## 4 Results and Discussion
### Heat Resistivity
#### 4.2.1 Heat Distribution  
The heat distribution was determined along the length (50 mm) of the specimen between the electrical bonding. The heating curves were recorded after the heat distribution reached a stationary state. Figure 14 shows the heat distribution along this measuring distance for the four materials PPC, 3 dk, FFE and M3D depending on the applied voltage each with the different raster angle orientations. The maximum voltage differs between the individual materials depending on the damage of the specimen caused by its softening due to resistive heating. Because of the lower resistance of FFE and M3D, the voltage was restricted to 12 V. In addition, the latter softens at much lower temperatures than the PLA-based materials due to the low melting point of its matrix polymer PCL (\(\approx\)60 \({}^{\circ}\)C).  
Due to the softening, the specimen was locally damaged and, thus, the maximum temperature was restricted to 43.5 \({}^{\circ}\)C and a narrow area (see Figure 15). In general, the temperature drop at both sides of the clamping was caused by the much higher thermal conductivity at the electrical bonding. Consequently, the heat was transmitted more quickly to the environment and via the cables. This results in a high thermal gradient along the measurement distance due to the comparatively low lengths of the test specimens.  
Figure 14: Heat distribution of the investigated materials along the specimen for material-specific voltages and the raster angle orientations of 0\({}^{\circ}\), \(\pm\)45\({}^{\circ}\) and 45\({}^{\circ}\) respectively.  
Figure 15: Damage of the specimen caused by softening of the PCL matrix due to resistive heating at a voltage of 12 V and a maximum temperature of 43.5 \({}^{\circ}\)C.  
Overall, the FFE specimens show the highest temperature increase of 88.5 K for the raster angle of 0\({}^{\circ}\), even though the voltage was restricted to 12 V. It is followed by PPC and 3 dk as higher voltages were applied. The maximum temperature of M3D is limited by the softening of the matrix polymer and, owing to this, only reaches a temperature increase of 23.5 K. Furthermore, FFE exhibits a more homogenous heat distribution compared to the other tested materials. This probably results from a higher thermal conductivity of the CNT filled composite polymer compared to the CB filled materials and, thus, a lower temperature gradient.  
The influence of the raster angle orientation on the heat resistivity is most evident for FFE; however, this dependence was also observed with the other materials. Heat radiation capacity is highest at a raster angle orientation of 0\({}^{\circ}\), followed by \(\pm\)45\({}^{\circ}\) and 90\({}^{\circ}\); however, 3 dk is an exception as the resistance of the specimen with an infill pattern of 0\({}^{\circ}\) is increased in extrusion direction due to the defects in the filament strand. The heating curves of the PPC show a smaller range for the different raster angles due to a higher temperature increase of the 90\({}^{\circ}\) specimen from 24 V to 30 V as the glass transition temperature is exceeded. This most likely results from annealing because the resistance between the individual strands is decreased by a neck growth. 3 dk does not show this effect because the glass transition temperature was not reached. FFE shows the highest dependency on the raster angle orientation, whereby the 90\({}^{\circ}\) oriented specimen has a significantly lower maximum temperature than the other infill patterns. At a voltage of 12 V, its temperature increase is only 56.2 K compared to 76.3 K (\(\pm\)45\({}^{\circ}\)) and 88.5 K (0\({}^{\circ}\)). Thus, it is very likely that the resistance is mainly determined by the orientation of the CNTs. Although the sample was clearly deformed during heating (see Figure 16), no damage could be detected, so that a homogenous temperature distribution still occurs.