A method for joining two workpiece parts during a manufacturing process includes forming a first workpiece part having a first mating surface and forming a second workpiece part having a second mating surface. The first and second workpiece part may be separately distributed or may be joined for distribution. To join the parts, an energy director is formed in the first mating surface by an automated process, the energy director comprising an end portion projecting from the first mating surface such that the projecting end portion of the energy director engages the second mating surface when the first and second workpiece parts are brought into engagement with each other. In one example, the first and second workpiece parts are ultrasonically welded by pressing the parts together while vibrating at least one of the first workpiece part or the second workpiece part.