Ultrasonic Riveting Methods
Two methods are recommended for staking, depending upon the material and staking requirements: the conventional method, which is most frequently used, and the high-pressure method. Both methods of staking require the stud to be properly located and rigidly supported directly below to ensure correct alignment with the horn cavity, and that energy will be expended at the horn/stud interface rather than exciting the entire plastic assembly and fixture.
Conventional Method:
In this instance, the intent is to localize the ultrasonic energy at the top of the plastic stud, so only this area of the stud begins to melt. The mechanics used to generate this method of melt staking often require pretriggering (energizing the ultrasonic horn) prior to contacting the top of the stud. The horn descends onto the stud at a preset speed and pressure (bearing force) to allow surface melting of the stud to occur. It is important that the down speed of the horn is not faster than the melting rate of the plastic stud; this prevents the stud from being deformed or buckling.
The plastic stud melts into the cavity of the horn. A hold (dwell) time is then required to allow solidification of the reformed stud head. Sufficient clamp force between the formed head and the horn during solidification will keep the parts tightly locked together.
High-Pressure Method:
This method involves reforming the plastic stud without reaching its melting temperature on the surface, creating a condition that softens and forms the stud into a mushroom shape when using a flat-faced horn. (Cavities may also be used.) The high-pressure method works best with resilient materials such as ABS and high-impact styrene or polyethylene and polypropylene. However, it has also been found to work well with more rigid materials such as polycarbonate and acetal. The mechanics used to accomplish high-pressure staking require high force between the working face of the horn and the top of the plastic stud before energizing the ultrasonic horn, and using a low-amplitude horn. The horn reforms the stud to generate a mushroom head on the top of the stud. The travel distance selected can permit the parts joined to move freely or be tightly locked together. It is recommended that only low profile plastic studs be used to help prevent stud bending (deflection) when high pressure is applied between the top of the stud and horn. Figure 8 shows a cross section of a mush-room-formed stud.
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