Amplitude and clamp force during an ultrasonic weld can be held steady, which is most common, or varied, which is less common. When varying amplitude and clamp force, studies have shown an increase in pure strength when these factors are decreased as the weld progresses, which slows the heating rate and allows heat to penetrate more deeply into surrounding material. This results in less molecular orientation and residual stress. David Grewell has done a lot of great work on this topic. Generally, amorphous materials respond better to varying amplitude while semi-crystalline materials tend to respond better to varying clamp force. The increase in time to complete the weld using an approach reducing amplitude or force as the weld progresses, however, often makes it necessary to compromise a bit on optimizing for strength alone. Increasing clamp force near the end of weld time seems to produce an altogether different effect, which, while theoretically resulting in less strength, often produces tighter joints offering a greater likelihood of a complete seal and uniform gap closure with a shorter weld cycle. Which is the best approach, or whether to simply use constant amplitude and force during the weld, are very application dependent. Some ultrasonic machines can exert a higher clamp force during hold time which has also been shown to increase the probability of closing gaps and creating leak-proof seals in many applications, particularly with challenging materials such as high melt temperature resins or compounds having high proportions of glass or other reinforcement.