As disclosed shown, for example, in Non-Patent Document 1 and so on, a formed material having a tubular trunk portion and a flange portion formed on an end portion of the trunk portion is manufactured by performing a drawing process. In the drawing process, the trunk portion is formed by stretching a raw material metal plate. Therefore, the plate thickness of the trunk portion becomes thinner than the plate thickness of the raw material. Meanwhile, the region of the metal plate corresponding to the flange portion undergoes overall shrinkage in response to formation of the trunk portion, and therefore the plate thickness of the flange portion becomes thicker than the plate thickness of the raw material.
A formed material such as that described above may be used as a motor case disclosed shown, for example, in Patent Document 1 and so on. In this case, the trunk portion is expected to perform as a shielding material that prevents magnetic leakage to the exterior of the motor case. Further, depending on the structure of the motor, the trunk portion is also expected to perform as a back yoke of a stator. The performance of the trunk portion as a shielding material or a back yoke improves as the thickness thereof increases. Therefore, when a formed material is manufactured by a drawing process as described above, a raw material metal plate having a thickness greater than the required plate thickness of the trunk portion is selected in consideration of the reduction in plate thickness that occurs during the drawing process. The flange portion, meanwhile, is often used to attach the motor case to an attachment object. The flange portion is therefore expected to have a fixed strength.    Non-Patent Document 1: “Basics of Plastic Forming”, Masao Murakawa and three others, First Edition, SANGYO-TOSHO Publishing Co. Ltd., Jan. 16, 1990, pp. 104 to 107    Patent Document 1: Japanese Patent Application Publication No. 2013-51765