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Zinc plated parts get small vent holes during powder coating

Hi All,
I am currently facing problems in regards to the vent holes on powder paint. I have several pre treatment takes place which are clear chromating, iron phosphate and zinc plating. However, I am seeing the vent holes occur on the zinc plated parts. I do not face the same issue on the iron phosphate as well as aluminium clear chromating. I went through the paint evaluation and have done some eval where I reduced the oven curing temperature as well as reduced the speed of my conveyer oven. But yet, the problem still occur. Can anybody advice on this matter?

Abu Bakar Anuar
plating employee - Penang, Malaysia
May 16, 2011

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After pretreatment, but before application of powder, raise the temperature of the plated metal to 10 degrees C higher than your cure oven temp for 10 minutes, then allow to cool, apply powder, then cure in normal way.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

May 19, 2011

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First of two simultaneous responses -- May 21, 2011

Thanks Geoff,
I will start doing your proposed advice accordingly. As we are the powder paint service center, this would be an additional process to my line. Therefore, as we are the zinc plater for those parts as well, I would appreciate if anybody can let me know their experience of the problem and what is the possible cause of the issue. What are the things that I need to check and do accordingly.

Thanks,

Anuar Abu Bakar
- Penang, Malaysia

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Second of two simultaneous responses -- May 23, 2011

This is a common problem with Hot Dip Galvanised components and it is significantly reduced pretty much as Geoff describes. I would however point out that you could damage your chemical pre-treatment by overbake.
I have found it best to degrease the components then overbake even 30 degrees higher metal temperature than powder cure schedule to drive out any gas entrapped and then to chemically pretreat, coat and cure.
If they are Zinc electroplated components it is possible that you have the beginnings of "white rust" on the components.
If this is not too far progressed you can improve the situation by wiping down with dilute phosphoric acid, rinsing then chemically pretreat, coat & cure.
Hope this helps.
Regards,
Bill

William Doherty
Trainer - Salamander Bay, Australia