## 3 Results and Discussion
### Electromagnetic Interference Shielding Efficiency
Interestingly, the lowest apparent density was measured for FDM-PC components and the electrical conductivities of ABS/CNT composites were just an order of magnitude different; near 10\({}^{-3}\) S\(\cdot\)cm\({}^{-1}\) and 10\({}^{-4}\) S\(\cdot\)cm\({}^{-1}\) for the ABS/CNT composites processed via CM and FDM, respectively. Table 3 presents a comparative overview on various polymer nanocomposites prepared for EMI SE investigations. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & **Polymer Matrix** & **Filler** & **Thickness** & **Preparation Method** & **EMI SE in the X-Band Frequency Range** \\ \hline
[29] & Acrylonitrile-butadiene-styrene & carbon black (CB) and carbon & & & 44 dB & 5 wt.5\% ABS/CNT \\  & (ABS) & nanotubes (CNTs) & 2 mm & Melt mixing followed by & 87 dB & 15 wt.5\% ABS/CNT \\  & & nanotubes (CNTs) & & compression molding & 9 dB & 5 wt.5\% ABS/CB \\  & & & & 34 dB & 15 wt.5\% ABS/CB \\  & & & & & 10 wt.5 \\  & & & & compression molding & 50 dB & 10 wt.5 \\  & & & & & & UHWHW/C/CNT \\  & & & & & 30 wt  
To better understand EMI shielding properties, Figures 12 and 13 show the variation of the real (dielectric constant, \(\varepsilon^{\prime}\)) and the imaginary (dielectric loss, \(\varepsilon^{\prime\prime}\)) parts of the complex dielectric function (or permittivity) for the investigated composites in the X-band microwave frequency range. The addition of 6 wt.% of MWCNT considerably modified the dielectric constant of ABS (Figure 11). In fact, an improved \(\varepsilon^{\prime}\) value was measured for the ABS/CNT and both CM and FDM-PC specimens exhibited, despite the frequency dependence, comparable \(\varepsilon^{\prime}\) behavior. The increase of \(\varepsilon^{\prime}\) is due to the presence of mobile charge carriers and related to the ability of the component in transport the charges from a point to another one across the component. As far as the dielectric loss (Figure 12) is concerned, ABS/CNT composites processed via compression molding exhibited the highest \(\varepsilon^{\prime\prime}\) values over the X-band frequency range. The 3D-printed specimens showed improved dielectric loss values in this order: FDM-PC > FDM-H45 \(\approx\) FDM-HC. The values of \(\varepsilon^{\prime\prime}\) are related to the dissipation of the energy from the incident EM waves which are absorbed by the material and converted into heat [22]. Although not experimentally proven, the higher quantities of material/air interfaces created in the FDM-PC components could be the reason for their better ability of dissipating the energy from the incident EM waves. In fact, higher multiple internal interactions might have occurred, justifying the higher EMI shielding efficiencies for FDM-PC components even though they have a lower amount of material to interact with the EM radiation. Figure 12: Dielectric constant of ABS and its composites in the X-band frequency range; FDM-PC; FDM-HC; FDM-H45. The orientations PC, HC and H45 are described in Figure 2.  
In summary, MWCNT is the most efficient carbon-based filler when an electrically conductive polymer composite with enhanced EMI shielding purposes is desired. In fact, low quantities of MWCNT are required to improve the mechanical, electrical and EMI properties of ABS based composites without compromising the rheological properties [17; 18; 19; 20]. ABS polymer composites containing carbon black or xGnP could eventually reach total EMI SE higher than -20 dB corresponding to attenuations >99.9%, by simply increasing the content of these fillers. However, processing such formulations via FDM would be challenging due to changes on the rheological behavior. In addition, a recent study has shown the advantages of using carbon nanotubes over the traditional aluminum sheets in the development of polymer composites for EMI shielding purposes; energetic and economic savings are attributed to the polymer composites based on carbon nanotubes [36].