Surface Finishing Sheet-Metal Assemblies When Using Self-Clinching Fasteners

- Dec 05, 2018-

Steel and aluminum panels in assemblies often will undergo surface finishing for reasons ranging from cosmetics to performance, while stainless steel panels may require passivation. For any assembly subject to surface finishing and requiring self-clinching hardware, a central question will arise: How and when should a metal panel be surface finished without degrading the installed performance of the self-clinching fasteners?A basic understanding about self-clinching fastener technology and the issues associated with surface finishing can help guide the way toward best practices.

Self-clinching fasteners provide permanent and reusable load-bearing threads in thin metal sheets and historically have been specified in sheet metal too thin to allow for secure fastening by any other method. Upon installation (usually during the fabrication process), they become integral parts of an assembly, will not loosen or fall out (even when the mating thread is removed), and never have to be restrained from rotation with a tool. Clinching of these fasteners is enabled by a non-round displacer feature, which embeds into the host panel and displaces panel material into the fastener’s undercut. 

This results in the fastener’s permanent attachment in a panel with the capability to withstand rotation (because of the non-round displacer) and axial force (because of the displaced panel material in the undercut). Panels must always be sufficiently ductile to cold flow without fracturing, and fasteners must always be sufficiently harder than the panel to prevent fastener deformation, even as the panel is deformed during the installation process. Although it is always preferable to avoid surface finishing an assembly after self-clinching fasteners have been installed, sometimes this approach may be necessary.

Among the cosmetic reasons for applying a finish to an assembly after fastener installation, there may be concerns that a finished panel would be scratched or otherwise damaged when subsequently handled for fastener installation; there may be a desire to achieve the same color and texture both on the panel and the fasteners; and/or there may be a requirement to achieve surface properties on the fastener unavailable with the standard fastener finish. (Such surface properties may include low light reflectance, color fastness, and “non-stick” properties for clean room applications.)

Another typically encountered reason to apply a finish to an assembly after fastener installation is that some surface treatments on a panel may be detrimental to the self-clinching process when applied before fasteners are installed. These include hard-coat anodizing of aluminum panels and nitriding of steel panels. Such panel treatments increase the hardness and decrease the ductility of a panel’s surface—making the panel surface more resistant to the cold flow required during installation of a self-clinching fastener. Powder coating a panel, too, becomes an issue, because the typical thickness of .003"/ .076mm prevents full installation of the displacer feature on the fastener. Thin brittle platings (such as hardened electroless nickel) also will prove problematic, because they will crack and/or flake off from the severe deformation they must undergo during fastener installation. Thick hot dip galvanizing will likely exhibit some of the same hard panel and brittle plating issues.