Powder coating application process consist of 3 main stages.
SURFACE CLEANING AND CHEMICAL PRE-TREATMENT
The surface to be coated with powder should be free from all foreign substances such as oil, dirt, rust, and dust. In addition, coating the surface with a chemical substance provides various properties. If the material to be coated is coated with a layer such as phosphate or chromate after pre-cleaning, the powder adheres to the porous surface better and stops the progression of corrosion in case of impact. Depending on the process conditions among surface treatments, there may also be intermediate processes such as rinsing and drying.
Powder coating can be performed using different methods. Typically known and used methods:
- Fluidized bed method: The Fluidized Bed Method involves a booth, and an embossing base under which air can pass but powder cannot. Compressed air fed into this base lifts the powder and gives a boiling appearance. (as with powder tanks in gun systems).
The surfaces to be coated are immersed into the boiling powder and removed with rail systems circulating above the booth. Then, the powder-coated surfaces are transferred to the ovens. The powder can adhere to the surface in two different ways. First way, surfaces to be coated are heated and the powder adheres to the surface thanks to the heat. The second way, the boiling powder is electrostatically charged and the surface to be coated is grounded. The surface is coated with the principle that negative and positive charges attract each other.
The biggest disadvantage of the Fluidized Bed Method is that it is difficult to adjust the coating thickness. Due to very thick surfaces, powder consumption and the total usage cost is high. Although this is not a widely used method, it can be used for functions, such as coating small parts for which using a gun is difficult.
- Electrostatic spraying (gun) method: Powder particles are liquefied with air and carried to the spray guns, where they are charged with 10150 kV or with friction (tribo) in Corona type guns. The charged powders adhere to the grounded surfaces.
Powders, which cannot adhere to the surface and fall down, are collected by ventilation systems and filtered. Then, they are mixed with powder and used again.
There are 2 types of charging/guns which allow this method to be applied. The types of guns used in the electrostatic spraying method and their features can be listed as follows:
• Charging can be controlled thanks to the electrical current. All types of powder of all thicknesses are used.
• Environmental factors such as temperature and humidity have fewer effects.
• Guns and operating costs are low.
• The Faraday cage is formed. The film thickness can change with a voltage adjustment.
• High consumption.
• Powder replacement (system cleaning) is easy.
• High risk of back ionization.
• No need for high voltage. The powder is charged with friction.
• Special particle sized and tribo designed powders are used.
• Easily affected by environmental factors.
• High cost.
• No Faraday cage effect.
• The film thickness may vary with air adjustment.
• Low consumption.
• Difficult powder replacement.
• Low risk of back ionization.
Thanks to the recent techniques, the excess ion formed in the corona guns is poured into the ground with the collector placed in the gun’s barrel section, reducing the Faraday Cage effect. This particular type of gun is called Super Corona.
Also, guns are divided into two as Manual and Robot. Robot guns are preferred by companies that use large amounts of powder paint. They are fully automated and used with computer control. In this way, the user does not contact the powder.
Manual guns are completely manually controlled. They are used in Research&Development Laboratories, Quality Control Departments, or the Repair and Touch-Up Departments of the users.
The curing conditions are determined by the powder manufacturer in line with the information received from the user. What is important here is to keep time after the coated surface (metal) reaches the desired temperature in light of given information. Each powder has a specific temperature-time curve.
As shown in the example below, powder can be cured at various temperatures and times. The higher the temperature, the lower the curing time. The user company can select a safe temperature and time based on this curve according to their own parameters. However, as can be seen in the graph, proper curing is not possible at extreme temperatures and time such as 1 min. at 500°C and 1000 min. at 50°C. This is because these curing conditions are not suitable for the reactions which will occur in the bill of material.
The powder must be baked and cured in order to create a suitable surface based on its properties. The powder can be baked between 120°C and 250°C. The baking temperature depends on the type of powder used, the weight of the coated surface, and the curing time. Baking time at a certain temperature depends on the oven design and the type, shape, and thickness of the surface to be coated.
Choosing the right curing temperature is essential for powder coating. The lowest and highest temperatures must be observed during the baking process. Because while temperatures lower than normal cause incomplete curing, high temperatures may cause yellowing and cracks on the surface. Baking conditions should be determined in advance or provided by the powder manufacturer.