When resin is fed in the hopper, it is advanced by a screw and heated until it melts. At this point, when air is blown into the molten resin being extruded, a tube-shaped film is produced. The sides of this tube-shaped film are then cut away to create a flat sheet, which is rolled up to manufacture biaxially oriented film. This is the blown type process, while the cast method uses the same feeding approach, but the molten resin is cooled using a cooling roller.