## 2 Materials and Methods
### Flowability and Sintering Window  
Mixing different materials can potentially alter the flowability of a powder and therefore the properties of the final product [38]. In fact, good flowability is necessary to properly deposit the layers of raw material during the laser sintering process [39]. To measure the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristic** & **PA12** & **xGNPs** \\ \hline Average particle size—D50 [\(\mathrm{\SIUnitSymbolMicro m}\)] & 38 & 25 \\ Average particle size—D90 [\(\mathrm{\SIUnitSymbolMicro m}\)] & - & 30 \\ Mean thickness [\(\mathrm{\SIUnitSymbolMicro m}\)] & - & 14 \\ Specific surface area [\(\mathrm{m}^{2}\)/g] & - & 30 \\ Density [g/cm\({}^{3}\)] & 0.92 & 0.31 \\ Melting point [\({}^{\circ}\)C] & 185 & - \\ Carbon content [\%] & - & \(>\)97 \\ C:O ratio [-] & - & 44:1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main characteristics of the composite matrix (PA12) and reinforcement (xGNPs), as declared by the suppliers.  
flowability, the compressibility index (\(CPR\)) and the Hausner ratio (\(HR\)) were determined using the bulk density (\(\rho_{b}\)) and the tapped density (\(\rho_{t}\)), according to Equations (1) and (2):  
\[CPR=\frac{\rho_{t}-\rho_{b}}{\rho_{t}}100, \tag{1}\]  
\[HR=\frac{\rho_{t}}{\rho_{b}}. \tag{2}\]  
A compressibility index lower than 10 or a Hausner ratio lower than 1.11 is considered an excellent flow, whereas a compressibility index greater than 38 or a Hausner ratio greater than 1.60 is considered a very, very poor flow [40]. The densities were measured by marking a small cuvette with known volumes, then adding a small mass of powder to determine the bulk density. The cuvette was then gently tapped vertically against a padded benchtop 50 times to calculate the tapped density [41]. This process was repeated three times to ensure the repeatability of the results.  
The addition of xGNPs could alter also the sintering window of the powder, i.e., the temperature interval between the onset melting point (\(T_{m,On}\)) and the onset recrystallization temperature (\(T_{r,On}\)). In fact, it represents the temperature range within which the bed temperature of a semi-crystalline polymer is typically set to reduce eventual thermal stress following consolidation [42]. To evaluate any effect, a Differential Scanning Calorimetric (DSC) analysis was carried out using the DSC Q2000 supplied by TA Instruments (159 Lukens Dr, New Castle, DE, USA). Table 2 presents the DSC parameters adopted to perform the tests. The resulting DSC curves, given as temperature vs. heat flow, were finally analyzed with the TA Universal Analysis app according to the ASTM D3418 standard [43], allowing the definition of the onset temperatures by extrapolating the baseline and the leading side of the melting (or recrystallization) peak to their intersection [44], as schematized in Figure 1.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Heating/cooling rate [\({}^{\circ}\)C/min] & 10 \\ Nitrogen flow [mL/min] & 20 \\ Temperature interval [\({}^{\circ}\)C] & From 20 to 225 \\ Sample weight [mg] & Between 5 and 10 \\ \hline \hline \end{tabular}
\end{table}
Table 2: DSC parameters adopted to perform the tests for the evaluation of the sintering window.  
Figure 1: Schematization of the approach to evaluate the onset recrystallization temperature (\(T_{r,On}\)) and the onset melting temperature (\(T_{m,On}\)) for the identification of the sintering window.