Powder Coating Pretreatment

Q. We are a general purpose coating house. Our cleaning line consists of an alkaline cleaner, city water rinse, iron phosphate, sealer, RO rinse then RO halo. This is a spray system with roughly 45 sec dwell time. The cleaner temperature is 120°F. We are looking at a project involving a cast aluminum valve cover where the die lube is a paraffin wax. How do you recommend we clean the wax from the substrate and have a thoroughly cleaned casting? We end up powder coating the part and have had adhesion failures at different locations. S.M.

A. In general, you are doing many things right with your cleaning and pretreatment system, but there could be a few areas for improvement. In general, paraffin waxes need a significant amount of temperature and time to be removed in an aqueous cleaning system. In an ideal situation, you could allow the part to spend more time in the cleaning stage (at least two or three minutes). Additionally, if you could turn up the cleaning temperature to about 160°F, you would stand a much better chance of removing the paraffin wax.

However, not knowing what percentage of your overall production is dedicated to this one part, it may be difficult to modify the time and temperature of this one stage in order to accomplish this. If the part represents a relatively low amount of your overall production, you may want to consider a dedicated single-stage, heated immersion soak tank. Immersion cleaning systems can be used even hotter than spray cleaning systems, and parts can be immersed for a significantly longer period of time. Following that initial soak tank cleaning, you could then continue by processing it in your iron phosphate pretreatment and powder coat system.

Finally, whenever pretreating any castings prior to paint or powder coating, it may be advisable to pre-heat the casting to the cure temperature in order to outgas any contaminants from porosity that is inherent with virtually all castings. Even the most thorough cleaning job will not eliminate casting outgassing from porosity that can blemish an otherwise perfect coating job.