A method of joining substrate portions includes positioning the substrate portions such that the substrate portions overlap at an overlap area. The substrate portions each have a melting temperature and an outer surface. A fluid is heated to a temperature sufficient to at least partially melt the substrate portions. A jet of the heated fluid is directed from a fluid orifice onto the substrate portions at the overlap area. The heated fluid penetrates at least one of the outer surfaces of the substrate portions. The substrate portions are at least partially melted using the heated fluid. The substrate portions are compressed using a pressure applying surface adjacent the fluid orifice to join the substrate portions together at the overlap area.

An example system comprises a water-soluble unit dose article comprising a fibrous non-woven sheet shaped to form a sealed internal compartment and a granular laundry detergent comprised within the internal compartment. The sheet comprises fibres which comprise polyvinyl alcohol polymer. The granular laundry detergent comprises perfume particles. The system also comprises sealing jaws, whereby at least one of the sealing jaws is a heated sealing jaw. A top seal of the internal compartment is sandwiched between the jaws. A first of said jaws comprises a protruding portion along a longitudinal direction of the seal. The protruding portion has a base along a direction perpendicular to the longitudinal direction of the seal. The base has a dimension of more than 3 times and of less than 20 times a thickness of the water-soluble fibrous non-woven sheet.