## 4 Discussion
Similar to the heating series, both cooling test series reinforced the idea of a relationship between the substrate temperature and the resulting tensile strength. However, the tensile strengths of the widely cooled samples did not drop for lower temperatures, as expected by the hypothesis. The reason might be that the surface temperature is cooled to the measured temperatures, but a higher core temperature of the strands leads to slower cooling of the interface after deposition and thus may lead to a higher degree of bond and higher tensile strength as expected. Furthermore, this would suggest that the interface temperature remains at higher temperatures for longer, reducing the hindrance of molecular motion due to the increasing crystallinity in the polymer. While the higher substrate temperatures yielded only moderate increases of 1-7% compared to the unheated samples at identical substrate temperatures, the tensile strength of the lower samples was increased by up to 59%. The tensile modulus was also expected to be congruent to that of the unmodified samples; however, all three series showed that the tensile modulus remained slightly below the expected target values. All setups yielded tensile moduli between +4% and \(-11\)% of the unmanipulated reference samples. The authors suggest that this is due to the different morphology formed under the manipulated thermal conditions, which would best be analyzed through a thorough study of the crystallinity of the different experimental setups. Similar to the widely cooled experiments, the narrow cooling experiments with decreased cooling distance yielded higher tensile strengths compared to the unmanipulated reference samples. Numerically, the maximum increase of up to 23% in tensile strength was lower than in the wide tests, since only the processing window down to 150 \({}^{\circ}\)C was considered in the narrow experiments. The samples of the highest temperature and traversing velocity yielded even slightly lower tensile strengths than the reference values. This was attributed to the high temperatures potentially affecting the mesostructure and the final way of layer-to-layer adhesion, which should be further analyzed through more detailed 2D micrographs or 3D X-ray computed tomography scans. A limit of this study is the comparatively simple design of experiment. To fully characterize the effect of cooling and heating, a lot more temperatures and traversing velocities, the heating power, and the cooling volume throughput would have to be varied on a much larger scale. For the sake of a feasibility study, the scope of this investigation allows a first judgment about likely influences as well as the overall potential of the approach. Additionally, with the objective of optimizing processing parameters for maximum attainable layer adhesion, the potential of post-processing treatments such as annealing was not investigated. For standard FFF processes, high-temperature post-processing treatments slightly increased the tensile strength and even reduced the porosity while increasing crystallinity within the component [26]. This provides a basis for further investigation of post-processing treatments to improve mechanical properties. Since in all test series, there was a heating or cooling element between the trailing pyrometer and the extrusion nozzle, the final substrate temperature immediately prior to deposition had to be determined in separate tests. This did not affect the tensile measurements or the individual results, but it shifted the data points to higher or lower calculated target temperatures. Care has been taken to ensure that these separate measurements were aligned in the best way possible with the actual tests to reduce as much variance and uncertainty as possible. The thermal radiation from the extrusion unit may also have influenced the temperature signal, especially for the narrow cooling series with only 100 mm distance between measuring point and extrusion nozzle.