In order to reduce cost/environmental impact for prototype fins I tried filling 3D printed fins with Polyurethane expanding foam. The claim is it expands 20-25 times which makes 2Kg (1666ml) raw product purchased here in Perth at $45 (at the furthest reaches of the known universe) cost around 3cents per ml. But since this stuff expands (assuming max claimed expansion of x25) the cost reduces to $0.0012 per ml of finished product. Real world tests show that the actual expansion in a fin is not close to claimed but closer to 10-15% The fins I am using have total volume of around 350ml (including the tuttle) and with 15% infill thats about 36ml required. Add 14ml for wastage so required volume is around 50ml and the cost is approx $1. Even if my calcs are out you are still way ahead. The strength achieved far exceeds expectations.
