In the extruder cylinder, the resin is melted and passed through a circular die to extrude into a tubing state. Air is inserted into the tube to expand it, and at the same time, cooling air is placed in the external tube through an air ring to make the film using the form of a hollow balloon called “Bubble.” Bubbles are removed on the nip roll and post-processed through several guide rolls, depending on the function required by the film. The post-processed film is then rolled up in a double or single roll in a winder or put directly into a box using a Z-folder for commercialization.