Ultrasonic riveting and forming

The ultrasonic riveting makes possible an interlocking bonding of two thermoplastics or of thermoplastics with dissimilar materials, e. g. thermoset materials or metals.

In this process the sonotrode serves as a re-forming tool. It determines the forming of the riveting pin.

During the holding time (cooling phase) the sonotrode serves also as fixation stamp, thus the melted material cools down under pressure.

Advantages of ultrasonic riveting:

• solid and seamless fitting joint by overcoming the return elasticity of the plastic material
• short process times
• low-stress bondings
• quick thermal drain via the cold sonotrode; the sonotrode simultaneously serves as fixation stamp

• also deeper areas of a plastic component can be reached with the riveting sonotrodes
• reproducible bolding technology; dissimilar materials can be riveted
• use of basic sonotrodes with screw-in riveting sonotrodes; thus several riveting steps are possible with one head
• no connection elements

Disadvantages of ultrasonic riveting:

• Long process times and short cycle sequences lead to warming up of the riveting sonotrode. Here the sonotrode tip should be ventilated.
• Electronic components are very sensitive against ultrasonic waves. It can lead to destruction of the elements.

• If the parts to be riveted have thin walls, the visible side can show shiny spots and imprints
• If the parts to be riveted have thin walls and also in case of hooks or clips the ultrasonic waves can lead to fissuring

An optimal riveting joint should be based on the following preconditions:

• Principally provide radiuses or rounded punctures at the riveting root; this will reduce the indenting effect
• Do not exceed the relation of pin diameter and pin length of 1:2 in full pins, otherwise use hollow pins (see attachment ‘design of riveting head’)
• For easier parts assembly use inclined insertion surface
• When riveting reinforced plastics and bondings with metal parts the riveting sonotrodes have to be wear-resistant

In the following there are listed some established sonotrode materials:

• Hardened steel sonotrodes
• Hardened Ferrotitanite sonotrodes
• Titanium sonotrodes with coated riveting surface

Moreover, for an optimum riveting joint not only the sonotrode form and the material to be riveted are important, but also the process parameters like:

• amplitude
• melting time
• contact force
• contact speed
• cooling time

Sonotrode forms in titanium, aluminium and steel in the frequencies of 20, 30, and 40 kHz

Basic sonotrodes with several screw-in sonotrodes in a special machine

Basic machine with frame, generators, control cabinet and control

Exchangeable tooling with cylinders, converters, sonotrodes and fixture in a tool transfer trolley

Special machine for riveting a bezel with third brake light - 56 riveting spots

Complete machine with frame, generators, control cabinet, welding tool and control

Detailed view of the sonotrodes, holding-down clamp, cooling and contour fixture

Various riveting applications:

plastic clip of cell housing - 2 riveting spots

The design of the sonotrode operating surface and the riveting pin determine the form of the riveting head. Please see below some examples for an optimal design of the riveting head: