Although I too write for Products Finishing (Pollution Control Clinic), I am writing you on behalf of one of my clients. He has been approached by one of his customers on a potential project. Currently, the customer is producing an aluminum casting part that undergoes extensive and significant grinding, buffing and polishing followed by pretreatments and nickel/chrome plating. The customer’s product suffers from a high reject rate as well as field failures. We know of another aluminum cast product that is apparently clear powder coated to produce a very smooth finish (thus eliminating all the grinding, buffing and polishing) and then metallized to create a mirror finish. Here are my questions:
- Are you familiar with this type of process?
- What type of pretreatment (cleaning, phosphating, degreasing, etc.) is required before powder coating?
- What type of powder coating is or could be used? Does it have to be clear? The part would be outside 24/7.
- Would the powder coating application be by spray gun or fluidized bed?
- Are there powder coatings that are sufficiently conductive that one could use nickel/chrome electroplating instead of metallizing?
Nick, feel free to call me for further information on this project. Stephen Schulte P.E.
It was great talking with you on this subject. It is a very unusual application for powder coatings. Following are the answers that we discussed via telephone for the benefit of our readers:
- Yes, I have heard of this process, although I have no personal experience helping anyone who has this process. This method of finishing products is used to gain the appearance of a highly polished and buffed metal surface without having to actually buff and polish the substrate. The powder coating fills the small voids and levels the surface. The metallizing process provides a surface that is harder and more robust than the powder coated surface to closely approximate the performance of nickel/ chrome plating. One product that comes to mind that uses this technology is automotive wheels.
- Pretreatment methodologies would be the same for any other organic finishing method. The product must be cleaned of all organic and inorganic soils to ensure proper adhesion of the powder coating. Since the substrate is aluminum, I suggest using an alkaline cleaner, followed by a water rinse, followed by a deoxidizing chemical, followed by a water rinse. This method will clean both organic and inorganic soils from the substrate. What follows depends upon the corrosion protection you require. Since this product is outdoors 24/7, then a chromate conversion coating or chrome-free DIP (dried-in-place) polymer coating should be applied to the substrate to obtain the maximum corrosion resistance.
- The powder coating should be selected in consultation with the powder coating supplier to ensure the coating has the mechanical and corrosion properties required for this application, the metallizing process and the fielded conditions. I suspect that a clear powder coating is appropriate to provide a depth of finish or distinctness of image (DOI) for the metallizing process. Furthermore, a clear powder will be easier to hide with the metallizing than an opaque powder. The final product will require that the powder coating be very smooth, so be sure to ask the coating supplier for a formulation that has high leveling and smoothness characteristics.
- There is no doubt that this process will require the powder coating to be sprayed onto the substrate. Electrostatic guns will provide an even powder coating film controlled to tight thickness tolerances not available using a fluidized bed application process.
- There are no conductive organic powder coatings that will allow the substrate to be plated after powder coating. However, the converse is true where powder coatings have been applied to plated surfaces. Furthermore, there are powder coatings that can be used to substitute for the appearance of plating, but their performance is not the same as plating.
I hope that this information provides the answer your client needs to evaluate using this process as a substitute for buffing, polishing and plating his aluminum castings.
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