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Discolouration after metallising

I face a problem of metallising discolouration (in some cases it starts with rainbow colour and then metallised layer is wiped off) from reflector (poly carbonate reflector)if the head lamp (auto lamp) is kept on (bulb glowing)for a hour or so. The reflector is directly metallised after moulding and silicon monoxide protection is provided after aluminium deposition. There is no change in processing parameters. please comment. regards manish

manish
- nashik, maha, India
2003

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I suggest two problems to consider: first, the type of polycarbonate you are using may be outgassing. Discoloration may not happen during Al coating because the part temp does not rise to that level. If it did not happen in the past, ask supplier if there was a change in brand or chemistry. All PC, ABS or other plastics are not the same chemically although they come under same generic name, and there are hundreds of them.

The second, if the SiO is too thin, porous and discontinuous, which is typical of thin PVD coatings, air may be discoloring Al at higher temperatures.

Mandar Sunthankar
- Fort Collins, Colorado
2003