It goes by many names: prepainted metal, coil coated metal, 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 as a coated product in construction applications (metal walls and roofs are two examples), as well as 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 such a thing as 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, and 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 some way to keep the steady-state part of the coil line running while the entry end of 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, and it stores lengths of metal as the two roll banks are spread apart. The total stored length of metal depends on the design speed of the line—usually 60 seconds of steady-state metal processing time. When the entry end of the continuous coil line stops (is that an oxymoron?), the roll banks move toward each other, and the stored metal in the accumulator continues to feed the steady-state portion of the CCL.
Magura, Daniel & Fedák, Viliam & Kyslan, Karol & Sanjeevikumar, P. (2016). “Practical Experience with Control of Drives of an Accumulator in a Web Processing Continuous Line.”
In those 60 seconds, a lot of things have to happen at the entry end, which is now not moving at all. 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 may be made by mechanical press-type joint (sometimes called stitching) or by welding the two ends together. And the 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, the CCL must be stopped, which, of course, negates the concept of “steady state.”
As 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 (i.e., the length of metal in the serpentine arrangement of rolls is very small). 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 done 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 it is typically 25% up to 100% faster than the steady-state portion of the line.
So, imagine that you are standing near the accumulator and you see the steady-state portion of the line is 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 is fed into the accumulator. When the accumulator is filled, the line speed at the entry end returns to the same as 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 of the line after it has been cleaned, pretreated, and painted? You guessed it: There is not only an entry end accumulator, there is also an exit end accumulator. Basically, the same process occurs at both the entry end and the exit end, but in reverse of each other. The exit accumulator collects metal from the continuous process, allowing the non-continuous processes of changing coils at the exit end of the line. Fascinating stuff!
Other painting processes are also continuous. A spray line, for example, is nothing more than 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 the paint is applied. Then the parts travel into very large baking oven, where the coating is cured. Then the parts are cooled and unloaded.
The big difference between a CCL and a spray line is the rate at which the metal can be processed. A CCL typically can coat 10 times as much surface area as a spray line in the same amount of time, and it does so at essentially 100% efficiency (i.e., no loss of paint during the painting process). That’s why a CCL is emblematic of an engineer’s mantra: Better (consistent cleaning, pretreating, painting, curing, and cooling), faster (line speeds, meaning more metal area painted), cheaper (maximized utilization of energy)!
David A. Cocuzzi
NCCA Technical Director