## 3 Results and Discussion
### Electromagnetic Interference Shielding Efficiency  
The graphs given in Figure 10 refer to the total electromagnetic interference shielding effectiveness of ABS and its composites prepared via compressed molding and FDM along the three different growing directions described in Figure 1.  
The total EMI SE is clearly a function of both the filler type as well as the fabrication route. The higher EMI SE was achieved by the ABS/CNT independent of the fabrication route. On the  
Figure 10: Total EMI shielding effectiveness of ABS and composites: Compression molded (**upper left graph**), FDM-PC (**upper right graph**), FDM-HC (**lower left graph**) and FDM-H45 (**lower right graph**). The orientations PC, HC and H45 are described in Figure 2.  
other hand, the highest EMI SE values were achieved for the samples manufactured via compression molding whereas 3D-printed parts attenuated the incident EM waves in the following order: FDM-PC > FDM-HC \(\approx\) FDM-H45, independently from the composite formulation. For example, the total EMI SE of ABS/CNT composite processed via compression molding was around 45 dB, whereas the 3D-printed specimens of the same composite showed attenuation near 25 dB (PC direction) and 15 dB (HC and H45 direction). Differently, lower attenuations were observed for the ABS/GNP, approximately 5 dB, independently from the fabrication route. Considering that the EMI SE of electrically conductive polymer composites is improved from the increase in electrical conductivity [1; 5; 28], the higher values of EMI SE achieved for ABS/CNT composites with respect to other formulations are reasonable. Figure 11 compares the amount of the incident radiation attenuated by absorption and reflection by both ABS/GNP and ABS/CNT composites prepared by CM and FDM. The whole absorption and reflection spectra of tested samples are given in the Supplementary Materials (see Figures S2 and S3). It is worthwhile to note that ABS/CNT attenuated the incident EM waves mostly by absorption (approximately by 80%). For ABS/GNP composites instead, despite the low total EMI SE, the incident EM was shielded by absorption and reflection mechanisms in similar amounts. CM samples are assumed to have a negligible quantity of defects such as voids or holes, since the samples are well-compacted during the manufacturing process. In contrast, the deposition of layer-by-layer in FDM generates a microstructure with several material/air interfaces inside the 3D components. Moreover, in the case of 45 \(\times\) 45 \(\times\) 2 mm samples, less material was used to produce the specimens via FDM. In fact, compression molded specimens are denser, with an apparent density of 1.06 g/cm\({}^{3}\) (Table 2). 3D-printed parts manufactured along H45 and HC directions had a density of approximately 0.95 g/cm\({}^{3}\) whereas along the PC direction a density of 0.88 g/cm\({}^{3}\) was measured. This indicates that, as expected, a higher porosity is generated by the FDM process. The different microstructure can therefore explain the lower EMI SE for components manufactured via FDM with respect to CM. The main differences between samples obtained by CM or FDM are represented by the distribution and dispersion of the fillers besides the fact that, due to their higher porosity, 3D-printed components offer less material to interact with the incident EM waves. It is worthwhile noticing that 3D-printed ABS/CNT tested along the PC direction reached a total EMI SE near 25 dB representing an attenuation of the incident EM wave higher than 99.9% [8]. The magnitudes of the attenuation were approximately 20 dB lower than the respective CM sample  
Figure 11: Shielding effectiveness by absorption and reflection of ABS composites. (\(\sim\)45 dB) and 10 dB higher than the 3D manufactured samples along the HC and H45 directions (\(\sim\)15 dB).