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When an automotive molder wanted to upgrade from primer painting to multi-color coating, a big concern was costly downtime and material waste during frequent color changes. Disposal of solvent flushing chemicals was another potential obstacle. For this molder, a “piggable” piping system was the central element in designing an automated paint line that allows use of multiple colors on demand, with less solvent usage, paint waste, and color-change time than with conventional systems.

“Piggable” paint systems are used by captive car and truck paint operations but are not typical for a custom molder like Continental Structural Plastics (CSP). Based in Troy, Mich., it is reportedly the largest North American compression molder of thermoset SMC and thermoplastic GMT automotive composites.

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The “piggable” option is used at CSP’s Carey, Ohio, facility. The challenge was to upgrade from applying just a primer coat to multi-color finishes. The change was spurred by new contracts for the entire front hood assembly of the Hino medium-duty truck from Toyota’s Hino Diesel Trucks Div.

THE MOVE TO MULTI-COLOR

Explains paint operation manager Chris Twining, “We painted non-conductive parts with a conductive primer using conventional and electrostatic and air-atomized guns mounted on robots in a high-volume automated system. We now required reliable two-component flow control, total paint supply flexibility, and optimal performance.”

A.B. Myr Industries of Belleville, Mich., which specializes in industrial finishing systems, engineered and installed an upgraded system that allows parts to be coated with a primer or one of four paint colors as they pass through the paint spray booth. The system continuously recirculates paint to supply it at a steady rate free of pulsations and fluctuations. The system supports “piggable” pipeline cleaning to enable fast color changes and virtually eliminate waste. It also automatically heats or cools the paint as needed and reduces overspray by more than 50% through improved grounding and replacement of spray guns with rotary bells.

Results have been dramatic, says Twining. “We are now producing significantly improved wave, gloss, and color. Paint-related defects have diminished. Uptime and throughput have increased, and most overspray has been eliminated.” All this was achieved with 25% reduction in paint usage.

WHAT’S A ‘PIG’?

CSP’s new 400-ft paint line incorporates four piggable paint systems, each able to handle primer and three colors at a time, plus a hardening agent for the two-component paints. Each color has its own pumping system and piping network and is available for color changes several times a day.

The system minimizes color-change time and waste by recovering over 96% of the paint during each change and using less than 1 gal of solvent per change. Color changes in non-piggable systems can waste up to 6 gal of paint and a high volume of solvent.

The pig is a bullet-shaped polyurethane foam object that is shot through the paint system at the end of a color run with compressed air and a minimal amount of solvent to force paint out of the lines. The pig travels through the entire line in about 30 sec. Its O.D., typically 0.5 to 0.75 in., is slightly larger than the I.D. of the tubing to be cleaned, which ensures a good seal against the tubing wall. After the purge, a short burst of air and solvent through the system picks up the traces of paint left by the pig.

Special valves allow the pig to pass through the color-change manifold. Air-tight, quick-disconnect fittings and integrated pig launch and retrieval help cut color-change time to less than 15 min.