Coil Coating Line

Overview

A coil coating line paints metal strip while it runs continuously off a coil, so a full roll of bare steel or aluminum comes in one end and a roll of finished prepainted strip comes out the other. The work is done in a fixed sequence: the strip is cleaned, given a conversion coating, painted with a primer and a topcoat by roll coaters, baked in an oven until the paint crosslinks, cooled, and rewound. Because the coating, curing and cooling happen on the moving strip rather than on cut sheets, the line runs at a steady speed and never stops at the process section, which is why two accumulators are built into Entry Section and Exit Section to absorb the stops at the reels. Prepainted coil made this way goes on to become roofing, siding, appliance panels, and automotive trim, with the paint applied flat and cured under tight control before any forming happens.

Entry and continuous feed

The line starts at the Double Uncoiler, a turret carrying two Expanding Mandrel reels so a fresh coil can be loaded and threaded while the other is still paying off. When the running coil nears its end, the Strip Joiner crops both ends square with the Crop Shear and joins them with the Stitching Head, turning the supply into one endless strip. While that joint is made the uncoiler stops, but the process section keeps moving because the Entry Accumulator pays out stored strip. Its Accumulator Carriage rides up and down on the Carriage Hoist Cable cables, lengthening and shortening the loops formed over the Accumulator Roll rolls. The Entry Bridle sets the tension entering the accumulator, and the Entry Edge Guide keeps the strip centered using Edge Position Sensor feedback.

Cleaning and pretreatment

Bare strip arrives with rolling oil and metal fines on it, and paint will not stick to that. The Cleaning and Pretreatment Section removes it in the Alkaline Cleaning Stage, where heated solution from the Spray Header headers and rotating Scrub Brush Roll brushes scrub both faces. The strip then runs through the Rinse Tank cascade, with Squeegee Roll rolls wiping carryover between stages so chemicals do not migrate forward. The clean metal passes the Conversion Coater, where an Applicator Roll lays a thin chrome-free passivation film fed by the Pick-up Roll. This conversion layer is what the paint actually bonds to, and it also slows corrosion at any cut edge. The Pretreatment Dryer then drives off the rinse water with hot air from its Air Recirculation Fan so the strip enters the coater dry. Solution is kept circulating by the Chemical Circulation Pump pumps.

Coating

Paint is applied by roll coating, not spraying, because rolls put down a precise wet film with almost no overspray. The Prime Coater applies the primer first, then the strip is cured, and on the same pass the Finish Coater lays the topcoat. Each is a three-roll head: the Pick-up Roll dips into the pan, transfers paint to the Applicator Roll, and the Coater Backup Roll presses the strip against it to set the nip. Film thickness is governed by roll gap, roll speed ratio and contact load, all set by the Coater Head Actuator. Running the applicator against the strip travel, called reverse coating, gives the most uniform film. The Backside Coater adds a backer coat on the reverse face. Paint is kept ready by the Paint Supply Skid skid, where the Paint Circulation Pump circulates it, the Paint Filter catches skins, the Viscosity Meter trims solvent to hold viscosity, and the Paint Heater holds temperature so the film builds the same all day. The Coater Bridle holds steady tension so the strip does not flutter in the nips.

Curing

The wet-coated strip cannot touch anything until the paint is cured, so the Curing Oven floats it on air. Each Oven Heating Zone blows hot air through Air-Float Nozzle Box boxes above and below the strip, supporting it on cushions while the Oven Recirculation Fan recirculates the air heated by the Oven Gas Burner. A NIR Booster Module of near-infrared lamps at the entry gives a fast initial heat-up that flashes off most of the solvent. The cure target is a peak metal temperature, read by the Peak Metal Temperature Sensor pyrometers: the strip must reach the value the paint chemistry needs, held for the right dwell, for the resin to crosslink fully. Too low and the paint stays soft, too high and it scorches, so the zone setpoints and the line speed are matched to the coating and the strip gauge.

Cooling, exit and rewinding

Out of the oven the strip is near 230 C, so the Quench and Cooling Section cools it down. It runs through the Quench Tank, where water from the Quench Water Pump cools it fast, then the Air Knife knives strip off the water film, with the Quench Water Cooler rejecting the heat to the plant loop. The cooled strip enters the Exit Section, where the Exit Accumulator stores strip so the Recoiler can stop to cut a finished coil with the Exit Shear without halting the process. The Belt Wrapper starts each new coil onto the mandrel, and the Coil Inspection Station station checks the surface and reads cured thickness on the Dry Film Thickness Gauge.

Emissions, drives and control

The oven air carries the solvent evaporated from the paint, which cannot go to atmosphere untreated. The Regenerative Thermal Oxidizer burns it: the Induced-Draft Fan draws the fume into the Oxidizer Combustion Chamber, the Ceramic Media Bed beds recover most of the combustion heat, and the Oxidizer Burner tops up the temperature when the solvent load is low. Speed and tension across every section are held by the Drive and Control System system, where each AC Drive regulates a section motor and the Line PLC closes the loops using feedback from the Tension Load Cell cells. Operators run the line from the Operator HMI panels, calling up recipes that store cleaning, coater, oven and tension settings for each product. The Process Exhaust and Air Handling system supplies tempered make-up air through the Make-up Air Unit and ducts fume to the oxidizer through the Process Exhaust Fan fans.