General Design Principles for DuPont Engineering Polymers - Module

Figure 11.40 Typical ultrasonic welding machines, b
with magnetostrictive transducer, a with
piezoelectric transducer
Vibrations introduced into the parts by the welding
horn may be described as waves of several possible
types.
• Longitudinal waves can be propagated in any
materials: gases, fluids or solids. They are transmitted
in the direction of the vibration source axis.
Identical oscillatory states (i.e. phases) depend on
the wave length, both dimensionally and longitudinally.
During the operation of mechanical resonators,
the longitudinal wave plays almost exclusively
the role of an immaterial energy carrier (see
Figure 11.41a).
• Contrary to the longitudinal wave, the transverse
wave can be generated and transmitted only in
solids. Transverse waves are high frequency electromagnetic
waves, light, etc. Shear stresses are
required to generate a transverse wave. The latter is
moving in a direction perpendicular to the vibration
inducing source (transverse vibration). This type of
wave must be avoided or eliminated as far as
possible, particularly in the ultrasonic welding
applications, because only the superficial layer of
a
b
96
the welding horn end is submitted to vibrations and
thus, energy is not transmitted to the mating surfaces
of the energy users (see Figure 11.41b).
• Curved waves are generated exclusively by the
longitudinal excitation of a part. Moreover, the
generation of such waves in the application field of
ultrasonics requires asymmetrical mass ratios. On
the area we are considering, waves of this type lead
to considerable problems. As shown on Figure
11.41c, areas submitted to high compression loads
are created at the surface of the medium used, and
areas of high tensile strength also appear, meaning
the generation of a partial load of high intensity.
Figure 11.41 a) Longitudinal wave; b) Transverse wave;
c) Curved wave
Direction of
particle motion
Direction
of particle
vibration
B A B A B A
(b)
Wavelength
λ
(c)
Besides, during the transmission of ultrasonic waves
from the transducer to the welding horn, the wave
generates a reciprocal vibration from the ceramics to
the transducer which could cause the ceramics to break.
When designing welding horns, this situation and also
the elimination of the curved waves should be taken
carefully into account.
In the welding process, the efficient use of the sonic
energy requires the generation of a controlled and
localized amount of intermolecular frictional heat in
order to purposely induce a certain “fatigue” of the
plastic layer material at the joint or interface between
the surfaces to be welded.
λ
(a)
λ
Direction of wave
propogation
Direction of wave
propogation
Direction of
particle motion
Direction of wave
propogation