The Right Process for the Part?

Question:

I have recently been offered a job, for powder coating heater coils made out of copper. The copper tubing is 3/8 inch diameter and coiled 1/2 inch apart. The whole thing from top to bottom is roughly 36 inches. The diameter of the coil itself is about 10 inches around from top to bottom, with a smaller coil inside maybe 4 inches in diameter and it to runs top to bottom. The finish is a smooth epoxy coat. The company that is coating currently has a lot of fish eyes and defects. What is the best way to coat the product—spray or fluidized bed? Any info provided would be greatly appreciated. Thanks. B.M.

Answer:

The first question you should ask your customer is: How much coating do they want on the part? This will be the best factor in determining the method of coating the part. For instance, if they can allow more than five mils of powder coating on the coil, then electrostatic fluidized bed technology is most appropriate. However, since these parts are heat exchangers coils, I suspect that too much powder coating (5 mils or more) will inhibit the transfer of heat when the coils are put to use. Therefore, you will probably use spray technology to apply the powder coating and keep the film thickness between 1.5-3.5 mils.

Now the question becomes which type of spray gun should be used: Corona charging or Tribo charging? Since the coils have a half-inch spacing between the coil loops, I expect that a Corona charging spray gun would work. These are the most popular type of powder coating spray guns used in the market today with more than 95% of all installations using them. The voltage and current can be controlled to allow an even deposition of powder coating to complex and simple shaped parts. They are not severely affected by environmental conditions (temperature and humidity) or powder formulation or particle size, unlike Tribo charging guns.

If Corona charging guns cannot get the powder in all the recessed areas of the coil, regardless of voltage and current adjustments and the use of different spray pattern devices, then you have no choice but to use Tribo guns. It may seem harshly worded, but this is how Tribo guns are selected—only if all else fails. Without the problems of using a Corona electrostatic field, Tribo guns can put powder into areas where Corona guns don’t have a chance. Basically, all Faraday problems disappear completely. That’s the good news. The bad news is that Tribo guns cannot charge the same volume of powder as Corona guns, have problems with humidity and temperature conditions, don’t work with many powders and have more problems with impact fusion. Otherwise they are great to use for just about any application. However, you will find, as most others who have tried Tribo technology, that these drawbacks are just too difficult to overcome in everyday use, unless you have no other choice. I guess that is why less than 5% of the powder coating market uses Tribo technology.