# Functionally Graded Additive Manufacturing: Bridging the Gap between Design and Material Extrusion
## 3 Results and Discussion  
To explain the method and test the proposed workflow, the approach is applied to a part designed with a graded material distribution. The model was inspired by the SOL Republic model Plus HD headphone and is shown in Figure 2, in which it is possible to identify the main parts: a hard case and a soft ear pad.  
Figure 2: Headphone used as inspiration for the test case and parts identification.  
The shape of the earphone component was geometrically modeled in Rhinoceros 7 ([PERSON] and associates) using NURBS surfaces. The parts, i.e., the hard case and the soft earpad, were consolidated into a single component. The surface model was meshed, i.e., discretized by adjacent planar polygons with three or four edges that which approximate the NURBS surface, and was exported as an STL file adopting the following parameters:  
* Tolerance: 0.01 mm;
* Maximum aspect ratio: 6;
* Minimum edge length: 0.0001 mm;
* Maximum distance, edge to surface: 0.01 mm;
* Maximum edge length/angle and minimum initial grid quads: 0.  
Figure 3 shows the model with the main dimensions and a longitudinal section to show the internal detail.  
Two materials were selected: PLA Extrafill (polylactic acid) for the rigid portions, and FlexFill TPU 98A (thermoplastic polyurethane) for the compliant areas, both supplied by Fillamentum [32]. Table 1 shows the main properties of the materials selected in this work. The choice of this pair of materials is due to the wide range of properties that can be reached by varying the material fraction and the compatibility between them, while maintaining rather similar process conditions that are easy to achieve with standard equipment, as confirmed by the literature [11; 33; 34; 35].  
The material distribution and CAM parameters (e.g., infill density and number of perimeters) are closely related to the local stiffness of the part, and they can be defined by functional requirements in the design process. In this work, linear variations of the material fraction along the \(z\)-axis are assumed, as shown in Figure 4, where the material distribution is visualized by the color of the model. To prevent clogging issues during manufacturing, it could be useful to avoid material fractions (MFs) less than 5% and over 95%. Theoretical MF variation can be introduced in every direction, but at this first stage, the study was limited to the z-direction, to neglect the delay effects due to the volume of the material in the melting chamber, which will be studied in future works.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties** & **PLA Extrafill** & **Flexfill TPU 98A** \\ \hline Density [g/cm\({}^{3}\)] & 1.24 & 1.23 \\ Ultimate tensile strength [MPa] & 60 & 53.7 \\ Elongation at break (\%) & 6 & 318 \\ Elastic modulus [MPa] & 3600 & 160 * \\ Print temperature range [\({}^{\circ}\)C] & 190–210 & 220–240 \\ Bed temperature range [\({}^{\circ}\)C] & 55–60 & 50–60 \\ \hline \hline \end{tabular}  
* Measured by mechanical tests.  
\end{table}
Table 1: Main properties of the chosen materials [32].  
Figure 3: Surface model: main dimensions and internal detail.