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Corona charging operates by applying a high static electrical charge to an electrode, resulting in the charging of powder particles when they move near this electrode. Essentially, the particles gather free electrons from the electrostatic field generated by the corona electrode.
Every atom, in gases like oxygen, nitrogen, and carbon dioxide, holds an equilibrium of positive and negative charges, making it electrically neutral. Yet, when an atom either loses or gains electrons, it becomes electrically imbalanced, turning into an ion. Atoms that have shed electrons become positively charged cations, while those with extra electrons become negatively charged anions.
In the realm of powder coating, corona discharge is achieved by imparting a high voltage, sometimes reaching 100KV, to a pinpointed electrode at a spray gun’s tip. This action ionizes the surrounding air, usually producing negative ions. As powder particles traverse this ion-rich zone, they get charged and then stick to an earthed substrate.
However, corona charging isn’t flawless. A significant proportion of generated ions don’t adhere to the powder particles, resulting in an abundance of free ions. This surplus tends to accumulate, especially in intricate crevices, leading to what’s known as the “Faraday cage effect”. Another limitation is “back ionisation”, where excess same-polarity charges repel each other, causing discharges within the powder coat and an uneven finish resembling an orange peel.
Triboelectric charging, on the other hand, stems from the frictional interaction within the Tribo charge gun. Here, powder moves through a uniquely designed friction body in the gun, which is insulated. This method charges the powder particles positively, necessitating specially formulated powders for the Tribo charging technique.
The absence of a charging electrode minimizes the Faraday cage effect, making Tribo charging especially advantageous for intricate designs, like painting radiators. Additionally, since the powder charges internally within the gun, there’s no creation of free ions, addressing many challenges faced by the corona method.
However, Tribo charging does not generate the pronounced “wrap around” effect seen in corona charging, and the powder flow from the gun tends to be gentler. In scenarios where a product has complex geometries or requires a singular heavy coat without surface disturbances, the Tribo system, provided compatible powder is available, may outperform the Corona system.
One of the most effective methods for ensuring optimal powder performance during coating is by fluidizing it using compressed air. This process creates a blend of air and powder, which can be seamlessly transported within a sealed system. Notably, the flow of the powder can be easily regulated by adjusting the air rate.
When the transport air is activated, this air-powder blend moves consistently through the hose system. However, turning off the air causes the powder to settle at the base of the hose or gun nozzle. Reactivating the air introduces new powder into the system, which then needs to combine with the settled powder. This integration can lead to a minor back-pressure, resulting in a brief burst or surge of powder from the coating guns.
When selecting manual electrostatic spray systems, it’s crucial to ensure fluidization, stirring, or vibration mechanisms are in place for the powder within the hopper. Moreover, the system should be designed for minimal air consumption while providing a steady flow of powder.
An effective system should seamlessly integrate into a recovery cycle and maintain consistent coating quality. This is achieved by a constant influx of fresh powder from a refilling hopper. Additionally, for many operators, the flexibility to swiftly switch between different colors is an indispensable feature.
Automatic spraying systems can be designed with up to 10 gun control modules. For larger-scale operations or specific product dimensions and conveyor speeds, multiple cabinets can be linked together to accommodate additional modules.
These systems often utilize powder sourced from hoppers with integrated fluidised beds, ensuring a smooth and steady powder supply. It’s crucial for a module to monitor and manage powder levels, ensuring consistent replenishment. This stability is often maintained using sensors that track powder levels and guarantee a steady delivery rate.
Coordinating gun movement is vital. This can be achieved through reciprocating control strokes, tailored to match the shape and size of the substrate. These settings are often inputted via a keyboard, allowing operators to store multiple pre-programmed configurations, recalling them as needed.
While vertical reciprocation is the most common method for gun movement, especially for uniformly shaped substrates like flat panels or aluminum extrusions, there’s a growing trend towards mounting guns vertically. In this setup, guns employ a shorter reciprocating stroke, covering only specific sections of passing components.
To ensure an even coating without any striping, the following factors should be meticulously calibrated:
Powder coatings are optimal for high-efficiency, automated application setups. Nevertheless, they’re also versatile enough to accommodate more hands-on, manual operations.
Utilizing the electrostatic spray technique, thin powder films ranging from 25 to 125 microns are applied. In certain scenarios, even thicker films of 150 to 375 microns can be achieved. The powder, mixed with air, is channeled from a feed hopper to the spray gun. Here, a high voltage is applied, charging each powder particle electrostatically. These charged particles are then drawn to the grounded object intended for coating, accumulating on any conductive surface within the spray zone.
Yet, not all particles adhere to the object. This is where powder coating shines. The non-adhering, or “overspray”, particles are recaptured and funneled back to the feed hopper for subsequent use, boasting potential recovery rates of up to 99%.
After the coating process, the workpiece moves to the curing oven. Since powder coatings are devoid of solvents, there’s no need for additional zones to let solvents evaporate, nor is there a requirement for special environmental equipment – a typical necessity with conventional liquid paints. This not only results in cost savings but also translates to reduced fire insurance premiums and a more secure working atmosphere.
Another method of charging in some spray gun designs is Tribo Charging. Tribo Charging does not use high voltage, but instead uses the principal of high velocity friction to give the powder particles a static chargAnother method of charging in some spray gun designs is Tribo Charging. Tribo Charging does not use high voltage, but instead uses the principal of high velocity friction to give the powder particles a static charge
Dipping powder coatings are an innovative method where objects are submerged into a fluidized bed of powder. This bed is created by forcing air through a porous plate beneath the powder, making it flow similarly to a liquid. Before immersion, the item to be coated is typically pre-heated to a temperature ranging from 70°C to 200°C. Depending on the object’s heat retention capability, post-heating might be necessary to ensure the coating cures properly.
This technique is particularly favored for larger items like pipeline valves and fence posts, as well as smaller components from the electronics sector. Coatings applied using this method are renowned for their thickness and durability. They offer outstanding corrosion resistance, mechanical strength, electrical insulation, and resistance to chemicals.
An effective system should seamlessly integrate into a recovery cycle and maintain consistent coating quality. This is achieved by a constant influx of fresh powder from a refilling hopper. Additionally, for many operators, the flexibility to swiftly switch between different colors is an indispensable feature.
Powder coating is a versatile process that can be applied through various methods. The most common are the electrostatic spray and the fluidized bed techniques. Here’s a comparative analysis of both methods:
Advantages:
Disadvantages:
Spray Guns:
Advantages:
Disadvantages:
In summary, while both processes have their unique advantages, electrostatic spraying tends to be more adaptable and diverse in application. Conversely, the fluidized bed method excels in specific scenarios, particularly where thicker coatings are essential
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