A forming device is configured to form metal laminated sheet material into a tray-shaped case half while allowing the material to form pleats in the corners of the recess during the forming process while preventing formation of pleats along the flange. The device forms the flange so that excess material in the corners of the case halves is arranged in an undulating shape. As a result, the flange has a waved or ruffled configuration at each corner. Two such case halves are sealed together along the flange to form a reliably sealed package. A method of forming pouch cell is also described.