The objective of this work was to study the differences in the ultrasonic weld strength of polypropylene compounds with different fillers. The fillers were calcium carbonate, talc, mica, and glass fibers. The welder parameters were varied to determine the optimum set. These welder parameters were the weld time, weld force, trigger force, and amplitude. The results indicated that the weld time had the greatest effect on the weld strength of each of the filled compounds. Unfilled polypropylene had the highest weld strength under the optimum welding conditions, which were used as the baseline welding conditions. For each given filler, the weld strength was reduced as the filler loading increased. (C) 2004 Wiley Periodicals, Inc.