Factors affecting the joint strength of ultrasonically welded polypropylene composites

Ultrasonic welding of thermoplastic composites has become an important process in industry because of its relatively low cost and resultant high quality joints. An experimental study, based on the Taguchi orthogonal array design, is reported on the effect of different processing factors on the joint strength of ultrasonically welded composites, including weld time, weld pressure, amplitude of vibration, hold time, hold pressure, and geometry of energy director. Three materials were used in the study: virgin polypropylene, and 10% and 30% glass-fiber filled polypropylene composites. Experiments were carried out on a 2000-Watt ultrasonic welding unit. After welding, the joint strength of the composites was determined by a tensile tester. For the factors selected in the main experiments, weld time, geometry of energy director and amplitude of vibration were found to be the principal factors affecting the joint property of ultrasonically welded thermoplastic composites. Glass-fiber filled polymers required less energy for successful welding than the non-filled polymer. The joint strength of welded parts increased with the fiber content in the composites. In addition, a triangular energy director was found to weld parts of the highest strength for virgin polypropylene and 10% glass-fiber filled polypropylene composites, while a semi-circular energy director was found to weld the highest strength parts for 30% glass-fiber filled composites.