A high-throughput technique for welding thermoplastic
Welding tools have not wear, so it's possible the automatic production of millions of parts at low cost. Other applications include ultrasound occlusion metal parts, riveting and termplásticos combining porous materials.
Ultrasonic welding 40 kHz is suitable for mass production, for example for automobile taillights, film cassettes, slide mount, and fountain pens. Large parts such as automobile air deflectors are welded to 20kHz. Other frequencies such as 15 kHz, for welding engineering plastics are used.
The ultrasonic welding equipment are mainly made up of the generator, the piezoelectric converter and horn. The generator converts the normal current at 50-60 Hz to produce electrical oscillations of high frequency of 15-40 kHz. The piezoelectric ceramic converter converts electrical oscillations into mechanical ultrasonic oscillations. And the sonotrode transmits, through an enhancer, the oscillations to the parts to be welded, which are fixed between it and an anvil, thereby producing friction whose energy is dissipated as heat and raises the temperature at the interface required for welding between 0.02 and 2 s.
The maximum power of the big teams to 20 kHz ultrasound is 3 kW and more silent machine to 40 kHz produces 1.5 kW. Only a few materials to resist the sonotrode vibratory stresses for extended periods, such as titanium, possibly protected by carbide. A pressure of 2 to 5 N / mm \ 'b2 is required to ensure that the sonotrode not destroy the material to be separated from it. For small series aluminum it is also used: two materials combine good stiffness with low density.
Sonotrodes are manufactured in many ways and each must carefully optimized to ensure that resonance occurs purely axial vibration mode which is constant across the area of ??the sonotrode in contact with the plastic. It is essential that no axial adjacent resonances are avoided because they may interfere with the welding process.
By welding contact or near field filled packaging tubes, or the like are welded, making welding sonotrodes transversely with spider-like, irrespective of the nature of the filling material; is powder, paste or fluid.
The most common devices are arranged in a press, incorporating in its upper part the microprocessor and the central vibrated assembly including the generator, the piezoelectric converter and booster and the sonotrode
The major application area is remote welding. Vibration can lead from sonotrodes through the adjacent portion of the article for welding edges which are arranged to avoid overlapping parts of molten solder. In this way they can be welded to rigid plastics such as 40 kHz PS, SAN, ABS, PMMA, POM, PC, PET / PBT and PA dry, and are also mutually weldable partially.
The presence of fillers or fibers made more difficult a waterproof welding. There are various types of equipment, which are usually in the form of static type press, but also manual employees has basically devices for positioning inserts, do blísterso spot welding.
Automatic insertion of components, welding in precise areas and other works are made with rotary tables complement or coordinate to ensure good accuracy and repeatability of the operation.
Design procedures sonotrodes
It is understood that each type of welding involves the development of a specific sonotrode for each case. In the past it was used for this design basically technical experience and assimilation to similar cases, which often necessitated an iteration of the design and even the need for prototypes.
Many designs use slots machined through the sonotrode to ensure, varying their number and arrangement, the horn resonance at the desired frequency.
These settings allow the engineer afinarel instrument to produce a more uniform level of vibration on the exit surface and get as much separation as possible between the desired frequency and harmful axial transverse directions. It is not always possible to obtain satisfactory results with this technique and it is necessary to machine a new prototype with a different way that presupposes more satisfactory.
Understandably, this procedure is costly and time-consuming without a total guarantee that an optimal configuration is obtained and the most simplified form, since the weight is important in horn performance.