There are various options for applying paint to metal. In the field it is common to see spray or brush applications; in the factory, you can spray, powder coat or coil coat. However, when it comes to the performance requirements of today’s metal architecture, products like metal composite materials almost exclusively use the coil coating process to apply finishes that look practically flawless and can last.
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Coil coating is the method by which a metal coil is coated in a continuous process prior to fabrication. The metal coil is brought through an uncoiler, a line-loading accumulator, processed and painted according to requirements, brought through a finish line exit accumulator and then rewound on a recoiler. These coils are finished and cured in a continuous process. Though traditionally used for aluminum substrates, this procedure can be applied to other metals, such as steel.
According to the National Coil Coating Association, prepainted metal “holds up better over time than post-painted surfaces ...” With robust quality assurance programs, coil-coated metal has a clear advantage with regards to consistency. Continuous manufacturing also allows for lower costs and greener manufacturing with reduced waste, increased efficiency, and the collection of VOCs.
Application of paint to metal has moved beyond spray systems, which are now more typical in a single-sheet batch process. Continuous coil coating manufacturing utilizes either a roll coating or a die coating method. Reverse roll coating, common in metal manufacturing, applies paint with two rolls running in opposite directions. A metering roll loads and transfers paint to an applicator roll, which then applies the coating to the substrate. This allows for good control over the paint thickness but leaves the panel susceptible to “roping” marks, striations that form at the edges of the brush resembling a ship’s wake.
Die coating is a precision coil coating process that maintains the advantages of high-volume, continuous process coating, but with even greater paint control. Paint is applied directly to the substrate through a slot die extrusion system.
Paint thickness is meticulously controlled in a single, seamless application. Both the paint system and the coil require exacting quality standards in the die coating process. The result is the smoothest coating possible with the added bonus of being more sustainable because of the reduction in waste.
Paint systems evolved over the years as the industry began to require more from finishes than polyester and urethane paints could provide. The only paint systems capable of meeting the most commonly referenced architectural performance specifications laid out in AAMA are fluoropolymers. The unique chemical composition provides exceptional weatherability and durability.
The first generation of fluoropolymer paint introduced in was polyvinylidene fluoride (PVDF). This paint system requires an additional acrylic resin to formulate PVDF into a coating, with the highest performance dispersion rate at 70 percent PVDF and 30 percent acrylic resin. As the ratio of PDVF to acrylic drops, so does the paint performance. The paint is thermoplastic, melting at high temperatures, however no chemical cross-linking occurs. This makes PVDF susceptible to remelt. PVDF is a relatively low-gloss finish. As measured by a 60-degree gloss meter, it is limited to a 30 to 40 range. Because of the added acrylic, color palettes are limited and bright colors can be a challenge.
The second generation of fluoropolymer paint introduced in is fluoroethylene-vinyl ether (FEVE). This paint system is thermoset, chemically cross-linking the fluorocarbon components so that under high pressure and temperature they will not remelt. FEVE is a clear resin, allowing for brighter and more vibrant finishes and has a smooth surface capable of a gloss rate as high as 70.
Advancements in coil coating and paint systems have enabled a nearly limitless palette of finishes that can last for decades. The introduction of metal composite material (MCM) into the architectural landscape was another revolutionary innovation. MCM is versatile and easy to fabricate into complex forms without the weight or other issues, such as oil canning found in solid plate metal. Over the last 40 years, these advancements have radically redefined what is possible for fabricators and designers.
Jim Moses is the technical services manager at Mitsubishi Chemical Composites America’s ALPOLIC MCM division, Chesapeake, Va. He has more than 40 years in the construction manufacturing, engineering and operations field, and is an active member of industry associations.
High - purity aluminum ingots with few impurities, good ductility, and stability are chosen as raw materials. These aluminum ingots are then made into aluminum coil blanks with a certain thickness and width through processes such as melting - casting and casting - rolling.
Each roll of aluminum coil blanks undergoes strict weighing and quality inspection to ensure that it meets the production standards. For example, the thickness, width, and flatness of the aluminum coil are checked to see if they meet the standards.
Uncoiling and Pretreatment
The aluminum coil is placed on an uncoiler, and the uncoiler rotates to unwind the aluminum coil, preparing it for subsequent processing.
Chemical agents such as degreasers are used to remove grease and dirt from the surface of the aluminum coil to ensure good adhesion between the coating and the aluminum substrate. Common degreasing methods include immersion and spraying, and the appropriate degreasing process can be selected according to the actual situation.
After degreasing, the aluminum coil needs to be washed with water to remove the residual degreaser and impurities on the surface to prevent them from having an adverse effect on the subsequent coating.
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Chemical or electrochemical methods are used to treat the surface of the aluminum coil to make it more shiny and flat, further improving the quality and decorative effect of the coating.
The aluminum coil is washed again to ensure that the surface is thoroughly clean and free of any residual substances.
The aluminum coil after water washing is sent to a drying device for drying to remove the surface moisture and provide a dry surface condition for the subsequent passivation treatment.
A chemical conversion treatment is carried out to form a passivation film on the surface of the aluminum coil, which enhances the corrosion resistance and antioxidant property of the aluminum coil and prolongs the service life of the product.
The passivated aluminum coil is dried again to make the passivation film more stable and firm.
Coating Stage
The pretreated aluminum coil enters the primer coating process, and the roll - coating process is usually adopted. The prepared primer paint is poured into the paint tank of the roll - coating machine. When the aluminum coil passes through the roll - coating machine, the primer paint is evenly coated on the surface of the aluminum coil. The role of the primer is to enhance the adhesion between the coating and the aluminum substrate and provide a good foundation for the topcoat.
The aluminum coil coated with the primer enters a drying oven for drying to evaporate the solvent in the primer paint and solidify the coating. The drying temperature and time are adjusted according to the type and thickness of the primer. Generally, it is between 180°C - 250°C, and the drying time is 1 - 5 minutes.
After the primer is dried, the aluminum coil then undergoes topcoat coating. The color and performance of the topcoat are selected according to the customer's requirements. For example, fluorocarbon topcoat with high weather resistance and polyester topcoat with strong decoration are available. The roll - coating process is also used to evenly coat the topcoat paint on the surface of the primer to form the required color and decorative effect.
After the topcoat is applied, it is dried again to fully solidify the topcoat paint and improve the hardness, wear resistance, and weather resistance of the coating. The control of the drying temperature and time is crucial for the performance and appearance quality of the topcoat. Generally, the drying temperature is between 200°C - 300°C, and the drying time is 2 - 10 minutes.
The painted aluminum coil is cured at a high temperature to make the resin in the paint fully cross - linked and form a hard, wear - resistant, and corrosion - resistant coating. The curing temperature and time are set according to the type and performance requirements of the paint. Generally, it is between 200°C - 350°C, and the curing time is 5 - 15 minutes.
The cured aluminum coil needs to be cooled to around room temperature for subsequent processing and handling. Cooling methods such as air - cooling, water - cooling, or natural cooling can be used, and the appropriate cooling method can be selected according to the production efficiency and product quality requirements.
Quality Inspection:
During the production process, the colored aluminum coil undergoes multiple - quality inspections, including coating thickness inspection, adhesion inspection, appearance inspection, and color difference inspection. Professional testing equipment and tools such as coating thickness gauges and cross - cut testers are used to ensure that the product meets relevant standards and customer requirements.
The qualified colored aluminum coil is coiled into a coil with a certain specification for storage, transportation, and use. During the coiling process, attention should be paid to the control of tension to avoid deformation and scratches on the aluminum coil.
Changzhou Dingang Metal Materials Co., Ltd. stands as a highly professional factory with two decades of dedicated expertise in the production of top-notch color-coated aluminum coils. Our state-of-the-art manufacturing processes and strict quality control measures ensure the delivery of premium products. For all your requirements and inquiries regarding color-coated aluminum coils, do not hesitate to reach out to us. We are eager to establish a fruitful cooperation and provide you with the most professional service and solutions.