METALLIC FINISHES

Significance:
It is applied to plastic parts for decorative purposes, to provide electrical conductivity, to minimize electromagnetic interference and to provide surface with improved solvent and/or abrasion resistance.
Methods of Metallic coating:
1. Vacuum metalizing
2. Electroplating
3. Barrel plating
4. Hot stamping
5. Spray plating
The choice of the process depends on the type of the part and the environment in which it is used. The first two techniques are the most popular techniques.
Vacuum Metalizing:
Definition:
The use of a vacuum chamber to deposit a metal coating on a part or on both surfaces of a plastic film or sheet is called vacuum metallization. The metal coating is usually very thin Aluminium.
Process: It is performed in three phase: A) The base coat application B) Metal deposition and C) Top coat application.
A)The base coat application:
A clear, high gloss material is applied to the substrate. It is specially formulated to have high adherence to the particular substrate, to smooth out any small surface imperfection and to act as a mechanical and molecular bonding surface for the vacuum applied metal. Improper base coats or improperly applied coating can cause low adhesion or failure of the metal coating under humid condition hence the parts before coating should be thoroughly cleaned and preferably be moulded without mould release agent. Sections of the part that are not to be metalized should be masked prior to the base coat application.
B)Metal deposition:
The small wires of metal to be deposited (usually Aluminium) is placed on the tungsten filament which is heated to a temperature that causes vaporisation of the wires. The metal vapours condense on the parts that are on rotating racks inside the vacuum chambers. Since the condensation is a line-of-sight process correct placement, rotation of the part and racking is very important for uniform metal deposition.
C)Top coat application:
It can be a clear, abrasion resistant coating; or it can be a tinted coating, so that the final colour may have a simulation of Gold, Copper, Brass or any other transparent shade. It is specially formulated for maximum adhesion to the deposited metal and not affecting the adhesion of the metal to the base coat. It protects the metal from wear, oxidation or any environmental effect.
Though any metal can be theoretically deposited by this process, Aluminium is the most popularly and particularly chosen because of its low cost, high reflectivity and brilliant surface. Since the coating is very thin (several millionth of an inch) the top coat is required. However the transparent top coat is required when the metal is deposited on the rear or 2nd surface (hence 2nd surface metallization process) of a transparent plastic or glass part and a final protective coating of an opaque, heavily pigmented lacquer may be used.
Design guide:
a)Sharp corners, edges and serrations should be avoided. The top coat will fail at these areas first when exposed to abrasion.
b)Moulds should be highly polished.
c)Gates should be adequate to reduce stress which may result in craze marks on the application of the base coat.
d)Mould release should not be used if possible. Internal lubricants should also be minimised because they interfere with good adhesion of the base coat or the metal.
e)Low density areas in the part, caused by too rapid mould filling and insufficient dwell, may cause irregularities in the base coat such as dullness and loss of adhesion.
Recent Development:
The most popularly used plastic substrates are Polystyrene, Acrylics, Phenolics, unplasticized Vinyls and Polycarbonate. Several formulations of base coat are developed that can be used with variety of substrates e.g. ABS, Phenylene oxide based plastic. Some base coats are suitable for Polystyrene and ABS.
Fast curve base coats are developed especially for the high volume conveyorized shapes. Vacuum metalizing base coats which can be applied to primer less Polypropylene.