The Coil Coating Process
The metal substrate (steel or aluminum) is delivered in coil form from the rolling mills. Coil weights vary from 5-6 tonnes for aluminum and up to 20 tonnes for steel.
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The coil is positioned at the beginning of the line, then unwound at a constant speed, passing through various pre-treatment and coating processes before being recoiled.
Two strip accumulators, located at the beginning and the end of the line, enable continuous work, allowing new coils to be added and finished coils to be removed without slowing down or stopping 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
- Surface treatment by chemical conversion
- Drying the strip
- Application of primer on one or both sides
- Passage through the first curing oven (between 15 to 60 seconds)
- Cooling down the strip
- Coating the finish on one or both sides
- Passage through the second curing oven (between 15 to 60 seconds)
- Cooling down to room temperature
- Rewinding of the coated coil
It goes by many names: prepainted metal, coil-coated metal, and prefinished metal. Each of these descriptions refers to the product of a coil coating line, sometimes called a continuous coil line (CCL). Prepainted metal is commonly used in construction applications (metal walls and roofs are examples), as well as in appliances, HVAC units (air conditioners, furnaces, etc.), rainware products (gutters, downspouts, flashing, etc.), and many others. Prepainted metal is the product; a CCL is the application process used to produce prepainted metal.
As its name suggests, the CCL process for prepainted metal is continuous. But what does that actually mean? It probably seems obvious—continuous means "not stopping" or "never-ending"—but how does a coil coating line make this happen? It's not like there is an infinitely long coil of metal. Surely, some part of the process must stop when a coil of metal is completely processed and another new, unprocessed coil must be started.
The word "continuous" refers to that segment of a coil coating line that stays in a steady-state condition: the cleaning, pretreating, painting, curing, and cooling. Engineers love steady states because all the processing conditions remain constant throughout this segment. A constant, unchanging process is one of the secrets to producing high-quality prepainted metal at very fast line speeds.
Still, what happens when one coil has been completely processed and another must be started? Imagine watching the fascinating engineering that makes the CCL continuous.
Here's the scenario. You've now painted about 17,000 feet (about 3 miles of metal) from a coil that is at the entry end of the CCL. The tail end of this coil is about to run out, and another coil must be put in its place. To accomplish this, you must find a way to keep the steady-state part of the coil line running while the entry end of the line stops. The answer is genius: the CCL uses an accumulator, a piece of equipment that "accumulates" a certain amount of metal. It is a set of upper and lower banks of rolls through which the metal strip is threaded in a serpentine fashion, storing lengths of metal as the two roll banks spread apart. The total stored length of metal depends on the line's design speed—usually 60 seconds of steady-state metal processing time. When the entry end of the continuous coil line stops, the roll banks move toward each other, and the stored metal in the accumulator continues to feed the steady-state portion of the CCL.
In those 60 seconds, numerous activities occur at the now stationary entry end. The new coil is loaded onto the CCL. The front end of this new coil must be attached to the tail end of the earlier coil. This attachment can be made by a mechanical press-type joint (sometimes called stitching) or by welding the two ends together. This attachment must happen quickly. If the accumulator runs out of metal and if the two coils (the old and the new one) are not joined in time, the CCL must be stopped, negating the "steady state" concept.
Once the coils are successfully joined, the CCL continues to operate. The first task of the newly joined coils is to replenish the accumulator. Remember that it has fed the steady-state portion of the coil line while the new coil was loaded onto the line, but it is now nearly empty. It must be replenished in anticipation of the next time a coil is consumed and a new coil is loaded and joined to the old coil. This is achieved by running the entry end of the CCL at a much faster speed than the steady-state portion. This is referred to as overspeed and is typically 25% to 100% faster than the steady-state portion of the line.
Imagine you are standing near the accumulator and you see the steady-state portion of the line running at 600 feet per minute. The accumulator is empty, but that serpentine ribbon of metal and the rollers start rising higher and higher from the ground level as metal from the entry end of the CCL feeds into the accumulator. When the accumulator is filled, the line speed at the entry end returns to match that of the steady-state portion of the CCL. It truly is an engineering marvel.
But wait; there's more. We have explained adding a coil to the line, but how does one remove a coil from the exit end after it has been cleaned, pretreated, and painted? There is not only an entry end accumulator but also an exit end accumulator. Essentially, the same process occurs at both the entry and exit ends but in reverse. The exit accumulator collects metal from the continuous process, allowing the non-continuous process of changing coils at the exit end of the line. Fascinating stuff!
Other painting processes are also continuous. A spray line, for example, is essentially a giant conveyorized loop, where bare parts are hung onto the line and then cleaned and pretreated. The conveyor eventually carries them into a spray booth, where paint is applied. Then the parts travel into a large baking oven where the coating is cured, followed by cooling and unloading.
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The big difference between a CCL and a spray line is the rate at which metal can be processed. A CCL can typically coat 10 times as much surface area as a spray line in the same amount of time and does so at nearly 100% efficiency (i.e., no paint loss during the painting process). That’s why a CCL embodies an engineer’s mantra: Better (consistent cleaning, pretreating, painting, curing, and cooling), faster (higher line speeds, meaning more metal area painted), and cheaper (maximized utilization of energy)!
David A. Cocuzzi
NCCA Technical Director
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