Paper shopping bags are manufactured by folding a continuous strip of paper from a roll. Once the vertical shape of the bag is formed, it's cut into bag-sized pieces and the bottoms are folded in and glued. You can see this in action here:
At about 1:16 you can see how this paper is cut: A large guillotine blade strikes the paper strip, creating a new piece. The paper strip is continuously in motion, increasing the chance that the paper may tear when cut. The serrated cut evenly spreads out forces, preventing the paper from tearing in either direction.
From this point the bottom of this paper tube is folded, forming a complete bag. If a cut is made in-line with the motion of the paper it can be a smooth cut: Some bag designs have a small tab, called a gusset, that joins other pieces to form the bottom. The end of the tab will be serrated from the original cut but its sides will be smooth.
Some paper shopping bags with handles have a smooth opening. These still use a serrated cut but the top of the bag is folded down and glued around the handles to attach them. If you look inside the bag you will still see the serrated edges.
Posted 2980 day ago