Self-pierce riveting (SPR) has been around for decades, so most are probably familiar with its basic premise. SPR is a cold joining process used to fasten two or more sheets of material by driving an engineered rivet through the top sheet(s) and upsetting the rivet, under the influence of a die, into the bottom sheet without breaking through it.

What’s new is that, with a properly engineered system, SPR can produce high performance joints  in diverse and thinner materials that are often used in today’s applications.

Standard SPR can be used on materials 1.5 mm or 0.060 in. on a simple 2T joint. With more ductile materials, SPR can be used on joints down to 1 mm or 0.040-in. thick. In addition, special configurations can be designed for particularly demanding joints, such as the joining of a thick piece of plastic to thin metal or even plastic-to-plastic. Aluminum applications are common, as are those using stainless steel, pre-coated, pre-painted or pre-plated components.

Backside of SPR joint with trifuricating rivet.

SPR has evolved since its humble beginnings, but still maintains all the benefits of the original technology.

With SPR, no pre-punched/pre-drilled holes are required, and the process does not create a through-hole. No liquids or gases can pass through the resulting joint. This net-hole condition also adds stiffness to the product and provides better resistance to loosening. Unlike some mechanical fasteners, fall-out usually only occurs if the material immediately around the self-pierce rivet completely fails.

SPR can easily join dissimilar materials, including aluminum, stainless steel, high-strength steel, and various plastics ranging from nylon and PP to SMC. SPR can be used on unprepared materials like pre-lubricated steel. SPR can also be used on pre-painted, pre-coated and pre-plated materials without secondary operations to improve appearance. For purposes of matching the joined materials, rivet heads can also be pre-painted, pre-coated and pre-plated.

SPR can also be used on joints that include adhesives, sealants, or insulation. Instead of choosing between mechanical fasteners and an adhesive, one obtains the benefits of both by “rivbonding,” where SPR is used with intermediate layers of adhesives.

Self-pierce rivets are usually made from high-quality grades of steel wire that has been forged, hardened and tempered to provide optimal joint properties. However, the rivets can also be made from austenitic or martensitic stainless steel; copper for special applications; or aluminum when joining soft grades of aluminum material. A variety of head styles can be chosen to meet functional and/or aesthetic requirements. Even threaded and special studs are available, which are ideal for stand-offs and electrical grounding applications.

Applications

A standard rivet.

Standard rivet designs meet the needs of most applications, although rivet geometry and installation settings must be configured to ensure success.

Sometimes more advanced configurations are required for successful SPR joints. For example, to reduce materials to the bare minimum, one group of appliance engineers selected 2.2 mm plastic and 1.2 mm galvanized steel for a new dishwasher design.

They considered self-pierce fastening from the start, but variations in the thickness of plastic material caused inconsistent joint strength when using standard self-pierce processes. Also, the steel thickness was at the minimum required to prevent breakthrough of the fastener.

Testing of these SPR joints showed 2,800 lbs. of static pull strength and 480 in.-lbs. of energy to failure.

The challenge was solved by the development of an automated, trifurcating self-pierce riveting process, using 300 series stainless steel rivets. The process pierces the material, then splits the rivet into three legs on the tail side. Modifications to the die minimizes material drawdown during riveting and prevents sharp edges on the tail side of the joint.

In-house testing showed that this installation process consistently produces high-strength joints. The 300 series stainless-steel rivets and net hole (no tolerances) condition create corrosion-proof and leak-proof joints.

Trifurcating dies have been used successfully on other applications, as well. They have proved useful with thin sheet applications or those with a thick piece of plastic (top sheet) attached to a thin metal part (bottom sheet). One current use is automotive airbag applications, joining 3.0 mm-3.5 mm ABS to 0.7 mm-1.0 mm steel.

A joint after going through static pull testing.

Self-pierce riveting is still a great choice for heavier or more traditional materials. During the design of a new commercial laundry washer frame in mild steel, the project engineers determined that they wanted mechanical fasteners to maintain high quality, durable joints possessing high fatigue strength that could withstand heavy cyclic loads. They selected self-pierce riveting, and Henrob Corp. locations worldwide provided engineering support and joint development throughout the project.

Although the riveted joints held during an unbalanced load test, the engineers had to increase the steel thickness from 16 gauge to 11 gauge mild steel to overcome frame buckling.

Pan head rivets 5 mm diameter were selected for the design. Lap shear tests showed 2,824 lbs. of static pull strength and 480 in.-lbs. of energy to failure per self-pierce riveted joint. Tests of unbalanced loads at high speeds over 1,000 hours showed no distortion with performance equivalent to welded joints.

SPR joint in 300 series stainless-steel corner seam on a refrigerator door.

Another group of appliance engineers was looking for a new way to fasten the corner seams on 300 series stainless-steel refrigerator doors. Originally, a combination of automated and manual plasma welding was used, but with buffing and dressing operations needed to improve appearance, this method could not meet demands for a significant increase in production.

In addition, the decision to use pre-painted and laminated doors on some models completely eliminated welding as a viable assembly option. These engineers also selected a self-pierce fastening system.

SPR causes no damage to the pre-painted and laminated surfaces on the application. This eliminates the need for clean up, which in turn helps shorten production time. The SPR system can also rivet right through a protective sheet on the stainless steel without joint deterioration. This allows the protective sheet to stay in place during assembly and through delivery, minimizing possible surface damage during transit.

Design considerations

SPR is also used in models with laminated doors.

With standard SPR, the head side of the joint can be either flush or raised, while the tail side will have a 1 mm to 3 mm high protrusion, depending on the rivet size.

To employ SPR, two conditions are required:

1. 1. Access to both sides of application (no true blind applications). Tools can be configured with deep throat depths to accommodate wide applications.
2. 2. A minimum surface area for the die that helps set the rivet (3 mm rivets need 12 mm of flat surface area; 5mm rivets need 16 mm).

  Joint design considerations for self-pierce riveting include the following:

• Preferably, brittle material should not be used for the bottom sheet. Examples of brittle material include 7000 Series aluminum, martensitic steels, polycarbonate polymer, and cast iron.
• If possible, the bottom sheet should be ductile with an elongation factor above 12 percent.
• Fastening through the thin or soft material(s) into the thick or hard material is preferred. However, as a rule of thumb, if the joint must be riveted through the thick material into the thin, this can be achieved with standard rivets if the bottom sheet is approximately 1/3 of the total stack thickness.

  Current technology allows joining up to 12 mm total thickness in aluminum and 6 mm in steel.

There are no consensus industry standards for self-pierce riveting. For maximum performance, it is important to have the SPR rivets and dies engineered for a specific application. This up-front planning can result in a lower-cost joint design, minimal part numbers, and a more efficient assembly system.

A single rivet type is used in this application in several different joint thicknesses to simplify assembly. Tooling is engineered to prevent marking on the top sheet.

For example, SPR simplified one refrigerator design because a single rivet style meets the needs of several different sizes and types of joints. The designers originally considered adhesives and clinching to join pre-painted steel to uncoated steel. This method was not pursued because variations in material thickness, due to product size, complicated the manufacturing process.

The company now uses Henrob SPR, with a single rivet type installed with different dies to accommodate all joints, thin or thick. Rivet selection was based on testing the entire range of joints found in the application, ranging in size from 0.017 in./0.125 in. to 0.040 in./0.226 in. Each joint was steel with a pre-painted and embossed top sheet. Henrob engineered a special self-pierce rivet to meet the entire range. A non-standard countersunk head prevents damage to joints with very thin top sheets.

Further engineering developed a special nosepiece on the setting tool to prevent unnecessary marks on the external skin of the joint. The automated system can switch between two available setting pressures in order to select the appropriate pressure for each joint.

The flexibility of this system allows a reduction in fastener part numbers while satisfying strength and appearance needs and accommodating all joint thickness requirements. Using only one rivet type also eliminates the possibility of rivets being loaded into the wrong tool.

As these case histories show, SPR is a very flexible fastening system, in step with the trend towards more efficient material use and joint design.

For more information, email: sales@henrob.com