Powder coating

Powder coating is a type of coating that is applied with a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a “skin.” It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as whiteware, aluminum extrusions, and automobile and bicycle parts. Newer technologies allow other materials, such as MDF-medium density fiberboard-, to be powder coated using different methods.

There are several advantages of powder coating over conventional liquid coatings:

1. Powder coatings emit zero or near zero Volatile Organic Compounds (VOC).
2. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
3. Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.
4. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.
5. A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes.

While powder coatings have many advantages over other coating processes, there are limitations to the technology. These are

1. While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more and more orange peeled in texture due to the particle size and TG (glass transition temperature) of the powder.
2. Powder coatings will break down when exposed to UV rays between 5 to 10 years.
3. Powder coating will not hide defects which require putty filling and the performance of the coating is directly related to the condition of the substrate.

There are two main categories of powder coatings:
Thermosetting and Thermoplastic.

The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties.

The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating. The decision to what powder to use depends upon the specific appearance & performance requirements of the end products. The proper coating is a balance of following variables:

1. Cost of powder (applied)

2. Performance Characteristics

3. Application Characteristics

4. Appearance

1. Powder Application Processes

a. The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or Corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the work piece by the powerful electrostatic charge. There are a wide variety of spray nozzles available for use in electro static coating. The type of nozzle used will depend on the shape of the work piece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.

b. Powder can also be applied using specifically adapted electrostatic discs.

c. Another method of applying powder coating, called the Fluidized Bed method, is by heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometers. This is how most dishwasher racks are coated.

2. Curing

a. When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This curing process, called cross linking, requires a certain degree of temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200°C (390°F) in 10 minutes. The curing schedule could vary according to the manufacturer’s specifications.

b. The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.

The equipment’s used for Powder Coating are Powder Coating Booths, Coating Equipment’s, Baking Oven & Material handling systems.

The type of plant i.e. batch / conveyorized depends on the production volume & article configuration.