Methods for forming rotor blades, rotor blade mold assemblies, and cores for rotor blade mold assemblies are disclosed. A method includes providing a first shell substrate on a first mold, providing a generally hollow core on the first shell substrate, providing a second shell substrate on the generally hollow core, and providing a second mold on the second shell substrate. The method further includes flowing a resin into a mold interior defined between the first mold and the second mold.