What do window frames, automotive parts, machinery, and bicycles have in common? They are all powder coated. Powder coating improves both the mechanical and optical properties of metal surfaces. This widespread process consists of several steps: surface pre-treatment, coating, and curing in an oven. From electrostatics to phosphating – many exciting chemical and physical processes are involved in powder coating.
A bare bicycle frame made of steel or aluminium might look impressive, but in practice, it would be quite useless as it would quickly scratch and rust. Therefore, not only bicycle frames but also many metallic products are provided with a protective coating in the powder coating process.
This generally at least partially automated process is widely used both in industrial mass production and for individual small orders. Typical applications include window frames, doors, furniture, and facades. Industrial machinery and vehicle parts are also frequently powder coated.
In powder coating, an electrically conductive, metallic component is coated with a powder paint. This enhances surfaces both mechanically and optically. Powder coatings are organic and consist of binders, additives, and pigments, all as solid particles with grain sizes between 1 and 100 µm. Unlike liquid paints, they do not require solvents and are thus more environmentally friendly. The powder coating process consists of several steps, including surface pre-treatment (cleaning and creating a conversion coating), electrostatic charging of the powder, and coating the workpiece, followed by curing the powder coating in the oven.
Step 1: Cleaning the workpiece
Before applying the powder coating, the surface of the workpiece must be absolutely clean and dry. Otherwise, there may be adhesion losses or crater formation in the paint film. Mechanical pre-treatment such as grinding, brushing, and blasting removes coarse contaminants such as dust, rust, or scale. Blasting with stainless steel shot or glass beads also serves to roughen the surface. Chemical pre-treatment removes contaminants such as paints and greases.
Step 2: Conversion coating
In the next step, a conversion coating is created through a chemical change in the surface of the workpiece. This very thin, non-metallic, mostly inorganic layer enlarges the active surface and improves the adhesion of the powder coating, while also providing additional corrosion protection. In phosphating, the workpiece surface reacts with the metal ions (mostly iron or zinc) of an aqueous phosphate solution, so that the metal phosphates are firmly embedded in the uppermost layer of the workpiece. This method is suitable for steel, galvanised steel, and aluminium. Anodic oxidation (anodising) is an electrochemical process in which a homogeneous oxide layer is formed on aluminium parts.
Step 3: Charging the powder
The application of the powder coating to the electrically conductive workpiece is based on electrostatic adhesion. For this purpose, the powder coating must first be electrostatically charged. This can be done by two methods: in corona charging, a high-voltage electrode generates an electric field that ionises the surrounding air and thus charges the powder particles. In triboelectric charging, the powder particles are charged by friction in the spray gun.
Step 4: Coating
The powder coating is atomised through the nozzle of the spray gun and brought into the environment of the grounded, i.e., uncharged, workpiece in the spray booth. The mutual repulsion of the similarly charged powder particles creates a homogeneous powder cloud. When the ionised particles hit the workpiece, they generate a counter-charge on the workpiece surface at the moment of impact. The attractive force (Coulomb force) between the charge of the particles and the counter-charge on the workpiece ensures that the particles adhere to the surface. The electrostatic force must be stronger than gravity. Therefore, the layer thickness in powder coating is physically limited and typically ranges between 60 and 120 µm. The coating remains adherent for up to several hours before the powder gradually falls off due to charge equalisation. To prevent this, the curing process follows.
Step 5: Curing in the oven
After coating, the powder coating is cured in an oven at temperatures between 110 and 250 °C. This process, also called cross-linking, begins with the melting of the powder coating in the oven. Subsequently, the plastic particles interlink with each other as well as with the other solids in the powder coating to form a homogeneous and smooth powder coating layer.
Automation in powder coating
In modern powder coating, the process is often automated or at least partially automated. Automatic systems are capable of coating large quantities with high quality and consistent results. Individual small parts are often coated in manual powder booths and gas-heated batch ovens. For large series orders, fully automatic systems with conveyor belts and gas-heated continuous ovens are used. The products pass through the pre-treatment, coating booth, and oven while hanging on a conveyor belt.
However, implementing such automated systems involves significant investments. New machines and systems can be very expensive. Used machines, such as the powder coating system available for direct purchase at Surplex, offer a cost-effective solution. This system in Zaragoza has an estimated operating time of only around 1000 hours and is therefore in good condition.
To learn more, visit www.surplex.com.
