Powder coating aluminium profiles: process and design

What is powder coating?

Powder coating applies a finely ground mix of resin and pigment to an earthed metal surface by electrostatic attraction and then cures it in an oven. Unlike liquid paint it contains no solvent. The powder melts, flows across the surface and cross-links chemically into a hard film. This guide describes thermosetting powder coatings, which behave in this way during curing. On aluminium profile it serves everything from architectural facades to furniture and equipment.

Why is it so common on aluminium?

Aluminium’s thin natural oxide gives protection, but on its own it may not deliver the intended appearance or the long-term protection needed in aggressive environments. Powder coating offers a wide range of colours and textures, a hard film that resists scratching and no solvent emissions. Powder that does not stick to the profile and is caught by the collection system can often be reused, depending on powder type and quality requirements, so material loss is low. Durability depends on the paint bonding properly to the metal, and that starts with pre-treatment.

What are the steps?

The steps below describe a typical powder coating line in general terms; they are not the process specification of any particular plant.

1. Racking. Profiles are hung so that the contact points fall on a non-visible area.
2. Pre-treatment. The surface is cleaned, degreased and, where needed, chemically etched to remove loose oxide. After cleaning and the required rinsing stages, a suitable pre-treatment is applied. Chemistry and any subsequent rinsing depend on the selected system and the chemical supplier’s instructions. This stage governs adhesion and corrosion resistance.
3. Drying. Profiles are dried before they enter the booth.
4. Electrostatic application. A spray gun charges the powder with high voltage. The earthed profile attracts the charged powder, and film distribution depends on geometry and application settings.
5. Curing. The profile goes into an oven. Depending on the powder, cure typically takes ten to twenty minutes at roughly 180 to 200 °C.
6. Cooling, inspection and packing. After cure the profile cools, is inspected and is packed with the surface protected.

Each stage has to be controlled: bath concentration in pre-treatment, drying temperature, gun voltage and the temperature profile in the cure oven all show up in the result. The metal itself reaches oven temperature later than the air does, so on heavy sections the cure time must be counted from the moment the metal reaches its target temperature.

Which powder types exist?

• Polyester. The usual choice for outdoor and architectural use, with good resistance to UV and weather.
• Epoxy. High chemical and mechanical resistance, but it chalks outdoors, so it suits interior use.
• Epoxy-polyester hybrid. Used for indoor applications.
• Polyurethane. Chosen for particular finish and durability requirements.

Gloss, colour and texture options depend on the powder manufacturer’s range. For outdoor performance, architectural projects commonly refer to standards such as EN 12206-1 or quality labels such as Qualicoat. This describes general industry practice and does not mean that Solid Group Metal holds a certificate or a process specification of that kind. If your project requires a particular specification, state it in the enquiry; it is confirmed in the quotation.

How are film thickness and quality checked?

Quality is verified with a handful of measurements. Film thickness is usually taken with an eddy-current gauge, adhesion with a cross-cut test, gloss with a gloss meter and colour against a reference panel. Outdoor specifications commonly ask for a minimum film thickness, and 60 micrometres is a frequent figure. Which tests and acceptance limits apply is agreed in writing at order.

Solid Group Metal and powder coating

Solid Group Metal operates an electrostatic powder coating line with a capacity of 450 tonnes per month. The process and standards described above are general information; you can send a coating requirement together with your profile drawing, and the scope is confirmed in writing in the quotation.

What should a designer watch?

The finish is affected by the shape of the profile:

• Corner radii. On sharp edges the coat thins and wears early. A radius gives a more even film. The radius rules are in designing profiles for extrusion.
• Deep, narrow channels. The electrostatic field struggles to reach into a channel, so film is thin there. If a channel must be coated, open it up.
• Rack points. Where the rack touches, the surface may be bare or marked. Place that point on a hidden surface.
• Length. Profile length is limited by what the coating line and oven accept, so ask early about long parts.
• Surface quality. Die lines and extrusion marks can show through the coat. State the expectation for visible surfaces from the start.

How does coating affect dimensions and heat treatment?

The coat adds a few tens of micrometres per surface, and the amount varies in recesses. For mating surfaces, account for it. The extrusion tolerances guide covers how to handle this on the drawing. Cure temperature is also close to artificial ageing temperature, so on tempers that are not fully aged, such as T4, the cure cycle can change mechanical properties. See tempers T4, T5, T6 and T66.

Where do coating defects come from?

An orange-peel texture usually points to film thickness or cure that is out of range. Pinholes come from moisture in the powder or residue on the surface. Adhesion trouble most often traces to inadequate pre-treatment. Thin film on an edge is the result of a sharp corner. Most of these defects can be prevented at the design and process stage.

How should coated profile be handled and cleaned?

Coated profile should be packed with interleaving paper or film to prevent scratching. After installation, warm water and a neutral detergent are enough for cleaning. Avoid abrasives, strong solvents and acidic or alkaline cleaners. Light, regular cleaning extends the life of the coat outdoors.

Send your coating requirement, alloy and colour expectation through the quote request page.