## 4 Discussion
### Manufacturing
Although they were far away from the main hydrofoil portion of the blade section, manufacturing defects like these can become damage initiation points under loading of the structure [27]. They were caused by the sharp changes in geometry of the mold surface and root inserts, resulting in the triaxial fabric wrinkling. Further design of the root insert geometry or different glass fabric selection would help alleviate these wrinkles. The triaxial fabric used was relatively stiff and not easily draped over complex contours. The drapability of fabrics should be carefully considered for future designs when sharp contours are present. The scale of the blade section may have contributed to this issue. It is expected that the waves would be much less prominent on a larger-scale component. The final defect observed on the manufactured blade section was a large delamination along the leading edge of the transition region (see Figure 10). This was caused by the removal of the vacuum bag from the cured part. Ultimately, the 3D-printed mold will be intended to be a load-bearing component of the composite structure, so delaminations such as these would be unacceptable, and good adhesion between the 3D print and the composite overlay is critical. The mold surface was prepared in a comparable way to adhesive bonding surfaces but, clearly, that was insufficient. More research is required to develop surface preparation practices for bonding these dissimilar materials. It appears there is currently very little information available for such requirements. To summarize, the main goal of this research was to investigate the potential for additive manufacturing to be applied to the creation of marine energy composite structures through internal molding to produce advanced hybrid structures. Our investigation has indicated that additive manufacturing of internal composite molds can allow for a unique design approach which can have the potential to reduce costs and lead times. Future research will focus on a fully optimized structural design, which will then allow for proper in-depth techno-economic analyses for comparison with conventional manufacturing techniques. Ultimately, additive manufacturing of internal hybrid structural molds could help pave the way for a new generation of low-cost, highly reliable, recyclable, and advanced marine energy composite structures. Conceptualization, P.M. and D.M. ; methodology, P.M., J.D., and R.B. ; design, P.M., J.D., D.M., S.H., and R.B. ; manufacturing and validation, P.M., J.D., and R.B. ; writing--original draft preparation, P.M.; writing--review and editing, J.D., D.M., S.H., and R.B. ; funding acquisition, P.M., D.M., and S.H. All authors have read and agreed to the published version of the manuscript. This work was authored by the National Renewable Energy Laboratory, operated by the Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08 GO28308. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Water Power Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The authors would like to thank all the engineers and technicians who provided their knowledge and expertise for this project, particularly [PERSON] and [PERSON]. The authors would also like to thank [PERSON] from Sandia National Laboratories and [PERSON] from the U.S. Department of Energy's Water Power Technologies Office for their valued input throughout the project.