A method is disclosed that involves providing a sheet of thermoplastic belt material having a first end with a first radius of curvature, providing a mandrel having a second radius of curvature that is no more than 12% larger or smaller than the first radius of curvature, wrapping the sheet of thermoplastic belt material on the mandrel to form a material overlap region, and welding the material overlap region to form a seamed belt. In one embodiment, instead of cutting the belt to length, the material is then continued to be wrapped around the mandrel, forming a belt material overlap, and held under tension. As the ultrasonic welding horn is traversed across the width of the belt overlap, the horn not only joins the belt material wrapped around the mandrel but, as the trailing edge is held under tension, the horn also separates or severs the trailing edge of the material from the welded seam. A seamed belt is also disclosed.