An impeller is used in, for example, a compressor or a pump. This type of impeller is required to be small in size and highly efficient and is desired to be designed into a shape which satisfies the requirements. To impart kinetic energy from the impeller to a fluid with good efficiency, it is advantageous that the contact area of vanes of the impeller with the fluid becomes larger. Thus, demanding high efficiency results in larger vanes. Further, attempting to reduce the size of the impeller while satisfying the requirements by changing continuously the shape of the impeller from an entrance to an exit of the fluid, the impeller has a shape in which vanes are overlapped each other.
When fabricating an impeller having such a shape, a multispindle machine tool is used to skive it from a block material (refer to JP-A-2010-269417).
When the multispindle machine tool is used, however, the working time becomes long to reduce the manufacturing efficiency, and moreover, the manufacturing cost is increased.
Because of this, even though it is attempted to use a mold to mass produce impellers through molding (including resin molding and casting) in order to increase the productivity, the shape of the impeller prohibits the removal of the mold. Because of this, it is not possible to mold the impeller using the mold. Although it is possible to mold the impeller using the lost wax process, this process is expansive, and hence, it is actually difficult to apply the lost wax process to mass produce impellers of that type.