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Unable to Hard Anodize diecastings

We are manufacturers of textile machinery parts, we have manufactured one of the parts by using the pressure die-cast method. but we are having problem in giving it a hard anodizing coating. Is there a problem coating products which are derived from pressure die-cast method? The material we are using at present is an aluminium alloy and and 0.82% of silicon content in it. the problem is that the product is not accepting the coating, it becomes Dull grey which is also not uniform throughout; we want a black colored hard coating on it. please tell what should I do, to solve this problem.

Navneet Bakshi
- Surat, Gujrat, India
2005

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Dear Navneet Bakshi,

I think you need to check the alloy. You are telling that you are using an aluminium alloy with only 0.82 % silicon. Such an alloy should be easy to anodise but unsually very difficult to die-cast.

Please return with more info about the alloy.

Anne Deacon Juhl
- - Trustrup Denmark
2005

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For Anne Deacon Juhl

Thank you for replying to my query, mam could please suggest me, which grade of aluminum alloy can I use to get a black colored hard anodized coat. I want to derive it from a pressure die-cast process, so which alloy do you suggest and if possible do list down the constituents and their %, I'd be really grateful..

Navneet.S.Bakshi
- India
2005

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You should go for alloys with up to 5 % magnesium, without copper and with silicon max. 1 - 2 % or alloys with up to 5 % zinc without copper and silicon.

Hope this work.

Best regards

Anne Deacon Juhl
- - Trustrup Denmark
2005

adv.