description an installation capable of carrying out the method according to the invention has been shown diagrammatically this installation comprises basically a filling machine 1 which is symbolized by a parallelepiped and which is in fact a relatively complex machine copiously described in the literature see for example fr a 2 195 550 and not forming the subject of the present invention at the exit of this filling machine that is to say the location in the region of which the packs 4 are as already specified filled prior to their formation and cutting out and folding the said packs 4 emerge in a unitary manner on a conveyor 9 intended for conveying them into the region of a precutting station 2 this precutting station comprises a laser 5 in particular a co 2 laser intended for producing on the upper face 6 of the packs 4 a precut zone 7 for example of elliptic shape as illustrated in fig1 and 2b since the packs arriving at the precutting station 2 are held fixedly the laser it itself fixed however a precutting station is conceivable in the region of which the laser accompanies the pack at the same speed itself continuing its advance by means of the conveyor 9 the said laser being driven in a return movement as soon.