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Over 800 million tons of coil-coated metal are produced and shipped annually in North America alone. Coil coating (see Diagram I) is a very efficient way to produce a uniform, high quality, coated finish over metal in a continuous automated fashion. Coil coating is also referred to as pre-painted metal, because the metal is painted prior to, rather than after, fabrication.
In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape. Versus most other application methods, coil coating efficiency is nearly 100%. Application is at very high line speeds as modern coil lines can run at speeds as high as 700 feet per minute and cure the applied paint in 15 – 45 seconds. As opposed to a spray-applied coating, for example, a coil-coated, formed surface offers uniform film thickness rather than the thicker films on edges, corners and bends that is more typical of spray-applied coatings.
Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.
The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings. Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4 – 6 microns in thickness, whereas topcoats are normally applied to provide a dry film thickness of 18 – 20 microns.
Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. From a paint usage standpoint, polyesters make up the bulk of coil coatings. Table II details typical materials utilized in a polyester coil coating.
In summary, coil coating technology has grown more rapidly than more conventional application technologies due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.
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Manufacturers have long benefitted from the speed, precision, and non-polluting aspects of using coil coating lines to make prepainted metal panels. Some manufacturers, however, utilize a coil coater’s capability beyond the painting process. Here are three ways manufacturers benefit from using coil lines as an effective first-step operation beyond prepainting.
#1 Coil coating offers easier and more effective stamping. When it comes to stamping metal parts, lubes and oil are essential in facilitating the fabrication process. There are, however, many issues with the use of lubes and oil—and one of the biggest is safety. During the stamping process, oil often drips onto the floor, which can be dangerous for employees walking or operating a forklift. In some cases, using oil can even affect the manufacturer’s productivity. By switching to a coil-line–applied dry film lubricant (DFL), companies can increase productivity, keep plants clean, and minimize the hazard of slips and falls. A DFL helps to protect the tooling, and has the potential for substrate grade reduction (e.g., using a lighter gauge, but still being able to make the part).
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#2 Preprimed metals can help save time and money. Coil coaters offer a variety of options for manufacturers that post-paint products. A preprimed material that has been cleaned, pretreated, and primed at a coil line replaces the first steps in a post-painting operation—cleaning and pretreating the metal. Outsourcing these steps and eliminating the corresponding waste treatment necessitated by these processes makes for sound financial and environmental decisions. A coil-coating primer from a coil line can also provide protection from corrosion and debris for steel products when they are stored in warehouses before post painting. Prepainted primers can also be formulated when resistance welding is used to join metals. Weldable primers also protect products from rust.
#3 Outsourcing pretreatment offers environmental and regulatory benefits. Using pretreated metal from a coil line helps customers keep up with the growing number of environmental regulations. The coil coating process itself is considered the most environmentally responsible way to apply a pretreatment to steel and aluminum substrate. Also, when manufacturers outsource pretreatment, they also avoid exposing workers to chemicals or solvents—and any environmental issues can more easily be concentrated, controlled, and even eliminated.
Whether a manufacturer is looking for a cleaner and safer plant floor, a more precise pretreatment, or an easier way to abide by the latest environmental regulations, using a coil line can greatly improve a company’s productivity, lower costs in the long run and reduce the amount of inventory in warehouses. It’s not just for painting!
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