A packing unit for folding a blank on a packing machine; the packing unit comprising: a packing conveyor having at least one pocket for receiving the blank; and a feed device for inserting the blank into the pocket on the packing conveyor; wherein the packing conveyor comprises: at least a first conveyor belt, and at least a second conveyor belt separate from, parallel to, and alongside the first conveyor belt, said first and second conveyor belts defining a bottom wall of the pocket; a first retaining member, which projects perpendicularly from the first conveyor belt and defines a front wall of the pocket in the direction of the packing path; and a second retaining member, which projects perpendicularly from the second conveyor belt and defines a rear wall of the pocket in the direction of the packing path; and wherein the distance between said first retaining member and said second retaining member in the direction of the packing path defines the length of said pocket; wherein said length is variable according to the format of said blank; the packing unit being characterized in that the feed device comprises: a first suction pickup head, which engages a first panel of the blank inside a store; a second suction pickup head, which engages a second panel of the blank inside the store; and actuating devices (33, 34) for moving the pickup heads (21, 22) from a withdrawal position at the store to a release position at the packing conveyor, to insert the blank inside the pocket on the packing conveyor, and then release the blank, folded into a 'U', inside the pocket on the packing conveyor; and x wherein said actuating devices (33, 34) alters the distance between said pickup heads (21, 22) and said packing unit at said store according to the format of said blank. a packing conveyor having at least one pocket for receiving the blank; and a feed device for inserting the blank into the pocket on the packing conveyor; wherein the packing conveyor comprises: at least a first conveyor belt, and at least a second conveyor belt separate from, parallel to, and alongside the first conveyor belt, said first and second conveyor belts defining a bottom wall of the pocket; a first retaining member, which projects perpendicularly from the first conveyor belt and defines a front wall of the pocket in the direction of the packing path; and a second retaining member, which projects perpendicularly from the second conveyor belt and defines a rear wall of the pocket in the direction of the packing path; and wherein the distance between said first retaining member and said second retaining member in the direction of the packing path defines the length of said pocket; wherein said length is variable according to the format of said blank; the packing unit being characterized in that the feed device comprises: a first suction pickup head, which engages a first panel of the blank inside a store; a second suction pickup head, which engages a second panel of the blank inside the store; and actuating devices (33, 34) for moving the pickup heads (21, 22) from a withdrawal position at the store to a release position at the packing conveyor, to insert the blank inside the pocket on the packing conveyor, and then release the blank, folded into a 'U', inside the pocket on the packing conveyor; and x wherein said actuating devices (33, 34) alters the distance between said pickup heads (21, 22) and said packing unit at said store according to the format of said blank. a first suction pickup head, which engages a first panel of the blank inside a store; a second suction pickup head, which engages a second panel of the blank inside the store; and actuating devices (33, 34) for moving the pickup heads (21, 22) from a withdrawal position at the store to a release position at the packing conveyor, to insert the blank inside the pocket on the packing conveyor, and then release the blank, folded into a 'U', inside the pocket on the packing conveyor; and x wherein said actuating devices (33, 34) alters the distance between said pickup heads (21, 22) and said packing unit at said store according to the format of said blank.