PURPOSE: To obtain the forming force capable of forming integrally a form having protruding pieces on the circular side wall thereof by a method wherein a movable special core is equipped in the forming force.

CONSTITUTION: The forming force 23, utilized for forming an annular case 9 in a balancer provided on the upper part of a revolving tank for a dehydrating and washing machine, is consisting of a fixed force 24, a movable force 25 and the core 27. The fixed force 24 is formed with a plurality of guide holes 26 corresponding to the multiple protruding pieces 18 provided on the outer peripheral side wall 16 of the annular case 9 while the core 27, having a notched recess 27a for forming the protuberances 18, is inserted into and mounted to the guide holes 26 movably into the up-and-down direction thereof. After forming the annular case 9 and the protuberances 18 integrally by the forming force 23 and the core 27, the movable force 25 is moved upwardly to separate it from the mold at first, then, the annular case 9 is pushed out upwardly by a knockout member. In this case, the core 27 is moved upwardly by a rod 28 synchronizing with the motion of the knockout member. In accordance with this operation, the core 27 is moved to the peripheral direction thereof and, at the same time, is moved to the central direction to remove it from the force.