The Gema Guys
Tech Tip of the Month
(L) Mike Smith-
Senior Technical Services Specialist
(R) Phil Flasher-
Manager, Standard Products Technical Sales
Water Purification and Part Pretreatment
According to Powder Coating - The Complete Finisher's Handbook, "Proper cleaning is the single most important aspect of a powder finishing system should you desire to maximize powder performance." (Click here for information on ordering a copy of The Complete Finisher's Handbook www.powdercoating.org/home.htm). It is important to remember that the quality of water you use is crucial to achieving a quality finish. Water purification systems are essential for removing films and debris from parts, but which method of water purification is best for paint pretreatment? We've asked the experts and Craig Burkart, Industry Development Manager, Finishing, for Ondeo Nalco Chemical Company, has provided us with the following explanation:
Two common methods of water purification are Deionization (DI) and Reverse Osmosis (RO). DI units work by utilizing a process known as "ion exchange". DI systems do not produce totally pure water, but the output is sufficiently clean for most applications. RO systems, on the other hand, push water through membranes that trap impurities. RO systems are much more effective than DI systems and can remove up to 99% of the impurities. The type of membrane used will determine how pure the output of the RO unit will be. RO and DI systems are often used in conjunction with each other to obtain the purest water possible. Water is first sent through the RO system and is then processed by the DI unit for further purification.
The truth is that for most pre-paint applications, either will work. An exception is for electro-coat operations, where DI water purification is required. RO has gained favor recently because it is perceived to provide more consistent quality output over time. Also, the extra cost and hazard associated with the chemicals in a DI unit are eliminated when using RO. But don't be fooled into thinking an RO system has no cost to operate. The membranes in an RO system must be replaced, on average, every 3-5 years.
Got a question for Craig Burkart? Send it to gemaguys@gema.us.com. For more information, visit our website at www.gemapowdercoating.us/info/links.html and scroll down to "Pretreatment and Lubrication Materials" to find Ondeo Nalco Chemical Company contact information.
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The relationship of KV to ground and measuring the voltage going to ground.
The powder coating fluidizing hopper and how it can change film thickness.
Applying a second coat on a part that's already powder coated and the second coat will not stick to the part.
Options available for transfer of powder from the box to the feed hopper.
Theory of the corona charging system from the input AC power supply to the 100 KV at the electrode including the various system components.
Using electrostatic powder coating equipment to apply seasoning to food.
What type of powder hose to use.
Using a tribo gun with EasySystem™ manual powder coating box units.
Maintaining 45 to 60% relative humidity for ideal powder coating conditions.
Maintaining air quality in powder coating systems and what could happen if you don't.
How much compressed air is required to operate a PG-1 gun.
The ideal virgin to reclaim powder ratio.
The required airflow through the openings to contain powder in a spray booth.
What is a SuperCorona.
Powder is not adhering to the part to be coated.
Resolving inconsistent gun deliveries.
Causes of UV shutdowns.
Inconsistent fluidization.
The correct temperature and humidity for a powder application room.
Q: How does the relationship of KV to ground work and how do you measure the voltage going to ground? - M. Landers, Salem, VA
A: The relationship between KV (thousands of volts) and ground is the key to electrostatic application. Voltage is a measure of electrical potential energy and ground is zero volts or neutral. This energy from the corona discharge moves from the applicator towards the substrate during the powder charging process assuming that the part has the same potential as earth ground. To "move," the charge must overcome resistance. All matter has some resistance to the movement of charge. Air is highly resistive and its resistance is determined by the air's makeup, especially the moisture in the air. The movement of charge is termed "current" and the resistance of current is measured in "ohms." The powder particles are charged so that they will be attracted to a grounded substrate. If the substrate or part is not connected to ground, it will become charged and the charged powder will not have enough difference of potential to cause electrical attraction. To check the part for ground, use a mega-ohm meter or "megger." This meter sends a 500 volt signal through the part and measures the amount of resistance. The National Fire Protection Association has established a safety benchmark target of one mega-ohm or less resistance to ground. Remember, ground is the electrical potential of earth or zero voltage. When hangers are coated and do not provide a good ground connection to the part, the part cannot dissipate charge to earth ground through the conveyor. Therefore, the part will become charged when it is being coated and will quickly become at or near the same potential as the powder being applied. This prevents the powder from sticking on the part and can even be rejected, causing back-ionization. Regular cleaning of part hangers should be a standard part of your maintenance routine and will help you achieve the best ground and the best finish possible.
Q. Can you explain the purpose of the powder coating fluidizing hopper and how it can change the film thickness I put on my parts?
- K. Nolan, Miami, FL
A: Fluidization of the powder hopper is the process of preparing the powder in the gun feed hopper to become "fluid-like," enabling the powder to become easily transported through the suction tube and to the gun. Other methods for preparing the powder are vibration, agitation (stirring the powder), or combination of methods. Fluidization, however, is the most frequently used method for material handling and conditioning of the powder. To fluidize the powder, compressed air is brought into the hopper through a plenum chamber and membrane. The air is delivered through the membrane and then up through the powder in the hopper. As the air moves up through the powder, it lifts or aerates the powder, making it fluid-like. Most users do not take the time to adjust the level and fluidization air setting each time the power level changes substantially. As the level raises and lowers, the density of the powder/air mixture in the hopper can change. As a result, the density and distance from the pump also changes in the suction tube assembly. This means that the powder becomes more or less dense and the work load on the pump varies as the level changes, but the delivery air remains constant. The result is that the powder pump is actually delivering more or less powder to the gun. Users will notice that the gun outputs vary throughout the day as the powder level in the hopper rises and falls. The best approach is to maintain a constant level of powder in the hopper. This minimizes the number of adjustments that have to be made to the fluidization and delivery air, allows a consistent delivery of powder to the gun, and enables powder to be applied to the substrate in a more uniform manner.
Q. I am trying to put a second coat onto a part that is already powder coated and the second coat will not stick to the part. Any suggestions?
- J. Duncan, Perry, OH
A: When applying a second coating of powder or re-coating a part, application conditions differ from first-pass conditions. Modifications in equipment settings and/or technique are needed.The electrostatic charge should be reduced (either by lowering voltage or limiting current) and/or the gun-to-part distance should be increased. You may also need to adjust the powder delivery settings to increase the output by 10-20%. Be aware that the application will require more time. The equipment will not be as efficient, i.e. transfer efficiency will decrease when doing re-coats, but the application can be achieved.When coating a grounded metal surface, charged powder is attracted to the grounded surface. Once in contact with the bare metal, electrical charge will begin to dissipate from the powder to ground. All of the charge does not immediately go to ground. If it did, the powder would fall off of the part. Instead, the charge slowly bleeds off through the grounded part, hanger, and conveyor and eventually to earth.If the part has a cured powder finish over its surface and is connected to earth ground, the surface should be at ground potential prior to the application process. Once the charged powder particles are sprayed onto the part, they will be attracted to the surface, but will not be able to dissipate their charge through the cured coating and back to earth. So, the surface of the part will become charged quickly and will reach a point where the charged powder on the part is at, or near, the same potential as the powder being applied, thereby creating back ionization or rejection.In summary, applying a second coating to a part that has a cured powder coat finish requires adjustments in charge setting, time, and technique. With patience and proper technique, you should be able to achieve the finish you desire.
Q. We currently feed fresh powder to our hopper by hand with a scoop and would like to automate the process. Can you tell us what options are available for transfer of powder from the box to the feed hopper? - G. Ellis, Colorado Springs, CO
A: At Gema, we call the type of unit you are referring to a "Fresh Powder System," and the most common such unit we sell is the FPS-14. The FPS-14 looks similar in appearance to Gema's EasySystem™ manual box unit, which uses a vibratory table to agitate the powder. In addition, this system utilizes a special FPP01 Fresh Powder Pump that uses minimal air to move powder, thus keeping dust to a minimum. Depending on the amount of powder you wish to move, there are also other options available for moving powder. Gema also offers a 300 pound drum unloader or 1200-1500 pound palletized box feeder unit. Both of these units enable you to have automatic or manual control of the powder replenishment process. These two units incorporate a venturi style pump system that uses compressed air to transport. Once you have decided upon the type of unit you wish to use, you then need to decide if you are going to manually or automatically activate the transfer process. Manual activation requires a person to "flip a switch" or "turn a valve" to begin powder transport. Once this occurs, the unit will transfer powder until the container is empty or the operator turns off the unit. Typical problems are forgetting to turn the unit on and running the hopper empty. The opposite situation can also occur when the operator forgets to turn the unit off and overfills the hopper. Conversely, automatic replenishment uses level sensors in the hopper to determine the level of powder in the fluidized hopper and, at the appropriate time, activate and deactivate the fresh feed unit. Regardless of what kind of container your powder arrives in, Gema has a unit specifically designed to transfer the powder into your coating process.
Q. Will you kindly explain the theory of the corona charging system from the input AC power supply to the 100 KV at the electrode including the various system components? - J.H. Whitley, Cleveland, OH
A: All electrostatic corona applicators (or guns) utilize a power source for generating the voltage that is emitted at the electrode tip. For powder to be attracted to a part, it must have an ion charge imparted on the surface of the powder particle. In order to accomplish this, a powder gun is connected to a control unit giving the operator the ability to adjust the amount of energy emitted at the tip. As the operator adjusts the voltage, the control unit sends a low voltage high frequency signal to the gun. The low voltage signal is received at the transformer and steps up the signal to a higher voltage. At this point the voltage is passed through a diode/capacitor circuit called the "cascade multiplier," producing the desired voltage. Depending upon the design of the gun, most guns will range between 80,000 to 100,000 volts maximum capability (Note: 100,000 volts is the same as 100 kilo volts or 100 kV). To obtain a negative or positive charge, the polarity of the incoming voltage at the transformer is set to provide the desired charge. When all of this potential reaches the end of the gun tip, it has to go somewhere, and that somewhere is in the air. As the gun tip discharges the energy, the powder particles are charged in the form of ion bombardment. Water molecules in the air and on the particle play a role in the transfer of charge to the particle. The field of energy at the gun tip creates an area known as a "corona." This is where the name "corona charging system" is derived from. Once charged, the powder particle is relying on aerodynamic forces as it is pushed out of the gun barrel, and electrostatic forces created from the passing through the corona field, in order to be properly transferred to the part. Optimizing powder delivery pressure and electrostatic voltage, as well as achieving correct target distance and proper grounding, will ultimately produce the highest quality part.
Q. Is it possible to use electrostatic powder coating equipment to apply seasoning to food? - L. Brady, Binghamtom, NY
A: The answer is yes indeed! Electrostatic powder coating equipment from Gema has been used in a wide variety of food and cosmetic applications, as well as for lubrication of aluminum extrusion presses with boron nitride powder. As for the food industry, electrostatic spray equipment has been used to apply seasoning, flour, or sugar powder on a number of items such as chicken, bubble gum, potato chips, and donuts. Many of these powders offer unique challenges in material handling and "spray-ability," primarily due to the particle size, specific weight, and oil content of the powder.Although a standard spray gun and control unit is used, the powder delivery pump may need to be customized by the user in order to eliminate impacting. Modifications are usually made to minimize the impaction of powder in the pump and maintain consistent delivery to the gun. As for the gun, either manual "hand-held" guns or automatic guns can be used to apply the materials.
Q. I am a custom coater that sprays many different types of powder, including metallics. What type of powder hose should I use? -- B.Stanley, St. Louis, MO
A: There are several things to consider when selecting a hose to convey powder from the powder pump to the spray gun. For manual spray guns, hose flexibility is important. If you will be spraying different types of powders or different colors through the same hose, it will need to clean out easily. Abrasive materials will require the use of a very durable hose. Most powder hoses range in size from .375" inside diameter (ID) to .75" ID. Typically, 11mm or 7/16" ID hose is used. If your application requires high powder flow, you may need a hose with a larger ID. For applications of low powder output, a smaller ID hose will give better control of the powder delivery. Many different types of powder hoses are now available, and each one is designed for a specific purpose. For your application, I would suggest Gema's new conductive powder hose. Because you make frequent color changes, using this hose will help to speed up color change a great deal. The conductive nature of the hose resists impact fusion and is so easy to clean that you will no longer need to use cleaning cubes and a hose-cleaning gun. Please note, however, that this hose is not recommended for enamel powders. Gema offers several different hose materials and sizes for various application needs.
Q. We have two EasySystem™ manual powder coating box units and would like to use a tribo gun. Is this possible? - C.M., California
A: There is an adaptor for the TG-1 tribo gun to enable it to work with the EasyTronic™ control unit; however, there are a couple of drawbacks.A tribo gun uses rinse air from the control unit to create the tribo charge. With the EasyTronic, there are two settings for rinse air and neither one is really high enough to work properly. The highest setting available on the EasyTronic is the minimum necessary to operate a tribo gun, and there is no way to increase this setting. It would be better to be able to increase the rinse or swirl air to increase the tribo charge. Another problem is that we read the current feedback of the tribo gun to display a charge setting or amount of charge. Because the EasyTronic does not display current or KV, you will have no way to know how much charge the gun is creating. All is not lost - there is a better option available! If you wish to use a tribo gun, you should consider upgrading to an OptiTronic™ control unit. With the OptiTronic, you can adjust the rinse or swirl air setting and the unit displays the current feedback of the gun's charge.
Q. I've been told that I need to maintain 45 to 60% relative humidity for ideal powder coating conditions. Is this true? - DJ, Indiana
A: The realtive humidity of 45 to 60 percent is important for a few reasons. First, we are creating a corona field of charge around the gun via the electrode on the front of the gun. The moisture in the air is necessary to facilitate the ion bombardment on the particle to be charged. We are ionizing the powder particles and moisture in the air. if the moisture content is too low, the corona filed is not as effective and results in lower transfer efficiency of the application process. Also, if the humidity is too low, there is a tendency for the material to create a tribo charge, which strips away negative ions and leaves the powder with a positive charge. We typically use negative charge for our corona guns. Relative humidity of the environment is important because if it is too humid, the powder will not fluidize as well or will create impact fusion. Too much moisture may seem obvious, but the powder will agglomerate when too moist. Therefore, maintaining 45 to 60 percent relative humidity is most effective in creating the ideal powder coating environment for most powders. As in most manufacturing processes, controlling variables within the process produces fewer rejects and reduces costs associated with rework and scrap. The bottom line is that changes in humdity can cause fluctuations in application deposition weights, thus increasing your reject rate. Maintaining relative humidity of 45 to 60 percent contributes to a consistent finish and reduced rejects.
Q. What can I do to maintain air quality in my powder coating system and what could happen if I don't? - E.M.H., Virginia
A: It's important to maintain the correct volume of clean, dry, compressed air to your powder paint system. The air should be conditioned with a properly sized refrigerant or regenerative (if air volume is 500cfm or higher) air dryer. It's also recommended that a particle or coalescing filter be used and equipped with automatic drains or flags that indicate when service or replacement is required. Air quality requirements are three-fold: maximum oil content of 0.1ppm, dew point of 35 degrees or lower and particulate matter no larger than 10 microns. By keeping the powder free of moisture, oil and other dirt, powder fluidizes more evenly and the pump can lift the powder through the suction tube easier, transporting the powder to the gun more efficiently.
Q. How much compressed air is required to operate a PG-1 gun?
A. Required air volume for the PGC1 is 3 SCFM under normal operation. However, the unit is capable of flowing 5 SCFM. An MPS-1F unit (includes gun, pump, control unit, 45lb hopper and accessories) and may require up to 10 SCFM.
The ideal operating pressure is 0-65 psi. Minimum input pressure is 75psi
Q. What is the ideal virgin to reclaim powder ratio?
A. 60%-40 percentage maximum. That is 60% virgin powder-40percentage reclaim (a mix including a higher percentage of virgin powder is even better). This ratio should be obtainable if you have a good transfer efficiency of powder from the gun to the part being coated.
Q. In order to contain powder in a spray booth, what is the required airflow through the openings?
A. Not all booths are engineered the same. What is desired is a soft airflow, which will keep powder from drifting out of the booth. Too much airflow through the booth openings will cause a loss of transfer efficiency because the airflow is too strong and will overcome the electrostatic attraction of the charged particle to the grounded substrate. Our booths operate with an ideal airflow of 120 fpm on average through the booth openings. However there are other factors to take into consideration: gun location, filter pressure differential, etc.
Q. What is a SuperCorona?
A. The SuperCorona is an "add on" to the PG guns that are equipped with flat spray nozzles. Designed to provide a smother surface finish for applications requiring heavier film builds, or work pieces with recessed or grooved areas, the SuperCorona delivers excellent coverage.
Q. What can I do if the powder is not adhering to the part I am coating?
A. Make sure that the part is properly grounded. The maximum resistance between the part and the earth ground should not exceed 1 megohm. Make sure the gun is producing high voltage. If the problem continues, try new or fresh powder.
Q. How do I resolve inconsistent gun deliveries?
A. Check your fluidization. The powder should appear to be "simmering". Make sure that all O-rings and pump inserts are in good shape and that powder hoses are clean and clear. Confirm that the air supply is feeding the system with consistent and correct pressure (normally 100 to 120 psi). Check for possible blockage in check valves located on gun pumps.
Q. What causes UV shutdowns?
A. Most UV shutdowns are caused by powder build-up on part fixtures or hangers, which causes arcing between the fixtures and the parts. The detection system sees this arcing and shuts down the system. Shutdowns can also be caused by non-grounded objects in the spray area, or by external sources, including welding operations near the booth.
Q. My fluidization is inconsistent. Why is this happening?
A. Possible causes include an incorrect reclaim to virgin powder ratio, inconsistent air supply to the hopper, and changes in the environment, like humidity. All of these factors can influence fluidization. Due to a shift in particle size, new or virgin powder is easier to fluidize than powder that has been reclaimed. When new powder is first sprayed, more of the larger particles are transferred to the parts than the smaller ones, which is why reclaim powder has a smaller particle size distribution than virgin powder. The objective is to maintain a ratio that fluidizes and charges efficiently.
Q. What is the correct temperature and humidity for a powder application room?
A. The ideal temperature for powder application is between 60 and 80 degrees Fahrenheit. Humidity should be kept between 40%-60%. |
