A Laser Replaces.... Stamping
By using a laser to cut and weld steel strip rather than stamping and forming the material, Freudenberg is no longer producing tons of engineered scrap in a component-making operation.
The material savings alone are compel-ling for the utilization of a laser-based process at the Freudenberg-NOKFreudenberg Sealing Technologies (Freudenberg.com) plant in Weinheim, Germany: 1,800 tons of scrap steel—AISI 1008 low-carbon steel—per machine per year. Looked at in the context of producing the metal cases that are manufactured for radial shaft seals, it is a 73% material savings, says Dr. Juergen Ruhan, head of Corporate Lead Function Components, Freudenberg.
Ruhan explains that the previous operation—and one that continues to be used for some of the steel metal cases that they produce each year—they produce some 750-million annually—is stamping and drawing. They stamp a blank, form the cylindrical collar-shaped seal, and then crimp the bottom.
The parts that are being produced by the Schmalband-Umform-Laserschweiß (SUL) process (a.k.a., Laser Welded Narrow Band Forming technology) are from 90 to 400 mm outer diameter, though he points out that the 400-mm cases are comparatively low-volume, as they are used in windmills. The smaller seals are used for truck applications, as well as for agricultural and off-road equipment. For passenger cars, the outer diameters are on the order of 50 to 60 mm.
Prior to the use of SUL, they would start with coil of steel from 250-mm wide, then stamp and form it on hydraulic or mechanical presses ranging from 50 to 300 tons in capacity. The final product is ring shaped. Which means that as they stamp the rings in the rectangular sheet, they are left with a lot of waste in the remaining Swiss-cheese like sheet. “Of course,” Ruhan says, “there are techniques to reduce the amount of scrap, such as oscillating the feed to position the disc shapes on the sheet, which usually provides a 10 to 15% advantage in steel consumption.” They’d also looked at the possibility of using some of the remaining material to make smaller parts, but determined that there was a problem regarding material traceability.
So they ended up with a whole lot of scrap.
With SUL, they use a 2,700-W CO2 laser to cut a steel strip from the coil, usually 20 mm wide. The length is predicated on the OD to be achieved. The strip is formed into a circular shape, then the same laser, with a change in the focus of the optics, is used to weld the two ends. Then the bottom is cramped or chamfered, with the latter being preferred by Freudenberg’s European customers.
As for the processing speed, SUL is competitive with stamping: “The speed of a press for rings is usually about 40 strokes per minute. With SUL we achieve 44 pieces per minute.”
While this sounds rather straight-forward, Ruhan points out, “To make this happen in our new process, we need to control 72 linear axes in real time.” This necessitated a new controller that was capable of handling real-time data. The cost of the equipment, he admits, is high, as much as three times more expensive than a standard press.
However, there are the savings that are realized, not only as regards the material savings, but also in terms of tooling. “The tooling we use for stamping—compound, progressive, or transfer—usually cost between €25,000 and €120,000. The tooling for the SUL is about €3,000,” Ruhan says.
That said, the material savings are the primary reason why they’re not yet using SUL for the passenger car shaft seals. “The material savings,” he explains, “is not balanced by the higher equipment cost.” Not only is there less scrap with the smaller OD seals, but those smaller seals are made with thinner gage material.
“Health and safety are another argument for this technology,” Ruhan says, explaining that for the stamping operations they are regularly changing dies. While they do have assist devices for handling the tools, “At the end, you cannot ignore the weight,” he says. Secondly, stamping often leaves burrs on the edges of the workpiece. Gloves notwithstanding, cuts tend to occur. They have had no injuries related to SUL after two years in operation.
Another benefit is that SUL creates a more precise part. The metal cases have rubber molded into them to create the seal. If the dimensions of the case aren’t precise and repeatable, then the molding leads to a flash defect, which has to be manually removed. They’ve found that the Cpk for the SUL process is far better than that provided by stamping, which, as Ruhan points out, not only means they don’t have to perform the secondary flash removal, but that it “provides more quality to the final customer.”
By in large in Germany, it seems, there is a greater awareness of/concern about CO2 emissions. And Ruhan cites a savings from SUL even in that context. There is less steel required, so less CO2 generated to produce the steel in the first place; less steel has to be transported to the Freudenberg factory in Weinheim so there is less exhaust from the trucks; less scrap steel is shipped back to the steel supplier, again a CO2 reduction.
If there is a weak point in this, it is one that is more psychological that physical. Unlike the rings made with the presses, there is a weld seam on the OD of the SUL-produced part. Ruhan says that Freudenberg R&D personnel have tested and analyzed the welds and determined that the welds are actually stronger than the parent material.
“We now have produced more than 50-million rings over the previous two years. The seam is not the weak point,” Ruhan says.
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