Metal finishing Q&As since 1989

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Chrome plating on HIPS?

2002

Hi,

I am a mechanical engineer and the company I work for is interested in chrome plating for plastic parts as part of their higher end consumer product roadmap. I am interested to know about the possibilities of plating HIPS instead of the usual ABS. What are the pros and cons involved?

Are there any good recommendations for low cost chrome plating companies I can work with in the Asia region?

Loh Yeow Meng, Andy
- Singapore

ANYTHING can be plated, Andy, whether it be an orchid or a rock. The question is whether the plating will adhere sufficiently for the paticular application in question. When it comes to really difficult applications like automotive exteriors, a tremendous amount of development has gone into pretreatment processes for ABS, so that the plating will indeed stick under such difficult conditions. Some grades of fiberglass have been successfully electroplated as well. In the case of ABS the butadiene can be etched away, leaving great tooth.

Standards & Guidelines for Electroplated Plastic is a great introduction to plating on plastic.

I doubt that you can develop a really robust, ultra high adhesion pretreatment process in house, so you would need to ask the major suppliers if they have one for HIPS. If they don't, it still may be plateable, depending on the application severity. But if it's not going to be quite right, you might consider something simpler and cheaper like vacuum metallizing or perhaps the new generation of chrome-look paints.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

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DEAR Mr. Loh,

What is HIPS ? It is polystyrene WHY it is HIGH IMPACT? BECAUSE IT CONTAINS RUBBER [buta diene]. So IN MY opinion IT IS PLATABLE since it will etched either with chrome based sol. or alkayne sol. So the rest of ABS plating procedure could be done.

RAAFAT ALBENDARY
- Cairo, Egypt
2002

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2002

HIPS can be electroplated by etching it in chromic acid. As mentioned above, it contains butadiene, so this can be dissolved out, as it is in ABS. The simplest way is to soak it in hot (45 C) chromic acid for about 15 minutes and then neutralize it in sodium metabisulphite solution. Ensure it is totally neutralized because chromic acid is a very powerful oxidizing agent and gets everywhere and will contaminate the upstream baths. Thoroughly rinse the HIPS in good quality water and then activate it as you would for any electroless metal deposition process. The simplest way is to use the palladium-tin system and then deposit either electroless copper or nickel. It is also very easy to spray silver onto the HIPS. This is done by spraying two aerosols of ammoniacal silver nitrate and a reducing agent such as glucose and/or formaldehyde. The silver is reduced in the aerosol and deposits onto the HIPS. Once you have a metallizing layer, just build up the thickness with electrolytic nickel and/or copper so that the metal will be able to take the currents you need for chromium plating. However, be warned that if you use hexavalent chromium, any uncoated HIPS will be etched during the chromium plating process. You could of course always use trivalent chromium, but it depends on your needs.

An alternative method of etching the HIPS is to use a plasma discharge. This is very effective with HIPS and MIPS, is easier and environmentally far superior. If the HIPS is molded, any superficial surface damage can be cause a poor area of plating and it leaves a mark similar to that often seen at sprue sites on injection molded parts. I believe this to be due to the outer skin of molded HIPS being a slightly different composition to the bulk polymer in that the surface is depleted in butadiene. Any damage to the surface will expose the inner structure. The plasma discharge reacts differently with the polymer surface in that it makes the surface hydrophilic by bond cleavage, not butadiene dissolution and partial oxidation of the HIPS, as seen with chromic acid.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

I am looking for a paintable product that will give a Rotomolded Urethane part a high gloss Chrome appearance. I heard Silver Nitrate will do this, but I can't find anyone that uses it. Is Silver Nitrate still being used?

Steffen Hveven
- Broomfield, Colorado, U.S.A.
2005

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Two-component silvering, that is, simultaneously spraying a part with silver nitrate and a reducing agent, is a venerable process that still finds wide use, and is the right answer for many applications. Suppliers like G.J. Nikolas [a finishing.com supporting advertiser] would offer this. But I tend to think that the new generation of chrome-look paints from suppliers like Alsa and might be the answer for this application.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005