Basic Process Variables

The heating rate in ultrasonic welding is the product of the loss modulus of the material, frequency, clamp force, and the square of amplitude. Loss modulus is the ability of the material to convert repetitive variation in compressive load into heat. In other words, how easy it is to heat the material using ultrasound. Think of this as being inversely proportional to intermolecular lubricity. Slippery materials are harder to heat up than less slippery materials. This is an oversimplification, but it will work for now. Loss modulus “is what it is” when you go to weld a part, but knowing it is a factor can be useful in rectifying a troublesome application. Frequency is determined by the equipment and tooling and there is not much you can do about it if you are standing next to the machine with the tooling installed, but careful thought should go into selection of the right frequency for the job. More on that later. Amplitude, as we have seen, is determined electrically and acoustically. Clamp force is generally provided by an air cylinder and is adjusted by adjusting the pressure in the cylinder. The ultimate temperature of the joint is determined by the heating rate and the exposure time. In the simplest terms, for any given weld, one can increase the heating rate by increasing clamp force or amplitude and decreasing exposure time, or decrease the heating rate by decreasing ampltiude or clamp force and increasing exposure time. When changing the heating rate, it is important to remember that the heating rate is much more greatly affected by changes in amplitude than it is by changes in clamp force.