April 15, 2008
A variety of methods help push a roll forming operation's efficiency. Various coil handling methods offer different levels of efficiency and automation, each suiting certain applications. The key is to analyze an operation, calculate the time spent handling coil, and then determine the best level of coil handling automation to fit.
Editor's Note: This article was adapted in part from Dale Kroskey, "Improving Efficiency of In-Plant Roll Forming and Trends in On-Site Roll Forming," presented at FABTECH® Intl. & AWS Welding Show, Nov. 11-14, 2007, Chicago, © 2007 by the Fabricators & Manufacturers Association Intl., Society of Manufacturing Engineers, and the American Welding Society.
Roll forming operations haven't had the best uptime records. In front of the roll former itself, setup personnel spend significant time changing out coils—especially in the metal-building construction sector, where just-in-time thinking has changed the manufacturing landscape. Historically, companies would keep a variety of prepainted panel stock on hand. Today's lean efforts to reduce inventories have forced many to produce only what customers demand, delivering it exactly when they need it. For this reason, it isn't uncommon to see a roll forming operation change out coil as many as 15 times a shift, with a short run of beige panel, another run of green, then red, and so on.
That's a lot of coil changes.
It's no wonder that a typical roll forming production line runs at only 30 percent efficiency. Imagine if that facility, running a roll former at 120 feet per minute (FPM), added 360 feet to its production a day. With a selling price of $2 a foot, that equates to $720 a day, or $180,000 over 250 working days a year. That's serious cash, and it comes from getting the product to the customer faster.
To get that extra productivity takes some unconventional thinking when it comes to getting material to the roll forming system. A variety of methods help push a roll forming operation's efficiency. Various coil handling methods offer different levels of efficiency and automation, each suiting certain applications. The key is to analyze an operation, calculate the time spent handling coil, and then determine the best level of coil handling automation to fit.
Degrees of efficiency boil down to three levels. In the first, the roll form line must shut down as coils change over. The second level introduces increased automation so the operator can stage coils while the roll form line is running; however, it still requires coils to be manually stored, transported to and from the line's decoilers, and operators to cut and feed material. The third level, the highest degree of automation, takes that manual storing, transportation, cutting, and feeding out of the equation.
Straightforward, the single-mandrel decoiler has been an adequate option for years, but for operations requiring frequent changeovers, it has obvious drawbacks (see Figure 1). For each changeout, the entire production line must stop. A fork truck or overhead crane with a J hook must remove the old coil, put it down and retrieve the new coil, and insert it onto the decoiler unit. Then operators remove the J hook or similar device and expand the coil; cut the bands and remove the outer wrap; jog the decoiler and pull the strip into the entry table of the roll former; and finally thread the machine.
This process alone may take 15 minutes or more, depending on a shop's organization. Fork trucks easily can scratch sensitive coil in transit. If jostled, the coil can telescope and, even worse, crash to the floor, potentially ruining an entire batch of material. Still, these mandrel decoilers may be the most cost-effective option for roll forming lines with infrequent changeovers.
Double-mandrel uncoilers, another step up the efficiency scale, offer two mandrels to hold coils (see Figure 2). One side pays off into the production equipment while the other side is free to be loaded. The system still requires the same coil loading, unloading, cutting, and threading as the single-mandrel decoiler does. When ready to switch coils, an operator stops the line, winds up the remaining pup coil, restraps it, tapes it, perhaps marks it with a marker ("Forest Green, 26 gauge," for instance), removes it from the mandrel, spins the uncoiler 180 degrees, removes the band, and rethreads the line. While the roll former is running, the operator can retrieve the next coil, per the schedule. The double uncoiler still requires a fair amount of material handling, but because it uses two mandrels, it reduces downtime significantly.
For both double- and single-mandrel systems, various tools offer incremental increases in efficiency, while decreasing the possibility of damage. Coils often are damaged when a fork truck or other device takes coils shipped "eye to the sky" and rotates them "eye to the side" to load them on the uncoiler. With a fork truck or hoist, workers often tip the coils onto tires or other material to get them to the right orientation. If workers aren't careful, this can cause telescoping or coil damage. In these cases, upenders can help (see Figure 3). These units rotate the coil to the correct eye orientation, reducing the chance for damage and increasing efficiency.
Two more tools help push efficiency by assisting coil loading and unloading. Coil cars allow the roll form operator to load coils onto the uncoiler mandrels without a fork truck or crane, freeing those material handling tools to perform other duties, such as removing finished goods from the exit side of the roll forming system (see Figure 4). A crane or fork truck still must transport the coil from storage to the cradle of the coil car, but the car does hold the coil by the outside, giving better control when loading onto a decoiler, shaving a few minutes off load time, and reducing the possibility for coil damage.
Another tool, coil turnstiles, involves three or four arms mounted around a single rotation point. This allows coils to be stored very close to the point of use; using a coil car, operators can remove different coils from the turnstile as needed.
Some operations might have just a single uncoiler. Others may have a coil car transporting coils to that single uncoiler. Others may have double uncoilers, and still others may have multiple single uncoilers in a row. Each offers different levels of efficiency, but all require at least some operator intervention.
Traditional uncoil methods require an operator to take a coil from inventory, transport it to the roll former, and load it onto the uncoiler. With infrequent changeovers, this usually doesn't present a problem, especially with upenders, coil cars, and turnstiles to speed the process.
But as the number of changeovers rises to 10, 12, or even 15 times per shift, the time adds up significantly. And no matter the method, every handling of the coil—be it with a crane or fork truck—adds more potential for coil damage.
Within the uncoilers themselves there remains no way to know exactly how much sheet is left in a given coil. Before a run, operators can take the coil to a separate scale, but that adds material handling time. Separate production control monitoring systems do help keep track of coil inventory, footage remaining, and scrap rates, and offer an improvement over manual data gathering. Or it can be guesswork, with operators erring on the side of caution, thinking excessive scrap is better than having a line shut down midstream as a coil unexpectedly runs out of material.
For years single- and double-mandrel systems provided an adequate way to offload coil. But with JIT manufacturing promoting minimal inventories and the demand for shorter runs, coil handling has turned into a serious bottleneck on many production floors. Here's where the third level, automated warehousing and feeding, may help (see Figure 5).
Automated coil warehousing and feeding automate coil storage and loading into a roll forming or cutting line. After the coil arrives on the loading dock and is unpacked, workers insert a mandrel that expands to go flush with the coil inside diameter. (That mandrel stays with the coil until it's completely depleted.) Using a crane or fork truck, workers lift the coil into the cradle within the system. After this the coil material isn't touched again until it emerges as finished product at the end of the line.
The controller asks the inside diameter, outside diameter, gauge, color, and weight, and from that knows how many linear feet of sheet is in each coil. Next, a manipulator grabs the coil and transports it to a holding place (see Figure 6). The system then tracks where the coil sits and moves it to the right position for a smooth changeover.
A track and rack-and-pinion drive system runs along the floor. Those floor rails guide the coil manipulator, which has arms that clamp to the mandrels and move coils vertically, up and out of the way of preceding coils. The manipulator then moves along the track, shuffling coil order as necessary. After the coil is positioned for decoiling, the automatic docking station grabs the coil edge and feeds it into the roll forming or cutting line. If a coil is partly used, the system cuts off the coil, rewinds it, automatically clamps it, and moves it back into the warehousing system.
With a single-manipulator system, one manipulator does all the work, retrieving the coil, taking it to the threading station, and driving the uncoiler during operation. With two manipulators, the front manipulator performs the threading and uncoiling, while the other manipulator moves coils in back for optimal staging. If a rush job comes in the door, for instance, the back manipulator can move the needed coil to the front while the front manipulator continues to run the current job.
A roll former in some metal fabrication markets might run 18-inch-wide, 16-gauge material all day long. For this, a single-mandrel uncoiler would probably suffice. But within other markets, such as metal-building construction, demand for short runs has steadily increased over the years. Lean manufacturing has pushed companies to minimize their inventory wherever possible, so for many, having cash tied up in large inventories of different colored panels isn't an option.
When choosing an uncoiling system, metal fabricators need to consider factors like material cost and sensitivity, lot size, and customer delivery requirements. But the most significant factor is simply time. The more time a roll forming operation spends changing out coil, the more quick-change technology may be beneficial.
Also, if a growing business has a need to increase capacity, adding another roll forming machine may not be the only option, particularly in an operation with frequent changeovers. More efficient coil handling may make it possible to increase uptime and, hence, overall capacity for a company's existing roll forming system, and eliminate the need to invest in another roll forming line.
To compete, roll forming operations will need to maximize value-added time—that is, production time—and to do that requires a careful look at everything that's preventing the roll forming line from doing what it's supposed to be doing: making parts. Efficient coil handling is sure to remain a big part of that equation.
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