Taking the (Oxide) Edge Off
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
The cut and the finish—those manufacturing processes have changed greatly over the last 40 years. Laser cutting and powder coating are two technologies that have emerged during this time to make metal manufacturers rethink the way they produce parts.
Laser cutting machines provide manufacturers with an effective means to cut blanks of all shapes with burr-free edges in a matter of seconds. Powder coating systems give manufacturers an environmentally friendly and cost-effective way to apply a finish to metal products that stands up to the rigors of the most challenging environments. The two technologies are commonplace in metals-related supply chains around the world.
Having said that, they don’t always play well together. There’s an edge to their relationship.
Specifically, it’s an oxide edge. Fabricators typically use oxygen as an assist gas during laser cutting of mild steel. The oxygen causes an exothermic reaction that creates heat, which helps the laser cut through the metal. In addition, not as much pressure is required to feed the oxygen to the cutting head. It’s a cost-effective way of laser cutting when compared to the use of nitrogen, for example, which will be discussed later. The oxygen, however, can mar surface-sensitive finishes, remove surface coatings altogether, and—of most concern for fabricators who need to powder coat mild steel parts—create an oxidized edge that makes it difficult for powder coatings to adhere to the area exposed to the laser cutting.
This is not a new problem, but for metal fabricators that don’t have tons of experience producing parts destined to receive a high-quality powder coating finish, this edge quality issue can be vexing. Here are the stories of three fabricators that have figured out just what they need to do to take the oxide edge off.
Case Study No. 1: Pass the Problem Through
Art Harrison, vice president, manufacturing operations, General Sheet Metal Works, recalled the powder coating adhesion problem one of the company’s customers had with thick mild steel parts cut at the South Bend, Ind., fabricating shop. The metal parts looked fine prior to powder coating, but once the powder was applied and cured, large chunks of the finish fell off just with a simple knock to the metal.
So back in 2009 the company purchased its first oxide removal machine. When operators aren’t tending to the laser cutting machine or breaking out parts from skeletons, they feed laser-cut parts through the LISSMAC Corp. machine. Four brush belts, two on the top and two on the bottom, run perpendicular to the direction of the conveyor that feeds the parts. With one pass through, the belts are able to remove the oxide layer on all edges—even on interior shapes. The brushes are even able to keep any protective oil film on the sheets.
General Sheet Metal Works later purchased a second machine for another building where it houses two more laser cutting machines. The original machine resides in a nearby building where five other laser cutting machines are slicing up sheet metal blanks.
Even at the speed at which laser cutting machines operate, Harrison said, the oxide removal equipment can keep up. It takes about 20 or 30 seconds for a part to process through the machine. Harrison added that typically one of the laser operators in the cell has time to send parts through the machine, avoiding potential bottlenecks.
“We’ve had no customer complaints about paint adhesion using this process,” Harrison said.
Cast Study No. 2: Enter the Sandblast
Bermo Inc. used to rely on a company down the road from its Circle Pines, Minn., plant to sandblast its larger laser-cut parts and tumble-finish the smaller ones—all in an effort to remove that oxide from the edges. In those instances when there wasn’t time to send parts out, employees used hand tools to clean the edges.
Bermo decided to purchase a sandblast booth, but it was small. It could accommodate a stack of parts of only about 10 inches. That pallet could take as long as 20 minutes to undergo an entire sandblasting cycle, according to Tim Gerrits, the company’s short-run prototype department manager.
Gerrits said the clean edge the sandblasting operation produced was exactly what was needed to meet customers’ demands for a surface that would be receptive to powder coatings. Bermo just needed a more efficient way to process those parts.
Luckily, Bermo officials learned that a nearby diesel engine remanufacturing company had a larger sandblasting chamber and were receptive to visitors. The Bermo team liked what they saw.
“They had a larger one to blast their castings. It showed how clean the edge got,” Gerrits said. “So we thought, let’s bring one in and try it.”
Bermo contacted a local equipment dealer and had a Goff shotblasting machine with a 60-in. bed delivered in the summer of 2011. The metal fabricator constructed its own tables with grated tops to put the laser-cut parts on.
The only other piece of equipment that Bermo needs for the oxide removal is a lift truck to place the pallet of parts in the chamber and to remove them when the cleaning process is complete.
“The cycle time is about two minutes,” Gerrits said. “It’s amazing.”
In the near future, Bermo might have to look at another means to remove the oxide from internal features laser-cut into mild steel parts. Currently the sandblasting operation removes only the oxide layer from the outside edges.
“That’ll be the next step that we have to address,” he said.
Case Study No. 3: Switching Gases
Cummins Power Generation, Fridley, Minn., has produced thousands of generator enclosures over the years. Customers expect the generators to operate in the most stressful environments—from the deserts of Afghanistan to the frigid landscape of Alaska. They also expect them to look like a robust piece of equipment, even after exposure to these rugged conditions.
That led the fabricating team to undertake a continuous improvement project to investigate paint corrosion problems in the field, according to Amith Pinapala, a former manufacturing leader, paint operations, at Cummins. Complaints about corrosion were coming in from the field, and those reports revealed that the paint on the edges was most likely flaking off, leaving the metal exposed and increasing the chance of corrosion.
Engineers performed a design of experiments study covering all factors involved in the production process—material substrate, laser cutting assist gas, stud welding, seam welding, and surface treatment chemicals. Prototype panels were fabricated using these processes and subjected to accelerated corrosion testing.
The testing revealed the same type of corrosion being reported from the field. Further testing eliminated material, paint, and pretreatment chemicals as potential culprits. That left engineers to take a closer look at the method of metal blank production. As it turned out, Pinapala said, these problem parts had one major thing in common: A laser was used to cut 90 percent of the steel blanks, and oxygen was used as the assist gas during the cutting process.
Laser cutting with nitrogen as the assist gas was identified as the way to go. Testing on a 3.2-kW laser cutting machine with a single nitrogen tank produced smooth and shiny edges, not the dull, dark look common to oxidized edges. Because the nitrogen produces no exothermic reaction, no oxidation occurs on the metal. But using nitrogen requires higher cutting gas pressure, which typically results in higher processing costs when compared with oxygen.
Even with the added expense associated with using nitrogen, the Cummins Power Generation management team approved the shift in laser cutting practices. Over a couple of days a facilities crew ran refrigerant-quality copper piping to the laser cutting machines, and a 6,000-gallon nitrogen bulk tank was installed just outside the shop.
When the day came to switch over to nitrogen cutting, the fabricating team focused on one workhorse laser cutting machine. It was fed new cutting programs, and over a 72-hour period, the machine performed as expected. Attention was then turned to another workhorse machine, and similar results occurred.
The start of the fabrication process was now as strong as the final finish for the power generator manufacturer. n
Fabricator Editor-in-Chief Dan Davis can be reached at firstname.lastname@example.org. For more information from the companies mentioned, visit Bermo Inc. at bermo.com, Cummins Power Generation at cummins.com, General Sheet Metal Works at gsmwinc.com.