The Powder Coating Process
Powder coating is one of the most durable finishes that can be applied to industrial manufactured products, and offers excellent corrosion protection and is very safe because of its lack of volatile organic compounds.
#basics #curing #masking
Powder coating is a dry finishing process used to apply a dry coating material. The coating material is made up of finely ground particles of resin and pigment for color, along with other additives for specific functions such as gloss or hardness. The dry powder coating is delivered to a spray gun tip that is fitted with an electrode to provide an electrostatic charge to the powder as it passes through a charged area at the gun tip. The charged powder particles are attracted to a grounded part and are held there by electrostatic attraction until melted and fused into a uniform coating in a curing oven.
Since its introduction more than 40 years ago, powder coating has grown in popularity and is now used by many manufacturers of common household and industrial products. In North America, it is estimated that more than 5,000 finishers apply powder to produce high-quality, durable finishes on a wide variety of products. Powder coated finishes resist scratches, corrosion, abrasion, chemicals and detergents, and the process can cut costs, improve efficiency and facilitate compliance with environmental regulations.
Because powder coating materials contain no solvents, the process emits negligible, if any, volatile organic compounds (VOCs) into the atmosphere. It requires no venting, filtering or solvent recovery systems in the application area such as those needed for liquid finishing operations. Exhaust air from the powder booth can be safely returned to the coating room, and less oven air is exhausted to the outside, making powder coating a safe, clean finishing alternative and saving considerable energy and cost.
Theoretically, 100 percent of the powder overspray can be recovered and reused. Even with some loss in the collection filtering systems and on part hangers, powder utilization can be very high. Oversprayed powder can be reclaimed by a recovery unit and returned to a feed hopper for recirculation through the system. The waste that results can typically be disposed of easily and economically.
Powder coating requires no air-drying or flash-off time. Parts can be racked closer together than with some liquid coating systems, and more parts can be coated automatically. It is very difficult to make powder coating run, drip or sag, resulting in significantly lower reject rates for appearance issues.
Powder coating operations require minimal operator training and supervision when compared with some other coating technologies. Employees typically prefer to work with dry powder rather than liquid paints, and housekeeping problems and clothing contamination are kept to a minimum. Also, compliance with federal and state regulations is easier, saving both time and money. In short, powder coating can provide the five “Es:” economy, efficiency, energy savings, environmental compliance and an excellent finish.
There are two types of powder coatings: thermoplastic and thermosetting. Thermoplastic powders melt and flow when heat is applied, but they continue to have the same chemical composition once they cool and solidify. Thermosetting powder coatings also melt when exposed to heat, but they then chemically cross-link within themselves or with other reactive components. The cured coating has a different chemical structure than the basic resin. Thermosetting coatings are heat-stable and, unlike thermoplastic powders, will not soften back to the liquid phase when reheated. Thermoset powders can also be applied by spray application to develop thinner films with better appearance than some thermoplastic powder coatings.
The main driver in the development of powder coating materials was the pursuit of an environmentally friendly alternative to solvent-laden paints. In pursuit of a spray-able, low-VOC coating, Dr. Pieter g. de Lange of The Netherlands developed the process of hot melt compounding in a z-blade mixer. This made powder coating materials much more consistent and provided the opportunity for thinner-film thermoset products that could better compete with liquid coatings. De Lange also developed the electrostatic spray application method for thermoset powder coatings in 1960. Using an addition of compressed air to the dry powder to “fluidize” the material, he was able to spray the coating and provide a decorative film. The process was introduced in the United States in the 1960s, and rapid growth continued for the next 30 years.
Pretreatment for Powder
The first step in the powder coating process is to prepare or pretreat the parts. The product to be coated is exposed to cleaning and pretreatment operations to ensure that surfaces to be coated are clean and free of grease, dust, oils, rust and other contaminants. Chemical pretreatment normally takes place in a series of spray chambers. Parts are first cleaned using an alkaline, acidic or neutral cleaner. In many cases the part is surface-treated with a conversion coating of iron or zinc phosphate or a transitional metal conversion coating such as a zirconium oxide product. Each stage is typically separated by a rinse stage to remove residual chemistry. Spray systems enable pretreatment of a wide variety of part sizes and configurations; dip tanks may be used instead of spray for some applications.
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 or non-chrome treatments for aluminum substrates. In addition to traditional phosphate processes, a new group of technologies has emerged that uses 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. Non-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.
Mechanical blasting can be used alone or along with a chemical treatment. The blast operation creates an excellent surface for bond but does not add any additional corrosion protection. In many cases, the blasted surface is first coated with a suitable primer to add additional corrosion protection for blast-only surfaces. The primer can be further enhanced by using a zinc containing material.
The most common way to apply powder coating materials uses 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 oversprayed 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 collects a charge. The powder bell provides a high level of charging efficiency and transfer efficiency. The larger pattern from the bell is very efficient for coating larger parts.
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 Systems
In 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 1953.
In fluidized-bed coating, parts are preheated to 450–500°F and then dipped into a tank filled with powder material that has been “fluidized” by addition of compressed air through a porous membrane at the bottom of the tank. 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 Spray Booths
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.
Curing of Powder Coated Parts
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.
Powder Technology Advances
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.
Powder Coating Markets and Uses
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. Several automakers are now applying powder clearcoats over liquid exterior basecoats, and there are some automobiles that are powder color-coated. Clear acrylic topcoats have been used on BMW and Mercedes vehicles.
In the automotive aftermarket, high-heat resistant powder coatings are used to finish mufflers to resist corrosion, protect against nicks and prolong the life of the muffler. Light truck and SUV owners can purchase powder coated running boards, bed rails, luggage racks and toolboxes as dealer add-ons or from aftermarket suppliers. Powder manufacturers are also working with the automotive industry to perfect powder coating on plastic items such as wheel covers, rear-view mirrors, door handles and air conditioning vents.
As more powder coaters are able to accommodate large parts, off-road vehicle frames such as those used in agricultural and construction equipment are being powder coated, with good UV and weather protection, and high resistance to salt spray and fertilizer.
Manufacturers of architectural components and building supplies powder coat aluminum extrusions used on windows, doorframes, storefronts and shelters.
In the U.S., recently installed vertical lines for powder coating aluminum extrusions, commonplace in Europe for many years, have improved speed of production as well as finish quality. Many highway and building projects use powder coating on light poles, stadium seating, guard rails, posts and fencing.
Many lawn and garden implements, including wheelbarrows, lawn mowers, lawn sprinklers, snow blowers, snow shovels, barbecue grills, propane tanks and garden tools, are powder coated, as are everyday products such as lighting fixtures, antennas, and electrical components. Sporting goods applications include powder coated bicycles, camping equipment, exercise equipment and golf clubs.
Powder coating is widely used for office furniture and equipment including file drawers, computer cabinets and desks. Parents use powder coated baby strollers, cribs, playpens, car seats and toys; consumers also own electronic components, bathroom scales, toolboxes, laptop computers, cell phones and fire extinguishers with powder coated components.
Functional powder applications are an ever-growing market where powders are applied to rebar used to strengthen bridges, buildings, retaining walls and roads. Fusion-bonded epoxy powder coatings are applied to protect both the inside (ID) and outside (OD) diameter of gas and oil transmission pipe, valves, potable water applications and springs.
Applications for powder coating are expanding. More applications continue to develop in the areas of powder on plastics and powder on wood, specifically medium-density fiberboard. Ongoing development in powder coating materials and new methods of applying powder promise even more uses that may be unimaginable today.
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