Ultrasonic Welding Machine

The device sounds like one from science fiction – a welding machine that can weld plastic or metal using ultrasound to bond the two materials together – yet the fact is that ultrasonic welding machines exist in our prosaic modern-day world and not in an interstellar civilization filled with spaceships, exotic weapons, thinking robots, strange perils, and alien adventures. The first ultrasonic welding machine was developed and patented in 1960, and the technique gradually gained ground over the following several decades. Recently, ultrasonic welding has become sharply more common, but research is ongoing into how to utilize and improve this method of assembly, making it, in some ways, an even fresher technique than laser welding.

Ultrasonic welding operates by causing focused, mechanical vibrations at the same point in two pieces of plastic or metal that are to be joined. A completely different ultrasonic welding machine design is needed for welding plastic and for welding metal. The process can also be used to embed metal parts into plastic, by softening the plastic to the extent that the metal will sink into it at the precise point desired. The main uses are bonding plastic to plastic or metal to metal, however.

An ultrasonic welding machine’s design may vary depending on the substance it is meant to weld, but some features are common to all of the machines.  Most include an anvil – a surface on which the work is positioned for welding – and a sonotrode – the “hammer” which clamps onto the other side of the work and emits the ultrasonic pulse that causes welding. Small, portable ultrasonic welding machines omit the anvil, and feature a hand-held sonotrode, which is pressed to the two sheets of work material (usually, fairly thin plastic) that have already been clamped firmly together.

Welding plastics with an Ultrasonic welding machine

Ultrasonic welding machines that are designed to weld plastic parts together into a single workpiece fires its pulse of intense ultrasound vertically, straight through both pieces of plastic. The plastics should have close to, or exactly, the same melting points for the ultrasonic welding process to offer its best results. High-frequency vibrations are used to produce the sharpest and most intense vibration possible in the plastics, with sound frequencies anywhere from 20 to 70 kilohertz. These are outside the range of human hearing, but will probably raise the hackles on any dog unlucky enough to be nearby.

The vibrations, together with the friction between the two pieces of plastic as they rebound against each other, melt both pieces of plastic at the point where the ultrasound is concentrated. The liquefied plastic of the two pieces mixes together, then cools and hardens almost immediately, creating a very strong, clean bond between the parts.  Weld time is often no more than a single second, and there are no byproducts such as heat or fumes.

Welding metals as well

Metals can also be welded using an ultrasonic welding machine, although, in this case, the metal is not actually melted to create the weld. The sonotrode is built to introduce the oscillations horizontally, in the same plane as the two pieces of metal to be joined rather than at right angles to them.  The metal is under pressure, and the shock of sound along its horizontal plane disrupts the structure of both pieces of metal enough so that they ‘diffuse’ into each other – that is, they literally merge together without melting. Different kinds of metal can be welded in this way, and the bond is strong and lasting. This type of ultrasonic welding requires very precise control, however, and at the moment, it is practically limited to very large welding operations with technicians and many resources.