Introduction of Ultrasonic Machine in Plastic Bonding/Embedding/Inserting/Staking/Spot Welding/Forming/Sealing/Slitting

Introduction of Ultrasonic Machine in Plastic Bonding/Embedding/Inserting/Staking/Spot Welding/Forming/Sealing/Slitting

Ultrasonic machine in 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 subject to ultrasonic vibration usually at a frequency of 15khz, 20khz, and 28khz, the ability to weld a component successfully is governed by the design of the ultrasonic machine, the mechanical properties of the material to be welded and the design of the components, since ultrasonic machine in welding is very fast (welding times are typically less than one second) and easily automated, it is 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 machine in welding comprises four main components, power supply/ultrasonic generator, converter/transducer, amplitude modifying device/booster and acoustic too/horn/sonotrodel, the power supply changes main electricity at frequency of 50,60hz,into a high frequency electrical supply operating at 15khz, 20khz, 28khz and 35khz, this electrical energy is supplied to the converter, within the converter, discs of piezoelectric material are sandwiched between two metal section, the converter changes the electrical energy into mechanical vibration 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 machine of welding process

Heating rate: the heating rate in ultrasonic machine of welding is the result of the combined effects of frequency, amplitude and clamping force, in the heating rate equation, clamping 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 clamping force applied, when more clamp force is applied, the heating rate increases in directly 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, here is an inversely proportion relationship between the frequency of an ultrasonic machine in welding, and its output amplitude, if the highest available amplitude yields consistently acceptable results is used, minimal part damage and long sonotrode life usually is desirable

Plastic material: An important consideration in the ultrasonic machine of welding process is the material, soften materials do not carry sound wave 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 temperature will require more amplitude to reach welding temperature before the joint details is going, choosing a machine that is lower in frequency and therefore higher in amplitude is often advisable with soft or high temperature materials, stiffer materials maybe 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 limitation: 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 (low frequency), if can have larger transverse dimensions, a lower frequency tool will be simpler and potentially more durable that a high frequency tool doing the same application

Machine: High frequency machine of welding typically run small tools, making small, delicate parts with great precision, they typically have small, light slides, driven by small air cylinders, low frequency welder typically run large tools at high amplitudes, making large parts made of softer materials, they typically have large, heavy slides driven by large air cylinder

Type 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, which 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 machine in 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 circle 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

Swagging/Forming: Mechanically capture another component of an assembly by ultrasonic 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 ability to overcome material memory

Spot welding: An assembly technique for joining two thermoplastic components at localized points without the necessity for preformed holes or 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 material, smooth. Sealed edges that will not unravel are possible with this method, this 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, localized 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

Limitation of ultrasonic machine in welding: The material for ultrasonically welded components are possibly the most 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 compatible) related ultrasonic joining techniques should be considered, if neither materials is suitable for welding (eg. Thermoset plastic) then another jointing method needs to be used, the size of a continuous ultrasonic weld depends on the horn that makes it, but horns are limited in size by physical constraints based on the wavelength of the ultrasonic used, some typical “rules of thumbs” for axial-mode horn (as used in plastic welding)are:

Sonotrode length is have of a wavelength

Maximum diameter (or other lateral dimension) is one third of wavelength, to avoid interference from other modes of vibration

The wavelength depends on the operation frequency and sound velocity of the sonotrode material, in most cases (using minimum frequency 20khz and common materials such as aluminum, titanium or stainless steel) the maximum wavelength is around 250mm, hence, the lateral dimension of a sonotrode cannot ever be larger than about 80mm, lower frequencies, down to 15khz or less, permit a large sonotrode size but with significant increased audible noise, larger sonotrode 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 prototype work will be required to arrive at a successful sonotrode design

The power required for an ultrasonic machine in welding depends mainly on the size of the weld, the material being welded and the efficiency of transmitting power through to the weld, most ultrasonic systems use control system to adjust power input automatically as the process demands it, but obviously within the capacity of the generator and transducer, with modern electronics used in ultrasonic generator it is the transducer that dictate the the maximum power the system can handle, because o the same constrains on physical size discussed above for sonotrode, modern ultrasonic transducers can often handle 3kw, and some claim as much as 6kw,which should push out the boundaries of ultrasonic machine in welding ability, it is difficult to achieve in axial-mode systems, as used for plastic welding, unless the transducers can be applied to completed separate ultrasonic system, thus multiple ultrasonic system can make discrete weld in several location 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