An experimental design is presented for optimizing the ultrasonic welding process in series and mass production, which requires a quantitative test method for measuring product quality. In addition to measurements of an average value, the standard deviation is also important, Small standard deviations lead to reduced scrap in production. For the optimization study, a simple welding process was chosen. Welding machines usually have adequate accuracy for a repeatable machine setting with respect to weld quality. In the cases investigated, they were not sufficiently accurate to allow for a simple quality control using the microprocessor in the machine control as a measuring device. Apart from the insufficient accuracy of the welding machines for quality control, there proved to be no correlation between weld quality and weld-time measurement under energy control.