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