## 2 Materials and Methods
### Additive Manufacturing  
In this study, widely used additive manufacturing technology, the FDM process, was employed to manufacture both the polymer gears and hybrid polymer gears. An Ultimaker S5 printer (Utrecht, The Netherlands) with a build volume of 330 mm \(\times\) 240 mm \(\times\) 300 mm, where single or dual extrusion can be performed, was used to produce the gears. The Ultimaker S5 print head features two separate nozzles with a diameter of 0.4 mm, one for the base material and the other for the support material. The thermoplastic material used to make the polymer gears and the polymer matrix of the hybrid polymer gears was PA, based on PA6/66, in the form of a filament with a diameter of 2.85 mm. PA filament is strong and durable, and is characterized by reduced moisture absorption. In order to achieve an optimal balance between the adhesion and warpage of the PA gears, the printing parameters recommended by Ultimaker were applied. The printing parameters and other PA material properties are given in Table 2.  
The layer height is a measure of the quality of the model in additive manufacturing, and, in this study, gears were produced in two qualities. Higher-quality gears, referred to as Extra Fine, were produced with a layer height of 0.06 mm, while lower-quality gears, referred to as Fine, were produced with a layer height of 0.1 mm. In the FDM method, the gears are produced in a layered structure so that the orientation of the layers is in the direction of the gear facewidth, which affects the surface roughness of the tooth flanks. To prevent the model parts that have overhangs from collapsing, support structures must be created during printing. The support material for gear manufacturing was not required due to the asymmetrical shape of the fourth hybrid polymer gear hub design. With a symmetrical design, there would be parts of the gears that overhang, and it would be necessary to generate a support structure. In comparison to the symmetrical design, the asymmetrical hub design reduced the printing time by 5 h and consumed around 10% (5 g) less filament. No waste material in the form of supports was used in this hub design, making the additive manufacturing more energy- and cost-efficient. For the other hub designs, a support structure was unnecessary anyway. Figure 4 shows all four designs of the hybrid polymer gears with the metal inserts attached. Each pair of polymer and hybrid polymer gears was produced in a set of 3 pieces.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter/Material** & **PA** \\ \hline Material brand name & Ultimaker Transparent Nylon \\ Printing temperature [°C] & 245 \\ Build plate temperature [°C] & 40 \\ Print speed [mm/s] & 70.0 \\ Infill density [°s] & 100 \\ Infill pattern & Grid \\ Tensile (Young’s) modulus (ASTM D3039 [58]) [MPa] & 2331 \(\pm\) 55 \\ Tensile stress at yield (ASTM D3039) [MPa] & 63.1 \(\pm\) 1.1 \\ Flexural strength (ISO 178 [59]) [MPa] & 82.9 \(\pm\) 3.1 \\ Charpy impact strength (at 23 \(\lx@math@degree\)C) [Jk/m2] & 13.7 \(\pm\) 1.2 \\ Hardness (ISO 7619-1 [60]) & 81 Shore D \\ Thermal resistance [°C] & 89 \\ Glass transition temperature (ISO 11357 [61]) [°C] & 55.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: FDM method process parameters and material properties [57].