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Plating on screws--appearance problem in heads

We are using zinc plating for our screws (M5). After plating inside of heads black marks had come (our supplier said that it is due to forging), but after plating also it had come. If you replate the same screws, 50% only passed. Kindly suggest to short out this problem in screw appearance or any other special cleaning before plating is preferable?

Bilal Mansoor Akbar
quality engineer - Pondihcerry, India
September 2, 2009

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Hello Mr Bilal,

It might be far better to ensure proper cleaning of the screws prior to Heat Treatment, in fact a thick phosphating before Heat treatment will be more helpful.
This will prevent such thick scale formation on the screws in the first place, it is nearly impossible to remove such scales left from carbonaceous pitches caused by HT.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

September 6, 2009

I understand it is due to forging process. But why this black dirt particles are not removed from the screws even after anodic degreasing for 20 min. Is any special process is required to remove this black dirt?

ANANTHA RAMAN.S
Senior Engineer - Pondicherry
September 14, 2009

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Hi

I agree with Vahanwala that we have to attack the problem before HT.

In my opinion if you heat treat the parts along with rust preventive oil the scales due to volatile nature and subsequent oxidation of the organics is more. Hence, try the HT after removing rust preventive if you earlier did so.

Or

I am sure if you preclean the parts by vibro polishing before zinc plating process you will solve the problem

Good luck

Vijendran Sudeendran
- Chennai, Tamil Nadu, India
September 15, 2009