## 3 Results and Discussion  
### Thermal Monitoring of the FFF Process  
IR thermography directly provides the thermal history of a deposited layer monitored over time. Figure 2 portrays an example of a recorded temperature profile for Layer 10 of Condition 1 with three distinguishable zones. Initially, cyclic heating and cooling can be observed, which is due to deposition of new layers onto the monitored layer, resulting in sharp peaks (Zone 1). Here, the layer experiences high heating and cooling rates up to 200 \({}^{\circ}\)C/s within a very narrow time window. Earlier studies reported cooling rates around 400 \({}^{\circ}\)C/s and even 700 \({}^{\circ}\)C/s for fused deposition of PP and PPS, respectively [17; 56]. It should be noted that the recorded peak temperatures are several tens of degrees lower than the set liquefier temperature of 260 \({}^{\circ}\)C, as was previously reported in the literature [27; 28; 33; 36]. Other authors have attributed this discrepancy to the limited residence time of the polymer filament in the liquefier, especially at higher print speeds, resulting in a difference of 20 \({}^{\circ}\)C between the set liquefier temperature and extrudate temperature at a print speed of 4.5 mm/s, which is still considerably lower than the print speed set at 11 mm/s in this case [57]. Although convection with ambient air is the most dominant cooling mechanism, the conduction from the build plate becomes visible through the increasing lower limit of the peaks at longer time scales. After deposition of multiple layers, the influence of newly deposited layers loses intensity and the layer temperature cools down to the build plate temperature of 110 \({}^{\circ}\)C (Zone 2). Once printing is finished, i.e., the last layer is deposited, the layer temperature cools down even further, yet more gradually, since the build plate itself is cooling down (Zone 3). An overview of the resulting temperature profiles for the different applied print settings is displayed in Figure 3. All thermal profiles exhibit the three distinct zones. When comparing the temperature profile of the 10 th and 40 th layers for all conditions, the peak temperatures are similar, whereas the lower limits of the peaks have shifted downwards. This phenomenon can be attributed either to the reduced conduction originating from the build plate at layers printed higher above the build plate or to the decrease in ambient air temperature further away from the build plate, which will enhance convective air cooling, or to a combination of both. The second zone is obviously shorter for Layer 40. The cooling rate in the third zone, when printing has stopped, is also lower for Layer 10 than for Layer 40, since the build plate cools down slowly due to its large thermal inertia. Setting the build plate temperature, \(\text{T}_{\text{build plate}}\), to 40 \({}^{\circ}\)C instead of 110 \({}^{\circ}\)C has a significant impact on the temperature experienced by a monitored layer as demonstrated in Figure 3a, comparing Conditions 1 and 2. Although, initially, the first peak temperature lies in the same range, temperature decreases quickly. This is due to the limited conductive heat transfer from the build plate, and the lower ambient temperature. Hence, the overall temperature profiles for Condition 2 are shifted downwards by approximately 50 \({}^{\circ}\)C. Figure 3b compares Conditions 1 and 4 and shows the effect of halving the print speed, \(\text{v}_{\text{print}}\), from 11 mm/s to 5.5 mm/s. As a consequence of reducing the print speed by 50%, the required printing time is doubled. Since the internal temperature of the IR camera cannot exceed 45 \({}^{\circ}\)C, it had to be taken off the build plate directly after the print had finished. Therefore, only the first and second zone could be measured for Condition 4. However, the cooling in the third zone is assumed to be similar to Condition 1, as for both conditions the build plate temperature is identical.