Heating, generation ultrasonic vibration

Many cells, especially bacteria, yeast, etc., require very drastic measures to disrupt them and various cell disintegrators are available for this purpose. Ultrasonic vibrations cause the formation of minute bubbles, a phenomenon known as cavitation, which is caused by the extreme variations in pressure generated by the sound waves, although the generation of heat may cause problems unless the samples are cooled frequently during the treatment. An additional... 

X Zhang, T Inada, A Yabe, S Lu, Y Kozawa. Active control of phase change from supercooled water to ice by ultrasonic vibration 2. Generation of ice slurries and effect of bubble nuclei. International Journal of Heat and Mass Transfer 44 4533-4539, 2001. 

For higher production rates, the above principles apply, but the heat source is generated by ultrasonic vibration. 

Figure 8.8 Stages in the ultrasonic welding process. In Phase 1, the horn is placed in contact with the part, pressure is applied, and vibratory motion is started. Heat generation due to friction melts the energy director, and it flows into the joint interface. The weld displacement begins to increase as the distance between the parts decreases. In Phase 2, the melting rate increases, resulting in increased weld displacement, and the part surfaces meet. Steady-state melting occurs in Phase 3, as a constant melt layer thickness is maintained in the weld. In Phase 4, the holding phase, vibrations cease. Maximum di lacement is reached, and inter-molecular diffusion occurs as the weld cools and solidifies. ...

Ultrasonic head forming and welding is a fast assembly technique. It is a very rapid operation of about 2 seconds or less and lends itself to full automation. In this process high-frequency vibrations and pressure are applied to the products to be joined, heat is generated at the plastic causing it to flow, and, when the vibrations cease, the melt solidifies. The heart of the ultrasonic system is the horn, which is made of a metal that can be carefully tuned to the frequency of the system. The manufacture of the horn and its shape is normally developed by the manufacturer of the equipment. The results from this operation are not only economical, but also most satisfactory from a quality control standpoint. 

Its formation is accompanied by the generation of a spray, resulting from the vibrations at the liquid surface and cavitation at the liquid-gas interface. The quantity of spray is a function of the intensity. Ultrasonic atomization is accomplished using an appropriate transducer made of PZT located at the bottom of the liquid container. A 500-1000 kHz transducer is generally adequate. The atomized spray which goes up in a column fixed to the liquid container is deposited onto a suitable solid substrate and then heat treated to obtain the film of the material concerned. The flow rate of the spray is controlled by the flow rate of air or any other gas. The liquid is heated to some extent, but its vaporization should be avoided. 

Heating or heat sealing can also be achieved by impulse, ultrasonic (beware particle generation), induction, radio or high frequency, electromagnetic, vibration or laser methods. 

Ultrasonic welding is a sohd-state process in which the actual bonding between the two parts to be welded is obtained through the decaying into heat of the fi-iction forces work. The latter are generated by a vibration induced by an ultrasonic transducer. As it is well known, ultrasonic elastic waves are characterized by frequencies which can reach 10 kHz. To obtain the welding, electromechanical generators are used ... 

Vibration welding is similar to ultrasonic welding except that it uses a lower frequency (120 to 240 Hz) of vibration. In this way, very large parts can be bonded. The two thermoplastic parts are rubbed together, under pressure and at a suitable frequency and amplitude, until enough heat is generated at the joint interface to melt and diffuse the polymer surfaces. 

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