A device for folding an edge portion (72) along a circumferential direction of a mantle of a closure cap (70) for a container, for forming a retaining ring on the closure cap (70), comprises a first transport device for transporting the closure cap (70) along a transport path, which comprises a processing path, wherein the first transport device comprises a driver (60) rotatable about an axis of rotation for receiving a closure cap (70). A stationary folding rail (10; 50) with a folding angle that changes along the processing path is arranged along the processing path in such a way that during transport of a closure cap (70) by the first transport device along the folding rail (10; 50), the edge portion (72) can be folded along the circumferential direction of the mantle of the closure cap (70) in the direction of the axis of rotation about a folding axis that is orientated orthogonally to the axis of rotation and spaced apart from the axis of rotation.