Provided is a method of thermoforming a molding material to have a bent portion and a flat portion extending from the bent portion. The method includes: placing the molding material on a bending mold, the bending mold having a curved surface; and forming the bent portion of the molding material by heating a portion of the molding material at least to a fluidization temperature such that the portion of the molding material bends due to the weight of the flat portion of the molding material to form the bent portion according to a shape of the bending mold.