Evaluating in-house coil slitting
Key areas to consider
As the cost of steel rises and profit margins shrink, small to medium-sized stampers are looking for ways to reduce costs and streamline operations while maintaining quality.
One area to consider is the cost of slitting steel.
Shops that run 10,000 to 50,000 tons a year usually outsource slitting. Typical costs for outsourced processing have been fairly steady at 1.5 cents to 5 cents per pound (based on a minimal 1.5-cent rate). Smaller operations processing 10,000 tons normally budget about $300,000 per year for outsourcing.
A stamper running 50,000 tons can expect to spend $1.5 million per year on slitting. As these numbers increase, a shop may save mon
ey by slitting on-site. Although each job shop must consider its own unique situation, consider the benefits and costs associated with slitting in-house to see if this move is right for you.
Weighing the Benefits Against the Costs
Stampers considering in-house slitting first must decide whether to invest in used or new equipment. Used slitting lines are readily available but may require rebuilding and upgrades. In some cases, the final investment can result in 70 percent to 80 percent of the cost of a new line.
New small- to medium-capacity slitting lines range from 24 to 60 inches wide and can cost $300,000 to $600,000. With this type of prepiped, prewired slitting line, minimal foundation work is required. Compared to high-capacity lines found in toll processing facilities, these lines generally have more manual functions, yet employ the latest controls technology.
Some advantages of in-house slitting are:
- Control coil inspections to prevent processing flawed materials
- Control delivery times
- Build in-house expertise
- Purchase raw material for less through a broker or directly from the mill
- Eliminate one leg of freight expenses
- Sell edge trim material as scrap to offset costs
A Basic Slitting Line
A basic slitting line includes coil handling, material processing, and scrap management equipment.
Coil Loading Equipment. Depending on coil size, loading can be performed by a forklift or an overhead crane. Large slitting lines typically use a four-arm turnstile, coil loading car, and an uncoiler. Small to medium-size slitting lines use a stationary, floor-mounted coil saddle or a floor-mounted, four-station rotary storage system.
The uncoiler is a floor-mounted, traversing style that also acts as a loading car. It traverses to the coil storage location, picks up the coil, and moves back into position at the slitting line. The coil saddle and uncoiler are connected mechanically, which reduces overturning, rail, and foundation work associated with a four-arm turnstile, coil car, and stationary uncoiler.
On the exit end, two methods of coil removal are common. The most economical is physically pushing a coil off the recoiler and onto an exit arm. With this method, coil damage can result. Another method is to remove the coil with a coil unloading car traversing to an exit turnstile. The exit turnstile can have one, two, three, or four arms, depending on the number of slitter cuts. A two-arm turnstile is sufficient for small- to medium-duty lines.
Material Processing Equipment. Uncoilers can be tension-braked or driven by a DC or AC regenerative motor, depending on the material processed. If the coils are loosely wound, surface-critical, very thin or soft material, or are grain-oriented silicone electrical steel, then a driven uncoiler paying off into a shallow loop before the slitter is the appropriate choice.
For other applications, a tension brake uncoiler with a tight passline to the slitter head is sufficient. A tension brake uncoiler can use water-cooled brakes or hydrostatic braking. The economic advantage of hydrostatic braking is commonality of hydraulic services and the elimination of an additional water utility required at the line and its attendant water cooler.
The slitter, truly the heart of the system, consists of a drive, slitter head, and, in certain applications, a dedicated outboard support. In loop slitting, the head is driven by hydrostatic, DC electric, or AC vector electric.
Hydrostatic drives are well-suited for all applications and usually are more cost-efficient. A hydraulic drive is quiet and compact and offers good sightlines.
A DC drive offers ease of operation, improved housekeeping, and is better-suited for certain applications. It also is universally acceptable as an industry standard because maintenance is readily available.
AC vector drives typically cost 10 percent to 20 percent more than DC drives, but require little maintenance because they don't have brushes. Another advantage is their ability to share power between an uncoiler and slitter. During unwind, the uncoiler AC motor acts as a brake and regenerates current. This regenerated current can be supplied to the slitter, which results in less energy consumption.
The actual slitter head should be chosen based on coil width to be slit, gauge range, shear strength, maximum number of cuts, and anticipated processing speed. All of these factors dictate arbor diameter and horsepower.
Slitter arbors and tooling can be keyed or keyless. In most applications of 0.187-in. or thinner mild steel material, arbor keys are not needed. By avoiding arbor keys and keyways, tooling setups can be quicker.
Jet nuts are commonly used to lock tooling on slitter arbors. However, an alternative such as hydraulically deployed dogs, which emerge from the slitter arbors and bear against the tooling stack, requires shimless precision tooling but can save 10 to 20 minutes on every setup.
The number of required setups per shift dictates which slitter head style is appropriate. The most efficient style of slitter setup change is to use indexing slitter heads so one is online while the other head is offline being set up. If setups per shift are few, the economical choice is one slitter head with online setups. An indexing head can be added should the need arise.
The key to tight-wound, straight-walled slit coils is the tension stand. It's essential to provide consistent tension across all of the slit strips. This tension causes the thinner strips to accumulate in a loop in the pit, as they are wound more slowly than thicker strands.
Some tensioning methods include:
- Tension Pad. Tension can be supplied by a drag board, a set of wooden boards covered in carpet or felt and pressurized together to provide pinch pressure across all of the strips. The advantage of this system is simplicity and low capital cost. The disadvantages are the cost of consumables (felt or carpet strips) and the high potential for surface damage.
- Roll Only. This type of tensioning is critical for surface-sensitive applications, but it can be more costly than pads. Eliminating the drag board eliminates one potential for surface damage and simplifies the procedures for the operator. Tension rolls are available with polyurethane covers for clean, dry applications or compressed, nonwoven fabric covers for oily and dirty surfaces. Roll sets can be changed out in less than 30 minutes using a crane and sling or in minutes with a tension stand index system.
- Combination of Tension Pad and Roll Only. Pretensioning is provided by a drag board, immediately followed by a set of tension rolls.
|Benefits of toll processing|
|Toll processing offers many advantages and benefits:|
| No capital investment or depreciation associated with equipment ownership|
Just-in-time delivery/fast turnaround
No maintenance or operator expenses
No additional personnel
No need for extra floor space
No installation costs
No detailed engineering requirements or building permits
No spare parts to inventory
High-production lines can process many materials, weights, and thicknesses and typically utilize powered features
Customers have the flexibility to offer new products without committing funds for equipment modifications or additional investment.
The drag boards and rolls also can be used independently. This system is used mainly for very thin material and a large number of cuts.
The last processing step in a slitting line is the recoiler. Recoiler drums are designed to accommodate IDs of 16, 20, or 24 in. and should be sized to match the smallest-ID coil. The drum consists of expanding segments with a gripper mechanism to hold the coil tails.
To adapt the diameter of the recoiler drum to accommodate larger IDs, risers also can be used. Risers are bolt-on plastic segments that add dimension to the recoiler diameter; for example, increasing the ID from 16 to 20 or 24 in. Risers are inexpensive but create a long ID tail from the gripper to the first wrap and require bolt-on segments.
Scrap Management Equipment. Disposing of edge trim is necessary in any slitting operation. The slitter operator seeks to dispose of scrap with as little added labor and downtime as possible. High-density scrap management saves labor, space, and commands a higher scrap price.
Three main ways to handle scrap are:
- Scrap Winders. Scrap winders accumulate scrap under tension from the edge trimming knife sets. If trim breaks, the line must stop for rethreading. A winder is good for most gauges and low- to medium-yield-strength material on coils with good edges. Higher-strength and thick materials tend to reduce density of full wind-ups.
- Scrap Baller. A scrap baller generally is mounted offline and fed through a scrap trench from a shallow scrap pit in the floor. The baller takes in two strands or a tangle of scrap. If trim should break during the run, it's likely that it will accumulate in the pit only until it becomes tangled with the trim pulled into the baller, thus re-establishing continuity almost immediately.
- Scrap Choppers. Scrap choppers or slicers usually are mounted under the line below trim knife sets, where infeed guides are mounted. Trim break is not a concern because the trim has nowhere to go but into the chopper knives. Scrap is chopped into relatively consistent lengths because chopper speed is controlled by a signal from the slitting line. The scrap then is discharged onto a conveyor and dumped into a container. In most areas of the country, chopped scrap commands a premium price over balled, bailed, or wound scrap.
The key to specifying the ideal slitting line is finding a balance between the investment level and the highest possible productivity.
Other Potential Costs
In addition to the slitting line itself, other key factors to consider are material handling, space and foundation requirements, labor and training, and operating costs.
Material Handling. To achieve maximum productivity, stampers should rely on the equipment supplier to help direct the process flow from the minute raw material arrives until it is slit and moved to the stamping line. Consideration also must be given to the coil transfers from delivery trucks to the storage location and then to the slitting line. Depending on a coil's weight, a forklift or overhead crane will be needed.
Space and Foundation Requirements. A slitting line can be scaled to fit almost any area—even as small as 60 by 30 ft. For high-capacity lines, foundation preparation can account for up to 25 percent of the total cost of installation. For small- to medium-capacity slitting lines, foundation costs are less, typically only the cost of a looping pit.
Labor and Training. Unlike high-production toll processing lines, small to medium-size lines can run with one or two operators. However, these operators must be trained and receive ongoing safety and education to reap the full benefits of the line's capabilities. Stampers should make sure training is included as part of the support package the equipment supplier offers.
Operating Costs. Most maintenance centers around grease and oil, and depending on the tension unit, felt/carpet usage or roll dressing. Slitter knives can be ground for a long service life—sometimes up to 10 years. Of course, similar to stamping presses, slitting lines also require energy to run.
With a clear understanding of how all the various parameters and components interact, stampers can have the tools they need to select the best combination of components to meet their slitting needs if they determine an in-house slitting operation is right for them.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.