Plastics joining

Ultrasonic Welding technique

The process

Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20, 30 or 40kHz. The ability to weld a component successfully is governed by the design of the equipment, the mechanical properties of the material to be welded and the design of the components.

Since ultrasonic welding is very fast (weld times are typically less than 1 second) and easily automated, it is a widely used technique. In order to guarantee the successful welding of any parts, careful design of components and fixtures is required and for this reason the technique is best suited for mass production.

An ultrasonic welding machine consists of four main components: a power supply, a converter, an amplitude modifying device (commonly called a Booster) and an acoustic tool known as the horn (or sonotrode). The power supply changes mains electricity at a frequency of 50-60Hz, into a high frequency electrical supply operating at 20, 30 or 40kHz. This electrical energy is supplied to the converter. Within the converter, discs of piezoelectric material are sandwiched between two metal sections. The converter changes the electrical energy into mechanical vibratory energy at ultrasonic frequencies.

The vibratory energy is then transmitted through the booster, which increases the amplitude of the sound wave. The sound waves are then transmitted to the horn. The horn is an acoustic tool that transfers the vibratory energy directly to the parts being assembled, and it also applies a welding pressure. The vibrations are transmitted through the workpiece to the joint area. Here the vibratory energy is converted to heat through friction - this then softens or melts the thermoplastic, and joins the parts together.

Benefits of the process include: energy efficiency, high productivity with low costs and ease of automated assembly line production. The main limitation of the process is that the maximum component length that can be welded by a single horn is approximately 250 mm. This is due to limitations in the power output capability of a single transducer, the inability of the horns to transmit very high power, and amplitude control difficulties due to the fact that joints of this length are comparable to the wavelength of the ultrasound.

Types of joining

Ultrasonic vibratory energy is used in several distinct assembly and finishing techniques as described below:

Welding

The process of generating melt at the mating surfaces of two thermoplastic parts. When ultrasonic vibrations stop, the molten material solidifies and a weld is achieved. The resultant joint strength approaches that of the parent material; with proper part and joint design, hermetic seals are possible. Ultrasonic welding allows fast, clean assembly without the use of consumables.

Staking

The process of melting and reforming a thermoplastic stud to mechanically lock a dissimilar material in place. Short cycle times, tight assemblies, good appearance of final assembly, and elimination of consumables are possible with this technique.

Inserting

Embedding a metal component (such as a threaded insert) in a preformed hole in a thermoplastic part. High strength, reduced moulding cycles and rapid installation with no stress build-up are some of the advantages.

Swaging/Forming

Mechanically capturing another component of an assembly by ultrasonically melting and reforming a ridge of plastic or reforming plastic tubing or other extruded parts. Advantages of this method include speed of processing, less stress build-up, good appearance, and the ability to overcome material memory.

Spot Welding

An assembly technique for joining two thermoplastic components at localised points without the necessity for preformed holes or an energy director. Spot welding produces a strong structural weld and is particularly suitable for large parts, sheets of extruded or cast thermoplastic, and parts with complicated geometry and hard-to-reach joining surfaces.

Slitting

The use of ultrasonic energy to slit and edge-seal knitted, woven and non-woven thermoplastic materials. Smooth, sealed edges that will not unravel are possible with this method. There is no "bead" or build-up of thickness on the slit edge to add bulk to rolled materials.

Textile/Film Sealing

The use of ultrasonic energy to join thin thermoplastic materials. Clear, pressure-tight seals in films, and neat, localised welds in textiles may be accomplished. Simultaneous cutting and sealing is also possible. A variety of patterned anvils are available to provide decorative and functional "stitch" patterns.

Typical applications

Ultrasonic assembly is the method of choice for many applications in the automotive, appliance, medical, textile, packaging, toy and electronics markets, among others. The basic advantages of ultrasonic assembly - fast, strong, clean and reliable welds - are common to all markets. However, each market has specialised needs that they rely on ultrasonic assembly to meet.