Protecting surface-sensitive materials in coil processing
Beware of three potential danger areas
As manufacturers worldwide strive to reduce costs and streamline their production processes, the market for surface-sensitive materials continues to expand. Surface-sensitive materials include all prepainted steel products and nonferrous decorative materials, such as copper, brass, and stainless steel.The use of surface-sensitive materials is especially prevalent in the automotive industry, which commonly uses galvanized steel. In this industry, metal surfaces that have a high-quality finish are known as automotive-exposed materials.
Furniture and appliance manufacturers also are using more surface-sensitive materials, and manufacturers of electrical components have particularly high quality standards. Manufacturers of motor laminations, transformer stock, and core plates require material that is free of burrs and scratches because such flaws can alter the material's conductivity or cause a component to short out.
For metal coil processing service centers and original equipment manufacturers (OEMs), surface-sensitive materials present some unique challenges. Because the surface of prefinished metal products is easily scratched or damaged, coil processors must pay extra attention to their equipment and handle these coils with great care.
In a coil processing line, wherever the material's finished surface touches another surface is an opportunity for damage. By understanding and assessing the risk of damage in three key areas—the tension stand-recoiler, material processing, and coil handling—coil processors can make certain each component of the slitting line is working properly and qualified to handle surface-sensitive materials.
Roll-only tension stands often are used by coil processors because they produce tight rolls without scratching the coil surface.
Tension stands keep coils tight and must be used in a majority of coil processing lines, but they also represent the most potential for tension scratches on the coil's finished surface (see Figure 1).
The four tension stand designs are pad, roll-pad combination, bridle roll, and roll. Pad and bridle roll tension units pull material through; as the material is pulled, pads also catch debris that becomes embedded and can cause ongoing damage to material. The roll tension stand, which employs two rolls moving in synchronization with the material, is the only configuration designed specifically to prevent damage to the material surface.
With a roll tension stand, consideration also must be given to the material the tension rolls are made of. Rolls covered with a nonwoven synthetic material grip well; provide squeegee action for oily, dirty surfaces; and are self-healing, so debris caught on the roll surface is less likely to damage the coil. Polyurethane rolls are best-suited for running clean and dry materials because debris that adheres to polyurethane will cause surface damage.
Movement between adjacent laps of a coil causes tension scratches, also called pull marks. Similar to the tightening or release of a clock spring, this movement can be quite detrimental to a coil of surface-sensitive material.
Large openings in recoilers can create reel marks, which are heard as "thumps" as the coil is being wound. The use of precision-ground drums that provide an almost seamless surface can help decrease the potential for damage. For processing critically sensitive materials, such as those for the automotive industry, a visual inspection should show no reel marks after one wrap, while an inspection by "stoning" should show no reel marks after five to seven wraps.
Excessive pressure from the overarm support on the recoiler also can be a culprit in surface damage. If the overarm support is not counterbalanced to control down pressure and designed with the proper geometry, banding grooves can mark the coil. A properly aligned slitting line often will allow a processor to raise the overarm support out of harm's way after a good core has been established early in the recoiling process.
Beginning with the uncoiler, many of the steps in actual material processing can cause surface damage to the metal. To prevent tension scratching at the uncoiler (or payoff reel), processors must uncoil with about the same amount of tension that was used originally to wind the coil. Loosely wound coils are particularly difficult to process because it is so easy to pull one wrap over another.
Any movement of the marked line during processing is an
immediate indication of improper brake settings.
A straight line marked on the side wall of a coil before it is processed can help coil processors identify the correct payoff tension during their operation. Any movement of the marked line during processing is an immediate indication of improper brake settings (see Figure 2).
Uncoilers have an adjustable brake to ensure proper tension. Both insufficient and excessive braking will cause movement in the coil and subsequent scratching.
Uncoiler braking systems are either mechanical or electromagnetic. To prevent excessive braking and the movement that will result from a coil that is too loose, tension must be constant as the coil is processed.
The hold-down roll can prevent the outer lap of a coil from loosening and damaging the finish. To help prevent damage, hold-down rolls may be polyurethane or chrome-plated with grooves for banding. The grooves should have generous radii to reduce sharp edges, which can dent or scratch the material being processed.
In a mechanical braking system, adjustments are performed automatically or manually, in contrast to electromagnetic braking systems, which are driven by direct current (DC) and designed to compensate more accurately for decreasing coil size. The operator sets the DC drive system at the beginning of each coil run, and the braking system requires few adjustments.
The hold-down roll, also referred to as a snubber roll, is an important feature of the uncoiler because it prevents the outer lap of a coil from loosening and subsequently damaging the finished surface of the coil. For safety reasons, hold-down rolls are critical in lines that process heavy-gauge products. In lines that process light-gauge products, hold-down rolls prevent scratches caused by the outer wraps loosening and rubbing on the inner wraps.
To help prevent surface damage, hold-down rolls may be polyurethane or chrome-plated with grooves for banding. The grooves should have generous radii to reduce sharp edges, which can dent or scratch the material being processed (see Figure 3).
An entry loop between the uncoiler and slitter is another choice processors have for reducing surface damage, particularly in light-gauge, surface-sensitive applications.
First, the entry loop eliminates cinching of the coil because tension between the uncoiler and slitter is nearly zero. Second, there is no squaring of the coil ID or OD, which produces better cuts because the slitter head doesn't slip or chatter. Third, near-zero tension helps slitter knives last longer and produces a better slit edge. Last, no edge damage occurs with an entry loop because generally no edge control is required.
Low-friction transfer table technology, which initially was developed to handle lightweight products in the paper industry, has been applied to coil processing lines to reduce scratching. Rolls and the supports between the rolls potentially can contact the metal as it passes across transfer tables. Most scratching occurs when roll speeds differ from the speed of the metal or when the metal contacts a support surface that is not appropriate for processing surface-sensitive materials.
As with hold-down rolls, chrome-plated rolls remain hard, smooth, and easily cleaned throughout the life of the transfer table. Nonmetallic surfaces on transfer tables allow material to move without marking its surface.
As with other rolls in the coil processing line, feed rolls should be made from a nonstick material to ensure cleanliness. If the feed rolls are driven, they should match the speed of the metal to prevent scratching. When used as strip support at the crop shear, feed rolls should provide adequate support to protect the metal from contacting the bottom knife.
When operating properly, the slitter head is the least likely to scratch or damage the finished surface of a coil. When marking does occur at the slitter head, it usually appears as knife marks that can be seen more easily with a strobe light.
To prevent coil damage at the slitter head, the knife stripper rings must be the proper diameters. If their relationship relative to the gauge and hardness of the strip is not correct, the knife edge is likely to mark the material.
Cascade rolls must support material into the pit and prevent the metal from bending under its own weight. The radius formed from the cascade rolls always should be larger than the minimum wrap of the coil. Geometry related to the placement of the cascade rolls is critical.
Many surface damage problems are related to improper alignment in the processing line. Tooling bars that are free-running, precisely located, and designed to minimize pickup of metal debris eliminate these problems.
Meticulous coil handling practices throughout the coil processing operation will have measurable payback. Damage, such as a dent or gouge, to the first wrap of the coil can be carried to inner wraps of the coil, causing a large section of expensive material to become scrap.
Because prefinished metal coils are more valuable than unfinished coils, their surfaces must be properly protected. Paper often is wrapped with the metal to protect the finished surface. For stainless steel and nonferrous products, vinyl often is wrapped with the metal. Protective films also may be applied before slitting.
Various cushioning materials effectively protect coils while they are moved, placed in inventory, or shipped. Polyurethane is one of the most effective materials for these purposes, but felt and previously used conveyor belting also have been used effectively.
A polyurethane coating or thick felt is recommended for C-hooks. The cushion usually is held in place with Velcro® straps. More costly, but more reliable, are precast polyurethane boots that fully cover the ends of the C-hooks.
Coil car design is yet another area for consideration. To guard against damage, coils are lifted from turnstiles with a V-shaped coil car. The surface of the coil car that is in contact with the coil should be made of a nonmetallic material.
It is important to avoid sharp corners on four-arm turnstiles that can damage coils as they slide on and off. Turnstiles with modular, round supports made of hardened steel for coil support help reduce damage.
The Future of Surface-sensitive Materials
Typically, the profit margin for processing surface-sensitive materials outpaces other products', and this is expected to continue, even in a weak economy. By paying close attention to equipment, tension practices, material processing methods, and material handling, coil processors can participate in the market for these materials. Even for processors who do not specialize in surface-sensitive materials, some shifts to processing methods associated with these materials can help reduce scrap.
The key is to consider each step in the process, assess the risk for coil damage in each step, and take steps to prevent coil damage. More and more coil processing plants are purchasing new equipment or modifying and retrofitting existing equipment with features that ensure the plant's ability to process prefinished materials. For processors who look forward, this will be key to continued profitability.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.