How is powder coating applied

What is powder coating? How does powder coating work? Powder coating process Powder coating is usually applied electrostatic-ally using a process called electrostatic spray disposition ESD. But high-gloss finishes should be left to regular paint. Powder coating cost As mentioned above, powder coating has a much higher initial cost than painting. In some cases, the powder is electrostatically charged. Another option is flame-spray application. In flame-spray, which is used to apply thermoplastic powder materials, powder is propelled through the flame in a heat gun using compressed air.

The heat of the flame melts the powder, eliminating the need for ovens. Yet another method of application is called hot flocking. In this process, the part to be coated is preheated so that the sprayed powder will gel when it comes in contact with the hot part surface. Hot flocking is often used for functional epoxy applications because it builds a thick film that will provide exceptional performance.

These fusion-bond epoxy FBE products are often used to coat valves and pipe used in extreme conditions such as oilfield or offshore applications. Powder booths are designed to safely contain the powder overspray. Booth entrance and exit openings must be properly sized to allow clearance for the size range of parts being coated, and airflows through the booth must be sufficient to channel all overspray to the recovery system, but not so forceful that they disrupt powder deposition and retention on the part.

There are booths designed for limited production batch operations and larger booths designed for volume operations where parts are conveyed through on some type of hanger. Batch booths are used for coating individual parts or groups of parts that are hung on a single hanger, rack or cart. Conveyorized booths can provide continuous coating of parts hung on an overhead conveyor line in medium- to high-production operations. Chain-on-edge booths are designed for use with an inverted conveyor featuring spindles or carriers for holding the parts.

Parts are rotated on the spindle as they pass the stationary powder guns. Flat line booths and conveyor system are used for one-sided coating of sheet metal and similar parts of minimal thickness. Flat-line booths use a horizontal conveyor that passes through the powder booth carrying the part to be coated on its surface. Properly designed, operated and maintained powder systems can allow color changes from a reclaim color to another reclaim color in anywhere from 45 minutes to less than 15 minutes.

For color changes that do not reclaim the overspray the color-change time can be reduced to a very few minutes for automated systems and as short as one minute for manual systems. A powder booth can include special features that facilitate color changes such as non-conductive walls that do not attract powder, curved booth walls to discourage powder accumulation in corners, or automated sweepers that brush powder particles to the floor and into the recovery systems.

Fast color change can also be facilitated using blow-off nozzles set up at each gun barrel and easily changed connections at the back of the gun outside the booth.

Guns can have the outside of the barrels blown off automatically, and also use an automated purge system for the interior of the hoses and gun barrels. Powder recovery systems use either cyclones or cartridge filter modules that can be dedicated to each color and removed and replaced when a color change is needed. Equipment suppliers have made significant design improvements in spray booths that can allow both fast color changes with minimal downtime and recovery of a high percentage of the overspray.

The use of the right powder recovery technology can increase powder utilization. The decision of whether or not to reclaim powder for reuse depends on the value of the powder that has been oversprayed when compared to the time and cost associated with the recovery process.

In the case of a long run of expensive powder, it can be very economical to conduct a 15 minute or longer color change, but in the case of a short run or low-value powder, the time may not be justified.

Thermoset powder materials require a certain amount of thermal energy applied for a certain time to produce the chemical reaction needed to cross-link the power into a film. The powder material will melt when exposed to heat, flow into a level film and then begin to chemically cross-link before ultimately reaching full cure.

Various methods can be used to supply the energy needed for cure. Convection ovens use a heat source usually natural gas and fan to distribute and circulate air through a duct inside the oven. The heated air will in turn heat the part and then the coating. Convection ovens are the most common type of cure oven used for powder. As the part reaches peak temperature it will conduct heat into the coating and cause the powder to cure. Infrared IR ovens, using either gas or electricity as their energy source, emit radiation in the IR wavelength band.

This radiated energy is absorbed by the powder and substrate immediately below the powder without heating the entire part to cure temperature. This allows a relatively rapid heat rise, causing the powder to flow and cure when exposed for a sufficient time. Parts can be cured in less time in an IR oven, but the shape and density of the part can affect curing uniformity. Combination ovens generally use IR in the first zone to melt the powder quickly. The following convection zone can then use relatively higher airflows without disturbing the powder.

These higher flows permit faster heat transfer and a shorter cure time. A variety of radiation curing technologies are available, including near-infrared, ultraviolet UV and electron beam EB. These processes have the potential to open up new applications for powder coating of heat-sensitive substrates such as wood, plastic parts and assembled components with heat-sensitive details. UV curing requires specially formulated powders that can be cured by exposure to ultraviolet light.

The powder first needs to be exposed to enough heat so it is molten when exposed to UV energy; the initial heat source is typically infrared, but convection heating can also be used. The coating is then exposed to a UV lamp. A photo initiator in the coating material absorbs the UV energy and converts the molten film to a solid cured finish in a matter of seconds.

Powder coatings can also be applied by dipping into a fluidised bed of powder, made to flow like a liquid by bubbling air through it. A bed of powder is fluidised by blowing air from below through a porous plate. The object to be painted is pre-heated to, typically C, and immersed in the bed. This method is often used for large work pieces such as pipe line valves, fence posts etc. However, the harder finish can also limit the impact resistance of thermoset coatings, and over-hardening can cause the coating to become brittle, particularly in thicker coatings.

Thermoplastic powder can be applied via both the ESD and the fluidized bed coating method, and generally can produce thicker, more flexible and impact resistant coatings than thermoset powder. While the ability to be remelted offers some advantage in regards to material costs, it also makes thermoplastic powder coatings less suitable for high and intense heat applications as the coating material may soften or melt off.

Powder coatings are primarily applied to metal substrates, such as steel, stainless steel, and aluminum. However, they can also be applied to non-metal substrates, such as glass, wood, or medium density fiberboard.

The range of suitable materials for the powder coating process is limited to materials that can withstand the temperatures required to melt and cure the powder coating material without melting, deforming, or burning itself. The chosen material also helps determine the coating method employed. Since metals can be electrically grounded, the coating material is generally applied to metal substrates via the electrostatic spray deposition method, but they can also be applied via the fluidized bed method.

On the other hand, since non-metals cannot be sufficiently grounded, they require that the powder coatings be applied through the fluidized bed powder coating method. Powder coatings can be applied in a wide range of colors, finishes, textures, and thicknesses that are not readily achievable through conventional liquid coating methods.

Capable of being manufactured in virtually any color, powder coating materials can be formulated for both protective and decorative applications. The final finish achieved by the powder material ranges from matte to glossy, and clear to glittered or metallic. Various textures are also available for decorative purposes or hiding surface imperfections. The powder coating process allows for a wider range of coating thicknesses. Compared to the liquid coating process, powder coating can more readily produce thicker, even coatings, especially when using the fluidized bed coating method.

Using the ESD method, it is also possible to achieve thin, even coatings; albeit, not as thin as the coatings achieved via the liquid coating process. The powder coating method offers several advantages over conventional liquid coating methods, including increased durability, capabilities for more specialized finishes, less environmental impact, faster turnaround time, and lower material costs. In addition to being available with a wide range of finish options, powder coatings are generally more long-lasting and durable than liquid coatings.

They demonstrate higher resistance to impact, moisture, chemicals, and wear, and offer greater protection from scratches, abrasion, corrosion, fading, and general wear. These characteristics make them well-suited for high use and high traffic applications. Another advantage of the powder coatings is the lack of solvent and carbon dioxide emissions, hazardous waste material that requires disposal, and, generally, surface primer requirements.

These exclusions limit the amount of toxic and carcinogenic substances being released into the environment throughout the process and contribute to the recognition of powder coating as a more environmentally-friendly alternative to liquid coatings.

The process can have much lower long-term costs compared to the liquid coating process due to its having a generally quicker turnaround and greater coating material utilization. Since the curing stage of powder coating allows powder coated parts to be assembled, packaged, and shipped immediately after the part is cool, parts spend less time in inventory which enables manufacturers and finishing service providers to have a faster turnaround and less storage space requirements.

The process also allows for overspray material to be collected and recycled instead of wasted, which decreases the amount of waste product requiring disposal, increases the coating material utilization rate, and lowers the cost of materials over time. Although the powder coating process offers several important advantages over liquid coating, there are also limitations to the process.

Limitations of powder coating include a restricted range of suitable substrate materials, difficulty producing even, thin coatings, longer lead times for color changing coatings, longer dry and cure times for large parts, and higher start-up costs.