Aluminum coil coating line

Aluminum is a perfect substrate for color coating because of its flat matte finish and high corrosion resistance. Aluminum consumption in construction is growing due to the weight difference compared to steel, making it suitable for large span roof panels, mezzanine panels, composite panels and many other construction applications.

Coating methods for aluminum in construction applications are generally similar to steel; however, sensitive surfaces, soft alloys, different microstructures and electrical conductivity mean that some subtle differences must be considered when designing coating equipment for aluminum. These requirements will be highlighted below.

Stitching/slitting and roll cleaning
As with all continuous processing lines, it is necessary to join the aluminum rolls together. To do this, a mechanical seamer is required; however, unlike steel, aluminum is prone to cracking during the seaming process. A blowing system can reduce these fragments and also reduce the level of residue. However, it is best to perform roll cleaning throughout the line to reduce any further carryover along the line. A squeegee applied to each roll on the inlet accumulator tower can greatly reduce line contamination, however, a separate roll cleaner covering the bridge roll can optimize line cleanliness and performance of the highest quality coating. Both of these items can be retrofitted to existing lines where cleanliness is an issue.

Tension leveling - 6 layers high
Since aluminum is cold rolled, it is usually uneven (unlike steel, which is usually leveled on a galvanizing line prior to color coating),. Therefore, tension leveling can often be found in the continuous processing part of the aluminum coil coating line. This avoids marking due to stopping/starting the strip in the leveler and ensures optimal surface flatness and condition prior to coating. Again, due to the surface sensitivity of aluminum and soft alloys, a 6 high leveling roll cassette is required.

This design avoids any "tram lines", a marking problem that can be seen on the strip (potentially through color coated products), which is created by the backup rolls coming in contact with the work rolls, and can be eliminated by adding intermediate rolls and using a 6 high machine design.

Strip cleaning (cleaning section) and pretreatment
In general, the cleaning zones of steel and aluminum are similar, if not identical, in structure. Aluminum usually has oil in it, but it is usually dry and easily removed with alkali-based cleaners. Many older aluminum coil coating lines were set up with only one tank, however, an additional tank combined with a hot rinse and a well-designed cascade system can limit both water consumption and the amount of effluent that needs to be treated with water prior to treatment. The main difference in aluminum cleaning is the absence of a scrubber, which is typically required when cleaning steel.

Today, many pretreatment chemicals are common to both steel and aluminum, and there is a shift to chromium-free products driven by environmental legislation. The use of roller coaters can improve chemical application, reduce chemical consumption and provide more accurate coatings. Today, both roller and spray/scraper systems tend to use dry-in-place chemicals. This avoids any post rinsing and again reduces the efficiency level and environmental impact of the line.

Coating - (3 roller heads, PVDF's)
Generally speaking, the coating heads are the same for colored coatings on aluminum or steel. However, the high quality requirements of architectural aluminum products, which often require PVDF coating, dictate the need to include 3 roller coating heads (with metering, picker and applicator). The flexibility of the coating head design allows for versatility in coating applications, ensuring forward coating/reverse coating, tray feeding and nip feeding to suit the coating and the required surface finish.

Recently, coated aluminum has become the material of choice for decorative architectural products, especially in the curtain wall/cladding of new modern buildings. This has led to an increase in the demand for patterns. Applying patterns using inks or coatings with a transparent protective layer can both differentiate the product and increase its use as an alternative to other materials. Wood grain is the most common pattern, but the market is increasingly demanding other effects as manufacturers look to create a niche product compared to their competitors.

Aluminum coil pattern coating can be achieved through a number of different production techniques. Rotary screen printing is a technology that offers flexible patterns, but the investment cost is high and requires a high volume production line/demand to justify this investment. bronx uses a simpler process as it can be easily integrated into a new production line or retrofitted to an existing coating technology at a much lower cost than other processes, but still offers an equivalent product.

Curing Ovens
Aluminum color coating lines can use gas convection furnaces, floating furnaces and electric infrared furnaces. Because aluminum has higher electrical conductivity than steel, it can reach the required PMT (peak metal temperature) much faster. This means that for a dedicated line, smaller burners or electricity can be used than for steel, however, the required dwell time as well as the length of the oven will be determined primarily by the coating rather than the substrate.

Care must be taken when using infrared curing ovens, as uncoated surfaces (commonly found on the reverse side of coated aluminum products) can reflect infrared wavelengths, reducing the effective heating capacity of the oven and resulting in under-curing of the coating. In high-speed aluminum lines, air float may be preferred over convection ovens due to their shorter oven length. This is particularly beneficial for high-speed light rail applications where there is an increased risk of strip breakage within the furnace. Bronx technical experts can help you decide which oven is best suited to your processing requirements.

Cooling and Quenching
Cooling of the aluminum strip prior to quenching with water is a critical part of the line design. In aluminum coating lines, ambient air is typically applied to the aluminum strip in the exit area of the oven or in a separate air cooler located after the oven and prior to quenching. This is necessary to avoid thermal shock; strip shape caused by metallurgical changes resulting from too rapid cooling of the strip. Air cooling is used to reduce the PMT (peak metal temperature) of the strip to about 150-170 degrees Celsius. Additional benefits can be seen when adding hot film lamination, which typically requires strip temperatures around 180 degrees Celsius.

Water quenching ensures that the color coated strip does not recoil at too high a temperature. This can lead to coil collapse, clogging and scratches, which are common defects associated with poor temperature control. Water quenching is similar for both steel and aluminum, but aluminum will cool faster and must be cooled uniformly by water because hot spots can create defective spots on some coatings, especially PVDF, which is commonly used for aluminum color coatings.

Line Tension
Strip tension control is a key aspect of any processing line, but aluminum requires a high degree of control, especially for lighter gauges. Aluminum color coated coils can be as light as 0.18mm (equivalent to 0.06mm steel). Unless excellent tension control is applied, band tearing/breakage can occur. This is accomplished by using a wire bridge with a rubber covered coil. However, in some applications, the use of tungsten coated rolls can increase the coefficient of friction, improve performance, reduce slippage and improve strip tension control.

When considering the design of any processing line, it is important to have a realistic idea of the range of specifications to be processed, however, this becomes even more critical for aluminum as the roll size and motor power need to be optimized to provide satisfactory performance across the entire range of specifications.

Metal embossing and laminating
Color coated aluminum offers a wide range of different surface effects through the application of different coatings. These effects can be further enhanced by the introduction of metal embossing, where a pattern is pressed into the surface of the coated aluminum. While this can be used for aesthetic purposes, it also offers the advantage of increasing the hardness of the material. In some applications, such as roofing, this can increase the load-bearing capacity of a coated panel, allowing a lighter gauge embossed material to do the same job as a heavier gauge non-embossed product. It can also reduce production line throughput by masking small surface defects that are very visible on the flat sheet and cause scrap for visual reasons.

Lamination is often used as an alternative to color coating/patterning because it allows PVC, polyester and PET films to be adhered to the surface of the strip. These pre-coated laminates are available in different thicknesses and patterns. They can offer a greater range of designs than color pattern coatings, but can be more costly than standard pattern coating processes for large quantities.

The combination of laminates with subsequent embossing can again produce different and interesting effects that stimulate the imagination of architects, and it is these niche products that are becoming increasingly popular in steel and aluminum coated products worldwide.

Summing up
Color-coated aluminum offers a versatile product for the architectural industry. Equipment design and supplier selection require careful consideration, as this is a high-value product that is coated and finished with expensive coatings and finishes. Incorrect line design can result in high scrap rates or inability to process the full product range, resulting in much lower yields than predicted. Depending on market conditions, production lines can operate effectively from 5,000 to 60,000 tons/year. End-user product requirements are constantly changing and quality levels are increasing, so choosing the right plant and equipment will ensure the sustainability of your aluminum coating line into the future. Please feel free to contact us to discuss your requirements.

Disclaimer: This information is for general purposes only and should be treated as such. For detailed and precise information on your upcoming coil coating line, it is best to go to df-alu.com.