In ultrasonic staking, also referred to as ultrasonic “heading or riveting”, the controlled flow of the molten plastic is used to capture or retain another component, usually of a different material. Ultrasonic staking provides an alternative to welding when the two parts consist of dissimilar materials which cannot be welded or when simple mechanical retention of one part relative to another is adequate (i.e. as opposed to molecular bonding). The most commonly used application involves the attachment of metal to plastic. A hole in the metal part receives a premolded plastic boss. Vibrating at high frequency, the horn tip contacts the boss and through friction creates localized heat. As the boss melts due to frictional heat, the light pressure from the horn forms a head to a shape determined by the horn tip configuration. When the vibrations stop, the plastic material solidifies and the dissimilar materials are fastened together. Unlike ultrasonic plastics welding, staking requires that out-of-phase vibrations be generated between the horn and the plastic surfaces. Therefore, a light, initial contact pressure is a requirement for out-of-phase vibratory activity within the limited contact area. It is the progressive melting of the plastic boss under continuously but light pressure that forms the head. When staking, low pressure rather than high pressure is usually recommended. Ultrasonic staking should be considered when the parts to be assembled are still in the design stage. Several configurations for boss/cavity design are available, each with specific features and advantages. Their selection is determined by such factors as type of plastic, part geometry, assembly requirements, machining and molding capabilities, and cosmetic appearance. The principle of staking is the same for each; the area of initial contact between the horn and the boss is kept to a minimum in order to concentrate the energy and produce a rapid melt. The integrity of an ultrasonically staked assembly depends greatly upon the geometric relationship between the boss and the horn cavity. Proper design will produce optimum strength with minimum flash. Whenever possible, the bosses should be designed with an undercut radius at the base to prevent fracturing or melting. Holes in the mating parts should be radiused or at least deburred. Long bosses should be avoided, but if used, they should be tapered from the base to the top. The boss should be properly located and rigidly supported from below to ensure that the energy will be dissipated at the horn/boss interface rather than exiting the entire plastic assembly and fixture. Best staking results are obtained when the ultrasonic vibrations are started before the horn contacts the boss. This prevents “cold forming” and allows for the gradual reforming of the boss. The pretriggering of the ultrasonic vibrations is normally accomplished using a pretrigger switch.