A second factor
contributing to failure in this instance is again related to the microsctructure
of the polymer used. Semicrystalline polymers can be thought of as being
relatively hard crystalline particles in a relatively soft, rubbery matrix.
This rubber matrix produces high attenuation of the ultrasound.
For this reason
semicrystalline polymers are most suited for use with “near field” ultrasonic
welding techniques. In essence this means that the ultrasonic horn (the energy
source) should be no more than 6 mm away from the weld surface. In this
instance the distance was approximately 10 mm. The implication being that the
weld face would see a relatively low proportion of the applied energy.
The "far field" technique employed in the socket
assembly in question is far more suited to amorphous thermoplastic materials.
Their glassy structure conducts ultrasound well with little attenuation.
For such materials the ultrasonic horn can be placed some distance (up to
200 mm) from the weld face.
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