# Non-Destructive Porosity Measurements of 3D Printed Polymer by Terahertz Time-Domain Spectroscopy
## 4 Conclusions  
The effects of 3D printer settings on the porosity and homogeneity of the produced materials were examined using terahertz time-domain spectroscopy. A set of PETG samples were 3D printed by systematically varying the printer parameters, including layer thickness, nozzle diameter, thick/thin printing, extrusion, and printing pattern. The effective refractive indices and loss coefficients of the 3D printed samples were measured and compared with those of solid PETG. porosity was calculated from the difference in the refractive index of solid and 3D printed material. For the first time, a Bragg diffraction feature was observed in the loss spectrum of all 3D printed samples; material inhomogeneity was inferred from the amplitude of this feature.  
When fabricating 3D printed materials, it is desirable to minimise their porosity and inhomogeneity in order to improve their mechanical and optical properties. It was found that porosity and inhomogeneity are strongly influenced by printer settings, and that a \"sweet spot\" exists where these are minimised for an optimal combination of printer settings. In particular, increasing extrusion appears to be most effective in reducing porosity and inhomogeneity. Moreover, there is a positive correlation between porosity and diffraction, such that both are minimised simultaneously. A simple geometrical model was proposed to explain the findings.  
In this study, we have demonstrated the use of THz TDS for examining 3D printed materials. We have measured porosity and diffraction loss and have observed their variations with printer settings. Porosity is known to be related to the mechanical properties of 3D printed materials, such as elastic modulus. Therefore, THz TDS measurements can provide insight into the mechanical performance of such materials.  
It should be noted that in the simple implementation described here, THz TDS is a bulk technique that measures the mean porosity over the volume of material (area and depth) exposed to the beam. As such, it cannot provide a 3D image of the internal structure of the object, nor depth variation of properties such as porosity. As with other optical techniques, tomographic implementations of THz TDS that can reveal internal structure are possible. However, these require complex equipment and are time-consuming, and therefore may not be suitable for routine quality control.  
THz TDS is a non-contact, non-destructive technique that can be implemented in an industrial environment to provide in-line or near-line testing, and therefore offers a useful testing tool to the additive manufacturing industry.  
Conceptualization, M.N. and R.A.M.; data curation, G.S., M.N., P.F. and R.A.M.; methodology, G.S., F.A., R.A.M. and M.N.; software, G.S.; validation, G.S. and M.N.; formal analysis, G.S. and M.N.; investigation, M.N., G.S., F.A. and R.A.M.; resources, G.S., R.A.M. and M.N.; writing--original draft preparation, M.N. and G.S.; writing--review and editing, P.F., R.A.M., F.A., G.S., G.L., M.F. and M.N.; visualization, G.S. and M.N.; supervision, R.A.M. and F.A.; project administration, R.A.M.; funding acquisition, R.A.M. and M.N. All authors have read and agreed to the published version of the manuscript.  
Funding: The Etaphase contributions to this research were funded by the National Science Foundation, grant number NSF SBIR-1534779. MN's work was funded by the National Measurement Office of the Department for Business, Energy & Industrial Strategy (BEIS) of the UK government. [PERSON] and [PERSON] acknowledge support from the EPSRC (United Kingdom) Doctoral Training Partnership grant EP/R513350/1, EPSRC (United Kingdom) Strategic Equipment Grant No. EP/L02263X/1 (EP/M008576/1) and EPSRC (United Kingdom) Grant EP/M027791/1.  
Not applicable.  
Savvides Design provided access to the Prusa-3D printer as in-kind support.  
The authors declare no conflict of interest.