Reducing Waste in Your Powder Coating Facility
Finising industry consultant Rodger Talbert explains ways to reduce waste, heat in your powder coating facility.
Q: We have a small job shop that does a lot of short runs and single-part projects. Our operation is a batch system and we do not reclaim any of our powder for reuse. Is it possible to improve efficiency in a batch operation with no powder recovery?
A: Let’s do a review of some of the basics on how to get the most from your box of powder. Like all discussions on application efficiency, it starts with confirmed and consistent earth grounding. Grounding affects efficiency in several ways. It does reduce first-pass transfer efficiency (FPTE) directly but it also has a profound impact on Faraday cage coverage, edge coverage and film build consistency. This forces the operator to use higher flow rates and expose the part to more powder to get the needed coverage in the critical areas. That combination adds up to more powder use than necessary both on the part and on the floor. So make sure you have good ground by keeping your hooks and the hook point on your rail or cart clean and attaching a cable to the ground source if necessary. It also means that you need a good ohm meter to regularly measure the resistance to ground. It should be below 1 mega-ohm for safety but for efficiency it should be as low as possible, preferably no resistance. A high-voltage megger is often recommended because it will overcome a little resistance and get a reading. I personally prefer a good quality ohm-meter that shows more minor resistance because I am looking for a clean path to earth ground, no resistance. Definitely do not use a cheap device with a simple 9V battery because it is not going to be robust enough to give you a reliable reading.
After you confirm that the ground is always good, you need to focus on spray gun setup. Flow rate should be optimum, starting somewhere in the midrange. If it is too low, the efficiency may be good but it could take too long to build film and too long to finish the part. If it is too high, the transfer efficiency will be lower and the overspray volume will be higher. On a volume-based system, you should start with a setting of about 40% powder and 4.0 total air. A pressure-based system should be around 25 to 30 PSI with just enough secondary air to create a smooth and consistent pattern. Basically, pump enough powder to get the job done in a reasonable amount of time without excess velocity and minimize overspray. Check film thickness regularly to make sure you are getting the correct range.
You should also optimize the voltage/micro-amp settings. If the surface is simple to coat, you can run at full voltage and not worry about limiting current. If the part has Faraday areas or a more complex shape, you should use factory presetting for better penetration or limit the current. If you are doing a second coating, you should limit current, move the gun a little farther from the target and increase the flow a little.
Make sure the parts are racked so that the operator cans see well and reach all surfaces as best as possible. Poor rack position can cause increased overspray.
Finally, you need to make sure the operator has the proper training. You can teach them yourself, have a good teacher among your crew or send them to a training program offered by the Powder Coating Institute (PCI). Find a way to make sure they understand proper technique and settings. You can coat as a self-trained operator but formal training always helps. Add up all this and you should be able to get more powder on the parts and less on the floor.
Reducing Heat in the Building
Q: We installed a powder coating system in our manufacturing facility a little over a year ago. The heat from the system is increasing temperature in our facility by 10-15 degrees. What are some ways to reduce heat in the building?
A: We have had this concern expressed before and seen it many times in the field. Obviously, the ovens and washer in a powder system generate a lot of heat. You need to consider ways to contain it and/or move it out of the building.
The first consideration is to look at heat roll-out from the ovens. Heat containment can be improved in some cases with powered air-seals. Powered air-seals are pretty commonly included with powder coating dry-off ovens and cure ovens. A fan drives air across the product opening at high velocity to create an “air-curtain” and reduce the heat roll-out. They can help if the opening is not too big. As the opening size increases, the impact of the air-curtain is reduced. Above 4' wide x 6' high, the air-curtain is not very effective. It is also important to remember that high-velocity air is not very welcome at the entrance to a powder cure oven because it can blow un-cured powder off the parts. Also, generation of the necessary high-velocity air takes a pretty good sized motor (usually 10 to 15 HP) so there is a cost factor to consider.
Another option is a heat-relief hood positioned about the openings to capture a good portion of the heat as it rolls out of the oven and allow it to flow up and out of a stack in through the roof. Heat relief hoods can be effective but they depend on the building having reasonable pressure from air-make-up (more on that later) and they do not save any energy, they just get rid of most of the heat. They do not need a large motor so there is no additional cost factor.
The most effective oven-heat containment is a long entry and exit vestibule. The vestibule is the oven tunnel area between the discharge duct (hot zone) and the actual opening into the oven. That unheated space can act as a good space to reduce the heat in the building. It also makes the oven bigger and may not be possible to add to an existing oven.
To make any of these approaches work properly, you need air-make-up (AMU). An air handler that adds air to the building will move heated air out instead of allowing it to build and raise the temperature. Adding a source of air into the build provides make-up for the exhaust and helps make these devices work a lot better. If the building has negative air pressure, the air will come in through open doors and other areas of the building and upset the function of the exhaust and the heat containment. It will affect heat containment in the ovens and also vapor containment in the washer.
Speaking of the washer, if it has heated stages, it is a giant radiator. Get your air balance worked out and that will help some. The best thing you can do for a washer is to insulate the tanks and tunnel. I did a study of the impact of insulation a few years ago and the gas used is much lower if both the tanks and tunnel are insulated. If it uses less gas, that means the radiant heat loss is lower and that means your building is not as hot.
So, if you can, extend the oven vestibules. If you do not have enough room to do that, you should consider heat-relief hoods. Insulate the washer if you can. And it will all work better if you have enough AMU to turn-over the air in the building.
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