Powder Coating Process
Powder Coating Process
Powder Coating is just Thick Paint, right? Powder coating and painting are two entirely different things. Painting is a method of depositing pigment and binder directly onto a part. The wet solvent that carried it there evaporates away. Powder coating involves encapsulating a part in a thin plastic shell. The plastic is pigmented and nothing evaporates away. Both change the color of something so it’s common to interchange them freely, but they are completely different compositions and the powder coating process is intriguingly unique. Remember as a kid when you rubbed a balloon on your head until your hair stuck to it and then it clung to whatever you placed it on – the wall, the ceiling, your cat… That’s because you literally rubbed some of the electrons off the balloon (you removed some of the negative charges) and it became positively charged as a result.
Why are we talking about static cling?This is the operational principle behind powder coating. Powder coating uses an electrostatic charge to attract powdered bits of plastic to a conductive metal part. Once the part is covered, it’s heated to fuse the powdered particles together and onto the surface of the part. The result is a smooth, thin, non-porous plastic coating that is very durable.
What is the Process:Successful powder coating contains three main steps: 1. Prep 2. Powder 3. Cure
PrepIf the surfaces of the parts aren’t in the best condition, you won’t get the best result. The best condition is dry, bare metal. While the plastic powder will tend to stick to any conductive metal part fairly well through static forces, it may not bond to the surface well when heated. You want to clean the surface of parts to be powder coated to remove any foreign debris or rust. Media blasting with sand, steel shot or other media is a common method for doing this effectively.
PowderThe finely powdered pigmented plastic is applied to the part electrostatically. This is accomplished by creating a large electrostatic charge imbalance between atomized powder and the part using a powder spray gun. The part side of the equation is bonded to an electrical ground using conductive hooks or clamps, metal hanging rods, cables, and wires. In the vast majority of cases, if it doesn’t already exist, a small hole is made available on the part with which to hang it by a small “S” shaped hook. Multiple hook holes may be required if part dimensions are exceedingly long in at least one dimension. Long parts may also be prohibited from going through an automated powder coat line. Such systems move parts on a continuous overhead conveyor through different zones for prep, powder, and curing steps to occur. Any layout of this conveyor other than a long straight line might have restrictions when turning long parts through corners. The area used for applying the powder is usually at least partially enclosed by a cabinet, booth, or room where fans and filters draw excess powder out of the air to prevent it from floating around and covering the whole shop. Or from choking everyone who decides they want to breathe within a 2 block radius. You’re essentially spraying plastic dust around when you’re powder coating. None of this is desirable except as a means to stick the powder to your part, so a basic cabinet and particle filters are standard practice. On the powder side of the equation, the powder spray gun does a couple of things. First, it uses a very small amount of air pressure (usually just 4-7psi) to stir up the powder and get it moving. The powder is kept dry (VERY important) and sits in a reservoir either attached directly to the gun in small manual systems or remotely connected through a tube in larger systems. Pressurized air is used to stir up the powder into an atomized state so it can flow in suspension more like a liquid. The atomized powder is then directed through the gun where the second important function happens. It gets a HUGE positive charge. Either by friction – like the balloon on the head trick (tribo gun), or from a high voltage electrode (corona gun). As it exits the gun, the atomized powder then has a positive charge between 15,000-100,000 volts and you had better believe it wants to be neutralized. It sticks to the nearest source of electrons, the closest grounded object around – which just so happens to be your part. It’s probably important to talk about a little additional detail here. This is the part where the design of the part, the skill of the powder coater and the capabilities of their equipment come into play. Fundamentally, the issue is the very strong yet very invisible electrostatic field. If you’ve ever seen iron filings arrayed around a magnet, you can get an idea of how to visualize this field. A magnet has field lines that curve from its north pole to its south.The field lines in powder coating flow between the gun and the part. The geometry of the part and location of the gun at any time will shape these field lines. Some part geometry makes it difficult for the powder to coat certain areas because of the Faraday cage effect. This is where the shape of the field is bent in a way that blocks the flow of electrical charge even though it is open to light, air and everything else. Deep, shallow cervices and enclosed spaces with small openings or sharp return bends are some such challenging shapes to powder coat.If powder coating is a design requirement, good design practices will work to minimize such troublesome areas. Where this cannot be fully eliminated, the powder coating technician will need to adapt the voltage and air pressure settings of the gun, and maybe use other spray distances or physical blocking methods to ensure proper powder coverage of these areas. With a lower voltage and higher flow rate of powder, ballistic forces can overcome electrostatic fields. Doing this properly though requires an experienced technician who is quite familiar with their equipment. Even if the difficult recesses do receive a coating of powder, it can be easy to make surrounding areas overly thick. Sometimes, you don’t want powder everywhere though… Tapped holes, studs, critical mating part surfaces, and areas for electrical conduction will need to be masked from powder. Since the powder coating process involves high heat, masking materials must be made to withstand the ovens. Common materials here are silicone thread protectors, hole plugs, and high-temperature paint masking tape. Once the powder is applied, it will only stick to the part for a relatively short time and will not withstand wind, shocks or vibration. The electrostatic force is weak, so it doesn’t take much to dislodge the powder. Immediately after covering the part with powder, it has to be moved on to the next phase.
CuringThe part now needs to be heated to melt and fuse the plastic powder particles to each other and to the surface of the part. Curing normally involves a brief flash stage at a higher temperature (around 400 °F) followed by a more moderate bake temperature (around 250 °F) for a longer period. As long as the prep has been done properly, the cure will be successful. Curing is done in industrial ovens. The temperatures are not excessive, but the size and construction are. While a proper cure could possibly be obtained in a home convection oven right alongside a tray of grandma’s chocolate chip cookies, powder curing batch ovens are built more like a garden shed. Large doors swing open to allow a wheeled rack of hanging parts to be smoothly rolled inside along an unbroken concrete floor surface. Since the temperatures required are not too excessive, the walls can be safely insulated and typical concrete floors can safely dissipate the excess heat across a short distance. Automated powder coating lines, mentioned earlier, carry parts through zones by a continuously moving overhead conveyor. The curing oven zones in these systems are like oven tunnels open at both ends. In both types of curing oven systems, heated air is provided by gas-fired burners or high-powered infrared lamps are distributed along the inside.
ExceptionsThe entire time, we’ve covered the powder coating of conductive metal parts and electrostatic charge causing the powder to cling. There are a few other process variations that can be used to powder coat non-metals. The simplest is ceramics. Ceramics and glass parts can handle the heat, but they’re not conductive. To powder coat these materials, the parts are first pre-heated and then the powder is applied while the part is hot. This partially fuses the atomized plastic powder on contact to stick it to the part until it can later be fully cured in the oven. Surprisingly, some wood can also be powder coated. The furniture industry has widely adopted a method of protecting low-cost MDF board (a partially synthetic wood product) with powder coating finishes. MDF can handle elevated heat, but not quite as high as is needed for typical metal or ceramic finishes. For these parts, a conductive spray is applied to the boards first to make them at least partially conductive, and they are preheated slightly in an effort to aid in the electrostatic charge and possible fusing of the plastic powder. After the powder is sprayed on then, the curing takes place at lower temperatures, or specialized plastics are cured with UV light. All conditions for powder coating wood require application-specific powders that differ from the typical electrostatic metal processes.
Hey, It’s Green!The powder coating process is the most environmentally respectful surface finishing processes when compared to painting, plating, anodizing and the like. Oven systems are on par with wet solvent-based paint systems, but air-polluting VOCs are not a concern since no solvents are used. Not only are these irritating VOCs not created, but the high-volume filtration and exhaust systems required by other systems are not necessary, so energy costs and related effects are reduced. Only low-flow air systems are needed for the powder spray areas. Also, excess powder overspray can be collected and reused. If its kept dry and not mixed with other powders when colors are changed, excess powder that did not cling to the parts can be put right back in the hopper to spray another day.
Finish AttributesA powder coat finish can give all the cosmetic effects as wet solvent-based paint. Even including clear coats, metallics, and candy-apple finishes! The nature of the coating though is much more durable. Powder-coated parts have far superior corrosion resistance to painted parts; however, if a chip or scratch occurs that exposes the bare metal to the elements, corrosion will still work its way out beyond that area under the powder coating and damage the surface beneath. This process can also modify the surface texture. Applied thinly, the powder will closely resemble the underlying metal surface with all its irregularities. A bit thicker and the powder can tend to smooth out some of those small bumps and scratches. This can be a great benefit for cosmetic parts. Applied heavily and the resultant powder will have a dimpled look called “orange peel” because it looks just like uh… some kind of fruit – can’t remember. Anyway, this effect can be desired to hide even larger surface blemishes or give the part a rugged look. Thicker is not necessarily better for corrosion protection though – in fact, the opposite can be truer. Provided the part has been prepped properly, coated for an unbroken surface, and protected from scratches or chipping damage, it will last a good long time without ill effects from corrosion. Thicker doesn’t improve on this property, it actually makes it more brittle = easier to chip, less durable.
SummaryThe powder coating process is an excellent option for all sorts of metal parts. You won’t be limited in your desired color or luster for your finish as there is an extensive variety of powders available. While some shapes can be a bit tricky, a good powder coater will be able to handle what is needed. In the end, you’ll have a great looking part with good surface durability and corrosion resistance – far better than paint. If this is a process you’re considering as a core part of your manufacturing line, you can benefit from the positive ecological and economic attributes. The EPA even publicly and specifically encourages powder coating systems over other surface finishing options. That’s one less headache for you and a better environment for your employees. If you haven’t done so already, consider powder coat finishes on your new product designs. It’s a great way to get a durable and great looking finish.
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