# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 3 Results and Discussion
### Fused Deposition Modeling  
Fused deposition modeling (FDM) is a highly productive AM technology for plastic parts. A polymer fiber is melted and extruded through a heated nozzle. Traces with a width of 0.2 mm can be readily printed using cost-effective printers, including five-axis systems like the 5 axismaker, which was employed for prototyping in this paper and is depicted in Figure 4. Conductive filaments are also available; however, the specific resistance of conductive FDM materials is generally quite high. These polymer materials are filled with either carbon or graphite particles or fibers, or with metallic particles. Notably, standard filled filaments are typically non-electrically conductive, even when heavily loaded with metal. Materials from 3 DXTECH (Conductive/ESD--PLA), ColorFabb (Bronzefill), ColorFabb (Copperfill), Formfutura (Metalfill Classic Copper), METALFIL (Ancient Bronze), Proto-Pasta (Composite PLA Magnetic Iron), and Proto-Pasta (Composite PLA Stainless Steel) are filled but remain non-conductive. The fillers primarily serve optical purposes, even at high concentrations (e.g., 80% copper). Only three materials  
Figure 3: Transient thermal simulation of radome heating. (**a**) Radome surface temperature after 10 min for a radome of 90 mm \(\times\) 58 mm \(\times\) 3 mm. (**b**) CAD drawing of the radar sensor. (**c**) Infrared image of the experimental prototype using wires glued on the radome into CNC-machined grooves. (**d**) Rise time of average radome surface temperature and average heating wire temperature. The heating power was 4 W. The experimental temperatures were roughly 4 \({}^{\circ}\)C below the simulated one which is caused by the fact that the experimental test system was not in a still air box and therefore more air movement could be expected. In addition, the emissivity setting of 0.9 in the simulation might be underestimated.  
were electrically conductive. Materials from Material4 Print and Proto-Pasta are designed for ESD protection and offer only very low conductivity. The sole material providing a sufficient low specific resistance of 6000 u\(\Omega\)cm was sourced from Multi3D (Electrifi Filament), which is a non-hazardous, proprietary metal-polymer composite primarily composed of a biodegradable polyester and copper powder. However, the polymer's melting point is only 125 \({}^{\circ}\)C, limiting its use to low-temperature applications.  
The meander structure depicted in Figure 4, as anticipated, exhibited an excessively high resistance experimentally; specifically, a resistance of 960 \(\Omega\) was measured. This outcome aligned with expectations based on the datasheet's specified specific resistance of 6000 u\(\Omega\)cm. Furthermore, it is worth noting that the filament was extruded at a low temperature of 60 \({}^{\circ}\)C, while increasing this temperature might further reduce the resistance. The experimentally calculated bulk resistivity of the printed trace was 6400 u\(\Omega\)cm, which is over 3500 times that of bulk copper and consistent with the datasheet. Consequently, the filament was also unsuitable for the heating structure, despite its respectable adhesion rating of 4B.  
Therefore, instead of one serial meander structure, a parallel heating structure was designed, achieving a sufficiently low resistance of 20 \(\Omega\). However, the trace temperatures became excessively high. The maximum current had to be limited to 100 mA due to the low thermal conductivity, which caused the conductor to exceed the polymer's melting point. Therefore, solely 0.2 W of heating power could be generated. Multi3D is currently developing new filaments with higher melting temperatures. Thus, it can be concluded that, so far, no suitable filament material has been identified to realize the heating coil on the radome. Nevertheless, given the ongoing developments, it is anticipated that the application of FDM will become feasible in the near future.