The powder coating process involves three basic steps: part preparation or the pre-treatment, the powder application, and curing.
Part preparation processes and equipment Removal of oil, dirt, lubrication greases, metal oxides, welding scale 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 impurities to be removed and the performance requirement of the finished product. Some heat-sensitive plastics and composites have low surface tensions and
plasma treating can be necessary to improve powder adhesion. 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 and new nanotechnology chemical bonding. The pre-treatment process both cleans and improves bonding of the powder to the metal. Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the environment.
Titanium,
zirconium and
silanes offer similar performance against corrosion and adhesion of the powder. In many high end applications, the part is electrocoated following the pretreatment process, and subsequent to the powder coating application. This has been particularly useful in automotive and other applications requiring high end performance characteristics. 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. The online consumer market typically offers media blasting services coupled with their coating services at additional costs. A recent development for the powder coating industry is the use of
plasma pretreatment for heat-sensitive plastics and composites. These materials typically have low-energy surfaces, are hydrophobic, and have a low degree of wetability which all negatively impact coating adhesion. Plasma treatment physically cleans, etches, and provides chemically active bonding sites for coatings to anchor to. The result is a hydrophilic, wettable surface that is amenable to coating flow and adhesion.
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 negative charge to the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece 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 workpiece 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. Another type of gun is called a
tribo gun, which charges the powder by the
triboelectric. In this case, the powder picks up a positive charge while rubbing along the wall of a Teflon tube inside the barrel of the gun. These charged powder particles then adhere to the grounded substrate. Using a tribo gun requires a different formulation of powder than the more common corona guns. Tribo guns are not subject to some of the problems associated with corona guns, however, such as
back-ionization and the
Faraday cage effect. Powder can also be applied using specifically adapted electrostatic discs. Another method of applying powder coating, named as 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 micrometres. This is how most dishwasher racks are coated.
Electrostatic fluidized bed coating Electrostatic fluidized bed application uses the same fluidizing technique as the conventional fluidized bed dip process but with much more powder depth in the bed. An electrostatic charging medium is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.
Electrostatic magnetic brush (EMB) coating A coating method for flat materials that applies powder with a roller, enabling relatively high speeds and accurate layer thickness between 5 and 100 micrometres. The base for this process is conventional
copier technology. It is currently in use in some coating applications and looks promising for commercial powder coating on flat substrates (steel, aluminium, MDF, paper, board) as well as in sheet to sheet and/or roll to roll processes. This process can potentially be integrated in an existing coating line.
Curing Thermoset When a thermosetting 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. The architecture of the polyester resin and type of curing agent have a major impact on crosslinking. Common powders cure at object temperature for 10 minutes. In European and Asian markets, a curing schedule of for 10 minutes has been the industrial standard for decades, but is nowadays shifting towards a temperature level of at the same curing time. Advanced hybrid systems for indoor applications are established to cure at a temperature level of preferably for applications on medium-density fiberboards (MDF); outdoor durable powders with triglycidyl isocyanurate (TGIC) as hardener can operate at a similar temperature level, whereas TGIC-free systems with β-hydroxy alkylamides as curing agents are limited to approx. . The low-temperature bake approach results in energy savings, especially in cases where coating of massive parts are task of the coating operation. The total oven residence time needs to be only 18–19 min to completely cure the reactive powder at . A major challenge for all low-temperature cures is to optimize simultaneously reactivity, flow-out (aspect of the powder film) and storage stability. Low-temperature-cure powders tend to have less color stability than their standard bake counterparts because they contain catalysts to augment accelerated cure. HAA polyesters tend to overbake yellow more than do TGIC polyesters. The curing schedule may vary according to the manufacturer's specifications. The application of energy to the product to be cured can be accomplished by
convection cure ovens,
infrared cure ovens, or by laser curing process. The latter demonstrates significant reduction of curing time.
UV cure Ultraviolet (UV)-cured powder coatings have been in commercial use since the 1990s and were initially developed to finish heat-sensitive medium density
fiberboard (MDF) furniture components. This coating technology requires less heat energy and cures significantly faster than thermally-cured powder coatings. Typical oven dwell times for UV curable powder coatings are 1–2 minutes with temperatures of the coating reaching 110–130 °C. The use of UV LED curing systems, which are highly energy efficient and do not generate IR energy from the lamp head, make UV-cured powder coating even more desirable for finishing a variety of heat-sensitive materials and assemblies. An additional benefit for UV-cured powder coatings is that the total process cycle, application to cure, is faster than other coating methods. == Removing powder coating ==