## 3 Results
### 3D Topology and Related Temperature Distribution on the Powder Bed Surface  
In order to obtain constant measurement results, a reproducible reference method was defined for each evaluation process. The surface data was generated by 3D topology, contrast shape detection, and grayscale visualization. It was referenced throughout the manually defined zero plane at the top surface of the powder bed. By manually defining and referencing the zero plane, deviations due to inclination of the profilometry system could be avoided. The 3D topology provides structural data of the current process quality by lining up 2-dimensional profile sections to generate a height map of the powder bed surface. In addition, maximum height values in the positive z-direction are automatically tagged by the system, enabling real-time process monitoring by adjusting process parameters if the defined threshold for curling defects is exceeded. The powder bed surface was measured simultaneously by laser triangulation and thermal imaging 12 s after recoating in an ongoing process when curling was visible to the human eye. The process temperature in the installation space and the cooling time of the previous layer were manually adjusted according to Section 2.1. Considering the thermal recording in Figure 13b, and as described in Section 3.1, lower temperatures were suspected as an indication of curling at build part sections, compared to the surrounding temperature distribution. The evaluation of the height map generated by the laser profilometer, as seen in Figure 13a, clearly proves that actual curling occurred at values of up to 80 \(\upmu\)m (c) and thus, thermal imaging is capable of curling detection in powder bed fusion of polymers. The height map is based on grayscale imaging, highlighting sections of the components lifting up from the surface of the powder bed.  
Iteratively generated height maps at different post-coating progressions revealed curling failure starting at 23 \(\upmu\)m in an average range over the entire powder bed surface. In addition to the post-coating measurements, further height maps and related thermal images were evaluated after laser exposure to confirm that curling can occur in every  
Figure 13: Grayscale height map of the powder bed surface (**a**) after recoating. Related temperature distribution (**b**). Clearly visible coherences of the measured curling height (**c,d**) and temperature fields at the free edges of build parts (**e,f**). Slight visibility of striping at the 3D topology (**a**) due to non-filtered vibrations by the linear unit.  
process step, even in addition to the visible melting depth at the pre-coating state after laser exposure. Since grayscale images only highlight topologies in the positive z-direction, an alternative height map was applied (as previously seen in Figure 9) to reveal both positive and negative values relating to the zero plane at the powder bed surface. Regarding the 3D topology in Figure 14a, a clear expression of curling appears close to the free edges at almost every transition point from sintered areas to the powder bed surface. In Figure 14b, the related temperature distribution at the pre-coating state is shown, revealing similar expressions of curling as within the post-coating state in Figure 13b.