Powder coating of aluminium products

Learn about the advantages of powder coating of aluminium. This eco-friendly process eliminates the need for organic solvents and ensures high efficiency.

Why use powder coating of aluminium

Powder coating is a dry technology

Coating liquid paints require the use of solvents, to deposit organic compounds on the surface of paint. These liquid solvents necessitate use in the production of equipment for venting, filtration and capture environmentally hazardous volatile organic compounds. Powder painting is essentially the same dry technology, which does not use organic volatile substance nor at its one step.

Powder coating is cheaper

Generally, powder coating lines include efficient recirculation system and returning the sprayed powder for reuse. Therefore, the overall efficiency of the use of powder 95-98 %. color change on the line powder coating takes place fairly quickly. Near 99 % powder, which was deposited, but I did not get into the finished powder coating, returned to the dyeing cycle, and again sprayed. Therefore, waste paint lines powder coating are minimal.

Modern line powder coating coating provide a wide range of colors, shine, textures and thicknesses.

The two types of powder coatings

The main types of powder paints are:

• thermoplastic and
• thermosetting.

Thermoplastic powder coatings

Under the influence of heating thermoplastic powder paint melts and flows over the surface in the form of a film. This film has the same chemical composition, which had a starting powder. The thermoplastic powder coatings based on thermoplastic resins with high molecular weight. These resins are difficult to grind into small particles and therefore they are usually used to create special thicker coatings. Powder coating on a product usually occurs during its immersion in a tank of liquefied powder. Liquefaction of the powder produced by means of compressed air.

Thermoplastic powder coatings are not used for mass production of aluminium products, such as extruded aluminium profiles or rolled aluminium sheets.

Thermosetting powder coating

Thermosetting powder paints are significantly different from the thermoplastic. They are based on solid resins with low molecular weight. Under the influence of heat melt thermosetting powder, spreading over the surface and are connected with each other as a result of chemical reactions to form high molecular weight organic compounds. This process is called polymerization. Therefore, the finished powder coating has a chemical composition, which differs from the chemical composition of the starting resin in powder paint.

The main types of thermosetting powder paints, which are used for coloring aluminum products and semi-finished products, are:

• epoxy,
• polyester,
• polyurethane,
• acrylic,
• hybrid (mixed).

Surface pretreatment

The technology of chemical preparation of the aluminum surface usually includes the following stages (Figures 1 and 2):

• Degreasing
• Rinsing
• Alkaline etching
• Rinsing
• Desmutting
• Conversion coating
• Rinsing in deionized water
• Drying

 Figure 1 – A horizontal chemical pretreatment line [2]

Figure 2 – A vertical powder coating line [2]

Powder spraying method

Aluminium products with the prepared surface are feeded to a powder coating line (Figure 3). The first step is a powder spraying on the surface of the product, for example, aluminium profile. Aluminum profile enters the electrostatic spraying chamber, wherein the electrically charged particles “stick” to surface.

Figure 3 – Powder coating on aluminum profiles [1]

The electric charge of a powder particle is obtained when passing through special pistols (Figure 4). All of the powder, who did not or could not “stick” to profile, goes to the powder recycling system, and is returned for reuse.

Figure 4 – Principle of electrostatic powder coating of aluminium products [1]

Aluminum profiles with the deposited powder layer further tested on the processing chain to the furnace, termed polymerization oven. In this furnace are profiles during 15-20 minutes at a temperature of from 160 to 210 ° C depending on the type of powder paint. Under the influence of heating the powder particles soften and form a continuous uniform coating. When this polymeric compound, powder consisting, undergo polymerization with the formation of new chemical bonds.

After exiting the furnace the profiles given time to cool to room temperature to control, packing and shipping.

RAL color scale

Thermosetting powder coatings are available in a wide range of colors. When aluminum powder staining, including, aluminum profiles, apply the international RAL color scale (picture 5). RAL – an organization in Germany, which is engaged in standardization and certification of paint colors and coatings. Furthermore RAL colors on a scale typically also specify gloss coating – Matt, Honed, brilliant. Matte coatings are especially popular for powder coating of building products made of aluminum profiles, for example, windows and doors, so they hide the most pressing defects, such as scratches.

Figure 5 – Example of the international RAL color scale

Aluminum for powder coating

Typical aluminum alloys, which are often subjected to powder coating, are alloys 6060 и 6063. Of these alloys, the vast majority are manufactured aluminum profiles, including, for aluminum windows and doors. Moreover, product with enhanced strength properties of aluminum alloys used 6061, 6082 и 6005. Aluminum sheets 1xxx, 3xxx and 5xxx are also satisfactorily powder coated.

Aluminum casting alloys may also be painted with powder paints, however, in this case, the quality of the painted surface largely depends on the quality of the aluminum casting. Powder painting castings with high porosity may lead to the formation of bubbles at the coating surface. These bubbles are formed when gases exit from the casting [1].

Designing for powder coating

If the aluminum profile will be subjected to powder coating, except the destination standard characteristics, such as the type of powder paint, coating thickness, color and gloss, it is also necessary to consider the following questions, which are associated with powder coating technology features.

Visible and significant surface

On the drawing of the profile (or other product) visible or significant surfaces must be indicated. All requirements for the quality of the powder coating are presented only to such surfaces.

Hanging points

To be sprayed on the aluminum profile powder coating, it must be properly hung on the conveyor line powder coating. To do this on the profile indicate the place, on which you can drill holes or install a pointed contact the conveyor so, so as not to damage the visible or significant surface.

Drainage

Usually chemical preparation of products (for example, profiles) requires immersing them in baths with working solutions. It is therefore important, profiles to immersion in a bath is not floated on the surface, as well immersed in the bath. After removal from the bath, they should not contain a solution or leaching residues. If the product cavity, for example, aluminum profile, remain such residual moisture, they may boil in the polymerization furnace. This will lead to damage or even a powder coating oven accident. In most cases diameter holes 6 mm is sufficient to ensure complete drainage of residual solutions and washes.

Faraday cage

The particles of powder paint adhere to the surface of metal products due to electrostatic forces. However, the phenomenon can occur in some cases,, which is called the “Faraday cage” (figure 6). This phenomenon prevents the penetration of powder into the narrow grooves, slit or interior acute angles. Generally for normal coloration width of such slits should be greater, than its depth.

 

Figure 6 – “Faraday’s Cage” [1]

Sources:

1. Powder Coater’s Manual by Roger Talbert, 1998
2. Aluminium Extrusion Manual, 4th Edition, Aluminium Extrusion Council, 2014
3. User’s Guide to Powder Coating, 4th Edition / N. Liberto, 2003