Here is a synopsis of the top powder coating questions and answers we have been asked over the past 15 years about the industry
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Powder coating is increasingly accepted as the preferred finishing process for many applications. Increasingly stringent environmental regulations, rising costs in all areas, and demands by consumers for better quality and more durable products are among the challenges facing today’s finishers. Powder coatings provide a solution to these challenges and others. Powder coating is the technique of applying dry paint to the component.
The powdered paint is normally applied by using a powder feed system and gun to electrostatically charge and spray the powder onto the part. For some applications, the part being coated is dipped into a fluidized bed of powder. The coated part is then heated in an oven, or via infrared panels, to melt and cure the paint. During the curing process, a chemical cross-linking reaction is triggered and it is this chemical reaction that gives the powder coatings many of their desirable properties.
We know that powder coating can seem intimidating and overwhelming for the new powder coater. For you, we have compiled this section with the top questions asked by new powder coaters as well as information for setting up your new powder coating system.
The use of powder coatings vs. liquid solvent-based coatings results in significantly fewer emissions of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). They can also reduce energy consumption, can be more cost-effective, and provide better properties.
Powder coatings have been applied in the US since the mid-s when they were being used to coat pipes for corrosion protection and electric motor parts for insulation. These powders were applied using a fluidized bed into which a heated part was dipped. These coatings were functional coatings and did have the greatest appearance.
Thermoplastic powder coatings will melt and flow when heated but do not change chemically. They will be remelt when reheated. Thermoset powder coatings will melt and flow when heated and then chemically ‘crosslink’ or react. These will not be remelted when reheated and tend to have better physical properties than Thermoplastics. Most powder coatings are Thermoset.
It all depends on your application. Most chemistries can be custom formulated to meet your specific needs. For most applications, the best bet is a TGIC Polyester, these can be used on parts that are outside and have good physical properties. For interior applications, a Hybrid powder coating is cost-effective and has good properties.
An Epoxy powder coating has great properties and excellent corrosion and chemical resistance but is also only for interior applications. A Urethane Polyester has good flow and physical properties, is for exterior applications but can be expensive, and has a higher oven temp requirement than TGIC coatings. When in doubt, go with a TGIC. Remember, never use Epoxy or Hybrid for outdoor applications. They will fade.
Although Porcelain is applied with similar equipment as standard powder coatings, it is very different. Porcelain powder is called Frit and is much more expensive a process that requires an oven temperature of 1,200 F to fuse the Frit (powdered glass) to the metal surface. Sinks, bathtubs, and some appliance parts are coated with Frit.
Most coatings need some degree of pretreatment in order to achieve acceptable coating properties and appearance. With powder coatings the pretreatment is critical. Pretreatment refers to cleaning parts and leaving a ‘conversion coating’ for the paint to adhere to. At a minimum, parts need to be clean and dry with no residual oils or chemicals on the parts.
Depending on the final use of the part additional processing may be required before powdercoating. Interior applications, noncorrosive environments ( chairs, furniture, decorative finishing): For these applications, you may be able to get by simply degreasing the parts. The finish may remain for a number of years, or come off in sheets after a few months. Interior applications, corrosive or heavy use environments (washers, auto under hood, equipment) You must at a minimum, degrease and apply an iron phosphate pretreatment.
A one-step cleaner/phosphatizer may work for basic applications. Exterior applications or highly corrosive environments. Do not even consider simply degreasing the part. The coating will fail, sooner than later. A minimum of a three-stage pretreatment is required. Your customers will eventually be very disappointed if you do not do this
Powder coatings are generally applied via an electrostatic spray gun. Compressed air moves the powder coating through the feed tube from the box, drum, or hopper to the gun. It is possible to move the powder with air because the air ‘fluidizes’ the powder making it act like a liquid. The gun then applies a charge to the powder particle by the use of a high-voltage, low amperage electrostatic field which it generates between the gun electrode and the grounded part.
This charge is usually negative. There are guns available that do not use an electrode to generate the charge, they use a special insulator/conductor inside the gun which generates a charge through the use of friction. This is a Tribo gun and it generates a positive charge.
Most guns used are Corona guns. The powder sticks to the part the same way a magnet sticks to metal or lint to a static-charged shirt. This charge will diminish or decay over a few hours. Most parts enter an oven as soon as they are powder coated. In the oven, the powder melts, flows, and crosslinks.
The main function of the spray booth is to safely contain the powder coating so that the overspray generated does not go into other areas of the plant. You must ensure that the airflow through the booth is sufficient to contain all the overspray into the recovery chamber without disrupting the powder depositing on the part. A general rule of thumb is to maintain a face velocity of 100 ft/min across all openings.
Since there is no solvent in the coating, the exhaust air can be safely circulated back into the plant. The powder can be reused by simply blending the overspray with virgin, or new, powder. The reclaimed powder is of value only if it is free of contamination from other coatings. Color changes in a reclaim-type booth can be simplified by the use of separate recovery chambers.
The main types of ovens used for baking, or curing, powder-coated parts are Convection, infrared, or a combination of the two. Convection ovens are gas or electric, which heat air and circulate the heated air in the oven. Infrared ovens are also gas or electric and emit radiation energy in the IR wavelength band. This energy is absorbed and causes the powder to cure.
Transfer efficiency is defined as the ratio of powder applied to the parts to the total amount of powder sprayed. Reclaimed, or recovered powder is not considered in this calculation.
Factors that affect transfer efficiency include the operator skill, shape of the part, booth design, powder quality, spray techniques, grounding of the parts, and condition of the equipment. The high transfer efficiency rates and ability to reclaim overspray contribute to the large economic advantage powder holds over liquid
Powder-coated parts are routinely recoated for a variety of reasons. To recoat a part with the same product it was coated with you usually can simply rehang the part, turn the gun voltage down, respray it and cure as needed. Certain chemistry powders cannot be recoated, such as high chemical resistant epoxy powders. These products need to be lightly sanded to obtain the proper level of adhesion.
Powder coatings can be stripped with Mechanical methods, Thermal/Bake methods, Chemical methods. The method used will depend on the speed, degree of stripping, effect on the parts, and environmental concerns. Mechanical methods included sanding or blast stripping the powder coating with abrasives or fluidized aluminum oxide (sand).
Thermal methods may adversely affect the hardness of the parts stripped due to the operating temperatures required. This is however often used for noncritical parts. Most of these systems are batch-type systems in which the oven is ramped up to 750 F and the organics in the coatings are volatilized.
These ‘VOCs’ are usually destroyed by an afterburner in the exhaust. Chemical methods included hot or cold chemical solutions of aqueous caustic or non-caustic alkalis. Most solvent systems such as Methylene Chloride are no longer viable due to environmental and health concerns.
Properly cured powder coatings are resistant to most chemicals, however, DO NOT allow MEK or Trichloroethane to come in contact with the powder as it will attack the surface. You can wax powder-coated pieces if you want to but it’s not necessary.
Whenever possible, use washers beneath both nuts and bolt heads. Excess powder can be removed with either an oiled tap/die for threads or with a die grinder affixed with an abrasive wheel or cone. Unless using a high heat powder, do not expose cured powder coatings to temperatures over 400 degrees for an extended period of time. Use care in packaging powder-coated parts, they are durable but do not like cardboard packing.
The first issue is to consider the compatibility of the two powder coatings. A compatibility test of the two powders can determine if problems will result when the two powders are mixed. Generally your powder coating supplier can run this test for you. The test consists of blending the two powders in ratios of 90:10, 50:50 and 10:90, then spraying a test panel with each blend. If the two powders are compatible (compatible is when minimal gloss change and no craters occur) the change over can be rather simple:
We have put together a complete maintenance checklist for your use. This is available at the end of our manual.
The inadequate ground is the most frequent problem encountered in powder coating systems. A sufficient ground path must be provided for all equipment associated with powder applications for safety and efficiency reasons
Safety Reasons: Grounding is necessary to carry the charge away from the application equipment to prevent static discharge or sparks which provide an ignition source and may lead to a dust explosion.
This is one of the most serious safety hazards associated with powder coating. All automatic powder systems must be equipped with spark detection equipment to shut down the spray guns in the event of a discharge. The grounding of the operators helps to prevent shocks as well as sparks. Conductive shoe soles and grounding straps can help prevent shocks from static build-up in operators.
Efficiency reasons for grounding. Good ground is required to carry away charge which is delivered to the parts by the powder or through ionized air. This allows more powder to be deposited and better penetration into Faraday areas. The main reason is that good ground improves transfer efficiency.
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As powder builds up on the surface of the part, charge separation begins to occur in the part. That is, in the case of a negatively charged powder, positive ions in the part flow toward the powder coated surface and electrons carrying a negative charge flow to ground. In order to continue powder attraction to the part the ground circuit must stay intact. Thus ground maximizes the powder charge holding capacity of the part.
What causes poor ground? Most ground problems are the result of poor design or inadequate maintenance. For some powder booths, there may not be enough air the movement to capture the over-sprayed powder. This powder may drift out of the booth and settle on the conveyor or load parts. The powder is a very effective electrical insulator and it will interfere with the ground if deposited at contact points.
This is especially true if coated hooks or hangers go through the oven and the powder is cured in place. An important design feature involves hook/hanger configuration. S-shaped hooks may provide only a single contact point and allow the part to change position. V-shaped hooks, for example, can offer at least two contact points and hold the part more firmly in place to prevent undercoating with powder. When possible, the hooks/hangers should be designed to shield the contact point from the coating.
Hooks made from square stock should be turned on an angle to have a sharp contact point. It is also important to maintain metal-to-metal contact at all points. This is usually done by cleaning the hooks/hangers, via burn-off, chemical stripping or blasting. For the following burn-off, it is also important to insure that any residue or ash is removed. This ash may also be an electric insulator and as such it will interfere with the ground.
To prevent powder build-up on hooks/hangers ensure they are fully loaded to prevent empty contact points from being coated. Maintenance of proper gun placement and aiming as well as booth capture air flows will also prevent a build-up of powder on hooks/hangers. Cleaning excess powder off of the conveyer is also a good practice. Ensure that your gun control panels and booths are properly grounded using ground cables instead of depending on connections to the control cabinet.
Testing for sufficient ground is preferably done with a Megohm meter or megger. It is important to follow the entire ground path. For parts, it is best to start with the conveyer and check every link in the chain to the part (from the conveyor to load bar, load bar to the rack, rack to hook, hook to part). This will tell you if and where any problems may exist.
The NFPA Bulletin 33 states that less than one megohm is needed to minimize the chance of sparks, for efficient coating resistance of 0.5 megohms or less is best While testing, if a wire lead is attached to the ground and directly to a clean part, the effect of good ground can actually be seen by coating the part with this connection in place. If the noted improvement in the deposition/thickness or penetration is dramatic then the benefits of good ground become more clear.
If the improvement is slight, but resistance readings are one megohm or higher, chances are still good that better ground will improve harder-to-measure characteristics such as transfer efficiency and consistency.
Obtaining good ground:
A good ground starts with the earth connection. Using water pipes may provide you with reasonable success. The optimum ground, however, is achieved by driving a copper rod into the soil to a depth of 8 to 10 feet. This should then be connected to the conveyer at multiple points for insured good contact. Minimizing connections between the conveyer and the part can be very helpful in optimizing ground.
A solid rod has fewer contact points to worry about than a chain of multiple connections. The addition of a well-grounded rub bar or brush above the booth helps to insure contact with fixtures in the event that the conveyer is not providing adequate ground due to a build-up of lubricants and contaminants. Modify hook designs to encourage contacts and frequently clean hooks
Powder coatings are frequently used as basecoats or primers for liquids. The reasons for this can be:
It is sometimes needed to touch up areas left by hangers or hooks or to repair damage resulting from abuse of the coating during handling or assembly. Most room temperature cured epoxy coatings can be successfully used to touch up the powder with excellent results. These epoxy coatings generally consist of two parts: A base and a hardener which you mix together before using. You can spray or paint this on.
To powder coat parts you will need the following:
The purpose of application rooms, or environmental rooms, are multi-purpose rooms designed to isolate the application process from the harsh, dirty plant environment as well as to provide consistent temperature and humidity control to provide the optimum application efficiency of the process. In addition to Federal and local fire codes (which typically include a 1-2 hour fire rating) incorporate the following ideas into your design:
Presuming your system has been properly set up, which for a hobby coater is as simple as:
The basic rule of thumb is that you can powder coat anything which you can bake in an oven. Current powder technology allows a minimum cure cycle of 15 minutes at 250F. This means you can powder coat:
If you’re running a successful powder coating shop that uses “batch” powder coating equipment, you need to move parts quickly and efficiently between the various stages of the coating process: loading, parts cleaning and preparation, powder application, powder curing, cool-down, and unloading. Well-designed parts racks can make the problem much easier to tackle. A parts rack (also called a parts hanging rack or a parts cart) is a sturdy metal rack on wheels. Parts are typically hung from the rack using slender metal hooks. Once they have been hung in place, the parts can easily be moved from one stage of the powder coating process to the next.
To maximize your shop’s efficiency, is important that you build or buy good quality powder coating parts racks that are suitable for the way your shop is laid out. Unless you have experience working with a particular type of rack and know exactly what you want, you may need to start out by testing just one rack at a time and refining the design as you go. If you already have orders that need to be filled and you want to hit the ground running with your new powder coating equipment, then building multiple racks that can be stationed at every stage of the coating process might be the best solution.
Whether you buy or build racks—one rack at a time or in multiples—these 10 tips will help assure you get the right racks for the job:
Try to use only 6” or larger wheels. The rack’s wheels will pick up powder overspray from the floor. Small wheels clog up faster with powder build-up and require more frequent maintenance. Because of the increased space between the wheel and the wheel’s mounting plate, large wheels don’t clog as fast and are easier to clean. Smaller wheels also tend to be harder to steer when the rack is loaded with parts. It’s no fun fighting with a set of wheels that won’t turn smoothly or that wander back and forth under heavy loads. To prevent the rack from constantly trying to spin as you move it, mount wheels that turn only on one end of the rack and attach wheels that track straight on the other end.
A T-shaped rack design prevents parts from accidently bumping into the frame of the rack. It also keeps the frame out of the way of the operator trying to spray the parts. To prevent fatigue, joints at 90° angles need to be cross braced. It will see plenty of use, so make sure your rack has only good welds. If you are spraying very heavy parts, you may need a square cage type rack, but try to design a T-rack whenever practical, even if you have to use much heavier material for the rack itself.
The “ideal” spraying zone for manual powder application is somewhat limited in size. Loading parts into this zone whenever practical will assure good throughput, high powder transfer efficiency, and consistent results. While standing on the floor, an operator can typically spray in a rectangular area that extends from about 2’ off the floor up to about 6’ from the floor. He can do this comfortably and with good visibility of the powder on the part. Side to side, the operator can spray in sections of about 4’ before having to change foot position. Imagine a 4’ x 4’ active coating area in front of the operator where he can do his best work without using a ladder, kneeling down, or having to move his feet side to side. Hanging parts in this zone will net optimum results. For longer parts, imagine another 4’ X 4’ area beside the first one, creating a 4’ X 8’ ideal coating zone. The operator can cover this entire zone by slightly changing his foot position as he applies powder. This approach to racking and coating parts will provide optimal throughput and prevent operator fatigue. There may be times when a part must be hung with the bottom close to the floor or the top surface near the ceiling of the booth. These parts can still be coated, but the amount of time it takes to apply an acceptable finish will be much longer than with parts that are hung so that it is easy for the operator to coat them without climbing a ladder or having to kneel down.
Parts hung near the floor may need extra dwell time to cure adequately. The temperature at the top of the oven may also be a bit hotter than average, especially on warm days where the oven sees non-stop use. You need to be on the lookout for color consistency issues or other signs of overbaking if you have parts near the ceiling of the oven. With parts that are so large they extend both above and below the ideal curing zone, you will probably have to tweak your curing routine to get top-quality results.
It should be obvious what will and won’t fit in your booth and oven, but sometimes people don’t consider what will actually work best with their shop layout. It’s important to think about the turning radius of the rack. More than one business has fabricated their own racks without testing them, only to find out the racks couldn’t turn sharply enough to move from appliance to appliance because the shop was so crowded.
We’ve also encountered shops where the racks were based on the equipment’s internal dimensions and ended up having to be cut down to work properly. Don’t get carried away trying to fill every inch of your appliances when you build or order racks.
The rack will need to be at least 1’ away from all surfaces inside the oven. If not, your operators will spend too much time trying to avoid bumping parts into the walls or discharge ductwork as they move the rack in and out of the oven.
Keeping the rack smaller than the oven is also a good idea because you want to avoid jamming parts against the wall or floor-mounted ductwork (or ceiling in ovens with overhead discharge of heated air). You risk having the powder blown off the parts or impairing the performance of the oven’s recirculation or exhaust fans, which can cause the oven to shut down. Your rack will also need to allow 2’-3’ or more of working space inside the powder booth. If the rack fills the booth, there’s no place for the operator to work. Multiple small racks give you more flexibility when coating and can be stacked into the oven side by side or one in front of the other when you want to do several parts in a single cure cycle.
Don’t rely on your cart’s wheels for grounding. Weld a beefy grounding tab to the bottom section of your cart. You may want to also weld a small shield a few inches above the tab to prevent falling powder from quickly coating it. This tab is where a grounding clamp will attach. When planning your grounding tab, remember it will need to be easy to clean because powder overspray will build up on it over time. Having a grounding tab on your cart only makes sense if you have a good grounding point in your booth. Don’t count on the wimpy ground that is integrated into your powder gun and don’t hook your ground strap to the wall of your booth. Install a grounding rod near the wall of your booth. Install two or more in a large booth so your operators don’t have to deal with dragging around a long ground strap. If possible, use an 8’ long copper rod, as opposed to a shorter one or one made from a cheaper material. Use 14-gauge or heavier wire and a stout spring-loaded clip to make your ground strap. Permanently attach the ground strap to the grounding rod and use the clip to attach to your cart’s tab. This grounding method will improve powder transfer efficiency, saving you time and money. Remember to keep your racks clean. Don’t saddle operators with racks that are so badly caked with baked-on powder that the parts can’t be properly grounded. Good rack maintenance practices will improve throughput and reduce powder waste.
This isn’t a design tip, but it is just as valuable. One of the most frustrating things we encounter when talking to a customer who needs more throughput from their batch powder coating shop is a rack shortage. Racks are one of the cheapest components you can get for your powder coating process, yet smaller shops sometimes treat them like gold. In fact, one of the biggest differences we see between shops that are thriving and those that seem to be struggling with throughput is the way they go about racking and staging their parts.
Successful shops don’t let themselves get delayed by having their operators waiting around for racks. Once you build or buy a rack that works well for your shop, get enough of them to let your business thrive!
Spending time to design and build racks that work well in your shop environment will increase your productivity and save you from headaches down the road. If you have experienced operators, get feedback from them, especially workers who have a good bit of powder spraying experience. They can add valuable input and you can learn about the challenges they’re facing by working alongside them as they test new rack designs.
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