The coil coating process

The coil coating process

The coil coating process

The metal substrate, whether steel or aluminum, is initially provided in coil form from rolling mills. Coil weights range between 5-6 tonnes for aluminum and up to 20 tonnes for steel.

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At the beginning of the process line, the coil is unwound at a consistent speed and moves through various pre-treatment and coating stages before being recoiled.

Two strip accumulators, located at the start and end of the line, ensure a continuous process. This allows for new coils to be added and finished coils to be removed via a metal stitching process without halting the line.

The following steps take place on a modern coating line:

• Mechanical stitching of the strip to its predecessor
• Cleaning the strip
• Power brushing
• Chemical conversion surface treatment
• Drying the strip
• Application of primer on one or both sides
• First curing oven pass (15 to 60 seconds)
• Cooling down the strip
• Applying the finish coat on one or both sides
• Second curing oven pass (15 to 60 seconds)
• Cooling to room temperature
• Rewinding of the coated coil

The Evolution of Coil Coatings and Paint Systems for Metal

Numerous finishes redefine what's possible for fabricators and designers

There are various ways to apply paint to metal. Generally, spray or brush applications are common in the field; in a factory, you can use spray, powder coat, or coil coat methods. However, for modern metal architecture, products like metal composite materials overwhelmingly rely on the coil coating process to achieve a nearly flawless, long-lasting finish.

Coil coating involves coating a metal coil continuously before fabrication. The process includes uncoiling, processing, and painting the coil, then rewinding it. This method is traditionally used for aluminum but can also apply to steel and other metals.

According to the National Coil Coating Association, prepainted metal "holds up better over time than post-painted surfaces..." Comprehensive quality assurance in coil coating ensures consistency. Additionally, continuous processes enable cost-effectiveness and green manufacturing by minimizing waste, increasing efficiency, and capturing VOCs.

Methods of Coating

The application of paint to metal has evolved beyond traditional spray systems. Modern continuous coil coating processes use roll coating or die coating methods. Reverse roll coating, commonly used in metal manufacturing, employs two rolls moving in opposite directions to control paint thickness accurately. This method can leave "roping" marks, vertical lines resembling a ship's wake at the panel's edges.

Die coating, on the other hand, is a high-precision method that retains the benefits of continuous coating. Paint is applied through a slot die extrusion system, resulting in a uniformly controlled paint thickness with minimal waste.

Additional resources:
by Spectra Metals - Trim Coil - Goering Hardware

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Paint thickness is precisely controlled in a single-cycle application. Both the paint system and the coil must meet stringent quality standards in the die coating process, resulting in exceptionally smooth finishes and enhanced sustainability through waste reduction.

Paint Systems

Over the years, paint systems have evolved to meet the growing demands for more durable finishes. Initially dominated by polyester and urethane paints, the industry has now shifted towards fluoropolymer systems that comply with the architectural performance specifications laid out in AAMA 2605, known for their excellent weatherability and durability.

The first generation of fluoropolymer paint, introduced in 1948, was polyvinylidene fluoride (PVDF). This paint required an additional acrylic resin to achieve its final form, with a maximum performance ratio of 70% PVDF to 30% acrylic resin. PVDF is a thermoplastic that can remelt under high temperatures, limiting its performance. It provides a low-gloss finish, ranging from 30 to 40 on a 60-degree gloss meter, and has limited color palette options.

Fluoroethylene-vinyl ether (FEVE), introduced in 1982, represents the second generation of fluoropolymer paint. FEVE is a thermoset paint that forms chemical cross-links under high pressure and temperature, preventing remelt. This clear resin allows for more vibrant colors and glossier finishes, with gloss levels reaching up to 70.

Advancements in coil coating and paint systems have expanded the possibilities for durable and decorative finishes, lasting decades. The rise of metal composite materials (MCM) in architecture offers new opportunities, allowing for complex forms without the drawbacks of traditional solid metal sheets, like oil canning.

Jim Moses is the technical services manager at Mitsubishi Chemical Composites America's ALPOLIC MCM division in Chesapeake, Va. With over 40 years of experience in construction manufacturing, engineering, and operations, he is an active member of several industry associations.

This article was featured in Metal Construction News.

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