## 3 Results  
### Thermal Visibility of Curling Defects on the Powder Bed Surface  
Revealing the relationship between the thermal distribution and curling defects on the surface of the powder bed was one of the main research aims of this study. Curling, as the thermally induced in-process distortion of the free edges of build parts, is still one of the challenges in the additive manufacturing of polymers. Considering that productivity and efficiency are key aspects in AM, the reduction of single-layer build time by optimizing  
Figure 9: Topology of the complete powder bed surface after laser exposure (**a**). The profile section of the melting depth (**b**) results in a height difference of 54 \(\upmu\)m between an average value of the powder bed (**c**) and an average value of the melting depth at the triangular shaped build part (**d**).  
process parameters enhances the further potential prospects of this technology. Nominal layer build time correlates directly to the processing steps such as heating, laser exposure, layer cooling, and recoating. According to the scan strategy, the laser exposure time is primarily influenced by the part bed size and varies with the complexity of the part. The duration of heating is related to the powder's properties and mainly influenced by the thermal conductivity of the powder deployed, while recoating time depends on spreader speed and geometry [17].  
Utilizing the above-mentioned process parameters, a reproducible approach of artificially generated curling was employed as part of this study. Curling, as described by [PERSON] and [PERSON] in [48] and [PERSON] in [20], is generally defined as the in-process lifting of the free edges of a component due to divergent shrinkage and associated residual stresses between the top and bottom of the component. This phenomenon is suspected to develop when the surface of a sintered layer is exposed to a new layer of coated powder due to heat transferring by conduction, or by heat transfer to the surrounding circulating gas by convection and radiation [49]. The temperature drop at the surface induces shrinkage at the top, while the bottom surface is kept below the higher temperature powder bed surface. In addition to other related process factors for curling, such as scanning strategy, laser power, part dimensions, and location/orientation, the curling effect is highly related to time and temperature during process, according to the elementary principles of heat transfer. As described in Section 2.1, both process parameters, time and temperature, were iteratively adjusted until constant and reproducible process conditions prevailed. Under set conditions, the visible appearance of curling was investigated within two different process stages, after recoating and after laser exposure during layer cooling time, revealing interesting new effects.  
As can be seen in Figure 10, the curling effect gets clearly visible at the thermal image after recoating with a new layer of powder. Sections close to the free edges of all depicted build part geometries (a-c) occurred in distinct highlighted temperature changes compared to the rest of the geometry. The visibility arose because the temperature of the curled edges very quickly took on the manually pre-lowered temperature of the circulating surrounding gas, due to the combination of the effects related in the section above: heat transfer by conduction into a new layer of powder and by convection and radiation into the surrounding gas. In addition to lowering the process temperature, the layer cooling time was extended (>100%) to provoke the appearance of curling.  
Figure 10: Thermal visibility of curling phenomenon during process after coating. Distinct lifting of the free edges of (**a**) circular, (**b**) cross, and (**c**) triangular-shaped build parts and lower intensity of curling at the top of the powder bed due to less temperature variation.