Job Knowledge 61
Ultrasonic welding, for thermoplastic injection moulded components, is a process that uses mechanical vibrations above the audible range. The vibrations, produced by a welding sonotrode or horn, as it is generally known, are used to soften or melt the thermoplastic material at the joint line. The components to be joined are held together under pressure and subjected to vibrations, usually at a frequency of 20 or 40 kHz.
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 and joint. Ultrasonic welding times are short (typically less than one second), which makes the process ideal for mass production. The process is widely accepted in many applications ranging from automotive light clusters to consumer electronics products, such as mobile telephone casings.
An ultrasonic welding machine is shown in Fig.1.
Ultrasonic welding equipment consists of a machine press, generator, converter or transducer, booster, sonotrode or horn, and component support tooling. A schematic of an ultrasonic welding machine is shown in Fig.2.
The generator converts electrical power from the single-phase mains to the correct frequency and voltage for the transducer to convert into mechanical vibrations. The microprocessor unit controls the welding cycle and feeds back key welding information to the user, via the user interface. The user interface also allows the operator to enter the required welding parameters.
The machine stand is designed to hold the welding system or stack and apply the force necessary for welding. It consists of a base-plate, to hold the tooling jig, and a pneumatic cylinder to apply the force.
The machine has a pressure gauge and regulator for adjustment of the welding force. It should be noted that a particular gauge pressure set on one piece of ultrasonic welding equipment will not necessarily provide the same welding force as another machine set at the same gauge pressure.
Welding force should be calibrated using a load cell so that direct comparison of welding forces can be made from machine to machine.
There is also a flow control valve to allow adjustment of the speed at which the welding head approaches the component being welded. Some equipment manufacturers have introduced an electromagnetic force application system in place of the traditional pneumatic cylinder. This gives better control of the approach rate, and can be beneficial when welding small or delicate components.
This is the part of the machine that provides the ultrasonic mechanical vibrations. It is generally a three-part unit consisting of transducer, booster and welding horn, mounted on the welding press at the centre-point of the booster section. The stack is a tuned resonator, rather like a musical instrument tuning fork. In order to function, the resonant frequency of the tuned welding stack must closely match the frequency of the electrical signal from the generator (to within 30Hz).
The transducer, also known as the converter, converts the electrical energy from the generator to the mechanical vibrations used for the welding process. It consists of a number of piezo-electric ceramic discs sandwiched between two metal blocks, usually titanium.
Between each of the discs there is a thin metal plate, which forms the electrode. As the sinusoidal electrical signal is fed to the transducer via the electrodes, the discs expand and contract, producing an axial, peak-to-peak movement of 15 to 20µm.
Transducers are delicate devices and should be handled with care. Once the elements are broken, the transducer will not function.
The booster section of the welding stack serves two purposes, primarily to amplify the mechanical vibrations produced at the tip of the transducer and transfer them to the welding horn. Its secondary purpose is to provide a location for mounting the stack on the welding press.
The booster expands and contracts as the transducer applies the ultrasonic energy. Figure 3 shows a range of ultrasonic welding boosters.
The booster, like other elements in the welding stack, is a tuned device therefore it must resonate at a specific frequency in order to transfer the ultrasonic energy from the transducer to the welding horn. In order to function successfully, the booster must be either one half of a wavelength of ultrasound in the material from which it is manufactured, or multiples of this length. Normally, it is one half wave length.
The welding horn is the element of the welding stack that supplies energy to the component being welded. A typical welding horn is shown in Fig.4. Design of the welding horn is critical to successful welding. It is strongly recommended that welding horn manufacture should only be carried out by companies specialising in ultrasonic welding.
The welding horn, like the booster element, is a tuned device, which, in the majority of applications, also provides mechanical gain. It is typically manufactured in either aluminium or titanium. Aluminium welding horns tend to be used for low volume applications since wear can be a particular problem with this material. Some welding horns have specially hardened tips to reduce wear during welding.
As with the booster element, the length of the welding horn must be either one half of a wavelength of ultrasound in the material from which it is manufactured, or multiples of this length. This ensures that there is sufficient amplitude at the end of the welding horn to effect welding.
The amplitude is typically between 30 and 120µm. The shape of the welding horn is important since stress, caused by the axial expansion and contraction of the horn, could lead to cracking in high amplitude applications. In some applications the welding horn is manufactured with slots in the axial direction. This is to ensure that the maximum vibration amplitude is in the longitudinal direction.
The tip of the welding horn delivers the ultrasonic energy to the component being welded. The tip should be specifically designed to match the component. This will ensure that maximum energy transfer between the horn and the component is achieved. Usually, the tip of the horn is profiled to match the contours of the component.
Finally, the base of the machine press supports the tooling that supports the components during the welding operation. The support tooling is designed to prevent movement of the lower component while the ultrasound is applied. It is often machined to match the contours of the component surface intimately.
The next article will cover component design and welding parameters.
Part 2: Component design
See further information about polymer welding or please contact us.