Coil coating

Coil coating is the continuous and highly automated industrial process for efficiently coating coils of metal. Because the metal is treated before it is cut and formed, the entire surface is cleaned and treated, providing tightly-bonded finishes. (Formed parts can have many holes, recessed areas, valleys, and hidden areas that make it difficult to clean and uniformly paint.) Coil coated metal (often called prepainted metal) is often considered more durable and more corrosion-resistant than most post painted metal.

Annually, 4.5 million tons of coil coated steel and aluminum are produced and shipped in North America, and 5 million tons in Europe. In almost every five-year period since the early 1980s, the growth rate of coil coated metal has exceeded the growth rates of either steel and/or aluminum production.

The definition of a coil coating process according to EN 10169 : 2010 is a ‘process in which an (organic) coating material is applied on rolled metal strip in a continuous process which includes cleaning, if necessary, and chemical pre-treatment of the metal surface and either one-side or two-side, single or multiple application of (liquid) paints or coating powders which are subsequently cured or/and laminating with permanent plastic films’.

The metal substrate (steel or aluminum) is delivered in coil form from the rolling mills. Coil weights vary from 5-6 tons for aluminum and up to about 25 tons for steel. The coil is positioned at the beginning of the line, then unwound at a constant speed, passing through the various pre-treatment and coating processes before being recoiled. Two strip accumulators at the beginning and the end of the line enable the work to be continuous, allowing new coils to be added (and finished coils removed) by a metal stitching process without having to slow down or stop the line.

The continuous process of applying up to three separate coating layers onto one or both sides of a metal strip substrate takes place on a coil coating line. These lines vary greatly in size, with widths from 18 inches to over 60 inches and speeds from 100 feet per minute to 700 feet per minute; however, all coil-coating lines share the same basic process steps.

A typical organic coil coating line consists of decoilers, entry strip accumulator, cleaning, chemical pretreatment, primer coat application, curing, final coat application, curing, exit accumulator and recoilers.

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