The primary cause of
failure is the joint design used and itís applicability to the materials
The use of an energy
director is common practice in ultrasonic joint design. Using this geometry
vibrations caused by the application of ultrasound are focused to a point. The
point source also assists in increasing stress at the contact point. The design
relies on the compressive vibrations generated by the ultrasound generating
frictional heating of the polymer at the energy director tip. Molten polymer
flows across the weld face as the joint is compressed, allowed to cool to freeze
the weld face and form the bond.
This joint geometry works
well for amorphous thermoplastics such as polycarbonate or ABS due to a broad
melting range allowing for significant polymer flow. However, semicrystalline
polymers such as the Nylon used in the socket have a limited melting range.
Using the energy director design the tip melts as expected but when removed from
the heat source rapidly freezes before it can flow over the entire weld face.
For semicrystalline polymers a far better
joint deign is to use a shear of scarf joint.
Using this joint design a
point contact is still maintained on the corner of the tongue. However,
relative movement of the two weld faces is converted from compression to shear.
As the heat is produced at the weld front molten polymer is smeared across the
weld face. This joint design eliminates the need for polymer to flow
across the weld face and hence produces a significantly stronger joint for
rapid freezing semi crystalline polymers.
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