How it’s done – the powder coating process 101
Powder coating is a versatile and high performance coating, not just in durability and protective properties, but also in application methods. Whatever the part that needs to be coated, there is a powder coating process that will suit the application. The two main powder coating application methods are spraying and dipping, and within these categories there are a several options including fluidised bed, corona, and tribo methods.
In this article we describe the different types of powder coating application methods, and the uses, advantages and disadvantages of each.
Method 1: Powder coating spray application
Spray application is the most common powder coating application method. It is typically used to apply thin films (25-125 microns), but can also be used for slightly thicker film applications. The basic principle is that a powder/air mixture is fed from a hopper through a spray gun, which blasts it onto the substrate to be coated. The coated piece is then sent to the oven for curing. The powder is attracted to the substrate through electric charge, and this process is also called electrostatic spray. There are two types of electrostatic spray, based on the method by which they impart an electric charge.
- Electrostatic spray application with corona charging gun
Corona charging guns apply a charge to the powder as it leaves the gun. A high voltage generator produces up to 100,000 volts, creating a high voltage potential at the electrodes near the tip of the gun. As the voltage increases, the electrode generates an electric/ion field between the gun and grounded substrate. This field is the corona. The ion field transmits a charge to the powder particles as they pass through. The newly charged powder is attracted to the grounded substrate and will attach long enough to undergo the curing process.
Most corona spray guns impart a negative charge to the powder, but some can produce a positive charge. These are commonly used for nylon powder coating.
- Tribostatic spray application with tribo charging gun
Tribo charging spray guns use an entirely different method to impart charge to the powder. Tribostatic application uses friction as the source of charge. The powder is funneled through a surface lined with PTFE (a non stick fluoropolymer), each particle gaining more charge the more times it collides with the walls. These frictionally charged particles are then attracted to the grounded substrate. The speed and volume of the powder affects its charge.
Choosing the right type of charging system for the right powder is crucial to avoid problems with powder coating application.
Method 2: Powder coating process by dipping
Dipping is the powder coating process used by a number of specialist industries, particularly in cases where the parts being coated are complex, need far thicker coating films, or need consistent edge coverage. They are more commonly used for thermoplastic powders. In the fluidised bed dipping methods the part to be coated is pre-heated before the powder is deposited. Dipping principally uses two methods.
- Fluidised bed application
The ‘bed’ in fluidised bed refers to a large open box or container which holds the powder. Air is pumped into this container uniformly, reducing the density of the powder and causing the powder to flow and behave like a liquid. Powders intended for fluidised bed application tend to have more regular, and larger, particle sizes to facilitate this behaviour. The pre-heated part is then dipped into the bed of fluidised powder, and the thickness of the coating is determined by the temperature of the part and the time spent in the powder.
- Fluidised bed with electrostatic charge
Using the same fluidising principles as fluidised bed coating, electrostatic fluidised bed coating adds a charge to the powder, causing it to form a cloud of charged powder above the bed. When the parts move through this cloud the powder is attracted to them. Preheating is not required. This form of coating is used for continuous coil coating, wire screen coating, and other simple configurations without deep recesses that only require a thin film.
Comparing the pros & cons of corona, tribo and fluidised bed
Each of the different powder coating application methods have their own advantages and disadvantages, but what is important to remember is what is being coated. Using the right technique for the right job is key for a successful, high quality coating application. What you need to consider are the following 4 factors:
- The specific film thickness requirements of the project
- The powder coating chemistry
- The shape, size, and thickness of the part being coated
- The line speed
Below we have outlined some of the pros and cons of the most common powder coating application methods.
|Powder Coating Application Method||Advantages||Disadvantages|
|CORONA CHARGING ELECTROSTATIC SPRAY||
|TRIBOMATIC CHARGING ELECTROSTATIC SPRAY||
|ELECTROSTATIC FLUIDISED BED||
The powder coating process for other than metal substrates
As well as the more traditional powder coating application methods described above, the advancements in technologies and chemistries has led to new ways of applying powder coat. Powder coating is no longer just solid tones on metal substrates. It is now possible to coat wood, glass, plastic, and other low temperature substrates (with special low temperature powders), as well as more decorative powder coating, including special effects like wood grain, marble, pictures, and designs. These designs are transferred onto a substrate through a powder coating process known as Sublimation.
Interpon’s “decoration coatings” line, the Interpon D series, is one coating range used for this process. It typically involves three layers of coating: a primer layer (Interpon STF), the film transfer of the print, then finally a clear protective topcoat. Another powder coating process used for wood effects is powder-on-powder. In this process, the effects coating is applied with a special roller which is perforated in such a way as to allow powder coating through in a particular pattern – in this case, wood texture. A second powder coat is applied on top to complete the effect.