Abstract: A method for making a rotating part of plastic material for a permanent magnet motor and a rotor arranged on the outer circumference of the rotating part, said rotor comprising an annular pan (1) and a permanent magnet unit fitted around it, in which method the rotating part is cast in a mold tool (4). The annular part (1) together with the permanent magnets (2) is placed inside a plastic injection mold tool (4), said plastic injection mold tool (4) being divided radially into several sectors (4a-4d) which are provided with slots (6) allowing them to be moved radially by an actuator. When the plastic injection mold tool (4) is being closed, the sector are tightened radially inwards, specifically closing the air gap between the outer surface of the permanent magnet unit (2) and the sectors of the plastic injection mold tool (4), whereupon plastic injection molding is performed.