No More Welds: Fasteners for Composites and Other Rigid Materials

Ann R. Thryft, Senior Technical Editor, Materials & Assembly

When I met with the folks from PennEngineering at the Pacific Design & Manufacturing show earlier this year, I asked them what was in the pipeline since I know they’re always looking ahead. Director of global marketing Leon Attarian told me in confidence that the company expected to launch a self-clinching fastener for composites later this year.

That fastener is now available. It’s the PEM VariMount Fastening System, and it works on more than composites: as PennEngineering’s engineers discovered during R&D, it was easy to tweak the design a little so it could work on any rigid material. The VariMount is designed to mount on panels made of composites, as well as metals or plastics, Attarian told Design News.

Some ongoing research by the Fastening and Joining Research Institute, which investigates both adhesive and fastening technologies, is looking at how to design threaded fasteners for use in bolted assemblies made of several different kinds of materials, such as metals, composites, and plastics. A specific area of research is comparing mechanical fastening and adhesive bonding of composite and polymer joints. But many fasteners, if not most commercially available ones, simply don’t work in plastics or composites.

Outside of some very highly specialized fasteners designed for automotive and aerospace applications, components made of composites and other plastics are usually joined together using adhesives. That’s because, assuming it’s even possible to drill holes in composites without breaking them, these materials have a much lower bearing strength than metals. Consequently, fasteners used in aerospace composites, for example, must be designed differently from those used with metals, and require a larger bearing area under the head of the fastener and also under the nut or the collar.

The VariMount assembly can be mounted to a variety of materials using a variety of mounting methods, Attarian said. It consists of a self-clinching PEM fastener permanently mounted on a round steel or stainless steel baseplate, without the need for welds. The entire assembly itself can then be mounted on a composite panel on the front or through the panel’s back.

The baseplate’s wide footprint and multiple radial holes provide for multiple methods of mounting the assembly, including mold-in or laminate with layers, adhesives, standard fasteners, or spot welding. The holes are sized to accept universally standard diameters of rivets, self-clinching fasteners, and loose hardware such as nuts, bolts, and screws. Several different PEM self-clinching fastener types for accepting mating hardware in the application can be specified for VariMount assemblies. You can download a product bulletin here, and find out more about the PEM VariMount Fastening System here, including detailed specifications, fastener drawings, and models.


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