## 2 Design Methods
### Key Features  
Figure 2 shows the general process steps for manufacturing a traditional tidal turbine blade mold set. The mold set generally consists of two halves: one for the low-pressure side of the turbine blade and one for the high-pressure side of the blade. First, a master plug is machined from structural foam, which is then sanded, finished, and prepared to create a splash mold from the low-pressure side of the blade plug. For the splash mold, fiberglass/epoxy or a material similar to the final blade is typically laid over the plug, cured, and removed from the plug. The same is repeated for the high-pressure side. Support structures are then built around the splash molds and their surfaces are sanded and prepared for composite manufacturing. Overall, the mold manufacturing process is slow, expensive, and labor-intensive. Figure 2 also shows how additive manufacturing can significantly simplify this process. In addition, using a permanent internal mold means that only one mold is required instead of two halves. After printing, molds typically require a moderate amount of machining and sanding to smooth the filament lines, which sometimes can be integrated into the printing process [5; 7; 12]. Finally, the molds are typically sealed to promote vacuum integrity for typical composite manufacturing techniques. [PERSON] et al. have demonstrated these techniques at a large scale for female molds and shown that large-scale additive manufacturing processes can significantly reduce the costs and lead times of large composite molds [5; 7]. The disadvantage of using an internal mold as a permanent part of a hybrid structure is that one must be printed and prepared for every component to be manufactured, whereas a pair of high-quality traditional female molds can be used to produce hundreds of components, but structural or hydrodynamic design iterations cannot be implemented. Manufacturing composite tidal turbine blades typically requires several significant process steps (see Figure 3). Blades are made in multiple components: a high-pressure skin, a low-pressure skin, and a shear web or box spar depending on the length of the blade [4; 13; 14]. These components are often manufactured from fiber-reinforced composite and composite sandwich structures. The components are bonded together using structural adhesives. Several postprocessing steps are also required, such as trimming excess composite from the two bonded skins, and sanding and grinding the outer surfaces to their final hydrodynamic shapes. The root end of the blade must also be prepared for connection to the hub of the tidal turbine, typically in the form of a bolted connection. Bolted connections in composite laminates and composite/metal interfaces can be particularly problematic [15], and a variety of different designs exist in this area. The composite layup in the root section is built particularly thick, so that lengthwise holes can be included or drilled through the laminate and threaded sleeves can be bonded or secured inside. This can be a particularly time-consuming and expensive manufacturing step, as a result of expensive preforms, adhesives, and specialized equipment. Figure 3: Comparison of the generalized traditional process steps required to manufacture composite tidal turbine blades [4; 13] vs. those that are proposed using the additively manufactured internal molds with novel integrated features. Figure 2: A comparison of the general process steps required to produce a traditionally manufactured composite mold vs. an additively manufactured composite mold. Manufacturing a tidal turbine blade with a single internal permanent mold may allow for significant innovations to reduce the manufacturing steps shown at the top of Figure 3 and potentially increase the reliability of the structural design. A major advantage of internal molding is that composite plies can be wrapped around the outside of critical design features, such as leading and trailing edges, to create a continuous construction in failure-prone areas, with continuous fibers.