Q: As you know, most industrial powder coating systems have about 60-70% transfer efficiency on average, which means about 30-40% of the powder does not adhere to components being powder coated. Therefore, powder coaters tend to work on reusing the excess overspray powder (aka reclaimed powder) in order to increase efficiency and cost effectiveness.
Consequently, we have started evaluating the possibility of reusing reclaimed powder in a thermal spray (flame spray) coating process. We prepared some sprayed metal components with 100% reclaimed powder and tested them against some sprayed metal ones with 100% virgin powder. The results showed no differences between them, in terms of other parameters such as coating adhesion, wear performance, etc. Also, the chemistry of the reclaimed powder remained within our spec.
As we are still evaluating this, I’d like to get your opinion and thoughts on reusing reclaimed powder. I know you are very experienced and have seen many different applications. Have you seen others using reclaimed powders (particularly in a flame-spray coating process)? Is this something that you recommend? If yes, what mix ratio of reclaimed powder vs. virgin powder would you recommend?
A: You offer an interesting path to managing overspray powder coating. You are correct that typical application systems operate with 60-70% first-pass transfer efficiency. Of course, this depends on what you spray and the quality of your application equipment and controls. For example, transfer efficiency is poor when spraying wire racks (mostly air), but pretty good when coating flat panels.
As for flame spraying reclaim or a mix of reclaim and virgin powder, it sounds like a good idea and may work. Here are my comments and concerns:
1. Powder sprayability is mostly tied to particle size distribution (PSD). Virgin powder, if properly manufactured, will have an ideal PSD. Reclaim contains the less-than-desirable particles, such as a higher concentration of fines and coarse particles. This makes the reclaim more difficult to fluidize, transport, and electrostatically charge.
Consequently, reusing reclaim powder involves blending it with virgin powder. How much depends on how the blend behaves in the application process, namely in terms of fluidity, transport, and charging. With flame spray, you obviously eliminate the requirement to charge the powder.
2. To be honest, I’m not a big fan of the flame-spraying powder process. Why, you may ask? I think it’s a bad idea to create a cloud of fine organic particles with lots of air (i.e., oxygen), move it through space, then pass it through a flame. I can see the generation of a flamethrower effect or, worse yet, rapid combustion (Ka-BOOM). That said, there may be alternate thermal-spray processes such as plasma or high-intensity IR that may be safer, but I am not sure.
3. Regarding the ratio of virgin to reclaim powder, you should incorporate a fluid-flow additive to help the non-ideal PSD fluidize and transport better. This can impair electrostatic charging; however, if you’re using a thermal spray process, that should not be an issue. Fluid-flow additives are typically based on fumed silica or fumed alumina and should be incorporated at low levels (say 0.1-0.2% by weight).
Good luck with your inventiveness. It’s always encouraging to see someone thinking outside of the box…of powder.