This invention relates to a method for sealing film layers to form a sealed portion and a notch within the sealed portion, comprising: providing film layers between sealing jaws comprising a heat sealing element, with a sealing face having a length and a protrusion extending partly along the length of its sealing face; inducing a temperature gradient along the of sealing face length, wherein the temperature proximate to the protrusion is higher than at a distance from the protrusion; closing the jaws and simultaneously clamping the film at a location adjacent to the sealing face; holding the jaws closed for a period of time; and opening the jaws to release the layers of film.