February 20, 2012
Scrap is a necessary stamping byproduct, but it doesn’t have to be an unprofitable one. In recent years the high price of scrap has spurred the use of some sophisticated scrap handling and separation technologies.
The teeth alone can intimidate.
Think of a paper shredder the size of a bungalow, and instead of paper, think cars. Such massive car recycling operations use some of the latest in scrap handling and separation technology. To separate metals, magnets first separate ferrous from nonferrous; eddy-current systems then separate out the nonferrous. Sophisticated metal detectors or sensor sorters separate the stainless steel. Ultimately, the choppers, conveyors, and advanced material separators help the operation sort the automotive remains into dense packets of material that are easy to work with and transport.
Ease of transport also matters in the metal manufacturing arena, which is why scrap handlers and remelt facilities usually dot metro areas across the country. The longer it takes to transport, the more that scrap costs. To reduce cost further, scrap houses want to pack trucks tightly. Typically, hauling “air”—that is, the gaps between large pieces of irregularly shaped sheet metal—adds to transportation costs, because it reduces the amount of scrap a hauler can fit on the truck.
When scrap handlers receive finely chopped, separated, densely packed metal, they view it as high-value, or upgraded, scrap, simply because they have less to do on their end to prepare it for further processing. In short, upgraded scrap costs scrap handlers less to process, so they pay more for it.
Scrap is a necessary stamping byproduct, but it doesn’t have to be an unprofitable one. In recent years the high price of scrap has spurred the use of some sophisticated scrap handling and separation technologies inside manufacturing facilities. The more a stamping shop can get for its scrap, the more an investment in scrap handling technology begins to make sense.
Scrap will always be a relatively low-value product compared with the metal components a stamping operation sells. But scrap doesn’t need to be as low-value as it often is. It still is a product that’s sold, just to different customers—that is, scrap handlers—and these customers often are willing to pay more per pound if the scrap is clean, separated, chopped, and densely packed.
So how can you improve or expand your scrap handling capabilities to make that scrap more valuable? It, of course, depends on your situation and local scrap market, but here are some basic first steps.
Ask your scrap handlers how much additional value they would add to scrap if your stamping operation performed additional scrap processing. In most cases, scrap handlers will pay less for irregularly shaped parts and commingled material, which is multiple material types piled together. If you can provide nicely chopped material, with ferrous metals separated from other materials, the scrap handler probably will pay more, because it costs less for the handler to transport and process.
Efficient scrap handling does help make a stamping operation more efficient and reliable. But scrap handlers are customers in their own right, and their needs should help shape your scrap handling strategy.
As with most things in manufacturing, it costs less to overcome problems farther upstream in the process. Consider scrap handling when designing the part and its tooling, or when discussing design-for-manufacturability (DFM) factors with your customer. For instance, an extra prog-die station that chops scrap into manageable pieces not only can make scrap handling easier, but also may make the stamping process more reliable. Oddly shaped or unwieldy pieces of scrap can be difficult to evacuate and can jam, leading to some expensive press downtime.
Orientation of specific scrap pieces also matters. A long, thin piece of scrap may be difficult to evacuate in one orientation but easy to eject if the part nest is rotated. Scrap handling shouldn’t be the only factor when determining part and tool design, of course, but it at least should be part of the conversation.
Chopping is a science unto itself. Pieces cut to the wrong size can jam conveyors, while consistently chopped metal pieces make for reliable conveying (see Figure 1). Chopper types vary. Some resemble simple shears; others are shredders. A slitting operation that involves scrapping the edges of a coil might require only a simple shear chopper. But a stamping press that processes different parts of varying shapes, material types, and thicknesses may call for a shredder-type chopper. (Grinding-type choppers are also available, but these usually process metal chips from machining.)
It may be most cost-effective to install a chopping station into the tooling—or, at the very least, ensure the scrap dropping off the press is of a regular shape (squares, rectangles, etc.) that won’t catch easily. If chopping the scrap in the press isn’t an option, you need to chop scrap soon after it exits the stamping die. Oblong scrap shapes catch easily, potentially jamming downstream conveying equipment.
No one wants a line to stop, and scrap jammed on a conveyor can certainly be a cause. Various factors affect how reliably scrap is transported from the press to the scrap bin, including the material type and thickness, part shape, as well as moisture. Coolants, lubricants, and films remaining on scrap cause the cut pieces to clump, and these clumps can get caught on conveyors.
How these factors affect scrap handling reliability depends on the conveyors you have and whether you separate scrap materials downstream. If you’re stamping strictly ferrous metal, a magnetic conveyor can be a reliable choice. Vibratory conveyors, which handle all materials, also work well. A vibratory drive underneath a tray or pan moves materials forward. This eliminates pinch points, hinges, belts, and other areas where the material could become trapped.
Unfortunately, these vibratory systems work only for horizontal motion; you cannot incline them (see Figure 2). This is why these conveyors often act as tributaries of a larger scrap handling system, feeding scrap directly from the press to a central hinged-belt conveyor or similar system that can incline the material for further processing. This way, you have only one robust belt-style conveyor system to invest in, maintain, and monitor.
When possible, design systems and procedures so that scrap separates at the press itself. Anytime you can dedicate a press to a material family—be it mild steel, stainless, or aluminum and other nonferrous metal—adds value to scrap, because the metal being collected is already separated.
If dedicating a line to a material family isn’t an option, then you can factor in scrap handling during job changeovers. Consider a line running stainless and then aluminum. Taking a few minutes more to evacuate scrap from the system and switch over to a new scrap bin may be time well spent. A movable chute can speed the changeovers. Between jobs, the operator cleans the chute to avoid cross-contamination of metals, and then moves the chute to a different bin or conveyor.
But what if you’re running high volumes of bimetal parts? In these cases, you just can’t avoid having commingled scrap emerging from the press, and the high volume may make additional scrap separation in the stamping plant worthwhile.
This is where scrap separation systems come into play. To separate the ferrous material from everything else, magnetic-drum separators can help (see Figure 3). These rotating cylindrical systems have an arc of magnets in their core. As material flows over the drum, the ferrous metal sticks and is diverted, while the remaining nonmagnetic material continues on.
When using such separators, consider the material’s moisture content. Small scrap chips coated with lubricant tend to stick together. Lubricant viscosity plays a role, but so does the fluid’s properties after it dries. Some low-viscosity flood coolants and other liquids can dry and become sticky. The fluid remnants cause some nonferrous scrap chips to stick to the ferrous chips, which in turn decreases the purity of the separated scrap.
Sometimes getting a little nonferrous scrap on the drum separator just isn’t avoidable in one pass. To attain clean nonferrous scrap may require further processing, such as an additional wash and dry cycle, or simply sending the nonferrous scrap over the drum separator again.
A magnetic drum won’t, of course, separate stainless steel from the ferrous material, or separate out different types of nonferrous scrap. At this point, though, it usually doesn’t make business sense for a typical stamping operation to perform additional scrap separation. True, the scrap handler will pay more for cleaner scrap, but probably not enough more to justify a stamper’s additional equipment, time, and labor investment. Still, separation technology does exist to automate the process, and today many scrap handlers have adopted the technology (see sidebar).
Say you’ve determined that your plant could indeed benefit from a scrap separator. Where should you put it? It depends, of course. Scrap separation investments hinge on volume; the more scrap you send over a magnetic drum in less time, the quicker your return on investment. On the other hand, that ROI can vanish if it’s integrated into a poorly designed system that causes frequent work stoppages.
Placing a scrap separator on a conveyor connected to just one press may make sense if that press stamps a lot of bimetal product that leaves commingled scrap. But if other presses also produce commingled scrap, placing the separator in a centralized location may give you a quicker ROI. Scrap from the press can ride a vibratory conveyor to a central shredder, after which a centralized magnetic drum separates the ferrous from the rest.
Consider a press shop with a dozen lines; half stamp bimetal, half do not. Even those other presses that don’t stamp bimetal product still can send scrap to a centrally located separator. Not only does this simplify scrap handling procedures at those other presses (no chutes to shift between jobs, and so on), it also can make your operation more flexible. Presses may no longer need to be dedicated to specific material families, for instance.
Of course, a centralized system isn’t always the best option. In a high-production situation, one central conveyor could become overwhelmed with a deep pile of heavy scrap, more than the system was designed to handle. If it jams, the entire operation can grind to a halt. As always, the application, including scrap volume and weight, dictates a system’s design.
Regardless, scrap shouldn’t be an afterthought. Some level of scrap is unavoidable in any stamping operation. Its volume will determine which scrap handling technology makes business sense, but some scrap handling practices—dedicating a press line, job changeover procedures, and so on—can apply regardless of volume.
Always present, scrap should be a consideration during your company’s improvement efforts—first by working to minimize it, then by optimizing its handling and value. Both strategies help stamping operations get more out of what remains the most expensive line on the balance sheet: raw material.
Say a stamper chops and magnetically separates out the ferrous scrap. The company then sells a scrap handler a batch of purely ferrous metal along with a chopped mix of various nonferrous metals and stainless steel.
To process the nonferrous and stainless mix, the scrap handler uses eddy currents to repel the nonferrous from the material flow. From here, the operation uses metal detection or sensor-sorter technology. An electromagnetic coil senses a metal type in a certain zone, or lane, on the conveyor. Upon detection, the system sends a signal to paddles that physically remove that material, be it stainless steel, magnesium, or other nonferrous materials not recovered by the eddy-current separator.
Certain stamping operations may use a magnetic drum to separate ferrous metal from nonmagnetic metal. At present, though, most do not have the volume to justify additional material separation equipment. But as scrap handling technology progresses, and if scrap prices rise, more separation technology eventually will find its way to the stamping shop floor.
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