Over 800 million tons of coil-coated metal are produced and shipped annually in North America alone. coil coating (see Diagram I) is a very efficient way to produce a uniform, high quality, coated finish over metal in a continuous automated fashion. Coil coating is also referred to as pre-painted metal, because the metal is painted prior to, rather than after, fabrication.
In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape. Versus most other application methods, coil coating efficiency is nearly 100%. Application is at very high line speeds as modern coil lines can run at speeds as high as 700 feet per minute and cure the applied paint in 15 – 45 seconds. As opposed to a spray-applied coating, for example, a coil-coated, formed surface offers uniform film thickness rather than the thicker films on edges, corners and bends that is more typical of spray-applied coatings.
Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.
The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings. Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4 – 6 microns in thickness, whereas topcoats are normally applied to provide a dry film thickness of 18 – 20 microns.
Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. From a paint usage standpoint, polyesters make up the bulk of coil coatings. Table II details typical materials utilized in a polyester coil coating.
In summary, coil coating technology has grown more rapidly than more conventional application technologies due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
Share this article:
In the mid to late 1930s, Joseph Hunter, the founder of Hunter Engineering, installed the first coil painting line. He used this method to paint the shutters as part of a continuous strip. Later, Hunter Engineering went on to design and build more coil coating lines. It was eventually acquired by the Italian global engineering company FATA Spa, which is known for building coating lines all over the world.
Today, the coil coating process offers a wide range of opportunities for the world's coating companies. The coil coating process is like one of those giant printing presses that produce evening newspapers. The process does not stop, it runs at a constant speed and the metal coils are constantly spliced together so that the moving web can be infinitely long.
Aluminum Coil
According to EN-10169:2012-06, the coil coating process is a method of applying a continuous coating of organic coating material to a rolled metal strip substrate. The process consists of
Unrolling and cleaning of the coil, if necessary
Chemical pre-treatment of the metal surface (one or both sides), and
Single or multiple applications of (liquid) paint or coating powder, followed by curing or lamination with plastic film.
Cooling and rewinding of the same coils for transport to the cutter, slitter or manufacturer.
Aluminum Coils for 3D
From the oldest and most basic lines to the newest and most modern, all web coating lines have many steps or processes in common.
A basic principle of web coating materials is that the material passes through the various process sections in a continuous manner without stopping or changing speed!
As a result, the material being coated has extremely consistent properties.
In addition to the obvious labour efficiency, environmental benefits and energy savings; coil-coated materials are inherently more consistent than those produced part by part.
Aluminium Coils for Can-end & Tab Application
The two main types of coatings used for coil coating include
Liquid coatings
Powder coatings
Litong offers different types of coil coating and meets different aesthetic requirements.
In addition, films are used instead of lacquers or additional lacquers where a very high aesthetic quality is required. Films are available in a wide range of film thicknesses, colours and finishes (smooth, structured or printed).
Aluminum Ceiling
Liquid coatings must meet a number of basic requirements to be successfully applied. Easy application even on the fastest production lines (up to 220m/min) to provide a homogeneous appearance and a defect-free film.
Very short curing times for pigment colours to remain stable - the time spent in the oven can vary between 6 and 60 seconds depending on the production line, while the substrate temperature varies between 210ºC and 250ºC depending on the paint used
Flexibility to bend the coated metal without cracking or loss of adhesion of the paint film.
Dry film thicknesses are typically between 5 and 35 microns for primers and between 15 and 200 microns for topcoats. The thickness depends on the quality of the chosen paint and the final requirements of the pre-painted metal product.
PVDF / FEVE - Coated Aluminum Coil
Powder coatings can be described as "solid coatings" which melt to form a continuous film on the substrate.
These coatings can easily be applied by electrostatic or frictional electrostatic spraying to cover the item being painted. As the thickness of the coating increases, the coating is self-limiting and therefore a uniform film thickness can be achieved. The powder coated article then cures as the powder melts and flows out to form a continuous film of paint followed by a chemical reaction to form a solid inert coating.
Colour coated aluminium coils are one of the most popular decorative materials due to their stable performance and corrosion resistance. Its superior performance is attributed to the coloured coating.
There are four main types of paint used for aluminium at home and abroad: PVDF, acrylic paint, epoxy paint and PE. what is the best paint for colour-coated aluminium rolls? Let's find out one by one.
PVDF / FEVE - Coated Aluminum Coil
PVPD coating is a coating with fluorine resin as the main film-forming substance. Of these four coatings, PVDF coating has the best anti-ageing properties and the strongest UV resistance. It also has excellent weather resistance, heat resistance, low temperature resistance, chemical resistance, unique non-stick properties and low friction properties. Its drawback is the limited choice of colours.
PE, or polyester, is another type of paint that we are familiar with. Polyester paint is mainly made from polyester resins. It offers good adhesion, rich colours, excellent outdoor durability and moderate chemical resistance. It is relatively less expensive than PVDF paints.
There are two improved polyesters, HDP and SMP. HDP has excellent colour retention, UV resistance and is cost effective. SMP has good hardness, abrasion resistance and heat resistance, but is moderately flexible and moderately cost effective.
PE Coating Coils
It is a self-drying weather resistant anti-corrosion coating based on traditional acrylic resin with polyethylene rubber and weathering fillers and additives. It is easy to apply and cure at room temperature.
Its main components are epoxy resin and curing agent. Epoxy paint film has good adhesion, corrosion resistance and wear resistance, but is brittle. It is generally used as a backing paint for aluminium coils.
The choice of different paints for coated aluminium coils also means that the price will vary. So it is important to choose the right paint for the application and occasion of the aluminium coil. This will be the best paint for aluminium colour coated coils.