Ultrasonic Welding:
Ultrasonic
welding is represented as a friction welding method, where oxides and
other contaminants present on the material surfaces are broken up and
also the components to be welded are brought together under simultaneous
pressure. Molecular bonding, just like the conventional cold-press
welding, then takes place. Ultrasonic welding is the conversion of high
frequency electrical energy into high frequency mechanical energy. In
ultrasonic welding spot welds in thin steels are produced by the local
application of high frequency vibrating energy to work pieces held
together under pressure. The work pieces are clamped together under a
moderate static force applied normal to their face and oscillating shear
stresses of ultrasonic frequencies (1 KHz to 40 KHz) with a power
ranging of 700 to 6000 watts are applied parallel to the interface. The
vibrating probe called “a sonotrode” induces lateral vibrations and slip
between the surfaces fracturing the brittle oxide layers and softening
the asperities because of localized heating. The combined effects of
pressure and vibrations cause movement of metal molecule bringing about a
sound weld.
The
bonding is achieved in solid state without application of external
heat, filler rod or high pressure. There is also no need for any
thorough cleaning before welding because all contaminants, oxides,
moisture etc are removed by the vibrating motion.
Ultrasonic Welding Equipment:
The ultrasonic vibrating unit consists of following main components:
- Frequency converter,
- Booster,
- Horn or sonotrode,
- Pneumatic Press /Actuator,
- Ultrasonic power supply, and
- holding fixture
This
converts 50 Hz – 60 Hz line power into high frequency electrical power
and a transducer which changes the high frequency electrical power into
ultrasonic vibratory motion that is transmitted to the joint. The weld
is completed in 0.5 to 1.5 seconds.
Ultrasonic welding of plastics:
Plastics
are typically engineered materials consisting of polymers. Polymers are
shaped by polymerisation that may be a chemical action during which two
or more molecules are combined to make a larger molecule. Polymers are
often classified as either thermosets or thermoplastics. Thermosets
aren’t appropriate for ultrasonic assembly because they degrade when
subjected to intense heat. Thermoplastics on the opposite hand soften
when heated and cool when hardened and are thus ideally fitted for
ultrasonic assembly.
Materials for Ultrasonic Welding of Plastics:
Most
of the thermoplastic materials can be ultrasonic weldable. Teflon with
low coefficient of friction and high melting temperature is impossible
to weld using this process.
Welding Temperature Achieved:
Ultrasonic
welding produces a localized temperature rise from the combined effects
of elastic hysteresis, interfacial slip and plastic deformation. The
weld interfaces reach roughly 1/3 the temperatures required to melt the
metals. Since the temperature doesn’t reach the melting point of the
material, the physical properties of the welded material are preserved.
As the ultrasonic welding method is an exothermic reaction, as welding
time will increases so does weld temperature.
The
ultrasonic welding process has the advantage that since no bulk heating
of the work pieces is involved and there is no danger of any mechanical
or metallurgical bad effects. Although metals have up to 2.5 mm thick
have been welded by this process. It is used mostly for welding foils.
This process is suitable only for thermoplastics with the exception of
thermosetting resins and Teflons. The process can be used on a variety
of metals including the refractory metals. Even dissimilar metals can be
welded because there is no fusion. The process can also be used on
temperature sensitive materials because temperature rise is limited.
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