Abstract: A method for fabricating a bag with a flat bottom comprises: making a flattened tubular element with lateral bellows; obtaining a quadrilateral bottom of a single piece, the perimeter of which has a size equal to that given by the sum of the tubular element and the bellows; folding the bottom in a single piece along one of its first axes, keeping the outer sides parallel to the axis and of a size equal to the initial one; folding the other two sides to identify two opposite bellows, bringing a central folding line of the bellows thus identified into a position perpendicular to the first axis; inserting the bottom thus folded into the flattened tubular element, bringing the two opposite bellows of the bottom so that they are astride of the lateral bellows of the flattened tubular element; and welding the ends facing outwards of the flattened tubular element and of the corresponding lateral bellows respectively to the outer sides, which are parallel to the axis and are of a size equal to the initial size of the

Abstract: An improved-efficiency welding device in a plastic bag production machine, of the type including an upper welding bar and a lower welding bar, at least one of which is driven with reciprocating vertical movement between a position separated from The plastics material to be welded and a position in contact with the plastics material and the lower or upper other bar, the movement of the at least one welding bar being imparted by a pair of vertically guided rods driven by a first eccentric linkage to which they are connected by a pin, the first linkage obtaining its movement from a central shaft, there being provided a second eccentric between the rod and the first eccentric linkage and driven by an additional motion transmission system which rotates it to cause the at least one welding bar to undergo a further lowering and/or raising movement at a predetermined stage, with resultant welding engagement between the an least one welding bar and the other welding bar.