An ultrasonic welding method which requires no external pressure during the welding step is disclosed. A plurality of multi-layer strips can be welded such that the weld area is not thinner than the rest of the strip and has good UV protection. The method comprises: stacking a plurality of strips; providing external pressure; removing the external pressure, positioning the ultrasonic welding apparatus so that it places no external pressure on the stack; welding at least two strips; and removing the welding apparatus.