Making of Ultrasonic Weld:
Although
the theoretical method of manufacturing an ultrasonic weld is
uncomplicated, the interactions of the varied weld parameters are vital
and may be understood. When manufacturing an ultrasonic weld, there are 3
primary variables that interact;
They are:
• TIME the period of applied ultrasonic vibration
• AMPLITUDE the longitudinal displacement of the vibration
• FORCE the compressive force applied perpendicular (normal) to the direction of vibration
Power needed initiating and maintaining vibration (motion) throughout the weld cycle will be defined as:
P = F x A
Where:
P = Power (watts)
F = Force (psi)
A = Amplitude (microns)
Force = (Surface Area of the Cylinder) X (Air Pressure) X (Mechanical Advantage)
Energy is calculated as:
E = P x T
Where:
E = Energy (joules)
P = Power (watts)
T = Time (seconds)
Thus the complete ‘Weld to Energy’ process would be defined as:
E = (F x A) x T
A
well designed ultrasonic metal welding system can compensate for normal
variations within the surface conditions of the metals by delivering
the required energy value. This is often achieved by permitting time (T)
to regulate to suit the condition of the materials and deliver the
required energy.
How Ultrasonic Welding Works:
Step 1: The parts to be welded are placed into a locating holder
Step 2: The ultrasonic tool descends to apply a clamping pressure between the weld parts.
Step 3:
The tool then vibrates at a frequency 1 – 40 KHz. (The weld parts are
thus scrubbed together under pressure causing surface oils and oxides to
be dispersed)
Step 4: The base metals are
then mechanically mixed causing a metallurgical bond between the parts.
The parts are immediately welded. There is no hold time or curing time.
In
Ultrasonic welding electrical power supply is applied to a Transducer
at a frequency of 50 to 60 Hz, into a high frequency electrical supply
operating at 20, 30 or 40 KHz. Here transducer converts electrical
energy into mechanical energy. This electrical energy is supplied to the
converts, which converts to mechanical energy at ultrasonic
frequencies.
The
vibrating energy is then transmitted through the booster that will
increase the amplitude of the acoustic wave. The acoustic waves are then
transmitted to the horn. The horn is an acoustic tool that transfers
the vibrating energy directly to the components being assembled, and it
additionally applies a welding pressure. The vibrations are transmitted
through the workpiece to the joint area. The parts are “scrubbed”
together under pressure at 20000 cycles per second. Here the vibrating
energy is converted to heat through friction this then softens or melts
the thermoplastic, and joins the components together. As the atoms are
combined between the components to be welded, a real metallurgical bond
is made.
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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