Letter 32016

Poor powder paint adhesion on aluminum [U.K.] 

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Question: how do we get powder paint to adhere to aluminium castings? We are at our wits end. We have been doing our stuff for 15 years and are getting items returned from all over the world. The paint just comes off in sheets. the castings are clean and they are dipped in a heated solution called granodine 606. problem is that even the makers of the powder don`t seem to know why it's happening. We only follow the directions we have been given by our suppliers.

Richard Hayne
cast metal signs - Dolgellau, Gwynedd, UK

Dear Richard,

We came across your problem a few months ago - our customer drove all the way down from Lancaster to Birmingham to bring a casting just showing exactly the same defect on a aluminium casting - the powder was coming of in sheets just like yours. We put the item in our lab oven and baked it for a further 15 mins at 200C - the results were perfect - no loss of adhesion etc. What had happened was the sprayers (??)had run the track faster in his absence it was as simple as that.

Try stoving a little longer - have the oven checked out using a Grant or similar temp gauge (your powder supplier should help there). You require at least 10-12 mins metal temp at 200C to cure polyesters - are you sure obtaining this?

If this does not improve matters pls advise and we will then give advice on temp etc to adhere to for pre-treatment etc.

Terry Hickling Martex Paints Ltd    Birmingham, UK

In order to guarantee good adhesion, during the pretreatment stage of phosphate, you need to add a flour additive, you can ask for it from your pretreatment supplier. If the paint doesn't show adhesion after some machining of the casting, maybe you are using a wrong paint.

Miguel Segoviano
- Qro, Mexico

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