In order for the powder coating to adhere to the workpiece, various charging systems are used. The best known types of charging in powder coating are:
- Corona charging (electrostatic or ionization) (high voltage)
- Tribo charging (triboelectric or electrokinetic) (friction)
When best to use which system depends on the application and the user's coating requirements.
Ionization charging or corona charging is technically understood to be the escape of free electrons from an electrical conductor. In powder coating, the electrical voltage is generated by the high voltage (HV) cascade, which is installed in the powder gun. The voltage (30 to 100 kV and max. 100 µA) is delivered via the needle electrode, which is located at the outlet of the gun, to the powder-air mixture flowing past, whereby the air surrounding the powder coating particles is ionized. So that the powder can be charged optimally and constantly, the needle electrode is flushed by a small air stream. Without this air flushing, the powder would sinter ("stick") to the electrode and the electron flow would be significantly disturbed.
Figure 2 - Schematic diagram of a corona gun.
As the gun exits, a cloud of powder is created and the ionized air (in the form of electrons) accumulates on the powder particle surface. Of the free electrons, 1 - 3 % strike powder particles, the rest ionize in the ambient air or remain free, together also called space charge. The ionized powder particles, like the free ionized air particles, are attracted to all grounded objects. In practice, the workpiece is the first grounded part that the powder particles encounter and they remain attached to it, creating a counter charge. Since the powder is electrically non-conductive, the charge cannot dissipate and the attraction remains until the baking process. The current flowing away from the grounded workpiece consists of the impinging free electrons, of "air ions" and of the influence charge of the counter charge. The influence charge (back-spray effects), which attracts or repels newly arriving powder differently, causes irregular powder distribution and orange peel (see Low Ion Charging).
Likewise, the air velocity has a great influence on the coating quality. The air is mainly used to transport the powder from the powder container to the gun, via the mouthpiece to the object. High air velocities can help to increase the penetration depth, but they also increase the tendency to blow off. Therefore, the combination of the correct charging with the suitable air speed is decisive for an optimum result.
Retrofittable attachments, such as grounded metal rings or metal tips on the coater, reduce the number of free electrons (space charge) reaching the workpiece. With conventional electrostatic coating, it is not always possible to achieve a high surface quality when depressions have to be covered and the front edges have to be smooth at the same time. In the worst case, orange peel (uneven surface) can occur even with thin layers of 60 - 70 µm. This results from an excess of free electrons which cannot be discharged over the workpiece due to excessively thick powder layers. Their use is not only recommended for more complicated workpiece geometries, but also for textured powder coatings, where the use of these attachments results in a much more uniform surface structure. At the well-known manufacturers, for example, the attachments are called "SuperCorona" or "CoronaStar", which are put on the coating gun to increase the quality of the coating.
In the Tribo process, the charging of the powder coatings takes place via friction in the powder tube; there is no high-voltage supply. For this reason, the powder tube of a tribo gun is, for example, longer, usually coated on the inside with Teflon and arranged in a ring-gap or spiral shape, in order to increase the contact area as much as possible. The powder particles are passed through the spiral-shaped powder tube and in the process emit electrons to the surface of the inner tube. The excess electrons are absorbed by the metal outer tube and discharged via the inlet piece and gun cable. Due to the "loss" of electrons, the powder exits the gun positively charged. To control and measure the charge of the powder, the leakage current is fed to the control unit and measured there. Basically, this results in a smaller number of free electrons or "air ions" in the powder cloud. Compared to corona charging, therefore, the space charge is significantly reduced, which gives tribocharging a principal advantage in terms of surface quality.
Fig. 3 - Schematic diagram of a tribo gun
The tribo gun process requires a higher air velocity, so that more turbulence is formed in the inner tube. This increases the number of contacts between powder particles and results in a higher charge. The velocity of the conveying air (from the container through the gun to the outlet) is usually not sufficient to generate the required charge on the powder, therefore the powder passage velocity is increased with an additional air. This results in more difficult regulation of the desired charge on the powder, which reduces flexibility and increases complexity of operation.
List of sources
Text: Judith Pietschmann (2013). Industrial powder coating: fundamentals, processes, practical application. 4th, revised and expanded edition. Wiesbaden: Springer Verlag. S. 61-67.
Photo: Powder coating (2021): Wikipedia. Permanent URL: https://de.wikipedia.org/wiki/Pulverbeschichten. Access date: Jan. 30, 2022.
Graphics: Own representation.