## 5 Application Based on an Incorporated Heating Surface  
To indicate an exemplarily application of a heat radiating surface with a heat-activated optical signal, a surface made of FFE and a color-changing PLA of DAS FILAMENT (Emskirchen, Germany) was realized by using multi-material MEX. FFE was used because of the low and temperature independent electrical resistance and the high electrical power for provide heat radiation. The dimensions of the heat radiating surface were set to 53 \(\times\) 38.2 \(\times\) 0.8 mm\({}^{3}\). It was fully integrated into the color-changing PLA. The surface was designed as a honeycomb structure to maximize heat dissipation. As an additional design element, the logo of the Institute for Engineering Design of the Technische Universitat Braunschweig was integrated into the surface. The utilized process parameter set was equal to the FFE specimen of a raster angle orientation of 0\({}^{\circ}\) (see Table 5). To demonstrate a transferability of the experimental results to other AM-machines, the incorporated heating panel was manufactured using an Ultimaker 3 by Ultimaker B.V. (Geldermansen, The Netherlands) with a filament diameter of 2.85 mm. Figure 21 represents the mean value and the standard deviation for current-voltage characteristic, electrical power and resistance of the heating surface and selected images of the color-change from red to white. The heating curve at an applied voltage of 6 V with selected images of the heat distribution is shown in Figure 22. The time that voltage was applied was limited to 55 s to avoid a softening or a distortion of the color-changing PLA. The heating curve was recorded by the VarioCAM(r) HD head 800 thermographic camera (InfraTec GmbH, Dresden, Germany). Besides the mean value of the temperature measured over the total heating surface of the demonstrator, the minimum and maximum temperature are represented. In comparison to the experimental results shown in Figure 19, the resistivity of the heating surface is approximately 58% of that of the test specimen and, thus, corresponds with 0.0077 \(\Omega\)m, almost the resistivity of the feedstock material indicated by the manufacturer. Consequently, the electrical power is significant higher than in the experiments in Section 4.2.2. Probably the orientation of the CNTs in the extrusion direction is stronger because of the taper in the nozzle from 2.85 mm to 0.4 mm. The heating curve shows a large deviation between the minimum and maximum temperature due to the colder areas on the electrical bonding of the heating panel. Due to this, the mean temperature decreased. However, the surface temperature sharply increased to its maximum temperature of about 109 \({}^{\circ}\)C within 55 s, whereby the mean value still exceeded the glass transition temperature of PLA with a temperature of almost 64 \({}^{\circ}\)C. Thus, the power supply was switched off at 55 s. In contrast, the minimum temperature at both sides of the heating surface at the electrical bonding was still below 40 \({}^{\circ}\)C. In Figure 21, it is also shown that the PLA changes its color rapidly from red to white depending on the temperature due to resistive heating. The needed temperature of about 32 \({}^{\circ}\)C to activate the color change effect was reached within 10 s. The heat distribution was homogeneous except for a small area in the middle, in which the maximum temperature was reached. This heating spot results from an irregularity in the manufacturing process due to the varying friction in the Bowden tube that was lower in this area so that more material was processed. After switching off the power supply, the structure  
Figure 21: Representation of current, electrical power and resistance depending on voltage of the incorporated heating surface and the color change depending on the temperature. Figure 22: Representation of the heating curve at an applied voltage of 6 V with corresponding images of the heat distribution.