# Non-Destructive Porosity Measurements of 3D Printed Polymer by Terahertz Time-Domain Spectroscopy
###### Abstract  
The porosity and inhomogeneity of 3D printed polymer samples were examined using terahertz time-domain spectroscopy, and the effects of 3D printer settings were analysed. A set of PETG samples were 3D printed by systematically varying the printer parameters, including layer thickness, nozzle diameter, filament (line) thickness, extrusion, and printing pattern. Their effective refractive indices and loss coefficients were measured and compared with those of solid PETG. Porosity was calculated from the refractive index. A diffraction feature was observed in the loss spectrum of all 3D printed samples and was used as an indication of inhomogeneity. A \"sweet spot\" of printer settings was found, where porosity and inhomogeneity were minimised.  
3D printing: PETG polymer; THz time-domain spectroscopy; porosity; homogeneity; non-destructive testing 2022 12 927 2022 2021 2022 2022 2021 2022 2021 2022 2022 2021 2022 2022 2021 2022 2022 2021 2022 2022 2021 2022 2022 2021 2022 2022 2021 2022 2022 2022 2021 2022 2022 2022 2021 202  
The printer head moves in a pre-programmed pattern, creating the desired object [17; 18; 19]. It is well known that 3D printing in general, and FDM in particular, produces materials that are inhomogeneous, anisotropic, and contain significant porosity; these affect the mechanical, electrical, and optical properties of the printed materials. For that reason, much work has been devoted to the analysis and optimisation of 3D printing process, aimed at minimising inhomogeneity and porosity [20; 21; 22].  
3D printing studies predominantly employ mechanical tests, such as tensile strength and elastic modulus, which are destructive and time-consuming [20; 21; 22], and often require specially manufactured samples. X-ray computed tomography (XCT) and X-ray diffraction (XRD) have also been employed to reveal the microscopic structure of printed materials [23; 24]. Although XCT can produce detailed high-resolution images of the internal structure of the material, it is unsuited to widespread deployment at manufacturing stations and for routine quality control. Optical techniques cannot be employed, since with very few exceptions, 3D printed materials are opaque to visible wavelengths due to strong scattering. In this regard, terahertz (THz) spectroscopy (see Section 2.3 for a description of the technique) has an advantage, because the most common class of material utilised by FDM is polymers, which are transparent or semi-transparent at THz frequencies. Moreover, THz time-domain spectroscopy (TDS) can provide a simple and direct measurement of porosity [25; 26; 27], as well as revealing inhomogeneity and anisotropy. A crucial advantage of THz TDS is that it can serve as an in-line or near-line non-destructive tool for process monitoring and quality control.  
THz TDS has long been used for examining polymers [28; 29; 30; 31; 32]. Moreover, AM is being increasingly considered as a technology for fabricating THz components [33; 34; 35; 36]. The THz transmission properties of 3D printed polymers have been also examined [37; 38]. However, neither study considered the effects of printer settings or compared the transmission of 3D printed and solid materials.  
In this work, we employ transmission THz TDS to systematically investigate the effects of 3D printer settings on the porosity and inhomogeneity of FDM-fabricated PETG samples. A set of 3D printed PETG samples was prepared by systematically varying the printer parameters, including layer thickness, nozzle diameter, filament (line) thickness, extrusion, and printing pattern. Their effective refractive indices and loss coefficients were measured and compared with those of solid PETG. The porosity was calculated from the refractive index. We observed clear relationships between printer parameters and material properties, helping to identify the \"sweet spot\" where porosity is minimised. In doing so, we demonstrate the utility of THz TDS for the non-destructive evaluation of FDM-printed materials.