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Having emerged from the two-stage oven, finished Prowler surfaces undergo a thorough final inspection.
The image that comes to mind when you visualize an automotive finishing opera tion is a scene where a constant flow of cars – arranged end to end – pass through spray booths where finishing robots toil constantly to apply the various colors and clearcoat finishes. Colors are changed frequently, and there is little or no break in the conveyor line of car bodies traveling through the automated system.
This image is a surprising contrast to production at MSX International in Detroit, Michigan. You won't find a finishing robot or a conveyor line at this specialty automotive paint facility, because each finishing step is performed by employees who sand, polish, and paint the vehicles manually. Calling on its extensive experience, it maintains the highest standards in show car finishing capabilities. It's this expertise along with its attention to detail that prompted Chrysler to select MSX to paint its high performance, classic-designed roadster — the Chrysler Prowler which was produced from 1997 through 2001. (The last Prowler was sold at auction in May, 2002. See sidebar.)
“The Prowler appeals to a very specific driver,” said Gary Cibula, MSX Plant Operations Manager. “Production numbers were significantly lower than the typical production volume of automobiles, so it didn't make sense for a large automaker to dedicate production space for these custom products. Instead, companies like ours performed special services for them, such as painting body parts.”
MSX International offers the automotive industry a complete range of specialty build services from fabrication to assembly, including full-body paint capabilities. They have facilities worldwide.
“Even in our situation, a robotic or automated system was not feasible,” said Mr. Cibula. “Our daily production of Chrysler Prowlers was 14-17 vehicles. It did not make economical sense to install an automatic finishing system with robotic capabilities for such a short daily run.”
This lack of automation does not mean that MSX didn't take advantage of new advances in finishing technology. If new equipment or production methods are cost effective, and can increase production and quality, they will look at the product or process closely.
For instance, when MSX designed the finishing facility for the Prowler in 1995, it needed a system to accurately mix two-component clearcoat. It wanted something that was economical, precise, and easy to operate. After careful evaluation and input from the paint supplier, contractor, Chrysler and local finishing equipment supplier, MSX selected Graco's PrecisionMix electronic proportioning system.
The PrecisionMix proportioner sequentially metered and dispensed the two-component clearcoat—a 1:1 ratio, high gloss formula from PPG. The patented digital control unit provides a maximum flow rate of 4,000 cc/mm and assures proportioning accuracy within +1%. Volume ratios vary infinitely from 0.6:1 to 30:1.
MSX installed the proportioner outside the clearcoat spray booth so operators had easy access to it. According to Richard Underwood, paint job supervisor, the system is simple and reliable. “Our production people don't have to worry about off-ratio problems, because they're getting the same pre-selected recipe whenever they pick up their spray guns. They can verify this input on the proportioner's screen monitor. This allows our painters to focus on what they do best and that's applying a quality finish.”
“We haven't had any problems with the system since it was installed,” he added. “It does exactly what we expect it to do—day in and day out.”
In fact, the proportioner did such a good job mixing and dispensing clearcoat that MSX added a second system to proportion clearcoat in another booth. According to Mr. Underwood, “We were hand-mixing the 1:1 ratio material and spraying it from pressure pots. It wasn't a very economical operation. The system has allowed us to reduce labor and material expenses.”
Another benefit of the system is its reporting capabilities. The system allows MSX to monitor material usage. It used the information to analyze production and VOC trends.
The clearcoat components are transferred from a central pump room where they are pumped from 55-gallon drums to the respective proportioner. Graco Glutton 4:1 stainless steel diaphragm pumps were recently installed to transfer the plural component materials. “We replaced a number of other pumps that didn't have the output we required. The Glutton pumps are providing with the pressure and flow we need to meet our system requirements,” said Mr. Underwood.
Prowler System Operation
Parts that arrived at the MSX facility had been e-coated by a company that fabricated the metal body parts. The parts were loaded onto portable carts (each cart carried all the body parts to complete one Prowler) and were wheeled to the scuff sand booth where imperfections were sanded out. After the sanding process, the parts were pressure washed with DI water and then baked at 290F. This step removed excess gas from the substrate that otherwise could release later, causing blemishes to the finish.
The first material MSX sprayed was a single-component primer. Painters using conventional air spray guns applied three coats to attain a coverage of 1.5 mils. The primer is fast-drying, allowing the second and third coat to be applied 3-5 minutes after the previous coat. After the primer was applied, the painter wheeled the cart into a heated area where the primer cured at 95F for 20 minutes. From this heated area, the parts were moved into a two-stage oven. Temperatures were 240F in the first stage and 290F in the second stage. Parts remained in each stage for 20 minutes. According to Mr. Underwood, this provided optimum cure rate and also kept the product moving smoothly through the system.
After the parts were removed from the primer oven, they were inspected. Any imperfections caused by dirt or other airborne contaminants were sanded out of the finish. These parts were washed and retouched. Then the cart was wheeled into the color booth where the base coat was applied.
“We had a unique situation here” said Mr. Cibula. “Many automotive finishing operations have to contend with frequent color changes during the day. Our color batches were for 800 vehicles, so once we start painting a color we stayed with that color for approximately three months.
“To say the least, a color change was a big event here,” he added. “After spraying the same color for three months, our painters really looked forward to a change!”
After the color coat was sprayed, the parts were transported to a flash zone to drive the solvents out of the base coat. They remained there for 20 minutes at 110F. From the flash zone, the carts were rolled into the clearcoat booth where painters applied a polyurethane coating of 2.2 to 2.5 mils. After the parts were sprayed, they were rolled into an infrared flash zone for 20 minutes and then into another two-stage oven where the parts were heated at 195F and 285F, respectively.
Next, every inch of the parts finished surface was inspected. Upon passing inspection, the high gloss parts were meticulously polished. The parts were transported to a final inspection area. When they passed this inspection, they were transported to another part of the facility and were “signed” back to the company that fabricated the parts for assembly. Typically, a cart carrying the parts of a complete Prowler was in MSX's control for 48 hours.
“When the parts left our facility they carried the best finish available today,” Mr. Underwood concluded. “And the Graco PrecisionMix proportioners helped us attain that finish. They have met our quality requirements and have proved over the last six years to be very reliable.”
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