## 4 Discussion
### Mold Design  
The process of designing the additively manufactured mold itself did not have a significant influence on the design process from a manufacturing perspective. Once the composite ply layout had been designed, the composite thicknesses were subtracted from the intended net external geometry of the blade section, leaving the final outer surface geometry of the mold to be printed. This required some intuition during the CAD drawing process but could be much more streamlined with the use of specialized composite design and ply draping software. The implementation of the integrated shear web, resin infusion lines, root fasteners, and mold segmentation were also straightforward in the design for manufacturing approach, although ensuring that the complex internal structures of the mold could be perfectly sealed required forethought. Nonetheless, there is plenty of room for further optimization of this design in the future, especially from a structural integrity perspective.  
Another area for extensive optimization in the additively manufactured mold design is the in-fill and wall thickness parameters for the internal cavities of the 3D print. We chose a sparse, double-dense in-fill to reduce material usage and print times, and also to minimize distortion of the mold as a result of vacuum pressure (see Figure 9). In future designs, this aspect must be carefully considered, especially if the mold is intended to remain as a load-bearing component in the full-scale hybrid additively manufactured/composite structure. Segmented optimization or even full topology optimization of mold designs could give way to highly efficient, advanced hybrid composite structures. In addition, print and lead times, as well as material costs, could be significantly reduced for prototype and large-scale-manufacturing marine energy structures. This will be a specific focus for future research.  
Figure 10: The final finished blade section with visible out-of-plane waves stemming from the root inserts and the delamination along the leading edge of the transition region.  
Material selection for the blade section design and manufacturing was relatively conservative. The mold was printed with ABS plastic because of its low cost, and the composite was fiberglass infused with a room-temperature-cure epoxy. We chose this specific epoxy because of its low exotherm temperatures during curing, so it was compatible with the ABS mold. ABS has a relatively low heat deformation temperature and may have permanently deformed if a higher temperature cure resin was used. To 3D-print a mold also intended to be a structural element, chopped fiber-filled or even continuous fiber thermoplastic filaments would be a better choice, because of their higher stiffnesses, strengths, and heat deformation temperatures. Their lower coefficients of thermal expansion would also be beneficial. In turn, this would allow for a much broader selection of resin systems for the VARIM process. Novel, infusible thermoplastic resin systems are showing great promise for renewable composite applications [13]. They exhibit similar mechanical properties to epoxy resin systems, with the added benefit of being recycled more easily [24]. The combination of a thermoplastic, 3D-printed hybrid mold with a thermoplastic composite would allow for fully recyclable structures at the end of their service life [25]. This would be an important economic and sustainable benefit for the marine energy industry.