How do powder coatings work

The specific pretreatment process selected depends on the characteristics of the coating and substrate materials, and on the end use of the product being coated. Pretreatments most often used in powder coating are iron phosphate for steel, zinc phosphate for galvanized or steel substrates and chromium phosphates for aluminum substrates. In addition to traditional phosphate processes a new group of technologies has emerged that use transition metals and organo-metallic materials or other alternatives.

These alternative conversion coatings can be applied with little or no heat, and they are less prone to sludge buildup in the pretreatment bath than conventional iron or zinc phosphate formulations. The result is greater operating efficiencies in terms of lower energy costs, reduced floor-space requirements and decreased waste disposal requirements.

Other advances include non-chrome seal systems, which can yield improved corrosion protection on steel, galvanized steel and aluminum alloys.

Dry-in-place pretreatment products, such as a seal rinse over an alkali metal phosphate, can reduce the number of stages required before powder coating application. Chrome dried-in-place treatments are effective on multi-metal substrates, and may be the sole pretreatment required for some applications. On-chrome technologies are commonly used as well. Non-chrome aluminum treatments have become very popular over time with excellent performance properties. After the chemical pretreatment process is complete, parts are dried in a low-temperature dry-off oven.

They are then ready to be coated. For many functional applications, a mechanical pretreatment such as sand or shot blasting can be used. With this method, high-velocity air is used to drive sand, grit or steel shot toward the substrate, developing an anchor pattern on the part that improves the adhesion of the powder coating to the substrate.

Mechanical cleaning is particularly useful for removal of inorganic contaminants such as rust, mill scale and laser oxide. The most common way to apply powder coating materials requires a spray device with a powder delivery system and electrostatic spray gun. A spray booth with a powder recovery system is used to enclose the application process and collect any over-sprayed powder.

Powder delivery systems consist of a powder storage container or feed hopper and a pumping device that transports a mixture of powder and air into hoses or feed tubes. Some feed hoppers vibrate to help prevent clogging or clumping of powders prior to entry into the transport lines. Electrostatic powder spray guns direct the flow of powder. They use nozzles that control the pattern size, shape and density of the spray as it is released from the gun.

They also charge the powder being sprayed and control the deposition rate and location of powder on the target. Spray guns can be either manual hand-held or automatic mounted to a fixed stand or a reciprocator or other device to provide gun movement.

The charge applied to the powder particles encourages them to wrap around the part and deposit on surfaces of the product that are not directly in the path of the gun. Corona charging guns, the most commonly used, generate a high-voltage, low-amperage electrostatic field between the electrode and the product being coated.

Powder particles that pass through the ionized electrostatic field at the tip of the electrode become charged and are deposited on the electrically grounded surface of the part.

An alternative charging mechanism is a tribo charging spray gun. In such a gun the powder particles receive their electrostatic charge from friction which occurs when the particles rub a solid insulator or conductor inside the gun. The insulator strips electrons from the powder, producing positively charged powder particles. Powder can also be applied by a spray device called a bell or rotary atomizer. Powder bells use a turbine that rotates in an enclosed powder bell head.

Powder is delivered to the bell head and spread into a circular pattern by centrifugal force. The powder passes through an electric field between the bell head or an externally mounted electrode and either the grounded object to be coated or a counter-electrode positioned behind the bell head.

Use of oscillators, reciprocators and robots to control spray equipment reduces labor costs and provides more consistent coverage in many applications.

Gun triggering turning the gun on and off using a device that can sense when parts are properly positioned can reduce over-spray, which results in lower material and maintenance costs. Other Powder Application SystemsIn addition to spray application with electrostatic guns, powder coating materials can be applied by a dip method called fluidized bed.

Fluidized bed powder coating was developed by Edwin Gemmer for application of thermoplastic resins and patented in 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 application 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 over-spray. 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 over-spray 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 handled 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 in anywhere from 45 minutes to less than 15 minutes. Booth can include spec features that facilitate color changes such as non-conductive walls that repel rather than attract powder, curved booth walls to discourage powder accumulation and automated belts or 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. 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 over-spray.

The use of the right powder recovery technology can increase powder utilization. Thermoset powder materials require a certain amount of thermal energy applied for a certain time to produce the chemical reaction needed to crosslink the power into a film.

The powder material will melt when exposed to heat, flow into a level film and then begin to chemically crosslink before ultimately reaching full cure. Various methods can be used to supply the energy needed for cure. Convection ovens that use a heat source, usually natural gas, a fan and air distribution duct to circulate air inside the oven and heat the part, 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. 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 paints. 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 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 the powder coating industry 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 generally quicker turnaround and greater coating material utilization. Since the curing stage of powder coating allows 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 dry 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.

As mentioned previously, substrate materials must be able to withstand the temperature requirements of the curing stage to be suitable for powder coating applications. Even if a material can withstand the heat, achieving an even coating can still prove to be problematic, especially for thin or multi-color coatings.

Thin coatings are difficult to produce as it is challenging to control the amount of powder material that is applied to the substrate during the application stage while still ensuring an even coating.

Multi-color powder coatings are difficult to produce quickly because any overspray must be thoroughly gathered and cleaned from the spray area between color changes; otherwise, it may cause cross-contamination in recycled or reused materials. While the powder coating process can have lower costs over time, for specific coating applications, it may be more cost-effective to use liquid coatings. For example, while powder coated parts typically have faster turnaround, large, thick, or heavy parts tend to require higher temperatures and more extended curing and drying times; not only would these lengthened cure schedules delay the production process, they would also lead to higher energy costs.

For manufacturers and finishing services providers starting up, the initial investment in powder coating equipment also tends to be higher than with liquid coating as the process requires a spray gun, specialized spray booth, and a curing oven.

Image credit: Shutterstock. Powder coating can be used in a wide variety of manufacturing applications. The specific production requirements of an application help determine the finishing service provider that is best suited to consider. For manufacturers who cannot perform in-house powder coating operations, their prototype, short, and long production run jobs can be handled by a job shop or finishing service provider who offers powder coating services.

Job shops exist in all sizes from one person operations to businesses with hundreds of trained employees and with a wide range of coating capabilities and specialties.

For high volume coating applications, finishing service contractors can also prove to be a viable option. These contractors can design and build custom systems for coating specific parts, which ensures that the parts are coated consistently and to the required specifications. Although expensive as measured by the initial investment, over the course of several years the latter option can demonstrate much lower cost-per-part. Some manufacturers may choose to complete finishing operations in-house, in which case they would need to invest in obtaining or purchasing powder coating equipment.

The initial investment in equipment is high, and workers must be trained in using and maintaining the machines, but, in the long-term, especially if operations are performed routinely, it may prove to be a more cost-effective option. Finishing equipment manufacturing vendors can offer standard powder coating equipment and design and build services for custom powder coating systems, as well as provide the necessary employee training and maintenance services for the systems.

Whether a manufacturer is looking to invest in purchasing standard equipment or building a custom system, trained powder coating consultants can lend some insight and assistance, as they can provide both disinterested knowledge and vendor connections.

In deciding between completing powder coating operations in-house or outsourcing operations to a job shop or contractor, it is important for a manufacturer to understand the costs and benefits of both options in order to choose the one best suited for their powder coating application. Outlined above are the basics of the powder cutting process and equipment, and some of the considerations that may be taken into account by manufacturers when deciding whether powder coating is the most optimal solution for their particular coating application.

For more information on domestic commercial and industrial suppliers, visit the Thomas Supplier Discovery Platform , where you will find information on over , commercial and industrial suppliers. Guides Romina Ronquillo Share:. Select From Over , Industrial Suppliers. Receive Daily Industry Updates. Search Over 6 Million Products. 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. Near-infrared curing also uses specially formulated powders coupled with high-energy light sources and high-focusing reflector systems to complete the powder coating and curing process within several seconds.

Heat-sensitive assembled parts such as internal gaskets, hydraulic cylinders and air bag canisters can benefit from this technology. Induction ovens are normally used to pre-heat parts before powder coating to help accelerate film build.

They are often used in fusion-bonded epoxy coating applications such as concrete rebar and coating of pipe used for gas transmission. Such systems operate at high line speeds, and film builds of greater than 10 mils are common. Recent developments in several areas of powder application equipment and processing have significantly increased productivity and quality throughout the process, and expanded applications for powder coated parts.

These include application on medium-density fiberboard MDF , pultrusions, glass and other unique substrates. Lower-temperature cure products have been developed to accommodate heat-sensitive substrates. An in-mold powder coating process for plastic parts has been developed in which powder coating material is sprayed onto a heated mold cavity before the molding cycle begins. During the molding operation, the powder coating chemically bonds to the molding compound, resulting in a product with a coating that is chip and impact resistant.

Multi-layer processes have been developed to provide exceptional performance combined with a very high-quality appearance. Primers, basecoats and color coats are being combined with clearcoats on automotive products, boats and other products that demand exceptional quality.

Advances in microprocessors and robotics are also facilitating increased production in powder coating facilities. Robots are typically used where repeatability and high production of a limited variety of components are factors.

When combined with analog powder output and voltage controls, robots can adjust powder-delivery settings during coating, maneuvers too difficult to be accomplished manually. Today, powder coating materials are available in virtually every color and a variety of textures and glosses.

Powder coatings are used on hundreds of types of parts and products, including almost all metal patio furniture and the majority of metal display racks, store shelving and shop fixtures. Wire-formed products such as springs and storage baskets for the home and office are often powder coated. For thermosetting powders, the appliance industry is the largest single market sector. Thermosetting powder materials provide even, thin films with high levels of resistance to chips, impact, detergents and chemicals, which are critical to the appliance industry.

Applications include refrigerators, washer tops and lids, dryer drums, range housings, dishwashers, microwave oven cavities, freezer cabinets, and external air conditioning units. Automotive applications for powder coated parts include wheels, grills, bumpers, hubcaps, door handles, decorative trim, radiators, air bag components, engine blocks and numerous under-hood components, along with trailers and trailer hitches.