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Automotive Industry

Coils, powders, and other beneficial approaches to painting

Automotive Design & Production, Jan, 2003 by Christopher A. Sawyer

"Cut costs. Reduce emissions. Save weight. Make the top coat more durable." The challenges facing paint companies that supply the auto industry are often at odds with each other, and usually require new coating technologies applied using tried-and-true methods.

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There are Four layers to a typical OEM paint job: electro-coat, primer/surfacer, base (color) coat, and clear coat, and the sum total measures just 0.1-mm. thick. In terms of weight, the breakdown, according to the folks at BASF Automotive [Southfield, MI], looks like this: electro-coat, 2 kg--3 kg; primer/surfacer, 1.2 kg--1.7 kg; base coat, 0.6 kg--1.2 kg; and clear coat, 1.5 kg--1.8 kg. That means the average vehicle in the fleet has 12 lb--17 lb of covering on its body, with the lightest layer being the one that gives it its color. And OEMs want to bring this down in an effort to cut vehicle weight and cost. According to Dennis Taljan, director, Decorative Product/Technology/Management, PPG Industries (Troy, MI), "The weight and cost of the finish are paramount to the OEMs, yet consumers want coatings that are more colorful and durable." And while these concerns would appear to be diametric opposites, the paint industry is moving forward on developing technologies that meet these conflicting needs.

Coil coating is one way in which automakers and coating suppliers can control emissions, film thickness, and cost. It significantly reduces paint usage because it has a 99.5% paint transfer efficiency, zero VDC [volatile organic compound) emissions, and greatly reduces in-plant floor space requirements. "The coating line moves at 500 ft./min, and the cure time is 24 seconds for coil coated steel," says Robert White, director, Substrate Protection Systems, Automotive Coatings, PPG Industries. "Compared to painting a formed part, this process cleans the surface better, allows for better preparation and coating, coats both sides of the sheet in one pass, and has better edge-to-edge film control." The A- and B-pillar covers on the previous generation of the Ford Expedition and the windshield base molding of the first generation Chrysler Sebring convertible used steel coated by PPG. The company also has produced many prototype parts--including four-layer (base coat, paint, mica-flake, clear coat) door panels for Chrysler's now-defunct Prowler--and suggests the process is well--suited for many current applications, especially for producing ready-for-assembly roof sections. (The SMART car is the only current application for coil-coated roof panels, though other programs are under development. BASF (coating) and Arvin-Meritor (forming and assembly) collaborate to produce the coil--coated module, which includes an integrated fabric sunroof.) Unfortunately, the range of colors, including metallics, is in need of further development. This limits its application, and many OEMs are taking a wait-and-see attitude as development continues. "None of them want to be the first to use it in a large volume application," says White, "even though most of the large appliances on the market have used this technology for years." Undaunted, PPG says there is a low-cost, low-exposure way for automakers to begin using coil-coated panels.

"If you planned it properly," says White, "you could use coil-coated steel for an entire vehicle as long as you controlled the color palette." While development work is focused on increasing the number of colors available, and reducing appearance harmony issues (coil-coated panels often have a smoother finish and richer appearance due to the lack of "orange peel" and uniformity of the coating), its use might require tighter logistical control, including the batch-build of like-colored vehicles. The reason is simple. A steel coil weighs 40,000 lb., while the average body-in-white tips the scales at or below 1,000 lb. This suggests OEMs would be required to build vehicles in batches of 40 before swapping out coils and colors. However, since this is supposed to be a low-cost, low-exposure example, White suggest that OEMs would be better served by using the coil-coat technology to build, "a modular spaceframe structure with bolt-on panels. This," he says, "would let them stamp a complete set of panels From a single coil, and draw the color needed from inventory as needed."

Because spot welding has the potential to seriously mar the painted surFace, inner and outer panels would be held together via a combination oFadhesive bonding and self-piercing rivets. In what sounds like a precursor to the Saturn program, PPG says that prototypes were built by GM using this technology in the late 1 980s, though at the time the color palette was limited to Four colors: black, white, blue and red. This was a major stumbling block, despite some significant advantages to be had from using coil-coated steel. "Because you can completely eliminate the paint facility,' says White, "you can radically alter the plant Footprint, and eliminate a number oF processes." But with most automakers unwilling to throw out known processes and technologies in Favor oF an unproven but potentially profitable production method, the Fully coil-coated car, when it arrives, probably will Follow White's low-exposure recipe: a nodular spaceFrame niche model designed around the technology's process capability.

Automotive Industry

Coils, powders, and other beneficial approaches to painting

Automotive Design & Production, Jan, 2003 by Christopher A. Sawyer

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