What do window frames, car parts, machines and bicycles have in common? They are powder-coated. This is because powder coating improves both the mechanical and optical properties of metal surfaces. The widely used process consists of several steps: surface pre-treatment, coating and baking in the oven. From electrostatics to phosphating – many exciting chemical and physical processes are involved in powder coating.
A bare steel or aluminum bike frame might look impressive, but would be pretty useless in practice as it would quickly scratch and rust. This is why not only bicycle frames, but also many metal products are given a protective layer of paint using the powder coating process.
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The process, which is generally at least partially automated, is widely used both in industrial series production and for individual small orders. Typical areas of application are windows, doors, furniture and facades. Industrial machines and vehicle parts are also frequently powder-coated
Powder coating involves coating an electrically conductive, i.e. metallic, component with a powder coating. This enhances surfaces both mechanically and visually. Powder coatings are organic and consist of binders, additives and pigments, all as solid particles with grain sizes between 1 and 100 µm. In contrast to liquid paints, they do not require any solvents and are therefore more environmentally friendly. The powder coating process consists of several steps, including surface pre-treatment (cleaning and creation of a conversion layer), electrostatic charging of the powder and coating of the workpiece, as well as the final baking of the powder coating in the oven.
Step 1: Cleaning the workpiece
Before applying the powder coating, the surface of the component to be coated must be absolutely clean and dry. Failure to do so may result in loss of adhesion or cratering of the paint film.
Mechanical pre-treatment such as grinding, brushing and blasting removes coarse impurities such as dust, rust or scale. Blasting with stainless steel grit or glass beads also serves to roughen the surface. During chemical pre-treatment, impurities such as paint and grease are removed.
Step 2: Conversion layer
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In the next step, a conversion layer is created by chemically modifying the surface of the workpiece. This very thin, non-metallic, usually inorganic layer increases the active surface area and improves the adhesion of the powder coating, while at the same time providing additional corrosion protection.
During phosphating, the workpiece surface reacts with the metal ions (usually iron or zinc) of an aqueous phosphate solution so that the metal phosphates are firmly incorporated into the top layer of the workpiece. This method is suitable for steel, galvanized steel and aluminium. Anodic oxidation (anodizing) is an electrochemical process in which a homogeneous oxide layer is formed on aluminium components.
Step 3: Charging the powder
The powder coating is applied to the electrically conductive workpiece on the basis of electrostatic adhesion. To do this, the powder coating must first be electrostatically charged. This can be done using two methods: In corona charging, a high-voltage electrode generates an electric field that ionizes 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 atomized through the nozzle of the spray gun and applied to the grounded, i.e. uncharged, workpiece in the spray booth. The mutual repulsion of the equally charged powder particles creates a homogeneous powder cloud. When the ionized particles hit the workpiece, they generate a countercharge on the workpiece surface at the moment of impact. The attractive force (Coulomb force) between the charge of the particles and the countercharge on the workpiece causes the particles to adhere to the surface. The electrostatic force must be stronger than the force of gravity.
This is why the layer thickness is physically limited during powder coating and is typically between 60 and 120 µm. The coating remains adherent for up to several hours before the powder would fall off again due to the gradual equalization of the charge. To prevent this, the stoving process follows.
Step 5: Baking in the oven
After coating, the powder coating is baked in an oven at temperatures between 110 and 250 °C. This process, also known as cross-linking, begins with the melting of the powder coating in the oven. The plastic particles then cross-link with each other and with the other solids in the powder coating to form a homogeneous and smooth powder coating layer.
Automation in powder coating
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In modern powder coating, the process is often automated or at least partially automated. Automatic systems are capable of coating large quantities in high quality and with consistent results. Individual small parts are often coated in manual powder booths and gas-heated chamber ovens. Fully automatic systems with a conveyor system and gas-heated continuous oven are used for large series orders. The products pass through pre-treatment, the coating booth and the oven while hanging on a transport rail.
However, the implementation of such automated systems involves considerable investment. New machines and systems can be very expensive. A more cost-effective alternative is to buy used machines. High-quality, newer machines are coming onto the used machine market, particularly from plant closures. This was also the case with the insolvency of Pulverlackierung Sarnoch GmbH. At the Surplex subsidiary HT, the 03.07. The entire production of the powder coater was auctioned off.
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