Friday, April 29, 2011

ELECTROPLATING

The coating can be quite thick and many different metals can be used like copper, nickel, gold, silver, chrome etc. The most popular substrate plastics are ABS and PP. Polysulfone, phenylene oxide based resins and polyethers are also used.
Process:
It involves a series of Preplating treatments followed by the electroplating operation.
Preplating treatment:
It consists usually of cleaning, etching, neutralizing, catalyzing, accelerating and electro less nickel or copper deposition.
a)Cleaning:
To remove soil, particles, fingerprints, etc. The cleaning solution consists of mild alkaline cleaners or detergent solution depending on the type of plastic.
b)Pre Etching:
Sometimes a conditioning or pre etching step is considered to make the plastic more receptive to the etching process or to overcome the thermal problem involved with the exposure to the heated etch bath. For ABS organic solvent is used as a pre etch step and for PP special silica filled PP is required.
c)Etching:
The etchant solution (solution of chromic acid and other strong oxidising agent) softens the surface of the material and then the etchant will selectively attack the butadiene domains on the surface. This produces porosity or micro etches and the surface is rendered hydrophilic. Any remaining foreign matter on the surface is also removed.
d)Catalyzing:
The catalysts are used to introduce acidic solution of tin and palladium salts. The tin compounds are absorbed in the surface of plastic and also carry with them the palladium ions which act as ‘seeds’ to catalyse the electro less deposition of nickel or copper.
e)Accelerating:
It removes excess of tin salts and the accelerators also intensify the effectiveness of the catalyst particles.
f)Electro less nickel or copper deposition:
A thin (0.0010 to 0.0025 mill inch) conductive metallic coating is deposited on the surface of the treated plastic to provide the electrical conductivity necessary for the electroplate operation. Generally the electro less metal is chemically reduced from solution on the catalysed surface. The parts at this stage should be handled with great care to prevent degradation of the extremely thin and fragile conductive coating otherwise it would be reflected in the final electroplated coating.
Electroplating Operation:
The Preplating plastic moves in a mild H2SO4 etch bath. The n the 1st electroplate layer of copper is deposited. This soft, ductile copper coating is deposited from an acidic copper solution. This soft copper provides a smooth coat for the succeeding coat and it acts as a stress (induced by the nickel and chrome deposit and thermal) absorbing layer for the next coating. The thermal stresses are induced by the differences in thermal coefficient of expansion of plastic and the metals. Nickel and then chrome is deposited on the plastic substrate which is electroplated with copper. Sometimes gold, silver or tin is deposited over the copper or nickel if the functional requirement of the application dictates. When more corrosion protection is required a semi bright nickel is used instead of a bright nickel. Sometimes a dual nickel system (stress free semi bright nickel and a bright nickel) or a triple nickel system (semi bright nickel, bright nickel and a thin deposit of a stress inducing nickel to provide additional protection between the nickel and the chromium plate) are used.
Design Guide:
a)Part should be designed for moulding in one piece because mechanical welds are difficult to plate.
b)Gates should be hidden on non critical surfaces.
c)Deep recesses or blind holes should be avoided because plating solution may build up in cavity.
d)Non critical surface areas should be designated and designed for cathode contacts because plating will not be as uniform or a bright in these areas.
e)Sufficient rigidity should be allowed to prevent part warpage under elevated temperature during plating.

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