These rigid foam fins are die-cut from a unique rigid foam material that is super lightweight and has a thin skin on both surfaces for strength. Much more rigid and sturdy than foam meat tray type fins. Each fin is precisely cut to fit into a center ring. After the three fins have been slid into place and the alignment carefully checked, you apply a fillet of low temp hot melt glue to each fin/ring joint. This allows you to get perfect alignment each time. And for the SAFEST fin attachment system ever, the completed fin unit is held onto your PET bottle by the molded rocket nozzle (99-7321). Fins are NOT required to be glued to the surface of the PET bottle.