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Aluminium is a perfect substrate for colour coating, given its flat matt finish and high corrosion resistance. Aluminium consumption in the construction industry is growing, due to its weight differential compared to steel, making it suitable for long span roofing sheets, sandwich panels, composite panels and many other construction applications.
Methods of coating aluminium for construction applications are typically similar to that of steel, however, the sensitive surface, soft alloys, different micro structure and conductivity means some subtle differences must be considered when designing equipment for coating aluminium. The points below are aimed at highlighting these requirements.
As with all continuous process lines it is necessary to join the aluminium coils. To do this a mechanical stitcher is used (see Figure 1), however, unlike steel, aluminium is prone to creating slivers during the stitching process. Air blowing systems can reduce these and the level of carryover. It is however preferable that roll cleaning is also included throughout the line to reduce any further carryover along the line. Scraper blades applied to each roll on the entry accumulator tower provides a significant reduction in line contamination, however, individual roll cleaners on the covered bridle rolls optimises line cleanliness and performance on highest quality coatings (see Figure 2). Both of these items can be retrofitted to existing lines where cleanliness is an issue.
As aluminium is supplied cold rolled, it is usually supplied unlevelled (unlike steel which is often levelled in the galvanising line before colour coating), . It is therefore common to see tension levelling in the continuous process sections of aluminium coil coating lines. This avoids marking due to stopping / starting of the strip in the levelling machine and ensures optimum surface flatness and condition prior to coating (see Figure 3). Once again due to aluminium’s surface sensitivity and the soft alloys a 6 high levelling roll cassette is required.
This design avoids any ‘tram-lining’, a marking problem that can be seen on the strip (and potentially through the colour coated product) created by back up rolls contacting the works rolls, which is eliminated by use of the 6 high machine design (ee Figure 4) by the addition of intermediate rolls.
In general, cleaning sections (see Figure 5) for steel and aluminium are similar if not the same in construction. Aluminium is usually supplied oiled but often this is dry and easily removable with alkali based cleaner. Many older aluminium coil coating lines were set up with only one tank, however, an additional tank combined with hot rinses and a well-designed cascade system can both limit water consumption and volume of effluent requiring water treatment prior to disposal. The major difference in an aluminium cleaning seciton is the absence of a brush scrubber that is more typically required for cleaning steel.
Today, many pre-treatment chemicals are universal for both steel and aluminium and a move to chrome free products is being driven by environmental legislation. The use of roll coaters (see Figure 6) can allow for improved chemical application, reducing chemical consumption and provides a more accurate coating. With both roll coating and spray / squeegee systems dry-in-place chemicals are preferred today. This avoids any post rinsing again reducing effuent levels from the line and environmental impact.
Generally, coating heads are the same for colour coating of aluminium or steel. However, the high quality demands of the architectural aluminium products, which often request PVDF coatings dictates that 3 roll coating heads (with metering, pick up and applicator) are included (see Figure 7). Flexibility in head design allows versatility in coating applications, ensuring forward coating / reverse coating, tray feeding and nip feeding to suit paint and required surface finish.
In recent times coated aluminium has become a choice material for decorative construction products, especially in curtain walling / cladding around new modern buildings. This has led to an increase in demand for patterns. Application of patterns using inks or paints with a protective clear coat can both differentiate the product and increase its use as a substitute for other materials. Woodgrain is the most common pattern (see Figure 8) but increasingly the market is demanding other effects as producers’ look to create a niche product when compared with their competitors.
Aluminium coil pattern coating can be achieved by serveral different production techniques. Rotary screen print is one that offers flexibility of patterns but has a high investment cost requiring a high volume line / demand to justify this investment. Bronx uses a simpler process, as this can be readily integrated into new lines or retrofitted to existing coating technology at much lower costs than alternative processes, but still providing equivalent products.
Aluminium colour coating lines can use gas convection, floatation and electric infra-red ovens. As aluminium has a higher conductivity than steel it reaches the desired PMT (Peak Metal Temperature) quicker. This means that for a dedicated line, smaller burners or electrical power can be used than that required for steel, however, the desired dwell and thus length of the oven will be determined predominantly by the coating, not the substrate.
Care must be taken using infra-red curing ovens, as an uncoated surface (common on the reverse side of coated aluminium products) can reflect the IR wavelength and reduce the ovens effective heating capability, resulting in under cure of the coating. In high speed aluminium lines, air floatation may be preferred due to its shorter oven lengths when compared with convection ovens. This is especially beneficial on high speed light gauge applications where risk of strip breaking in the oven is increased. A Bronx Technologist can help you decide which oven is best for your processing requirements.
Cooling of aluminium strip prior to quenching with water is a critical part of the line design. In aluminium coating lines this is typically done with ambient air being applied to the strip at either the exit zone of the oven or in a separate air cooler located after the oven and prior to the quench. This is necessary to avoid thermal shock; created by too rapid cooling of the strip resulting in a metallurgical change that induces strip shape. Air cooling is used to reduce the strips PMT (Peak Metal Temperature) to around 150-170 deg C. The added benefit of this is seen when incorporating hot film laminating, typically requiring a strip temperature around 180 deg C.
Water quenching ensures that the colour coated strip is not recoiled at too hich a temperature. This can lead to coil collaspe, blocking and marring, common defects associated with poor temperature control. Water quenches are similar for both steel and aluminium, however aluminium will cool quicker and it is imperative to have even cooling by water, as hot spots can create defect patches on some coatings, especially PVDF’s which are commonly used in aluminium colour coating.
Tension control in the strip is a critical aspect of any process line, however aluminium requires a high degree of control, especially for lighter gauges. Aluminium colour coated coil can be as light as 0.18mm (equivalent to 0.06mm steel). Strip tear offs / breaks will occur unless excellent tension control is applied. This is done by the lines bridles using rubber covered rolls. However, in some applications the use of tungsten coated rolls may provide increased coefficient of friction, improving performance, reducing slip and improved strip tension control.
It is important when considering any process line design to be realistic about the gauge range to be processed, however, this becomes more critical for aluminium, as roll sizes and motors power need to be optimised to provide satisfactory performance throughout the gauge range.
Colour coated aluminium offers a multitude of different finishes by variation of coatings applied. These can be further enhanced with the introduction of metal embossing that presses a pattern into the surface of the coated aluminium. Whilst this can be used for aesthetic purposes, it also offers the advantage of increasing the materials rigidity. In certain applications, like roofing, this can increase the coated sheets load bearing capacity, allowing a lighter gauge embossed material to do the same job as a heavier gauge non-embossed product. It also covers minor surface defects that would be apparent on a flat sheet and result in a reject for visual reasons, therefore reducing line yield.
Lamination is often used as an alternative to colour coating / patterning, as it allows PVC, Polyester and PET films to be glued to the surface of the strip. These pre-painted laminates come in varying thicknesses and patterns. They can offer a greater scope of designs to that produced by painted pattern coating but for large volumes, costs can be higher than a standard pattern coating process.
Combinations of laminates with subsequent embossing can again produce varied and interesting effects that stimulate an architect’s imagination and it is these niche products that are gaining in popularity for both steel and aluminium coated products globally.
Colour coated aluminium offers a versatile product for the construction industry. Equipment designs and supplier selection needs to be carefully considered, as this is a high value product being coated with expensive coatings and finishes. Incorrect line design can result in much lower production yields than predicted by creating high scrap levels or inability to process full product range. Lines can operate effectively, subject to market conditions, from 5,000 tonnes / annum up to 60,000 tonnes / annum. End users product demands are constantly changing, quality levels are being raised and therefore selection of the right plant and equipment will ensure aluminium coating lines sustainability into the future. Feel free to contact us to discuss your requirements.
Disclaimer: This information is for general purposes only and should be viewed as such. For detailed, precise information for your upcoming coil coating line, its best to speak to a Bronx Technologist.
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