Tips for processing surface-sensitive materials
November 20, 2003
Savvy stampers are purchasing new equipment or modifying and retrofitting existing equipment to include prefinished materials features. By paying close attention to equipment, tension practices, material processing methods, and material handling, stampers can participate in the market for surface-sensitive materials.
As appliance manufacturers strive to reduce costs and streamline production processes, additional pressure is placed on coil processing to produce high-quality, surface-sensitive materials. These materials include pre-painted steel products and decorative metals such as stainless steel, copper, and brass.
Because it's easy to scratch or damage the surface of prefinished metal products, stampers must pay extra attention to their coil processing equipment and handle these materials with great care.
Every point in a coil processing line at which the finished material surface touches another surface is an opportunity for damage. By understanding the potential for and assessing risk in four key areas, stampers can ensure each component in the processing line is working properly to handle surface-sensitive materials:
To prevent tension scratching at the uncoiler, stampers must uncoil with approximately the same amount of tension that was used to wind the coil originally. Loosely wound coils are difficult to process because it's easy to pull one wrap over another.
A straight line marked on the side wall of a coil before it is processed can help stampers identify the correct payoff tension. Any movement of the side wall straight line during processing indicates improper brake settings.
Uncoilers, or payoff reels, have an adjustable brake to ensure proper tension. Insufficient or excessive braking can cause movement in the coil and subsequent scratching. Uncoiler braking systems can be mechanical or electromagnetic. To avoid excessive braking and the movement that results from a coil that is too loose, tension must be constant as the coil is processed.
In a mechanical braking system, adjustments are performed automatically or manually. Electromagnetic braking systems are driven by direct current (DC) and designed to compensate for decreasing coil size. Although the operator sets the DC drive system at the beginning of each coil run, these braking systems require few operator 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 coil's finished surface.
For safety reasons, hold-down rolls are critical to 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 can be polyurethane or chrome-plated with grooves for rebanding. The grooves should have a generous radius of 116 to 14 in. to reduce sharp edges, which can dent or scratch the material being processed.
Low-friction transfer table technology, which was initially developed to handle lightweight products in the paper industry, can reduce scratching in coil processing lines. Rolls and the supports between them can potentially contact the metal as it passes across transfer tables. Most scratches occur when roll speeds differ from the coil speed, or when the metal contacts a support surface that is not appropriate for processing surface-sensitive materials.
Like hold-down rolls, chrome-plated rolls remain hard, smooth, and easily cleaned throughout the life of the transfer table. Nonmetallic surfaces on transfer tables can move material without marking its surface.
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 protect the metal from contacting the bottom knife.
If the material requires leveling, choosing the correct roll configuration for the leveler will reduce or eliminate surface damage. In surface-sensitive metal coil processing, a five- or six-high leveler is appropriate. The key is the extra set of intermediate rolls between the work rolls and backup rolls. This reduces wear on the work rolls and decreases the chance for surface markings. A five-high leveler should be used if only one surface is critical, and a six-high if top and bottom surfaces are critical.
If no loop is present between the leveler and press feed, constant speed matching from the leveler to the press feed is necessary to reduce scratching.
If the line must stop to change or index pallet stacks, a creep mode can be used. Creep mode accumulates material from the unwind stand and leveler into a pit at a low speed. This prevents the leveler rolls from introducing coil set or creating roll marks.
In any case, ensuring that leveler rolls are cleaned frequently decreases the chance for contaminants to damage the material.
By maintaining proper friction between the roll feed and the material, stampers can prevent surface damage. Because the feed roll pulls material out of a shallow loop, roll preparation, such as a fine grit finish, must be chosen properly to ensure no markings on the material. Roll location and diameters also become critical with surface-sensitive material.
Rolls should be free-spinning to allow easy movement and constant material speed. Adding nonmarking backup rolls to support the feed rolls also can help prevent surface damage.
The feed roll alignment to the shear also affects surface-sensitive material. It must be within ±0.00l inch per foot to eliminate the possibility of material damage during feed start or stop. Squaring guides also should be aligned correctly. Supporting the material into the shear with low-friction rolls or tables can reduce marking of the material's underside.
Machine design, material support, and cushioning are key when dealing with a stacking system. Stackers should be designed to eliminate marking or denting a material.
Magnetic stackers carry material via magnets to the drop zone. They should allow the material drop location and speed to be adjusted to accommodate size and width variations. Adjusting these parameters controls the drop zone onto the stack and the material force when contacting the backstop. The backstops also should provide sufficient cushioning to prevent the material from buckling.
Flipper stackers drop material from overhead by rotating L-shaped supports, similar to a horizontal door. The supports should be designed with driven rollers or material that eliminates material marking. Driven rolls should be tightly controlled between the feed rolls to eliminate a speed differential.
Drop stackers feed the material out and drop it when the shear cuts the sheet. Long cut blanks should be well-supported, so the newly cut piece does not scratch the material already in the stack when it is pushed forward. One technique is to use an air current to "float" the sheet to rest.
Meticulous coil handling throughout the processing operation has a measurable payback. Damage such as a dent or gouge to the first wrap of the coil can be carried to the inner wraps, which can cause a large section of expensive material to become scrap.
Because of the increased value of prefinished metal coils, proper protection of the coil's surface is important. Paper often is wrapped with the metal to protect the product's finished surface. For stainless steel and nonferrous products, vinyl often is wrapped with the metal.
Protective films may also be applied before slitting. Various cushioning materials can protect coils while they are moved, placed in inventory, or shipped. Polyurethane is one of the most effective materials for this purpose, but felt and previously used conveyor belting also work well. To protect coils stored in inventory, floor pads made of polyurethane or an equivalent cushioning material should be used.
A polyurethane coating or thick felt is recommended for C-hooks. The cushion usually is held in place with Velcro® straps. Precast polyurethane boots also are available to fully cover the ends of the C-hooks.
Coils often are lifted from turnstiles with a V-shaped coil car. The surface of that coil car should be made of a nonmetallic material. Sharp corners on turnstiles can damage coils as they slide on and off. Turnstiles with modular round supports made of hardened steel for coil support help reduce damage.
Savvy stampers are purchasing new equipment or modifying and retrofitting existing equipment to include prefinished materials features. By paying close attention to equipment, tension practices, material processing methods, and material handling, stampers can participate in the growing market for surface-sensitive materials. The key is to consider each step in the process, assess the risk for coil damage in each step, and take action to prevent coil damage.