Reprinted from Powder Coating Magazine, October 1995
CASE HISTORY Company plugs into powder for surge in corrosion resistance
Salt-spray resistance rises tenfold when a welding equipment supplierswitches from solvent-borne paint to powder coatings. In addition, VOCemissions plummet.
Miller Electric, Appleton, Wis., is an international supplier of weldingequipment and metal working accessories. Its products include 115-voltwelding machines for small shops, diesel-powered units for heavy industry,and robotic equipment for high-speed metal fabrication.
The company was founded in 1929 by Nels Miller, a Wisconsin native whobuilt his first welder in the family basement. Today, the company employsabout 1,500 people and operates several business units. The mechanicalsupport unit supplies coated parts to the assembly areas of the Appletonoperation.
Competitors up the ante
In addition to finishing the parts, the mechanical support unit safeguardsthe quality of the components. "We're constantly evaluating the job wedo," said Jim Kujawa, mechanical support unit manager. "If something isn'tto our standards we look at how to improve it." To do this, the unitoperates a performance test laboratory where technicians monitor finishquality. In one series of tests, Miller found that its competitors hadincreased the corrosion resistance of their parts. Not to be outdone,Miller re-evaluated its finishing process and searched for ways to improveit.
At the time, the company used a reciprocating disk (installed 16 yearsearlier) to apply a solvent-based, heat-cured paint with a solids contentof 67 percent. Although the disk still worked efficiently, the company'sdesire to improve corrosion resistance required a different coatingtechnology. At first, the company considered overhauling the system, whichalso included a manual spray paint booth and a paint dip tank. The diptank applied one paint color; the manual booth applied almost 30.
After looking at several finishing technologies, Miller focused on powdercoating. It promised a more durable, corrosion-resistant finish than thecompany had been getting and would reduce the release of volatile organiccompounds (VOCs) into the air. According to Kujawa, these and otherbenefits made powder coating the clear favorite among many finishingtechnologies.
Some liquid coatings remain
The original system proposal called for coating every part with powder.However, about 80 percent of the parts are coated in two colors.Twenty-eight other colors are applied in low volumes to the remaining 20percent of the parts as demand requires. For this reason, the companyjettisoned its high-volume reciprocating disk and spared its low-volumemanual liquid paint booth and dip operation. That way, the company couldcontinue applying the 28 colors manually to 20 percent of the partswithout buying new equipment and apply the two main colors automaticallyto 80 percent of the parts with a powder system.
Once the company chose powder, Miller teamed its engineers with equipmentsuppliers to integrate powder coating into the existing finishing system.The five-stage surface preparation system, which had been modified toinclude a deionized water rinse as a sixth stage, didn't require furtherchanges. The dry-off oven was also fine. (It dries parts for 5 minutes at260°F.) The conveyor and curing oven, however, needed modifications toaccommodate powder coating. Miller also needed an environmentallycontrolled room to house the powder booths.
Transition done during holiday shutdown
Because business is good at Miller and schedules are tight, slowing orstopping the finishing line would have created immediate shortages at theassembly lines and a logjam of parts in the production area. To avoidthis, Miller installed the system in stages before and during a 9-dayshutdown over the Christmas and New Year's holidays. I was impressed,"said LeRoy Schultz, production coordinator, paint department. "They did agreat job."
The first stage was the environmental room and powder booths. Installedoff line from the liquid paint booth, they were erected without disruptingproduction. The climate-controlled environmental room is 68 feet long, 40feet wide, and 14 1/2 feet tall. The fiberglass-reinforced, modularplastic walls are 4 inches thick with a smooth interior and a pebblyexterior. The room includes a roll-up door large enough for forklifttrucks to carry in pallets of powder coatings. Workers enter throughself-closing doors at either end of the room.
Installation of the two roll-on, roll-off powder booths coincided with theenvironmental room installation. Each self-contained booth is 26 feet longand 12 feet tall with a ware opening 2 1/2 feet wide and 6 feet tall. Foreven airflow, the booths use cartridge filters that are centrally locatedin the booth wall. The booth afterfilters are below the booth floor toprevent swirling air from disturbing powder application.
Each booth includes 12 automatic corona charging spray guns divided intotwo banks of six guns per side. Each gun bank is connected to an automaticgun mover, which cycles up and down when the powder booth is turned on. Aset of sensors at the booth entrance signals when parts enter the booth.It also communicates part dimensions to a gun controller. The controllerthen starts only the number of guns necessary to coat the part. The boothsalso include two manual spray guns for touch-up.
IR booster oven gives curing a jump start
The next installation stage began when the holiday break stoppedproduction. First, the system installer extended the 4-inch I-beamoverhead conveyor from the liquid paint area through the powder coatingarea. The conveyor is now several hundred feet longer than it was andincludes several additional turns.
Because all parts travel on the same conveyor, curing had to fluctuatedepending on whether the parts were coated with liquid or powder:Liquid-coated parts cure in 15 minutes at 400°F; powder-coated partscure more slowly. Instead of scheduling the parts according to their coatingtype and changing the conveyor speed, Miller added an infrared (IR)booster oven to the roof-mounted oven.
Located at the oven entrance, the 5-foot-long booster looks like a toasteroven set on edge with a bank of 18 IR elements on either side. The IR ovenis idle until powder-coated parts pass sensors similar to the ones mountedon the powder booths. As a part passes the sensors, a controller tells theoven how many elements are needed to cure it. A medium-sized part, forinstance, might require five or six elements; two small parts racked oneatop the other might require all the elements. When liquid-coated partsare on the way, an operator disengages the IR booster so that the partsdon't overcure. After curing, the parts descend from the oven, cool, andare removed from the line.
Powder boosts corrosion resistance and slashes rework
After the holiday break, workers returned to a finished shop equipped witha working powder coating system. Production started immediately. Within aweek, the equipment settings were locked in. Powder application wasuniform, film thickness was consistent, and the parts were fully cured,Schultz said. In the weeks that followed, salt spray test confirmed theanticipated quality results: a tenfold increase in corrosion resistance."Quality is definitely superior," said Kujawa, the mechanical support unitmanager.
More benefits followed. According to Schultz, a 24-year veteran of thepaint shop, rework dropped 50 percent. That's because powder coatingsdon't create runs, sags, or drips, which are a common defect in a liquidpaint operation. Line density, and thus production, also increased thanksto powder coating, he said. That's because parts on the powder line arehung closer together than parts hung on the disk line.
Added to these benefits are a decrease in annual VOC emissions, down36,000 pounds, and a reduction in hazardous waste. Now, instead ofshipping one barrel of waste each day, the finishing shop ships one barreleach week.
Schultz said that he has found only one coating problem since productionbegan in January 1995, and it was quickly resolved. Three months intoproduction, workers noticed fisheyes, or cratering, on the powder-coatedparts and told Schultz about it. Recollecting that an assembly line hadbeen recently added nearby, he and another worker went to look at it. Sureenough, they found the problem. Workers there were spraying a mold-releaseagent on metal tubes before slopping rubber handgrips over them. The spraywas wafting though the air and landing on the uncured parts. When thecompany eliminated the mold-release agent, it eliminated the fisheyeproblem. "Other than that," Schultz said, "we have not had one problemwith this system. Not one."
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