The openable discharge spout (35) of a relatively elastic and fluid-proof inner bag (32) of a double-walled bag of high loading capacity comprises four first lines (37) which are parallel to the longitudinal axis of the inner bag forming the spout itself, and four second lines (38) which extend between the discharge spout and the corners of the bottom of the outer bag when the double-walled bag has been filled. Those first and second lines are created by heat welding plies (39-42) of an originally tube-shaped material and removing waste material from it to create the spout.
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