Angular bottom formation modulo of the synthetic resin make square bottom bag, and airtightness bag body null of the rectangular parallelepiped forma which uses

摘要

PROBLEM TO BE SOLVED: To provide a method of forming a square bottom of a square bottom bag from a flat tube. SOLUTION: The method comprises a forked part forming process wherein a symmetric forked part 12 is formed by tucking in the opposite corner parts 10a of the flat tube, starting from the end parts 11a of a bottom part folding line 11 to be positioned at 1/2 of a tube width (w) from an opening edge side 10 of the flat tube 1; an insertion space forming process wherein an insertion space 13a is formed by raising up the opening edge side 10 located on a bottom edge line 13 parallel to the bottom part folding line in a triangular piece 12a on one side and by putting it over the triangular piece on the other side from the bottom edge line; a bottom plate setting process wherein a bottom plate 2 is inserted into the insertion space; a cutting-welding process wherein the insertion edge side 13b of the insertion space and the triangular piece on the other side are welded by heating and a portion extending from the heat-welded position is cut off; and a process wherein the square bottom is formed by spreading the tube. In the cutting-welding process, it is also allowable that the opening edge side is exposed and held and the exposed portion is heat-melted and then welded, being held with pressure.