# Mechanical and Tribological Performance of Carbon Fiber-Reinforced PETG for FFF Applications
## 3 Results  
Similar to other polymeric materials, PETG undergoes modifications in its properties when it reaches a certain temperature. Being a thermoplastic material, it exhibits softening and demonstrates highly viscous behavior [14; 15]. This happens during the extrusion process, that is why it is very common to find some typical defects in FFF parts obtained in PETG. Consequently, as can be seen in Figure 5a,b, specimens show a tendency to accumulate material resulting in voids and porosities. In the same way, it is very common for the specimens to exhibit the formation of residual threads that need to be eliminated. Both defects are mainly due to non-homogeneous material extrusion caused by the mentioned viscosity. When CF is added, these defects are modified and accumulations appear in the trajectory changes (Figure 5c), in addition to other defects such as lack of cohesion. Figure 5d shows the reinforcement filaments and their alignment in the extrusion direction, which shows a lack of cohesion with the matrix. In addition, viscosity impacts the potential variations in the size of the specimens. When assessing the RMS of deviations observed in the specimens (as shown in Figure 6), a noticeable trend towards increased deviation becomes evident with rising temperatures and layer thickness. This trend is observable in both the in-plane deviations (Figure 6a) and the Z-deviations (Figure 6b). However, as observed in the case of unreinforced PETG, while the in-plane deflections are positive, the Z-deviations are negative. This phenomenon may be attributed to the flow of the molten material, causing it to spread and widen while simultaneously decreasing its height. A similar effect has been noted in various other thermoplastic materials [32]. It is noticeable that the most significant deviations occur at higher temperatures, where the material exhibits greater fluidity. When analyzing this effect on the CF-reinforced specimens (Figure 6), it can be seen that the deviations are higher. Therefore, in this case the reinforcement induces a significant macroscopic defect, which is in agreement with what was previously observed. In this case it can also be seen that there is no clear proportionality between the deviations, although it can be seen that the increase in temperature and thickness affects the specimens negatively. Figure 4: Pin-on-disc tribometer. This may be due to the extrusion mechanism of the filament, since it must be heated and extruded by passing it through a nozzle whose lower orifice is 0.4 mm. In this way, there is a reduction in sections that can lead to the accumulation of reinforcements, which would cause a non-uniform extrusion. This effect is very common in reinforced composites [33]. Higher temperatures cause a higher fluence, but the addition of the increased layer thickness, and therefore the speed at which the material is extruded, causes greater extrusion problems. The analysis of the extruded filaments has revealed that the addition of CF reinforcement causes an increase in the roughness of the filament surface, as shown in Figure 7. The increase in the roughness values, combined with the abrasive effect of the reinforce  
Figure 5: Defectology: (**a**) overall appearance of parts of PETG parts; (**b**) zoom of the surface texture of PETG parts; (**c**) overall appearance of CF-reinforced PETG parts; (**d**) zoom of the surface texture of CF-reinforced PETG parts. Figure 6: Dimensional deviations of PETG and PETG-CF specimens: (**a**) width and (**b**) thickness. ment particles, results in an increase in the friction between the filament and the extruder. Additionally, the higher friction results in the growth of the volume of detached particles, causing wear phenomena and the obstruction of the extruder. The challenges related to the extrusion of this material, leading to nozzle damage and premature nozzle wear due to material adhesion on the inner nozzle walls, hold significant importance.