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Ultrasonic Joint Design Joint Design

Revised 03/07/2008 .


The primary cause of failure is the joint design used and itís applicability to the materials selected. 

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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