Getting Better Powder Coverage in Inside Corners
Powder expert Rodger Talbert offers his advice on improving inside corner coverage and other powder coating issues.
Q: We seem to be having some trouble getting the coating to cover in the inside corners. Any advice?
A: The problem you describe is related to electrical resistance in the recesses of the parts. Voltage tends to follow the path of least resistance, and the powder tends to follow along the voltage lines, preferentially building more readily in the flatter areas and not building as easily inside the recessed corners. This problem is commonly called Faraday cage effect, based on research by scientist Michael Faraday on the behavior of electricity and the flow of current and voltage. There are a few things that should be considered to help overcome this natural electrostatic behavior.
First and foremost, make sure that the parts are earth-grounded. The electrons flowing in the corona field from the gun must have a clear path to ground. If the part has poor ground, it will magnify the Faraday cage effect and make it more difficult to control the electrostatic process for uniform coverage. The hook that the part connects to must make bare-metal contact with the part and have a clean path to ground-throw the frame or cart that it is hung from, and the frame must be connected to earth ground with a cable or bar of some kind.
Secondly, make sure that he voltage and micro-amps are set to an optimum level for your parts, and experiment with lower levels of current (micro-amps). Lower current draw can help avoid pronounced Faraday cage effect. Try something between 20 and 40 micro-amps, and also use the factory preset for Faraday areas if you have that feature on your gun.
Also, set the flow rate so that it is high enough to get into the corners but not so hight that it is turbulent when it hits it the part. High flow rates are not helpful.
Finally, work to discover the right gun-to-target distance in which you are close enough to get the powder into the corner but not so close that it sets up higher velocity and current draw. The rough surface you described is most likely back ionization from getting the gun too close to the part.
Q: How can we effectively measure the cost of the powder material to determine if we have the right material for the job?
A: In order to truly understand the cost of powder, you need to stop focusing on the cost per pound and start focusing on the applied cost per square foot. There are features built into a powder that can make it hide the metal at a thinner film build, or cover edges or Faraday areas better. Some materials are more likely to impact fuse (build-up in the delivery path from friction), and the particle size distribution may make one powder apply at a higher percentage than another.
So, how can you make sure that you are using the best powder for the job? Measure the cost per applied square foot by determining how much powder it takes to produce a batch of parts with a given material. For example, one powder that cost $2.50 per pound takes 50 pounds to coat 1,000 parts (2.5 x 50 = $125). Another powder of the same color that cost, $2.60 per pound takes 45 pounds to coat 1,000 of the same parts (2.6 x 45 = $117). The lower-cost powder actually is more expensive in applied cost per square foot.
There may be other cost advantages to using a better powder product as well. For example, if the transfer efficiency is higher, there is less powder going to waste, less to clean up during color change, lower compressed air use and less wear on the powder application equipment. Overall, buying the best powder may cut your costs vs. buying the lowest cost powder.
Q: What is the best level of humidity for application of powder? How can we make simple adjustments to the changes we see during different seasons?
A: The correct range for optimum application is between 40 and 60 percent relative humidity (RH). When it is high, the powder absorbs moisture, is harder to fluidize and is resistant to smooth flow in the delivery system. When RH is very low, it can create a field of static that interferes with the normal development of the corona field and makes it hard to cover critical areas of a part like the edges and Faraday areas.
What can you do to offset the impact of changes in humidity? When humidity is high, be sure that you prepare the powder by fluidizing for 29 minutes or so before you start to spray. This will help remove moisture from the powder and make it flow better. It also will make the gun easier to adjust and allow more standard settings. Blow out the delivery system when convenient, and make sure the area around the electrode is clean. Check the gun tip and electrode frequently to make sure there is not powder building up in the air cap. Store the powder in an atmospherically controlled storage room to avoid clumping. Of course, it goes without saying, also make sure your parts are grounded, and make sure your compressed air is clean and dry.When the humidity is very low, you need to add humidity to the surrounding area. A humidifier can help a lot, and if the area is enclosed, it can help even more. And on the topic of enclosure, a powder room is very helpful. Some climates are relatively stable and do not need atmospheric control for relatively consistent settings and results. Some areas, like the one you describe, have wide swings in temperature and humidity. In these areas, it is recommend that a powder room be built around the powder booth and application area. It is not necessary; you can apply powder without one. But as you pointed out, it means having to adjust the guns, and it means you will have more rejects. The powder room cost, money to install but saves a lot in film-build control, reduced powder waste and lower rejects. Having a room around the area elevates your control over the system and provides a more reliable result.
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