## 2 Materials and Methods
### Design of the Experiment Based on the RSM Method  
The RSM method [31], based on the circumscribed central composite design (CCCD) was adopted, to study the relationship between the process parameters and the output response, to construct a mathematical model that can predict the output response of an \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material Properties** & **Test Method** & **Value** \\ \hline Tensile modulus (flat X-X orientation) & ISO 527 & 7625 MPa \\ Stress at yield (flat X-X orientation) & ISO 527 & 112 MPa \\ Strain at yield (flat X-X orientation) & ISO 527 & 2.5\% \\ Stress at break (flat X-X orientation) & ISO 527 & 110 MPa \\ Strain at break (flat X-X orientation) & ISO 527 & 2.2\% \\ Tensile modulus (flat Y-X orientation) & ISO 527 & 2720 MPa \\ Stress at yield (flat Y-X orientation) & ISO 527 & 63 MPa \\ Strain at yield (flat Y-X orientation) & ISO 527 & 3\% \\ Stress at break (flat Y-X orientation) & ISO 527 & 58 MPa \\ Strain at break (flat Y-X orientation) & ISO 527 & 4.5\% \\ Printing temperature & & \(270\pm 10\)\({}^{\circ}\)C \\ Melting temperature & & 200 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties for Novamid® ID1030 CF10 [30]. Figure 1: Dimensions of the specimen according to ISO 527. Figure 2: CAD model sliced into the layers with defined printing path for each layer. actual process. To develop the mathematical model for tensile strength, an experiment was implemented in accordance with CCCD with four input parameters (\(\mid\alpha\mid=2.0\)). The influence of four process parameters, namely, layer thickness, printing speed, raster angle and wall thickness on tensile strength, was assessed in this study. Other process parameters, such as infill, printing temperature, part orientation and build plate temperature, were kept constant and are depicted in Table 2. The printing temperature and build plate temperature were adopted according to the recommendations of the material manufacturer. The research recognizes that high tensile strengths can be achieved with flat orientation at a 100% infill [13; 32]. Variations of the input parameters were performed on 5 levels. The zero level (center point) was created in between the high and low levels. The range of parameter variation was selected through a preliminary experiment so that it was possible to create a test sample with any combination of selected parameters. Table 3 shows the control process parameters, along with their levels in terms of uncoded form in accordance with CCCD. The experimental data for 31 runs with four control input parameters and one output response are given in Table 4. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Exp. No.**} & \multicolumn{4}{c}{**Process Parameters**} & \multirow{2}{*}{**Tensile Strength (MPa)**} \\ \cline{2-2} \cline{5-6}  & **A** & **B** & **C** & **D** & \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 4: The experimental data. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Process Parameters**} & \multirow{2}{*}{**Level**} & \multirow{2}{*}{**Unit**} \\ \hline Printing temperature & 270 & \({}^{\circ}\)C \\ Part orientation & Flat x-x direction & degree \\ Build plate temperature & 110 & \({}^{\circ}\)C \\ Infill & 100 & \% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fixed parameters and their levels. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Process Parameters**} & \multirow{2}{*}{**Symbol**} & \multirow{2}{*}{**Unit**} & \multicolumn{5}{c}{**Levels**} \\ \cline{3-6}  & & & \(-\)**2** & \(-\)**1** & **0** & **1** & **2** \\ \hline Layer thickness & A & mm & 0.1 & 0.15 & 0.2 & 0.25 & 0.3 \\ Printing speed & B & mm/s & 60 & 70 & 80 & 90 & 100 \\ Raster angle & C & \({}^{\circ}\) & 0 & 30 & 45 & 60 & 90 \\ Wall thickness & D & mm & 0.8 & 1.1 & 1.4 & 1.7 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Control process parameters and their levels. Design Expert software was used to analyze the experimental data.