Abstract: A disposable self-opener apparatus for opening a tubular bag cartridge and squeezing a pasty composition therefrom includes a conical tube shoulder having an orifice, with the conical tube shoulder, preferably, made by a plastic injection molding procedure. A self-opener device is able to be screwed with the screw cap for joining the self-opener device with said one end of a tube portion of the conical tube shoulder. The self-opener device has gable-shaped pairs of cutting edges on an inner side thereof with the peaks of the gable-shaped pairs of cutting edges pointing in the direction of screwing. A retainer basket for receiving a metal clip of a tubular bag cartridge to be opened is centrally located on the inner side of the self-opener device, so that the metal clip does not enter a nozzle attached to the orifice of the conical tube shoulder for dispensing the pasty composition, thereby preventing the metal clip from obstructing the flow of the pasty composition through the nozzle.