## 3 Results and Discussion
### Simulation of Thermal Profiles Using FSC
#### 3.2.4 Approximation of Thermal Profiles and Simulation in FSC
The melting enthalpies reported for Condition 2 with \(\text{T}_{\text{build plate}}\) of 40 \({}^{\circ}\)C amount to less than 50% of those obtained for Condition 1 printed with \(\text{T}_{\text{build plate}}\) of 110 \({}^{\circ}\)C. A layer printed closer to the build plate will likewise be more crystalline due to annealing above the material's glass transition temperature through conductive heating from the build plate. This effect is again less pronounced for Condition 2, explained by the fact that the third zone for both monitored layers of Condition 2 almost coincide as a result of the lower build plate temperature in this case, as demonstrated in Figure 3a. For the other conditions, however, there is a clear difference in temperature experienced by the 10 th and 40 th layers. As a consequence, a gradient in crystallinity is present in the printed wall with crystallinity decreasing when moving away from the build plate. This can again be attributed to a considerably weaker heating of the deposited layers by the build plate, either through conduction or through convective heat transfer by surrounding air heated by the build plate. This suggests the possibility of using an environmentally controlled build chamber, which would surround the printed part and ensure uniform annealing of the deposited layers to mediate the observed gradient. Despite higher peak temperatures associated with a higher liquefier temperature, hardly any change in the degree of crystallinity can be attributed to a higher \(\text{T}_{\text{liquefier}}\). Similarly, halving the print speed has little to no impact on the melting enthalpy values. As a matter of fact, Conditions 1, 3, 4, and 5 possess very similar degrees of crystallinity after the third zone due to the fact that they all experience a similar thermal history during the third zone with a \(\text{T}_{\text{build plate}}\) of 110 \({}^{\circ}\)C. Findings by other authors support these trends [43; 44; 45; 46; 47]. The large difference in melting enthalpy between Condition 6 using LMWPA and the conditions printed with HMWPA illustrates the substantial effect of a decrease in molecular weight on the total degree of crystallinity that is developed over the course of the FFF process, already demonstrated in the literature for PLA [30]. Figure 9: An overview of the resulting melting enthalpies after each zone in the thermal profile for the different print conditions. Applying the correction for reflection originating from the nozzle of the print head onto the simulated thermal profile has very little influence on the eventual melting enthalpies for both Conditions 1 and 6, as can be seen in Figure 10, for both monitored layers. This signifies that, besides the annealing effect of the build plate, the gradual, slower cooling rates after the sharp initial peaks are mainly responsible for the development of crystallinity during the first zone for both conditions. The sharp initial peaks due to reflection from the nozzle will impose significantly high heating and cooling rates over a very short period of time which are insufficient to induce any crystallinity. During simulation of the approximated thermal profiles for Conditions 1 and 6, the FSC samples were remelted after each peak and linear segment in the approximation to get a more in depth picture of the development of crystallinity over time during the FFF process. Figure 11a,b shows the evolution of the resulting melting enthalpies over the course of printing for Conditions 1 and 6, respectively. The corresponding thermal profiles are depicted as well for comparison. It can again be noticed that a considerable portion of the total crystallinity is already acquired during the first zone in the thermal profiles. Besides the evident difference in the total degree of crystallinity after printing between both conditions as mentioned earlier, both samples exhibit a considerably less steep increase in crystallinity after the first two zones. Especially, the HMWPA sample almost reaches a plateau in crystallinity after 300 s of printing.