because the printing conditions, such as nozzle temperature, chamber temperature, printing speed, and print layer thickness, directly affect the characteristics of printed objects [19; 37]. [PERSON] et al. [22] reported that PEEK printing is technically complex compared to other thermoplastic materials melted at low temperatures. These authors performed a scanning electron microscope analysis of FDM-engineered PEEK samples and found irregularities created by material dragging by the nozzle during print. Optimizing printing parameters is crucial for controlling the mechanical properties of printed PEEK parts [22]. According to [PERSON] et al. [14] and [PERSON] et al. [16], the print layer thickness is the parameter with the greatest influence on some mechanical properties of the printed structure. They claim that increasing the print layer thickness reduces the printed part's geometry precision and contour perfection and showed optimal mechanical properties in PEEK in samples produced with a layer thickness of 0.3 mm [14]. Some authors recommend a print layer thickness of 0.1 mm to reduce internal defects and improve the surface finish [21]. The print layer thickness seems to be a printing parameter that significantly affects the structure roughness. According to various authors, these two properties are directly related, because a decrease in print layer thickness generally causes a decrease in surface roughness [8; 21; 25; 38]. The authors attribute these surface roughness results to the interlayer gaps caused by the FDM printing [38]. The results of the present study contradict those of the authors mentioned above [8; 21; 25; 38] since the specimens with the highest roughness had the lowest print layer thickness (0.1 mm). Nonetheless, as mentioned previously, other printing conditions, such as nozzle temperature, chamber temperature, and printing speed, directly affect the characteristics of printed objects and may explain the difference between the present study and previous ones. Another hypothesis that could be raised is the utilization of multiple brands of PEEK across these studies and on different printers. This variation in materials and printing methods may have influenced the behavior observed, potentially contributing to the disparity in the results. The printing conditions should be described and protocolized to enable a comparison with future studies' results. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Specimens** & \
\begin{tabular}{c} **Arithmetic Mean Roughness (Ra)** \\ **before Polishing \(\pm\) SD (\(\mu\)m)** \\ \end{tabular} &
\begin{tabular}{c} **Arithmetic Mean Roughness (Ra)** \\ **after Polishing \(\pm\) SD (\(\mu\)m)** \\ \end{tabular} \\ \hline PEEK–Group A & 0.98 \(\pm\) 0.07 & 0.11 \(\pm\) 0.02 \\ PEEK–Group B & 0.33 \(\pm\) * & 0.04 \(\pm\) 0 \\ PMMA & 0.26 \(\pm\) * & 0.11 \(\pm\) * \\ \hline \hline \end{tabular} SD–standard deviation; * Not possible to calculate standard deviation because there is only one specimen.  
\end{table} 
Table 5: Arithmetic mean roughness (Ra) before and after polishing PEEK and PMMA specimens.