General Design Principles for DuPont Engineering Polymers - Module

Heat is generated throughout the parts being welded
during the welding process. Figure 11.42 describes an
experiment in which a 10 × 10 mm by 60 mm long rod
is welded to a flat block of a similar plastic.
An ultrasonic welding tool for introducing ultrasonic
vibrations into the rod is applied to the upper end of
the rod. The block rests on a solid base which acts as
a reflector of sound waves travelling through the rod
and block. Thermocouples are embedded at various
points along the rod. Ultrasonic vibrations are applied
for 5 sec. Variation of temperature with time at
5 points along the rod are shown in the graph. Maximum
temperatures occur at the welding tool and rod
interface and at the rod to block interface; however,
they occur at different times.
When sufficient heat is generated at the interface
between parts, softening and melting of contacting surfaces
occur. Under pressure, a weld results as thermally
and mechanically agitated molecules form bonds.
Welding Equipment
Equipment required for ultrasonic welding is relatively
complex and sophisticated in comparison with
equipment needed for other welding processes like
spin welding or hot plate welding. A complete system
includes an electronic power supply, cycle controlling
timers, an electrical or mechanical energy transducer,
a welding horn, and a part holding fixture, which may
be automated.
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a) Power Supply
In most commercially available equipment, the power
supply generates a 20 kHz electrical output, ranging
from a hundred to a thousand or more watts of rated
average power. Most recently produced power supplies
are solid state devices which operate at lower
voltages than earlier vacuum tube devices and have
impedances nearer to those of commonly used transducers
to which the power supply is connected.
b) Transducer
Transducers used in ultrasonic welding are electromechanical
devices used to convert high frequency
electrical oscillations into high frequency mechanical
vibrations through either piezoelectric or
electrostrictive principle. Piezoelectric material
changes length when an electric voltage is applied
across it. The material can exert a force on anything
that tries to keep it from changing dimensions, such as
the inertia of some structure in contact with the
material.
c) Welding Horn
A welding horn is attached to the output end of the
transducer. The welding horn has two functions:
• it introduces ultrasonic vibrations into parts being
welded and
• it applies pressure necessary to form a weld once
joint surfaces have been melted.
Figure 11.42 Variation of temperature along a plastic that has been ultrasonically joined in a tee weld to a plate of
the same material. a) Schematic diagram of transducer, workpieces and thermocouples; b) Variation of
the temperature with time at various points along the rod; c) Temperature readings when the weld
site temperature is maximum (dashed line) and peak temperature produced in the rod (solid line).
Welding horn
15 15 15 15
Reflector
(a)
N 1
N 2
N 3
N 4
N 5
Temperature, °C
200
100
0
5
2
4
Weld
3
1
10
20
t, sec
(b)
Temperature, °C
250
200
140
100
(mm)
30 40 0 15 30
l, mm
45 60
Thermocouples Welding horns
p p
N 1 N 2 N 3 N 4 N 5
(c)
Reflector
240