Nice job so far. I helped start Hydro Epic, and have made hundreds and hundreds of hollow composite boards. A few tips I can pass along:
• Molds can’t be too stiff or well supported. You don’t want them to move around and distort your parts.
• Post-curing the parts in an oven will greatly increase their strength. Epoxy gets soft and pliable when it gets hot. Once the epoxy reaches a certain temperature, it will be “cured” at that temperature - it won’t get soft and pliable. I tested our full carbon boards in direct sun on a 105* day and their surface temperature reached 180*. Boards that were post-cured to above 180* were unaffected by the heat (except expansion), boards that weren’t got all rubbery and were distorted when they cooled down.
• Use your molds as a gluing jig when the time comes to join the halves. At least to tack the parts together so the finished board is true to your original.
• The seams are also critical to the overall strength. 90% of our problems came from poorly glued seams. All it takes is a hairline crack to compromise the integrity of the whole board. If the seam goes, the board will be able to over-flex and fold at that weak point… and you’d have a heavy, water-filled, SHARP object attached to the end of your leash.
How are you planning on laminating the skins? We typically used 8oz carbon (carbon/kevlar on the deck) on the outside layer, 3/8" aluminum honeycomb in between, and 8oz carbon on the inside layer. The rails had a flange molded into the parts that added two additional layers of 8oz carbon/kevlar. The flange requires another complex part for your mold, so unless your going to make tons of these its easier to join the halves from the outside with 2" carbon tape.
If you try to make your skins using only cloth, you’ll have a major weight problem. Our 9’1" longboards weighed between 9.5 and 12 lbs. Our Merrick 6’1" MBMs weighed between 4.5 and 6 lbs - depending on our lamination schedule. Aluminum is expensive and has other inherent problems, but you could use divinicell or balsa or… We made our boards so most of the strength was in the skin. Whatever internal structure we used just helped keep the parts from over flexing (flattening out). Another thing to consider when joining the halves is - as the board flexes, the deck and bottom will want to flex at different rates. That’s going to put a lot of stress on your internal structure. The parts that connect the halves together will get pulled on top, and pushed on the bottom. Design in some give - either in it’s structure, or where it attaches to the skins.
Speaking of flex… I made a set of about twenty 9’1"s with varying flex patterns. The boards came from the same molds, and were identical in weight and dimensions. The difference in the way they surfed was HUGE.
The boards with the most flex would bend in turns, then not want to unflex. The result was a board that felt spongy and sluggish. They would initiate turns and not want to let you out of them.
On the other end of the spectrum were the boards that didn’t flex much at all. These boards felt lifeless and stiff. They were slow or impossible to initiate turns, and they just weren’t any fun to surf.
Somewhere in between was the magic board. With the right flex characteristics, the board would load up in a turn, then spring back quickly, launching you out of the turn and helping generate speed. These boards surf unreal. They turn on a dime, generate speed from turns, and are ready to react to your input when you give it. These boards work so well that I’ve surfed them in juicy triple-overhead waves in Hawaii and California with no drama.
Your setup will give you the opportunity to do the same kind of testing. I don’t envy how much work you still have in front of you though. Carving foam is way easier.
