Powder coating is a type of coating that is applied as 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". The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is used for coating most types of metals
TYPES OF POWDER COATINGS
THERE ARE TWO MAIN CATEGORIES OF POWDER COATING:
THERMOSETS AND THERMOPLASTICS
The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder to polymerize, improving 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 most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.
The polymer granules are mixed with harder pigments and other powder in a mixer.
The mixture is heated in an extruder.
The extruded mixture is rolled flat, cooked, and broken into small ships.
The chips are milled and sieved to make a fine powder.
THE POWDER COATING PROCESS
THE POWDER COATING PROCESS INVOLVES THREE BASIC STEPS
Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.
Chemical pre-treatments involve the use of phosphates or chromates in submersion or spray application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal. This is what Preferred Powder Coating uses. Titanium zirconium and silanes offer similar performance against corrosion and adhesion of the powder.
Another method of preparing the surface prior to coating is known as abrasive blasting or sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass. The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance.
Silicon carbide grit blast medium is brittle, sharp, and suitable for grinding metals and low-tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface finishing. Sand blast medium uses high-purity crystals that have low-metal content. Glass bead blast medium contains glass beads of various sizes.
Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.
Different powder coating applications can require alternative methods of preparation such as abrasive blasting prior to coating.
PART PREPARATION AND EQUIPMENT
POWDER APPLICATION PROCESSES
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 is a wide variety of spray nozzles available for use in electrostatic 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 then 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.
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 cure process, called crosslinking, requires a certain 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. Generally the powders cure at 200°C (390°F) for 10 minutes. The curing schedule could vary according to the manufacturer's specifications. Preferred uses verified convections ovens.