BMW i3 employs new joining technology for CFRP parts

  • Combination of bonding & screwing, based on stud welding
  • Adhesive cures within 4 seconds
  • Now in series production at BMW

Composite materials, such as CFRP and GFRP, take established joining technologies such as welding, screwing and riveting to their limits. Assembly specialist Böllhoff and adhesive manufacturer Delo have now developed a new joining method suitable for composites, thin sheet metals, plastics and other materials. The so-called Onsert technology, which is based on a bonded bolt, has been tested in a series of pilot projects. The last one was about fixing of cables, claddings and other components of the BMW i3. After this technique has proven its capabilities in series production, it will now be launched.

Combination of stud welding, bonding and screwing

This technology, based on stud welding, combines bonding and screwing by equipping fastening elements, such as threaded metal bolts, with a transparent plastic base and by bonding it to fiber composite materials. The threaded rod of the Onsert can be screwed and, if necessary, removed again.

Threaded bolt with plastic base and adhesive

Onsert: Threaded bolt with plastic base and adhesive (colored red) on CFRP plate (source: Böllhoff)

A main feature of this technique is its speed. The adhesive is cured within about 4 seconds using an LED lamp. It is possible to fully automate this process. The bonded connection can be loaded immediately. Bonded bolts with a base diameter of 25 mm achieve a pull-off strength up to 2,000 N on CFRP. When applying higher forces, a predetermined breaking point in the base ensures that the laminate of the composite material remains intact and no fibers are torn out. The Onsert elements are very robust as shown in salt spray and VDA climate tests.

Onsert is also flexible in design and production: first, the structural shape can be defined by the user to a great extent, especially in terms of rod length and base diameter. Second, they provide more freedom in production as bonded bolts can be attached to the raw body and prior to final assembly of lacquered elements.

VIDEO: BMW’s automated production process using Onsert

Video of BMW’s automated production process using Onsert.

Developed for the automotive & aviation industry

The Onsert technology is suitable for a wide range of applications where components must be joined. Possible applications include cable harnesses, control units, sensor retainers, brake lines, claddings or exterior attachment components. Cover plates, insulating materials or floor panels in airplanes are also possible.

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