# Non-Destructive Porosity Measurements of 3D Printed Polymer by Terahertz Time-Domain Spectroscopy
## 2 Materials and Methods
### 3D Printer Settings  
The samples were fabricated with a Prusa i3 MK3S (Prusa Research, Prague, Czech Republic) model 3D printer, employing the FDM technology. The fabricated samples were opaque white, indicating strong scattering of visible wavelengths.  
The raw PETG materials were in the form of 1.75 mm-diameter filaments. Different sample geometries were used for the three different print patterns listed in the last column of Table 1. The linear pattern, where all the layers of parallel lines are aligned along the same axis (Figure 2a), was printed as a 125 \(\times\) 50 mm rectangular slab. The circular pattern (Figure 2b) is comprised of a series of concentric rings forming a 50 mm-diameter disc. The cross-hatch pattern, which consists of alternate layers of parallel lines aligned perpendicular to the adjacent layer (Figure 2c), was printed as a square with a side length of 50 mm. All samples were around 2 mm thick.  
The standard nozzle temperature for PETG is 220 \({}^{\circ}\)C and the print speed is 55 mm/s [46]. The surface of the print-bed was coated with polyetherimide (PEI) to improve adhesion without causing warping, and its temperature was kept at 85 \({}^{\circ}\)C [46].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Nozzle** & **Layer Height** & **Thick/Thin** & \multirow{2}{*}{**Extrusion**} & \multirow{2}{*}{**Pattern**} \\  & **Diameter (\(\mu\)m)** & **(\(\mu\)m)** & **(Width)** & & \\ \hline A & 200 & 100 & Thick & 1.0 & Linear \\ B & 200 & 100 & Thick & 1.0 & Circular \\ C & 200 & 100 & Thick & 1.0 & Cross-hatch \\ D & 200 & 100 & Thin & 1.0 & Cross-hatch \\ E & 200 & 150 & Thin & 1.0 & Cross-hatch \\ F & 200 & 150 & Thick & 1.1 & Cross-hatch \\ G & 200 & 150 & Thick & 0.9 & Cross-hatch \\ H & 200 & 150 & Thick & 1.0 & Cross-hatch \\ I & 400 & 300 & Thick & 1.0 & Cross-hatch \\ J & 400 & 150 & Thick & 1.0 & Cross-hatch \\ \hline \hline \end{tabular}
\end{table}
Table 1: The printer settings for the 3D samples studied.  
Figure 2: Photographs of 3D printed PETG samples: (**a**) linear pattern; (**b**) circular pattern; (**c**) cross-hatch pattern.  
Table 1 lists the 3D printer parameters that were varied in order to investigate their effects. Two samples were printed for each combination of printer settings; therefore, in the figures below, there are two data points at each setting. The nozzle diameter is the size of the opening that extrudes the heated material. This is typically 400 \(\upmu\)m in most consumer FDM printers, so using a smaller diameter requires a higher nozzle temperature (240 \({}^{\circ}\)C) and a slower print speed (35 mm/s maximum; volumetric speed set at 3 mm\({}^{3}\)/s). The layer height refers to the height of a single printed layer. The \"thick/thin\" column refers to the nominal width of the printed lines. Thick is meant to be 250 \(\upmu\)m and thin is meant to be 225 \(\upmu\)m; however, due to the numerous factors involved (such as extrusion rate and material cooling rate), this should only be taken as a rough guide. The extrusion multiplier can be varied to control the extrusion rate: 1.0 indicates no change, 1.1 indicates over-extrusion by 10%, and 0.9 indicates under-extrusion by 10%. Other parameters, such as infill density and overlap, were controlled indirectly by the printer settings mentioned above.