SERVO-DRIVEN ULTRASONIC WELDING SYSTEM AND METHOD FOR WELDING TO A THIN PART WITHOUT READ-THROUGH

An ultrasonic welding method for joining a first thermoplastic part and a second thermoplastic part without causing visible read-through on an exposed surface of the second part. The method includes arranging the first part on an inner surface of the second part. The inner surface is opposite the exposed surface. The first part has an interface portion contacting the inner surface. The method includes causing a horn of an ultrasonic welding stack to be pressed against the first part by applying ultrasonic energy oscillating at a frequency in a range of 45-70 kHz through the horn, to thereby join the first part and the second part together. The horn has at least one protruding distal portion configured to penetrate through the first part as the ultrasonic energy is imparted through the horn. The distal portion has a length longer than a thickness of the first part. A collapse distance of a weld formed at the interface portion is less than the thickness of the first part, to avoid read-through effects on the exposed surface of the second part.