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 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. 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. Benefits of the process include: energy efficiency, high productivity with low costs and ease of automated assembly line production.
An ultrasonic welding machine comprises 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 50-60 Hz, into a high frequency electrical supply operating at 20, 30 or 40 kHz. 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 work piece 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.
Following are the factors for consideration in the ultrasonic welding process:
Heating Rate
The heating rate in ultrasonic welding is the result of the combined effects of frequency, amplitude and clamp force. In the heating rate equation, clamp force and frequency appear as multipliers. Frequency is usually fixed for a given machine. The heating rate in plastic varies directly and in proportion to the clamp force applied. When more clamp force is applied, the heating rate increases in direct proportion to the change. However, the heating rate varies with the square of the amplitude – if the amplitude is increased, heating rate increases dramatically. Hence, there is an inversely proportional relationship between the frequency of an ultrasonic welder and its output amplitude.
If the highest available amplitude yields consistently acceptable results is used, minimal part damage and long sonotrode/horn life usually is desirable.
Plastics Material
An important consideration in the ultrasonic welding process is the material. Softer materials do not carry sound as well as harder materials and will require more amplitude from the tool to get a usable amount of amplitude to the joint. Materials with higher melt temperatures will require more amplitude to reach upto weld temperature before the joint detail is gone. Choosing a machine that is lower in frequency and therefore higher in amplitude is often advisable with soft or high temperature materials. Stiffer materials may be damaged by high amplitude, and may heat so quickly that the process becomes uncontrollable. Welding too quickly also can result in weak welds.
Tool Design Limitations
The laws of physics that govern sonotrode/horn design are related to wavelength. Most of the factors that reduce acoustic performance have to do with transverse dimensions - dimensions perpendicular to the direction of amplitude. If a tool has a longer wavelength (lower frequency), it can have larger transverse dimensions. A lower frequency tool will be simpler and potentially more durable than a higher frequency tool doing the same application.
Machines
High frequency welders typically run small tools - making small, delicate parts with great precision. They typically have small, light slides driven by small air cylinders. Low frequency welders typically run large tools at high amplitudes, making larger parts made of softer materials. They typically have large, heavy slides driven by larger air cylinders.
Types of joining
Ultrasonic vibratory energy is used in several distinct assembly and finishing techniques such as:
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.
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.
| Appliance |
In this high-volume market, hermeticity, strength and also cosmetic appearance are important. Applications include: steam iron, pump housing, vacuum cleaner wand, and dishwasher spray arm. |
| Automotive |
Hermetic seals in applications such as lenses, filters and valves. Other applications include: glove box door, instrument cluster, air diverter and mass airflow sensor. |
| Business |
"Clean" assemblies with reduced particulate matter are produced on information storage discs. Other applications include the assembly for ribbon cartridges, and audio and video cassettes. |
| Consumer |
Precision welding, staking and forming operations are used in the manufacture of the Swatch®. |
| Electrical |
Multiple staking and inserting applications are often automated for high-volume production requirements with consistent reliability. Applications include: terminal blocks, connectors, switches (e.g. toggle, dip, rotary quick and diaphragm), and bobbin assemblies. |
| Medical |
Non-contamination and the ability to be operated in a clean room are as important as the strength of the weld. Reliable, repeatable assemblies for critical life-support devices are produced with new capabilities in process control. Applications include: arterial filter, cardiometry reservoir, blood/gas filter, face mask and IV spike/filter. |
| Packaging |
From aseptic packages to toothpaste tubes, the ability of ultrasonic assembly to seal through product contamination in the joint area is a major advantage. In addition to good cosmetic appearance, ultrasonic assembly provides tamper-evident seals for blister packs. Applications include: condiment dispenser, blister package, juice pouch, juice carton and plastic coated paper cups. |
| Toys |
In this highly competitive industry, the elimination of adhesives, screws and solvents, or other consumables is a bonus added to strong, safe, flash-free assemblies. |
Limitations of ultrasonic plastic welding
The materials for ultrasonically welded components are possibly the most important limitation - the process works best when both components are made from similar amorphous polymers. If only one of the components is suitable for welding (or they are incompatible) related ultrasonic joining techniques should be considered. If neither material is suitable for welding (eg. thermoset plastics) then another joining method needs to be used. The size of a continuous ultrasonic weld depends on the horn (sonotrode) that makes it. But sonotrodes are limited in size by physical constraints based on the wavelength of the ultrasound used. Some typical "rules of thumb" for axial-mode sonotrodes (as used in plastic welding) are:
• sonotrode length is half a wavelength
• maximum diameter (or other lateral dimension) is one third of a wavelength, to avoid interference from other modes of vibration
The wavelength depends on the operating frequency and sound velocity of the sonotrode material. In most cases (using a minimum frequency of 20 kHz and common materials such as aluminium, titanium or stainless steel) the maximum wavelength is around 250 mm (10 inches). Hence, the lateral dimension of a sonotrode cannot ever be larger than about 80 mm (just over 3 inches). Lower frequencies, down to 15 kHz or less, permit a larger sonotrode size but with significantly increased audible noise. Larger sonotrodes are often constructed using a series of slots, dividing them up into sections each of which individually obeys the rules. Or alternative modes of vibration (eg. radial) may be used which completely eliminate these limitations. In most cases though larger sections will have further, more complicated rules of their own - finite element analysis and a significant amount of prototyping work will be required to arrive at a successful sonotrode design.
The power required for an ultrasonic welding process depends mainly on the size of the weld, the materials being welded and the efficiency of transmitting power through to the weld. Most ultrasonic systems use control systems to adjust power input automatically as the process demands it, but obviously within the capability of the generator and transducer. With modern electronics used in ultrasonic generators it is the transducer that dictates the maximum power the system can handle, because of the same constraints on physical size discussed above for sonotrodes. Modern ultrasonic transducers can often handle 3kW, and some claim as much as 6 kW, which should push out the boundaries of ultrasonic welding viability. It is difficult to achieve in axial-mode systems, as used for plastic welding, unless the transducers can be applied to completely separate ultrasonic sytems. Thus multiple ultrasonic systems can make discrete welds in several locations on the components. It is not possible to compensate for limited power by increasing the weld time, as more time permits greater heat transfer out of the weld zone.
(Source Courtsey : TWI Ltd, powerultrasonics.com)
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