The hold time in ultrasonic welding refers to the duration of time that the force is applied after the ultrasonic energy has stopped. It is the period during which the plastic cools down and solidifies, forming a strong bond. The hold time is an important parameter in ultrasonic welding as it affects the strength and quality of the bond. If the hold time is too short, the plastic may not cool down and solidify properly, resulting in a weak bond. On the other hand, if the hold time is too long, the plastic may become over-cooled and brittle, also resulting in a weak bond. The optimum hold time depends on the specific materials being welded and the parameters of the ultrasonic welding process.