The weld zone melts instantaneously by internal friction. The mechanical
strength of an ultrasonic weld may reach a value of 70 to 80% of the original
strength of the material, however the actual strength depends on the specific
geometry and the materials being welded.
In general, a shear joint is advised for semi-crystalline thermoplastics
because of their short melting range. Note that the weld can be hidden either
on the inside or on the outside corner to improve appearance. The efficiency
of energy transfer to the weld surface depends largely on the type of
thermoplastic. Stiff parts with low mechanical damping properties can be
easily welded. The distance between horn and weld surface may be larger than 6
mm (0.24 in) (distant welding) in that case. Parts with a relatively low
stiffness should be welded under near field conditions [less than 6 mm (0.24
in)].
Internal sharp corners cause stress concentrations. The use of fillet radii is
strongly advised when using ultrasonic assembly. Proper welds always give
flash.
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