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 Advanta
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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