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Powder coating is a dry finishing process that has become extremely popular since its introduction in North America over in the 1960s. Representing over 15% of the total industrial finishing market, powder is used on a wide array of products. More and more companies specify powder coatings for a high-quality, durable finish, allowing for maximized production, improved efficiencies, and simplified environmental compliance. Used as functional (protective) and decorative finishes, powder coatings are available in an almost limitless range of colors and textures, and technological advancements have resulted in excellent performance properties.
How Powder Coating Works
Powder coatings are based on polymer resin systems, combined with curatives, pigments, leveling agents, flow modifiers, and other additives. These ingredients are melt mixed, cooled, and ground into a uniform powder similar to baking flour. A process called electrostatic spray deposition (ESD) is typically used to achieve the application of the powder coating to a metal substrate. This application method uses a spray gun, which applies an electrostatic charge to the powder particles, which are then attracted to the grounded part. After application of the powder coating, the parts enter a curing oven where, with the addition of heat, the coating chemically reacts to produce long molecular chains, resulting in high cross-link density. These molecular chains are very resistant to breakdown. This type of application is the most common method of applying powders. Powder coatings can also be applied to non-metallic substrates such as plastics and medium density fiberboard (MDF).
Sometimes a powder coating is applied during a fluidized bed application. Preheated parts are dipped in a hopper of fluidizing powder and the coating melts, and flows out on the part. Post cure may be needed depending on the mass and temperature of the part and the type of powder used. No matter which application process is utilized, powder coatings are easy to use, environmentally friendly, cost effective, and tough!
Powder coating is a high-quality finish found on thousands of products you come in contact with each day. Powder coating protects the roughest, toughest machinery as well as the household items you depend on daily. It provides a more durable finish than liquid paints can offer, while still providing an attractive finish. Powder coated products are more resistant to diminished coating quality as a result of impact, moisture, chemicals, ultraviolet light, and other extreme weather conditions. In turn, this reduces the risk of scratches, chipping, abrasions, corrosion, fading, and other wear issues.
It’s tough. It looks great. And it lasts a long, long time. In addition to being durable, powder coating is an attractive choice due to environmental advantages.
What role does static electricity play in powder coating?
There is static electricity from more than one source when dealing with powder coating application. As you probably already know powder coatings are electrostatically charged typically by a corona mechanism near the tip of the business side (outlet) of the spray gun. This works by very high voltage (80,000 to 100,000 volts) being delivered to the electrode. This high voltage ionizes the air creating a field of electrons or negative charge. (Don’t worry the current is very small, typically micro-amps so the process is relatively safe.) The powder is pneumatically conveyed into this electrostatic field, picks up a charge then heads to the nearest ground. The object of the game is to make your parts the closest ground and then the powder deposits on the surface of said part.So lots of static electricity is generated by the spray system. It’s important to note that static electricity is generated elsewhere in the finishing system. The mere fact that powder particles are being conveyed into fluidizing hoppers, through powder pumps, hoses and spray guns creates static charges. This extraneous generation of charge affects powder deposition and can influence transfer efficiency.
Very characteristic defects are caused by electrostatics. Excessive charge can build up in a powder coating layer if the powder is applied too thick, too quickly. This phenomenon is known as back ionization. The build-up of charge causes powder particles to microscopically burst away from the substrate. This causes micro defects that resemble volcanoes. These defects cause a rumpled appearance that looks more like localized macro orange peel for lack of a better description.
Fisheyes are caused by a different mechanism and are more distinct in their appearance. Fisheyes are created by a significant differential in surface tension between a molten powder film and a contaminant. Common contaminants are lubricants and oils. Silicone lubes and penetrating oil (e.g. WD-40) are some of the worst actors. Fisheyes are gross defects characterized as deep circular voids in the coating film that reach the substrate. Craters are ambiguously defined as defects similar to fisheyes but smaller in diameter. Craters often do not reach the surface of the substrate and typically look like dimples.
As for “bits” I imagine you are referring to unmelted protrusions in the finish of the cured powder coating. These are not caused by static electricity. Common sources of “bits” are environmental dirt (oven, spray area, unclean application equipment, etc.), unclean substrate or dirty powder coating. They are typically more prominent at thin films (i.e. < 1.5 mils). If the preponderance of “bits’ decreases with thicker coating films then you can suspect either the powder or a contaminated substrate surface. If film thickness is not a factor then the contaminant is probably environmental and is deposited on the powder after it has been applied.
Regarding repair – fisheyes, craters and “bits” need to be buffed with an abrasive (Scotchbrite or 200 grit sandpaper), the surface wiped (e.g. acetone), dried and recoated. Alternately you can consider using a liquid paint based touch-up if the repair is small and localized. Be careful however as the touch-up may not meet the performance of the original powder coating finish.
Can you use a heat gun to cure powder coating?
Powder coating is an alternative to painting in which powdered, pigmented particles are given an electrostatic charge and sprayed over a grounded item using a special gun; the powder is then cured using heat. The two main types of powder used in powder coating are thermoplastic powders and thermosetting powders. Thermoplastic powders can be melted when reheated, whereas thermosetting powders can't be melted again once they're cured. In traditional wet painting, the coating "cures" as the liquid part of the paint evaporates. Since there's no liquid involved in a powder coating, it's considered dry paint. For these reasons, powder coating is also referred to as electrostatic powder painting and non-solvent painting.
The curing process for powder coating is normally done in a special oven; the coating has to be exposed to a temperature range of 350 to 400 degrees Fahrenheit (160 to 210 degrees Celsius) for 20 minutes. When melting the more common thermosetting powder, it bonds chemically to form a hard, permanent layer of paint. Thermoplastic powders harden, but if they are heated again, they become malleable and only harden again when they cool.
To cure powder coating, both the powder and the substrate have to reach the desired temperature and stay there for the allotted time without any temperature fluctuations. For this reason, the best way to cure powder coating is with a curing oven. However, a kitchen oven works just as well, as long as you don't plan to use it for cooking food ever again. For small items, a toaster oven is just as good. While some people may choose to use a heat gun to melt the powder so that it stays in place while the item is transferred to an oven, a heat gun can't produce the necessary stable, all-encompassing temperature required to cure powder coating.
The Different Types of Powder Coatings
Whether you’re new to powder coatings or an experienced pro, you’re probably well aware that powder coating is a fantastic painting method that creates a tough, durable and beautiful finish. No matter whether you’re using a single gun and small oven or a large fully automated line, the process is basically the same. A powder spray gun is used, which when triggered gives an electrostatic charge to the powder particles passing through it. As the parts to be coated are grounded, the powder is attracted towards the part via the electrostatic process. Once the powder is applied the parts are cured in an oven, which melts and cross-links the powder over the surface of the part and creates a tough, scratch resistant and beautiful finish.
There are literally thousands of different applications for powder coatings. Powder coating is used mainly on metals, so is perfect for industrial parts, medical devices, architectural applications, automotive refinishing, bicycle parts, household appliances, furniture, enclosures, trailers, lighting…the list goes on!
There are many different types of powders used, each with their own characteristics and applications, so making sure you have the right type of powder, and THEN choose the color is very important for a successful application. Check out the different types of powder and the benefits they will bring to the parts.
There are so many advantages to using powder coatings – too many to cover in this blog post – but some of the obvious ones include lack of solvents. This means no volatile organic compounds (VOCs) are released, a great advantage especially when compared to traditional liquid paints. This makes them safer to work with, dispose of and transport and brings unbeatable environmental advantages to you and your customer.
Powder coating allows for much thicker coatings than liquid paint, without running or sagging. With liquid paint, horizontal and vertical painted surfaces often have differences in appearances, but powder coating typically provides a uniform visual appearance regardless of orientation.
Powder coating allows for much thicker coatings than liquid paint without running or sagging. With liquid paint, horizontal and vertical painted surfaces often have differences in appearances, but powder coating typically provides a uniform visual appearance regardless of orientation.
Powder coating also offers a wide range of specialty effects that are hard to achieve through traditional methods. The list goes on, and we could get into better scratch resistance, toughness, and hardness than traditional liquid paints, but let’s take a look at some of the different powders used that help achieve these characteristics.
Why Use Powder Coat Paint Instead of Baked Enamel?
Most steel products manufactured by the Activar Construction Products Group are finished with a powder-coated paint, providing a more durable coating with a better finish than baked enamel, without the environmental concerns of a solvent paint process.
The Process: steel products are pre-treated to remove oils, metal oxides and welding scales. This pre-treatment both cleans and improves the bonding of the powder to the metal, eliminating the need for a primer. Powder coating is applied as a free-flowing dry powder to steel that is electrostatically charged to hold the powder to the substrate, providing excellent coverage on the inside and outside of corners. After application, the product is exposed to heat in an oven for a specific amount of time, allowing the powder to flow and thermally bond to the substrate, forming a durable hard finish.
Benefits of using powder coat paint technology include:
Powder coating emits zero or near zero volatile organic compounds (VOC’s).
Powder coating can produce much thicker coatings than conventional liquid coatings.
Powder coating overspray can be recycled and allows nearly 100% use of the coating material.
Paint application is typically very even with no runs or sags.
Powder coating is thermally bonded to the metal substrate, which provides superior adhesion and prevents chipping.
What about baked enamel?
The Process: steel products are pre-treated to remove oils, metal oxides and welding scales. This pre-treatment both cleans and improves the bonding of the paint to the metal, eliminating the need for a primer in some cases. The coating is paint in a solvent-based suspension which is applied to the steel by spray, brush or roller. After application, the product is exposed to heat in an oven for a specific amount of time, allowing the VOC’s to be eliminated and the paint to cure, and form a durable finish.
How does powder coating compare to baked enamel?
Enamel wet paint systems use a solvent carrier to convey the paint to the substrate. Elimination of the solvents during the curing process results in high levels of VOC’s and hazardous air pollutants. Powder coating produces no VOC’s.
When an enamel is applied in a wet form, the surface appearance often shows spray patterns or brush strokes due to the application method, or can pull away from edges as the paint cures. Powder coating produces a smoother finish and better edge coverage.
Paint thickness can vary and running and sagging are more noticeable. Powder coating has a more uniform application of paint than baked enamel.