There are basically two methods to produce a blind rivet body. One is by cold heading wire and extrude a rivet body and the other method is by stamping and forming the blind rivet body from sheet raw material. These two methods of producing rivet bodies are completely different methods of manufacturing and tooling or molds.
The extrusion method uses cold heading machines having one die and two blows, up to, 6 dies one blow. The extrusion method uses wire as the raw material and offers a very low scrap waste factor. The only material waste is a slug equal to the diameter of the hole in the rivet body having a thickness of as low as .030 (.076mm). The production speed of extrusion is less than stamping, but the tooling or mold costs are less for extrusion. The benefit of extrusion is low scrap and low tooling costs.
The stamping method uses high speed stamping presses and strip sheet raw material. There is a very high scrap factor with the stamping method, but the production rate is higher than the extrusion method. A stamping tooling die having 5 rows of tooling mounted in a press that can stroke 300 times per minute will produce 90,000 rivet bodies per hour. I have seen presses that can stroke 600 cycles per minute producing 180,000 rivet bodies per minute. Tooling costs is very high but the production rate is very high.
I have been asked many times “How can I know by looking at a blind rivet if the rivet body is extruded or stamped”? There are some visible features between extruded and stamped rivet bodies.
The extruded rivet body will have square corners at the end of the barrel, as shown at No.1. The extruded rivet body is produced by cold heading machines and it is made from wire.
The stamped rivet body will have a radius at the end of the barrel as shown at No.1A. The stamped rivet body is produced by a stamping press and it is made from sheet material.
The stamped rivet body will have a rough looking surface at the edge of the outside diameter of the flange. This edge is formed when the rivet body is punched and cut out of the strip of metal. The barrel of the rivet body is formed in a downward direction and you may be able to feel a slight burr at the circumference of the flange at edge “A” and “B”. The size of the burr at “A” and “B” is dependent on the tolerance of the punch and die when the rivet body is punched and cut from the strip of material.
The extruded rivet body has a consistent inside diameter for the entire length of the barrel and therefore, the wall thickness of the barrel or shank is constant. During the extrusion process a slug or wafer is punched out from the inside diameter of the barrel. After the slug or wafer has been removed, the area where it was attached will be approximately .0004 (.010mm) smaller in diameter.
The stamped rivet body does not have a constant inside diameter and therefore the wall thickness of the barrel is not constant. The barrel wall is thicker near the flange then it is at the end of the barrel. For example: a size 66 rivet body has a body diameter of .183 (4.64mm) to .191 (4.85mm) with a .375 (9.52mm) Maximum grip range. At point “A” the wall thickness is .031 (0.78mm) at point “B” .027 (0.68mm) and at point “C” .024 (0.60mm). This wall thickness difference is the result of forming the barrel from a sheet of metal. As you move the metal and star to form the barrel length the thickness of the metal elongates and stretches, thus reducing the thickness of the metal. After the barrel length is formed the pierced hole is punched out. The pierced hole diameter is the only diameter constant to the same rivet body diameter. The 66 rivet body will have a pierced hole diameter of .115 (2.92mm) to .117 (2.97mm). The pierced hole diameter is in relation to the diameter of the mandrel wire used to produce the mandrel. The 66 rivet body uses mandrel wire diameter of .113 (2.87mm) to .115 (2.92mm). to achieve a secure rivet body and mandrel retention that will keep both components assembled, projections had to be formed on the mandrel to increase the diameter of the mandrel.
The outside diameter of a stamped or extruded 66 rivet body is .183 (4.64mm) to .191 (4.85mm). The stamped rivet body with its wall thickness variations, leaves only the pierced hole diameter as the only diameter that is holding the rivet body and mandrel as an assembly during shipping. The .120 (3.04mm) to .123 (3.12mm) dimension of the 66 mandrel is what gives the assembled rivet body and mandrel retention. When the .120 (3.04mm) to .123 (3.12mm) diameter passes through the thin pierced hole diameter of .115 (2.92mm) to .117 (2.97mm) the assembly is complete. Because the pierced hole is so thin, it is possible to move the rivet body by hand the distance between the mandrel dimensions of .120 (3.04mm) to .123 (3.12mm) diameter to the head of the mandrel.
Extruded rivet bodies cannot be moved by hand after they have been assembled, because the .115 (2.92mm) to .117 (2.97mm) inside diameter is the full length of the rivet body and the .120 (3.04mm) to .123 (3.12mm) diameter is forced into the inside diameter of the rivet barrel.
Mandrels for extruded rivet bodies can be the same as the mandrel for the stamped rivet body, but as additional increase of mandrel diameter can be introduced to the extruded rivet body mandrel to increase the rivet body and mandrel retention. The additional increase of mandrel diameter is achieved by forming two short flats in the outside diameter of the mandrel.
These flats increase the outside diameter of the mandrel and also increase the retention between the rivet body and the mandrel. Automatic blind rivet feeding systems that transport the blind rivet through a hose by means of a blast of compressed air, to a blind rivet setting tool, require a high rivet body and mandrel retention force. If the retention force is too low, the rivet body and mandrel will dis-assemble when the blind rivet reaches the blind rivet setting tool.
I prefer extruded rivet bodies over stamped for achieving high rivet body and mandrel retention, uniform rivet body wall thickness, better mandrel head upset when setting the blind rivet and no burrs on the flanges.
Some blind rivet manufacturers are still using stamped stainless steel rivet bodies. I believe this is so because of the tooling breakage when extruding and heading stainless steel.
When blind rivet manufacturing first started stamping rivet bodies were used. Then extruded rivet bodies started to be produced and in my opinion, giving the user a better blind rivet.