Powder coatings can provide improved performance over liquids when applied to a properly pretreated part. Solvent-borne paints are usually more forgiving of organic soils left on the work piece by sub-par cleaning. Because powder does not have solvents, you need to make sure the washer does a good cleaning job. This is just good operational practice and is not an unusual requirement. Iron phosphate is the most frequently encountered pretreatment used with powder coatings. However, if the highest level of performance is required, zinc phosphate will work admirably with powder as well.
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There is no stock answer for this question. Many factors go into deciding which cleaner is the best for your application. Base metal of the work piece, soil being removed, bath temperature, contact time, oil removal capabilities, environmental considerations, etc. must be taken into account when deciding. The bottom line is that you need to get your parts clean, safely, so the steps following in the pretreatment process will function properly. Check with your pretreatment supplier to determine your best course of action.
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There are powdered and liquid cleaners. There are acid cleaners that may be best for inorganic (metallic) soils. Alkaline cleaners are often recommended for organic soils. Neutral cleaners may be used to remove soils on substrates that react with strong acids or alkalis (aluminum or zinc). Oil splitting cleaners may be desirable where high levels of oil build quickly in the process bath. Work with your pretreatment supplier to determine which cleaners are best for your application.
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There are many tests to determine cleanliness. The most widely used is the "water break free" test. This test is a visual observation of whether water fully sheets over the clean part or draws away from portions of the surface (like water on a waxed car). Other tests may include wiping with a white cloth, alcohol drop tests, or other more sophisticated laboratory testing like coulometry (organic soil is burned off the substrate and measured). Again, your pretreatment supplier can assist with evaluating your part cleanliness.
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The generic term "phosphating" is a process where an acid attacks the metal of the work piece and re-deposits a material that is a combination of the metal substrate (and other metals - like zinc) along with phosphate. This process creates a surface that is tightly adherent to the base metal, has more surface area, provides improved corrosion inhibition, and helps the powder coating stick better. It provides a good coating base so the finished part has increased usable life.
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Iron phosphate is the most frequently used pretreatment for powder coating. It can be used with almost any substrate. Powder coatings will often give improved performance compared to liquid paint over the same pretreatment. For "high end" applications, zinc phosphate may be the pretreatment of choice. However, this process is more costly to operate, uses heavy metals, and can produce considerable sludge. The washer typically requires added stages versus an iron phosphate washer. If you cannot achieve the desired performance with iron phosphate, zinc phosphate or an improved paint system may be your only choice.
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Troubleshooting a process line can be complicated. First determine if anything has changed in the incoming work piece. This would include the metal itself as well as the incoming soil. Then determine if anything has changed in the wash process. Concentrate heavily on the cleaning stage. Many of the downstream problems can be traced back to the efficiency of cleaning. Look for the proper concentrations, nozzle performance or alignment, temperatures, and bath cleanliness. You can often see visual signs of cleaning efficiency at the exit of the rinse stage following the cleaner. If cleaning is good, verify that the pretreatment coating is still up to snuff. Often there are visual changes if the pretreatment varies. This may show up as a color difference, blotchy appearance, etc. Keep in mind that the variation of the pretreatment may be caused by poor cleaning. Lastly, check the quality of the rinse water. The coating is only as good as the last liquid the part sees. Contaminated rinses can leave salty deposits that will dramatically reduce Salt Spray performance and may also affect coating adhesion.
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Generally, oil and grease, metals (zinc from the substrate, or the zinc phosphate bath), and pH are the concerns from an environmental reporting standpoint. However, sludge or contaminant buildup in the cleaner and phosphate stages may force regular cleanout of these stages. The waste treatment process is usually a simple pH adjustment and flocculation. However, consult your treatment chemical or waste equipment supplier to assure acceptable results.
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Possible Causes:
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Possible Causes:
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As virgin powder is applied, the larger particles more readily stick to the part. This leaves the over sprayed powder with a lower average particle size. As this occurs during each cycle through the system, the recovered material may become laden with fine particles. Eventually, the reclaimed material will be nearly unmanageable in both fluidization and application.
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Friction may cause powder particles to fuse along the walls of the powder hose and other components in the powder delivery path. Impact fusion is the combining of these powder particles to form a solid mass during the delivery and application process.
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The fluidized material should take on fluid-like characteristics. It should have an appearance similar to that of water simmering on low heat. The material should not be boiling hard.
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Electrode-rinsing air is a feature that provides circumferential air that keeps the electrode free of powder build-up to ensure consistent maximum charging efficiency of the gun.
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Zone triggering typically relates to the guns automatically spraying when a photo eye or a light curtain is used to determine the vertical zone is broken or that a part is present in that particular zone (the part opening may be broken into multiple zones).
Style triggering generally uses multiple means of sensing in order to identify the part type or style. The guns then trigger on or off based on a recipe or pre-set parameters pre-programmed by the user.
The fundamental difference between zone and style is that zone formats do not care what the part is, since they paint by size. Style must recognize each part or family in order to know how to paint.
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Uncoated metal parts, clean hangers, and clean conveyors have very little resistance to ground and are excellent electrical conductors. What can degrade this ideal path to ground is powder build-up on the contact points of the hangers or conveyor. Also the conveyor may have swivels or indexing assemblies that, when contaminated, can impede grounding. All of these in total must not exceed one megohm resistance to ground. A Megger (an ohm-meter that has a megohm scale), which has a power source of 500 volts or higher, is typically used to measure continuity to ground.
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Practically all powder coatings are not hazardous waste by definition of the Resources Conservation and Recovery Act regulations (RCRA). Disposal methods for waste powder are the same as for non-hazardous wastes, in most states. However, there may be some exceptions and your powder supplier should be contacted regarding proper disposal. Always renew disposal needs with the local authority having jurisdiction over your facility location.
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Powder storage for many months is often normal practice. Ideally, powder should be stored in the optimum conditions of less than 80°F and approximately 50 - 60% relative humidity. Under these conditions most powder should be readily usable for at least a year. Avoid placing powder inventory in close proximity to any heat source such as an oven, washer, furnace, space heater, etc. Powder packaging is designed to protect the contents from compression that can result in lumps that are not readily broken. Do not stack packages that are broken or collapsed.
Some powders with special appearance, performance, or cure properties may have greater sensitivity to storage conditions, and should receive greater attention when stored to ensure the desired results.
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How do I get good powder coverage into Faraday areas? Faraday cage effect defines a condition that occurs when parts are coated that have recesses, inside corners, channels, or protrusions on their surfaces. The Faraday cage is the area of the part where the external electrical field does not penetrate. Powder coating of recessed areas is often complicated. For the successful coating of Faraday areas, the following conditions must be met: powder has to be well charged, airflow must be sufficient to deliver the powder inside a recess but not excessive to preclude powder deposition, and the external electric field must be controlled to reduce the "push" for powder particles to deposit on the edges of a Faraday cage. Because most Faraday areas involve some type of "pocket " areas, it is important that the method for directing the powder flow in allows for displacement of the air in the pocket area to allow penetration.
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There are two conditions that must be met to achieve proper cure of a powder coating. The first is temperature, referred to as metal temperature, and the second is time. The time/temperature requirements of a particular powder material must be achieved to obtain a full cure. A cured thermoset powder coating will not re-melt upon further heating. To ensure proper time and temperature it is often best to test using a Datapaq or similar device to provide actual performance data.
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Particle size plays an important role in the application process. Size distribution affects many other properties critical to consistent performance including: fluidization, powder transport in hoses, uniform cloud density, powder movement in air, electrostatic charging, deposition and build rate, transfer efficiency, Faraday cage penetration, wraparound, and film smoothness. Size distribution produced by the powder manufacturer must take the coating system design and anticipated changes into account to provide a suitable product. Final cured surface appearance may also be affected by the particle size distribution and flow rate.
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One of the most important considerations in defining the end use performance is weatherability. Outdoor exposure results in absorption of ultraviolet energy. This energy can attack the organic binder and result in gloss loss and color change. Due to a tendency to chalk, epoxies and epoxy containing hybrids are generally not recommended for outdoor use when aesthetics is a primary concern. Polyesters and acrylics on the other hand provide excellent UV light stability and typically find use in architectural, automotive, lawn and garden as well as outdoor furniture markets.
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Adhesion is very dependent on the cleanliness of the substrate, the appropriate selection and maintenance of pretreatment, and the proper melt and cure of the powder coating. Adhesion may be tested by several different methods. A commonly used technique is the cross hatch and tape method. In this method, crossed cuts are made through the coating. Adhesive tape is then applied to the cut coating and then removed quickly. The coating adhesion is then measured by its ability to remain on the substrate and to not be removed by the tape.
Powder coatings vary in their ability to provide intercoat adhesion when recoated. Epoxy powders tend to be harder and more prone to intercoat adhesion problems, particularly if they have been over baked. On the other hand, polyesters and epoxy-polyester hybrids usually recoat well. Chemistry for recoat is also important and should be tested with your powder supplier.
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The selection of a coating should be undertaken as a cooperative effort between the end user and the powder formulator (supplier). In developing a coating for a particular application, the formulator must consider a number of factors. As an example, if the coating requires high pencil hardness and mar resistance along with weatherability, then flexibility may be reduced. If superior chemical resistance is required, then weatherability and over bake yellowing resistance can be compromised. Other considerations may be cure time and temperature, substrate type and treatment, and application method. Only when all requirements of a particular application are considered can a formulator recommend the proper coating type.
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The oven cycle time is comprised of the bring-up time plus the dwell time for a proper powder cure. The bring-up time is the time required to attain the desired substrate cure temperature of the part. The dwell time is the time required to hold the substrate at cure temperature. These times and temperatures are available from the cure schedule for the powder coating. In some applications, shorter oven cycle times are possible by rapid heating of the substrate to a higher cure temperature for a shorter dwell time.
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Next to proper grounding, clean compressed air is the most important thing. Oil and water do not mix with powder. In a powder system, air plays a big role. Powder is fluidized with air, pumped to the guns with air, and the cartridge filters are back-pulsed with air. The equipment and powder manufacturer should be consulted regarding the air quality for optimal performance. In general, a minimum 35°F dew point and less than 0.1 ppm contaminate or oil is a good starting point.
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Always consult your compressor manufacturer. You should have good oil and water extractors along with a refrigerant or a desiccant dryer system. With either system type selection, an oil coalescing filter with automatic drain and particulate filter are recommended.
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Drain extractors daily and check internal filters monthly. Replace desiccant beads when indicated and have your refrigerant dryers serviced once a year. Depending on size and volume, an automatic indicator may be used to ensure proper performance. Again, always consult the manufacturer's manual.
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Place the dryer as close to the room as possible, but not in the powder room. This will give the compressed air less of a chance to form moisture in the pipe as it travels to the booth.
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If the room is too cold, condensation can form on supply ducts and vents, then drip on parts or the powder booth.
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This is air that is supplied at the nozzle to twist the pattern and close the donut hole pattern that is common with diffuser or round-type nozzles. This can provide better direct powder penetration into corners. Additional air will increase the pattern size, potentially reducing the need to change tips.
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Guns should be cleaned after each color change. At the end of the day, you should purge the gun, pump, and hose to prevent moisture from settling in the powder and clogging things up for the next use. Depending on the volume of powder sprayed and time between color changes, deep cleaning and worn part replacement should be reviewed weekly.
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It is recommended that you change your powder hose at least once a year depending on wear caused by the powder or if you are having contamination problems. Frequently inspect for cracking or impact fusion that may shorten the hose life.
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Drive the rod vertically into the ground and attach a grounding strap directly to the booth. Contact your local electrical contractor for a standard grounding rod, typically copper.
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Precoated or recoated parts may already be insulated with 1 to 2 mils of coating. Powder starts to repel itself around 4 to 5 mils. Turn down the gun voltage (Kv), by at least half, to prevent the repelling or back ionization of the new powder material. Also verify with the powder supplier to ensure proper intercoat adhesion.
Here is a synopsis of the top powder coating questions and answers we have been asked over the past 15 years about the industry
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.
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:
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