## 4 Discussion
CT scans of the LW-PLA samples revealed mean Hounsfield Unit (HU) values ranging from \(-\)702.7 HU to +141.4 HU. Similarly, the regular PLA samples yielded comparable results. However, the STD\({}_{\rm xy}\) values obtained from the regular PLA samples were expectedly found to be higher than the values observed in the LW-PLA samples. The magnitude of this difference reaches threefold for the flow rate of 20%, as indicated in Table 4. These results demonstrate that, due to its foaming feature, the LW-PLA filament is superior to regular PLA filaments in printing low-density parts with a homogeneous texture. The axial and longitudinal homogeneity (STD\({}_{\rm xy}\) and STD\({}_{\rm z}\)) of the LW-PLA samples were statistically evaluated, and neither of the STD datasets showed any correlation with the flow rate, printing temperature, or mean HU. For STD\({}_{\rm xy}\), the median value was \(\pm\)9.3 HU, with a third quartile value of \(\pm\)11.9 HU. This suggests that, for the majority of the printing jobs, the axial homogeneity is comparable to that of distilled water, measured at \(\pm\)7.5 HU. On the other hand, the STD\({}_{\rm z}\) of the LW-PLA samples ranged from \(\pm\)1.1 HU to \(\pm\)9.0 HU, with a median of \(\pm\)5.4 HU, while that of the regular PLA samples was observed up to \(\pm\)10 HU, with a median of \(\pm\)8.4 HU. For water, the STD\({}_{\rm z}\) was recorded as \(\pm\)0.6 HU. These findings imply that the homogeneity of 3D-printed samples along the longitudinal axis is slightly below the reference level measured in water, regardless of the filament used. This could potentially be attributed to the minor fluctuations that may occur in the printing parameters during the printing process. The LW-PLA filament is sensitive to changes in the exerted heat energy, primarily governed by the printing temperature, flow rate, and print speed. Additionally, the axial dimensions of the print job can be considered as an important factor influencing the heat transfer experienced by consecutive layers. For this reason, the cylindrical LW-PLA samples were printed with different diameters and at a lower speed than the previous print jobs (27 mm/s print speed used instead of 33 mm/s) to investigate the effects of the sample dimensions and print speed on the radiological property. The results revealed that the print speed plays an important role in the physical and radiological properties of a print job. It was observed that reducing the print speed from 33 mm/s to 27 mm/s for the sample printed at 250 \({}^{\circ}\)C and a 60% flow rate resulted in an almost 50% reduction in both the physical density and mean HU value. This difference can be explained by defining the relationship between the print speed and absolute volumetric flow rate, rather than the percent flow rate, which is defined as a multiplier of the absolute volumetric flow rate. For a print job, the absolute volumetric flow rate is determined by the multiplication of three factors, which are the layer height, line width, and print speed. So, reducing the print speed while keeping the layer height and line width constant would result in a reduction in the absolute volumetric flow rate as well. This is clearly indicated by the mass measurements taken from the samples. The sample printed at 250 \({}^{\circ}\)C, a 60% flow rate, and 33 mm/s weighed 2.72 g, while the sample printed at an identical printing temperature and percent flow rate but at a 27 mm/s print speed weighed 1.53 g. This shows that, despite keeping the percent flow rate constant at 60%, the absolute volumetric flow rate decreased due to the execution of a lower print speed, resulting in the extrusion of less amount of filament. The statistical analyses conducted across the findings of the print jobs with different diameters revealed significant reductions in the STD\({}_{\rm x}\) values with the increasing diameter of the print job.