## 2 Design Methods
### Conceptual Design  
We selected a conceptual tidal turbine blade section for this manufacturing demonstration with an additively manufactured internal mold, incorporating all of the novel features and unique innovations described earlier. The blade sections' geometry was based on the blades from the Reference Model 1 (RM1) tidal turbine from the U.S. Department of Energy Reference Model Project [3]. RM1 is a turbine with 9-m rotor blades with NACA 631-424 primary airfoil shapes. The blade section was designed to include the root section through to the maximum chord region (~1.5 m outboard). It should be noted that we simplified the overall blade geometry to some degree for this research. The main goal was to focus on the manufacturing aspects of the blade design, so it is not structurally or hydrodynamically optimized in any way. The demonstration is primarily meant to be representative of a tidal turbine blade. The additively manufactured mold was to be printed in four separated sections (see Figure 4): a root fixture (a), a root section (b), a transition section (c), and a main foil section (d). The four sections were designed to join together with steel dowel pins for easy and fast construction of the mold. Internal resin tubes were designed to be printed directly into the internal cavities of the sections, which were aligned by the dowel pins (see Figure 5b). They were designed so that during the VARIM process, resin could flow from the resin inlet ports in the root section along the length of the blade and then to the high- and low-pressure surfaces into the composite layers. Figure 4: Models of the four mold sections to be printed for the conceptual tidal turbine blade section design where (**a**) is the sacrificial root fixture, (**b**) is the root section, (**c**) is the transition section, and (**d**) is the main hydrofoil section. Figure 5: Full assembly drawings of the blade section mold, where (**a**) is fully assembled, (**b**) is exploded to show the connections between sections, and (**c**) includes the composite overlay. A shear web fixture was also integrated into the main foil section. It was designed so that a premade web could slide into the fixture, which was then sealed when the mold sections were assembled. The integrated resin tubes were located so resin could flow into the remaining free volume around the shear web before continuing to the surface of the mold, co-infusing the web in place with the resin. The root section and fixture are particularly unique. The root fixture is a sacrificial feature, functioning as an alignment and fixturing member for the 12 threaded root inserts around the printed root section. The 3D-printed root mold section is designed to account for the glass/carbon fabric and the cylindrical shape of the inserts. To build up the composite root, half of the root plies are laid down before attaching the root inserts, then the remaining root plies can be laid over the top of the inserts. The inserts themselves are tapered at the end to prevent a sudden change in geometry for the overlaying plies. The root fixture is a sacrificial print to be faced off after the composite manufacturing process is complete, leaving a clean, circular root section with the threaded fasteners in situ. Overall, the outer additively manufactured mold shape and composite layup must be designed concurrently. The outer shape of the mold accounts for the varying thicknesses of the composite layup to preserve the intended hydrodynamic shapes and profiles. The root of the blade has a thick composite buildup that gradually tapers in thickness toward the transition section, hence the reverse taper of the root mold section to be printed. Thicker spar cap sections are also included in the composite design to taper in thickness along the length of the transition and foil sections. Their influence on the mold shape can be observed at the center chord of the main foil section (Figure 4d). The blade is also designed to have a thin, continuous skin layer covering the entire outer surface of the blade section.