Ask an Expert
From: Products Finishing, David S. Peterson
I would like to start powder coating the aluminium housing of bus bar and need to know how to pretreat the parts before spraying.
Q. I would like to start powder coating the aluminium housing of bus bar and need to know how to pretreat the parts before spraying. Some say we need a three-stage system. Can you help? W.M.
A. Probably the most important part of your pretreatment prior to powder coating will be to provide an adequate cleaning. It will be important to remove all grease, oils, and process lubricants from the surface. This is most effectively done in-line with a paint system through the use of a conveyorized spray wash system. This is assuming that the size and geometry of your parts are amenable to this type of system. Most cleaners are moderately to highly alkaline to aid in buffering, saponification of fatty oils and because of a wider choice of surfactants are available for formulation.
A three-stage pretreatment system would modify that first stage significantly. In that first stage, it would be required to both clean (remove oils) and provide a light phosphate coating. The purpose of the phosphate coating is that it provides a better surface for adherence of paint, which will promote better durability and performance of the coating on your part. In the three stage scenario, the second stage is a rinse and the third a seal of the phosphate coating.
There are two potential obstacles you need to be aware of prior to committing to this type of pretreatment system, and they both focus on the first stage of the system. As mentioned, the chemistry of that first stage is modified significantly to both clean and coat the product. As a result, there are compromises that need to be made in order to perform both functions. As a result, the first stage has a lower ability to clean and generally provides a lower coating weight of the phosphate.
The first concern is related to the amount, type and tenacity of process oils and lubricant on your surface. For example, if there is a significant amount of a heavy drawing lubricant that may have a tendency to sit on the part for a while before pretreatment, it is unlikely that the first-stage chemistry will adequately remove that contaminant.
The second concern is only applicable if you have addressed the first one of cleaning. In order to provide a phosphate coating on aluminum, the chemistry of the bath needs to be modified in order to break down the stable aluminum oxide layer and form an aluminum phosphate coating. That bath chemistry will include a fluoride modification in order to accomplish this. Again, this can be done, but your chemistry alternatives are fewer.
The alternative to these compromises would be a more expensive five-stage pretreatment system where the first stage is a dedicated cleaner that can be formulated to remove even the most difficult to remove contamination. The second stage is a rinse followed by the third-stage phosphate coating. Again, having a dedicated process stage to this step allows for a more focused chemistry to provide the pretreatment you may need to satisfy the performance and durability requirements of your painted product. The fourth and fifth stages are rinse and seal, respectively.
Prior to embarking on the purchase of any type of system, you should better understand the chemistry of your pretreatment alternatives and their capabilities.
I suggest looking at PFOnline and starting a search under the Supplier tab for Cleaning and Pretreatment Chemicals.
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