Cause of Defects

[Toefuzz]

Greetings! I work for a small liquid/powder coater and am looking for advice on how to calm the eternal debate between die-casters and coaters as to what is actually causing a defect. We use an immersion zinc phosphate system consisting of a cleaner, pickling agent, conditioner, micro-crystal zinc phosphate, city rinses (less than 200 ppm) and a final DI rinse. Parts being coated are zinc die-castings for the automotive industry that must meet pretty standard Class "A" quality standards. The defect we're seeing consists of tiny specks, generally .3 mm in diameter or less, scattered about the casting. I've tried sanding the defect with 400 grit sandpaper under 40x magnification and do not see anything indicating gas out (mainly metal or a crater), nor do I see any contaminants (dirt, hair, etc). I simply keep sanding until the powder is gone and all that is left is the metal substrate. The defect is present at varying thicknesses (2 - 5 mils), which seems to rule out seeds in the powder. The parts are pre-baked prior to coating.

I see this defect on parts from different die-casters (though always on zinc castings), processed through different wash lines, using different powders. Through various experiments we've ruled out contaminents from the oven. Is it possible (or likely) that this defect is gas out? If the part gasses out early enough in the oven could the powder flow back over, thus hiding the typical metal specks we expect to see when sanding? If it's not gas out, do you have any other ideas? These 'specks' account for approximately 75% of our defects (though overall defects are only 5 - 10%) so we are obviously anxious to have the problem solved. Any ideas?

[Blademan]

Re: Cause of Defects

I'm willing to bet the farm on your prob is with out gassing....The manufacture that I powdercoat for pours there own secrete sauce aluminum (not a speller just a pc'er) . for their castings rangeing from dog handles to hatches for the govt and yachts, some batches are worse than others, for the most part when i get a problem batch I just thro em in the oven and really crank the heat up....most of the time that works, when it doesnt I will soak tyhe parts in zep-a-lum for three or four min, rinse with water throw it back in the oven and dry it and sttart all over..

[customandsound]

Re: Cause of Defects

Some times with the casting of the metal you use flux to help get the metal pure. and i found from the little parts i did sand cast. that if you use to much the metal will set like swiss cheese. The little pockets do form in the casting if the casting is cooled to quick or if it is under pure. But that stuff blademan is talking about works.

If you got a lot of pockets busting out you may want to try to use metal to metal or lab metal filler ..

You can also preheat the part to 130f for 15 minutes pull and shoot put back in the oven cure for 25 minutes at 400f That may help you

[Toefuzz]

Re: Cause of Defects

Just thought I'd update everyone as to what my problem actually was. As I said, sanding/knifing the defect under magnification didn't indicate gas out, and seeing as how I am contantly arguing with zinc diecasters about gas out I've become pretty good at spotting it. The actual problem was that the acrylic powder we were using contained ecap (a blocking agent, usually found in urethene coatings), which wasn't being released until after the powder had started to gel... hence the 'micro pinholes'. I've experienced problems with ecap in the past, but always on urethene coatings, so this was a new one to me... Just a little FYI to store in the back of your mind.

[PowderVisions]

Re: Cause of Defects

These type of problems can really be a pain. There are some good suggestions here. I have had these same type of problems, and while I didn't find the cause, I did change my pretreatment to rid the problem. Try stepping up your cleaning temperatures, change your time in the bath to a little longer and maybe step up the concentration level of the cleaner. Each one or all together may help you avoid the problem.