Abstract: A rotor for driving, or being driven by, a fluid has rotor blades that follow a screw thread shape, all portions of all blades having the same screw pitch. This enables the rotor, or a molding pattern for use in making a mold, to be withdrawn from a mold part by a screw motion without damage to the mold part. This helps to make it economically viable to manufacture the rotor by a molding or casting process in which the mold parts are not destroyed to release the rotor, such as injection molding, for example with reinforced plastics. Additionally, in the case of manufacturing methods in which the mold is destroyed to release the rotor, the process of making or assembling the mold may be improved. In some molding processes, the ability to remove the rotor or the pattern by screw motion improves the economic viability of the process by reducing the number of mold parts required.