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Follow-up question-powder coat problems

In question # 10377, a fellow named Harshad was having trouble with air bubbles forming in powder coat on an Al casting, even after chromatizing. The respondent suggested that he pre-heat the castings to a temperature above that of the cure temp. We are having the identical problem and I'm wondering how that worked for Harshad.

A vendor also told us that curved surfaces (our casting is a cylinder ~4 inches in dia.) are more prone to bubble formation than are flat surfaces. Anybody ever hear that one before? To me it sounds like a contrived excuse for the vendor.

James Bonnema
- Bedford, Massachusetts
2003

We have a similar problem. We have a company that produces their own aluminum castings. They admit that they have some porosity problems with their castings that do not effect the strength of their product but certainly affects our coating of their products. We gas off their castings before we powder-coat them, but this does not help. Re-sanding and then recoating does not help. I suspect that there must be a primer that will seal off the porosity prior to the powder coating, but I have no clue what will solve this customers problem. I should point out that this is NOT a customer looking to avoid paying an invoice. They are a VERY good customer looking for a solution...and so are we!

Steve Sarich
- White City, Oregon
2003

From the research I've done, the standard treatment for sealing up the pores in aluminum castings is called Alodining. I don't know the specifics, but I believe it is a type of chromate finish for aluminum. Others on this site surely would know more about it. However, in Taiwan (where our parts are made)nobody was familiar with that term, so I can't comment on how effective Alodining really is. After trying several unknown (to me)sealing pre-treatments that didn't work, we ultimately put an EDP (electro-deposited paint)coating on the casting before powder coating. We are also changing over to a vacuum casting process, which required us to build a new mold. If your customer isn't afraid to spend the bucks, that would help your situation.

James Bonnema
- Bedford, Massachusetts, US
2003