dirk broekhoven

Topic: powder coating of extruded aluminium

Hello everybody,

We currently have a problem with a failure that looks as a dust-inclusion after powder coating on a aluminium extruded profile for automotive exterior purpose.

After some further investigation and improvements in our painting line, we decided to clear-coat some parts to see if the failure is coming from the substrate (also see pdf document in attachment).

On the pictures you can clearly see black points, that are to my opinion small holes in the aluminium causing out-gassing.

My question, is it possible that there are air inclusions in a extruded aluminium profile?

If so, how can they be reduced?

We also found a lot of aluminium particles in the coating coming form the sanding operation after stretch-bending of the profiles. What is the best possible way to remove them? Power-wash is not a good way to remove sticky particles from the substrate.

Currently we wash the parts manually with scotch bride while flowing water over the parts. This gives moderate improvement.

Thanks,

Dirk
Attachments:

  dust inclusions.pdf

DustinGebhardt

Re: powder coating of extruded aluminium

Dirk,

  To see if the parts are out-gassing, you can try to pre-bake your parts.  Run them through your curing cycle without any powder, then run them again as normal.  You may also want to try 2 pre-bake cycles for very stubborn out-gassing.  

Regarding the cleaning of the parts, you may want to try different cleaners.  Perhaps a phosphate/cleaner (which would also improve adhesion and corrosion protection).  For very troublesome parts, you may want to go with an immersion process using ultrasonics.  Or perhaps a good non-etch cleaner.  Or perhaps a mild-etch cleaner followed by a deoxidizer/desmutter.  Two that I've commonly used for many types of aluminum are nitric acid (24-50%/vol) or tri-acid (I don't recall the exact formula, but it has nictric, sulfuric, plus 1 more acid). 

-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC

dirk broekhoven

Re: powder coating of extruded aluminium

Hi Dustin,


Thanks for your answer.


So you think out-gassing is possible on an extruded aluminium profile (I've known this problem from die-casting alu). I will give it a try in our drying oven. I will set it up with the same temperature as the curing oven.


we already have a pre-treatment with different de-greasing stations and a phosphate/cleaner before chromating (CR3). We also tried the ultra-sonic cleaning on several 100 parts. Without any succes. Now, I'm not shore if we're mixing the 2 problems together. What I meen is, even if we can reduce the alu inclusions, we still might have problems with outgassing. It's very hard to see the difference after painting. I will do some further investigations on this point.


Regards,


Dirk





Jay V

Re: powder coating of extruded aluminium

dirk broekhoven wrote:

(SNIP) I will give it a try in our drying oven. I will set it up with the same temperature as the curing oven.(SNIP)

For efficient outgassing, I would increase the temp 50+ degrees over cure temp and increase time significantly as well.

This type of operation is a lot easier on a small scale using batch equipment vs. a conveyor.

But I don't know if that could even be the problem - I was under the impression that outgassing should only be an issue with cast parts.

Jay

dirk broekhoven

Re: powder coating of extruded aluminium

Hi Jay,


I had the same thought about the out gassing on cast alu vs. extruded. Maybe the lower quality of the Chinees (I work in Beijing) alu might have an influance. As far as the pictures shows, it looks like small holes. I will send send some parts to our head-office for further investigation with a microscope to make shore.

I give it a try with a higher temperature in the curing oven. We don't have a batch oven, so I have to use the curing oven.


Thanks,


Dirk