Coe Press Equipment Study:
Coil Feeding vs. Blank Feeding

As current economic realities force the metal forming industry to innovate, economize and streamline their stamping operations, the question of which is the superior method of feeding material through the stamping operation—coil versus sheet—takes on new urgency. Coe Press Equipment, a leading manufacturer of coil-handling and material feeding equipment, recently did a head-to-head comparison of coil feeding and sheet feeding to see what, if any, efficiencies could be brought to bear on the overall stamping operation. What they found was that the choices made at the material feed level impacted not only the press operations, but the overall plant floor environment as well.

Coe used a transfer press operation as the basis for the test. Technical and operational comparisons were based on processing similar parts using both methods. The part was a sheet metal stamping requiring flat stock material that is 36" wide x 24" long x .050" thick. The feeding operations required a progression of material to be presented to the first stamping operation on the coil feeding side, and a loose blank to be presented to the first stamping operation on the sheet feeding side. In this scenario, a 72" O.D. coil was used, containing a specific lineal footage of material. Sheet feeding was based on an optimum number of pallets of flat blanks that would contain an equal number of pieces as the coil.

Material Requirements:

Coil MaterialBlank Material
- 72" OD x 32" ID- 36" x 24" area
- 36,000 lb. coil weight- 3,600 lb. weight per stack
- .050" Thickness- .050" Thickness
- 6,000 feet/coil- 15" stack height
- 3,000 pieces/coil- 300 pieces/stack

The results of this comparison played out in several areas, including:

Annual Usage

There is no 'typical' production volume for sheet metal stamping. Annual volume can vary from a few hundred pieces for "short-run" type stampers to millions of pieces for "high-volume" type stampers. For the purposes of the comparison, an annual volume of 300,000 pieces was used. Coil material would require approximately 100 coils to be purchased from the supplier. Blank material would require approximately 1000 stacks to be purchased. Thus the coil material method provides a 10x factor of less purchasing, receiving, tagging, and handling of "raw" materials.

Material Costs

As a rule of thumb, coiled cold rolled steel (CRS) can be purchased for approximately $27 per hundred pounds of material (CWT), and blanked CRS can be purchased for approximately $30/cwt. In the case of the blanked material, the stamper is paying for the "value added" for the steel service center to blank, stack, band, store, and ship the material. This $3/cwt difference adds up to approximately $108,000 in overhead charges that are not required in the coil feeding process. The cost savings associated with directly processing coiled material into the press can add up to hundreds of thousands of dollars per year.

Operations

The process of cutting the blank to length or developing its shape is an additional operation in the stamped parts development process, which comes at a measurable cost. As shown, it can be reflected in the cost to purchase the 'raw' material. If the 'raw' material is in the form of a blank, it will cost the stamper approximately 10% more than if the material were in the form of coil steel. To offset these cost premiums, many stampers have "in-house" capabilities to make blanks. This comes at a cost as well. There are initial capital costs for the cut-to-length (CTL) or blanking equipment, direct labor costs, overhead costs, and operation costs. If the "raw" material for the stamper is in the form of coiled stock it will reduce the number of operations required for the stamper to make the part. This provides benefits such as lower tooling costs, lower per piece costs, improved payback on capital equipment investment, and reducing the "cycle time" required to flow materials and parts through the manufacturing operation.

Work In Process (WIP)

Blank processing can present considerable WIP costs and constraints to the manufacturing operation not found in coil stock processing. If blanks are purchased, release orders and inventory must be maintained to support the mix and volume of production required for a given work center. One week's production volume of the example part is approximately 6,520 pieces. The WIP for this time period is 21 stacks of blanks. If the blanks are stacked in a "two-high" configuration, this WIP will require approximately 66 sq. ft. of floor space for storage. Coil processing will result in certain WIP costs and consideration as well. The same one-week production volume will require 2 coils of steel. These coils will occupy approximately 36 sq. ft. for WIP storage. This is about 50% less floor space than would be required by blanks. The blank feeding method also requires additional WIP handling to store the stacks until demanded at the stamping press. From the Receiving Area or Blanking Work Center, the blank stacks must be moved to a designated WIP Area. Once demanded at the transfer press the blank stacks must be moved again. Multiple handling of the same materials adds indirect costs to the final parts and often results in increased requirements for hilos and operators within the plant.

Dunnage

Blank stacks are typically maintained on some type of wood dunnage or material pallet. Some manufacturers use rough-sawn 4 x 4 wooden skids banded to the stack that are discarded after use. Others use structural fabricated metal pallets that are continually used in the material handling process. Either method requires handling of dunnage for the blank stacking and destacking process. In the case of wood dunnage, the stacks must often be removed from the 4x4 or skid prior to entry into an automated destacking machine. The process of cutting the stack banding and handling a loose stack of blanks can present a safety hazard to the plant operations. Work cell productivity can suffer while the press waits for stacks to be accurately and safely loaded. Fabricated pallets offer a benefit of improved handling of the blank stacks. Many versions of fabricated pallets offer adjustable locating pins to suit the exact blank dimensions and these pallets can be loaded directly into automated destacking machines. The trade off of this method is the high initial investment cost and ongoing maintenance cost associated with the fabricated pallets. Again, the additional handling requirements and investment costs of blank processing are avoided by the direct coil feeding system. Coil handling requires no dunnage, since the coils are self-contained (banded to themselves).

Quality Control

Directly processing coil stock into the press where the stamping operation is performed improves the stamper's quality control potential. To process blanks, the coil strip must be unwound, straightened, cut and stacked by the CTL equipment. The stacks are then maintained as WIP. The blanks must then be loaded, destacked, and fed into the press performing the stamping operation. The redundant stacking and destacking processes increase the potential for damage to the blank surface or edges. The blanks are also subject to the plant ambient conditions for the time kept as WIP. Dirt and oils on the blanks can prohibit efficient destacking. To solve these problems the coil material can be delivered on a JIT basis to the presswork center. It is then unwound, straightened, and fed directly into the tooling. Exposure to redundant handling and potential damage is minimized. Not only does the material quality improve with direct coil feeding, but the press tooling can also benefit. By eliminating potential for dirts, oils, and foreign materials from being introduced, tool life can be extended and the quality of parts produced is improved. Another measure of improved quality control is in the part itself. Today's critical stamping operations demand the right combination of material composition, die conditions, press and handling operations. As a stamper experiences part shape, dimensional, or structural problems decisions must be made as to the source of the problem. If the parts are processed from pre-cut blanks, there is potential for thousands of pounds of compromised WIP throughout many locations in the manufacturing operation. If the parts are processed directly from coil material, the potentially compromised material is usually located in the coil storage area.

Flatness Control

A specified blank flatness tolerance must be maintained for most automated destacking machines to work efficiently. The blank flatness is established by the leveler or straightener in the cut-to-length (CTL) equipment. This equipment is often in a different plant location or, in the case of purchased blanks, a different company altogether. Production efficiencies are compromised and WIP can contain substantial volumes of bad stock. Direct coil feeding eliminates this potential pitfall. The material flatness required to feed the material is established directly at the press at the same time as the stamping process is being performed. Any adjustments required to maintain targeted flatness can be made immediately.

Material Handling

A coil of steel can contain 10-20 times more material, and therefore pieces, than a stack of blanks. Once the coil is loaded and threaded through the coil processing equipment, the material handling is complete for an extended period of production running. At a production rate of 20 sheets per minute, the example coil will run for approximately 2.5 hrs. The production operator can now focus on the press, tooling, scrap and other factors to keep the ram going up and down. This provides the stamper with the potential for a very productive work center. In contrast, to maintain continuous production with stacks of blanks, a new stack must be loaded every 15 minutes. While most destackers are provided with certain 'continuous run' features, each stack must be properly located, bands must be cut, dunnage must be handled, and the machine must be properly set up. This increased material handling requires increased labor from the operator, set-up personnel, hi-lo drivers and crane operators for non-value added operations.

Production Efficiency

The production efficiency of the transfer press is dependent on the capability and efficiency of the auxiliary devices to move material into, through, and out of the press. Coil feeding equipment, as compared to destacking equipment, offers a simpler solution and the potential for improved production efficiencies—a point demonstrated in the Material Handling section of this comparison. The process and equipment required to unwind, thread, straighten and feed coil stock are proven and accepted technologies. Most pressroom set-up and operations personnel have high comfort levels with this equipment. The process and equipment required to handle unbanded stacks of blanks, spread the blanks, destack the blanks and feed the blanks is often complex and less user-friendly to the pressroom operator. Process problems such as poor toleranced stacks, misaligned stacks, double blanks and rejected blanks can adversely impact the machine's production efficiency.

Blank Shapes

The traditional benefit to the destacker and blank feeding methods is the ability to process a wide variety of blank shapes in the same machine. A destacker can be designed to process square, trapezoid, parallel and developed shaped blanks. Machine design complexity often increases as these capabilities are integrated into the same machine, but this is offset by the production capacity and flexibility to process blank shapes that optimize both die design and material usage. Traditional coil feeding systems have been limited to feeding the strip into "cut-off" tools under the ram or "cut-off and draw" tools under the ram. The drawbacks to these methods are that additional tooling is required and valuable space "under the ram" is used for the cut-off process. Integrated coil feeding and CTL lines can provide the capability to feed square cut blanks into the first station of the press. This method eliminates the need for cut-off operations to be performed "under the ram". Unfortunately, neither of these direct coil feeding methods provide the potential material savings of trapezoid, parallel, and developed shape blanks.

Integrated Technology

The fundamental requirement of the upstream material handling equipment is to present material at a rate and with the precision required to optimize all downstream operations. Recent machine developments have integrated the production benefits of coil processing with the material utilization benefits of blank feeding. This combination of equipment utilizes the inherent benefits of both methods to provide optimum production efficiency and flexibility for transfer press operations. Conventional coil processing equipment has now been combined with the cut-to-length and material handling equipment to process a wide variety of blank types and shapes. This new type of direct coil feeding system is capable of processing square cut, parallel cut, and trapezoid cut blanks into the first station of the transfer press. The end user benefits from the long list of cost savings, production efficiency, and quality improvements as well as the potential material savings associated with parallel cut and trapezoid cut blanks.

Coil vs. Sheet

The decision to choose either sheet feeding or coil feeding is something that ultimately depends on the unique circumstances of the given situation. While there may certainly be specific manufacturing situations wherein sheet feeding would be the preferable method, it becomes apparent that there are plenty of factors, as spelled out above, that would seem to favor coil feeding. Coil feeding brings with it many advantages, and has a built-in flexibility that lends itself to many applications. While it may not be the perfect solution for every challenge faced in running press operations, coil feeding has proven itself to be a viable and reliable means of bringing added efficiencies and value to today's competitive manufacturing environment.

About The Company

Coe Press Equipment Corporation, with headquarters in Sterling Heights, Michigan, is a leading producer of pressroom feed equipment including servo roll feeds, power straighteners, coil reels and cradles, and complete coil processing systems. They also design and engineer fully-integrated coil processing systems for metal stamping and processing operations.

For more information, contact Jim Ward at Coe Press Equipment Corporation, 40549 Brentwood, Sterling Heights, MI 48310, (586) 979-4800, fax (586) 979-2970.

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