Powder Coating: Everything You Need to Know
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 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.
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Because powder coating materials contain no solvents, the process emits negligible, if any, 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 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.
Powder Application
Powder application can be manual or automated.
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 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. Electro Galvanising 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.