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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

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