Devices and methods for fusing materials using a heating element, where the overall mass to be sealed varies along the length of the seal. According to the invention, the heating element has a different profile in different areas. According to some aspects, the thickness and/or cross section of the heating element is different in different areas so that when a current is passed through the heating element, each area heats to a different degree. In some aspects, the heating element is shaped to conform to the shape of the parts to be fused together. The transition between areas of different thickness or cross-sectional area, or between areas of different shape may be sharply defined. This abrupt transition may be created by machining the heating element to a finished shape rather than bending flat stock to shape.
